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November 3

Time dilation? Time speeding?

The beginning of the section of {Time dilation and length contraction} under Special Relativity is confusing to me. Hope someone can help. It states:

See also: Twin paradox

Writing the Lorentz transformation and its inverse in terms of coordinate differences, where for instance one event has coordinates (x1,t1) and (x'1,t'1), another event has coordinates (x2,t2) and (x'2,t'2), and the differences are defined as Δx = x2 − x1, Δt = t2 − t1, Δx' = x'2 − x'1, Δt' = t'2 − t'1 , we get

△t’=γ(△t-(v△x/c^2)) △x’=γ(△x-v△t) and △t=γ(△t’+(v△x’/c^2)) △x=γ(△x’+v△t’)

Suppose we have a clock at rest in the unprimed system S. Two consecutive ticks of this clock are then characterized by Δx = 0. If we want to know the relation between the times between these ticks as measured in both systems, we can use the first equation and find: △t’=γ△t (for events satisfying Δx = 0). This shows that the time Δt' between the two ticks as seen in the 'moving' frame S' is larger than the time Δt between these ticks as measured in the rest frame of the clock. This phenomenon is called time dilation.

I am confused by the last two sentences. To explain △t’=γ△t in more detail, we can assume the unit time in S is Ut and in S' is Ut', then the time of the {two consecutive ticks event} measured in S is △t*Ut=(t2-t1)Ut, the t2-t1 can be 1; while measured in the S' is △t'*Ut'=(t'2-t'1)Ut', the t'2-t'1 may not be 1. Since they are for the same event, we have △t*Ut=△t'*Ut', then, put △t’=γ△t in it, we have Ut=γUt', that means the clock in S' is running quicker than the clock in S. This phenomenon should be called time speeding, isn't it?

When Einstein used x'=0 to derive t’=t/γ, so that △t’=△t/γ, we have time dilation; how can we call △t’=γ△t as time dilation too? Please help me to understand that paragraph. Thanks.

Regards, John HuangJh17710 (talk) 04:53, 3 November 2010 (UTC)

Relativity of simultaneity actually has some nice, non-mathematical, explanations of the problem. If there are any two events, seperated by some arbitrary distance, which are perceived in one reference frame to occur at the same time, there will exist other reference frames which perceive them not to be simultaneous, some in which one event occurs first, and some in which the other event occurs first. This perception is dependent not only on the location of the reference frames but on their relative motion to each other. In other words, for your problem, lets assume that you and I are moving relative to one another. If we both start our stop watches at what we perceive at the same time, there will exist three different classes of reference frames relative to us:

• Some will perceive our clocks to run at the same speed.
• Some will perceive your clock to run faster than mine
• Some will perceive my clock to run faster than yours

That's the key problem with Special Relativity, there is no reference frame which is "right". All are equally valid. By convention, we hold the reference frame of the designated observer to be "stationary" and measure time changes relative to that one; thus since everyone else is moving relative to THAT observer, everyone elses clock is running a little bit slower. But this convention holds regardless of who you choose as the observer. If two spaceships are moving away from each other at some fraction of c, but each is in communication with the other, each will perceive the other's clock to be slowed down. If they then, at some later time, change speeds to be stationary relative to each other, the perception of which clock slowed down will depend on which ship had to speed up or slow down to "match speeds". If the two ships performed identical, mirror image paths and speed changes, then the clocks will match when they reach the same relative motion. If one ship decelerated while the other maintained a constant speed, you will get different results. Special_relativity#Lack_of_an_absolute_reference_frame and Consequences of special relativity also discusses some of this. --Jayron32 05:34, 3 November 2010 (UTC)

The OP is makingn a crucial mistake. He states "Since they are for the same event, we have △t*Ut=△t'*Ut'". That's wrong. In fact both observers will be using similarly constructed clocks which will be using the same time unit, say a second: Ut=Ut'= 1 second.67.78.137.62 (talk) 15:22, 3 November 2010 (UTC)

Yeah, the phrase "time dilation" is potentially confusing, because that phrase by itself doesn't do a great job of helping you remember which observer will measure a greater amount of time between two events under which circumstances. Fortunately, there's another phrase that expresses the time dilation phenomenon that's more helpful in that regard, namely "moving clocks run slow". That phrase is particularly useful if you think of a "clock" as being a light pulse bouncing between two mirrors, as described at time dilation#Simple inference of time dilation due to relative velocity. In this case, the clock in question is stationary in the unprimed system, so it's moving from the perspective of the primed system. Therefore, the "seconds" as ticked out by the clock in question will appear to be longer than a "real" second, according to the primed observer. That's time dilation.

The reason you're thinking of this problem as being a "time speeding" is essentially because you're thinking that the clock in the primed system that's measuring the time between the two events in question is running faster than it ought to, as considered from the perspective of the unprimed observer. The crucial flaw in that line of reasoning is that there is no such clock in the primed system.

You're making the mistake of thinking that there's a clock in the unprimed system that measures the time of events in the unprimed system, and a clock in the primed system that measures the time of events in the primed system. But the time of events can't be accurately measured that way. Because the speed of light is finite, trying to measure the time of all events with one clock would run into the problem of there having to be a delay between when an event occurs, and when news about the event reaches the clock, informing the clock to measure the current time. You might think you could just take the time delay into account, but that would involve needing to know the distance that the light traveled in between the event and the clock, which opens up a whole other can of worms. So what's done instead is that times of events are measured by a system of synchronized clocks, stationed at every location of interest. It is possible to synchronize a system of clocks, as long as all of the clocks in the system are stationary with respect to each other. However, to an observer that's moving with respect to those clocks, the clocks won't appear to be properly synchronized.

In the problem in question, the two events involved can indeed be measured by a single clock that's stationary in the unprimed system, because the two events occur at the same location as measured by the unprimed observer. However, the two events are not at the same location as measured by the primed observer, so the primed observer, who measures events with clocks that are stationary with respect to the primed observer, must measure the two events with two different clocks. The reason the unprimed observer thinks that the primed observer measured too much time in between the two events is not at all because the unprimed observer thinks that the primed observer's clocks are running too fast. Indeed, the unprimed observer will think that the primed observer's clocks are running too slow. Instead, the unprimed observer will consider the primed observer to have measured too much time between the two events because the primed observer's clocks aren't properly synchronized. Red Act (talk) 18:37, 3 November 2010 (UTC)

Heart muscles

• What property of the heart muscles are exploited in the heart transplant?
• In blood groups,why is it imposible for one individual to carry all 3 alleles?
• why is the oxygen dissociation curves of animals living at high altitudes located to the left of most of those of other animals?

— Preceding unsigned comment added by 196.0.7.4 (talk • contribs)

Hmm, these look suspiciously like homework questions. Try reading "Heart", particularly the "Functioning" section. Axl ¤ [Talk] 10:41, 3 November 2010 (UTC)

"ABO blood group system", section "Inheritance". Axl ¤ [Talk] 10:44, 3 November 2010 (UTC)

"Oxygen-haemoglobin dissociation curve", section "Factors that affect the standard dissociation curve", will point you in the right direction, although it does not contain the answer that you seek. Axl ¤ [Talk] 10:50, 3 November 2010 (UTC)

Well an individual could carry all 3 groups if they were triploid... 128.143.170.201 (talk) 19:26, 3 November 2010 (UTC)

...or is a Chimera (genetics). DMacks (talk) 01:23, 5 November 2010 (UTC)

The proportion of swine flu to all flues

Of those people having flu in late 2009 in the UK, what proportion of them would have had swine flu, diagnosed or not? 2009 flu pandemic. Thanks 92.24.178.95 (talk) 18:41, 3 November 2010 (UTC)

It's a bit vague ('late 2009') yet also specific (UK). Not sure of that exactly. If it's any help the WHO Centre on Influenza in Melbourne says 90% of flu viruses in Australia and NZ in 2010 were H1N1 2009. The WHO influenza page may hold the information somewhere, but I could only see data going back for this year - the map there for January 2010 for example indicates about 80-90% of flu cases in Western Europe were H1N1 2009, but it doesn't seem to refine down to countries. The European influenza section of the WHO site may contain some information, as may the WHO Global Atlas of Infectious diseases, but that seems to want a login. --jjron (talk) 02:33, 4 November 2010 (UTC)

Try Google Flutrends, which should show the influenza cases and past predictions for individual countries. ~AH1^(TCU) 18:27, 7 November 2010 (UTC)

limestone make-up and other soft stone

(Copied from the Earth Portal's talk page)

I don't know if this will reach anyone but am going to try anyway. I am an authenticator of North American Indian artifacts. I have been sent several items recently by a man that are no brainers to identify but have one really odd piece that I have not come accross before. It is a statuette of an Aztec or Central Amer. pre-columbian art. It appears to me to be Peperino Tuff. Whis is not from this hemisphere I don't believe. More likely Italy. My question is, could I be mistaking this for some sort of Limestone. Under high powered magnification it shows fine crystal like clusters covering the surface with isolated black pepper like crystal inclusions. It also appears to have a pinkish hue deep with-in the stone. It is porous with a conglomerate look to it. It also has a slight sodium taste to it when touching your tounge to it. Can anyone help itentify this stone type??? How long does it normally take to get a response for questions? Many thanks, Bob 11/03/10 —Preceding unsigned comment added by 74.197.184.126 (talk) 18:51, 3 November 2010 (UTC)

Your description sounds very similar to that in our Peperino article (which needs to be merged with our peperite article I see) and much less like a limestone, but note that that rock does contain fragments of limestone. Peperites are found all over the world however, Nigeria, Spain, Wales, France, Italy, California, Australia, Argentina, Canada, Chile, Japan, Antarctica, Kenya, South Africa, Scotland (just a quick selection from [1]), so it may not tell you much about provinance. Mikenorton (talk) 21:21, 3 November 2010 (UTC)

If you want more confirmation, maybe try contacting one of these geologists at your local university (I worked that out from checking where your IP locates). If you register an account here and take a photograph of the piece and upload it, then someone may be able to help further as well. SmartSE (talk) 11:52, 4 November 2010 (UTC)

if the moon and moon landings are real

why didn't they run around the moon to prove it is a sphere and not a disc like it looks? 85.181.145.78 (talk) 08:58, 3 November 2010 (UTC)

We can observe 59% of the moon's surface, thus it is not a disk Tidal_locking#Earth.27s_Moon. Also there have been plenty of satellites (inclusing the moon landers themselves) that orbited the moon, observing it completely. See e.g. SELENE. Furthermore, the moon is simply too large to run around, especially with a limited oxygen supply. 157.193.175.207 (talk) 09:28, 3 November 2010 (UTC)

To put it in context, the equatorial circumference of the moon is 10,921 kilometres (6,786 mi). Ghmyrtle (talk) 09:35, 3 November 2010 (UTC)

The Apollo rover had a top-speed of 18 miles per hour (29 km/h), and a range of 57 miles (92 km). IIRC, NASA didn't want the rovers to go out of walking-distance of the lander, incase they broke down. CS Miller (talk) 09:52, 3 November 2010 (UTC)

The following is imported from a duplicate question

It's rather a long way! They did fly round it - will that not suffice to convince you? (later) sorry edit conflict. The much better answer above wasn't there when I started to edit. Dbfirs 09:31, 3 November 2010 (UTC)

See Apollo 10. BTW that also confirmed that the Earth is (roughly) a sphere in case you doubt that too. Cuddlyable3 (talk) 13:38, 3 November 2010 (UTC)

Actually Apollo 8 of 1968 was the first mission in which humans circled the Moon and observed its far side; and, a decade previously, the Soviet Luna 3 mission of 1959 was the first probe to circle the Moon and send us photos of the far side. Comet Tuttle (talk) 17:17, 3 November 2010 (UTC)

Besides, the nature of lunar phases is sufficient to demonstrate that the moon is spherical. This has been understood for thousands of years. — Lomn 13:42, 3 November 2010 (UTC)

In other words, the moon doesn't look like a disc. Simple observation of the moon with the naked eye makes it abundantly clear that it is nearly spherical. Nimur (talk) 15:42, 3 November 2010 (UTC)

Trolling questions are best ignored. Looie496 (talk) 17:41, 3 November 2010 (UTC)

Observe this film animation of the Libration of the Moon and ask yourself whether a flat disk could do that. Cuddlyable3 (talk) 21:30, 3 November 2010 (UTC) I restored the question, see Talk page.Cuddlyable3 (talk) 21:30, 3 November 2010 (UTC)

Technically, that's not a film, it is a simulation generated from composite texture photographs by Clementine (spacecraft) (you can plainly see the orbit-track artifacts!) But the point stands. Careful long-term observation of the moon with the naked eye reveals exactly the same behavior. Moon enthusiasts regularly photograph this behavior. Nimur (talk) 21:43, 3 November 2010 (UTC)

Thank you Nimur for the correction. The animation is for a theoretical observer at the center of the Earth.Cuddlyable3 (talk) 08:03, 4 November 2010 (UTC)

Why does the moon get larger and and smaller in that animation? Does the apparent size from earth change that way over a month? Ariel. (talk) 22:28, 3 November 2010 (UTC)

Yes. As you can see in the infobox at Moon, the apparent angular size of Moon as observed from Earth fluctuates between about 29.3 to 34.1 arc-minutes - a visible difference of around 20% ! This is because its orbit, while "nearly" circular, actually has a perigee of 360,000 km and an apogee of 400,000 km - a pretty huge difference! Nimur (talk) 22:31, 3 November 2010 (UTC)

Why on earth would they run around the moon? That is very nearly the most useless thing they could do with their brief and incredibly expensive time on the moon. And it wouldn't have occurred to them that anyone would doubt that they were there. -FisherQueen (talk · contribs) 22:35, 3 November 2010 (UTC)

If they had "run around the moon", why would any of the moon-landing-deniers believe them?! I can't see why that would be "proof" of anything to anyone for whom their having orbited around the moon isn't "proof" of anything. Do you really think that would have "proven" to you that the moon is a sphere[oid], OP...? WikiDao ☯ (talk) 01:12, 4 November 2010 (UTC)

The OP's question caused some initial confusion because its premise "if the moon and moon landings are real" has nothing to do with the real question and instead provokes some to think this is about Moon landing conspiracy theories. The question itself is reasonable because (i) the full Moon looks disc-like because of its even illuimination, and (ii) until the late 1950s the Far side of the Moon was an unseen mystery. On October 7, 1959 the Soviet probe Luna 3 did indeed "run around the Moon" and took the first photographs of the lunar far side. Comet Tuttle already mentioned this. 08:20, 4 November 2010 (UTC)Cuddlyable3 (talk)

A flat-Earther would point out that a single circumnavigation will not prove that the Moon is a sphere. You've got to circumnavigate it longitudinally and latitudinally. That would be a lot of running. APL (talk) 14:30, 4 November 2010 (UTC)

And people have circled the earth and the flat-earth people don't believe them, so I don't see circling the moon as convincing to people dedicated to believing the moon landings were a conspiracy. Googlemeister (talk) 14:34, 4 November 2010 (UTC)

They have an explanation for longitudinal circumnavigation of a flat Earth, but if someone flew from South America to Australia via the south pole, then their theory would be in trouble. —Arctic Gnome (talk • contribs) 16:14, 4 November 2010 (UTC)

The mere act of doing that would not disprove the Frisbee model of the Earth. It would be entirely possible to start at South America, fly out to the outer edge of the world (Known to round-earthers as "the south pole"), fly about halfway around the edge, and then fly back inwards to arrive in Australia.

Admittedly, the observations made along the way would pretty much prove it (Man, We've been flying for hours and were still over the south pole?), but who trusts observations made by scientists?!? APL (talk) 20:23, 5 November 2010 (UTC)

The manned probes that used a Lunar orbit rendezvous course did indeed complete a loop around the Moon. ~AH1^(TCU) 18:25, 7 November 2010 (UTC)

From childhood, it never seemed reasonable to me to perceive the Moon as a disk when looking at it with the naked eye. After all, it looks like a ball, and being a ball, the pattern of light and dark makes sense, whereas otherwise, why would these patterns be there? So cartoon depictions of somebody setting on a crescent moon, etc. seemed only like some bizarre comic convention to me when I was growing up. I'm curious how the OP ended up seeing it the other way, and if it's actually common for people to do so. Wnt (talk) 12:04, 8 November 2010 (UTC)

November 4

emissivity

physical meaning of emissivity —Preceding unsigned comment added by Quiet.mp (talk • contribs) 02:15, 4 November 2010 (UTC)

See Emissivity. -- kainaw™ 02:18, 4 November 2010 (UTC)

Chemistry

What substance melts at Negitive 97 degrees and boils at 76 degrees? —Preceding unsigned comment added by 67.182.212.131 (talk) 03:34, 4 November 2010 (UTC)

Seems like that might depend on whether you mean degrees Celsius, Fahrenheit, etc. DMacks (talk) 03:36, 4 November 2010 (UTC)

Regardless of which temperature scale you use, there are likely to be hundreds of substances which broadly meet these requirements. Given the millions and millions of possible substances in existance, it is quite impossible to identify a substance given only those two bits of info. --Jayron32 03:38, 4 November 2010 (UTC)

See acetyl bromide for a common substance with those specific melting/boiling points (within 1 degree). -- kainaw™ 03:43, 4 November 2010 (UTC)

Why isn't the acetyl bromide boiling point a sharp point? Is it because there are actually multiple species upon boiling, or that there are different microphases? John Riemann Soong (talk) 04:01, 4 November 2010 (UTC)

Probably because no one has bothered to measure the boiling point any more specifically. If you want to get an exact boiling point (where vapor pressure = 1 atm), you're going to need to control both the temperature and pressure pretty well. Anyway, my CRC handbook gives 76 C. Buddy431 (talk) 04:32, 4 November 2010 (UTC)

Just to explain in a bit more detail, while melting point is usually vital information for identifing a compound (melting point analysis is usually a standard analytical technique for identifying both identity and purity of a compound), boiling point usually is not, being that boiling point tends to be had to control for, as it will vary slightly due to a LOT of different factors. So, while someone could nail down a more exact boiling point (equipment and standard techniques do exist for this) they may not have, at least in the literature used as a reference for the Wikipedia article. --Jayron32 05:06, 4 November 2010 (UTC)

Is this slightly harder (than for say, water) because of the tendency of acyl chlorides to be impure? John Riemann Soong (talk) 06:34, 4 November 2010 (UTC)

Autoclave naming

Autoclave means self-locking device. But why did they get this name? It seem to me they got named after an insignificant feature compared to their general purpose. —Preceding unsigned comment added by Jib-boom (talk • contribs) 04:39, 4 November 2010 (UTC)

I guess that the self locking devices to prevent blow-out on the early pressurised autoclaves were unusual enough for it to be included in the name of the machine. Perhaps something like "Modern Autoclave Sterilising Machine. Like all new words 'autoclave' became the significant part of the name and possibly it became 'The Autoclave', with the purpose now well known. The English language is a strange and infinitely mutatable medium. There are many examples of names that have left their original meanings back in the mists of obscurity. ?Chauffeur,[2] for example. Caesar's Daddy (talk) 07:48, 4 November 2010 (UTC)

See google books: The London literary gazette and journal of belles lettres.. the name was invented by fr:Pierre-Alexandre Lemare, ironically a linguist.213.249.225.56 (talk) 11:52, 4 November 2010 (UTC)

His linguistic skills may have been a bit week as "auto" is Greek and "clavis" is Latin. Further, "self-lock" (using his Greek-Latin system) would be something more like "autoclaus", which my personal limited Latin knowledge tells me is close to meaning "self-contained". -- kainaw™ 12:18, 4 November 2010 (UTC)

Just like the French and German mix in the "YesYes" board game -- Sjschen (talk) 19:45, 4 November 2010 (UTC)

So if it sanitizes would it be a "saniticlaus?" Edison (talk) 18:31, 4 November 2010 (UTC)

There ain't no saniticlause! APL (talk) 13:04, 5 November 2010 (UTC)

Escape Trajectory

I understand an orbiting mass around a larger mass can be pulled into a escape trajectory by a passing mass. But what would be the simplest trajectory that would illustrate this interaction?

Just to clarify with an example: what would the trajectory of the Moon following its last orbit around the Earth look like if a planet size mass passed by and pulled the moon into an escape trajectory, avoiding a collision with both bodies? TheFutureAwaits (talk) 09:16, 4 November 2010 (UTC)

Even if we ignore the gravitation of the Sun, what you ask is a Three-body problem. They can be difficult. Cuddlyable3 (talk) 10:48, 4 November 2010 (UTC)

Sure but we know there are solutions that involve putting a mass into an escape trajectory...I'm just asking what the simplest case would be? It's really just the opposite of a capture trajectory which involves a moving mass passing near another mass at hyberbolic speeds when a 3rd body removes enough velocity through its own gravity to reduce the eccentricity <1. For seem reason I have a much harder time visualizing the reverse... TheFutureAwaits (talk) 11:34, 4 November 2010 (UTC)

Newtonian gravity is time symmetric. So, if you can visualise the geometry of a capture trajectory, just run the film backwards to visualise an escape trajectory. Gandalf61 (talk) 11:40, 4 November 2010 (UTC)

But the starting condition in this case has one body already "captured" by another. I think TFA is imagining eg. the moon being "captured" from the earth by a third body? So running that backwards gives you another "capture", not the case where all three bodies gravitationally escape each other. WikiDao ☯ (talk) 16:12, 4 November 2010 (UTC)

Simulations of three-body problems and passing rogue stars are available from this astronomy site. ~AH1^(TCU) 18:05, 7 November 2010 (UTC)

Tropical Laserbeam

If I were to take a green laster pointer and point it at the night sky will some of its light make it out of the atmosphere? What about if I use a red or blue laser pointer? Or a flashlight? TheFutureAwaits (talk) 12:38, 4 November 2010 (UTC)

Unless it is cloudy at least a few of the photons should be able to make it out into space, but unless it is a very powerful light, you would not really be able to differentiate those photons from other light "clutter". The atmosphere is pretty transparent to the visible portion of the EM spectrum. Heavy airborne pollution or dust would limit the amount escaping, but without anything to absorb the light or bend it back to earth, it will go into space. Googlemeister (talk) 13:15, 4 November 2010 (UTC)

Lasers are used to precisely measure the Earth-Moon distance, so clearly they can make it out of the atmosphere (and back); see Lunar Laser Ranging experiment. Sure these are not just your garden variety laser pointers though, but as Googlemeister says, undoubtedly some the photons from those would escape as well. The article linked above states that "...out of 10¹⁷ photons aimed at the reflector [on the moon], only one will be received back on Earth every few seconds, even under good conditions (they can be identified as originating from the laser because the laser is highly monochromatic)." --jjron (talk) 13:37, 4 November 2010 (UTC)

(ec)To avoid confusion, it should be noted that the reason so few photons return is not because so few make it out of the atmosphere, but because the the diameter of the laser beam will be very large by the time it hits the Moon (kilometers across). Most of the photons which reach the Moon strike the lunar surface instead of the retroreflector; similarly, most of the photons that do get reflected back end up spread over the Earth's surface rather than entering the (comparatively) small aperture of the receiving telescope. I wouldn't be surprised if the answer to the original poster's question is actually "most of the light makes it out of the atmosphere" — provided that it isn't cloudy. TenOfAllTrades(talk) 14:14, 4 November 2010 (UTC)

Here is a simple thought experiment: The laser pointer, or flashlight, or torch in your hand appears much brighter than Sirius. Light from Sirius easily makes it through the atmosphere, so light from your local device will make it up, too. In numbers, Sirius puts out about 10²⁸W. Your local light source puts out ~1W. Sirius is about 80228200000000000 times farther away that the flashlight (2.6 parsec vs. 1 m), so it appears 6.4e+33 times fainter than it would at one meter. So Sirius is ~60000 times fainter than you local light, and still is clearly visible through the atmosphere. --Stephan Schulz (talk) 14:03, 4 November 2010 (UTC)

Sirius is spreading light in all directions, while the laser sends all it's light down a narrow path, so I don't think your math works out that easily. Googlemeister (talk) 14:32, 4 November 2010 (UTC)

You are right, but the laser light is just as attenuated by 1/r² as Sirius. So yes, we lose a small constant factor because we do not see the backside of Sirius, but that's a minor effect. --Stephan Schulz (talk) 14:48, 4 November 2010 (UTC)

I am not sure that you mean attenuation because (a) that only happens in a medium, not in a vacuum and (b) it follows an exponential law. Are you saying that the intensity of a laser beam in a vacuum decreases as 1/r² ? I find that hard to understand. I can see there would be a some fall-off in intensity with distance due to a small amount of beam spreading, but I don't see how it could be anything near 1/r². Gandalf61 (talk) 15:52, 4 November 2010 (UTC)

Well, there's nothing out there focusing the beam, so the sides of the beam form straight lines, so the diameter of the beam is proportional to r, so the area of the beam is proportional to r², so the intensity of the beam is proportional to 1/r². Red Act (talk) 16:12, 4 November 2010 (UTC)

But the beam has already been focussed - it doesn't need continual refocussing. If the sides of the beam are almost parallel then your r is not the distance from the source - in effect, r is the distance from a "virtual" source, a long way behind the real source, where the sides of the beam would meet if projected backwards. Gandalf61 (talk) 16:48, 4 November 2010 (UTC)

That is true, but only important if r is of the same order of magnitude as (or smaller than) the distance back to the virtual source. On astronomical scales, the virtual source of the laser beam and the physical location of the laser aperture are in essentially the same place. TenOfAllTrades(talk) 18:04, 4 November 2010 (UTC)

A beam with a wavelength of 600nm and an original diameter of 2mm has a divergence of 0.0003 radians (even larger if the beam is partial spatial coherent).[3] Still assuming a 2mm aperture, that means that the "virtual" source is only about 6m behind the "real" source, which is completely negligible compared to the 100km thickness of the atmosphere. Over the 100km thickness of the atmosphere, the beam will widen to a diameter of about 30m, compared to which the original 2mm diameter is negligible. Red Act (talk) 18:10, 4 November 2010 (UTC)

I follow your argument, but something is askew in your calculations or model or assumptions, because a beam width that increased by a factor of 15,000 (30m/2mm) over 100km would increase 6 x 10⁷ times over 400,000 km, and then a 2mm aperture would give a beam width of 120km at the Moon's surface, whereas the actual beam width at the Moon's surface in the Lunar Laser Ranging experiment was only 6.5 km - see below. That's different by a factor of 20. Gandalf61 (talk) 11:54, 5 November 2010 (UTC)

The diffraction limited divergence angle of a laser beam is inversely proportional to the initial diameter of the beam; see the link I provided in my earlier post. The OP, and Stephan Schulz's first post, refer to the laser as being a laser pointer, so for my calculation I assumed an initial beam diameter that would be reasonable for an inexpensive battery-powered handheld laser pointer. The observatories participating in the Lunar Laser Ranging Experiment use much more expensive, much higher power lasers, with a much wider initial beam diameter. My point would still stand for the LLRE lasers, though. Even if the distance from the virtual source to the real source is 20 times bigger than in my calculation (in the ballpark of 120m), that's still negligible compared to the 100km thickness of the atmosphere. Red Act (talk) 17:39, 5 November 2010 (UTC)

Actually, I just found out that the divergence of the laser beam used by the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) isn't even diffraction limited, because they use a beam expander to expand the beam to a huge 3.5m initial diameter. Instead, atmospheric divergence dominates in their case.[4] Red Act (talk) 18:48, 5 November 2010 (UTC)

In light of this discussion, here is another question: If I were sat atop the retroreflector used for the lunar range finding experiment, would I be able to see the beam with the naked eye? HappymulletukHappymulletuk (talk) 22:44, 4 November 2010 (UTC)

Well Lunar Laser Ranging experiment says "the reflected light is too weak to be seen with the human eye", but nothing about your question. However it also says "at the Moon's surface, the beam is...about 6.5 kilometers wide", and as stated above "out of 10¹⁷ photons...only one will be received back on Earth every few seconds". Now allowing for a pupil diameter of 8mm and assuming the 6.5km wide spread of the laser is circular, my back of the envelope calculation says that some 6,059 photons would enter your pupil 'every few seconds' (which is horribly imprecise). However, that many photons would most likely be detectable (technically you could 'see' one photon), but I doubt that it would be intense enough that you would determine its source to be that laser. (Willing to be corrected here...). --jjron (talk) 07:24, 5 November 2010 (UTC)

Could you be a bit more specific about your assumptions here? Your number feels like it's a bit high. (Here's my envelope; feel free to compare with the back of yours.) Given 1e17 photons spread uniformly (er...) over a 6.5 km diameter circle on the moon, and assuming a 1-square-meter retroreflector (I think the actual exposed reflective surface is smaller, but we'll go with that), then we're down to 3e8 photons striking the retroreflector. If we then assume that the reflected beam spreads no more than the original beam (a generous assumption, I suspect) and reaches a detecting telescope with an aperture of 1 meter diameter, then the scope picks up just 70 photons. Meanwhile, the astronomer next to the telescope is looking in vain — with his tiny 8 millimeter pupils, he gets 0.005 photons per shot.

In practice, the actual return rate is much worse; the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) project boasts a 3.5-meter telescope, and is very happy to see on the order of one photon returned per pulse. I suspect that this is largely due to divergence of the beam after it strikes the retroreflector, rather than inefficiency in the ground-based detector. TenOfAllTrades(talk) 18:39, 5 November 2010 (UTC)

The question asked by Happymelletuk is about someone sitting on the retroreflector, not about someone standing next to the telescope.

The divergence of the reflected beam is indeed somewhat worse than the incident beam, but not enormously so. Due to thermal deformations of the retroreflector, the reflected beam has a divergence of about 7 to 10 arcseconds.[5] Red Act (talk) 19:08, 5 November 2010 (UTC)

Yeah, my calc was done based on someone sitting on the retroreflector on the moon as was asked. FWIW I only calculated it for one eye, so open both eyes and you could double those photons! :) (Mind you it could still be out - just quickly retried and got a different answer, but still in the magnitude of ≈10³). --jjron (talk) 16:38, 6 November 2010 (UTC)

You can easily compute the apparent magnitude of a laser as a function of distance. For a black body radiator at a temperature of roughly 5500 K (e.g. the Sun) the formula is:

$m=-2.5\log _{10}\left({\frac {F}{{\text{W/m}}^{2}}}\right)-18.9$

where F is the flux produced by the black body source. Now our eyes are about 7 times more sensitive to green light of the wavelength produced by green laserpointers than to sunlight. Our eyes are most sensitive to light of these wavelengths, while any black body radiator will always emit a lot of energy to either longer and shorter wavelengths. So, in case of a green laserpointer, we have:

$m=-2.5\log _{10}\left({\frac {F}{{\text{W/m}}^{2}}}\right)-21$

Taking the beam divergence to be 0.0003 radians (as Red Act explains above, this is the ideal beam divergence that can be obtained by a laserpointer with a 2 mm initial beam diameter), the flux of a 5 milli-Watt laserpointer a distance d away is:

$F={\frac {5\times 10^{-3}}{\pi \left(1.5\times 10^{-4}d\right)^{2}}}{\text{W}}=7.07\times 10^{4}{\frac {\text{W}}{d^{2}}}$

Inserting this in the above formula for visible magnitude gives:

$m=5\log _{10}\left({\frac {d}{\text{m}}}\right)-33.1$

So, even at a distance of 1000 km, it will still appear to be magnitude -3.1, approximately similar to Jupiter in the sky right now. Count Iblis (talk) 20:10, 5 November 2010 (UTC)

The laser ranging beam will appear to be magnitude -5.9 for someone looking into it from the Moon. Count Iblis (talk) 21:40, 5 November 2010 (UTC)

Although the light from hand-held laser pointers may not escape the atmosphere, they can still be bright enough to cause temporary blindness in the cockpits of aircraft kilometers above the surface, so much that shining a laser at a plane is illegal. The laser beam usually expands as the distance increases, so the light may be less focused. However some non-laser lights such as the xenon lamps used to light IMAX screens would apparently be visible from Earth, were it placed on the Moon. ~AH1^(TCU) 18:04, 7 November 2010 (UTC)

algae and plankton

Would destruction of all the algae and plankton on the Earth destroy human civilization as we know it, and if so how? --96.252.213.127 (talk) 14:04, 4 November 2010 (UTC)

I can't find a reliable source quickly, but it's usually said that the ocean's algae is responsible for some large percentage of Earth's oxygen. Numbers between 50% and 80% are often mentioned in this context. Even assuming it's only 50%, that kind of oxygen loss would probably be devastating. Especially if it happened suddenly. APL (talk) 14:22, 4 November 2010 (UTC)

Not to mention that the whole food web would be shot to pieces. --Stephan Schulz (talk) 14:25, 4 November 2010 (UTC)

Based on the previous two answers: Yes. -- Sjschen (talk) 19:43, 4 November 2010 (UTC)

I'm not so sure. Most of Earth's population gets its food from land-based food sources (i.e. Maize), so the collapse of the marine ecosystem wouldn't really matter so much. The oxygen's a bigger deal. This document shows what happens to people in low-oxygen environments. If algae do really contribute this significantly to oxygen levels, things could get bad. Humans are pretty inventive though, so I bet enough people would create oxygen collecting and breathing devices (we already have them for Mountain Climbing) to prevent the total extinction of the human species. Buddy431 (talk) 00:58, 5 November 2010 (UTC)

Wouldn't there also be a massive upshot in atmospheric CO2? That could be bad on its own. (Though, I suppose it would help the maize in the short term.) APL (talk) 13:03, 5 November 2010 (UTC)

The Earth system's two main sources of oxygen are the oceanic plankton (and cyanobacteria) and the tropical rainforests. The oxygen synthesis and carbon dioxide removal capacity of both are declining. The rise of Carbon dioxide in Earth's atmosphere would also simultaneously cause a depletion of atmospheric oxygen at twice the rate that the carbon dioxide is rising. ~AH1^(TCU) 17:59, 7 November 2010 (UTC)

Superinfections?

How Many HIV Superinfections are there. I understand this is a second strain of the virus. and I also understand that the second stage of the virus may cause more rapid disease progression or carry resistance to medicines. How long is the life expectancy if someone is infected with a superinfection.

Questions are: How many Hiv Superinfections are there? How long is the life expectancy is someone is infected with a Superinfection? —Preceding unsigned comment added by 91.111.85.243 (talk) 16:16, 4 November 2010 (UTC)

According to HIV superinfection and the reference it cites, HIV superinfection is when an infected person becomes infected with a second strain of the virus, making the disease harder to treat. This seems to be different to what you understand of the term, at least from what you've written anyway. Because of this, there aren't a certain number of superinfections that could be counted. There are however, many Subtypes of HIV, but I think that it is impossible to say how many strains there are since it mutates so frequently. Life expectancy will vary depending on many factors, the HIV article has a section on prognosis and superinfection isn't going to make it any better. SmartSE (talk) 16:35, 4 November 2010 (UTC)

Also take a look at SuperAIDS. ~AH1^(TCU) 17:57, 7 November 2010 (UTC)

Acceleration at event horizon

In the wikipedia's article on gravity acceleration is written that on event horizon the gravity acceleration is infinite. But isn't infinite only at singularity? The event horizon is a few kilometres from singularity, so shouldn't be infinite. Francesco —Preceding unsigned comment added by 95.234.212.148 (talk) 16:52, 4 November 2010 (UTC)

Wikipedia does not have an article called gravity acceleration. There is an article on gravitational acceleration but this says nothing about any event horizon. Please could you state exactly which article you are talking about?--Shantavira|^{feed me} 18:06, 4 November 2010 (UTC)

Yeah, I thought maybe there was just a missing comma between "gravity" and "acceleration", meaning the article in question was Gravity. But that article also doesn't talk about acceleration near black holes. Neither do General relativity or Black hole, for that matter. At any rate, near black holes, the Newtonian gravity approximation completely breaks down, and you have to use general relativity. And in general relativity, gravity isn't even viewed as being an acceleration. Instead, objects travel in a straight line (a geodesic) along a curved spacetime. Red Act (talk) 19:23, 4 November 2010 (UTC)

The event horizon is typically the edge where the acceleration or speed of matter is moving toward/around the black hole or away from the observer at faster than the speed of light, and it is not necessarily infinite but may be in certain equations. ~AH1^(TCU) 17:56, 7 November 2010 (UTC)

Capillary DNA sequencers on eBay?

How much do sequencers cost? The department I've joined (as a PhD student) sends out samples to a company within another university for sequencing and it just makes me wonder why most large universities that carry out such research don't have their own internal sequencer for internal use. Wouldn't that work out cheaper than outsourcing, even with the cost of a technician to operate it? ----Seans Potato Business 18:03, 4 November 2010 (UTC)

Many financial decisions in research are very opaque to a graduate student. It is very possible, for example, that your research group or department has received funding grants with specific line item requirements - for example, $ X to be spent on outsourced technical work; $ X to be spent on capital equipment purchases. (If the funding comes from the government, this may be in support of a wide-reaching policy of using research to foster certain economic activity, or other totally opaque objectives from your point of view). Maybe the best resource to ask is your PI or Ph.D. advisor; depending on your research conditions, they may be willing (and even happy) to discuss research-funding details with the students. At my school, one detail was that money granted to individual professors was "taxed" (by the University!) at a different rate depending on its purpose: capital equipment investments were taxed differently than money granted for student labor, research expenses, and so forth - so the grant-writers, knowing this, played all kinds of mind-bending games to make sure they maximized the impact of their money. Nimur (talk) 18:13, 4 November 2010 (UTC)

(ec) I'm not sure what they cost, but remember that fully-operational sequencing services can't just spring out of nowhere. Someone has to write the grant application, find space, hire the staff, and supervise the facility. Unless there exist a few principal investigators who each do a lot of sequencing (and therefore see sequencing as big line items in their budgets) and who get along well enough with each other to write such a grant proposal, it's unlikely that it will happen at your university. It may also be that unless utilization of the sequencers remains high (50% of the maximum throughput? 80%?) and there are enough tasks to fully occupy a full-time technician (how many sequencers can one full-time tech operate?) then any theoretical per-sequence savings will be smothered in operating overhead costs, and then you've gone to a tremendous amount of time and effort to build a facility that's less cost-effective than the original outsourcing option.

As well, outsourcing 'future-proofs' your sequencing needs. If someone introduces a new technology then it's up to your contractors to figure out how to pay for it — you don't have to write a new grant. If the output from one service isn't up to your requirements, then you can change to another lab (or send piecework off to other labs in cases where specialized treatment or tools are required). TenOfAllTrades(talk) 18:35, 4 November 2010 (UTC)

Oh, right, I see. Well, I'll put £50 in the pot to buy Edinburgh Uni's own sequencer. :) I'll ask my supervisor what it's all about. ----Seans Potato Business 18:48, 4 November 2010 (UTC)

Keep in mind that the cost of the machine itself is a *very* small part of doing sequencing. There's disposables/reagents associated with each run. (You replace the acrylamide in the capillary after each run, and the capillary itself is only good for a limited number of runs.) Then there's the maintenance costs associated with the equipment (usually 5-10% of the list price of the equipment *per year*). Finally, there's the time cost for the the technician to run it, and personnel costs tend to be the most expensive part of any business. Given that there are a number of companies who have brought assembly-line efficiency with razor-thin profits to DNA sequencing, you'd have to be doing a lot of sequencing to make it cost effective: think enough work for several full time technicians. You say you are using "a company within another university" currently, which makes me guess that you're probably already getting the same near-at-cost price the other university gets. It's highly unlikely that your university would be able to do it cheaper by itself. Indeed - after evaluating the price, quality, and turn-around time, for us it's actually better for to go with a private, commercial provider for sequencing, rather than use either of the two on-campus sequencing services we have. -- 140.142.20.229 (talk) 21:59, 4 November 2010 (UTC)

Are we very tiny in the universe?

I don't mean this as a silly question, but I can't get over how tiny the earth is compared to the sun and how tiny the sun is compared to Arcturus. So, are we like the tiny people living in the locker of the movie Men in Black, where we don't even know how small we are compared to the universe around us? Are there other planets as big as Arcturus, and is it possible there are giant lifeforms and we are just a speck of dust compared to them? (If I sound insane, please let me know. Thanks.) AdbMonkey (talk) 18:31, 4 November 2010 (UTC)

I think I get what you're saying. You're asking whether we are to some ET lifeform as, say, small insects are to us? ----Seans Potato Business 18:42, 4 November 2010 (UTC)

I think the OP is referring to the film closing, which zooms out on our planet/solar-system/etc. to reveal that it's just the inner molecular structure of a giant alien. While it's tempting to look at solar-system models and compare them to bohr atom models, they are not the same. Different physical laws govern the behavior of our solar system (mostly, gravity), as opposed to the electromagnetic and nuclear effects that govern atomic behaviors. So, the very large structures in the universe are probably not just bigger versions of atoms. What we can see in the universe around us is large-scale structure of the cosmos - things like galaxies, and then at larger scales, clusters and filaments, and so on. We are not capable of "zooming out" to arbitrary limits - we are bound by our experimental capabilities. But we have some pretty good, albeit incomplete, ideas about the physics that governs the extremely large processes in our universe. It would be unlikely that any "macro-world" built out of the larger structures of our universe could exist and be compatible with what we do observe. Nimur (talk) 18:48, 4 November 2010 (UTC)

Well, thank you for answering, but I didn't think that the universe was a atom, or that our tiny planet was an atom, but I know what you're talking about from that ending scene. I was actually wondering if there could be gigantic life and if we would be super, super tiny compared to them. I was wondering if there were giant planets as big as Antares, because all I hear of are gas giant stars. So, to reframe my question, are planets capable of getting to be the size of Arcturus? Thanks for explaining that we're not likely a tiny component in a "macro-world." AdbMonkey (talk) 19:00, 4 November 2010 (UTC)

No, you couldn't get planets as big as a star, because once you get a collection of that much matter in that small a space, the pressures in the core are enough to get nuclear fusion going, which is what makes a star a star in the first place. (Hmm, actually, thinking about it I don't know whether you could theoretically in a very metal-rich environment have ridiculously big planets, but you'd have to artificially create the conditions or wait a very long time for enrichment of the ISM to get you to that point ...) --86.130.152.0 (talk) 19:37, 4 November 2010 (UTC)

So does that mean there is an upper limit of how big a matter based planet can become before undergoing "auto-fusion"? -- Sjschen (talk) 19:42, 4 November 2010 (UTC)

Would depend very much on the exact elemental composition, I would think. Some elements are much easier to fuse than others, and many need energy putting in for it to happen. But the (baryonic) matter in the universe is overwhelmingly hydrogen, so you do get stars. --86.130.152.0 (talk) 20:53, 4 November 2010 (UTC)

So if I were to construct a planet "brick-by-brick" with some kind of baryonic matter, say clay bricks, is there a way to calculate the critical mass (in the way they discovered the critical mass for U-based nuclear fusion) in which the pile would start crushing itself so much to start undergoing fusion? Or directly collapse into some kind of neutron star, black hole, or "weird"? -- Sjschen (talk) 05:38, 5 November 2010 (UTC)

The size of a biological creature is related to 3 things: Gravity, Heat, and Energy. A larger creature will retain heat better (good in the cold), but will also have a harder time getting rid of heat created by exercise. Gravity is obvious, but the strength of biological matter is unlikely to change much, so if gravity was lower you would have taller (since it's easier to support), but not heavier (what would be the point of over-muscling the creature, i.e. the strength of muscles are likely to be about the same). (So it's up to you if that counts as bigger.) And energy from various redox (oxygen burning) reactions is more of less the same, but a bigger creature will need more energy which could be hard. Of course what I write here could just be a failure of imagination, but I suspect that our size is likely to be typical (for internally supported creatures anyway - if you have water dwelling, or gas filled creatures things could be very different). Ariel. (talk) 20:01, 4 November 2010 (UTC)

So what's the maximum size a lifeform can get? Since there's an upper limit on planet size? And life can't exist in a star, right? And the gravity is why roaches on this planet aren't big, right? AdbMonkey (talk) 01:46, 5 November 2010 (UTC)

The answer is that we don't know. We're pretty sure that the maximum size for earth-like life is represented by animals like the blue whale and Apatosaurus, which are among the largest animals ever to have lived on earth. However, consider that Earth-based life does some pretty outrageous stuff, chemically speaking. We seem to have this sense that aliens on other planets will have a chemistry that resembles ours, a body plan that resembles earth-based animals, etc, etc. The truth is, it is much more likely that alien life will bear absolutely no resemblence to what is on earth. Considering the rather exotic and convoluted ways that life on Earth has come up with to solve Earth-type problems, there's no telling what sorts of exotic ways that life on other planets may arrive at. For example, the heat-dissipation problems and musculature issues that Ariel discusses still asumes a roughly Earth-like body plan and chemistry, if you untether your mind from those limitations, it becomes almost boundless the sorts of life that may exist. My favorite statement along these lines was made by Douglas Adams, who once postulated on a "hyper-intelligent shade of the color blue". While that is plainly farcical, it does demonstrate that life could really be just about anything. --Jayron32 04:38, 5 November 2010 (UTC)

Solaris describes this very nicely.--131.188.3.20 (talk) 11:52, 5 November 2010 (UTC)

I thought the biggest living thing was some fungus somewhere. Sequoiadendron giganteum trees could be bigger than blue whales. If you are floating rather than being affected by gravity, and are in a medium of the right temperature or are 'cold-blooded', then weight and heat might not matter to much. 92.15.10.141 (talk) 13:17, 5 November 2010 (UTC)

A better candidate for 'largest' organism on earth is Pando_(tree). Also probably oldest. All hail Pando! SemanticMantis (talk) 15:54, 5 November 2010 (UTC)

Another thing to think about, which Jaryon alludes to above. How are you defining life? Depending on your definition, the Great Read Spot might qualify (homeostasis, reproduction, growth etc). Along these lines, some sci-fi have speculated about things like 'living' nebulae. What Nimur and Ariel say above is generally correct though, if you're looking for carbon-based biology basically similar to life on earth, there are some pretty solid limits at large scales. SemanticMantis (talk) 14:41, 5 November 2010 (UTC)

I am defining life as something capable of thought. So probably like something rarely seen on this planet. AdbMonkey (talk) 22:38, 5 November 2010 (UTC)

Gas giant planets may get up to 13 Jupiter masses; beyond that they condense into the approximate size of Jupiter and become a brown dwarf. That being said, it's certainly possible that large bodies of life live on the suborganic gases of the atmosphere of those planets, getting their energy from chemical reactions similar to photosynthesis while allowing their size to be comparable to those of entire countries on Earth. It's all theoretical. ~AH1^(TCU) 17:53, 7 November 2010 (UTC)

THANK YOU AH1! That is exactly what I was looking for 13x Jupiter. Is mass and size the same? Or could it be heavier and smaller that 13 put together Jupiters? And is that just for our solar system, or for the entire universe? AdbMonkey (talk) 14:36, 8 November 2010 (UTC)

Definition of planet. Brown dwarfs are usually roughly Jupiter-size, regardless of mass. ~AH1^(TCU) 03:15, 13 November 2010 (UTC)

Why car radiators are called so.?

I know because it radiates heat..... but i think the prime mode of removal of heat is convection not radiation in a car. —Preceding unsigned comment added by 59.95.101.211 (talk) 18:41, 4 November 2010 (UTC)

Many thermal loss processes are happening simultaneously to cool the engine. The radiator is radiating heat directly (as infrared emission); and it is also undergoing a (very complicated) conduction/convection process, wherein it conducts heat to nearby air, and then the air flows away, both laminarly and convectively (usually with the assistance of an air intake duct and/or a fan). This doesn't even begin to address the complex cooling mechanism of the fluid that is circulating inside the radiator - again, a mix of conduction, convection, and radiation, all rolled into one. If you needed to calculate very accurately, you could measure geometry and material properties to determine how many watts are attributable to each process.

Regarding the etymology: I'd rack it up to "loose use of terminology." Radiators existed (for various applications) long before thermodynamic theory developed and formalized specific definitions. Nimur (talk) 19:09, 4 November 2010 (UTC)

The importance of conduction/convection versus radiation is shown by the speed with which the engine overheats and stalls if the fan quits blowing. That interferes with convection more than with radiation. Edison (talk) 17:11, 5 November 2010 (UTC)

Note that domestic radiators also heat mostly by convection, while electric fires that do substantially radiate are not called radiators! --ColinFine (talk) 17:31, 7 November 2010 (UTC)

Entirely different forms of power

I've been watching the BBC series on the National Grid and it got me thinking. Is there any conceivable power delivery mechanism that's better than electricity? I don't mean like nuclear power or hydro or whatever where power is generated and turned into electricity (or wind), I mean the actual end thing. I.e. where we no longer have electrical powered devices we have 'x' powered devices. I don't want to get into whether it's economical or realistically going to replace electricity, purely thinking - is there research going on to look at different non-electricity based ways of powering devices. ny156uk (talk) 18:42, 4 November 2010 (UTC)

There is some interest in both extra-orbital and surface-to-surface microwave power transmission. -- Finlay McWalter ☻ Talk 18:46, 4 November 2010 (UTC)

(EC)Please provide some metrics of how "better" is defined. There are doubtless special cases where superior systems might be proposed. , or provide a test case, such as "Transmit 1 mw of power to a spaceship 1 million km from Earth for 1 month."Some sort of laser might get the power there better than a pair of copper wires. "Transmit 10000 watts of power from the gasoline engine of a car to the rear axle." A hydraulic and mechanical transmission might do a good job at a lower cost. or "Transmit 80,000 BTU of energy from a furnace to the living space of a house." A steam or hot water boiler might be cheaper and lower maintenance than a steam turbine and generator with electric radiators. "Transmit energy from a solar power satellite to the ground." Microwave might work, copper wires are doubtful. Getting 600 megawatts of power 100 miles from a power station to a city? A drive belt, or a rotating shaft would be expensive and have very high friction losses. A hydraulic or compressed air system would be subject to leaks and friction losses. A laser or microwave or hydraulic system would probably be more expensive than electric wires and transformers. Edison (talk) 18:53, 4 November 2010 (UTC)

Purely of historical interest, many factories were powered by a central power source (stream, or sometimes a waterwheel) and this power was transmitted to many users inside the factory mechanically, using belts and shafts (info). Even then the mechanical losses were severe, but I guess that (if electrical power just didn't work for some science-fictiony reason) we could all get power delivered to our homes by means of miles of rotating shafts. -- Finlay McWalter ☻ Talk 18:55, 4 November 2010 (UTC)

• I believe Finlay meant to say "steam, or sometimes a waterwheel". --Anon, 03:59 UTC, November 5, 2010.

(ec) Energy is never really "produced" - it's moved around, and it changes form. Pumped storage uses gravitational potential energy, in the form of mechanically conveying (pumping) water - moving energy from the (electric) pump into the water. Many large institutions use a steam plant, instead of delivering heating via electric distribution; and large pipes full of steam deliver hot gas for use in water-heaters, building heating, and other energy-needs - they move thermal energy around. Natural gas lighting delivers energy in the form of chemical (delivering gas by pipeline), instead of running copper wire to power an electric light bulbs. Compressed air systems exist, and are in wide use to deliver enormous quantities of energy to industrial equipment, dentist-drills, construction sites, and so on. All of these schemes convey energy from a point of production to a point of use. But ultimately, what do you want to use energy for? If you want to power electric devices (like transistors, computers, and mp3 players), you need electricity at some point (whether you convey the energy in the form of compressed air or an electromagnetic wave, transistors can't run on compressed air. You can think about on-site electric generation: but there are few options that make things easier than electric distribution. There might be a future in filling up your iPod with alcohol so that it can run a fuel cell and produce electricity internally. Barring truly revolutionary changes in the way we understand physics, I don't think there is much foreseeable future in replacing transistors with non-electronic equivalents. They are simply the best, smallest, most energy-efficient, easiest-to-manufacture, devices, and we have an entire technology infrastructure set up to use them for all kinds of neat applications. Nimur (talk) 18:59, 4 November 2010 (UTC)

London had a high-pressure water system for powering small sites from 1872 to 1977. The UK grid and local distribution network is over 92% efficient at getting electricity from power station to the customers. 62.56.53.12 (talk) 19:51, 4 November 2010 (UTC)

A hydraulic power station, note the water tank on top, for more info see the image description

An alternative is Hydraulic power network, systems of which were quite common in cities during the early electricity age - a good method of tranmitting torque, etc, but not for lighting.94.72.205.11 (talk) 19:57, 4 November 2010 (UTC)

In Ada, Vladimir Nabokov presents an alternative history in which electricity has been banned and telephones and certain other devices operate by means of hydraulics. He doesn't explain how airplanes and automobiles (which exist in the fictional world) function without electricity, though. Deor (talk) 00:37, 5 November 2010 (UTC)

You don't technically need electricity for airplanes or automobiles, do you? I don't know how you'd use hydraulics for them, though, but if good-old-fashioned internal combustion engines are still viable then I think you're OK. --Mr.98 (talk) 00:57, 5 November 2010 (UTC)

Compressed air could be used see this article. Strong laser light in fiber optic "wires" could be a possibility. I'm not sure if those are "better" though. Ariel. (talk) 20:06, 4 November 2010 (UTC)

In Micropower are described some present day research projects aimed at using fuel to generate electricity at the point of use, such as for powering "Land Warrior" electronic devices used by soldiers in high tech wars, rather than batteries which have short lifetimes and are heavy. It would be easier to deliver liquid fuel to refill the fuel reservoir than to deliver enough batteries to supply the same watt hours for computers and communications devices on the battlefield. Some research has looked into using hydrocarbon fuel to power laptop computers via fuel cells [6], [7]. Just as in cars, chemical fuel provides greater power density or greater runtime than batteries. Natural gas is being used to power fuel cells or microturbines for "distributed generation" at the customer's building rather than at a power plant. The waste heat, perhaps 2/3 of the energy in the fuel, can be used for space heating, air conditioning, or hot water rather than being a waste product with negative value at the power plant. So a pipeline or fuel can can be a form of delivering energy to the point of use rather than by a power line. Amish farmers, at least in the movie Witness (1985 film), have used a windmill to power a mechanical power transmission system consisting of a pair of cords alternately pulled back and forth to transfer mechanical power to the house of barn to perform useful work. It would work much like alternating current. Such back and forth motion could be mechanically converted to rotary motion. It's unclear how the transmission efficiency would compare to a simple drive belt. [8] describes how a 3/4 inch hydraulic line can transmit 100 horsepower on a farm, via the "PowerCube". The PowerCube has a "Lovejoy Coupling" which sounds like fun. It should be noted that natural gas, liquid fuel, rotating mechanical shafts, tractor powertakeoff shafts, hydraulic lines, and compressed air have killed many workers, and are not intrinsically safer than electricity. Edison (talk) 17:02, 5 November 2010 (UTC)

It's worth mentioning that steam or hot water produced by cogeneration plants really is a "better" method of distributing power, in the sense that a Carnot engine inevitably leaves waste heat behind. If you don't distribute the heat in the form of hot water used to heat homes, you have to build a cooling tower or cooling pond or kill the fish in a heated river instead.

I have speculated that in the future fissile isotopes will be broken down through a managed cascade of nuclear isomers eventually converting much of their nuclear energy into usable chemical, mechanical or electrical forms. Wnt (talk) 11:28, 8 November 2010 (UTC)

Flavours and colours of quarks

I was wondering how many different quarks there are, which led me to three questions. First, can any flavour of quark be any colour (excluding anticolours)? This would allow six flavours times three colours times two for regular and anti for a total of 36 different types. However, I noticed in our article that quarks can change flavours and change colours. Can they also change from regular to anti, meaning that any quark can be any of the 36 types, or are regular and anti completely seperate? As an unrelated question, could I make a regular-matter proton using three anti-down quarks? It wolud have a charge of +1, but I assume that something would be wrong with it. Thank you. —Arctic Gnome (talk • contribs) 19:24, 4 November 2010 (UTC)

Quarks are in fact constantly changing colour -- every time gluons are exchanged (which mediate the strong force between the quarks, so ALL THE TIME), they're transmitting colour between the quarks. They can't change directly from "regular" to anti, though of course in pair production you get quarks and antiquarks created at the same time. And three anti-down quarks wouldn't be a proton, they'd be an anti-Delta-baryon (you may find list of baryons an entertaining article -- see also list of mesons). Note that you can't have three quarks all of the same flavour in a baryon of angular momentum 1/2 due to exclusion principle effects -- you have to be in the J=3/2 mode. HTH. --86.130.152.0 (talk) 19:35, 4 November 2010 (UTC)

supernova

How far away would a supernova be if it had an apparent magnitude on earth of -10? Googlemeister (talk) 19:52, 4 November 2010 (UTC)

That depends on the type of supernova. Type Ia supernovae are considered to be fairly uniform (with some caveats), with an absolute magnitude of -19.3. (That last article, incidentally, shows how to calculate distance given absolute and apparent magnitudes.) Type II supernovae are more varied, but all are dimmer than Type Ia. TenOfAllTrades(talk) 20:05, 4 November 2010 (UTC)

(edit conflict) That of course depends on the supernova. According to supernova, all type Ia supernovae have an absolute magnitude close to -19.3. Thus they would have to be about 724 parsecs away to have apparent magnitude -10. Type IIs are dimmer than that, though. Our articles don't seem to say much about typical magnitudes of type Ibs and Ics. Algebraist 20:06, 4 November 2010 (UTC)

The eventual supernova of Spica could have roughly that magnitude. ~AH1^(TCU) 17:49, 7 November 2010 (UTC)

nuclear war

Would a nuclear war kill all the algae and plankton on the Earth? --96.252.213.127 (talk) 20:51, 4 November 2010 (UTC)

No. To achieve that, you'd need many orders of magnitude more energy than the total kilotonnage of the world's nuclear stockpiles. --86.130.152.0 (talk) 20:58, 4 November 2010 (UTC)

Direct energy released matters very little for this question. What is more relevant are the radioactive and meteorological effects — nuclear fallout, nuclear winter. But it seems fantastically unlikely that even under the worse conditions it would kill off all algae and plankton — I'm not sure what level of blocked sunlight you'd have to get to do that, but it must be fantastically high, probably worse than anything humans could actively do in the short term. --Mr.98 (talk) 00:34, 5 November 2010 (UTC)

Forget about algae - a nuclear war would have a hard time even killing all the humans. Despite fears, we don't actually have enough nuclear weapons to kill everyone, and the earth is very very big. Ariel. (talk) 05:58, 5 November 2010 (UTC)

Furthermore, life persists in some of the most harsh places, even if we could exterminate all humans, or even all higher animals, extremophiles show that life would probably persist in some form or another. Though, as Ariel notes, despite alarmist views, even the sum total of nuclear weapons couldn't directly kill all humans. --Jayron32 06:27, 5 November 2010 (UTC)

We could make life pretty miserable for people, though. Radioactive climate change... it's not a good thing. I think we could probably say with confidence that it would kill human civilization "as we know it." It's not really an "alarmist view" to say that a full nuclear exchange (which fortunately today is a lot less probable than it was) would be devastating for the species on par with an asteroid impact. --Mr.98 (talk) 14:09, 5 November 2010 (UTC)

Note that the latest thinking is that life on Earth existed during the late heavy bombardment:

The Late Heavy Bombardment and the "re-melting" of the crust that it suggests provides a timeline under which this would be possible; life either formed immediately after the Late Heavy Bombardment, or more likely survived it, having arisen earlier during the Hadean. Recent studies suggest that the rocks Schidlowski found are indeed from the older end of the possible age range at about 3850 Ma, suggesting the latter possibility is the most likely answer.[11] Schidlowski's argument remains a topic of heated debate.

More recently, a similar study of Jack Hills rocks shows traces of the same sort of potential organic indicators. Thorsten Geisler of the Institute for Mineralogy at the University of Münster studied traces of carbon trapped in small pieces of diamond and graphite within zircons dating to 4250 Ma. The ratio of carbon-12 to carbon-13 was unusually high, normally a sign of "processing" by life.[12]

Three-dimensional computer models developed in May 2009 by a team at the University of Colorado at Boulder postulate that much of Earth's crust, and the microbes living in it, could have survived the Bombardment. Their models suggest that although the surface of the Earth would have been sterilized, hydrothermal vents below the Earth's surface could have incubated life by providing a sanctuary for heat-loving microbes.[13]

Count Iblis (talk) 17:34, 5 November 2010 (UTC)

Radio waves

I read the article on crystal radio (radios powered entirely from radio waves, with no need for a power source) but it doesn't explain the process? How much electricity is there in radio waves? Could they power a light bulb for example? 82.44.55.25 (talk) 21:01, 4 November 2010 (UTC)

According to the article, the power received by crystal radio antennas is at most measured in microwatts (millionths of a watt). To get an understanding of how much power a microwatt is, that’s 100,000,000 times less power than a standard 100W incandescent light bulb uses, and about 30,000 times less power than is used for a rather dim LED. Red Act (talk) 22:20, 4 November 2010 (UTC)

And if you've ever used a crystal radio, you know that the volume is very low. Immediately following the discovery of radio waves, a lot of inventors put a lot of effort into inventing a quality amplifier for use in loudspeakers. Nimur (talk) 23:24, 4 November 2010 (UTC)

If you were right next door to the transmitter,or even a kilometer or more away, a crystal radio could certainly drive a loud speaker. I have heard such demonstrations. A lightbulb of the right sort next to the transmitting antenna could be made to light up. 19th century scientists from Joseph Henry [9] in the 1830's to Thomas Edison (Etheric force)in 1875's and Heinrich Hertz in 1886 demonstrated that radio waves could produce a visible spark many meters from the transmitter in a receiving loop. Tesla did demonstrations of "wireless power" over short distances. The power drops off dramatically as you move farther away. Edison (talk) 16:33, 5 November 2010 (UTC)

Electricity

We're currently studying electricity, and while I can use the equations (Ohm's law and all that) just fine, i'm having some trouble intuitively 'getting' how electricity really works. Although analogies like the Hydraulic analogy are useful, they can break down when looked at closely.

My question is: What is voltage? It's the amount of energy that each coulomb of energy has, yes, but how is that energy stored? it's not like each electron has a liitle pouch they can store energy in, and the energy can't be stored as kinetic energy of the electrons, because that is current! And how does the application of a voltage make a current flow?

Thanks for helping sort my confusion, --HarmoniousMembrane (talk) 21:35, 4 November 2010 (UTC)

You can get offtrack thinking of electricity as something belonging to electrons. Applying a voltage to a circuit and the current flowing through it is NOT well represented by a mental picture of little "electron men" running into one end of a tunnel with some amount of momentum and eventually emerging from the other, and a resistance is not well modelled by the little electron creatures climbing a hill. The hydraulic model is not bad if you visualize the pipe as always filled with fluid, whether there is a flow or not. Think of voltage as pressure: in general the more voltage applied to an ohmic circuit path, the more current will flow. A higher resistance is like a longer or skinnier fluid filed tube. The tube is always filled with fluid, so it starts flowing out the distant end as soon as the valve is opened or the pressure is applied. Picture the voltage source or battery or generator as a pump which receives back into its input fluid as fluid is pumped out the outlet, for current electricity. (Such a model breaks down if carried too far, but helps with many circuit problems). Do lots of problems and get so you find the right answer easily, and it will all become more intuitive. Edison (talk) 00:15, 5 November 2010 (UTC)

I guess that no one else has answered this, because they are flabbergasted that the educational system has collapsed to the point that no one understands anything any more; rather they just have to memorises stuff. What happened to education and the long-winded pons asinorum. Never mind, the Chines are ready to take over world trade and economic leadership. --Aspro (talk) 00:22, 5 November 2010 (UTC)

Voltage is pressure, not just in the hydraulic analogy but literally. Mobile electrons repel each other and, given the choice, will space themselves out evenly in a wire. If there's a local excess/deficit in one part of the wire, that part will have a negative/positive charge that will repel/attract electrons to restore the equilibrium. Liquid water behaves in the same way, though for more complicated reasons, which is why the hydraulic analogy works. Imagine a closed loop of pipe with a pump in one location and a water wheel in another. The water downstream from the pump and upstream from the waterwheel is under pressure and its density is slightly higher than the average, while the rest of the water is under tension (negative pressure) and its density is slightly lower. Replace the water with a sea of electrons and the pump and wheel with a battery and electric motor respectively, and the same thing happens. The part of the wire that's under pressure has a slight net negative charge and the other part a net positive charge, so there's a macroscopic EM field (outside the wire), and that's where the energy is stored. -- BenRG (talk) 02:56, 5 November 2010 (UTC)

(The following assumes you're a GCSE-ish level student, apologies if that is a faulty assumption.)

Take heart, OP. There are studies that show that people's ability on the electricity part of school Physics courses is completely uncorrelated with the rest of the subject, so if you're struggling with this one particular bit it doesn't mean you've suddenly got much worse! I know that personally speaking, all this stuff only really clicked for me when I got to university and did Maxwell's equations -- once I understood those (and Gauss's Law and so on) I was able to go back to the "simple" circuit stuff and it made a lot more sense to me, but I wouldn't advocate trying to learn lots of calculus just on the off chance your brain happens to work the same as mine.

I would suggest that you look at all the different analogies out there (hydraulic, the "electron men"/"buckets of charge" carrying certain amounts of energy ideas -- which latter does come with several important caveats!) whilst always bearing in mind that they are just analogies and none of them is going to give the complete picture -- different ones are helpful for understanding different situations (simplistic presentations of the hydraulic analogy, for example, have a hard time with series circuits because your pipes are suddenly changing length all the time when you add new components). One thing you may find helpful is to read up on drift velocity, which is often left of Physics at lower levels these days, but makes a strong connection between the overall flow of current and the behaviour of the individual electrons in the electric field across the wire, and might help you disentangle some of the things you're confused about. --86.130.152.0 (talk) 06:53, 5 November 2010 (UTC)

Thanks For all the great ansers, I definately feel like I've got a handle on the Voltage issue now. And yes, that assumption was spot on, I am a GCSE student. Incidentally, Aspro, it's not as if my teacher has not tried to help us understand electricity, with analogies like the aforementioned 'Buckets of Charge', it's just that a full understanding of it takes quite advanced maths, and given the fact that definately at least half of us, if not more, will stop physics after GCSE, it's frankly much more constructive to inforce simple understandable analogies and simple ideas about power and resistance, which will help in everyday life, than try and go the whole hog and cover all of the subject in excruciating detail, so we can 'understand'; which will be pretty much useless unless you are going into a highly specialised field, in which case it can be assumed that you will be taking A-level Physics, where you will learn the 'truth' anyway. Again, thanks for all the very illuminating answers!--92.10.28.38 (talk) 16:10, 6 November 2010 (UTC) Oops, that was me: I forgot to sign in.--HarmoniousMembrane (talk) 16:12, 6 November 2010 (UTC)

Yes, think of voltage as a pressure or as a potential energy, and think of current as a kinetic energy, and compare the whole situation to Bernoulli's principle in relation to fluid flow. David Tombe (talk) 17:27, 7 November 2010 (UTC)

It may also be helpful to think of voltage as electric potential difference. ~AH1^(TCU) 17:48, 7 November 2010 (UTC)

Entropy

Hi! I'm in a 1st year physics course and now we're looking at entropy. In my text it says "If the temperature changes in the process of the addition of heat to a system, the change in entropy can be calculated using advanced mathematics. This chapter deals primarily with changes in entropy in isothermal systems. For a discussion of how a change in entropy is calculated when the temperature changes, see the Supplemental Text on the next page" I looked on the next page(s) and.... no supplemental texts :( I think they got lost somehow when the book was edited. But how is entropy calculated when the temperature changes? I tried your Entropy article but as I'm only in first-year physics that was not very helpful :( —Preceding unsigned comment added by 24.92.78.167 (talk) 22:03, 4 November 2010 (UTC)

Taking the heat to be added slowly (and reversibly), so that the system remains all at one temperature throughout, $dS={\frac {dQ}{T}}$. So, you can integrate (let's say over time) $\Delta S=\int _{t_{0}}^{t_{1}}{\frac {{\dot {Q}}(t)}{T(t)}}dt$. The trick is that you have to know how the temperature changes; if you know the heat capacity C of the system, it becomes nicer to integrate over the added heat: $\Delta S=\int _{0}^{Q}{\frac {dq}{T(q)}}$, where $T(q)=\int _{0}^{q}{\frac {dq'}{C(T(q))}}$. That last equation is the tricky one: $T(q)$ is on both sides, so unless C is a constant you actually have to treat it as an autonomous differential equation ${\frac {dT}{dq}}={\frac {1}{C(T)}}$. Solve that with the "advanced mathematics", then you have $T(q)$, and you can do the S integral. If C is constant, then $T(q)=T_{0}+{\frac {q}{C}}$ and you can directly write $\Delta S=\int _{0}^{Q}{\frac {dq}{T_{0}+q/C}}=C\log \left(1+{\frac {Q}{CT_{0}}}\right)=C\log {\frac {T_{1}}{T_{0}}}$. If you then try to start from absolute zero ($T_{0}=0$) you get nonsense; as the third law of thermodynamics says, C can't be constant at absolute zero. --Tardis (talk) 00:32, 5 November 2010 (UTC)

It's a tricky question. As a chemist, I start by telling my students that, for an ideal gas, dS = C_V. That puts them into an adequate state of confusion, because they can't believe it's as simple as that. Of course it's not, not for a real system! But practical measurements of entropy on pure substances still rely on heat capacity measurements to very low temperatures, and on finding the various phase changes. Physchim62 (talk) 01:21, 5 November 2010 (UTC)

You must have left something out: one and only one side of your equation is a differential. For constant C I get ${\frac {dS}{dT}}={\frac {C}{T}}$, but that's not very much like $dS=C_{V}$. --Tardis (talk) 13:53, 5 November 2010 (UTC)

How to control this old oscilloscope

I have this old oscilloscope but it didn't come with a manual. I hope it might be a simple enough machine that someone with relevant experience might be able to tell me what I'd need to know to operate it. I'd like to be able to connect a circuit driven by rectified but fluctuating current from a dynamo and determine the voltages present. What's that "vert" socket, bottom left for? --90.209.7.130 (talk) 23:52, 4 November 2010 (UTC)

You lucky man..

The good news is that all Oscilloscopes operate the same. This might get you going.Oscilloscope--Aspro (talk) 00:01, 5 November 2010 (UTC)

That article doesn't say anything about the 'vert' socket... when plugged in and switched one, should the 'pull on' light come on or is that only under certain conditions? Why does the on/off switch have a 1 and a !0 instead of a 1 and a 0? --90.209.7.130 (talk) 00:25, 5 November 2010 (UTC)

The "Vert" connector is just where you connect the signal. The switch with 1 and 0 looks like a custom add-on. The light you speak of is just a power-on indicator. Good luck. (Did the scope come with probes?) PhGustaf (talk) 00:40, 5 November 2010 (UTC)

If you have $32 plus my substantial finder's fee that you may donate directly to the Wikipedia Foundation then you can have the handbook you lack. Cuddlyable3 (talk) 14:46, 5 November 2010 (UTC)

One thing to watch out for in an oscilloscope is whether the input "ground" terminal is actually an earth ground, or whether it is floating. If it is truly a ground then you cannot connect the scope across a component which has some voltage to ground on each side, or something will make smoke. Similarly it is important to note if the grounds for each channel are independent of each other or connected together internally. Edison (talk) 16:18, 5 November 2010 (UTC)

November 5

Lethal overdose of Caffeine - biological process involved?

Recently a young man in the UK accidentally killed himself by ingesting two spoonfuls of caffeine powder. I was wondering about the mechanism by which a caffeine overdose actually kills you, as the news report didn't mention really how he died. The caffeine article mentions extreme amounts of caffeine can cause ventricular fibrillation. Does this mean your heart just kinda starts beating all out of rhythm, screwing up your blood flow, and thereby leading to death via asphyxiation? The Masked Booby (talk) 00:37, 5 November 2010 (UTC)

As the article you linked to states, v-fib results in "cessation of effective blood circulation". Asphyxiation means the air supply is cut off. In this case, that is not true...the oxygenated blood is just mostly sitting there rather than flowing to cells. The cells are starved for oxygen and soon start dying (or at least some of the most important ones do), so it's a similar effect to having no air supply, but a different chain of events. DMacks (talk) 01:16, 5 November 2010 (UTC)

Here's the article about the death. When are the ACMD going to issue a report on caffeine then? "This should serve as a warning that caffeine is so freely available on the internet but so lethal if the wrong dosage is taken” reminds me of other things. SmartSE (talk) 01:28, 5 November 2010 (UTC)

Other caffeine overdoses can cause "rhabdomyolysis and acute renal failure" [10] but reading the abstracts on google scholar it has caused deaths in a wide variety of ways. SmartSE (talk) 01:35, 5 November 2010 (UTC)

Thanks everyone! The Masked Booby (talk) 04:53, 5 November 2010 (UTC)

mosquito

Hello,

pls i would like to cofirm if it is true that an average mosquito has 47 teeth?

Thanks —Preceding unsigned comment added by 62.173.41.242 (talk) 00:53, 5 November 2010 (UTC)

Mosquitos have mouthparts designed to pierce into skin, so they wouldn't have teeth in the normal sense. --Chemicalinterest (talk) 00:57, 5 November 2010 (UTC)

Mosquitoes are actually nectar feeders - see Mosquito#Feeding habits of adults. In female mosquitoes (only) they can also pierce the skin and suck blood - see Insect mouthparts#Piercing and Sucking Insects. They don't have teeth as such. This site gives some pretty nice 'close-ups' of their mouthparts. --jjron (talk) 07:14, 5 November 2010 (UTC)

There are 47 SPECIES of mosquitoes. I know because where I live there are all 47 of them! —Preceding unsigned comment added by 165.212.189.187 (talk) 17:58, 5 November 2010 (UTC)

In case anyone is wondering, the OP has probably stumbled across one of those trivia list that infest webpages and inboxes with information of dubious quality. I've seen the 47 teeth thing a number of times, and virtually uniquely among these sorts of list the quantity is actually attributed (to Isaac Asimov of all people). Here is an example. 64.235.97.146 (talk) 19:19, 5 November 2010 (UTC)

Which of Isaac Asimov's two jobs (that of a research biochemist and that of a science fiction writer) makes him trustworthy regarding the mouthparts of insects? --Jayron32 20:36, 5 November 2010 (UTC)

Asimov was a wide ranging and prolific science populariser (as well as dipping into several Humanities topics) by virtue of being a good textual researcher - much like we here on the RefDesks! Just because the claim is attributed to him doesn't mean he actually said it - I've read a good deal (though by no means all) of his non-fiction and would have noticed such a ridiculous mis-statement, but never have. Until someone comes up with a citation I'll assume the attribution is as erroneous as the "fact." 87.81.230.195 (talk) 22:09, 5 November 2010 (UTC)

Mosquitoes may not have the same kind of teeth that vertebrates have, with enamel and dentin, but the mouthparts of female mosquitoes do indeed have structures on their maxillae which are called teeth, which look like little saw teeth.

The question is complicated by there being 3,500 species of mosquitoes, whose maxillae vary considerably. Indeed, it has been suggested([11], page 508) that the number of teeth a mosquito has could be used as an aid in taxonomy. Mosquitoes with 14 or fewer teeth per maxilla can be expected to feed primarily on man, whereas those with more teeth could be able to feed on domestic animals.(ibid) Also, the number of teeth in individuals within a species can vary a bit, for example, the W. smithii mosquitoes examined in the given reference had between 8-10 teeth per maxilla. Since those mosquitoes that have maxillae have two of them, 14 teeth per maxilla amounts to a mosquito having a total of 28 teeth. However, due to the huge number of mosquito species, and due to a bit of variability of number of teeth per individual within a species, there almost certainly do exist at least some individual mosquitoes which have 47 teeth, as that’s easily within the typical ballpark. Red Act (talk) 08:39, 6 November 2010 (UTC)

Oops, I missed the word "average" in the OP. From what I've read, 47 sounds a bit high for the number of teeth an average mosquito that bites humans has, but not by a huge amount. And it's a bit vague as to what "average" means here; are you counting by number of species, or number of individuals, or number of bites of humans? Red Act (talk) 08:58, 6 November 2010 (UTC)

Yes, as was displayed in the external link I gave in my original answer. These are not teeth in any sense of common usage of the term though, that is in the sense of their biological function, they are just given that name. But I spose you could ask "how many teeth does the average comb have" and get an answer as well. However, even if we took that as reasonable, the 47 still adds further silliness, as since mosquitoes are bilaterally symmetrical we would expect them to have an even number of teeth. So then we would have to expect that this is some overall average (of what?), with as much meaning as saying the average mammal has 19.574* teeth (* invented figure). --jjron (talk) 16:55, 6 November 2010 (UTC)

I disagree that mosquito teeth have no similarity in biological function to vertebrate teeth. Consider the description "a series of features on an animal's maxilla which are used to sever the flesh of another animal during the process of feeding". That description could be referring to the incisors on a human maxilla (our upper jaw) during the process of biting off a piece of fried chicken. That same description could also be used to describe the teeth on a mosquito's maxillae and their function during the piercing process. That wouldn't be obvious from the picture in the link you gave in which the maxillary teeth are identified, because in that picture the proboscis is still covered with the labium. But the labium does not enter the skin during the piercing process. During the piercing process, the maxillae are on the outside of the fascicle, with the teeth facing the flesh, slicing it. The article you linked to says that "the teeth cause more damage once inside the delicate tissues below the skin and thus cause a greater haemorrhage than would a straight, smooth, needle-like appendage". And the page I linked to says "Gordon and Lumsden (1939) and Waldbauer (1962) considered that the maxillary teeth may assist the entry of the fascicle by cutting through the tissue during protraction". The functioning of incisors and a mosquito's maxillary teeth is a bit different in that incisors cut flesh kind of like a pair of scissors, whereas a mosquito's maxillary teeth cut flesh more like a saw or a serrated knife, but they still are both used to cut the flesh of another animal while feeding. And if a difference in the details like that of exactly how two things perform a particular biological function is enough for you to say that it's a completely different biological function, then you'd also have to say that "mosquitoes don't have eyes as such", because the details of how mosquito eyes perform their function differs considerably from the way human eyes perform their function.

I'm using the phrase "maxillary teeth" above because I found out that some mosquito larvae also have mandibular teeth, which "serve as masticating agents by grinding food"[12]. That description would also work as a description of molars in vertebrates, so that's another example of there existing mosquito teeth which perform a similar biological function to a vertebrate's teeth.

Saying that the average of 47 adds further silliness because mosquitoes have an even number of teeth is very much like saying that the statistic that the average family has 2 1/2 children is silly, because no family has a half a kid. Unless otherwise specified, "average" generally means the mean (as opposed to the median or the mode), and there is no presumption that the mean of a set will coincidentally happen to be an element of that set. Red Act (talk) 04:37, 7 November 2010 (UTC)

Whatever became of the "super focused sound" guy and his invention?

I recall reading several years ago about an inventor, perhaps in California?, who invented a way to focus sound such that if two people were standing shoulder to shoulder only one would hear the music projected from a device in front of them. The effect was so profound as to be at first unbelievable. (You guys really didn't hear that too?!?) I also recall advertising companies being very interested it, as it was purported to allow them to play targeted audio ads without filling the ambient environment with background noise. One of the articles mentioned walking past posters in a subway with each poster talking "to you" as you passed, but people outside the "beam" hearing nothing. Does this jog anyone's memory? I'd love to know what the current status of that invention is... The Masked Booby (talk) 04:50, 5 November 2010 (UTC)

The guy you're talking about is Woody Norris; see Sound from ultrasound. Red Act (talk) 05:40, 5 November 2010 (UTC)

The guy you're thinking of perhaps also might be F. Joseph Pompei of Holosonics, who is the one who actually solved the distortion problems of earlier systems, and beat Woody Norris to the market by a couple of years. But Woody Norris appears to have gotten the most famous for it, so you're probably thinking of him.

This is consistent with my impression of inventions in general. Contrary to popular perception, if you look at any major invention, the invention process is typically a complicated mess in which there really isn't one person who is really the inventor of the thing, and the person who becomes famous for being "the" inventor of the thing is not at all necessarily the first person to have created that type of thing. See, e.g., Invention of the telephone, Invention of radio, Electrical telegraph or Incandescent light bulb#History of the light bulb. Red Act (talk) 06:41, 5 November 2010 (UTC)

I don't know about that specifically, but some museums have systems where a parabolic dish in the ceiling projects sound down so that only the person standing directly in front of a particular exhibit can hear it. The effect is a bit startling. APL (talk) 13:38, 5 November 2010 (UTC)

Sound Refreshment Station, of which six are located in the departure areas at Oslo Airport, Gardermoen, are sound "showers" that make refreshing sounds audible only to a person immediately under them. Cuddlyable3 (talk) 14:33, 5 November 2010 (UTC)

"The decision to axe the Ark will leave the Navy without the capability of launching fixed wing aircraft."

What about its sister ship, the HMS Illustrious? Did the BBC become so unreliable, or is there something I missed? By the way, why would the British Navy decommission all their fixed wing aircrafts? If Argentina invades the Falklands again, the plan is to use only helicopters against their air force? --131.188.3.20 (talk) 11:21, 5 November 2010 (UTC)

According to HMS Illustrious, "As part of Strategic Defence Review, Britain's Harrier fleet is to be retired. Therefore, Illustrious will no longer launch any fixed wing fighter aircraft.". The reference (BBC News) says that the Navy will be unable to launch planes from aircraft carriers until 2019. As I understand it, the plan is that a new fleet(?) of aircraft will be eventually be built to replace the Harriers, so as long as they don't start the second Falklands War for ten years, it should be okay. --Kateshortforbob _talk 11:47, 5 November 2010 (UTC)

(edit conflict)HMS Illustrious (R06)#2010 says (citing Janes) that the Harriers themselves will be retired. Military of the Falkland Islands#Royal Air Force notes that the Falklands are defended by four Typhoons; that doesn't sound like a lot, but it's proportionately much more than defend the UK. -- Finlay McWalter ☻ Talk 11:50, 5 November 2010 (UTC)

Or decommissioning doesn't mean outright scrapping? Like, in case of some extreme emergency other solutions could be found, like investing more resources to reactivate them or complete the new models sooner, or just lease some from other countries? --131.188.3.20 (talk) 11:57, 5 November 2010 (UTC)

I would guess they also have ground defences, no? Nil Einne (talk) 12:04, 5 November 2010 (UTC)

Yes; there's a Rapier missile battery, an infantry company (rotated every 6 weeks) and support units[13]. Alansplodge (talk) 18:49, 5 November 2010 (UTC)

If they really wanted, they could probably buy the USS John F Kennedy which the US is finished with. Googlemeister (talk) 12:59, 5 November 2010 (UTC)

Using what for money? Lend-lease? Edison (talk) 16:09, 5 November 2010 (UTC)

Well the UK does own about something like $300 billion of US government debt... Googlemeister (talk) 18:18, 5 November 2010 (UTC)

In the next few years, all they will need will be something to lauch drones (Unmanned aerial vehicles) from, and helicopters. 92.15.10.141 (talk) 13:34, 5 November 2010 (UTC)

Could the four UK fighters based in the Falklands be expected to defeat the 41 combat aircraft and 24 ground attack aircraft of Argentina listed at Argentine Air Force ? Edison (talk) 16:09, 5 November 2010 (UTC)

The 4 Typhoons flew to the Falklands via Ascension Island with air-to-air refuelling[14]; therefore they could be reinforced at short notice by the same route. Several weeks of bombardment from air and sea failed to close ther runway at Stanley, so it should be possible to keep the new one open for a few days - excluding a successful amphibious assault. BTW in 1982 there were 28 Sea Harriers against 220 Argentinian combat aircraft - so rather better odds then than now. Alansplodge (talk) 17:16, 5 November 2010 (UTC)

The combat aircraft of the Argentine airforce are all designs that first flew between 40 and 50 years ago, and many have been small or downgraded platforms to begin with. Also, they probably have very limited supplies of modern ordnance. So I would not bet against the Typhoons, which are considered to be among the best current fighter aircraft. However, I also suspect that the risk of a rerun of the Falklands war is minuscule at the moment. --Stephan Schulz (talk) 17:33, 5 November 2010 (UTC)

But not inconcievable[15]. Alansplodge (talk) 19:10, 5 November 2010 (UTC)

Political posturing doesn't mean they have the capability to deliver military effect at range. One of the main issues that the Arg forces had during Corporate was the distance from base they were operating at, so their mission generation rate was quite low compared to the mission generation rate of the defending aircraft. Also time on target was very limited.

The level of threat to FI is negligible at the moment.

ALR (talk) 19:27, 5 November 2010 (UTC)

OK, but everyone in HMG thought the threat was negligible in 1981. Alansplodge (talk) 08:44, 6 November 2010 (UTC)

Even if Argentina do invade, is it likely they will commit their entire air force to the invasion? This seems a bad idea to me. According to Falklands War there were 75 aircraft of various types + 25 helicopters that time around. This is far less then the 220 quoted above. Argentine air forces in the Falklands War also seems to support the idea that Argentina did not deply their entire air force to the Falklands. In terms of the other point, it seems to me the nature of government in Argentina is quite different then from now. Nil Einne (talk) 09:44, 6 November 2010 (UTC)

The Argentine air forces in the Falklands War article says that about half were retained for defence against Chile. The list of units engaged gives 105 Air Force combat jets, of which 34 were lost, and 18 Navy jets with 5 lost. It also says that a lack of tanker aircraft was a limiting factor. Alansplodge (talk) 18:19, 6 November 2010 (UTC)

(edit conflict) The AAF is largely comprised of the same aircraft that badly lost the 1982 conflict; only the compliment of reconditioned USMC Lockheed Martin A-4AR Fightinghawks was acquired after that, and they're not very impressive. Argentina has been unable to acquire (and in fairness mostly unwilling to spend money on) F-16s or Mirage 2000s. In addition to the Typhoons, the Royal Navy routinely deploys an attack submarine submarine in the South Atlantic (example), armed (depending on mission) with spearfish torpedoes and TLAMs. -- Finlay McWalter ☻ Talk 17:59, 5 November 2010 (UTC)

Plus a standing patrol by a destroyer or frigate and an auxilliary called the Atlantic Patrol Task South, APT(S). Alansplodge (talk) 08:52, 6 November 2010 (UTC)

One would assume that the radar facilities on the Falklands are much better than they were in 1982. Not just for military purposes either, but to monitor the fishing industry that now provides much of the islands' income. Physchim62 (talk) 19:24, 5 November 2010 (UTC)

Different types of radar. Monitoring the while picture would be done using AIS as surface search has a very limited range. IN practice the fisheries industry is monitored by ships anyway. That's part of the role of HMS Clyde (P257).

ALR (talk) 19:30, 5 November 2010 (UTC)

The current Harrier capability is Ground Attack and Recce anyway, the aircraft aren't fighters and wouldn't be used as such. In practice all of the capabilities currently delivered by the Harriers could be delivered by UAVs using current technology.

ALR (talk) 19:27, 5 November 2010 (UTC)

Childhood

Why does childhood only last 18 years? jc iindyysgvxc (my contributions) 13:37, 5 November 2010 (UTC)

It doesn't. Maturity in humans doesn't end until the mid-20's. Then, after a period of stabilization, many humans go through a relapse into a second childhood. -- kainaw™ 14:16, 5 November 2010 (UTC)

Childhood is the age span ranging from birth to adolescence, typically between the ages 13 and 19. Biologically the development from child to adult is the process of Puberty whose onset is usually between 10-13, having dropped from an average 16.5 in England in 1840. A significantly Delayed puberty is regarded as an abnormality with an underlying cause that needs investigation and possibly correction by hormone treatment. Age 18 or a range of other ages are used in formal thresholds in laws that define Age of consent, Coming of age and in the age of reason rules for baptism in Western Christian churches. In many countries, there is an age of majority when childhood officially ends and a person legally becomes an adult. The age ranges anywhere from 13 to 21, with 18 just being the most common. Cuddlyable3 (talk) 14:25, 5 November 2010 (UTC)

I think kainaw's first two sentences refer to studies suggesting that brain development isn't complete until the early 20s. Comet Tuttle (talk) 17:43, 5 November 2010 (UTC)

This article discusses recent trends in extended child-like behaviour [16]. In short, the claim is the average twenty-something today doesn't really fit into classic 'child-adolescent-adult' classification schemes. SemanticMantis (talk) 14:53, 5 November 2010 (UTC)

Numerous US politicians have dismissed things they would prefer to forget as "youthful indiscretions." Several back as far as Bill Clinton said their use of drugs while in college was a "youthful indiscretion[17]." Unsuccessful Supreme Court nominee G. Harrold Carswell said his vow of "segregation forever" at age 27 was a "youthful indiscretion[18]." Bobby Kennedy serving as Counsel for Joseph McCarthy's redbaiting House committee at age 27 was dismissed later as a "youthful indiscretion[19]." George W. Bush said his drunk driving arrest at age 30 was a "youthful indiscretion". Grover Cleveland dismissed the fathering of a child out of wedlock when he was 37 as a youthful indiscretion [20], [21]. Vice President Dan Quayle, while a candidate said his vote against a Veteran's Administration cabinet position at age 41 was a "youthful indiscretion.[22]" Representative Henry Hyde said his five year affair with a married woman from age 41 to 45 was a "youthful indiscretion." Childhood among politicians thus may extend to 45. Edison (talk) 15:26, 5 November 2010 (UTC)

Chemistry

CH3-CH=CH-CH2-CH3+HBr -> ? —Preceding unsigned comment added by 121.245.138.117 (talk) 13:46, 5 November 2010 (UTC)

Hbr#Uses_of_HBr may be useful, we don't do you homework though. SmartSE (talk) 14:20, 5 November 2010 (UTC)

Also Electrophilic addition, and even more specifically, Hydrohalogenation. Buddy431 (talk) 17:31, 5 November 2010 (UTC)

genetically modified foods

what are the most commmon genetic modifications to produce? 70.241.22.82 (talk) 13:56, 5 November 2010 (UTC)

According to this article: GMO corn (maize), GMO soy, rBGH milk, GMO canola (oil), and, interestingly, aspartame, which is produced these days using GMO bacteria. That sounds pretty plausible to me. --Mr.98 (talk) 14:04, 5 November 2010 (UTC)

If you were meaning what are the actual modifications that are made, the most common are resistance to glyphosate (a herbicide) and adding bt toxin (an insecticide). SmartSE (talk) 14:13, 5 November 2010 (UTC)

time constant for capacitor in both parallel and series

Suppose there is a resistor R1 after a battery, that then forks into a capacitor and a resistor R2 (that is they are both at the same potential difference). The circuit is then closed.

I am having a real hard time finding a succinct derivation of the time constant for this circuit. When t=0, most of the current (that is, E/R1) flows through the capacitor, but as time goes on, the current flows through R2 as the resistance of the capacitor becomes infinite.

How do I solve for the time constant? Neither google or the articles are helping me. John Riemann Soong (talk) 15:17, 5 November 2010 (UTC)

I don't get our explanation for charging a capacitor in parallel with a resistor. How does the time constant change in this case? John Riemann Soong (talk) 15:21, 5 November 2010 (UTC)

Mmmh fiddly. How far have you got:

1. If Q_t is the charge on the capacitor at time t (at time=0 charge=0)

2. The the voltage across the capacitor V_c is Q_t/C

3. The voltage across R₂ must be the same, they are in parallel (more on this later)

4. So the voltage across R1 is V-Q_t/C , and this voltage equals R₁I_t where I_t is the current through R1 at time t, (I_t is also the net current)

5. So I_c,t+I_r2,t=I_t (the currents through R2 and the capacitor equal the net current)

Also

6. I_r2,tR₂=V_c (see no. 3)

7. Therfor I_r2,t=Q_t/CR₂

8. So I_c,t+I_r2,t=I_c,t+Q_t/CR₂ , which equals I_t

9. Combining 8 and 4 gives:

(I_c,t+Q_t/CR₂)R₁=V-Q_t/C

But Q_t is got from the integral of I_c,t.dt (from 0 to t)

So 9 is a differential equation, and all that needs to be done is solve it.

Before that is attempted - did that make sense, are there any obvious mistakes in the above? (I haven't completed it)>94.72.205.11 (talk) 18:54, 5 November 2010 (UTC)

Wait what is the resistance of a capacitor as a function of time? Every time I look it up it says "resistance is infinite" but this is for a charged capacitor, not a capacitor that is charging. John Riemann Soong (talk) 19:09, 5 November 2010 (UTC)

If the charging voltage across a capacitor is greater than the voltage the capacitor is charged to then I would assume that the resistance of the capacitor is zero. (Have you tried the deriving the case when there isn't a R2 ie battery,resistor,capacitor ? - the current in that simpler case is (V_battery-V_capacitor)/R₁ eg not divided by R₁+R_c because R_c=0 ).19:28, 5 November 2010 (UTC)

Wait doesn't the resistance of the capacitor start at zero uncharged and increase to infinity when it's fully charged? I'm just trying to characterise the current division as a function of time. John Riemann Soong (talk) 19:44, 5 November 2010 (UTC)

(reply) that's what I just said -the resistance is zero until the capacitor is fully charged, at which point it becomes infinite.94.72.205.11 (talk) 20:12, 5 November 2010 (UTC)

How do I obtain a formula for the resistance of the capacitor as a function of time? John Riemann Soong (talk) 20:15, 5 November 2010 (UTC)

Welllll, That's actually simple - no need to solve - as you already noticed the resistance is zero until the capacitor is is fully charged. So the next question is "when is it fully charged" - hopefully it will be obvious that the capacitor never becomes fully charged - or to put it another way it becomes fully charged at t=infinity.

However if you want to calculate the 'effective resistance' (can't quite remember what that's called) - then all you need to do is solve the above equation - ie from step 9 onwards - to get the capacitor charge as a function of time, this gives you the capacitor voltage, the current through the capacitor is the rate of change of charge with respect to time, and the resitance (effective) is V/I not sure if that last bit is right nevertheless, with the solved equation for capacitor voltage and charge as a function of time you should have enough information to calculate simply any other parameters you desire... 94.72.205.11 (talk) 20:46, 5 November 2010 (UTC)

Imagine that you let your circuit charge the capacitor to equilibrium, and then set the battery voltage to zero. Assuming that your battery is an ideal voltage source, i.e. with an impedance of zero, then it will be equivalent to a short circuit. Your circuit then looks like a capacitor in parallel with two parallel resistors. Therefore the capacitor will discharge through R1||R2. So the time constant is 1/2π(R1||R2||C). If my life depended on it then I would use PSpice to verify my answer. --Heron (talk) 19:55, 5 November 2010 (UTC)

I don't get your derivation. Can you explain it to me? John Riemann Soong (talk) 20:10, 5 November 2010 (UTC)

Try the one I gave you ? 94.72.205.11 (talk) 20:13, 5 November 2010 (UTC)

Sorry. That was the formula for cutoff frequency, not time constant. I confused you needlessly there. I meant τ = (R1||R2)C. Does that make any more sense? I'm trying to avoid nasty differential equations by using a bit of lateral thinking. --Heron (talk) 20:32, 5 November 2010 (UTC)

Speaking of horrible differential equations, the above solution (9) appears to be of the form dx/dt=k+ax_t , where x is a function of t, x_t=0=0 dx_t/dt=k when t=0 , which possibly has a solution x=ke^ax-k (though as I mentioned before . I haven't checked the derivation) .. 94.72.205.11 (talk) 21:25, 5 November 2010 (UTC)

I get tau = R2+R1/(R2*R1*C). Does this seem correct? John Riemann Soong (talk) 21:36, 5 November 2010 (UTC)

I'm guessing you solved for ..e^{-tau x t}.. instead of ..e^{-t / tau} , as you answer is so close, if not how the hell did you get that?? :) 94.72.205.11 (talk) 23:07, 5 November 2010 (UTC)

No, dimensional analysis will tell you that that is wrong, but it works if you add a pair of parentheses around the first two terms and then turn the fraction upside-down. I just solved the circuit using Laplace transforms and it confirms my original guess: τ = (R1||R2)C. Translating that from electronics-speak into maths-speak:

$\tau ={{R_{1}R_{2}C} \over {R_{1}+R_{2}}}$

I had to refresh my knowledge of Laplace transforms from this very helpful lesson. Example 1 is almost the same as your problem, but you have to add another resistor. I'm leaving out a lot of details because it would take forever to type it all in Tex format, but I will supply as much information as you want on request. --Heron (talk) 22:29, 5 November 2010 (UTC)

question

If an airoplane was crashing, why don't people wait til it's a few feet from the ground and then just jump the short distance? —Preceding unsigned comment added by 85.140.85.64 (talk) 15:30, 5 November 2010 (UTC)

No, because you'd still be traveling towards the ground at hundreds of miles per hour. (You cannot possibly jump hard enough to even come close to counteracting that.)

You're better off in your seat, buckled in. Let the plane absorb as much of the impact as it can. (Of course, if the plane is just plummeting straight down, that won't save you either.) APL (talk) 15:43, 5 November 2010 (UTC)

And even if you could jump with enough force to counteract most of the speed you are going down with, you are going to hurt yourself with you hit the ceiling. And then there is the horizontal portion of your vector, so you will hit the front of the plane at high speed. Jumping is not a great way to avoid getting hurt here. Googlemeister (talk) 15:51, 5 November 2010 (UTC)

An ejection seat is your answer. It might even save a troll who could not jump very high. Edison (talk) 15:58, 5 November 2010 (UTC)

I had assumed he meant jumping out of the plane. (Wait until the plane is a foot from the ground and then jump! A one foot fall never hurt anyone! Sorry. Doesn't work like that. ) APL (talk) 16:08, 5 November 2010 (UTC)

Note that Mythbusters did an episode on a similar idea: jumping right before a free falling elevator crashed to the ground. - Akamad (talk) 16:34, 5 November 2010 (UTC)

I think that the OP meant to jump out of the airplane when it's a few feet off the ground; jumping a few feet wouldn't hurt you like falling a few hundred feet. --75.33.217.61 (talk) 16:55, 5 November 2010 (UTC)

1- Falling hurts because, by the time you hit the ground, you're going really really fast.

2- An airplane that has just fallen 40,000ft is going really really fast.

3- If you jump off something that is moving really really fast, you will hit the ground really really fast.

So yea, the speed you gain during that last foot won't hurt you, but it doesn't magically get rid of the speed you gained during the previous 39,999 feet. APL (talk) 17:00, 5 November 2010 (UTC)

To give a concrete example, suppose the plane is falling downward at 300 kph. You, also, are falling downward at 300 kph. When you're 1cm off the ground, you leap upward. Let's say your upward speed right at the fastest point of your jump is 20 kph. The net downward speed of your body is still 280 kph, which is enough to break your bones and rupture important organs like your heart, brain, and liver. Now, if you had some way to leap upward at a speed of about 300 kph, as in the ejector seat suggestion above, then your net speed would be 0 kph and you could land on your feet quite gently (though of course there are lots of airplane parts bouncing into you at a speed of 300 kph). Comet Tuttle (talk) 17:39, 5 November 2010 (UTC)

With one centimetre to spare and travelling at 300 km/h, an ejector seat would do exactly zip to save you: decelerating enough for the impact not to potentially kill you, in such a small time frame, would be lethal anyway. You would not land gently; using any real ejector seat, you'd crash almost exactly as hard as you would without, and using a hypothetical ejection seat that decelerates you to non-lethal velocity in 1 cm, you'd be pancaked against the seat. In other words, unless Scotty beams you up in those last few microseconds, you will go splat. --Link ^(t•c•m) 19:24, 5 November 2010 (UTC)

Agreed, sorry, I shouldn't have mentioned the ejector seat as being able to achieve my desired outcome. Comet Tuttle (talk) 20:35, 5 November 2010 (UTC)

That and most ejector seats have a minimum altitude and speed for effectiveness, since you usually need time for the canopy to clear. Googlemeister (talk) 19:42, 5 November 2010 (UTC)

A blanket claim that ejector seats don't work if the crashing plane is near the ground is refuted by many videos of successful ejections. If not "1 cm," then how about a few meters?Youtube At an air show, the pilot is likely to wait until the last second to make sure the plane does not crash into the crowd. Edison (talk) 22:22, 5 November 2010 (UTC)

Leaving the aircraft at the first sign of trouble as Vesna Vulović did, worked for her. From aircraft speeds, she just slowed down to her own terminal velocity. --Aspro (talk) 18:36, 5 November 2010 (UTC)

She's not alone - see Free fall#Surviving falls. Alansplodge (talk) 19:15, 5 November 2010 (UTC)

For better advice, though, see List of sole survivors of airline accidents or incidents. Comet Tuttle (talk) 20:35, 5 November 2010 (UTC)

In addition to all of the points made above, in order to get out of the plane you first need to get to an exit. --Anonymous, 00:20 UTC, November 6, 2010.

Springs question

In another discussion it was said that when a spring was compressed its mass increases by a tiny amount due to the stored energy. If this is true, why not have two springs, compress the first one, move the uncompressed one forward, compress that one, uncompress the other one and pull it to the compressed one and create a reaction-less drive? The force would be too tiny to be practical, but this is a thought experiment [Trevor Loughlin]80.1.88.13 (talk) 15:41, 5 November 2010 (UTC)

Because the energy that you're using to compress the springs ALSO has mass. (And the kinetic energy released when the spring decompresses!) APL (talk) 15:46, 5 November 2010 (UTC)

No energy enters or leaves the system, so it has a constant mass, and thus the ZMF is inertial. As expected. —Preceding unsigned comment added by 129.67.37.227 (talk) 16:08, 5 November 2010 (UTC)

Oscars eating chicken

I fed my oscars some shreds of raw chicken cutlet and I see them defecating what appears to be similar-sized, fully formed strips of the same cutlet -- can they not digest bird meat? Or is it that the cutlets are too tough, and, for instance, the beef heart that I buy at the pet store is ground first? Mind you my oscars are about 2 inches long. DRosenbach ^{(Talk | Contribs)} 19:25, 5 November 2010 (UTC)

Carnivorous fish (I think carnivorous animals in general) have poor digestive systems ie very short intestines (unlike the cow which has an advanced digestive system). I'm not surprised. Can't definitely say nothing is wrong though.94.72.205.11 (talk) 19:34, 5 November 2010 (UTC)

Here's some basic links http://www.aqualex.org/elearning/fish_feeding/english/digestion/fr_structure.html (the diagram says it all I think - basically the intestine of a carnivorous fish is nearly a straight pipe.

Also http://www.umaine.edu/aquaculture/GeneralInfo/Biology/Anatomy/digestion.htm "The length of the intestine is dependent upon the type of diet. Herbivorous fish often have intestines twenty times longer than their body length. Carnivorous fish, such as the cod, have an intestine length about equal to their body length." - the intestine just isn't long enough to significantly break down the food.94.72.205.11 (talk) 19:42, 5 November 2010 (UTC)

I'm not familiar with Oscars but for for the Walking catfish I can confirm what the first link above says - eg that they have a mouth, then an (expandable) stomach, and after that, basically nothing - just a short pipe to to anus. It's like having a stomach, but not large or small intestine..94.72.205.11 (talk) 19:47, 5 November 2010 (UTC)

Hairy dandelion

I saw in the UK something like a dandelion, except its leaves are not indented, and they are hairy on both sides. What could it be? Thanks 92.29.112.206 (talk) 20:40, 5 November 2010 (UTC)

My bet would be Cat's-ear (or Catsear) Hypochoeris radicata from the same Compositae family as the Dandelion. "The leaves of dandelions are jagged in appearance, whereas those of catsear are more lobe-shaped and hairy". My trusty Observer's Book of Wild Flowers (1937, revised 1963) says "Plentiful in meadows, pastures and waste places throughout the country" (ie the United Kingdom). The pictures in our article seem to be of a half-dead one, but there's a nice illustration here[23]. Alansplodge (talk) 09:16, 6 November 2010 (UTC)

The edges of the leaves are not indented but irregularly wavey. Not like the linked illustration, but more like the as you say half-dead Wikipedia photo. Thanks. 92.15.28.27 (talk) 14:45, 6 November 2010 (UTC)

I haven't made a study but in dandelions there is a great deal of irregularity in the leaf patterns; some have wholly unindented leaves while others are deeply incised almost to the stem. The catsear illustration itself seems to show several distinct leaf forms. The Observer's Book says; "a circlet of rough hairy leaves, their edges scalloped..." which agrees with your description. Alansplodge (talk) 17:36, 6 November 2010 (UTC)

Cherry thieves

During the late summer, I left the cherries on a dwarf cherry tree to ripen a little longer before I picked them. When I returned they had all disapeared, including the stalks, except for one half-eaten cherry that was still on its stalk. There were no fallen cherries below the tree.

If they were eaten by birds, is it usual for them to remove the stalks as well? Thanks. 92.29.112.206 (talk) 20:48, 5 November 2010 (UTC)

I had a cherry tree that I did not protect sufficiently well and a couple of blackbirds took the cherries and the stalks. PR - but every little helps. Richard Avery (talk) 22:53, 5 November 2010 (UTC)

AESA vs Active-cancellation

Given the current lack of FTL CPUs, isn't Active-cancellation (Thales Spectra) worse than useless against Active Electronically Scanned Array radars? It's like trying to use Noise-cancelling headphones against White noise. Hcobb (talk) 20:50, 5 November 2010 (UTC)

Active RADAR is not "white noise" - though they may be broad-band and may seem like "noise" to the "untrained eye." Pulse patterns are deterministic and depend on the hostile system's design. If an ELINT aircraft positively identifies RADAR type, it can configure its countermeasures to match expected returns. The ultimate objective in such electronic countermeasures is to know what signal the enemy RADAR expects to receive (as an echo)- which is both a SIGINT and a HUMINT challenge - and then to send something else (as a transmitted signal that looks like an echo). The active response system then sends a spoof response based on known parameters of the enemy RADAR. Of course, if the enemy RADAR uses counter-counter-electronic-countermeasures, your spoof response might indicate your presence or even give away your location. In some cases, "active-noise cancellation headphone" style methods would work: for example, if the enemy RADAR were using a CW illuminator (an archaic, but possible scenario), then you could phase-match and retransmit a deconstructively interfering signal to "null" your echo. Nimur (talk) 21:02, 5 November 2010 (UTC)

Horripilation

Is there any way to induce goosebumps? I am kind of fascinated by them but (or maybe because) to my knowledge I have never experienced them. I have been cold, I have felt awe, and I have felt creeped out—nada. It is possible I have piloerectile dysfunction?--141.155.159.142 (talk) 22:00, 5 November 2010 (UTC)

Apparently it is possible to induce goosebumps at will. [24]. Mitch Ames (talk) 05:56, 6 November 2010 (UTC)

More than two parents

Just watched the new BBC series 'First Life' (new Attenborough one started) and it got to the point where it stated that once animals sexually reproduced it vastly increased the genetic mutation / variations and therefore is considered a big factor in the speed of change of animal development. Anyhoo as with my electricity question a few days ago...it got me thinking. It seems to be pretty much standard that every new born animal is created by the 'dna' of 2 parents - a mum and a dad if you will. Are there any animals that are the offspring of more than 2 parents? (I.e. their DNA is made up of 3 animals rather than 2 or 1)? ny156uk (talk) 22:11, 5 November 2010 (UTC)

There's at least one case of a human (two humans, actually - twins) being conceived of the DNA content of their mother's egg and two of their fathers' sperm [25]. They are genetic mosaics, which means that each given cell bears the DNA of only two parental gametes, not all three. Theoretically speaking, if the mother had had sex with two different men in very close succession, she could have borne mosaic children with genetic contributions of three different people (herself and two fathers). However, it should be noted that even if this does ever occur in the animal kingdom, it is probably a rare event that has not been selected for by evolution. In any mammal, having more than two parents by such a mosaicism-producing mechanism is problematic because it can create infertile offspring that are male/female mosaics, as is the case in the paper I cite above. Someguy1221 (talk) 03:22, 6 November 2010 (UTC)

Is it true that all matter gives off at least some radiation?

Topic says it all. ScienceApe (talk) 22:16, 5 November 2010 (UTC)

I think so. I think that matter gives off some radiation from the vibration of atoms and molecules. The only way to stop matter from giving off radiation would be to cool it to absolute zero. --The High Fin Sperm Whale 22:25, 5 November 2010 (UTC)

Did you mean "radiation" as in radioactivity ? (I think the answer for real world stuff is still yes) eg carbon 14 is present in almost all life, similar weakly radioactive isotopes exist for a wide variety of elements, so most rocks etc will also be at least very slightly radioactive.

If you meant "radiation" as in electromagnetic spectrum the answer is still yes (in particular blackbody radiation), the emission stops at absolute zero (-273C), but that's impossible to reach.94.72.205.11 (talk) 22:53, 5 November 2010 (UTC)

You might be interested in our article on background radiation. Physchim62 (talk) 00:24, 6 November 2010 (UTC)

Thermal radiation. --Wrongfilter (talk) 08:58, 6 November 2010 (UTC)

Unknown mushrooms

Anyone know what these strange mushrooms are? Thanks, --The High Fin Sperm Whale 22:16, 5 November 2010 (UTC)

Narrow it down a bit - where did you find them? I don't recognize them, but I imagine the mushrooms growing in Central Europe (that I'm somewhat familiar with) differ from those growing in Britain or Asia or over the Pond in the American landmasses. TomorrowTime (talk) 06:50, 6 November 2010 (UTC)

User resides in British Columbia.--Shantavira|^{feed me} 12:38, 6 November 2010 (UTC)

1. just look at my userpage, 2. look at the coordinates on the picture. But if you want a bit more information, I found them beside a pile of old sticks and shrubs near the entrance to a forest. --The High Fin Sperm Whale 17:06, 6 November 2010 (UTC)

November 6

Lizard Salivation

It was a particularity cold day today, even though I live Southern Florida. I have a Cuban Anole, and knowing that lizards are cold blooded desided to warm him/her up by placinh him/her in some lukewarm water (I was sure the water was not to hot). I left him there to warm up a rock and on my return I found him, seemingly sleeping, but more important he was what seemed to be salivating a lot. My Concern: Is this normal? Should I be concerned? And how do I now if it is a boy or girl? 66.229.227.191 (talk) 01:30, 6 November 2010 (UTC)

First of all, as evident by our article on Cuban Anoles, there are two types. Assuming yours is a Brown anole, then see if it has a dewlap to determine the sex (males do, females do not). And was the anole touching the water? If so, when the water evaporated the anole would be cooled even more (in the same way sweat cools us). If it was asleep, then we know it was just getting cooler. I suppose the drooling occurred because when it was asleep, it didn't know it was drooling. And I don't think you should be concerned as long as the temperature was above 0° C; reptiles, as the temperature drops, simply get less and less conscious. However, below 0° C, they will die, because the ice will damage tissues. --The High Fin Sperm Whale 02:25, 6 November 2010 (UTC)

I've never heard a brown anole called a Cuban Anole... but yea, its a Cuban Knight Anole. I understand that water evaporates, but thats normally under wind shear and he was inside and completely emerged. That's strange, I've never seen him/her do that before, maybe someone could elaborate more 66.229.227.191 (talk) 02:39, 6 November 2010 (UTC)

If it's a Cuban Knight Anole, then still see if it has a dewlap. Water evaporates all the time, albeit slowly. Even if not evaporated, the water would get as cold as the air around it after you took it out, so warming by using water is not a good idea, I would think. And if you're wondering about the drooling, then see my earlier comment. Hope this helps, --The High Fin Sperm Whale 03:27, 6 November 2010 (UTC)

percent algae death

What percent of oceanic algae death is necessary to reduce the amount of atmospheric oxygen by 10% and is there a curve for the relationship between oxygen and algae? --96.252.213.127 (talk) 03:37, 6 November 2010 (UTC)

Question one: I am not sure, but the amount would be enough to kill most organisms in the ocean. And even if all of those instantly died for some reason, they would reproduce so fast that in no time the population would be back to normal. Right now, I think the problem is to many algae, not to few.

Question two: Yes. There is a strong relationship between algae and oxygen.

Hope this helps, --The High Fin Sperm Whale 05:35, 6 November 2010 (UTC)

NO. It does not answer my question for I know that there is also a strong relationship between how much oxygen is needed and how many humans are asleep in a coal mine from which they must be rescued. What I am looking for is the numerical correlation or curve between oxygen production and algae on the planet Earth. --96.252.213.127 (talk) 14:42, 6 November 2010 (UTC)

It would be very tricky to set up a model to estimate a curve for the relationship. As oxygen decreases, carbon dioxide increases (at least as long as there are animals breathing), and this encourages the growth of plants, but they probably wouldn't fully compensate for the loss of algae. There are just too many unknown variables to derive an accurate relationship or to predict where an equilibrium would be reached for a given percentage of algae death. The coal mine is much simpler to model if there are only miners there. Someone might be able to make a guess at an answer for you, but experts are unlikely to agree or to be accurate because they will have to make many assumptions about what factors to include in the model. Dbfirs 17:47, 6 November 2010 (UTC)

The curve is very complex and needs to account for all the positive and negative feedback that occurs. You could try modeling it as a differential equation, but I suspect it will be very complex. John Riemann Soong (talk) 21:14, 6 November 2010 (UTC)

jeans

why do jeans feel different on the outside than the on the inside? insint it the same fabric —Preceding unsigned comment added by Kj650 (talk • contribs) 04:37, 6 November 2010 (UTC)

Jeans are made from denim, which is a type of fabric called twill. According to our article "Twill fabrics technically have a front and a back side", presumably because of the way they are woven (described in the article). The two sides are different: the technical face side (front) is usually more durable and attractive. Presumably (the article doesn't actually say) the material feels different on each side because of the asymmetric weave. Mitch Ames (talk) 05:51, 6 November 2010 (UTC)

how do u weave something that has 2 sides —Preceding unsigned comment added by Kj650 (talk • contribs) 08:01, 6 November 2010 (UTC)

How do you weave something that hasn't? --Stephan Schulz (talk) 09:56, 6 November 2010 (UTC)

Like this. Mitch Ames (talk) 12:15, 6 November 2010 (UTC)

I think he means "two sides that are different". Judging by the twill article, I'd say you'd need to know a bit about weaving to understand it... Vimescarrot (talk) 10:30, 6 November 2010 (UTC)

The front side of a twill should feel the same as the reverse side at right angles.

However jeans material will have had it's front surface treated differently, (confirmed by the presence of dye) - this will also affect the feel.94.72.205.11 (talk) 12:00, 6 November 2010 (UTC)

They won't necessarily be the same after rotation, even without differential treatment. On something like a 3/1 twill, one side will have mostly the warp fibers exposed, whereas the reverse will have the weft exposed. During weaving, there's different tension on the two, so the "lay" will be different. Examining a pair of jeans, this appears to be the case. The ones I looked at appeared to be a 2/1 twill, with the high-tension warp threads being prominent on the inside, and the lower tension weft threads prominent on the outside. This gives the inside a smoother, flatter texture than the outside. Although it doesn't appear to be the case on the pair I examined, different thread types can be used on the warp versus weft, accentuating the difference. -- 174.31.204.207 (talk) 17:25, 6 November 2010 (UTC)

that's right, good point. 94.72.205.11 (talk) 18:21, 6 November 2010 (UTC)

Geology abbreviation

While looking at a job application, in the job description it is mentioned: Grade: Band 7/6 (SO/HSO). What does this mean, and what other grades are there? Thanks in advance. --Danninja (talk) 07:11, 6 November 2010 (UTC)

These are (probably UK government) officer grades: Scientific Officer (SO); Higher Scientific Officer (HSO); Senior Scientific Officer (SSO)

According to this site, point 6 was £29,412 in August 2009, but you should check with the employer before assuming that this is correct because Medical Scientific Officers at point 6 on the Liverpool University scale are currently paid only £23,366, and if the employment is in India where there are also "SO" scales, then the remuneration will be completely different. Dbfirs 08:12, 6 November 2010 (UTC)

Coccyx decay rumor

If anyone searches about Coccyx decay, they will get many results on the top confirming this rumor without any scientific evidence:

"Coccyx tail is the only bone that doesn't decay after one's death"

Is there any scientific citation?--Email4mobile (talk) 09:57, 6 November 2010 (UTC)

What exactly does "after one's death" mean? 10 years? 100 years? 1000 years? There are many, many places all over the world where human remains from several thousand years ago have been excavated and many of them have most of their bones intact.[26] Just think logically about the "rumour", why is the coccyx any different from any other bones in the body? Correct, it's not. So why should it decay in a different way. Richard Avery (talk) 13:34, 6 November 2010 (UTC)

Schiller's experiment

Few days ago I posted this question on the Michelson–Morley experiment talk page, but will repeat here: I've read a related article and it was astonishing but I don't know if it were reliable to add it to Michelson–Morley_experiment article?--Email4mobile (talk) 10:08, 6 November 2010 (UTC)

What I find astonishing is not the article or the confirmation of the results, but the crank comments that follow. I'm not sure that this experiment really adds anything to a long-established result, but we could add a note that the result has recently been confirmed to at least one part in 10¹³. Dbfirs 11:38, 6 November 2010 (UTC)

health care

This request to diagnose the cause of a health problem has been removed. -- kainaw™ 14:35, 6 November 2010 (UTC)

sorry, but I just had to edit your comment. --Chemicalinterest (talk) 15:02, 6 November 2010 (UTC)

How is combustion sustained in a jet engine?

Compressed air flows through the combustion chamber of a jet engine at several hundred miles per hour or more. How do they keep the flame from being extinguished? Do they have some kind of heating element that continuously tries to restart combustion? --173.49.14.225 (talk) 11:59, 6 November 2010 (UTC)

No, they don't. It's not necessary. 76.123.74.93 (talk) 13:58, 6 November 2010 (UTC)

Once a jet engine (where "jet engine" means turbojet or turbofan, for now) is started, the ignition system is turned off (it's mostly just a ring of spark plugs). From then on the flame is sustained by (subsonic) deflagration - the burning mass of fuel/air in the rearward end of the combustion chamber thermally heats the non-burning fuel-air mixture forward of it, causing it to burn. This means there's a stable flame-front in the engine, which self-sustains the flame without an external source of ignition. This process is aided by the incoming stream already being highly pressurised (overall pressure ratio gives ratios > 10:1). A flame holder in the air path provides an eddy environment to maintain the position of the flame front. Note that if things go wrong with the supply of air, fuel, or with the pressurisation provided by the turbine spools, the engine can flame out and must be reignited. This all works because the speed of air through a turbine engine is subsonic (even on supersonic aircraft); deflagration only works in a subsonic medium. If you tried to inject supersonic air, the deflagration couldn't keep up, the fire-front wouldn't maintain its position at the flame holder, would be blown backward, and the engine would flame out. As for truly supersonic engines, I think a scramjet needs a continuous ignition source, and a pulse detonation engine relies on supersonic detonation rather than subsonic deflagration, and I think may need a reignition for each pulse. -- Finlay McWalter ☻ Talk 15:34, 6 November 2010 (UTC)

Air traffic control system

I may be getting the details of the story wrong. I think it was in the early '90s when I read about a problem with the ATC system in the US. The system was described as antiquated and running on obsolete hardware. Procuring repair parts for the hardware was getting difficult. And attempt to develop a modern software replacement was unsuccessful. I don't know if and how the situation have improved since, or whether the problem persisted.

My questions:

1. Has the US replaced the old ATC system with something modern and maintainable?
2. Why couldn't the US just buy whatever ATC system the Europeans were using at the time? Did the Europeans have the same problem?

--173.49.14.225 (talk) 12:15, 6 November 2010 (UTC)

The Wikipedia article Air traffic control will be of interest. It notes that In 1999, U.S.A. controllers began use of the Standard Terminal Automation Replacement System, which included new displays and capabilities for approach control facilities. Cuddlyable3 (talk) 23:10, 6 November 2010 (UTC)

ATC is extremely conservative. Over here, in Germany, the replacement system for the Upper Airspace Control in Karlsruhe has been under development for 15 years, but is still not operational. Of course, when they designed it they made it future-proof by using a client/server structure. And by picking the most advanced microprocessor for all components - the Alpha 21264, instead of one of the obviously dying Intel CISC kludges. For the US< buying an ATC system in Europe is hard to imagine. The US aviation market is very much cornered by the big US suppliers - Boeing, Raytheon, Lockheed Martin, and a few others - due to pork barrel spending policies and protectionism. See Next Generation Air Transportation System for the current transition in the US and Single European Sky for the next generation ATC systems in Europe. --Stephan Schulz (talk) 00:41, 7 November 2010 (UTC)

Possible future need for faster than light communication

What is to be done theoretically if the mankind becomes advanced to such an extent, when communication between remote human colonies and objects would require a faster-than-light transmission of signals? 85.222.86.130 (talk) 12:38, 6 November 2010 (UTC)

Why would it be required? If humans have extensive colonies thorough many different stars, they will learn to live with the fact contact between these will take several or many years (depend on how far we're talking about) and you may be dead before people in the other colony receive your message (although if you personally don't want that and with the tech required to set up these colonies who knows what may be possible, e.g. uploading yourself to a computer, some sort of stasis, a relativity time dilation device or even simple advances allowing near biological immortality). Considering it will take even more years for people to travel between these, I see no reason why people will somehow be unable to live with that. It may be nicer to have the soft SF near instant communication but there's no reason we can't live with the real physical universe we live in. Nil Einne (talk) 13:01, 6 November 2010 (UTC)

As far as we know, FTL communication is impossible — just plain ruled out by physics which seems pretty solid at this point. So I think we can throw that one out the window unless physics is totally wrong in some important respects. (Which is always possible — the quantum world certainly is weird and often surprising, even at this late date — but you can't bet on it.) What you more likely get are colonies that aren't in communication. That either means you have a limit on colonization, or you have a limit on how "connected" you expect your colonies to be, when they can only communicate with each other every few decades or so. The world of Ender's Game (or, rather, the sequels to that particular book) discusses these issues in great detail, although largely in reference to what FTL communication (through the fictional ansible) lets you do that otherwise you would not be able to do, politically, especially if you have it in absence of FTL travel (which means, in one book, that if you want to send a military fleet to an unruly colony, it'll arrive only after a number of decades from the colony's reference frame). In any case it seems pretty clear that you couldn't have the same degree of power projection between colonized worlds that you do between different nations on Earth, and your political models would have to reflect this reality. --Mr.98 (talk) 13:08, 6 November 2010 (UTC)

Browsed the superluminal communication, thanks. 85.222.86.130 (talk) 15:30, 6 November 2010 (UTC)

Solstice dates change from one year to another

I am wondering why the dates for solstices and equinoxes change from one year to another.

I'm assuming it is because the sidereal year is 365.256363004 days and our calendar is 365 days (3 out of every 4 years).

If it is because of that discrepancy, could a calendar be devised in which it did not change? What elements would be required to accomplish this? -Joel (talk) 14:26, 6 November 2010 (UTC)

The relationship between the earth orbit's and the earth's rotation would need to change to integers. It's a good plan. --jpgordon^{::==( o )} 14:34, 6 November 2010 (UTC)

By that do you mean that our numeric system would need to change? There's got to be an easier way... I'm not thinking we'd need to ditch leap-years or anything. Could not the placement of the leap-year prevent the solstice date change? Like, with leap years in 2000, 2004, 2008 it will change, but with leap years in 2003, 2007, 2011 it won't. (It's really hard for me to think about trying to answer my question.) -Joel (talk) 14:48, 6 November 2010 (UTC)

I think jpgordon's reply was a subtle of saying that nature is messy and won't conform to our strictures or constructions. In other words, the plan would only work if the time for the Earth to go around the sun was an exact whole number multiple of the length of a day. Since it's not, that 'perfect' calendar simply can't exist. Changing when leap years are would make no practical difference. --jjron (talk) 16:04, 6 November 2010 (UTC)

... and even the year is not a fixed length. Our calendar is based on the average time between vernal equinoxes, and this is not quite the same as the mean tropical year. If we ignored sunlight and all lived according to an artificial "day" (not exactly 24 hours, and varying slightly from season to season) then we could have a fixed calendar, but I don't think it would be popular except possibly with those who work night shifts. Dbfirs 16:56, 6 November 2010 (UTC)

A further point is that solstices and equinoxes are instantaneous events. This means that the date when they occur depends on your local time zone. Since time zones exist that are over 24 hours apart (in Kiribati and Samoa, for example), any particular moment in real time always occurs on at least two different dates by zone time, sometimes three. For example, the upcoming solstice is at 23:28 UTC; it will be on December 21 in the UK and the Americas, but on December 22 in Australia, Asia, and most of Europe and Africa.

It would be possible to arrange a calendar so that a particular solstice was always on the same date, say December 21, in a particular part of the world. Whereas now we have leap years mostly at 4-year intervals with a fixed sequence of 8-year intervals, it could be decreed that leap years would be assigned dynamically at mostly 4-year and occasionally 5-year intervals, whichever would produce the desired effect. (The Islamic calendar is attached to the phases of the moon in a similar dynamic fashion.) But because of the time zones, this would not work for other parts of the world. There would be always be countries where it was not always on December 21.

Likewise, the time between one solstice or equinox and the event is not an integer number of days. For example, from the solstice this past September to the equinox coming in December is 89.853 days (the exact amount varies from year to year). This means that even if the equinox was fixed on December 21, the solstice might vary between September 22 and 23, for example.

--Anonymous, 04:57 UTC, November 7, 2010.

Work done by Ice-Skater

A skater's body has rotational inertia 4.2 kg m^2 with his fists held to his chest and 5.7 kg m^2 with his arms outstretched. The skater is twirling at 3.0 rev/s while holding a 2.5-kg weight in each outstretched hand; the weights are 76 cm from his rotation axis. Fine the work done by the skater pulling the weights to his chest.

I plan to find the work by subtracting initial kinetic energy (when his arms are outstretched) from the final KE (arms at chest). But I'm having trouble understanding the various rules regarding rotational/translational KE. Any help?199.94.68.201 (talk) 15:01, 6 November 2010 (UTC)

0. Cuddlyable3 (talk) 22:39, 6 November 2010 (UTC)

It's not zero, Cuds. Work is done pulling the weights into the body, increasing the kinetic energy. You have the right approach, Mr. 199.94. You want to find the final rotational speed using Conservation of Angular Momentum. Formulas can be found in that article, and Rotational energy. Buddy431 (talk) 02:40, 7 November 2010 (UTC)

What Happens if the gravitational force decreases?

Bold textItalic text I am very eager to know that what happen when the gravitational force is reduces? i know that the gravitational force is acting indirectly to keep the percentage of oxygen in air. if it start reducing will the percentage of oxygen will remain same or not? If that force is reduced what will be the reason for that? Waiting for the reply. —Preceding unsigned comment added by Yuvraja (talk • contribs) 15:05, 6 November 2010 (UTC)

Fear not. The gravitational force is not decreasing. See Gravity of Earth and Newton's law of universal gravitation. --jjron (talk) 15:35, 6 November 2010 (UTC)

Gravitational attraction is lowest at the outermost layer of Earth's atmosphere, the Exosphere which is mainly composed of hydrogen and helium. That might become the composition of the whole atmosphere if the Earth's gravity were somehow reduced, though that would not be the first abnormality we would notice. Cuddlyable3 (talk) 17:00, 6 November 2010 (UTC)

(edit conflict)No, it is not decreasing, the only way to do that would be to get rid of a huge amount of mass. One much more common (in space) way to loose oxygen is when a planet looses, or never has, a magnetic field. This causes the solar wind to blow away the atmosphere. --The High Fin Sperm Whale 17:04, 6 November 2010 (UTC)

It is decreasing because the mass of the planet is decreasing. Think of all those spaceships we released... --Chemicalinterest (talk) 18:59, 6 November 2010 (UTC)

True, but think of all the rocks that have hit us! --The High Fin Sperm Whale 20:35, 6 November 2010 (UTC)

If the gravitational force were to "magically" decrease you would have a wide variety of effects. For example air pressure would be lower, more air would escape into space (but not oxygen specifically, it would be the regular mix of air). More interesting would be the effects on the orbits of the planets - the planets would suddenly be orbiting too fast, and will move to farther orbits. A more important effect would be on the sun. Unlike the Earth the sun is held together by gravity. Additionally the nuclear reactions are controlled by the density and pressure in the center of the sun. If gravity were lower the suns output would decrease, this would cool the sun, causing it to shrink, which increases the density and pressure. I'm not sure if the end result will be the same level of brightness or not. Ariel. (talk) 23:50, 6 November 2010 (UTC)

Velocity management

The question appears quite weird in terms of special relativity, but anyway. Is it possible to extract the cause of light speed and manage it so that it could be multiplied? For example we take the light speed-producing conditions and multiply them just like we are adding more fuel to intensify the fire. Something like speed power of two photons cross-added, yielding roughly 600,000 m/sec. 85.222.86.130 (talk) 15:49, 6 November 2010 (UTC)

No. See article. Cuddlyable3 (talk) 16:42, 6 November 2010 (UTC)

(ec) You are correct in saying that the question sounds weird. Even if we knew the "cause of light speed", it is unlikely that we could "manage it" in any multiplicative way. From a mathematical and scientific point of view, the whole concept sounds like gobbledegook. Are you writing a science fantasy story? Dbfirs 16:44, 6 November 2010 (UTC)

To a certain crude approximation, the "cause of light speed" is the absence of invariant mass. (The "speed of light" is a fundamental property of the universe, and isn't special to light in any way.) It's not meaningful to say that something is "twice as massless". Two times zero is still zero. -- 174.31.204.207 (talk) 17:07, 6 November 2010 (UTC)

Heating house - turn off when I leave the house?

My husband and I debate over what we should do when we leave the house for a couple of hours. Should we turn off the heat, lower it, or leave it as is? Obviously, if we leave the house for a week it would make sence to turn the heat way down. Likewise, if we leave the house for 5 minutes, we would leave it alone. There must be a physics formula to answer my question. I've asked my friends, who just give me their opinions. —Preceding unsigned comment added by 74.90.197.121 (talk) 20:27, 6 November 2010 (UTC)

Much depends on the house and the climate. There's no one-size-fits-all answer about the time interval that would be suitable, but most programmable thermostats use four blocks of time: morning (warm), work (cool), evening (warm) and night (cool). I'd say around four to six hours of absence would be a good guide, based on that precedent, unless your house gets cold really quickly (and is quick to warm up again). Acroterion _(talk) 20:36, 6 November 2010 (UTC)

(edit conflict) Lowering the temperature always reduces heat loss, but whether this is a saving depends on whether you then turn the heat higher when you return. If you don't do this, then there will always be a saving in turning the heat off, but obviously for five minutes the few cents or pennies of saving will seldom be worth the effort. The graph never crosses zero, so there is no "break-even" point unless you factor in the wear on the switch. You will always save money by turning the heat off (though beware of frost damage in extreme cold). Dbfirs 20:38, 6 November 2010 (UTC)

To rephrase what Dbfirs said, it's always worth it to turn off the heat - unless, when you turn it back on, the furnace runs at a higher temperature. Usually the furnace does not run at a higher temperature, except if you have a modulating boiler or a heat pump. Most furnaces are either on or off, with no in between, but modulating boilers can change fire, and heat pumps will turn on auxiliary heat. Ariel. (talk) 23:46, 6 November 2010 (UTC)

Even if the furnace runs at a higher temperature for a while, it will not cost more unless the thermostat fails to turn it off at the set room temperature. The extra cost would only occur if there is some over-run, raising the room temperature higher than normal. I cannot see circumstances in which this would occur only after switching off, but I'm not an expert on heating systems, so I'm just looking at the whole-house boundary with the environment to give a simple answer. Dbfirs 00:24, 7 November 2010 (UTC)

In a modulating boiler the water temperature is adjusted based on the heat necessary. Usually it measure the outdoor temperature and sets it that way, but if the boiler sees that the call for heat lasts for more then X minutes it raises the temperature. The thing is, the higher the water temperature the lower the efficiency. The range is about 85% (180 degree water) to 98% (110 degree). In my house I only need 180 degree water when it's -20f outside. Interestingly with a modulating boiler, the typical recommendation to install a digital setback thermostat can actually cost you more, because the boiler has to raise the water temperature in order to heat the house quickly enough in the morning. The poorer efficiency when this happen can eat away any savings. I would love hard data on this subject (so I know what to do with mine), but I have not found any. Ariel. (talk) 01:19, 7 November 2010 (UTC)

Ah, yes, I hadn't considered the lowering of boiler efficiency where more heat or wasted fuel pours out through the boiler exhaust. Do no manufacturers provide an optional temperature-limiting control? Dbfirs 08:17, 7 November 2010 (UTC)

Moles

QI said that all pictures of animal moles are of dead moles, posed in the ground and fluffed up. Is this true? Is there not pictures of live moles? —Preceding unsigned comment added by 78.113.39.122 (talk) 20:45, 6 November 2010 (UTC)

The photographer needs an enormous amount of patience and luck to get a picture of a live mole. Moles spend at least 99% of their time underground. I occasionally see one above ground, but often moving fast, so difficult to photograph. I've never tried to take a picture of one, but I think I would be tempted to cheat. Dbfirs 21:43, 6 November 2010 (UTC)

People catch live moles on video [27] [28] [29]. I changed the title from "question" which is not informative. Cuddlyable3 (talk) 22:55, 6 November 2010 (UTC)

I got a video of me holding one once. It came out of the ground, so a grabbed it and kept it in my cage for a few days, during which time I got pictures and videos. --The High Fin Sperm Whale 23:31, 6 November 2010 (UTC)

Yes, it would be easier to take video footage than to obtain a high-quality picture with a manually adjusted camera. I suppose it would be much easier to catch a mole in action (the classic pose is just pushing up a molehill) using modern fast autofocus. The problem with moles is that one never quite knows where they are going to appear, so it is difficult to have the camera ready focused. I'm sure there are many pictures of live moles in existence, but I understand the psychology of using a dead mole to get a good photo of a "mole in action"! Dbfirs 00:16, 7 November 2010 (UTC)

Touching neutron star matter

The core of a neutron star is not well understood from what I heard, but it's mostly just neutrons right? If you were to take a mass the size of a baseball from the core and put it in outerspace, would it still maintain its immense density or would it kinda dissipate into a cloud of neutrons? I guess what I want to know is, is pressure causing it to be dense, or is it just an intrinsic property that the stuff has? Would touching it kill you? If so, why? Also since just a small amount of it is so massive, is it possible to arrange a bunch of them in a line in outerspace and climb them like a ladder even though they aren't touching each other? ScienceApe (talk) 21:05, 6 November 2010 (UTC)

I think it's kind of moot; you'd be dead before you could get close enough to a neutron star to touch the stuff, for I imagine many many reasons, but tidal forces will suffice if nothing else gets you first. Sci-fi writers sometimes like to imagine that you could hack off a piece of neutron degenerate matter separately from the gravitational field that squishes it together, but I believe that is not true. --Trovatore (talk) 21:08, 6 November 2010 (UTC)

Why not? ScienceApe (talk) 21:34, 6 November 2010 (UTC)

I'll leave it to a physicist to answer in more detail. But see the linked article, especially at the top, where it talks about the dominant contribution to the pressure coming from the Pauli Exclusion Principle. Stands to reason, though I certainly haven't worked out the equations, that if you take off that pressure, it all blows apart. --Trovatore (talk) 21:39, 6 November 2010 (UTC)

Neutron_star#Structure has a diagram of an expected structure and make up of a neutron star - also the text next to it has a description. In general pressure is required to maintain the structure.94.72.205.11 (talk) 22:40, 6 November 2010 (UTC)

Free neutrons have a half-life of about 15 minutes before they decay into hydrogen, or a proton and a neutron. The only reason that in a neutron the stuff stays neutrons is that in the crushing pressures there is simply not enough room to have atoms (remember, the nucleus is about 1/100,000 the diameter of an atom). They are squeezed until they merge, forming neutrons. And you couldn't touch the stuff; if you magically got a piece of it outer space it would instantly explode. --The High Fin Sperm Whale 23:29, 6 November 2010 (UTC)

I thought the neutron-star-ladder was kind of a neat idea, though, ScienceApe. In principle. It would probably be difficult to arrange everything in order to get the sequential gravity assist to work just right, though. WikiDao ☯ (talk) 01:18, 7 November 2010 (UTC)

Suicide

What causes people to consider suicide other than depression? The Utahraptor^Talk/_Contribs 21:51, 6 November 2010 (UTC)

Suicide#Causes - specifically see the social causes, but also read the whole article for idea, also Assisted suicide Voluntary euthanasia has some ideas too. —Preceding unsigned comment added by 94.72.205.11 (talk) 22:23, 6 November 2010 (UTC)

November 7

Beyond us

Are the problems in the world simply beyond us? Poverty, AIDS, human rights abuses, war. I am just wondering if that is what people learn as they try to accomplish things with a futility. Perhaps this should go in the misc section, but I was thinking that science should have the answer, above all things, right? AdbMonkey (talk) 00:22, 7 November 2010 (UTC)

It remains to be seen, AdbMonkey. We are awaiting the empirical results on that. Your last question reminds me of something I heard on NPR the other day, I'll see if I can dig that up for you in a moment... WikiDao ☯ (talk) 00:38, 7 November 2010 (UTC)

All you mention are problems we ourselves have created. The real question is, do we have the will to undo them? Perhaps we should start with renaming everyone's "defense" department back to their "department of war." Unfortunately, from war to pharmaceuticals, all is driven by the quest for profits. Unless of course you're just power hungry. Not to mention that the last "politician" I can think of where that moniker was not a dirty word was (for me in the U.S.) Hubert Humphrey. PЄTЄRS J VЄСRUМВА ►TALK 01:56, 7 November 2010 (UTC)

(e/c) Okay, that was a discussion on "Science and Morality" with Steven Pinker, Sam Harris,Simon Blackburn, and Lawrence Krauss. The lead-in to the program is:

"Did we evolve our sense of right and wrong, just like our opposable thumbs? Could scientific research ever turn up new facts to resolve sticky moral arguments such as euthanasia, or gay marriage? In this hour of Science Friday, we'll talk with philosophers and scientists about the origins of human values. Our guests are participating in an international conference entitled “The Origins of Morality: Evolution, Neuroscience and Their Implications (if Any) for Normative Ethics and Meta-ethics” being held in Tempe, Arizona on November 5-7. Listen in to their debate, and share your thoughts."

So far, I have only heard about the first 15 minutes of the program myself. But it may be relevant. If so, I'll comment further after having listened to the rest of it. :) Regards, WikiDao ☯ (talk) 02:10, 7 November 2010 (UTC)

To the OP: Not at all. Instead of comparing the current world to an ideal perfect world, without any form of suffering, instead compare today's world to that of say 50 years ago. Or 100 years ago. Or 200 years ago. Or 1000 years ago. At any point in history any arbitrarily long distance in the past, there were a higher proportion of people who were abjectly poor, or diseased, or who died young, or any other number of miserable existences. We've known about AIDS for about 30 years. Smallpox and Plague we knew about for centuries before they were finally cured. Give it time. It only seems like things are bad because you are living through them. Things were infinitely shittier before you were born. --Jayron32 03:23, 7 November 2010 (UTC)

That's a good point, and a good way of putting it. But, Jayron, the world is today in arguably a fundamentally different position than at any previous time with regard to humanity and its impacts on itself (overpopulation, technology, etc) and the world in general (environmentally, etc). It's a complex system, and as it gets more complex it gets more difficult to tell what's going to happen next... WikiDao ☯ (talk) 03:47, 7 November 2010 (UTC)

I'm not so sure about that. Ever since people began living in cities, they have been creating problems for themselves that living in caves did not. As bad as pollution was at the height of the Industrial Revolution, it still didn't cause as much death and illness as something as plainly simple as not shitting in the middle of the street. Progress tends to, on the balance, result in a higher standard of living across the board. It is true that technology and advancement causes unique problems, but it solves more problems than it causes. Despite the problems with polution, the Industrial Revolution in the UK saw the greatest population explosion that country ever saw. And as the modern economy has evolved, polution has gotten better. Yes, it is still a problem, but not nearly the problem it was in the middle 1800s. Thomas Malthus's predictions haven't come true, because his assumption that food production growth would be linear doesn't hold up. Food production has kept up with population growth because of technological advancements. The only major problems with overpopulation are politically created; its not that the food doesn't exist to feed people, or that the technology doesn't exist to fix the problems of overpopulation, its just that the political will to actually fix the problems lags behind the technological advancements. But that has always been so, and what has also always been so is that it eventually catches up. --Jayron32 03:59, 7 November 2010 (UTC)

Okay, sure, and if it does work out for humanity (and we ought to find out within the next century or so -- if we get through the tail-end here of the population explosion that has gone hand-in-hand with technological and social progress, it'll happen or not within the next hundred years) – if it does work out, it will be because it is as you describe it. I take your points about cities and failed Malthusian expectations. Still, we are globally overpopulated now. It's a closed complex system, and we've run up against the boundaries. What happens in a petri-dish when the bacteria run out of nutrients and fill it with their waste-products...? WikiDao ☯ (talk) 04:16, 7 November 2010 (UTC)

Actually, technological and social progress tends to lead to LOWER population growth, not more. See Demographic-economic paradox, aka The Paradox of Prosperity. In highly developed nations, like most of Western Europe and North America, the birth rate is below replacement rate, and these nations have to import workers from less developed nations just to do all of the work that the kids they aren't having aren't doing. The real question is what is going to happen to the world when EVERY country is so developed that we're all operating at below replacement rate. The trend would indicate that we're going to have the OPPOSITE of a Malthusian catastrophe, in that as we become more advanced, we don't even have enough kids to maintain a steady population. --Jayron32 04:24, 7 November 2010 (UTC)

A sigmoid curve.

Human population growth.

Yes. We are aiming for something like the diagram (of a sigmoid curve) shown at the left. That region on the upper-right-hand side is the region we are just entering now (see the diagram on the right and the World Population article, which says, "In the 20th century, the world saw the biggest increase in its population in human history due to lessening of the mortality rate in many countries due to medical advances and massive increase in agricultural productivity attributed to the Green Revolution," which is one of the things I was saying, too). WikiDao ☯ (talk) 04:36, 7 November 2010 (UTC)

Ah, it's he or she of the very funny user page, again! Hi, User:AdbMonkey! You might like to have a look at The Revolution of Hope, by the sociologist Erich Fromm. In one of his books, The Sane Society, I think it was, he makes a case, based on sociological metrics like suicide rates, alcoholism incidence, etc. for the idea that current Western society is more screwed up than it ever has been before. I've never seen his numbers discussed anywhere else, but (as I recall, it's been ten or twenty years) he claims the occurrence of such signs of distress is astronomically higher than ever before in recorded history. He's of the opinion that our technological development has way, way outstripped our moral intelligence, that we're like toddlers who think we can use fire responsibly because we know how to start one. ( I agree with this assessment, FWIW; it seems obvious to me. ) Along that same line, Fromm points out that if something can be done with technology, then eventually it almost certainly will be done, by someone, somewhere in the world; in this way technology has it's own inertia that sweeps us all along without much conscious choice or deliberation about whether the changes it introduces are what we really want, how we really want to live on the Earth. This is no way to run a planet, in Fromm's view. ;-) Then, in The Revolution of Hope (subtitled, "Toward a humanized technology") he gives one part of his "prescription" for what ails us as a people. Good fun, his ideas stretch the intellect, imo. Best,  – OhioStandard (talk) 04:36, 7 November 2010 (UTC)

magazine

can u buy famous older issues directly from playboy? —Preceding unsigned comment added by Kj650 (talk • contribs) 01:59, 7 November 2010 (UTC)

This is science? Anyway, the Playboy store only has some issues available from the 1980s forward, plus a reproduction of the Marilyn Monroe one. Clarityfiend (talk) 03:34, 7 November 2010 (UTC)

IIRC, Playboy's website has digitized versions of every issue ever produced, so you can at least access it if you pay the subscription fee. There are other magazines which offer free digital versions of their back issues. Sports Illustrated has a full collection of scanned issues (not just text, but full digital scans of every page of every magazine) going back to their first issue, and it is entirely free to browse. It's also fully text searchable. --Jayron32 04:28, 7 November 2010 (UTC)

Interesting. The Playboy Archive lets you [cough, cough] "read" 53 back issues for free. And Jayron is correct; apparently you can get digital versions for each decade.[30] Clarityfiend (talk) 04:57, 7 November 2010 (UTC)

pollution question

How much chemical does it take for the environment to be labeled as contaminated? Is it the same for every chemical? —Preceding unsigned comment added by 75.138.217.43 (talk) 02:04, 7 November 2010 (UTC)

It's different. I assume it's related to how toxic the chemical is, how well the environment can handle it, and how long it takes for it to be biodegraded. For example oil in the gulf is not as big a problem as it would be near alaska. The gulf has lots of mechanisms to deal with oil (bacteria, warmth, etc), alaska doesn't. Salt would not be a problem near the ocean, but would be near the great lakes. Ariel. (talk) 02:21, 7 November 2010 (UTC)

I do have to respond to this by saying that everything in the environment is made entirely of chemicals. So, it's obviously not the same for every chemical. I'm guessing our questioner is referring to chemicals widely regards as pollutants, and the same is true for them. ANother perspective is that what is good for some plants will be deadly for others. So, huge variation. HiLo48 (talk) 03:58, 7 November 2010 (UTC)

Electronics/Physics question about laptop power supplies

Hi, all. Can any electronics/physics guru tell me what's likely to happen if I plug a newish Gateway laptop into mains via an old AC/DC power converter "brick" that's from a (much earlier, monochrome screen) Gateway laptop?

Specs for the converter/brick that came with the new laptop are DC Output 19V, 3.42 Amps, according to a label affixed to it. There's also a symbol between the "Volts" number and the "Amps" number that consists of a short, horizontal line with a parallel dashed line (in three segments) positioned just below it. I presume this has to do with the polarity of the bayonet-style connection jack that plugs in to the laptop? That jack is also represented figuratively by the familiar "two concentric rings" graphic that shows that its "negative" pole is on the periphery/outside of the jack, with the "positive" pole located at the center. A label on the bottom of the new laptop also says 19 Volts and 3.42 Amps, btw. The old converter/brick has the same parallel lines symbol as the new one, but no "concentric rings" graphic, and a legend that says it's specified to deliver 19 Volts and just 2.64 Amps. This would seem to mean that the "new" power supply is capable of delivering 30% more current at 19 Volts than the "old" one can, also at 19 Volts, right?

So if I try this, is something likely to melt, and if so, what? The power supply? The computer? Or might everything still be within tolerance? I could try it with the laptop battery installed and fully charged, of course; would that be safer in case the power supply fries? And if this would be a really dumb thing to attempt, would it be sufficiently less dumb if I were to try running the new laptop only in some very low-power mode while connected to mains via the old converter/brick?

No penalty for informed guessing: I probably won't try this, regardless of the advice I get here. But I'll formally state that, as an adult, I alone am responsible for the consequences of my actions. That means I won't blame anyone else if I try this and it disrupts the fabric of the space-time continum, sets my house on fire, or worse, fries my computer. If anyone wants to explain the physics of what's likely to happen, I'd be interested to know that, too, since that's at least half my interest in asking this question. Thanks!  – OhioStandard (talk) 04:04, 7 November 2010 (UTC)

Well, we don't yet have a guideline against answering disrupting-the-fabric-of-the-space-time-continuum advice questions so... ;) The solid-and-dashed-line symbol is a well-known symbol for DC. The AC symbol is a sine wave. It is 99% likely that the polarity of your DC output hasn't changed (keep in mind that 83% of statistics are made up on the spot). It is very rare that the "shield" (outer-most parts) of a device/plug is NOT designed to be the ground/earth/negative terminal. As to what could happen if you plugged it in...it depends. If your old power supply has overload protection built-in it might switch itself off if you try to draw too much current. Or it might just run a bit hotter than normal. Or it might overheat, melt something internally and catch alight. Either way using a full battery would lessen the current draw when you plugged it in. Note that even though the power supply is rated for X amps, it doesn't mean that the laptop draws X amps. It is likely that the power supply is over-designed by at least 10% compared to the maximum laptop draw current. YMMV. Regards, Zunaid 05:44, 7 November 2010 (UTC)

Thanks, Zunaid! That's the word I was reaching for, "overdesigned". I was wondering if the old power supply might be sufficiently overdesigned to allow the swap; good word. The laptop itself does have a sticker affixed to it that says 19 V and 3.42 A, just like the power supply that shipped with it, but I understand that may not mean much. E.g. maybe it only draws that high a current when bluetooth power is on, there's three PC cards inserted, it's ethernet circuits are busy, the DVD writer is writing, etc. etc., i.e. when the computer is operating at maximum load.

But can you also give me some feeling for what would happen, in terms of the relevant physics equations, if such a state occurred when I was using the old (2.64 A) power supply? In terms, for example, of Ohm's Law? Voltage is fixed, right? So if you start adding "loads" (fire up the DVD burner, turn on wireless networking, etc.) that does what, increase the overall resistance? ( Can you tell I'm no prodigy re this stuff? ;-) If that's correct, then ... Well, then I'm out to sea, I'm afraid. But I have the vaguely formed idea that one of the three Ohm's Law variables will somehow become too extreme, and bad things might happen.

I guess besides just knowing I could damage hardware, I'm also trying to get some glimpse about how the dynamic interaction of those variables might change with increasing load, how different parts and subsystems (or even running software routines?) might be adversely affected when one of those variables deviates too far outside normal limits. I know there's no unified, simple answer for all cases like this, but am I at least thinking of this at all correctly? What, for example, would happen in an exagerated instance similar to this case. What if I hooked up my "19 V, 3.42 A" laptop to a "19 V, 0.5 A" power supply? Apart from the smoke billowing from the power supply or the hard drive spinning at one-third of its normal speed (just kidding), is there any way to know what would be going on re the variables of Ohm's Law? Any way, from just the relevant equations, to demonstrate on paper why doing so would be bad? I could hardly be more ignorant about electricity, I'm afraid, but I'd like to be able to understand, just from formulae, if possible, what happens when a device "wants" to draw more current than a power supply can deliver.  – OhioStandard (talk) 06:43, 7 November 2010 (UTC)

Modern power supplies are Switch-mode power supplies with circuitry much more complex than simple Ohm's-law calculations, but the article doesn't say how they behave under overload conditions. My instinct is that they will just reduce the output voltage (and overheat slightly rather than bursting into flame), but I haven't run any tests to confirm or refute this claim. I have successfully run a laptop with the wrong power supply, but not under serious overload, and I wouldn't recommend the practice. Dbfirs 07:59, 7 November 2010 (UTC)

The key terms here are internal resistance and electrical power, the power supply has internal resistance (like a battery does - see the article) - and the power dissapated (as heat) is V²/R or I²R. For this it's probably easier to use the equation with I (current, amps) - try to draw 3.42A from a 0.5A supply and you will be generating about 7 over 40 times as much heat - hence it gets hot - and possibly breaks. There's more detail and explanation on this if asked. using the V²/R equation is more complicated than it seems because the power supply switches on and off - in short V is not 19V, but a higher voltage in pulses

actually the relationship between current and heat given off in the power supply is not quite the same as the example above .. in fact the heat will be about proportional to the current for your example (because of the way SMPSs work) - which means about 7 times in the above example.Sf5xeplus (talk) 08:58, 7 November 2010 (UTC)

In practice the power supply will have some sort of overload protection built in (probably by law). I'm not so sure that it would reduce the voltage as suggested by dbfirs above, but it might. What I'd guess is that it either a. detects that the current is over the maximum rated (using a hall effect sensor) and/or detects when the device gets to hot (temperature sensor) - and then shuts off.

Note if the power supply is an old transformer (heavy) type the situation is a bit different.Sf5xeplus (talk) 08:24, 7 November 2010 (UTC)

Oh. - it's the power supply that is likely to melt, not the computer - though if the computer is run using a lower voltage than designed it may work, but there is an increasing likelyhood of the processor malfunctioning (not a permanent effect - just a crash as it freezes up or gets its sums wrong..)Sf5xeplus (talk) 08:47, 7 November 2010 (UTC)

Cholera treatment

Somewhere several years ago I heard that Gatorade would be almost ideal for treatment of Cholera (presumably used for rehydration). I haven't been able to find anything to confirm this since, though, so how plausible of an idea is this? Ks0stm ^(T•C•G) 04:11, 7 November 2010 (UTC)

According to the article you link, in the lead " The severity of the diarrhea and vomiting can lead to rapid dehydration and electrolyte imbalance. Primary treatment is with oral or intravenous rehydration solutions." Presumably, in a pinch, Gatorade would work. From reading the article, it seems that the main problem with Cholera is the massive diarrhea and vomiting causes such rapid dehydration and electrolyte problems that that can kill you before your body has a chance to fight the infection. See also Oral rehydration therapy. --Jayron32 04:20, 7 November 2010 (UTC)

Well my main question rephrased was basically whether Gatorade (or Powerade, etc) would be effective for oral rehydration when stricken with Cholera, due to such drinks having the water, salt, and electrolytes needed to replenish those lost during the infection, or if there is something that would prevent their overall effectiveness as a treatment. I already read the articles in question searching to see if it mentioned anything about such drinks, but didn't see anything. Ks0stm ^(T•C•G) 04:31, 7 November 2010 (UTC)

I had a long, speculative post written out, then I did the google search. Gatorade was first proposed as a Cholera treatment in 1969 in the New England Journal of Medicine, and you don't get a better reliable source than that. See this. You could also find a wealth of information at this google search or this similar one. --Jayron32 04:44, 7 November 2010 (UTC)

Is the age of the universe relative?

Thanksverymuch (talk) 04:31, 7 November 2010 (UTC)

You'll want to read the articles Age of the universe and Comoving distance and Proper frame. The quoted age of the universe is the age given for the earth's current frame of reference, extrapolated back to the point of the Big Bang. In other words, we assume the age of the Universe to be for the Earth's current frame of reference (relative speed and location). We assume the earth to be stationary (what is called the "proper frame") and make all measurments assuming that. In reality, nothing is stationary. In a different reference frame (i.e. if you were moving at a different speed than the earth is), the age would of course be different. This is due to the issues raised by special relativity and general relativity. --Jayron32 04:37, 7 November 2010 (UTC)

Thanks! What is the maximum possible relative age of the universe? Thanksverymuch (talk) 04:46, 7 November 2010 (UTC)

That's impossible to answer, because of the way that time works. There is no universal reference frame for which we can measure against; there is no absolute time. There are an infinite number of reference frames which one could conceive of in which the universe could be literally any age. We choose the earth's reference frame because that's the one we're in. This is not the same thing as saying that the Universe is infinite in age, its just that we could arbitrarily choose any reference frame in which the Universe could be any age. --Jayron32 04:53, 7 November 2010 (UTC)

For an object moving at (over very close to) the speed of light since the big bang, how old is another object moving at the slowest possible speed since the big bang? Thanksverymuch (talk) 05:01, 7 November 2010 (UTC)

For an object moving at the speed of light relative to what? --Jayron32 05:02, 7 November 2010 (UTC)

A stationary object. Thanksverymuch (talk) 05:09, 7 November 2010 (UTC)

Stationary relative to what? --Jayron32 05:10, 7 November 2010 (UTC)

Let me ask the question differently - is there a reference frame that entails an infinitely old universe? If not, just how old can the universe get? Thanksverymuch (talk) 05:20, 7 November 2010 (UTC)

There is no reference frame that entails an infinitely old universe. There are an infinite number of reference frames that entail an arbitrarily old universe. There is a distinction between infinite and arbitrarily large. In every reference frame, the universe is a finite age. But as the number of possible reference frames is boundless, for any age you could pick, there is a reference frame for which the universe is THAT age. Does that make sense? --Jayron32 05:23, 7 November 2010 (UTC)

I was unaware of this distinction. It does make sense. Thanksverymuch (talk) 05:40, 7 November 2010 (UTC)

Actually, I was a little incorrect. The age of the universe is quoted not to Earth's reference frame, but to the reference frame of the Hubble flow, that is to the metric expansion of space. The universe is 13ish billion years old on that time frame. --Jayron32 05:30, 7 November 2010 (UTC)

Thanks for the clarification. Thanksverymuch (talk) 05:40, 7 November 2010 (UTC)

People meddling in the environment

Hearing a story about geoengineering on the radio today got me wondering if other attempts by people to "fix" the environment/Earth/ecosystems/etc have ever worked. (I'm open to various definitions of whether something can be said to have "worked" or not) What I was wondering about specifically was when we've introduced non-native species to an area to improve something. So, has this ever worked? Dismas|^(talk) 04:44, 7 November 2010 (UTC)

http://alic.arid.arizona.edu/invasive/sub2/p7.shtml Thanksverymuch (talk) 04:53, 7 November 2010 (UTC)

Living donor liver transplant multiple times?

After a living living donor liver transplant, both the donor and recipient should eventually have a full-sized liver each. If it is required sometime in the future, could the donor or recipient be a living donor again? Has this happened before? thanks F (talk) 08:32, 7 November 2010 (UTC)

Capacitor plague

Capacitor plague explains the problem, and repeats (what seems to be the common claim) that certain taiwanese manufacturers were to blame (due to using an incomplete electrolyte formula stolen from elsewhere..) - eg as repeated here [31] [32]

None of this I question; my question is: what about the fallout - ie what happened to the suppliers (eg I tried to find references to show that the manufacturers got their 'ass sued off' by the manufactures who bought from them) - but found nothing. As a side question - are compensation lawsuits uncommon in the far east? (sorry this isn't actually a science question - it's a science topic though..)

Also confusingly this Dell [33] page blames Nichicon, whereas the the other link says Nichicon was amongst those ".inundated with orders for low-ESR aluminum capacitors, as more customers shy away from Taiwanese-produced parts" ?. 94.72.205.11 (talk) 10:15, 7 November 2010 (UTC)

Well as you said, they used an incomplete electrolyte formula stolen from elsewhere. Given that happened in the first place, how likely is it they got their ass sued off by people who bought from them? Nil Einne (talk) 10:24, 7 November 2010 (UTC)

Aeroplane crash

I read a question on here about jumping before a plane crashes to save you. obviously that wouldnt work, but what if you flooded the cabin with some sort of liquid or foam to spread the force acrost the entire body, and also provide more time to stop (reducing the accl, and thus the force). going from 300 km/h to zero over the distance of a few cm would be fatal, but over a couple of meters, it would be the equivilent force of going from 3 km/h to zero over a few cm. Would that work? 98.20.222.97 (talk) 10:03, 7 November 2010 (UTC)

or the cabin seats could be on a track that lets them slide forward a bit, making the stopping distance for the people inside greater —Preceding unsigned comment added by 98.20.222.97 (talk) 10:04, 7 November 2010 (UTC)

In theory, yes, but it is very difficult to find materials that will provide a gradual deceleration. To some extent, the crumpling of the metal of the plane already does this. Air bags are probably the most effective for the human body. Dbfirs 10:24, 7 November 2010 (UTC)