Science desk
< May 7	<< Apr \| May \| Jun >>	May 9 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

May 8

Do spherical mirrors produce a sphere inversion?

I've been puzzling at this today... and keep running up against my mental limitations.

Do spherical mirrors produce a sphere inversion? Namely does $(r-p)(r+i)=rr$ ?

By sphere inversion I mean a three dimensional generalisation of a circle inversion like in this video of the inside of the outside of a horse.

I tried to solve the equation with $1/p+1/i=2/r$ but got something absurd for an answer. I'm still so confused about optics.

Yet looking at lensmaker's equation suggests that I'm not entirely incorrect since I see this bugger: $x_{1}x_{2}=f^{2},\!$

Could somebody please help clear up my confused thinking?

-Craig Pemberton 04:48, 8 May 2011 (UTC)[reply]

The equation you should be trying to get to is

(r+p)(r+i)=rr

(remember that i is negative for a virtual image). It would be really cool if that worked, but alas, the answer to your question is no. It's possible to get to the equation

(p-f)(i-f)=ff

, though. Dauto (talk) 13:04, 8 May 2011 (UTC)[reply]

With

2f=r

, it is in a sense, a sphere inversion with the central point replaced with smaller sphere nested inside the mirror of half the mirror's radius. I'm not sure if such a thing still obeys much of what one would expect out of a sphere inversion though. -Craig Pemberton 05:44, 11 May 2011 (UTC)[reply]

I think this is true, but don't take my word for it:

(r-p/2)(r-i/2)=rr

. That brings it as close as possible to my original (corrected) equation

(r-p)(r-i)=rr

. -Craig Pemberton 06:16, 11 May 2011 (UTC)[reply]

Where to obtain a HyperSim card?

(I won't trust eBay until they have phone support for all)

I brought home a Softbank 920p from Japan, and the phone and its features (like the camera) stopped working a few months later. (Why not immediately?)

I was told there was a Hyper-Sim card, but none at Best Buy.

Froogle.com only shows Ebay hits, but I cannot trust that site after some harsh experiences.

Where are any other reputable sites that sell Hypersim cards? --70.179.169.115 (talk) 05:22, 8 May 2011 (UTC)[reply]

I recommend you move this post to the Computer Ref Desk. StuRat (talk) 06:40, 8 May 2011 (UTC)[reply]

Oops, 4got about that section. Copying there now. —Preceding unsigned comment added by 70.179.169.115 (talk) 06:58, 8 May 2011 (UTC)[reply]

Cost of panels per square mile, and how many needed to power Earth?

Ok, look here: http://i.imgur.com/j9wrB.jpg

Now, how much $ are they per square mile, and how many square miles are needed to power the planet?

Next question: How many square miles are needed Just to shut down all the coal, oil and gas power plants (rather, all "dirty" power plants) on Earth?

Moreover, this is assuming how many % efficiency? (Then if the efficiency is doubled by 2030, would that halve the costs, or is there a more complex formula than that?) --70.179.169.115 (talk) 06:57, 8 May 2011 (UTC)[reply]

Before you get too excited about this, be aware that there isn't enough raw material on earth to make that many solar panels (pure silicon panels are too inefficient). Additionally making that many panels (assuming you could get raw materials) is most definitely not carbon dioxide free. Today, making a solar panel costs more energy than you get back for more than 10 years, plus there would be lots of pollution (not just CO2) in mining the necessary materials, transporting them, building infrastructure, etc etc. IMO photovoltaics (solar panels) is a dead end technology. Look at solar thermal instead. And finally doubling the efficiency is pretty much impossible. The only way to get a higher efficiency with a solar panel is what they call multijunction solar cells but they are extremely expensive and use exotic materials which for sure could never be done in quantity. Ariel. (talk) 00:36, 9 May 2011 (UTC)[reply]

This is somewhat of a red herring - for mass production of solar power to replace existing power plants, solar thermal energy would be more efficient. Isolated solar panels are most useful for providing decentralized power, especially in remote places. Wnt (talk) 06:24, 9 May 2011 (UTC)[reply]

I am very sceptic to such a claim ("there isn't enough raw material on earth to make that many solar panels") with out a good source that includes details such as the types of solar cells considered, the limited raw material needed and the method of estimating the available amount on earth. Many different types of solar cells exists and new types are being developed. These use more or less common elements.

Solar cells can not be made from only "pure silicon" but my understanding is that they can be made from silicon, aluminium, glass and very small qualities of other elements (a few parts per billion) so the bulk materials are very common on earth.

Do you have a source for "Today, making a solar panel costs more energy than you get back for more than 10 years"? This document suggests that the time to produce the energy needed to produce the solar cell is in the range 0.8 to 2 years:

[1]

It also calculates that solar cells generates between 15 and 700 times the energy used to produce them, depending of source and definition.

This is called EROI (Energy Return Of Investment.)

The cost per square mile of such quantities of solar cells is very hard to estimate, it could be higher than today due to lack of easily accessible raw material or more likely it would be much cheaper due to more efficient automated large scale production.

According to Cost of electricity by source the cost of power from solar cells are two to eight times the cost of coal (without tax and subsidies).

I think wind power and solar thermal power are more promising in the short term due to very well known and basic working principles and less need for basic research, it is all down to rational production. Solar thermal also has the advantage that the heat can be stored for use later when there are no solar power. Storing electrical power are currently more expensive and less efficient than transporting the power very long distances for example between continents. I think its very premature to declare solar cells a dead end.

I think this is one of the worst use of a mercator projection I have seen, at least a equal-area projection such as Gall–Peters projection should be used. Ideally a map were the area is proportional to the insolation should be used. Similary to this one for rainfall [2].

According to the OP:s link about 370 000 km^2 is needed to cover the current energy consumption, that are about 143 000 square miles. (I do not understand why anybody would want to know the value in such a strange unit.)

--Gr8xoz (talk) 21:54, 9 May 2011 (UTC)[reply]

where to get zsm 5 catalyst in india for my chemistry projectWickyjazz (talk) 07:41, 8 May 2011 (UTC)

pls help me out finding it as soon as possible it would be better if u tell me a place in north India near uttar pradesh

This is not really the place for such queries, but try browsing through this directory [3]. Usually each site listed in the directory requires you send a product enquiry under the correct category. Plasmic Physics (talk) 13:27, 8 May 2011 (UTC)[reply]

Blood test for CJD and donating blood/plasma

When it comes to donating plasma, why do they put up a blanket prohibition to anyone who's been to Europe in the 80s? Why can't they just say, "If you've been to Europe during these years, please get your blood tested and proved that there is no evidence of CJD."

So why can't a simple blood test determine whether it's safe to donate my blood/plasma? What kind of new tech innovations would be needed in blood-testing to make this determination? --70.179.169.115 (talk) 07:47, 8 May 2011 (UTC)[reply]

The short answer is "because there isn't one yet." See this BBC article, which perhaps might be usefully incorporated into our article at some stage. If the trials of the test described prove successful and the technique is approved by the various regulatory authorities, screening of donated blood will become possible - whether it's adopted is a political rather than a medical decision. Tevildo (talk) 11:10, 8 May 2011 (UTC)[reply]

Conversely in the UK they don't let you give blood if you've recently been in the US (except maybe Hawaii) over the last 28 days, in case you have pre-symptomatic West Nile Disease.(ref) Strictly the UK guidelines apply to the high-risk WND period, but they often just make it a blanket 28 days.(ref). The Republic of Ireland applies a similar restriction for people who've been to the US and Canada, and also applies a restriction similar to the US one on people who've been to the UK.(ref) The UK prohibits donations from people who have family members that have developed vCJD.(ref). If the UK rules were applied in the US it would eliminate all donors; if the US rules were applied in the UK it would eliminate most. So both blood systems have to suck it up (sic) and accept blood that other high-quality systems would not. -- Finlay McWalter ☻ Talk 21:03, 8 May 2011 (UTC)[reply]

We (U.S.) can test our blood for West Nile, though. 75.41.110.200 (talk) 03:24, 11 May 2011 (UTC)[reply]

What other bodily components of mine would I be allowed to give for $?

Having been in Germany as a toddler, I can't donate blood. Is there any other part of my body that I CAN donate for cash though? (If not, then please cite a source.) --70.179.169.115 (talk) 07:47, 8 May 2011 (UTC)[reply]

Coming from a place where no money changes hands for such activities, I struggle with the concept of getting paid for a donation. (Maybe I should take this to the Language desk ;-) ) HiLo48 (talk) 07:58, 8 May 2011 (UTC)[reply]

According to this article, sperm (standards are very high; they don't just take anybody), ova, hair (but you won't make much), and breast milk. Clarityfiend (talk) 08:19, 8 May 2011 (UTC)[reply]

Thanks, Clarity. Sorry about the ova and breastmilk. Now the problem is, would anyone please find me the closest sperm bank to Manhattan, KS? Google is, evidently, too squeamish to lend me a straight answer. By the way, how much are you talking, when it comes to donating hair, and where's the nearest hairbank? --70.179.169.115 (talk) 08:49, 8 May 2011 (UTC)[reply]

Re: your last question, did you look at the ref or think it was just provided for fun? Nil Einne (talk) 11:00, 8 May 2011 (UTC)[reply]

[4] This site seems to have you covered. Looks like if your family's genetic history is good enough, you could be making a small, but non-trivial amount of money by making a commitment to show up and donate 4 to 8 times a month for six months. It also looks like they don't pay out until the six months is over. APL (talk) 07:56, 9 May 2011 (UTC)[reply]

Donated hair must normally be pretty long. And how would they know that you were in Germany as toddler? Quest09 (talk) 12:53, 8 May 2011 (UTC)[reply]

Because they ask you. Lying about such things in order to get paid for blood would be a very serious thing. --Tango (talk) 15:23, 8 May 2011 (UTC)[reply]

Pardon my ignorance, but why would someone who wants to donate blood not be allowed to do so if they've been to Germany as a kid? Germans seem to have no problem with it themselves and I can't think of any major disasters that would spark such a response. - Mgm|^(talk) 19:35, 8 May 2011 (UTC)[reply]
- See Tort reform. I make no further comment. Tevildo (talk) 19:39, 8 May 2011 (UTC)[reply]
- See the immediately preceding question. The US, Canada, Australia and some others prohibit blood donors from European countries that had apparent outbreaks of vCJD (better known as Mad Cow disease). It is a fatal blood-transmissible prion disease with an incubation period that can be 20-30 years, and at present there is no effective way screen potential blood donors for exposure. Dragons flight (talk) 20:48, 8 May 2011 (UTC)[reply]
- [5] [6] suggest in the US you are ineligible if you lived in Germany (or some other European countries) for a cumulative time of over 5 years unless it was on a US military base. I'm guessing from his previous questions and comments the OP's time in Germany wasn't associated with a US military base therefore the suggestion it was just time spent as a toddler preventing them donating blood seems a little misleading. It's possible commercial blood donation services have stricter requirements (which the OP implied) and similarly there may be stricter requirements if you are trying to donate specific components only (which the OP didn't mention in this question). Also the criteria do change so it's possible the OP is thinking of older criteria. In any case the reason is of course vCJD. Nil Einne (talk) 20:52, 8 May 2011 (UTC)[reply]

Have you considered trading labor instead of body parts? 99.39.5.103 (talk) 18:15, 9 May 2011 (UTC)[reply]

What kind of insect is this?

What kind of insect could this be? It was found in South Germany, at an altitude of about 1300 meters. Thanks. --Edcolins (talk) 09:18, 8 May 2011 (UTC)[reply]

Probably from the Carabus genus, maybe Carabus auratus. Icek (talk) 09:41, 8 May 2011 (UTC)[reply]

Great, thanks a lot. --Edcolins (talk) 10:07, 8 May 2011 (UTC)[reply]

FYI: The German Project has a site dedicated to identifying animals and plants. [7] --79.219.83.160 (talk) 11:20, 8 May 2011 (UTC)[reply]

Bicycles (and other things) remaining upright

Hi. I've been trying to find out about the age-old question of why a moving bicycle stays upright with apparently almost no effort, and does not immediately tip over like a stationary bicycle does. According to random websites I've visited, the gyroscopic effect of the wheels is not very significant, and the explanation is mainly to do with a combination of deliberate and automatic steering, such that the bicycle adjusts and recovers as it begins to tip. However, and although I have nothing suitable to experiment with at the moment, I can sort of visualise that a small two-wheeled toy with no steering mechanism at all would scoot along upright for a few feet if given a push, yet fall over instantly when stationary. Is this right, and what would be the explanation? One thing I can experiment with is a coin. Coins can easily be made to roll along upright for quite some distance, and then they fall over when they reach a certain minimum speed. What is the reason? Is this also due to some kind of "automatic self-steering"? 86.181.204.130 (talk) 11:45, 8 May 2011 (UTC)[reply]

Wikipedia has an excellent article, and a lot of supporting external links, at Bicycle and motorcycle dynamics. You will find your answer there. Dolphin (t) 12:14, 8 May 2011 (UTC)[reply]

It's a surprisingly difficult question -- in fact the journal Science published a paper about this just last month. (you can find a Wired blog discussion of the paper here). Looie496 (talk) 16:24, 8 May 2011 (UTC)[reply]

Already covered in our article BTW and suggesting it (our article) may therefore be wrong. Nil Einne (talk) 19:12, 8 May 2011 (UTC)[reply]

Thanks, that's a meaty article. I haven't even begun to read it properly yet, but it looks very comprehensive. However, note that (despite the title) my question is not actually so much about the stability of real ridden bicycles - which I have already read up on a little - but about the stability of rolling coins and of two-wheeled toys that do not have any steering. 86.181.204.130 (talk) 19:29, 8 May 2011 (UTC)[reply]

The thing that keeps rolling coins upright is called the gyroscopic effect, which is shockingly a red link. Some of the information is in the article on bycycle dynamics above, and some is in Gyroscope, but the basic principle is that spinning objects resist torques which are perpendicular to their axis of spin. For a standing coin to fall over, there has to be a torque towards the ground (i.e. a force which acts to rotate the coin along an axis through the coin longitudinally). A spinning coin has an axis of rotation through the center, thin part of the coin. This spin tends to "fight" (for lack of a better term) spins along other axes, and so keeps the coin from turning in a way that would make it fall over. --Jayron32 20:45, 8 May 2011 (UTC)[reply]

Gyroscopic effect is a red link but Gyroscope#Properties is blue. Dolphin (t) 23:59, 8 May 2011 (UTC)[reply]

So, should a redirect be created? 86.166.40.199 (talk) 20:52, 9 May 2011 (UTC)[reply]

More: The concept of the gyroscopic effect is dealt with mathematically in Moment of inertia, which is defined as "a measure of an object's resistance to changes to its rotation". In other words, an object spinning along a certain axis will resist any change to that rotation. For a rolling coin to fall down, the axis of rotation must itself move; and the movement of the axis of rotation is goverened by the coin's moment of inertia. --Jayron32 20:56, 8 May 2011 (UTC)[reply]

It all makes sense. The bicycle article cited earlier asserts that the rider's actions keep the bike from tipping over. Even a rider on a stationary or near-stationary bike can keep it standing, with some effort. I suspect the same thing is going on with roller blades. If you watch either a bicyclist or a roller blader from straight in front or behind, you'll see that they are constantly shifting their weight to keep the "center of gravity" in a steady position. For bicyclists, this is more pronounced when they are "standing up in the stirrups", as it were. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:37, 8 May 2011 (UTC)[reply]

The "automatic steering" the OP mentions is caused by the Caster angle that is deliberately designed in to such vehicles: obviously it can't apply to an independent unmounted rolling disk such as a coin. Intuitively (he weaselled) I think it would have less effect the smaller the vehicle, but I wouldn't like to try to prove it mathematically. {The poster formerly known as 87.81.230.195} 90.201.110.223 (talk) 14:33, 9 May 2011 (UTC)[reply]

If the tyre (etc) of the rolling thing is curved in cross-section, would that be enough to make the rolling thing steer into the direction it is starting to fall? So if it starts to fall to the right, the curve of the tyre mean that it turns somewhat to the right (like a rolling cone) and stops the fall. I think I heard that on a BBC radio program recently about a scientist investigating why bicycles stay upright. If the radio programme could be identified, you maybe be able to download it. 92.15.20.127 (talk) 20:59, 9 May 2011 (UTC)[reply]

Building the Duomo di Milano today.

I'm not sure if this is the right place to ask this, but how much and how long would it take to build an exact replica of the Duomo di Milano today? --190.19.19.156 (talk) 15:22, 8 May 2011 (UTC)[reply]

The Frauenkirche Dresden was rebuilt a few years ago at the cost of € 180 million. The duomo is quite a bit bigger, and the masonry may more complex, so it'd probably more expensive to rebuild. Not sure why one would want to do that, anyway. --Wrongfilter (talk) 19:15, 8 May 2011 (UTC)[reply]

Finding bin Laden

If it took so long to find bin Laden when he was in Pakistan, how long will it take, now, that he is in the sea? I don't understand why the USA throw him there in the first place, after such a long search you'll better put him in a place that you cannot loose him again. And what about the bounty of several millions for bin Laden - dead or alive -? If I find bin Laden's body, will I be able to claim it? 212.169.190.212 (talk) 21:43, 8 May 2011 (UTC)[reply]

His is no longer in the top 10 most wanted by the FBI. So forget about any bounty. Wikiweek (talk) 21:51, 8 May 2011 (UTC)[reply]

Much much longer. By this time, the denizens of the deep will have nibbled on him. On the bright(?) side, after a long enough while, chances are the next fish stick you consume will stand a reasonable chance of containing some of his molecules. As for the bounty, the line forms here[8][9]. Clarityfiend (talk) 21:58, 8 May 2011 (UTC)[reply]

There are an infinite number of Bin Ladens, but unfortunately for the bounty hunters, the density is rather low, see here:

A generic prediction of inflation is an infinite ergodic universe, which contains Hubble volumes realizing all initial conditions - including an identical copy of you about 10^{10^29} meters away.

Count Iblis (talk) 23:12, 8 May 2011 (UTC)[reply]

It is misleading to say the USA threw him in the sea. Burial at sea is a dignified and respectful way of disposing of bodies at sea. Dolphin (t) 00:06, 9 May 2011 (UTC)[reply]

^{On the other hand I'm concerned that we have turned the entire ocean into a Homeopathic strength solution of Bin Laden. --Daniel 00:15, 9 May 2011 (UTC)}[reply]

Homopathic strength dilution would be much more dilute than that; no molecules of the diluted substance would be left. So, distilled water can be considered to be a homeopathic dilution of anything. Count Iblis (talk) 00:31, 9 May 2011 (UTC)[reply]

So then: according to principles of homeopathy, drinking small amounts of diluted sea water will now cure terrorist and religious fundamentalist extremist behaviours? Mattopaedia ^{Say G'Day!} 09:42, 10 May 2011 (UTC)[reply]

I wonder if the Navy has actually secretly trained dolphins or sharks or something to wear a communications device and sniff out recent human corpses deep underwater. Whether for sentimental purposes of recovering their own, or to avoid unfortunate diplomatic incidents... Wnt (talk) 06:27, 9 May 2011 (UTC)[reply]

I suggest you get yourself a wet suit and a scuba and start looking, you have nothing to lose and a huge amount of fame if you find the body. You could start around the shores of Spain and then move outward. !Venga, hazlo ahora¡ Richard Avery (talk) 07:44, 9 May 2011 (UTC)[reply]

You want to claim a bounty on someone who is already known to be dead? That's not how it works. That's not how it works at all. APL (talk) 07:48, 9 May 2011 (UTC)[reply]

Does anyone here know what the average depth of the Arabian Sea would be? ←Baseball Bugs ^{What's up, Doc?} carrots→ 11:16, 9 May 2011 (UTC)[reply]

Google Earth tells me that there's a narrow continental shelf, which quickly drops off to depths of over 3,000 metres. (Maybe 10,000 feet.) Is that what you wanted to hear? HiLo48 (talk) 11:32, 9 May 2011 (UTC)[reply]

That's sufficient. Of course, we don't know just where they dropped the body. But one would think they would have taken it to the deepest part. It's difficult enough finding an ocean liner or an airplane at those kind of depths, never mind a corpse. And that depth would pretty much rule out scuba. ←Baseball Bugs ^{What's up, Doc?} carrots→ 11:48, 9 May 2011 (UTC)[reply]

He could try Bugs, he could try ;-)) Richard Avery (talk) 13:18, 9 May 2011 (UTC)[reply]

Sure. No harm in trying, right? :) ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:22, 9 May 2011 (UTC)[reply]

It does raise the question of how long human remains can "remain" at such depths. Titanic sat at 12,000 feet for 73 years before it was discovered, and no evidence of human remains was found. The Air France plane that went down a couple of years ago was found at a similar depth, and around 50 bodies were located. For sensitivity reasons, there is little detailed information, and it's reasonable to assume that they have significantly decomposed. The point being that if someone wants to find a lone body in 10,000 feet of ocean water, they prpbably have no time to lose. ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:36, 9 May 2011 (UTC)[reply]

A set of human remains was found in the 1994 exploration of the SS Edmund Fitzgerald, almost 20 years after it sank, in about 500 feet of water. Granted, that's in Lake Superior, which "Never gives up its dead" because the cold waters inhibit bacteria growth that might otherwise cause a body to float to the surface. Our article doesn't talk about the temperature of the Arabian Sea, but presumably, it's somewhat higher (though it is pretty deep, so maybe not, at the bottom). Buddy431 (talk) 18:47, 9 May 2011 (UTC)[reply]

Bodies in the ocean tend to be eaten/decompose relatively quickly, even the bones. However, there are a few anaerobic zones where wood, at least, can last indefinitely. I wonder if the same is true of bodies. StuRat (talk) 18:49, 9 May 2011 (UTC)[reply]

The body of diver Deon Dreyer drifted to the bottom of Bushman's Hole, an extremely deep body of fresh water in South Africa. The body was discovered by David Shaw ten years later. The body was sufficiently intact for Shaw to return the following day to recover it from those great depths. See Deon Dreyer#Recovery of Dreyer's body. (Shaw was successful in reaching Dreyer's body and dislodging it from the bottom. Tragically Shaw died in the attempt but the bodies of both Dreyer and Shaw drifted to the surface a day later and both were recovered.) Dolphin (t) 23:23, 9 May 2011 (UTC)[reply]

pain and pressure

Several years ago I had a spinal tap. (They had to do it six times). Each time they gave a little local anesthetic shot before sticking in the big needle. It still hurt. They said "you aren't feeling pain - only pressure." Whatever it was, it hurt. Is there really a difference between pain and pressure like that, or were they just trying to convince me that it didn't hurt? Bubba73 ^{You talkin' to me?} 23:18, 8 May 2011 (UTC)[reply]

Maybe there is some clinical difference that I don't know about, but as a speaker of English, I have to say that if something physically hurts, it's painful. The two are synonymous to me. If there is no pain, it does not hurt. To me saying that "you aren't feeling pain, it hurts - that's all" is like saying "water's not falling from the clouds, it's just raining". Falconus^{p t c} 02:42, 9 May 2011 (UTC)[reply]

I agree with the above, but I definitely "get" what they were trying to do. Pain is a very subjective experience, so even just being told you aren't supposed to be feeling pain might have a bit of a placebo effect. I've seen people literally cry from pain when they're getting a shot, before the needle has even touched them.. Vespine (talk) 03:22, 9 May 2011 (UTC)[reply]

In dental school, we're taught that there are different sensations and that patients might feel a burning or pricking sensation (for which I will inject more local anesthetic) or a pressure-like sensation (for which we will continue, let's say, extracting a tooth) without administering more anesthetic. I'm no neurophysiologist, but there are many, many types of afferent fibers with many types of nerve tips and maybe anesthesia, if not profound enough, affects some more than others. DRosenbach ^{(Talk | Contribs)} 03:36, 9 May 2011 (UTC)[reply]

I suspect in this case, they were wrong. Everyone I've ever known that has had a spinal tap has said it hurts a great deal. They might have been a little over-optimistic about the profoundness of the local anesthesia or they might have just been trying to reassure you. Having had dental work that involved both sensations at once, as Rosenbach talked about, there is a definite difference. Namely one feels like pressure, like someone pressing your arm, the other... like pain! HominidMachinae (talk) 20:27, 12 May 2011 (UTC)[reply]