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November 1

Comet Shoemaker–Levy 9

Comet Shoemaker–Levy 9 hit Jupiter in 1994 at a speed of .0002c. What would have happened to the comet and to the planet if it had hit at:

• .02c
• .2c
• .9c
• .99c
• .9999c

?Magog the Ogre (t • c) 02:28, 1 November 2012 (UTC)

Jupiter is not a solid body like Earth. It is a giant gas planet, comprising mainly hydrogen and helium. It is the largest planet in our solar system but its density is very low. When the comet struck Jupiter the result was fireballs and plumes of gas. Dolphin (t) 02:45, 1 November 2012 (UTC)

I know what Jupiter is, and I know what happened in the actual collision. I'm not asking about that though. Magog the Ogre (t • c) 02:49, 1 November 2012 (UTC)

The first question is easy: the comet would easily be obliterated, because that's what happened to Shoemaker-Levy 9. What happens to Jupiter is harder to answer. Even at 0.0002c, according to our article, "Astronomers had expected to see the fireballs from the impacts, but did not have any idea in advance how visible the atmospheric effects of the impacts would be from Earth." Since even 0.02c is 100 times faster, and the kinetic energy of the comet would be 10000 times greater, I doubt there are reasonable models about what would happen to Jupiter, aside from "absolutely nothing in the long run, because the comet's mass is utterly negligible compared to Jupiter's". --140.180.252.244 (talk) 05:43, 1 November 2012 (UTC)

When you get up to near light speed, though, I wouldn't be surprised if it shot right through and out the other side (actually an equivalent mass of Jupiter material might shoot out the far side, similar to Newton's cradle). StuRat (talk) 09:43, 1 November 2012 (UTC)

I suspect that imacting a mass of gas with something moving near the speed of light would probably result in the destruction of most atomic nuclei involved and lots of high-energy radiation blasting out. That radiation would also have some pretty impressive effects on whatever it strikes nearby. Looking up relativistic impact on Google Scholar doesn't bring up anything that sounds like it would apply to your question. 209.131.76.183 (talk) 12:05, 1 November 2012 (UTC)

I had hoped the relativistic kill vehicle article would have some information, since it's the same concept, just aimed. But alas, it seems not. 146.87.49.21 (talk) 14:20, 1 November 2012 (UTC)

I'm not sure how reliable it is, but I found an estimate of about 7*10^11 kg for the mass of SL 9. According to that article, the realitivistic kinetic energy at .99c would be somewhere on the order of 10^13 megatons of TNT, which Orders of magnitude (energy) puts somewhere around the kinetic energy of the moon at orbital velocity. I have no idea what whould happen if you released that energy into Jupiter, but at least that gives a general idea of the energy involved at that speed. 209.131.76.183 (talk) 14:41, 1 November 2012 (UTC)

To me this sounds like an ideal question for XKCD what if? OpenToppedBus - Talk to the driver 14:58, 2 November 2012 (UTC)

A question about the lead section for the article quantum state

Consider a system with just 1 hydrogen atom. The wave function for the electron only requires 3 quantum numbers $\{n,\ell ,m_{\ell }\}$. This wave function can give hydrogen's probability density in position space and momentum space. So, what other information about the position or momentum of the electron are not specified? I think we should delete the last phrase about the position or momentum of the electron.--LaoChen (talk)04:57, 1 November 2012 (UTC)

"Probability density" is not the "position", but only the chance of finding it in a position or that it is usually or mostly in the vicinity of a position. That is, there is no "the electron is at the following coordinates:" type of answer. Likewise, there is not an exact solution for the momentum (maybe only an approximate range of likelihood). It is mentioned in these types of terms later in the lead itself, and then discussed further in the article using a bunch of math. One of the key statements there is "In quantum theory, even pure states show statistical behaviour." DMacks (talk) 09:00, 1 November 2012 (UTC)

Is it possible to heat granite by using dielectric- or induction heating ..?

Is it possible to heat rock like granite by using dielectric heating or induction heating ..? Electron9 (talk) 05:46, 1 November 2012 (UTC)

Of course. Any material has dielectric loss and some degree of electrical conductivity, and so does granite. However, I would expect the conductivity of granite to be sufficiently low that induction heating would be quite impractical. Dielectric heating certainly would be practical. If you google dielectric properties granite, you'll get all the info you need to determine the power level and optimum frequency needed. See http://mars.mines.edu/pub/76EPropertiesRocks.pdf. You could also of couse obtain a thin (a few mm) sample of granite polished for 2 sides parallel and test it with a Q-meter fitted with a dielectric loss adapter. Most university physics labs and electronics labs have Q-meters. Ratbone 121.221.211.20 (talk) —Preceding undated comment added 06:06, 1 November 2012 (UTC)

Perhaps "practical" was the missing keyword ;-), I just hope the microwave oven won't go up in smoke like it would if a spoon is forgotten inside it. If I tried with a rock instead. Electron9 (talk) 00:43, 2 November 2012 (UTC)

A microwave oven is only one kind of dielectric heater. A microwave oven canc ertainly heat granite, but its frequency and other parameters will not be optimum. If you are just going to try a piece of granite or whatever in a domestic oven, I suggest you incude a glass of water to aborb some of the enegery, until you have verified the the piece of granite is getting hot in a reasonable time. If the granite gets warm at about the same rate as an equal volume of water, it will be safe for the oven without the water.

There is another important consideration: DO NOT heat unknown pieces of granite to reach anywhere near 100 deg C. If you do, any included moisture will cause an explosion.

Ratbone 60.230.218.171 (talk) 01:42, 2 November 2012 (UTC)

Magnetic field vs various computer parts

What would the effect of a strong magnetic field on an old (2004) desktop computer be? I understand the effects of magnetic fields on CRT monitors and hard drives, but what would happen to the motherboard, videocard or audiocard, dvd/cd writer/reader and the other parts? Would they be able to continue to operate? They (talk) 09:40, 1 November 2012 (UTC)

If it is not powered on at the time, there will almost certainly be no effect. If it is powered on, I still doubt you'll have problems unless you're talking about an incredibly strong (and probably changing) field. The only components that are in theory affected by magnets are the speakers, fan and DVD drive motors, and possibly the inductors in the voltage regulator circuits. None of those should be affected by any magnets you'll encounter in a normal household. If you're interested in higher-strength fields I suppose you could research what sort of requirements there are for a computer that runs in an MRI lab. 209.131.76.183 (talk) 11:58, 1 November 2012 (UTC)

Magnetic fields can have a huge effect on PCs. About 5 years ago, I was installing Linux on a PC a work, and it would crash at different points in the install process. This machine was about 2004 vintage. I tried reseating and replacing components to no avail. After the 8th or 9th combination of parts, some old, some new, the install was successful. The difference was, that my radio/cassette wasn't sitting on top of the case for this install! Bench space was limited and it was the only place to put it. I put the radio on top of my CRT monitor, switched it on, and the picture went a bit "funny". Moral: don't use a PC with a radio/casette on top of it. What is puzzling is that the PC had a steel case, which I think should have acted as a Faraday cage, but perhaps there were holes in it, which let the magnetic field in. I guess if I hadn't been listening to the radio, the install would have gone a lot smoother... --TrogWoolley (talk) 14:45, 1 November 2012 (UTC)

Only Mu-metal stop magnetic fields in practice but is expensive. And Faraday cages only protect from electric fields. Electron9 (talk) 00:48, 2 November 2012 (UTC)

From our articles there appear to be many options - currently topping the charts at permeability (electromagnetism) is something called metglas. Wnt (talk) 15:38, 2 November 2012 (UTC)

Why do you think this was due to a magnetic effect? I would suspect RF interference triggered noise problems in the power supply. An unshielded AC power cord draped down from the radio through the cables on the back of the PC could also introduce noise, but crashes wouldn't be too likely from that. The systems we build here put standard PC components (including hard disks) very close to multi-HP electric motors without any problems. Random crashing type issues tend to come from poor routing of cables that carry significant current along side data cables. I've never seen anything we could attribute to the magnetic field of the motors. 209.131.76.183 (talk) 12:56, 2 November 2012 (UTC)

Speaker interference due the shape of the magnetic field and the frequency. The conductive path is also conceivable. Electron9 (talk) 21:37, 2 November 2012 (UTC)

Does flashing 12:00 damage the display

Does a flashing 12:00 damage an LCD display over time or it has no effect?Smallman12q (talk) 12:54, 1 November 2012 (UTC)

No effect. LED displays are not susceptible to burnout the way incandescent bulbs are. — Lomn 13:21, 1 November 2012 (UTC)

Ah, LCD (liquid crystal) and LED (light emitting diode) are different techonlogies.... (Note also that there are also VFD (vacuum fluorescent display) clocks used in some applications, which are yet another technology used in a variety of consumer goods.) I suspect that the unsourced answer above is incomplete at best, but I'm pressed for time to do the research right now. Does anyone want to offer some references? TenOfAllTrades(talk) 15:14, 1 November 2012 (UTC)

Ten is correct, and I botched my acronyms. Regardless, from a practical end-user standpoint, the blinking display has no functionally damaging effect on the device. I will note that the searching I have done indicate that datasheets list the MTBF for the backlight on an LCD assembly, but take as a given that the LCD itself will last indefinitely. At the same time, I'm confident that, at sufficiently fine scales, there is an effect caused by a blinking display that is distinct from the effects caused by either an always-off display or an always-on display. Therein lies the peril of ever answering simply "yes" or "no" on the Science desk, even if one or the other is likely the most useful response. — Lomn 15:33, 1 November 2012 (UTC)

What causes the flashing? My assumption is there is nothing mechanical doing it, hence nothing that will wear out quicker than a steady light would. Please correct me. μηδείς (talk) 21:34, 1 November 2012 (UTC)

Incandescent bulbs wear out faster when switched on and off despite the lack of a direct mechanical effect. Thus, "nothing will wear out quicker" should not be assumed as a general rule. — Lomn 23:14, 1 November 2012 (UTC)

LCD display segments (in the simple backlit and pasive types used in clocks & clock radios) have a square wave voltage applied to the backplane (not to be confused with the backlight), an electrode common to all segments. A segment required to be "on" has a second square wave voltage applied out of phase to the backplane - it thus has an AC voltage across it. A segment required to be "off" has an in-phase squarewave applied to it - it thus has zero voltage difference across it. In both cases there is no DC voltage ever across the display segments - this means there cannot be any electrolytic degradation. The flashing is caused by swapping over the phase every half second or similar time. The segments themselves emit no light and draw no current, so there cannot be any electrochemical action. There is no wearout mechanism. In theory, LCD displays of this type last indefinitely regardless of whether they are on or off. Flashing 12:00 indefinitely cannot do any harm. In practice, LCD's do have a quite limited life due to impurities in manufacture, and sealing may fail, but being on or off will not affect that. Ratbone 60.230.218.171 (talk) 01:33, 2 November 2012 (UTC)

Some OR, the timer clock on our oven has been flashing for over 10 years. It started out flashing 00:00 but now it only does something like -- __| (all the other "strokes" having worn out) 121.45.180.226 (talk) 11:06, 2 November 2012 (UTC)

In the battle of wills (you refusing to set the time, with flashing 12 indicating you should set it), it looks like you won. 82.131.132.190 (talk) 23:05, 3 November 2012 (UTC)

Batch image merge (microscopy software question?)

I have a number of micrographs of cells takrn under brightfield and UV illumination (with a green filter). I'd like to obtain merged images from each of these pairs. What might be the easiest way to batch process all these files? I have The GIMP and ImageJ but I am not familiar with scripting. — Preceding unsigned comment added by 129.215.47.59 (talk) 14:20, 1 November 2012 (UTC)

How many do you have? With GIMP you can merge a pair in a few seconds of work, using drag & drop (presuming they are aligned perfectly). Unless you have hundreds of pictures, it might not be worth the effort of looking for a more sophisticated solution. Looie496 (talk) 14:58, 1 November 2012 (UTC)

A good tool for batch processing of images is ImageMagick[1]. It has a command-line tool that can be used to perform all sorts of operations, so it is easily scriptable. 209.131.76.183 (talk) 15:08, 1 November 2012 (UTC)

ImageMagick is great. It's available as a command-line tool, or as a set of libraries that you can link into software that you write. It's also free software. On the other end of the spectrum is the commercial Image Processing Toolbox for MATLAB. While this software cam be very expensive, it is superior to ImageMagick in many respects: performance, ease-of-use, and depth of coverage of common tasks. If you're going to specialize in image processing, it's a good idea to learn both tools, as they're common throughout the industry and academic worlds. For almost any image-processing job, either MATLAB or ImageMagick is the right answer when you're in the "prototyping" phase. Nimur (talk) 16:26, 1 November 2012 (UTC)

For microscopy images ImageJ is something of a standard free tool. (I use it too). If you know a little (really little) programming you can hack together a ffew lines of java code to make a macro that will do this in imageJ. In fact for a batch merge job, I would expect googling to turn up plenty of ready to eat macros. good luck.Staticd (talk) 04:50, 2 November 2012 (UTC)

Is there any known plant that is pollinated by the predators of its parasites?

Greetings,

Is there any known plant whose main pollinators, or seed dispersers, actually visit it to feed on its parasites? I can vaguely imagine how such a route of dispersal could evolve with time, but I'm not aware of it actually being found anywhere. Thanks, ליאור • Lior (talk) 16:08, 1 November 2012 (UTC)

It would seem unlikely except by chance unless the parasite were eating the anthers, in which case they themselves might also be pollinators. μηδείς (talk) 23:34, 1 November 2012 (UTC)

There's a 3-way relationship between fig trees, pollinator wasps, and parasitic wasps: [2]. StuRat (talk) 03:27, 2 November 2012 (UTC)

Well, you have to be careful about how you assign causation here. For instance, the Braconid wasps are generally parasitoids of common plant parasites. They commonly lay eggs in caterpillars that eat plants. Specificity varies, but some species of wasp will seek out specific species of caterpillar, which in turn are highly selective, and only appear on certain plants. The catch is, adult braconids drink nectar, and in turn can pollinate by transferring pollen (mentioned here [3]). So yes, some individual plants are pollinated by insects that were attracted to the plant's parasites. This not necessarily a tight, or common relationship, but it does happen. SemanticMantis (talk) 17:21, 2 November 2012 (UTC)

Fascinating! I can't find any relevant theoretical paper which models the plant-parasite-braconid population dynamics. Many thanks, ליאור • Lior (talk) 17:06, 3 November 2012 (UTC)

"Hurricane" vs "Post-Tropical 'Super Storm'"

Many insurance policies reference "hurricane" damage separate from regular "storm damage, with different deductibles and claim criteria, etc.. Since Sandy was "side-graded" to a Post-Tropical storm doesn't that mean that the hurricane language in insurance policies is a moot issue in this case? I know that insurance companies would argue to the hill if the shoe was on the other foot. Technically the storm was NOT a hurricane when it made landfall as it did not meet the necessary criteria to be one.165.212.189.187 (talk) 16:24, 1 November 2012 (UTC)

Nevermind. googled: sandy not a hurricane.165.212.189.187 (talk) 16:32, 1 November 2012 (UTC)

I disagree with the nevermind. Interesting question. Do regular home insurance policies really distinguish specifically hurricane damage from other wind damage? Would it really make a difference for coverage if a storm was 73 mph or 74 mph? What if it was hurricane force winds but not a cyclone (not rotating) and thus not a hurricane, just a hard wind? Duoduoduo (talk) 20:21, 1 November 2012 (UTC)

Yes, there is wind insurance and hurricane insurance clauses/riders. Did you google sandy not hurricane? The govt is going to closely monitor the ins cos to ensure that they do not enforce the hurricane aspects of policies where applicable.68.83.98.40 (talk) 01:16, 2 November 2012 (UTC)

Specifically [4] Duoduoduo (talk) 14:57, 2 November 2012 (UTC)

Slowing neutrinos

Could an exceeding intense beam of very very low frequency radio waves slow down an occasional neutrino? I'm thinking intensity could help a little with the low cross section, and the low frequency would mean any interactions that did happen would absorb energy from the neutrino. I'd like to be able someday to have a pound or so of ultraslow neautrinos in a small box.Thanks, Ricfh Petrson198.189.194.129 (talk) 18:52, 1 November 2012 (UTC)

Others may be able to comment more authoritatively, but no, I don't see how that could work. Radio waves are purely based on the electromagnetic force, whereas neutrinos interact only via the weak force (and of course the gravitational force).

Supposing you did get your pound of neutrinos, what do you imagine would hold them in the box? --Trovatore (talk) 18:57, 1 November 2012 (UTC)

Somewhere in outer space in free fall, if the neutrinos were traveling sufficiently slowly, say .1mm/century, then they would stay in there for a while, although the box would only serve as a marker to tell you what they were inside, it couldn't prevent them seeping out at all.--Are we absolutely sure they don't interact with EM?? I guess if that's true, there is no way to slow them down.198.189.194.129 (talk) 21:35, 1 November 2012 (UTC)

There's something called electroweak unification, but per our article, it happens only at very high energies. I suppose there must be some remnant of it at lower energies. --Trovatore (talk) 22:37, 1 November 2012 (UTC)

The remnant at low energy is the EM force and weak nuclear force. Neutrinos only feel the weak nuclear force part of it. However, if you had something extremely dense that would slow neutrinos down (via exchange of W (electron neutrinos only) and Z bosons) but I think matter would become a black hole first before getting near 1mm/century.Dja1979 (talk) 03:37, 2 November 2012 (UTC)

Mercury can slow neutrinos down, but not by that much. 24.23.196.85 (talk) 05:08, 2 November 2012 (UTC)

Hmmm. Our article on deuterium says it's been used as a target for neutrinos - I wonder if enough of that ultra-dense deuterium we were just talking about could really do something fun with them? Wnt (talk) 05:30, 2 November 2012 (UTC)

The detectors for neutrinos are chosen not by how much they interact with neutrinos, but how easily you can detect interaction with neutrinos without getting distracted by other events. In particular, large underground chambers filled with water or chlorine is used, because those materials are not too expensive to get very pure in large quantities, and are transparent so the Cherenkov radiation of the fast electrons kicked off by neutrinos can be detected. Heavy water (with deuterium) is used for a similar reason. – b_jonas 18:53, 3 November 2012 (UTC)

Sun's magnetic field

Roughly, what's the stength of the Sun's magnetic field on Earth? 65.92.7.202 (talk) 19:02, 1 November 2012 (UTC)

According to Sun#Magnetic field,

The interplanetary magnetic field is much stronger than the dipole component of the solar magnetic field. The Sun's dipole magnetic field of 50–400 μT (at the photosphere) reduces with the cube of the distance to about 0.1 nT at the distance of the Earth. However, according to spacecraft observations the interplanetary field at the Earth's location is around 5 nT, about a hundred times greater.[80] The difference is due to magnetic fields generated by electrical currents in the plasma surrounding the sun.

Duoduoduo (talk) 20:26, 1 November 2012 (UTC)

Can someone paraphrase that in lay-English for us idiots? μηδείς (talk) 20:47, 1 November 2012 (UTC)

Not me. I was hoping the OP would understand it. But the article Earth's magnetic field#Intensity says

The intensity of the field is greatest near the poles and weaker near the Equator. It is often measured in gausses (G) but is generally reported in nanoteslas (nT), with 1 G = 100,000 nT. A nanotesla is also referred to as a gamma (γ).[10][11][12] The field ranges between approximately 25,000 and 65,000 nT (0.25–0.65 G). By comparison, a strong refrigerator magnet has a field of about 100 G.

So, although I know absolutely nothing about this, it looks to me that the sun's 5 nT at Earth location is dwarfed by the Earth's 25,000 to 65,000 nT. Duoduoduo (talk) 22:36, 1 November 2012 (UTC)

The strength is 5 nT. Almost all of that is due to electrical currents surrounding the Sun. --140.180.252.244 (talk) 22:37, 1 November 2012 (UTC)

Well, I am not sure why anyone wuld be talking about tens of thousands of nano teslas, but if the sun's field is 5 at the earth, compared to the earth's 25-65,000, that gives an idea of scale. What is the 100 G, 100 giga teslas? I suppose a tesla must be a local measure then, not a sum across the entire flux. Am I wrong? μηδείς (talk) 23:33, 1 November 2012 (UTC)

G is Gauss, the other metric unit of magnetic field density. (The Tesla is the SI unit, while the Gauss is the cgs unit.) --Carnildo (talk) 02:18, 2 November 2012 (UTC)

Capacitor charging question

I am working on a project which uses a solar panel, and some other electronics, to charge a 10 farad capacitor. I would like the microcontroller to display the time remaining for the capacitor to be fully charged, but cannot find or work out the necessary equation.

Is it possible to work out the time remaining, with the microcontroller having access to the following values:

• The supply voltage
• The current going into the capacitor
• The voltage across the capacitor
• The resistance between the supply and the capacitor
• The required voltage across the capacitor
• And, if necessary, previous values of the above.

85.210.44.120 (talk) 19:41, 1 November 2012 (UTC)

With a supply voltage $V_{s}$, resistance R and capacitance C the voltage over the capacitor will increase over time t according to (see Capacitor#DC_circuits)

$V=V_{s}\left(1-\exp \left(-{\frac {t}{RC}}\right)\right)$.

If the voltage over the capacitor now is $V_{1}$ the time is now

$t_{1}=-RC\ln \left(1-{\frac {V_{1}}{V_{s}}}\right)$.

The time at which it will reach the required voltage $V_{r}$ is

$t_{2}=-RC\ln \left(1-{\frac {V_{r}}{V_{s}}}\right)$,

so the time left is

$\Delta t=t_{2}-t_{1}=-RC\ln \left({\frac {1-V_{r}/V_{s}}{1-V_{1}/V_{s}}}\right)=-RC\ln \left({\frac {V_{s}-V_{r}}{V_{s}-V_{1}}}\right)$.

This formula should work if you have these values, but I wouldn't trust the voltage you meassure over the capacitor while it is charging to be correct since there is always an inner resistance in the capacitor. Either you stop charging temporarily to measure $V_{1}$ or you measure the inner resistance in advance and use this and the charging current to calculate the real capacitor voltage from your measured value. Ulflund (talk) 23:29, 1 November 2012 (UTC)

.

Unfortunately, Ulfund is incorrect. A solar panel operated below its open circuit voltage, which it necesarily will be until the capacitor is charged, act as as a constant current source, and the current is directly proportional to the intensity of light falling upon it. A capacitor has a voltage directly proportional to the product of current (in amps) and elapsed time (in seconds) divided by the capacitance (in farads). Therfore, approximately (approximate because the change over from constant current to constant voltage of a solar panel is not abrupt), the time remaining is given by:-

T_remain = C ( V_oc - V_cap ) / I

where V_oc is the open circuit voltage of the panel, and V_cap is the capacitor voltage at the measurment time.

A capacitor of 10 Fards is likely to be of the supercapacitor type (http://en.wikipedia.org/wiki/Supercapacitor) and these have significant internal series resistance compared to other types, as only supercaps are available in the size cited, and are the most suitable type wrt self discharge. However in this application it is small enough to be neglected in the calculation of time remaining.

Keit 58.169.250.75 (talk) 00:46, 2 November 2012 (UTC)

My calculations were based on the assumption of constant charging voltage. For constant current the formula given by Keit should be used instead, and I agree that this is more likely to be the best approximation if the capacitor is charged with a solar panel. Ulflund (talk) 07:43, 2 November 2012 (UTC)

spiders

I have a lot on spiders in my basement are there any traps for them, or what is a good way to get rid of them?--Wrk678 (talk) 19:51, 1 November 2012 (UTC)

I was going to say 'insecticide' - but does insecticide even work on spiders? I've never thought about that before... --Kurt Shaped Box (talk) 20:05, 1 November 2012 (UTC)

Why do you want to get rid of them? Do you enjoy having extra insects around? Marnanel (talk) 20:11, 1 November 2012 (UTC)

I think it would depend on what spiders the OP has. Some of them are horrible critters to have about the home. --Kurt Shaped Box (talk) 20:20, 1 November 2012 (UTC)

My first instinct would be to head to my favorite internet search engine and type in exterminate spiders. If faced with a serious infestation, beyond what internet remedies suggest, I'd look at the local yellow pages for pest/insect control (yeah, spiders are not insects, but those guys will know what to do, whether the enemy has six or eight legs.) 88.112.36.91 (talk) 20:15, 1 November 2012 (UTC)

I put the question to Google (sorry, no special knowledge) and got [5] which sounds plausible - they say pyrethrins, resmethrin, and allethrin work on them but only if you more or less spray them directly, which is near pointless, so use housekeeping measures. Bottom line is that if you beat the insects, you beat the spiders. But I also got a hit for the vastly less "reliable" source eHow (Wikipedia blacklists www.ehow.com/facts_5906696_safe-insecticide-spiders.html as a link), which makes the curious claim that natural oils of lemon and citronella, detergents, horse chestnuts, and osage hedge apples would repel spiders. Not sure if these are indeed horse chestnuts! But the claims are all over the Web, and not facially implausible. The Telegraph says, well, they don't know, but "conkers sound like bonkers". [6] There sure are a lot of biology projects people could get good studies published about if funding were greatly expanded instead of cut for a change, but that doesn't sound likely. Our article on osage/horse apples cites a study saying that it actually is an insect repellent [7] based on the presence of elemol, said to be comparable to DEET in the publication. Looking it up, I see it repels ticks [8] so I count this one as "probably confirmed", pending replication of results. Oh, and someone's investigating the conkers. [9] And that's why I like to consider non reliable sources seriously. Wnt (talk) 20:36, 1 November 2012 (UTC)

Horse chestnuts work quite well against spiders, if you've got good aim. --Carnildo (talk) 02:22, 2 November 2012 (UTC)

Cleaning the basement will also work. If you remove all the spider webs regularly, the spiders will leave. Count Iblis (talk) 20:33, 1 November 2012 (UTC)

• Spiders are a self-controlling problem--they'll either eat your pests or themselves. The underlying question is, what is their food source? You are surely infested with something else, unless this is just a recent hatching from one colonist. I, myself, never kill spiders (nor most insects unless they are dedicated pests like ants, roaches, and flies). Of course we don't have many dangerous spiders in the US NE--some areas do. If you must, use bug spray or other bug control measures. Spiders have the same underlying physiology as insects and are subject to the same poisons. If you are really desperate, I know an old lady who will help, but at a price. μηδείς (talk) 20:42, 1 November 2012 (UTC)

I dont have any insects down, there just spiders, and I would like to not use chemicals.--Wrk678 (talk) 23:54, 1 November 2012 (UTC)

Well, there's no such thing as a spider trap. And they aren't magic, they do have to eat, so there must be some non-spider infestation unless this is just a crop of very recent babies from an egg sac. Try cleaning thoroughly and making sure their is no water source, because you are obviously supporting some sort of little ecology with water and food of some sort. μηδείς (talk) 02:17, 2 November 2012 (UTC)

I've seen glue boards trap spiders, but you'd need a massive number of them to get all your spiders. If there's nothing flammable around I tend to use fire to kill them. Specifically, I use a fireplace lighter to burn them and their webs. This avoids me getting spider webs and spider guts on myself. I also use a broom to get at spiders and webs out of reach (I push the bristles into the spiders to kill them). And to answer the Q someone posed above about why not leave them alive, that person apparently never had spiders lower themselves onto their face while they were sleeping and driving. Either one can ruin your day or night, trust me. StuRat (talk) 03:18, 2 November 2012 (UTC)

Well, I don't sleep in my basement, either! LOL. But I have had the occasional spider walk on me while I was in bed. Is no biggy. "Spiders in the basement" is about as vague as "bugs upstairs." Are they web builders? Are they wolf spiders? Are they all adults, or is this a hatching? Basically all we have here is an expression of an ick factor with no factual facts to go on. Might as well suggest moving unless we get more concrete information. And the fact remains that you don't get spiders unless they have a food source, which itself needs food and water. μηδείς (talk) 03:24, 2 November 2012 (UTC)

Or they can come in from outside. Houses are amazingly transparent to insects. Gnats can come right through window screens, and much larger insects have ways of getting in, too, although I don't quite know how (or I would stop them). StuRat (talk) 04:28, 2 November 2012 (UTC)

Yes, that's entirely possible, in which case the OP will be really upset when all their egg cases hatch in the Spring. He'll need to find how they are getting in. I have had half-a-dozen of those cute stink bugs in the last two months. They seem to be able to teleport through walls. μηδείς (talk) 16:48, 2 November 2012 (UTC)

As with most questions posted on the reference desk, the correct answer is also the most obvious: tetramethrin, methoprene and cyanide will do the job equally well. 203.27.72.5 (talk) 05:37, 4 November 2012 (UTC)

Cyanide would be too dangerous. 24.23.196.85 (talk) 02:42, 6 November 2012 (UTC)

Has 24.23.196.85 seen the New York Times today? Israel and Iran 2010. Almighty God always proves his little mystic right ! Cinquefoil (talk) 03:26, 6 November 2012 (UTC)

Question regarding the conversion of one Tesla to one Earth atmosphere

Hi All, and thank you in advance for your consideration. Is it possible to convert one Tesla to one Earth atmosphere, and what would that value be?Gavinth (talk) 21:15, 1 November 2012 (UTC)

Well, if you set Planck's constant, the gravitational constant, and the speed of light all to a dimensionless value of 1, then all units become dimensionless numbers, so presumably you could make this conversion in that sense. See natural units. Whether the result would have any articulable meaning is another question. --Trovatore (talk) 21:18, 1 November 2012 (UTC)

This sounds like a pretty good argument against dimensionless units! But to be clear, one tesla (unit) = one kg s^-2 A^-1, and one atmosphere (unit) = 101325 Pascal (unit) = 101325 kg s^-2 m^-1. So the number of Teslas in 1 atm is equal to 101325 A/m. Now you just have to figure out what that means. :) Actually, it might be easier to think of it as: an atmosphere is defined as 101325 newtons per square meter, and a tesla as one weber per square meter; the ratio is the number of newtons of force per weber of magnetic flux emerging from the bounded region in question. Wnt (talk) 21:55, 1 November 2012 (UTC)

No, it is not possible to convert 1 Tesla to 1 Earth atmosphere, because the two measure different quantities. In the same way, a length cannot be converted to an area, a temperature cannot be converted to kilograms, and a position cannot be converted into m/s. --140.180.252.244 (talk) 22:33, 1 November 2012 (UTC)

You can convert anything to anything, given the right assumptions. For example, kilograms of weight can be converted to newtons - if you know the gravity of the planet you have in mind. Wnt (talk) 22:55, 1 November 2012 (UTC)

I'm just guessing, but maybe the OP meant the magnetic field in the Earth's atmosphere, rather than the air pressure commonly known as 1 atmosphere (that doesn't make much sense to me). If so, Earth's magnetic field at the surface ranges from 0.25-0.65 mT, so one Tesla would be 1538-4000 times this field. - Lindert (talk) 00:35, 2 November 2012 (UTC)

What's more likely is that the OP is a troll. He's using an unregistered name, he's not showing an IP address, and he's asked a question designed to see what you do with it. The best way to deal with trolls is to ignore them. — Preceding unsigned comment added by 60.230.196.221 (talk) 03:42, 2 November 2012 (UTC)

See here. Count Iblis (talk) 16:27, 4 November 2012 (UTC)

Is there a name for "hating forks aiming for someone"?

If a fork or any long object is "pointing" at you or someone else and you deliberately change its position so it's pointing at nobody, is there still hope? Moreover, does this behaviour have a name or reason? (The subject doesn't mistake the fork for a gun and this is not a request for medical advise) Joepnl (talk) 23:12, 1 November 2012 (UTC)

It is called Superstition and has no reason at all. Pointing a folk at someone won't cause anything to happen so neither will turning it. CambridgeBayWeather (talk) 23:43, 1 November 2012 (UTC)

Unless it's loaded. Clarityfiend (talk) 00:17, 2 November 2012 (UTC)

If you mean forks that are just sitting there with nobody holding them, that would be a type of obsessive/compulsive behavior, akin to refusing to step on cracks in the sidewalk. Looie496 (talk) 02:36, 2 November 2012 (UTC)

Yes, an obsessive-compulsive disorder. StuRat (talk) 04:22, 2 November 2012 (UTC)

Well, it doesn't really rise to the level of OCD unless it is so severe that it interferes with the ability to lead a normal life. Lots of people have a quirk or two of this sort. Looie496 (talk) 15:00, 2 November 2012 (UTC)

I believe many people with an OCD can lead a normal life. StuRat (talk) 17:32, 2 November 2012 (UTC)

According to DSM4, a diagnosis of OCD requires that: The obsessions or compulsions cause marked distress, are time consuming (take more than 1 hour a day), or significantly interfere with the person's normal routine, occupational (or academic) functioning, or usual social activities or relationships.. Looie496 (talk) 17:56, 2 November 2012 (UTC)

To lay off the medical advice, what do you call such obsessions/compulsions when they're not meeting those criteria? Obsessions and compulsions? Is there any distinction between those a person comes up with idiosyncratically, as opposed to, say, a Middle Eastern tradition that pointing your foot at someone is offensive? Wnt (talk) 18:15, 2 November 2012 (UTC)

They are still obsessions or compulsions, the word that is in play here is disorder. The Middle Eastern thing seems like more a matter of culture. Lots of cultures treat pointing at somebody, in various ways, as an assault. That may go back to our primate ancestry: in monkeys, merely looking at another monkey is often treated as a threat and leads to an angry response. Looie496 (talk) 22:57, 2 November 2012 (UTC)

November 2

Gravitational collapse

Hi, how big would a planet with the same composition as the Earth have to be before it collapsed into some kind of super-dense exotic matter? 86.151.118.165 (talk) 02:51, 2 November 2012 (UTC)

Well, something with the same composition as the Earth but with the mass to form a black hole wouldn't be a planet, it would be a star. That is, nuclear fusion would support it until everything that could fuse had fused, then it would collapse into a black hole. Around 10 times the mass of the Sun seems to be enough to form a black hole, but I'm not sure if the composition of that mass changes the equation. StuRat (talk) 03:07, 2 November 2012 (UTC)

I am not necessarily talking about a black hole. In fact, I specifically didn't mean a black hole. 86.151.118.165 (talk) 03:17, 2 November 2012 (UTC)

Like neutron stars? Plasmic Physics (talk) 03:45, 2 November 2012 (UTC)

Or white dwarfs perhaps... Whatever is the first stage of exotic matter that is far denser than any ordinary matter that we are familiar with on Earth could be. 86.151.118.165 (talk) 04:06, 2 November 2012 (UTC)

... also, I am specifically not asking about stars. I literally mean that you take a rocky planet, same composition as the Earth, scale it up by x amount, then how big before there's a big crunch and it squishes down to a lump of exotic matter. 86.176.211.219 (talk) 04:45, 2 November 2012 (UTC)

I just did a back of the envelope calculation which came as 78.2 times the radius of the earth for it to collapse into a neutron star (using 2.84 E 30 as the collapsing mass from the Netron star article and 5500 kg/m^3 as the density of the earth). But that doesn't sound right (also, apologies for the lack of math tags, its to early for me to remember how to do them) 80.254.147.164 (talk) 09:56, 2 November 2012 (UTC)

(edit conflict) Well, our white dwarf article says the smallest estimated mass for a white dwarf is 0.17 solar masses, which is about 56,000 Earth masses. So a rocky planet (I am assuming "rocky" means insufficient hydrogen or helium to support fusion reactions) with the same average density as the Earth but about 40 times the Earth's radius would collapse under its own gravity to form something like a white dwarf. I have no idea how such a massive "rocky planet" object would form in the first place - maybe we just have to assume it appears from nowhere. Gandalf61 (talk) 10:01, 2 November 2012 (UTC)

You don't necessarily need hydrogen or helium. Silicon and oxygen, the two most abundant elements in the Earth's crust, will undergo fusion if you get them hot enough and squeeze them tight enough.

Iron, on the other hand, will not, and I kind of doubt nickel will either; these are the main components of the core.

So the bottom line is that I kind of doubt StuRat's claim about fusion, but the question can't be disposed of completely trivially. --Trovatore (talk) 02:12, 3 November 2012 (UTC)

How's this for a paraphrase: How massive can a body be before degenerate matter appears at its core? —Tamfang (talk) 19:08, 2 November 2012 (UTC)

Metallic hydrogen is degenerate matter and is thought to be present in the cores of Jupiter and Saturn. Gandalf61 (talk) 19:24, 2 November 2012 (UTC)

Whenever the mass is large enough to cause a force that overcomes the atomic weak interaction ..?, But I also wonder what actually happens when a body of elements that are more dense than iron-26 gets forcefully compressed and heated ? Electron9 (talk) 18:29, 6 November 2012 (UTC)

Avalanche rescue

Is earth-moving equipment used to rescue avalanche victims? If so, how do they make sure that survivors are not accidentally killed by it? 24.23.196.85 (talk) 05:19, 2 November 2012 (UTC)

I've always wondered the same when it comes to rescuing survivors from debris after a tornado, building collapse, etc. Ks0stm ^{(T•C•G•E)} 05:27, 2 November 2012 (UTC)

Only a WAG, but I wouldn't think so. Time is of the essence and getting heavy equipment into position would take too much of it. Clarityfiend (talk) 07:35, 2 November 2012 (UTC)

Agreed, you have minutes, at most, to dig the person out. Also, if they are buried so deeply in snow that shovels won't work in time, then they are already dead, due to the pressure. And, the main problem isn't digging the hole to get them out, it's finding them in the first place. StuRat (talk) 17:28, 2 November 2012 (UTC)

The usual method is to for rescuers to form a line, poking through the snow with a long pole called an avalanche probe and hoping to be able to recognize the resistance when the probe hits a body. Trained dogs are used to refine the search. I found this photo of the Galtür Avalanche, which appears to be using this technique. This photo shows a dog guiding a team of rescuers armed with avalanche shovels. 20 people who had been buried were rescued but a further 31 died. Alansplodge (talk) 18:07, 2 November 2012 (UTC)

And I have just found our Avalanche rescue article. Alansplodge (talk) 18:08, 2 November 2012 (UTC)

Thanks for the input, everyone! Now, what if the avalanche also buries several buildings and cuts the only road into the area -- would a bulldozer be useful to uncover the buildings and/or clear the road, or would the same techniques be used as for the victims on the slopes? 24.23.196.85 (talk) 01:11, 3 November 2012 (UTC)

Yes, bulldozers or snowplows would be used to clear the road, provided there aren't any victims believed to be present, and in the area around buildings, but not the buildings themselves, if they were believed to contain victims. Buildings have more of a possibility of air gaps inside, so people might survive for quite some time. StuRat (talk) 01:18, 3 November 2012 (UTC)

Thanks a million, and clear skies to you! 24.23.196.85 (talk) 02:53, 3 November 2012 (UTC)

Mushrooms and mycelium

Hello there reference desk. I'm trying to work out what is known about fungal lifecycles in terms of the variety of types of mushroom a single 'lump' of mycelium can produce. I'm most interested in the Euholobasidiomycetes, as this seems to be the group containing most of the mushroom producing fungi.

I realise it's fairly hard to give good answers as the genetics of some fungi aren't that well understood at the moment. As far as I can tell, a single spore from a mushroom contains a haploid nucleus that will start a mycelium colony growing. Some species have asexual reproduction methods, but most will have to interact sexually with another 'compatible' mycelium colony before producing fruiting bodies. At this stage, I started getting bogged down in detail - but it seems that some species of mycelium have the ability to acquire and incorporate more and more genetic material. There was also talk about varieties of mushrooms previously though distinct that turn out to be genetically identical. My questions are:

1. When fungi are said to be sexually compatible, does this mean that they are the same species in a way analogous to plants and animals?

2. Many species seem to end up with multiple, genetically distinct, haploid nuclei in their cells after various complicated genetic processes. Will the spores dispersed by a mushroom represent the entire genetic material of a fungus, or are all spore from one mushroom the same (barring chance mutations.)

3. What kind of variety of mushrooms can one mycelium colony (assume indefinite sexual encounters if this is necessary) produce?

The main site I've been using is . I'm sure the information is in there, but it's very dense and I can only get comfortable with 10 or so terms of art per sitting...80.254.147.84 (talk) 08:46, 2 November 2012 (UTC)

1.Depends on how you choose to define species and what you mean by "sexual compatibility". Generally, a species is defined as "a group of organisms capable of interbreeding and producing fertile offspring". So...

• Yes, if by "sexual compatibility" you mean the capability of the individuals in the population to reproduce with another individual within the group (inclusive of mating type barriers between individuals among heterothallic fungi).
• No, if you mean sexual compatibility merely in the sense of individual mating type compatibility. In the same way that just because two human males can not reproduce with each other, does not mean they belong to different species (although the latter analogy is vastly oversimplified given that fungi can have thousands of "biological sexes").
• Overarching all that is the species problem. If you follow a morphologically defined concept of species, some fungi populations can remain morphologically identical (cryptic species), even when in truth they have diverged and achieved reproductive isolation from each other long ago. Indeed, until the very recently (2011 International Botanical Congress) fungi were given different scientific names for different morphs in their life cycles (see teleomorph, anamorph and holomorph), even if they are all the same species, largely because they can look radically different.

If you follow biological species concepts which relies on reproductive isolation, it can fail in complex reproductive relationships where some groups can reproduce with some other groups but not with others and vice versa; while others still are actually not reproductively isolated but will only mate under certain conditions that might be missed in laboratory environments; and it does not take into account where the capability of interbreeding may simply be the result of retained ancestral characters (plesiomorphy) when in truth the individuals themselves have become genetically isolated a long time ago.

Phylogenetic species concepts addresses the latter by restricting groupings based on shared ancestors, but it runs into the problem of where to place the boundaries. How do you divide into species what basically is a genetic continuum broken only by extinctions? The latter problem exists even in higher animals but is more evident among fungi, where even trying to define an "individual" itself is difficult due to the fact that they barely qualify as "multicellular organisms". So the similarities and differences on how higher organisms are classified depends on which system is followed or combinations thereof.

2.The most common, asexual spores, are of course geneticaly identical to the parent. Though apparently some long-lived fungi colonies have been discovered to essentially be genetic mosaics, possessing genetic variation within a sngle "individual", so spores from the same individual can be different in those circumstances even if produced asexually.

Sexual spores in contrast are the result of the meiosis of a typically brief diploid stage and thus will differ from each parent. Spores that result from mitosis after meiosis are of course genetically identical to each other, but not to the others.

Thus in Ascomycota, for example, the nuclei from the two parent individuals (not genetically identical) fuse to each other (karyogamy) and form the diploid zygote nucleus. The diploid zygote nucleus then undergoes meiosis, exchange genetic information here and there, and in the end divide into 4 genetically different haploid daughter nuclei. Each of the daughter nuclei then undergo mitosis, such that they create genetically identical copies of themselves. This usually results in 8 ascospores, with every two ascospore genetically identical with each other, but not to the other pairs. Basically, they're a set of 4 twins. This series of diagrams might help. Basidiospores are more or less the same, except that in Basidiomycota, the two haploid nuclei of the parent individuals coexist in the same cell (even vegetative ones) for a longer time without undergoing karyogamy.

3.I'm assuming you mean how many genetically different individuals can result from a single mycelium colony through sexual reproduction. Well as many as all the possible permutations I guess. :P That's a bit like asking how many genetically different children two human couples can produce assuming they have an indefinite number of sexual encounters. Barring twins, no two children will be genetically the same, though they can be genetically similar.

Disclaimer: not a mycologist, merely rehashing uni biology lessons. :P -- OBSIDIAN†SOUL 09:57, 4 November 2012 (UTC)

3d tracking tic-tacs

I have the following requirements. (Basically I would like to track the 3d location of a few things in near-field like bluetooth-distnace)

1. Glue ten tic-tac size devices to ten spots within a 40cm x 40cm x 40cm area.

2. Know in real time the location of each device #1 - #10 with a 2mm x 2mm x 2mm margin of error, within a 3m x 3m x 3m space (no tracking required if they leave this space). The response should be within 100 ms but preferably 15-20 ms.

Any technological solution could be considered.

--

My thoughts. The devices can be active (know their own location) or completely passive, like an RFID strip, and activated remotely by a reader or base station. It also doesn't matter if the location is only relative to some point of origin. And although the tic-tacs should be small, there can be a considerably larger base station or receiver.

Now, I realize there are many, many approaches here. Hollywood tends to use 3 or more cameras and interpolate exact position of the tic-tacs. The tic-tacs could have gps devices, then their precision and response is much, much worse than my requirements. The tic-tacs could be physically connected to rods that can move in 3d (in-and-out and pivot on a ball) that register their relative movements. Or they might somehow have radio signatures that the base station can interpret.

They could be magnetized and the magnetic signatures detected by a very large base station. They could be radioactive or emit xrays and be detected by their signatures on that basis. (Then the base station would have to have 3 planes, maybe).

What is the simplest, cheapest solution here? I would think that if there were some radiofrequency-based thing where there is a somewhat largish base station then it would work.

What do mimeo whiteboard digitizers use, for exmaple, like this - http://www.mimio.com/en-EM/Products/MimioCapture.aspx (scroll down)

or how do Wacom tablets digitize the location of a passive plastic stylus?

I don't really know the space of technologies that can meet the requirements. Your creativity is appreciated here, thank you. --91.120.48.242 (talk) 09:20, 2 November 2012 (UTC)

It sounds like a pretty traditional motion capture problem. I don't think there are many options out there other than optical systems like you mentioned. Does a marker and camera based solution work for your problem, or is there a reason you need something more technical? 209.131.76.183 (talk) 13:25, 2 November 2012 (UTC)

I just think that a passive radio emitter would be less technical, not more technical. RFID strips are incredibly low-cost. Isn't there a way to get a receiver to triangulate the position of a passive radio device? This seems like it's gotta be far less expensive from trying to get a line-of-sight and markets. I also just cannot believe that markers can be precise enough to my tolerances, e.g. 2-5 mm. --91.120.48.242 (talk) 14:58, 2 November 2012 (UTC)

That does sound like a sensible solution, though I think that's not "RFID" proper but some more basic type of radar signature - the key here is that waiting for digital operations to return a serial number seems like it should introduce too much timing uncertainty to measure the distance according to speed-of-light time delay. Triangulating the signal according to intensity or exact angle of displacement seems like it would be very tricky. But take what I say with a shaker of salt, because it's not my field. Wnt (talk) 18:06, 2 November 2012 (UTC)

You said they will be glued down, but glued down to what ? Rods ? StuRat (talk) 17:23, 2 November 2012 (UTC)

Hi! That is one possibility, but better is if they are just glued to the locations. The question is how to locate these in 3d space (not 6d, you don't need to know orientation at all). The rod idea is the worst one that I can think of, since it involves physically connecting these tic-tacs, instead of letting them move freely with the body they are on. 178.48.114.143 (talk) 17:47, 2 November 2012 (UTC)

What "body" are they on ? StuRat (talk) 18:02, 2 November 2012 (UTC)

Fine joints on a human body, glued on lightly thereto. In fact though they can be slightly tethered to some kind of slightly larger thing held on with a band, that is fine. this seems to be what the reference i found below was about. 178.48.114.143 (talk) 11:17, 3 November 2012 (UTC)

One potential problem, do we need to account for the possibility that one of the "tic-tacs" can be behind another, or behind a rod, from the POV of one of the detection devices ? StuRat (talk) 17:17, 2 November 2012 (UTC)

They can indeed be behind each other. I think something radio-based is the best idea. Does such technology exist? THey could be on different frequencies. So, the idea is if you have ten of these on ten unique frequency, say, and passive radio emitters that emit their frequency in response to a base frequency - then the receiver emits that frequency, and gets ten point sources corresponding to the ten different tic-tacs. THrough radio trangulation would it be possible to locate each to within 2mm by 2mm by 2mm? You don't need to know their orientation. 178.48.114.143 (talk) 17:47, 2 November 2012 (UTC)

The technology exists, but I don't think you can get that type of resolution out of it. Here's a company that offers such an RFID system, but to work on larger scales: [10]. StuRat (talk) 17:59, 2 November 2012 (UTC)

could you talk about the physics behind it ,and why it wouldn't work for radio, ultrasound, or magnetic-production and sensing respectively? 178.48.114.143 (talk) 11:17, 3 November 2012 (UTC)

The motion scan article claims that radio systems are less accurate, which makes sense when you start looking into the wavelength of the signals. Hollywood uses optical systems because they are relatively simple and have the best accuracy. Similar problems occur in industrial automation, and there is a large industry based around sensing locations in that market. If you have a good budget for this project, I recommend talking to motion capture or industrial sensing companies for ideas. If you're trying to make something from scratch on a low budget, optical is probably the simplest. 209.131.76.183 (talk) 18:08, 2 November 2012 (UTC)

This is an interesting contribution. Could you help me make sense of this research paper:

“ J Neurosci Methods. 1998 Sep 1;83(2):125-31.

Using miniature sensor coils for simultaneous measurement of orientation and position of small, fast-moving animals. Schilstra C, van Hateren JH. Department of Neurobiophysics, University of Groningen, The Netherlands.

Abstract

A system is described that measures, with a sampling frequency of 1 kHz, the orientation and position of a blowfly (Calliphora vicina) flying in a volume of 0.4 x 0.4 x 0.4 m3. Orientation is measured with a typical accuracy of 0.5 degrees, and position with a typical accuracy of 1 mm.

This is accomplished by producing a time-varying magnetic field with three orthogonal pairs of field coils, driven sinusoidally at frequencies of 50, 68, and 86 kHz, respectively. Each pair induces a voltage at the corresponding frequency in each of three miniature orthogonal sensor coils mounted on the animal.

The sensor coils are connected via thin (12-microm) wires to a set of nine lock-in amplifiers, each locking to one of the three field frequencies. Two of the pairs of field coils produce approximately homogeneous magnetic fields, which are necessary for reconstructing the orientation of the animal. The third pair produces a gradient field, which is necessary for reconstructing the position of the animal.

Both sensor coils and leads are light enough (0.8-1.6 mg for three sensor coils of 40-80 windings, and 6.7 mg/m for the leads, causing a maximal load of approximately 5.7 mg) not to hinder normal flight of the animal (typical weight 80 mg). In general, the system can be used for high-speed recordings of head, eye or limb movements, where a wire connection is possible, but the mechanical load on the moving parts needs to be very small."

”

if I understand it correctly, this is sensors lead by a small wire. THis would be fine for me - the sensors would be attached to the fine joints and lead to a wire on a band around a heavier body part. Would this be doable? very expensive? Could you help me interpret the science in this article? THanks. 178.48.114.143 (talk) 18:46, 2 November 2012 (UTC)

• Hmmm, I'm thinking two bar code scanners, if you have one with a red laser, one with a green laser. The tic tacs contain photosensors that can tell when red or green is detected with very good temporal resolution. If they know when the laser beam is supposed to reach a certain spot in a 2-d grid pattern, and they can do it based on two separated points with the red and green, then they know their 3d location. This assumes the laser makes a thorough grid sweep (enough to hit any given spot a tic tac can be) and that you have enough understanding of the software to know when it hits that spot with great temporal precision - not sure either is true for a standard scanner. Wnt (talk) 17:55, 2 November 2012 (UTC)

This is an interesitng thought, I will keep track of it along with the others. I wonder if making the actual end-point "sensors" dumber however and the receptor smarter isn't the better way to go maybe? 178.48.114.143 (talk) 18:46, 2 November 2012 (UTC)

Heat transfer and temperature

I'd like to know if it is possible to estimate how hot an object will get over time when placed near a heat source. My specific problem is this: I have a wood stove that will have a surface temperature of about 300-500 C (?). I also have paint for a concrete wall behind the stove and the paint can take about 120 C. What I'm wondering is how close to the stove I can paint this wall and not burn my paint later. The shortest distance between wall and stove is about 30 cm, the longest (from top of the stove to top of the wall) about 150 cm, and there is some airflow between stove and wall. The max time the stove would be used would be around 2-3 hours/day (the time it takes to make food & heat the room). I'd like to paint as close as possible. I've read articles about heat conductivity, radiation etc but I'm none the wiser.

I don't know if this question is enough about science, but I couldn't figure out another place to ask this. --88.148.221.134 (talk) 13:09, 2 November 2012 (UTC)

I suspect that the paint will be fine, especially if it is white to help reflect the radiated heat. The concrete wall should be able to wick heat out of the paint surface pretty quickly, and circulating air will also help cool the surface. Intuitively, I think the wall will get hot, but not so hot that you can't touch it, and not hot enough to boil water, so it should be below 120C. Higher-temperature paints are also available, but are more expensive. The best way to answer the question for sure is to run the stove before painting the wall and measure the temperature directly. One of those IR "laser" thermometers would work great, if you use something (a board, tin foil, pretty much anything) to shade the wall from the stove's radiative heat just before measuring in order to prevent interference in the reading. 209.131.76.183 (talk) 13:21, 2 November 2012 (UTC)

It is possible to calculate the wall surface temperature by using airconditioning formulae. There are formulae for estimating the heat flow via air flow and formulae for direct radiation. You need to estimate the emissivity of the paint surface, and look up the thermal conductivity of the wall - tables for all sorts of walls are published in airconditioning manuals. You will need high school algebra capability to use such formulae.

While you said you intend to only use the stove for 2 to 3 hours a day, for safety you should assume continuous operation with the stove well stoked. From my own experience, I suggest you view paint temperature specs with a GREAT deal of suspicion. In any case, if the wall is any sort of timber product, plasterboard etc, you want to keep it to 50 C or less. Concrete walls 90 C or less.

However, I purchased a wood stove some years ago, and it came with installation instructions which gave the minimum distance from walls. So check with the manufacturer of your stove.

Ratbone 121.215.21.99 (talk) 15:08, 2 November 2012 (UTC)

I suggest you place something between the wood stove and the wall, to protect it, since, even if the paint doesn't scorch, the constant heating and cooling will cause it to crack and peel before the rest of the paint. A chunk of drywall covered with aluminum foil on the side facing the stove should do the job nicely and cheaply. If you want to spend more and make something fancier, you can do that, too. StuRat (talk) 17:06, 2 November 2012 (UTC)

people mover using air pressure

Somewhere in India or near India is a people mover that uses air pressure to push a large box around a circular path. The air boxes are higher up and is attached by an arm to a lower box fills up with riders. The circular route has segments. The air from one is pumped from a forward void into an aft void. Does anyone has a photo of this mass transit system or any other printed information? I think I read about it in the Whole Earth Catalogue back in the 70’s. I've google it extensively with no results. Thanks. Dennis — Preceding unsigned comment added by Jdenwyatt (talk • contribs) 13:54, 2 November 2012 (UTC)

You might be interested to know that some of the earliest subways were pneumatic, like this: Beach Pneumatic Transit. The problem is, it only works for a small distance. StuRat (talk) 16:45, 2 November 2012 (UTC)

Taman Mini Indonesia Indah (TMII, an Indonesian theme park) has an elevated Aeromovel transportation system that is close to the system that you've described, though with the positions of the pumped air and the passenger compartment swapped. In the TMII Aeromovel system, there is a hollow concrete pipe (beneath the elevated tracks) through which air is constantly pumped. The vehicles have large paddles or 'sails' that extend down into this pipe; when the sails are perpendicular to the direction of travel the train is 'blown' along, whereas when the paddles are turned parallel to the airflow the train coasts. I've read about similar systems in the past (I have a vague memory of a Popular Science article from about 20 years ago), but I'm having difficulty locating additional references. TenOfAllTrades(talk) 19:46, 2 November 2012 (UTC)

Although it doesn't address the OP's specific query, the general topic of atmospheric railways might be of background interest. (The poster formerly known as 87.81.230.195} 90.197.66.19 (talk) 13:14, 3 November 2012 (UTC)

Ball point pens and pull-cord light switches

Is there a name for, and do we have an article on, the alternating up/down mechanism found in ball point pens, which I'm assuming is similar to that found in pull-cord light switches? Rojomoke (talk) 14:09, 2 November 2012 (UTC)

Some poking around at google indicates that retractable ballpoint pens use a type of cylindrical cam follower/ratchet mechanism, sometimes called a "Ratchet spring". --Jayron32 14:26, 2 November 2012 (UTC)

Fat soluble medicine

If a medicine is fat soluble, will we absorb more of it taking it with fat or without fat (just water)? 80.58.205.34 (talk) 16:19, 2 November 2012 (UTC)

Most such medicines are advised to be taken with food, read the label and consult with a doctor, pharmacist, or nutritionist. See also, fat-soluble vitamins. (I support re-openning this since it is a general question, not a request for specific individual advice.) μηδείς (talk) 16:38, 2 November 2012 (UTC)

I know that medicines will explain how do you have to take it. But my doubt was rather general. The requirement "take with food" could be motivated by other causes, like to protect the stomach from some medicine. I just wanted to know how it works physiologically, not in a real life scenario. If a medicine gets dissolved in fat, will we absorb more or it or excrete more of it? 80.58.205.34 (talk) 16:42, 2 November 2012 (UTC)

It should help to dissolve it in fat. A normal person's digestive system is quite capable of digesting fat, so there's no problem with it passing out of the body like that. Being more diluted in all that fat would also help to avoid irritating the intestinal lining by being exposed to high concentrations. StuRat (talk) 16:53, 2 November 2012 (UTC)

• This section really does border on giving medical advice. What if the IP changed how he took his medicine according to what was stated here and it had negative effects? IRWolfie- (talk) 16:56, 2 November 2012 (UTC)

The IP is not asking to change anything. if the IP takes a medicine, he will follow the label. 80.58.205.34 (talk) 17:04, 2 November 2012 (UTC)

• If they said they wanted advice on how to take their own meds, then you'd have a point. StuRat (talk) 17:00, 2 November 2012 (UTC)

Yes, the reason is for the presumed fat content of the food, some medicines are also taken with milk. The body preferentially absorbs fat (fat digestion), so unless you are just drinking two quarts of vegetable oil daily, or a eating the un-absorbable Olestra, it should lead to better absorption, not excretion. Of course this is biology so there are all sorts of caveats. You will find most actual pharmacists enjoy discussing this sort of thing (not register clerks) so I would suggest approaching a consultation window if you have questions about any specific medicine. μηδείς (talk) 17:01, 2 November 2012 (UTC)

• It appears that the answer depends on the specific vitamin or drug in question. See [11] where lutein appeared to require fat, but vitamin E didn't (but then again, a preceding study found it did...). It's important to remember in all this that chylomicrons are made, they're not just bubbles of free fat that carry intestinal contents into the blood, which would be bad! So I think the effect is not one where the vitamins hitch along for the ride the whole way - rather, the vitamin has to get in as an individual molecule, and then, if there are more blobs of fat on the far side, it's more likely to stick and make room for the next molecule. So I don't expect super dramatic effects here, unless the biology is being particularly creative (something which it tends to enjoy doing often). There are some fun details, like Xenical can reduce the rate of absorption of some things. Wnt (talk) 17:34, 2 November 2012 (UTC)

Why does a dead body float on water ?

Why does a dead body float on water ? (Not Homework) Thanks! 106.209.2.66 (talk) 18:05, 2 November 2012 (UTC)

Gas builds up in the intestines (mostly, I think), bloating the corpse. There's a specific "floating decay stage" which takes anywhere from a day to a week to begin depending on temperature. [12] Wnt (talk) 18:11, 2 November 2012 (UTC)

A fresh corpse will immediately sink in water[13]. After a certain amount of decomposition the bloating will float the body back up. After all the gas-containing cavity rot enough for the gas to leak out, the body will again sink into the water. A8875 (talk) 21:10, 2 November 2012 (UTC)

Human bodies differ in density, but are generally slightly more dense than water [14], for example, for males. A very fat corpse might be less dense than water and float initially. Air or CO2 in the stomach from consuming fizzy pop could also add to bouyancy. [15]. I have observed that I float with lungs full of air, but sink if I expel sufficient air. Air retained in clothing could also make a fresh corpse float (Which sounds a bit like the world's worst soda fountain confection!) Edison (talk) 23:02, 2 November 2012 (UTC)

But wouldn't a body normally have lungs at least partially full of air ? StuRat (talk) 23:04, 2 November 2012 (UTC)

Once you're dead the diaphragm relaxes, and the lungs deflate. Rojomoke (talk) 23:35, 2 November 2012 (UTC)

Archemedes principle190.56.105.253 (talk) 15:21, 3 November 2012 (UTC)

Is that when he went to take a bath, found a body in his tub, and yelled 'Eureka !' ?" StuRat (talk) 23:33, 3 November 2012 (UTC)

Cell Biology

Hi! Which are the most important discoveries in history of cell biology, important for medicine? Please name 10 to 15. Thank you in advance! --Atacamadesert12 (talk) 19:23, 2 November 2012 (UTC)

As you have been previously informed, we do not do people's homework for them. Please refer to that prior response for the reasons for our position. — TransporterMan (TALK) 21:15, 2 November 2012 (UTC)

I am surprised we don't have an article on Metachlorians. μηδείς (talk) 21:37, 2 November 2012 (UTC)

Midichlorians. Someguy1221 (talk) 21:46, 2 November 2012 (UTC)

Thought I must be misspelling it but one gets quite a few hits on the other. μηδείς (talk) 01:19, 3 November 2012 (UTC)

That would be the citizens of the planet Midichloria. ←Baseball Bugs ^{What's up, Doc?} carrots→ 19:32, 3 November 2012 (UTC)

November 3

Time is an illusion

Back when I was abducted, I got into a conversation with, well lets just call him "Ted". At some point, the subject turned to reality and the meaning of life. Ted started to explain his understanding of Time. He said that it was just an illusion, and that what we were experiencing was our conscious movement through the multiverse. He said that there is an infinite number of static Universes and that the illusion of Time was in fact the changes noted when passing though these static universes. Was Ted pulling my leg, or could this be true? 46.229.161.232 (talk) 01:34, 3 November 2012 (UTC)

Welcome to the Science Reference Desk. Do you need help finding a reference to scientific question? If you need assistance finding a general forum for discussion, please see our article on internet chat room. Nimur (talk) 01:47, 3 November 2012 (UTC)

I'm un-hatnoting this. I see nothing medical involved. Besides, I agree with Ted. :) (Oddly enough, I just linked to my comments in this regard in the "free will" thread on the Humanities Desk) I should disclaim, however, that things beyond the universe tend to be inaccessible to scientific investigation, so this may not be the right subsection for the question. Wnt (talk) 02:26, 3 November 2012 (UTC)

Quoting from Scientific American,

...the true arena of events in a relativistic universe is a big four-dimensional block. Relativity appears to spatialize time: to turn it into merely one more direction within the block. Spacetime is like a loaf of bread that you can slice in different ways, called either “space” or “time” almost arbitrarily.

The concept of time that you describe is one that some physicists believe in, or believe is required according to certain models of how the universe works. However, it's not actually obvious that this interpretation is true, testable, or even meaningful. At this point in...time, it seems like just a philosophical interpretation of physics, and one that's not universally held. Someguy1221 (talk) 01:51, 3 November 2012 (UTC)

While physicists rely on time being real in many of their formulas, one thing that does seem odd to them is the unidirectional movement of time. Nothing in physics limits time in this way, and the 3 other dimensions go both ways. StuRat (talk) 02:19, 3 November 2012 (UTC)

Time is real, but in a couple of months Newsweek will be an illusion. As regards backwards time travel, has anyone found any evidence confirming it? ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:23, 3 November 2012 (UTC)

If the OP is suffering from illusions he needs medical attention, not bullshitting. μηδείς (talk) 02:35, 3 November 2012 (UTC)

Only if he's not making it up. ←Baseball Bugs ^{What's up, Doc?} carrots→ 03:14, 3 November 2012 (UTC)

In other words, feed the birds, not the trolls. :-) 24.23.196.85 (talk) 03:35, 3 November 2012 (UTC)

Per Refdesk guidelines, please don't diagnose the visitors. The notion that a person must live his entire life in strictly the physical world, without ever experiencing a single vision, prophecy, revelation, or paranormal experience, or else be branded as permanently psychiatrically damaged, seems to me to be an extreme excess of materialism. Wnt (talk) 03:55, 3 November 2012 (UTC)

I much prefer this version of feed the birds ;-) Dmcq (talk) 10:35, 3 November 2012 (UTC)

Clearly a troll question, or at best a question with no good RD answer. Let's stop feeding please. Shadowjams (talk) 04:38, 3 November 2012Comment moved here after unhatting by Someguy1221 (talk) 04:53, 3 November 2012 (UTC)

Time is an illusion, in the sense that the traditional interpretation of time that the past and future are not real and only the present moment is real, is wrong (not just in relativistic physics, also in classical mechanics and quantum mechanics). The burden of proof is on those who believe that time does exist to prove this, not the other way around. If something does not exist, then there is nothing to prove.

Any set of laws of physics where information is conserved will have this property. If the information about the past exists in the future without any of that getting lost, then the past exists inside the future state. E.g., in principle you can measure who wins the US elections. The practical problem with performing such a measurement is that it involves a complicated interaction with every particle located in a sphere of about 4 lightdays radius. But the known laws of physics allow you to perform that measurement, which will collapse the wavefunction of the World into that state that under the time evolution will evolve to the state consistent with your measurement. Count Iblis (talk) 04:52, 3 November 2012 (UTC)

Time is an illusion; lunchtime doubly so. 82.131.132.190 (talk) 22:45, 3 November 2012 (UTC)

A tidal dam for New York?

We have seen lately how vulnerable New York is to these types of hurricanes, especially thanks to global warming and sea-level rise.

I hope those subways and tunnels will get pumped out and operational again.

Especially since the hurricane, have the officials for the city of New York considered tidal dams to protect the city? London already has the Thames River Barrier so what obstacles must New York surmount in order to install such barriers of their own?

1. Do we have an article on such concepts?

2. How much would such a barrier be estimated to cost?

3. How big / what type of political obstacles are there?

4. What other obstacles and considerations would they need to evaluate and overcome in order to get the tidal barrier installed around the city?

Good golly, I really hope plans for such barriers are being put forth in city board meetings these days because we can't expect these storms to get any gentler in the future. The sea-level rise coupled with land subsidence due to heavy skyscrapers and etc. will make such sea barriers imperative for the city's future. --Let Us Update Wikipedia: Dusty Articles 07:35, 3 November 2012 (UTC)

It might help in the short run, but, eventually, any such barrier will suffer a catastrophic failure as sea levels rise and storms become more severe. So, it might result in a false sense of security, ultimately leading to more deaths, similar to the levee failures in New Orleans when Hurricane Katrina hit. A more sensible approach, IMHO, is to use any funds slated for a barrier to rebuild on higher ground, further inland, as anything that flooded this week will surely flood again, barrier or no barrier. StuRat (talk) 07:41, 3 November 2012 (UTC)

The Geography of New York City is very different from that of London. Where are you proposing to place your barrier(s)?--Shantavira|^{feed me} 08:03, 3 November 2012 (UTC)

There isn't much subsidence risk in NYC. London is built on clay, which is good for tunnelling, but only moderately good for skyscrapers. New York, on the other hand, is built on granite. This is why its underground network is so much shallower than London's, but it can support awesome skyscrapers. Even PATH, which is much deeper than most of the subway, is only as deep as the foundations of the WTC.

As for a barrier, well, one could put one in the Narrows, but that wouldn't protect Staten Island, southern Brooklyn, or Rockaway. If you put a much longer barrier between Sandy Hook and Rockaway, that might well worsen the flooding risk to Rockaway and coastal New Jersey, while costing a huge amount for something that's not used often. Obviously if the frequency of such storms increases, this might have to be reconsidered. AlexTiefling (talk) 09:56, 3 November 2012 (UTC)

Another solution is to let water flow freely. That has worked for Venice for ages. Comploose (talk) 13:24, 3 November 2012 (UTC)

No, it hasn't. See Venice#History and MOSE Project. Rmhermen (talk) 13:59, 3 November 2012 (UTC)

In fact, they had some serious flooding just this past week.[16] ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:50, 3 November 2012 (UTC)

See here. And along the coast line where you don't have harbors, you can build dikes in the sea like this, so that you still have a beach. Count Iblis (talk) 16:28, 3 November 2012 (UTC)

There was an interesting article in GSA Today just last month on optimizing mitigation. See this. Zoonoses (talk) 02:28, 4 November 2012 (UTC)

The best enzymes for ligating DNA:

The best enzymes for ligating DNA

Select one:

a. come from humans, as they are compatible with the most complex types of DNA.

b. come from bacteriophages, as they don't need to be incubated at a specific temperature to work.

c. come from thermophilic bacteria, as they need to survive thermocycling.

d. come from retroviruses, as they integrate their own DNA into a host's genome most efficiently.

(a) seems unlikely, but doesn't this seems like a subjective question anyway? 130.56.94.164 (talk) 13:17, 3 November 2012 (UTC)

(a) is clearly the correct answer, as humans are the most evolved species living on earth. Except for those who try having others do their homework. 77.3.129.83 (talk) 13:53, 3 November 2012 (UTC)

I don't know the answer, but our article on DNA ligase indicates that the most widely used type for lab work comes from bacteriophage T4. Looie496 (talk) 16:27, 3 November 2012 (UTC)

How would you do this?

You are given a plasmid with Ampicillin resistance, and a recombinant GFP gene engineered to only be expressed in the presence of lactose. Explain how you would get this vector into E. coli and screen for successfully transformed cells without using UV light. --130.56.94.164 (talk) 13:27, 3 November 2012 (UTC)

Not only does this question read as a homework question copied out, and we are not here to do your homework for you, it is impolite to post a question without giving a name. A psuedonym will do, but please assign yourself a name. Wickwack 60.230.220.27 (talk) 15:07, 3 November 2012 (UTC)

Is a name really a requirement here? It looks like new rules are generated every minute. But OK, I'll sign WickedWhack 77.3.129.83 (talk) 15:18, 3 November 2012 (UTC)

As far as I am aware there has never been a requirement for an IP to invent a name for the purposes of asking a question at the RefDesk. IP editors are just as welcome here as account holders. We won't do your homework for you, but you don't have to jump through any extra hoops in order to ask, and you were not being impolite. - Karenjc 19:05, 3 November 2012 (UTC)

We have articles on Ampicillin, I have made a redirect for Ampicillin resistance (which goes to a stub which you can expand given the inclination), and we also have articles on Beta-lactamase, there is a MeSH code: MeSH G12.392.269.347.500.600.050, pComb3H, pGLO, plasmid, Green fluorescent protein (for GFP) and Escherichia coli (molecular biology). Graeme Bartlett (talk) 04:09, 4 November 2012 (UTC)

Inoperability of back-of-neck skin?

"As he aged, starting in the late 1970s, Kim developed a growth on the right-back of his neck which was a calcium deposit. Its close location near his brain and spinal cord made it inoperable. Because of its unappealing nature, North Korean reporters and photographers, from then on, always shot and filmed Kim while standing from his same slight-left angle to hide the growth from official photographs and newsreels, which became an increasingly difficult task as the growth reached the size of a baseball by the late 1980s."

I'm missing something here. Why couldn't they just operate on it like any other skin condition? Nyttend (talk) 13:42, 3 November 2012 (UTC)

I'm not quite clear what is meant by a "calcium deposit" but I suggest that the reason they give is not the real reason, (North Korea has some history in this department) Maybe he had a surgery phobia. It is possible he had one of these (scroll down a bit). By coincidence the first reference at the bottom of the page is from four Korean medics, so they are possibly common in Korea. Richard Avery (talk) 15:24, 3 November 2012 (UTC)

Are there any pictures? Given his obesity and habits it could also have been a gout tophus, although I think they are usually associated with joints. μηδείς (talk) 17:58, 3 November 2012 (UTC)

Sounds like an odd place for one, and when it is so far advanced as to lead to visible external tophi, gout is not a hard condition to diagnose or treat. I assume Kim had access to some kind of medical care. Wnt (talk) 13:36, 4 November 2012 (UTC)

Ko's

How come in a boxing or mma fight, when people who get hit in the exact same spot get knocked out, some of them go limp, and some of them get stiff and rigid? Shoulden't the body react the same if they are hit in the same spot? --Wrk678 (talk) 19:57, 3 November 2012 (UTC)

I don't see why the body would react the same. The damage done could be different, depending on the force of the impact, tiredness, or on previous injuries. Comploose (talk) 20:00, 3 November 2012 (UTC)

I have a semi precious stone and I want to know what it is.

I have a stone that is light blue that glows orange in the light. Can anyone tell me what it is? — Preceding unsigned comment added by 216.36.16.221 (talk) 20:36, 3 November 2012 (UTC)

I don't know, but if you can post a picture it may aid others. Falconus^{p t c} 20:43, 3 November 2012 (UTC)

Could you be a little clearer about what kind of light you are using to see the glow? Is it daylight, ultraviolet light, or light from an incandescent bulb or a fluorescent tube? - Karenjc 21:56, 3 November 2012 (UTC)

Sounds like fluorite to me, but it doesn't have to be. See what you think from the article. Wnt (talk) 22:08, 3 November 2012 (UTC)

I'm not absolutely sure that this is what the OP is talking about, but he or she may want to look at the lists in our pleochroism article. Deor (talk) 12:23, 4 November 2012 (UTC)

What is it the

As much as I try to understand what is it an electric charge, I can't understand it at all. What is it this charge? Is it a product or an effect of the magnet? מוטיבציה (talk) 21:07, 3 November 2012 (UTC)

It is a property of a particle, there are two kinds. Two electric charges of the same kind will push appart, two of the different kind will draw near. Unlike magnetism, the kinds electric charge can be found appart from one another. Plasmic Physics (talk) 22:34, 3 November 2012 (UTC)

Magnetism is caused by moving electric charge, so a magnet is an effect of charge not the other way around. As User:Plasmic Physics says electric charge is a property of some elementary particles. In most every day macro occurrences something that is negatively charged has an excess of electrons. Something that has positive charge has a lack of electrons (again I'm talking macro objects, this is due to protons, which have positive charge, being immobile in objects and electron being able to move). But there is only one type of electric charge (and it's anti charge) unlike colour charge which has 3 different types of charge (and there anti-charges).Dja1979 (talk) 01:04, 4 November 2012 (UTC)

Electric charge is something that causes an electric field. Electric charge is a property intrinsic to certain particles such as electrons, protons, and their antimatter equivalents. A moving electric field creates a magnetic field; conversly, a moving magnetic field creates an electric field. Without getting into the mathematics of the subject, there's not much point in trying to take it further than that. There are things that man does not have a visuallizable understanding of, and this is one of them. Physicists and engineers have long worked out mathematical models that verify and predict practical measurements. Once you have become accustomed to using the math, you become comfortable and confident with it, without ever really understanding the actual basis of the phenomena. Ratbone 124.178.138.168 (talk) 01:59, 4 November 2012 (UTC)

Thank you all for help. מוטיבציה (talk) 21:24, 4 November 2012 (UTC)

Gravitational potential energy

Start with two masses m1 and m2 at rest, separated by a distance r, and then release them.

At any subsequent (non-pathological) point, should the sum of the masses' kinetic energy (= 1/2*m1*v1^2 + 1/2*m2*v2^2) equal the change in potential energy, where potential energy is calculated using the forumla -G*m1*m2/r^2? 86.128.3.213 (talk) 21:47, 3 November 2012 (UTC)

The question appears to be asking if the sum of kinetic and potential energy in the system is conserved. In an ideal Spherical cow system, where there is no intervening gas or fluid and no electromagnetic effects, the sum of potential and kinetic energy should not increase or decrease. If the sum of potential and kinetic energy increased, we would seem to have a fine perpetual motion energy production device. The question does not specify that the masses are in a vacuum. If the total energy decreased, where did it go? If I release them in a zero G chamber filled with a gas or a fluid, they will do work as they converge and the sum of kinetic and potential energy at a subsequent point before impact would be decreased by the work done on the medium, which would heat the medium. Edison (talk) 22:55, 3 November 2012 (UTC)

Thanks, yes, I am aware of the principle of conservation of energy. My question is whether the numbers calculated by the above formulas ought to match, because in simulations I cannot get them to match. I wonder if there is some flaw in the reasoning of using those formulas in the obvious way, not whether there is some flaw in the principle of conservation of energy. You may assume no loss of energy due to other factors. The other possiblity is that I may be making a silly error that I cannot spot. Sorry if that was unclear. 86.128.3.213 (talk) 23:06, 3 November 2012 (UTC)

Yes, the numbers should add up. If they don't, please post your work, and we will see if we can find the error. StuRat (talk) 23:26, 3 November 2012 (UTC)

The program I am using is below. I am dumping it without a great deal of explanation, but I believe its operation should be fairly obvious to anyone who has a chance of finding the error which I have been unable to see. I do hope this isn't just some silly finger trouble.

G = 1.23 'arbitrary value; shouldn't matter
dt = 0.00001

m1 = 1
m2 = 10

'initial positions and velocities
x1_0 = 0
v1_0 = 0
x2_0 = 10
v2_0 = 0

'initial energies
ke_0 = 1 / 2 * m1 * v1_0 ^ 2 + 1 / 2 * m2 * v2_0 ^ 2
pe_0 = -G * m1 * m2 / (x1_0 - x2_0) ^ 2

'initialise variables
x1 = x1_0
v1 = v1_0
x2 = x2_0
v2 = v2_0
t = 0

Do

    r = Abs(x1 - x2)

    'calculate energies
    ke = 1 / 2 * m1 * v1 ^ 2 + 1 / 2 * m2 * v2 ^ 2
    pe = -G * m1 * m2 / r ^ 2
    dke = ke - ke_0
    dpe = pe - pe_0
    
'-----------------------------------------------------------
' at this point dke + dpe should equal zero (to reasonable accuracy), but it doesn't
'-----------------------------------------------------------

    'move one timestep
    t = t + dt
    F = G * m1 * m2 / r ^ 2
        
    a1 = F / m1 * Sgn(x2 - x1)
    v1 = v1 + a1 * dt
    x1 = x1 + v1 * dt
    
    a2 = F / m2 * Sgn(x1 - x2)
    v2 = v2 + a2 * dt
    x2 = x2 + v2 * dt
    
Loop

86.128.3.213 (talk) 02:22, 4 November 2012 (UTC)

This is expected, because you are calculating a solution using Euler's method. While this seems to be a simple and correct approach, it is well-known (amongst computational physicists, anyway) that Euler's method, applied to the formula for gravitation, propagates error, amplifying tiny calculation errors until they are exponentially larger than the quantities under consideration. Hold on, I'll dig up link to a page in our archives when I last discussed this topic. Nimur (talk) 11:57, 4 November 2012 (UTC)

Thank you for looking at this. I must admit that I am initially sceptical that the level of discrepancy I am seeing is due to numerical artefacts of the discrete approximation method. However, I know from seeing previous replies of yours at the ref desk that you are normally very knowledgeable, so I may have to stand corrected. 81.159.107.19 (talk) 12:07, 4 November 2012 (UTC)

Thank you for the compliment. But don't take my advice as authority: verify for yourself! Unfortunately, my link from the 2009 discussion is no longer active; the course website is now handwritten lecture notes with only a passing mention of Euler's method. There used to be an interactive applet, a breakdown of analytic and numerical sources of error, and all specifically for gravitation! No worries, there are other examples on the web, and I'll find some. And, all hope is not lost for your simulation: you'll just need to implement a different calculation method, such as a Runge-Kutta solver, that is appropriate for stiff equations like gravitation. The problem with numerical error isn't the roundoff or floating-point inaccuracy in any single calculation: it's the error that propagates from one step to the next. Each iteration of your loop amplifies a small deviation, and before long, you're very far from a reasonable solution. Let me keep looking for a suitable example on the web. Nimur (talk) 12:31, 4 November 2012 (UTC)

One of the reasons I am surprised is that I have used an essentially identical method to plot orbits, and it seemed to work very well (in terms of producing a sensible-appearing elliptic orbit). I have also used it to simulate n-body problems with visually sensible-looking results. See http://img834.imageshack.us/img834/5673/nbody.gif . 81.159.107.19 (talk) 12:50, 4 November 2012 (UTC)

Forget all the responses above, or at least, forget them until you read this one. You are calculating gravitational potential energy incorrectly. It is -G * m1 * m2 / r, not -G * m1 * m2 / r ^ 2; you're off by a factor of r. --140.180.252.244 (talk) 00:21, 5 November 2012 (UTC)

Good eye! The above post is correct. Fix that error, and then we can consider other issues like numerical accuracy. Nimur (talk) 02:45, 5 November 2012 (UTC)

Maximum Age for Becoming a Father

Is there a maximum age at which a man can become a father like there is for women/mothers? Futurist110 (talk) 22:25, 3 November 2012 (UTC)

No, not quite, but they do have a "best before" date. Plasmic Physics (talk) 22:37, 3 November 2012 (UTC)

So a man aged 115 could theoretically naturally become a father if he wanted to (and had the energy to have sex)? And what is the "best before" age? I know that it has to do with the condition of sperm deteriorating over time, but I don't know the exact age. Futurist110 (talk) 22:40, 3 November 2012 (UTC)

I doubt there is any "exact age". It would probably be a continous curve, which would in any case probably vary significantly from person to person. 86.128.3.213 (talk) 23:08, 3 November 2012 (UTC)

This page outlines some of the risks associated with being an older father. See also Man is 'world's oldest dad at 94'. Alansplodge (talk) 23:46, 3 November 2012 (UTC)

Thank you. Based on the info in your link, it would thus appear that someone like Jiroemon Kimura could still have another child if he had the energy to have sex right now, but that the odds of this would be pretty low due to sperm deterioration over time. I wonder if someone aged 95+ ever tried having a child or not? Futurist110 (talk) 23:51, 4 November 2012 (UTC)

One risk is confusing the baby's diapers with your own. :-) StuRat (talk) 00:33, 4 November 2012 (UTC)

Sperm don't sit around like eggs. They are produced and degrade regularly. If oldsters can still produce them they will be new sperm. μηδείς (talk) 03:52, 5 November 2012 (UTC)

November 4

Syria Rebels

I came across this YouTube video in which the Syrian rebels successfully attack and capture an army outpost, I am confused how they managed to do this because in similar situations in which the US and its allies encounter almost daily in Afghanistan, insurgents will try to attack army bases and the army will call in attack helicopters, which turn on their infrared cameras and can easily see the insurgents even in the day time and kill them with their 30 mm machine gun, or Rockets, Syria has some fairly advanced attack helicopters from Russia. I don't see why they can't do the same. Here is the video:

http://www.youtube.com/watch?v=IC8ENtjVp3I

--Wrk678 (talk) 00:03, 4 November 2012 (UTC)

Not really a science question, but there are several possibilities:

1) Attack is too rapid to allow such a response. Air responses take time, and the base must be able to hold out until then.

2) Communications are jammed or cut, so they can't call for air support.

3) Syrian outposts aren't designed to withstand attacks from all sides. Many outposts are designed just to defend against one type of attack. For example, posts on the border may be set up only to defend against cross-border attacks.

4) The Syrian air force is simply overwhelmed, and can't respond as quickly as is needed. Lack of spare parts and maintenance issues from constant use can have such an effect.

5) Multiple coordinated attacks stretch the Syrian air force too thin to respond.

Also note that often those bases are retaken by the Syrian military. In many cases, the rebels don't even attempt to hold them, they just raid them and then scatter. StuRat (talk) 00:29, 4 November 2012 (UTC)

Well, it looks like the attack lasts several hours, starting during the day and ending at dusk. Syria is a pretty small country so I think they could get their in time. I doubt the rebels have any kind of sophisticated jamming equipment that could jam military radios.--Wrk678 (talk) 09:35, 4 November 2012 (UTC)

Your premise that attacks on military posts in Afghanistan are always quickly suppressed by air attack may not be the whole truth. See 8 U.S. troops killed in Afghanistan, Militant attacks on two remote outposts also leave as many as half a dozen Afghan troops dead.; "It was precisely the kind of attack the top U.S. commander in Afghanistan is hoping to stave off by recently ordering troops to withdraw from such small outposts...". Also the Battle of Wanat and Siege of Sangin. None of these attacks were ultimately successful, but it was sometimes a "damned near-run thing"; although as yiou say, air power was used in a suppressive role in all cases, it's often hard fighting on the ground that wins the day. The training and equipment of the Syrian Army may well not be up to the standard of professional NATO troops. Alansplodge (talk) 10:07, 4 November 2012 (UTC)

Doing a quick search I find [17] "In recent days it has appeared that Syrian rebels have acquired heavy weapons that have forced the government's air force to bomb rebel-held zones from higher altitude, leading to claims of more indiscriminate bombing." The video shows a helicopter spiraling down and exploding in a ball of fire. Wnt (talk) 13:32, 4 November 2012 (UTC)

This outpost was a minor one possibly. I doubt they send even one aircraft to all fighting that is taking place. The Syrian_Air_Force#Aircraft_inventory Syrian Air Force has its limitations. What's the point of risking a crown jewel like an attack helicopter to save just half a dozen of trapped soldiers? Comploose (talk) 19:42, 4 November 2012 (UTC)

Well in Afghanistan fighting on the ground can stave off the assault but it's usually when the attack helicopters come that the insurgents will retreat. Usually there is a "Alamo" call that is given when the base is starting to become overrun that brings all airborne helicopters to the area.

Even though this was a small army base its capture involved the capture and probably the execution of several dozen or more Syrian army soldiers, so I would think the government would prioritize that and send attack helicopters over there. If you watch the end of the video after they capture the base, They hang around there for at least several hours afterwards and nobody shows up, if, for example, it was a priority issue, they probably would have shown up "late" a hour or so after the attack, which they did not.

The thing is I have watched numerous videos of these Syrian rebel attacks, many of them last for hours or more and it doesn't appear that any Syrian attack helicopters usually show up. This makes me wonder if in fact the attack helicopters the Syrians use do in fact have infrared cameras in them. Does anyone have any information on this? --Wrk678 (talk) 21:28, 4 November 2012 (UTC)

Attack helicopters are not an "I win" button, same as tanks. A chopper or a tank will rarely be sent to enter an engagement with no support. To be effective there has to be a co-ordinated attack. All it takes is one enemy soldier with a cold war era anti-air or a anti-tank weapon to be a serious threat to several million dollars worth of military hardware. With no ground support or good reconnaissance, sending a lone chopper might just be too risky. As is noted above, at least one chopper has already been lost and if I'm not mistaken there's been a few. Vespine (talk) 22:11, 4 November 2012 (UTC)

Well, it's pretty standard procedure to send attack helicopters to bases being attacked, regardless of anti-air weapons. They had those in Libya and Afghanistan too.--Wrk678 (talk) 22:55, 4 November 2012 (UTC)

That's an assumption on your part. I see no reason to believe it's "standard procedure". You don't know all the details, as I said, in the other circumstances, maybe there was ground support or reconnaissance available nearby, maybe the threat of anti-air was lower, maybe the pilots were more highly trained; there's just way too many factors involved to say something like "it's standard procedure". Warfare strategy is a very complex topic, it's not just a list of "standard procedures". Vespine (talk) 01:34, 5 November 2012 (UTC)

I disagree with that, military tactics are fairly universal. --Wrk678 (talk) 01:37, 5 November 2012 (UTC)

Well, this very scenario seems to disagree with your evaluation. There was a reason why they didn't send attack choppers, I suggested a few explanations, i'm not sure what other answer you would find more satisfactory. Short of asking the "people in charge", which no doubt would not tell you anyway.. I'm not saying that there aren't universal rules of engagement, I was trying to show that there would be lots of exceptions and finer details which civilains would not be privy to. Vespine (talk) 02:34, 5 November 2012 (UTC)

Well, I'm starting to suspect their helicopters are not equipped with infrared cameras, in which case they wouldn't be of much use in that scenario.--Wrk678 (talk) 05:47, 5 November 2012 (UTC)

Oxidative stress and immune system

Hi all,

Can someone tell me in which treatise (hopefully up-to-date) can I find something about general relationships between the oxidative stress and the cells of the immune system? Thank you very much!

151.71.165.245 (talk) 10:12, 4 November 2012 (UTC)

This is a very general question, almost too general to answer. (Do you mean the effect of oxidative stress on invading cells, for example, and what level of review do you want?) Get to know http://www.ncbi.nlm.nih.gov/pubmed for making general queries and narrowing them down; http://scholar.google.com has become a very serious alternative. Leaf through oxidative stress, phagocytes including neutrophil, monocyte, and macrophage, phagocytosis, myeloperoxidase, reactive oxygen species, hydrogen peroxide, hypochlorite... it's no small topic, yet one key idea is very straightforward: the immune system recognizes invaders and puts them in special bins where they are bleached to death (and otherwise digested). Wnt (talk) 13:25, 4 November 2012 (UTC)

Black holes at the speed of light

Hi, I have a sort of simple question. If you were to accelerate into a black hole would you travel at the speed of light as you cross the event horizon?

Or if you accelerated in at an angle, so your horizontal component of velocity would be say 100mph and your vertical component would be the increasing velocity due to the acceleration due to gravity of the black hole, could you even hit the speed of light before you hit the event horizon?

(This is assuming you have infinite fuel and are not crushed by the G-force and gravity) — Preceding unsigned comment added by 31.54.166.196 (talk) 14:51, 4 November 2012 (UTC)

From special relativity no particle with a rest mass can travel at the speed of light. So in answer to your question no you would not be travelling at the speed of light when you cross the event horizon.Dja1979 (talk) 15:58, 4 November 2012 (UTC)

What would your speed be as you crossed the event horizon? — Preceding unsigned comment added by 86.146.165.159 (talk) 16:17, 4 November 2012 (UTC)

For an outside observer your speed would be (surprisingly) equal to zero because of gravitational time dilation. Dauto (talk) 16:30, 4 November 2012 (UTC)

For this reason, it has been argued that black holes (as seen from outside) can never truly exist because they take forever to form! —Tamfang (talk) 20:12, 4 November 2012 (UTC)

In fact, any observer crossing the event horizon will travel exactly with the speed of light as measured in his own time. Ruslik_Zero 19:13, 4 November 2012 (UTC)

Is this so? From special relativity, we know that any thing in the universe, that has a rest mass, cannot travel at the speed of light. Thinking about it more, maybe the event horizon is outside the universe (i.e. it doesn't interact with the rest of the universe, it is inside the black hole), so special relativity doesn't hold.Dja1979 (talk) 20:27, 4 November 2012 (UTC)

So you would travel at the speed of light then? Also how long would it appear to the outside observer that you freeze for? — Preceding unsigned comment added by 86.146.167.230 (talk) 20:08, 4 November 2012 (UTC)

If an observer sees you freeze, then you never actually make it over in their reference frame, so the answer is forever. However the observation will be redshifted to extreme levels as well, with lower energy photons, and less of them, so eventually nothing will be observed. Graeme Bartlett (talk) 20:15, 4 November 2012 (UTC)

As time speeds up relative to how fast you are travelling, does time stop for the person at the event horizon travelling at the speed of light? — Preceding unsigned comment added by 86.146.167.230 (talk) 20:23, 4 November 2012 (UTC)

Black hole#Event horizon says

According to his own clock, [an observer falling into a black hole] crosses the event horizon after a finite time, although he is unable to determine exactly when he crosses it, as it is impossible to determine the location of the event horizon from local observations.

But Event horizon#Interacting with an event horizon says

An observer crossing a black hole event horizon can calculate the moment they have crossed it, but will not actually see or feel anything special happen at that moment.

"Unable to determine exactly when he crosses it" versus "can calculate the moment they have crossed it" is contradictory. Does anyone know which is right?

A related question: According to those articles someone inside the event horizon can see someone else who previously entered on the same path. Does the previous entrant appear to accelerate to infinite velocity or decelerate to zero velocity? Duoduoduo (talk) 23:49, 4 November 2012 (UTC)

The point is that the observer is unable to determine the location of the horizon with a local experiment but can calculate it's location if he has full knowledge of the black hole's mass, location, etc. For instance, we could at this very moment be crossing the horizon of a black hole so large that it's center is outside of the observable universe and therefore unknowable. There is no experimental test that we could use to find out whether that's true or not. BTW the guy falling in the black hole is not moving at the speed of light for any local observer as stated above by some people. That's just wrong. Dauto (talk) 00:01, 5 November 2012 (UTC)

So why is the nootropic market kind of small?

Why are students not tested for doping? Is there any genius known for taking nootropics. I know that the mathematician Erdos was known for taking amphetamines, and some artists do not hide their past consume of drugs (although art is an entirely different field, you can fuck up with your brain and still be among the best in the field). Comploose (talk) 20:04, 4 November 2012 (UTC)

At the moment, there are no prohibitions against taking "cognitive enhancers", even for standardized testing. If there are no prohibitions, there is no testing. If, at some point, the use of these is banned — a big if, as it is a complicated issue of performance, mental health, doctor's recommendations, and so on — then presumably testing will have to become part of any effective enforcement regime. --Mr.98 (talk) 20:47, 4 November 2012 (UTC)

Mathematicians and artists do not really compete with each other as directly as people do in sports, that's why I think perhaps our society doesn't see cognitive enhancement in as bad a light as physical enhancement. Whatever competition there is amongst artists and scientists, it's not the "ends" of those pursuits. Sure you compete indirectly, but you compete indirectly by being born. In competitive sport on the other hand, winning against your opponents IS why you play the sport in the 1st place. I suppose students at one stage will compete for university places and scholarships, but you don't get professional "test takers" who make a living just by taking tests. Vespine (talk) 22:00, 4 November 2012 (UTC)

Nootropics are generally debatable. Why would there be a way for a drug to improve on intelligence, that nature doesn't already take advantage of? I suppose that if they increased calorie expenditures or something it would be conceivable ... still, what we see are more often things like modafinil. Tell me ... if much of sleep can be conveniently avoided with a little pill, why does every animal species from fruit flies to man go through the process, despite obvious vulnerability to predation and decreased opportunity for food gathering? Wnt (talk) 22:06, 4 November 2012 (UTC)

Isn't that appealing to nature? The same argument could be made against the bifocals that I'm wearing right now: "Why would there be a way to compensate for myopia, that nature doesn't already take advantage of?". I concur with your overall stance, I just think a different argument would be more convincing.A8875 (talk) 22:34, 4 November 2012 (UTC)

When we are awake we are on natural stimulants. The reason why animals sleep is due to competetion. If you have some machine that has to repair itself that is performing at some level, then you can put it on stimulants and let it perform better at the expense of its repair capacity. So you need to balance that by having a downtime for repair. Also, performing repairs in downtime is more efficient. Count Iblis (talk) 23:02, 4 November 2012 (UTC)

I see this less as appealing to nature as appealing to evolution. Evolution, however, is notably poorly tuned for ages that people rarely reached in primitive times, or for activities like reading that were not important many millennia ago, hence the bifocals. Wnt (talk) 01:27, 5 November 2012 (UTC)

The analogous argument could be made about studying: "Evolution is poorly tuned for activities like memorizing large amounts of information.".A8875 (talk) 03:21, 5 November 2012 (UTC)

(after ec) Add to this that the enhancing effect of drugs, any drugs, is rather dubious. That's no drug that will make you wiser. Some will make you kind of restless, reduce your fatigue, keep you awake and so on. But, in my personal experience, people who use them are the kind who need to spend the night awake before exams or people who have mental problems and need to function normally. These groups are not really skewing things to their benefit. OsmanRF34 (talk) 22:12, 4 November 2012 (UTC)

Ritalin lengthens attention span, and as any student knows, not being distracted while studying is the most important factor in doing well. Some of that is due to the placebo effect, but there's a statistically significant portion that isn't. Testing students for doping is infeasible because students don't confine their studying to a single 1-minute competition every month or year; they study 6 days a week, for half of their waking lives. --140.180.252.244 (talk) 00:14, 5 November 2012 (UTC)

• Come to think of it, a main reason why nootropics are not yet popular is that there has been little effort to wage a "war" on them. (Unless you count methamphetamine, which has much in common with Ritalin, if that counts) I think if we had a major DEA push to put people selling modafinil in jail for twenty years, the merchants with good police connections would make a huge profit, giving them a motive to hype it as a "hip" thing in the media and on the street; before long people would be coming up with super-potent addictive analogs to take by injection and we'd be hearing stories of people going without sleep for six months until their brains turn to tapioca. After that a multibillion-dollar trade in it would never end. Wnt (talk) 01:27, 5 November 2012 (UTC)

engine diagram

Hi : Looking for a engine diagram and info.for a toyota avalon xl 2006 .

                 Thank You  — Preceding unsigned comment added by Carol9905 (talk • contribs) 20:28, 4 November 2012 (UTC) 

Confusion about how telescopes work

I don't really get how a Newtonian telescope works.

Rays of light reflecting off the object we're attempting to view enter into the telescope tube effectively parallel because of its huge distance from us. The rays then reflect off of a parabolic mirror, which focuses all the rays to a point. At least it would, were it not for another mirror obliquely oriented, which reflects these rays to the telescope's eyepiece, which magnifies the image.

Here's my problem: if all the rays are focused to the same point after reflecting off the parabolic mirror, how can we distinguish between different points on the object? Wouldn't the image of the object just be a point? 65.92.7.202 (talk) 22:25, 4 November 2012 (UTC)

All parallel rays focus to the same point. Non-parallel rays do not. If you imagine 100 rays travelling perpendicular to the mirror's axis, they focus at a certain point on the focal plane. If you imagine 100 rays travelling at an angle of alpha to the perpendicular, those rays focus to a different point on the focal plane. The distance between these 2 focal points is proportional to alpha. --140.180.252.244 (talk) 00:04, 5 November 2012 (UTC)

Even your eyeball focuses light into a point, but unless there's something wrong with your eye, the point isn't on your retina. Maybe an image like this will help. I think a similar thing happens in the telescope. Vespine (talk) 05:06, 5 November 2012 (UTC)

Light slow motion

Is this real http://www.youtube.com/watch?v=SoHeWgLvlXI&feature=related. If it is I want to see the effects of relativity. I'm sick of hearing "oh yea relativity has been experimentally verified countless times" I want to see it with naked eyes. Why didn't they do that? Money is tight (talk) 22:42, 4 November 2012 (UTC)

I don't think that clip shows anything not explicable by classical electrodynamics; you see a light wave propagating, which is impressive, but not so much related to relativity. The easiest way to observe relativity with your eyes would probably be like they did it in 1919. You'd need to observe the stars around the sun (with a telescope ofc) during a total solar eclipse, and then compare the positions of the same stars with a picture you took at night when there was no eclipse. You should see the effect of the sun bending the light of the stars. - Lindert (talk) 23:13, 4 November 2012 (UTC)

The most dramatic visual demonstration of relativity is probably the Einstein ring. That article has lots of photos. --140.180.252.244 (talk) 00:00, 5 November 2012 (UTC)

Einstein's prediction before the eclipse was not that gravity would bend light – that was predicted classically – but how much. An Einstein ring doesn't tell us anything about general relativity unless we know the mass of the galaxy in front — or does it? —Tamfang (talk) 00:10, 5 November 2012 (UTC)

November 5

Impulse (physics)

In this article, it states there are 2 different units. I don't understand how those 2 different units can end up expressing the same thing in the end?174.20.101.190 (talk) 01:21, 5 November 2012 (UTC)

They are equivalent: ${\rm {N\cdot s=kg~{\frac {m}{s^{2}}}\cdot s=kg~{\frac {m}{s}}}}$A8875 (talk) 01:50, 5 November 2012 (UTC)

And, they help remind physicists about the two commonplace ways we calculate impulse: the impulse of an event is often computed by multiplying the net force by the duration of the event; or, the impulse of an event is calculated by multiplying a mass by its total change in velocity. Nimur (talk) 03:07, 5 November 2012 (UTC)

Mechanism from DNAl to proteins etc?

Is the mechanism between DNA and actual proteins etc that are created mapped out?, such that one can use the DNA to calculate (dry run a cell) how the produced protein etc looks like? Electron9 (talk) 03:58, 5 November 2012 (UTC)

See Transcription (genetics), Translation (genetics), Genetic code, Protein structure prediction. It's probably best for you to make a quick first pass through these articles and say what parts of the topic still confuse you after that. Wnt (talk) 04:16, 5 November 2012 (UTC)

Yes, specifically genetic code. The genetic code is the recipe for translating from sequences of DNA bases to the sequences of amino acids that make up proteins. It is astonishingly simple. If you know the code, you don't have to know anything about the mechanism that implements it in order to predict the result. (Well, there are a few complications, but they can be ignored on a first pass.) Looie496 (talk) 05:12, 5 November 2012 (UTC)