Wikipedia:Reference desk/Archives/Science/2012 August 17

= August 17 =

what's the squiggly side group symbol stand for?
See the structural diagram for Methamphetamine. What does the squiggly side group stand for? μηδείς (talk) 00:47, 17 August 2012 (UTC)


 * See skeletal formula to learn what a bond being a wavy line means. DMacks (talk) 00:52, 17 August 2012 (UTC)


 * Thanks, that gives a general answer of unknown or stereoisomer. In this case its obviously a stereoisomer of some chiral sidegroup.  Can anyone specify it?  μηδείς (talk) 01:02, 17 August 2012 (UTC)
 * The active form is dextrorotary, which in this case is "out of the screen". Bizarrely, Levomethamphetamine is also drug, but functions only as a decongestant. Someguy1221 (talk) 01:05, 17 August 2012 (UTC) When I wrote this comment, I thought you were asking which isomer it was, not which sidegroup it was. Someguy1221 (talk) 01:17, 17 August 2012 (UTC)
 * So, the squiggly side group is a methyl group. Is that what you're asking? 203.27.72.5 (talk) 01:10, 17 August 2012 (UTC)
 * That it is a methyl group is clear from this image. 203.27.72.5 (talk) 01:14, 17 August 2012 (UTC)
 * Otherwise unannotated squiggly lines are always methyl groups, just as otherwise unannotated straight lines are always methyl groups. Someguy1221 (talk) 01:16, 17 August 2012 (UTC)
 * Okay, so it's a methyl group and a hydrogen then, since a methyl alone would require a missing double bond. Makes sense. I don't remember the squiggly side group. When did that convention come into style? I remember a wedge perhaps next to a line, or an R. μηδείς (talk) 01:19, 17 August 2012 (UTC)
 * The wedges represent specific isomers. The squiggle represents a racemic mixture. And if it is a methyl group there must be a hydrogen. If there were a double bond it would not be a methyl group. See methyl group. 203.27.72.5 (talk) 01:23, 17 August 2012 (UTC)
 * It occurred to me that you might mean a double bond like this; R-CH=C(CH2)R, rather then like this; R-CH2-CH(=CH2)R as I first assumed. If that's the case then the methyl group would still be indicated by a line even though there's no hydrogen there. So the line doesn't represent the hydrogen as well, just the CH3. The hydrogens are ommited by convention. 203.27.72.5 (talk) 01:45, 17 August 2012 (UTC)
 * Yes, I did mean what you first assumed, 203. And at this point I fully get what is symbolized for this specific molecule.  And I agree that there would have to be a hydrogen for the remaining side group if none other were signified.  My point is, I do not remember the squiggly mark from Organic Chem when I tested in or took it.  For the Bio part of my undergrad double-major I tested out of OC lecture in the early nineties. (Before that I got A's in Chem I & II and Biochem in high school and a V on the AP test.)  And I aced organic chem lab when I finally took it in the mid nineties.  I simply do not remember the squiggly convention.  Should it be interpreted as any stereoisomeric sidegroup?  (Any old racemic mixture?)  And when did the squiggly convention begin?  I am concerned that I had an Ivy League education with a sub-par Org Chem class. μηδείς (talk) 02:45, 17 August 2012 (UTC)
 * To quote the page I pointed you to earlier, "wavy lines represent either unknown stereochemistry or a mixture of the two possible stereoisomers at that point". I know I've seen it in literature from at least back to the mid-1980s, but I don't know the history of IUPAC or others officially blessing it. DMacks (talk) 03:14, 17 August 2012 (UTC)
 * Here it is being used in 1989, so it at least predates your college days as far as I can see. To be fair though, I don't think it was ever explicity pointed out by my lecturers when I was at uni, and I majored in chem. It was just something I picked up on from seeing examples where that was the only possible meaning. 203.27.72.5 (talk) 03:17, 17 August 2012 (UTC)
 * I found these   earlier examples but they all seem a bit odd to me. I'm not 100% sure that they're using the squiggly line in the same way that we do these days. 203.27.72.5 (talk) 04:42, 17 August 2012 (UTC)
 * I made the change to methamphetamine and someone quickly pointed out that the squiggle actually just means undefined, not racemic in any particular ratio. I should have thought of that, but I'll point it out now. Wnt (talk) 16:27, 17 August 2012 (UTC)

I very highly doubt the textbook edition I used in '90 that had been published in 1987 or 1989 was using a convention that we cannot find much prior to that date--textbooks tend to be conservative. I will accept this question as resolved. μηδείς (talk) 07:02, 18 August 2012 (UTC)

ET contact book
The question above about extraterrestrials reminded me of a book I read about on Wikipedia, and that I'm hoping someone could identify. The plot is that an enormous alien spacecraft has entered the solar system on a fast hyperbolic trajectory. Due to celestial mechanics, the only human spacecraft capable of docking with it is piloted by a young girl on her first solo flight (an average Joe in some ways), but her ship wouldn't have enough fuel to return to the solar system. The rest of the book describes her exploration of the mysterious artificial ecosystem inside the alien craft, which apparently had water, but many other features were extremely bizarre and non-Earth-like. At the end, there was no cheesy cop-out, and the girl did starve (or die some other way) on board, but the book was really more about the alien craft than about the explorer.

Before anyone mentions it, this is not Rendezvous with Rama (which is a great book, by the way; I highly recommend it). It seems to share a similar plot, but then again there are millions of books about aliens out there. --68.179.115.177 (talk) 06:43, 17 August 2012 (UTC)
 * Yeah Rendezvous with Rama was my first thought. I'd love to know too, sounds like a great read.-- O BSIDIAN  †  S OUL  07:29, 17 August 2012 (UTC)


 * I found something like it. Pushing Ice by Alastair Reynolds. Though in this case, it was an entire crew of a mining ship, but the captain is still female.-- O BSIDIAN  †  S OUL  09:32, 17 August 2012 (UTC)


 * The description reminds me somewhat of a short story by James Tiptree, Jr. It may have been the lead title (and certainly inspired the pbk cover illustration) of one of her short story collections, but I'm afraid I can't remember which and, being at work, can't consult my library. Hopefully this might give someone a helpful lead. {The poster formerly known as 87.81.230.195} 84.21.143.150 (talk) 12:57, 17 August 2012 (UTC)
 * [Later] False lead, mea culpa. I was misremembering "The Only Neat Thing to Do", whose plot is not close enough to that sought to be a candidate. {The poster formerly known as 87.81.230.195} 90.197.66.109 (talk) 21:15, 17 August 2012 (UTC)


 * Man, I hope someone finds this. I would like to read this, sounds interesting!

Which jobs require sharp vision?
I am endowed with exceptionally sharp vision, which enables me among else to read signs from a long distance. My motor skills are normal, and not as developed as those of an air pilot or a sniper. Which civilian jobs require sharp vision, besides relevant academic education? Thanks, 93.172.151.222 (talk) 10:36, 17 August 2012 (UTC)


 * Something in forensics, maybe? I don't think there are really many jobs where better than normal vision is particularly useful. --Tango (talk) 12:10, 17 August 2012 (UTC)
 * Off the top of my head: On the floor of the stock exchange; Any captain of a passenger vehicle (cruise ship, airliner, bus, etc.); Operator of heavy machinery (crane, bulldozer, etc.); Janitor; Quality control.165.212.189.187 (talk) 15:55, 17 August 2012 (UTC)


 * Private eye ? There one particular vision skill is needed, at which I seem rather deficient: facial recognition.  (I can only recognize people in context.  If I met my mother someplace unexpected, and she didn't talk, I wouldn't recognize her.) StuRat (talk) 15:54, 17 August 2012 (UTC)


 * That sounds like as a disability. Qpl87 (talk) 16:07, 17 August 2012 (UTC)


 * In general (not commenting on any one case) I suspect that prosopagnosia, or more properly prosopamnesia, is uncommonly common on Wikipedia... Wnt (talk) 16:49, 17 August 2012 (UTC)


 * Cool, so now I can get a handicapped sticker and park up front. :-) StuRat (talk) 16:51, 17 August 2012 (UTC)


 * Olympic archery champion. --TammyMoet (talk) 16:55, 17 August 2012 (UTC)


 * I'd say referee, but maybe sharp vision there would be a handicap. :) Also the security freaks who watch people at casinos and such? Wnt (talk) 17:10, 17 August 2012 (UTC)

Airline pilot? Sniper? Sniper spotter? 203.27.72.5 (talk) 21:15, 17 August 2012 (UTC)
 * Congratulations on your excellent vision, (this is from a resentful myopic) but I can't help noticing that all the occupations suggested need a good many other skills apart from good vision. Your exceptional visual acuity needs to be combined with sound judgement, knowledge, morality and experience, gather those 4 friends around you and you are on your way. Richard Avery (talk) 06:39, 18 August 2012 (UTC)


 * How are you at hitting a baseball? Ted Williams supposedly had 20/10 vision, which supposedly was helpful in his ability to hit. ←Baseball Bugs What's up, Doc? carrots→ 05:07, 19 August 2012 (UTC)

physics/5th Dimension
I don't understand what is defined as 5th Dimension ? I know the space and time is considered as Third and Forth dimension, but not sure about Fifth one? What the picture shows actually ?How can we observe/feel the 5th dimension? --Kesavan (talk) 13:09, 17 August 2012 (UTC)


 * First off lets pretend we are 2D (length and width) beings on the surface of a balloon (here time is the 3rd dimension). A spot on the other side of the balloon is a distance away, as we have to stick to the surface of the balloon. However, as we have 3 spacial dimensions (and a time dimension) to work with, we can take a "short cut" though the centre of the balloon, shortening the distance between the 2 points. So this is one way a 4th spacial dimension would revel itself, particles arriving at places sooner than expected. A second time dimension I'm not sure on how that would appear Dja1979 (talk) 14:38, 17 August 2012 (UTC).


 * This diagram, by itself, does not contain any useful information. It is part of a longer paper letter-to-the-editor-of-a-physics-journal, On the dimensionality of spacetime, available from its author's website: Max Tegmark, professor of Physics at MIT.  Have you read his paper?  It explains the author's views about space and time.  In my opinion, these views are not particularly rigorous, nor are the explanations of the assumptions particularly sound; but, nobody other than the author is responsible for his reasoning.  Nimur (talk) 17:18, 17 August 2012 (UTC)


 * "we are here " seems to me to be a bit short sighted because it only refers to our physical selves. When I look at the three dimensions of time and one of space I think of past, present and future, then I think what can travel in these dimensions?  A: thoughts.... my thoughts can travel back in time and recall real events and that "information" since it is obviously not lost is still "somewhere" in my head.  So is that information traveling ahead into the future to meet my consciousness in the present? Or do I have some intrinsic ability to travel at least in one dimension of thought to the past?GeeBIGS (talk) 02:39, 18 August 2012 (UTC)
 * when I think of one dimension of both space and time I think of light, two dimensions of space and one of time I think of the plane between positive and negative charge in a magnet, one dimension of time by itself or space without the other is like a singularityGeeBIGS (talk) 02:48, 18 August 2012 (UTC)
 * The information in your mind travels into the future along with your body. You do not travel back in time when you consider memories; you are only activating symbolic representations of past events. —Tamfang (talk) 01:26, 19 August 2012 (UTC)


 * Kesavan, the chart is a summary of what (the author thinks) existence would be like if the universe had a different number of dimensions. As it says, "we are here": our universe has exactly three spatial and one temporal dimension, so a fifth doesn't correspond to anything.
 * That's not to say, though, that five (or more) dimensions can never be useful. Any system with five continuous independent variables can be represented by a five-dimensional continuum; but these dimensions have no necessary relation to the 3+1 dimensions of our universe's spacetime. —Tamfang (talk) 01:26, 19 August 2012 (UTC)


 * See "Flatland".—Wavelength (talk) 01:37, 19 August 2012 (UTC)


 * In a hypothetical 4+1D universe, with one more spatial dimension than we do, the fifth dimension would be time. In a hypothetical 5+1D universe, with two more spatial dimensions than we do, the fifth dimension would be a spatial dimension. But we live in a 3+1D universe (well, on the macroscopic scale), and there aren't enough dimensions for the term "fifth dimension" to mean anything here.


 * But the fifth dimension is useful. See Kaluza-Klein theory and five-dimensional space. And of course, mathematically speaking, five-dimensional space is extremely useful if you are a 5-polytope. ;-) Double sharp (talk) 13:03, 19 August 2012 (UTC)

Approximately how much mass does the Milky Way lose each day?
Due to stars radiating energy into extragalactic space. Assume no mass is being gained from external sources. Goodbye Galaxy (talk) 14:22, 17 August 2012 (UTC)
 * and I just realized how appropriate my username is for this question


 * Since you ask only about radiated light, this is an easy calculation. Orders of magnitude (power) claims that the Milky Way radiates 5×1036 W (though it doesn't cite a source). Divided by c2, that's 5×1024 kg/day. -- BenRG (talk) 18:02, 17 August 2012 (UTC)
 * Which is remarkably close to the mass of Earth. That seems very small to me... just goes to show how big the speed of light is, I guess. --Tango (talk) 19:35, 17 August 2012 (UTC)


 * Ok, but how much does it recieve each day from intergalactic space? Sky Machine   ( ++ ) 03:39, 18 August 2012 (UTC)


 * Since the Galaxy is mostly empty space, I imagine that most mass "received" simply passes right through and keeps going. Someguy1221 (talk) 03:47, 18 August 2012 (UTC)


 * The net balance is ~zero, but can anyone estimate the daily incoming energy transaction?  Sky Machine   ( ++ ) 03:53, 18 August 2012 (UTC)
 * I disagree that the net balance is about zero. As the universe gets older and moves towards heat death, the amount of energy that exists outside galaxies in the form on photons (and other particles) moving through intergalactic space increases. That means galaxies are, on average and on balance, losing energy. As Someguy says, most of the energy from outside the galaxy that enters it never actually interacts (other than gravitationally) with it and just passes straight through. The actual energy absorbed by the galaxy from outside will be minimal. --Tango (talk) 20:26, 18 August 2012 (UTC)


 * By net balance I meant the difference in the ammount of energy originating from outside the galaxy that enters and leaves the milky way at any point in time. An analogy if you like: if a high rung meth dealer gets $100k each day as a result of drug dealing but spends ~$100k on hookers, cocaine, casino gambling, protection, and bribes, his net monetary gain is zero but alot of money is going through his hands on any given day. I want to know the energy influx from outside the galaxy that enters the milky way at any given time. Sky Machine   ( ++ ) 03:42, 19 August 2012 (UTC)
 * Yes, I know what "net" means and I disagree with you for the reasons stated. --Tango (talk) 12:01, 19 August 2012 (UTC)

Is an apple alive or dead when you eat it?
Three questions in one:

1. At the instant before you take the first bite of an apple is it considered alive or dead?

2. At any moment during consumption of an apple is it considered alive or dead?

3. After you've finished eating an apple, is the remaining apple core considered alive or dead?

Thank you for your time. — Preceding unsigned comment added by Wallywalrus (talk • contribs) 15:43, 17 August 2012 (UTC)


 * It's difficult to define alive or dead in plants. I will use the definition "it is alive if it can grow a new apple tree".  In that case, the apple, or more specifically the seeds, are still alive after all three steps (assuming the seeds were viable to begin with).  In fact, even if you ate the seeds, they might still be viable after you poop them out. StuRat (talk) 15:49, 17 August 2012 (UTC)


 * (ec) Technically one could say the apple is alive, in the sense that it can produce an apple tree. However, the only 'living' part are the seeds, the rest of the apple is actually food meant to be eaten, because when animals eat the apple, they unintentionally spread the seeds around, to the advantage of the apple tree that produced the apple. Certainly by eating an apple you do not 'kill' it, afterwards it has not lost its ability to grow (and eventually reproduce). - Lindert (talk) 15:51, 17 August 2012 (UTC)

What are alive in any organism are its cells. A fruit (excluding its seeds) is not an organism, but a part of one. The skin of an apple consists of live photosynthesizing cells which allow it to grow and produce various oils and pigments as the fruit matures. They remain living for some time after the fruit matures. There is not very much I can find at wikipedia, but see pome and exocarp. μηδείς (talk) 17:52, 17 August 2012 (UTC)


 * In multi-cellular animals, the organism can be dead even when most of its cells are still alive. This would be the case, for example, right after somebody was beheaded.  Some of the cells with low metabolic rates (like those which grow hair), can stay alive for quite some time.  However, with plants, there really isn't this distinction between the life of the cells and the organism.  The plant is basically "just the sum of its cells".  StuRat (talk) 18:02, 17 August 2012 (UTC)


 * Snopes and other debunkers deny old stories of fingernails and hair continuing to grow after death. When you die, your hair and fingernail production stops, but the flesh may dehydrate, exposing more of the hair or nails. Edison (talk) 03:03, 18 August 2012 (UTC)


 * Yes, but those cells don't die the very second your heart stops. StuRat (talk) 03:06, 18 August 2012 (UTC)


 * If they did, organ transplants wouldn't be possible. Organs such as hearts, livers, and kidneys can "survive" for several hours outside the body, if treated properly.Sjö (talk) 06:51, 18 August 2012 (UTC)


 * And a fruit is not an organism, so there is no sense in asking if it, as opposed to some of its parts, is alive. The apple is not alive as such, but many of its cells are, for quite a long time. So long as the skin is both open to air and not brown or frozen you should view its cells as alive. μηδείς (talk) 06:56, 18 August 2012 (UTC)


 * But, as previously explained, the concept of an organism being alive or dead, as opposed to the cells, only applies to multi-cellular animals. So, asking if an apple tree is alive is about the same as asking if an apple is alive.  With plants, perhaps asking "Is it still viable ?" would be the better question. StuRat (talk) 07:02, 18 August 2012 (UTC)


 * You could use that terminology, in which case apple skin itself is not viable, even though it can be cultured in a lab and produce a thallus. Otherwise the seeds, which are usually considered distinct, and not consumed, would be the only part normally considered viable.  Given that the seeds are not eaten, but the skin is, and it is alive, I would stick with yes as the answer. μηδείς (talk) 07:13, 18 August 2012 (UTC)


 * I read somewhere that most fruit you buy at a supermarket are hybrid varieties and as such sterile, anyway.Уга-уга12 (talk) 21:05, 21 August 2012 (UTC)


 * This adds yet another wrinkle. Certainly we can't consider the entire sterile plant to be dead, just because it can't reproduce.  So, I suggest the rule there should be "consider it to be alive if it could reproduce, were it not sterile".  StuRat (talk) 21:25, 21 August 2012 (UTC)

Different magnets
Is there an article in here about non-metallic magnetic materials? Is it possible to make a magnet which would repel the Earth? Is is possible to have a "laser" magnet? — Preceding unsigned comment added by Qpl87 (talk • contribs) 16:01, 17 August 2012 (UTC)
 * Plastic magnet is about non-metallic magnets. To repel the Earth an enormous and very heavy magnet would be required, but theoretically it is possible. I'm not sure what you mean by a 'laser magnet'. - Lindert (talk) 16:10, 17 August 2012 (UTC)


 * By "laser" magnet, I meant a magnet that instead of having a round field, would have a narrow field, concentrating its force on one point. Qpl87 (talk) 16:27, 17 August 2012 (UTC)
 * No, unless a magnetic monopole is discovered, magnetic fields are necessarily round, because according to Gauss's law for magnetism, total magnetic flux into any volume is zero. - Lindert (talk) 16:46, 17 August 2012 (UTC)


 * And why is a magnet that repels the Earth necessarily enormous and very heavy? According to your link polymers can be magnetic, so theoretically, should it be possible to construct a light-weight plastic magnet that would float around? Qpl87 (talk) 16:49, 17 August 2012 (UTC)


 * Yes, but to mainly repel the Earth instead of being repelled by it, it would need a lot of mass. If you are not concerned about getting the Earth moving, but moving away from the Earth, I guess you might use a lighter magnet. Anyway, I highly doubt any permanent magnet can be created in practice that is more repelled by the Earth's magnetic field than it is attracted by the Earth's gravity. - Lindert (talk) 17:09, 17 August 2012 (UTC)


 * Well any magnet can repel the Earth, it just can't repel it very much. :) Wnt (talk) 17:12, 17 August 2012 (UTC)


 * Technically, they repel each other by an equal amount, but, of course, the more massive object moves far less. StuRat (talk) 18:11, 17 August 2012 (UTC)


 * Magnetic fields are not "necessarily round", Gauss's law only states that the total flux through a closed surface is zero. See: Halbach array, Maxwell coil.&mdash;eric 18:19, 17 August 2012 (UTC)


 * Not round in the sense of prefectly circular, but magnetic field lines always form a loop (like in the Halbach array). - Lindert (talk) 18:26, 17 August 2012 (UTC)


 * So, back to the OP's question, yes, it is possible to have a narrow magnetic field, but I wouldn't describe such a field as a "laser magnet". 203.27.72.5 (talk) 20:49, 17 August 2012 (UTC)


 * This is all a bit dubious. The north poles of two magnets repel and likewise the south poles do the same. But if a magnet was sufficiently powerful that it could levitate by it's north pole repeling the earth's south pole, the magnet's south pole would be so attracted that it would change it's orientation towards the earth. You would need some mechanism to constantly keep the magnet in a particular orientation. If you want to achieve leviation, you'd be better off exploiting the Meissner effect with a superconductor, though I don't understand enough about this to know if it would ever work. 203.27.72.5 (talk) 20:44, 17 August 2012 (UTC)


 * Theoretically, if two magnets are perfectly aligned, neither should flip. However, placing a shaft between the two magnets will ensure that they don't (or just through the magnet you want to levitate above the Earth, stuck into the ground). StuRat (talk) 20:49, 17 August 2012 (UTC)


 * That's incorrect. See Earnshaw's theorem. 203.27.72.5 (talk) 03:19, 18 August 2012 (UTC)


 * There's no reason the alignment shaft shouldn't work. See Pseudo-levitation.  As for getting it to work without a mechanical constraint, that can be done with diamagnetism.  StuRat (talk) 05:53, 18 August 2012 (UTC)


 * Yes, that part is alright, but "Theoretically, if two magnets are perfectly aligned, neither should flip." is wrong. And as for diamagnetism, that' exactly what I was suggesting with superconductors. 203.27.72.5 (talk) 07:46, 18 August 2012 (UTC)


 * That part is also fine, so long as one is a diamagnetic, so I'm not sure what you're disagreeing with. StuRat (talk) 10:18, 18 August 2012 (UTC)


 * No, that statement is wrong since it doesn't specify that one be a diamagnet, and if it did, it would be nonsensical, because the repulsion of a diamagnet and a magnetic field is independent of alignment. 203.27.72.5 (talk) 20:27, 18 August 2012 (UTC)


 * "Wrong" and "not specific" aren't the same thing. And the diamagnet would need to be aligned.  If off-center from the object generating the magnetic field, it would fall off the edge. StuRat (talk) 21:25, 18 August 2012 (UTC)


 * Right, so "all cars are red and blue" isn't wrong, it's just not specific that I'm only talking about Domino's Pizza delivery cars. And in this case the object generating the magnetic field is earth, so how does something fall off? 203.27.72.5 (talk) 21:53, 18 August 2012 (UTC)


 * I didn't say "all"', and I wasn't talking about the Earth at that point. StuRat (talk) 23:33, 18 August 2012 (UTC)


 * I suppose you could do it with a gyroscope, or a very small motor with fast-reacting computer control, but by what margin would we need to improve on existing magnets to get one that levitates in this way? Wnt (talk) 23:40, 17 August 2012 (UTC)


 * You could make a blimp that's just slightly heavier than air, including an electromagnet, then use the electromagnet to lift off the ground. I don't see this being a very practical means of transportation, though. StuRat (talk) 02:06, 18 August 2012 (UTC)


 * Apparently, even that wouldn't work. See . I tried to calculate the field strength of a magnet that would be required to cancel out the force of gravity, but I ran into...er...complications. 203.27.72.5 (talk) 03:59, 18 August 2012 (UTC)


 * And would it work if you weren't near one of the poles? 203.27.72.5 (talk) 02:01, 18 August 2012 (UTC)


 * Not as well, no. StuRat (talk) 02:06, 18 August 2012 (UTC)

Invisibility
Is there any scientific theory that could potentially cloak objects or is that something that will be confined to sci go & Harry Potter. For example, could destructive interference of visible light be used? — Preceding unsigned comment added by Clover345 (talk • contribs) 16:13, 17 August 2012 (UTC)


 * Even if you destroy the light that is reflected by an object, you still would be able to see it, unless you reconstruct the light rays that would be going through the place this object is occupying, if it were not there. Qpl87 (talk) 16:35, 17 August 2012 (UTC)


 * Yes, invisibility is theoretically possible. They've even done it in a very limited sense.  You can either bend light around an object to hide it, or you can have cameras on one side of the object record the image on that side, and a screen on the other side display that (this only makes it invisible from one POV, however).  Or, of course, an object can be made of transparent materials, if that counts.  StuRat (talk) 16:39, 17 August 2012 (UTC)


 * The articles about the state of the art in invisibility: Transparency and translucency, and camouflage. Qpl87 (talk) 16:47, 17 August 2012 (UTC)


 * There was an even better article, but I can't seem to find it for some reason... StuRat (talk) 04:56, 18 August 2012 (UTC)

See Metamaterial cloaking, Adaptiv, Cloak_of_invisibility and Active_camouflage. 203.27.72.5 (talk) 05:38, 18 August 2012 (UTC)


 * Invisiblity is quite possible, current research have achieved near perfect invisibility using metamaterials to refract and reflect ambient light around an object. However, it has its limitations - currently, it only works in the microwave range, and on the microscale. IMHO, these are more technological limitations than scientific limitations. Plasmic Physics (talk) 05:47, 18 August 2012 (UTC)


 * Meaning "...more technological limitations than theoretical scientific limitations." StuRat (talk) 18:35, 18 August 2012 (UTC)


 * Yes, I had a brain-fart at that moment. Plasmic Physics (talk) 23:20, 18 August 2012 (UTC)


 * One issue with this approach to invisibility is that you would be effectively blind while under the "invisibility cloak." If all of the light is being moved around you, none of it reaches your eyes. For a Harry Potter style cloak that is implemented technologically you would need either imperfect invisibility or some sort of active solution that can make up for the light that reaches your eyes. 209.131.76.183 (talk) 14:51, 20 August 2012 (UTC)


 * Reminds me of the Cone of Silence. StuRat (talk) 05:03, 21 August 2012 (UTC)


 * So, poke some eye holes in it. 203.27.72.5 (talk) 20:39, 20 August 2012 (UTC)


 * The cloak bends only in a certain wavelength range, ergo it only blocks ina certain range. Plasmic Physics (talk) 23:44, 20 August 2012 (UTC)


 * But then you'll see the colours of the outside world wrongly. Double sharp (talk) 10:23, 21 August 2012 (UTC)

Tinned pineapple poisoning?
''Please note, this isn't a request for medical advice - I've already 'advised myself' to take it easy, and see a doctor if it doesn't resolve itself soon, though I'm already recovering rapidly. In any case, this is only a hypothesis, and "don't eat it if it tastes off" is hardly medical advice - doh!''

Could tinned pineapple in a leaky can ferment sufficiently to cause 'food poisoning like' effects without making the contents look inedible? Having consumed a tin of the stuff on wednesday evening that at the time I thought tasted a little odd, I found myself a few hours later feeling distinctly queasy, and by the following morning I was projectile vomiting spectacularly. Initially I thought that perhaps I was coming down with gastric flu, but given that I'm now almost recovered, this seems unlikely. So, purely hypothetically, could such effects result from a leaky pineapple tin? Could the contents ferment, and would it produce the quantities of alcohol(s) sufficient to bring about the results described? It doesn't strike me as an obvious breeding ground for food-poisoning type bacteria etc. I've found a little on the subject via Google, but it is all rather anecdotal. AndyTheGrump (talk) 16:21, 17 August 2012 (UTC)


 * It could certainly cause food poisoning, but not from alcohol. The bacteria consumed and/or their toxins would be the cause.  If it tastes or smells "off", don't eat it (it may not look rotten until later).  If the can has lost it's seal, don't eat it (the lack of the suction sound of the vacuum breaking when you open it clues you in to this). StuRat (talk) 16:46, 17 August 2012 (UTC)


 * I'm not sure whether botulism couldn't grow in tinned pineapple. The USDA says that it is limited to low-acid vegetables, but I see an anecdotal forum report claiming otherwise  ... I don't know what I believe.  There's no law of nature that says a bacterium can't learn how to resist acid, and doesn't have any strains that can.  Obviously other bacteria are possible.  But so are food allergies, or an unrelated cold. Wnt (talk) 17:07, 17 August 2012 (UTC)


 * I've seen plenty of moldy citrus, so it certainly is possible. I've noticed that it looks quite different on the outside of the moldy fruit, where you get furry bits of white, versus inside, where it tends to just get dark and mushy.  Was the pineapple dark and mushy ? StuRat (talk) 17:33, 17 August 2012 (UTC)


 * No - it looked fine. AndyTheGrump (talk) 18:00, 17 August 2012 (UTC)


 * I once found a jar of apple sauce, looked fine, was unopened, tasted alright, and was free, so I ate it. Later that night I was not feeling well at all - the apple sauce expired 6 years previous... can't always trust your nose/tongue. 65.95.22.16 (talk) 19:44, 17 August 2012 (UTC)


 * The OP mentions 'gastric flu', as if it were something distinct from 'food poisoning'; that is not the case. Both terms refer to the same thing: food borne illnesses. Food borne illnesses are almost always a result of infection by microbes (bacteria, viruses, other little beasties) or, more rarely, their metabolites, as with botulism. What I mean by this is that, whether the causative agent came from the tin or from something else entirely, it's virtually a certainty that it was infectious agent and terms like 'flu' vs 'poisoning' are mostly non-starters.
 * Now, a hearty bout of norovirus typically lasts a day or so with projectile vomiting as the central attraction. That's much more consistent with what went on than botulism, which presents rather differently (i.e. paralysis). There are plenty of other causative agents, though, so this should not be seen as an attempt at diagnosis.
 * Finally, since most foodborne illnesses result from cross-contamination, the visual or olfactory inspection of food is almost worthless for determining whether it's good to eat or not - think of it as a "no only" test; if it looks or smells bad, pitch it, but looking or smelling good means nothing. Matt Deres (talk) 01:56, 22 August 2012 (UTC)


 * Vomiting is bad enough, but projectile vomiting is just beyond the pail. :-) StuRat (talk) 02:02, 22 August 2012 (UTC)

Spider identification
What type of spider is this? I haven't been able to find any good information on how you're supposed to measure spiders, but I did include a best-effort photograph of a ruler in this composite picture. I'd say its length sans-legs is roughly 15–20mm. It was found in southern Saskatchewan, if that helps narrow it down.

As a follow-up question, I'm wondering whether my amateur pictures are worth uploading to commons in a less compressed form; although that's probably a help desk question, if anyone has an opinion feel free to voice it. The three shots on the top of my composite picture here are the best ones I got, and I have hi-res un-cropped versions of them, but I have my doubts as to the value of the photographs. BigNate37(T) 18:04, 17 August 2012 (UTC)


 * It's an orb-web spider, but that's not narrowing it down that much - perhaps an Araneus of some kind, such as the barn spider. Mikenorton (talk) 18:07, 17 August 2012 (UTC)


 * Wow, I actually saw the barn spider article soon after you mentioned orb-web spiders, but I didn't pay it much thought until you mentioned it explicitly. Upon closer examination, it may well be a barn spider, though several of the images at commons:Category:Araneus cavaticus look quite a bit different. Those listed at http://www.iowavoice.com/2009/09/13/barn-spider/ bear a lot more similarity. Thanks for taking a look, Mike. BigNate37(T) 18:38, 17 August 2012 (UTC)


 * The markings are unclear but it may also be the female bridge spider (or gray cross spider, see commons:Category:Larinioides sclopetarius), another orb-weaver and notable for being synanthropic (living in human habitations). Also the difference you see in the commons pictures for A. cavaticus is merely sexual dimorphism. Your spider is very likely female, since they're usually the ones which build webs.-- O BSIDIAN  †  S OUL  00:02, 18 August 2012 (UTC)


 * I see. The body proportions and general posture of the limbs of my spider seem to match up better with the barn spider, but the habits fit the bridge spider better: she would hide behind a piece of the sheet metal on the spare tire frame during the daytime, and sat in the centre of her web after dark. BigNate37(T) 00:18, 18 August 2012 (UTC)


 * Note though that the size of the abdomen varies depending on if the spider has recently fed or not. The pattern would be a better way to tell. For Larinioides sclopetarius see . For Araneus cavaticus, see . It also looks very similar to the *other* barn spider, Neoscona crucifera, see . The cross orbweaver (Araneus diadematus) is also another possibility, see.


 * One way to distinguish the four is that A. cavaticus has small "knobs" or "shoulders" on the front part of their abdomens, kinda resembling the cat-faced spiders (Araneus gemmoides). It also has curved shallow C-shaped or comma-like white patterns on the underside of their abdomens. L. sclopetarius also has elongated comma-shaped to quarter note-like white patterns on the underside but lack the "shoulders". N. crucifera has a broken L white patterns on the underside and also lack the "shoulders". A. diadematus are usually more colorful, have somewhat "squared-off shoulders", and have C-shaped white patterns on the underside as well as distinctive white cross patterns on the upper side of their abdomens.-- O BSIDIAN  †  S OUL  06:01, 18 August 2012 (UTC)


 * It not having been pointed out explicitly, Araneus cavaticus is the Barn Spider, the best known example of which is Charlotte, form Charlotte's Web. μηδείς (talk) 06:30, 18 August 2012 (UTC)

Archery Accuracy
I was once told that elite target archery can be more accurate than target pistol shooting - presumably only at standard competitive archery distances (70m in the Olympics), and presumably when neither sport uses optical aids. Is this true? ([|Archery] mentions "...the excellent accuracy of modern [archery] equipment..." but I cannot find anything that compares the two sports' accuracy. Tom Haythornthwaite 19:34, 17 August 2012 (UTC) — Preceding unsigned comment added by Hayttom (talk • contribs)
 * Of course some archer can deliver his arrows more accurately than some pistol shooter. But I would imagine that for someone with no prior experience with either, accurately shooting the pistol would be easier than shooting a bow and arrow. I shoot mainly rifles myself, but I do shoot pistol too. I've used a bow and arrow maybe twice ever, and I remember it being very difficult. Within the handgun dispilines there's also a wide array of different classes that are not all as easy to shoot accurately as each other, and they don't all have the same intrinsic accuracy. A black powder pistol that fires a lead ball is intrinsically less accurate than a revolver firing a bullet due to the aerodynamic aspects. A double action revolver is harder to shoot accurately than a semi automatic pistol because as the trigger is pulled in the revolver it cocks the hammer which causes the gun to want to move due to the hammer's inertia. You don't have that problem in the semi-auto because the firing pin was already cocked by the recoil from the previous round, or for the first round you would have cocked it manually. 203.27.72.5 (talk) 20:30, 17 August 2012 (UTC)


 * It's good to get the perspective of an actual gun shooter. I suppose I really want to know about the relative accuracy of top competitive archers and shooters; if they were aiming at the same target under the same conditions (within competitive archery range) who would win? Tom Haythornthwaite 20:37, 17 August 2012 (UTC)  — Preceding unsigned comment added by Hayttom (talk • contribs)


 * Well, I thought I would just compare the world record in archery against the world record in pistol shooting, but unfortunately the Olympic pistol event is over 50m and the Olympic archery event is over 70m, so it's hard to compare them. 203.27.72.5 (talk) 21:45, 17 August 2012 (UTC)


 * I teach some basic archery to kids and have been on courses where they talk about competition shooting. The standard competition archery "face" (archers have their own jargon) for FITA events at 50m is 80cm (32 in?) with the "gold" (which scores 9 or 10) being 155mm (6 in) across. However the 50m pistol target is 50cm (20in) with the centre being only 50mm (1.9in) in size.. Good archers do get pretty tight groups at 50 or 70m, but can't get too tight as the first arrows tend to obstruct the path of the later arrivals (although you can move sideways along the shooting line to try to minimize this problem). So based on the size of the target, I would say that the expectation is that pistols are going to be more accurate than bows. Alansplodge (talk) 00:02, 18 August 2012 (UTC)

Thank you, Alansplodge. Tom Haythornthwaite 05:27, 18 August 2012 (UTC)


 * Hmm, I think all the posts here have missed the point! The anecdote seems to work precisely because it IS counter intuitive! So you'd EXPECT the regular targets to be smaller for pistols, because they're for regular people; but the question is not about regular archers, but about ELITE archers.. I actually do think it's kind of a pointless anecdote that you probably can't really prove anyway. Come to think of it, perhaps this is a saying from the olden days when flint lock pistols 1st became popular. Only rich people could afford them and would have thought they were the bees knees, but someone who still shot with a bow could reply that elite target archery can be more accurate than target pistol shooting. Vespine (talk) 23:45, 19 August 2012 (UTC)
 * The dimensions of the targets are specified in the respective sport's regulations - the "bullseye" in archery is 6 inches wide, the pistol target "bullseye" is less than 2 inches in diameter. It has nothing to do with "regular" and "elite" archers or pistol shooters, the target regulations remain the same. You seem to have misinterpreted the word "regular" to mean "ordinary" while the actual meaning is "according to the regulations". Roger (talk) 21:34, 21 August 2012 (UTC)

Adrenaline rush
How do adrenaline rushes counter stress. For example, many people claim activities such as roller coasters or parachuting help relieve stress? Aren't prolonged adrenaline rushes dangerous as it leads to elevated heart rates which means blood is pumped less efficienty around the body or is that only in people with existing heart conditions? Note this is not medical advice and I'm not asking for an answer which contains advice. Clover345 (talk) 20:52, 17 August 2012 (UTC)


 * Short term it can be good, just like in exercise, as it gets your heart pumping, etc. Also, faced with a real or simulated life-and-death situation makes things we normally stress about seem insignificant, at least for a while. StuRat (talk) 20:55, 17 August 2012 (UTC)
 * thanks but what is the actual reason these activities relieve stress without being harmful? Clover345 (talk) 21:38, 17 August 2012 (UTC)
 * I don't think that adrenaline rushes per se counter stress. For example, suppose you are locked in a room with a hungry bear.  Each time the bear looks at you, you will get an adrenaline rush.  They don't counter your stress, though, they accumulate to increase it.  What counters stress in the examples you mentioned is more likely the pleasure associated with the experiences, manifesting itself as a dopamine rush. Looie496 (talk) 01:09, 18 August 2012 (UTC)
 * is there a source for where this notion that they relieve stress is coming from? I have sought and not found. μηδείς (talk) 04:44, 18 August 2012 (UTC)

Time Dilation Questions
I have been directed here by DVdm with regards to a comment that I made on the article Time Dilation. This is located at:

http://en.wikipedia.org/wiki/Time_dilation

The section of the article entitled “Simple inference of time dilation due to relative velocity” concludes by stating that Δt’ = γΔt where γ is the Lorentz factor.

I wrongly assumed that this equation was in error because I did not take the setup properly into account. (Out of context it reads that moving clocks run fast).

However, when I analyzed the proof in that section, I stated that the bottom drawing could not be Euclidean because there was relative motion between the rest observer O and the moving observer whom I will designate as O’.

DVdm stated that the analysis “…in this case is used in the context of a spatial vector triangle in the Euclidean geometry of a 2-dim strictly spatial schematic drawing”.

I do not understand his comment.

In my mind the following are problems with that statement:

1)	If the problem is strictly spatial then Δt and or Δt’ are 0 and the conclusion that Δt’ = γΔt is trivial.

2)	If one looks at the problem from the point of view of observer O (and ignores the fact that O’ is moving), then the conclusion that Δt’ = γΔt will not be valid because as DVdm has stated the conclusion is valid from the point of view of O’.

3)	If one looks at this from the point of view of O’ then the mirrors are moving and I can not envision a way to see this as a Euclidean space. This in turn means that the Pythagorean Theorem is not valid.  But the Pythagorean Theorem is critical to the proof.

I would appreciate any guidance you can give me on this matter.

Thanks. Emagnus3 (talk) 21:33, 17 August 2012 (UTC)


 * 3) The validity of the Pythagorean theorem in this context is an assumption of the argument. To say that "the speed of light is constant" is to say that $$\sqrt{(\Delta x)^2 + (\Delta y)^2 + (\Delta z)^2} / \Delta t$$ is a constant (namely c) for straight-line motion, where x, y, z, t are coordinates defined by some inertial reference frame (a network of metersticks and Einstein-synchronized clocks).


 * 2) Δt’ = γΔt is valid independent of reference frame, because all relevant reference frames are mentioned in the definitions of the variables. Δt and Δt' are elapsed coordinate times of frames O and O' respectively, and γ depends on the relative velocity of O and O'.


 * The argument is valid. It's fine to dislike it, though. I dislike it. It's superficially convincing because it exploits Euclidean/Newtonian intuitions, but special relativity shows that you shouldn't trust those intuitions. It's usually a bad idea to separate x, y, z from t in special relativity and pretend that you live in a quasi-Newtonian world with weird non-Newtonian "effects". -- BenRG (talk) 01:04, 18 August 2012 (UTC)


 * For what it's worth, an argument I like better starts from the assumption of "relativity of redshifts"—roughly that if two people are moving inertially away from each other in outer space, and they point identical radar speed guns at each other, both guns will report the same speed. See k-calculus. -- BenRG (talk) 01:27, 18 August 2012 (UTC)


 * Spacetime is still flat (ie it's a Minkowski space) in special relativity, so the Pythagorean theorem is valid. It's only when you start fiddling with the metric tensor in general relativity that you need to worry about how to take the norm.  --99.227.95.108 (talk) 04:42, 18 August 2012 (UTC)
 * Well, the Pythagorean theorem is valid on a Euclidean subspace of Minkowski space, anyway. The comparable theorem that holds on the entire Minkowski space is that if gαβUαVβ=0, then gαβ(U-V)α(U-V)β = gαβUαUβ + gμξVμVξ, where g is the Minkowski metric, instead of a Euclidean metric as would be needed to make those equations be an expression of the normal Pythagorean theorem.  Red Act (talk) 23:36, 18 August 2012 (UTC)
 * You do indeed need more assumptions than what is explicitly stated in that article section in order to conclude that it's reasonable to use the Pythagorean theorem in that section, and there's more than one way of specifying exactly what those postulates are; see Special relativity.


 * Part of what is indirectly being assumed from the unstated starting set of postulates is that there are multiple ways of decomposing spacetime into a product space of a one-dimensional "time" dimension, and a three-dimensional Euclidean space. Choosing that decomposition is part of what's being done when you choose an inertial frame of reference.


 * The bottom drawing in that article section is purely spatial in the O′ frame of reference, because it consists of a projection of the events involved along O′ 's time dimension onto a plane in O′ 's Euclidean space. The bottom drawing would not be purely spatial in the O frame of reference.


 * What's being called D in the bottom drawing and the equations should really instead be called D′, to make it clear that it's a distance measured in the primed frame. The L in the bottom diagram should probably also be labeled L′, with an accompanying argument based on the postulates as to why L′=L.  Red Act (talk) 23:07, 18 August 2012 (UTC)

Microwave plastic ring
My new microwave, like all others that I have seen so far, has a plastic ring below the glass plate. This is a combination microwave oven and grill and I don't know if its secure to use the grill (400 F) with the ring inside. The manual neither says it's possible, nor that it's not. Is it safe to use it? Qpl87 (talk) 22:20, 17 August 2012 (UTC)


 * First, knowing the model of microwave might help. Second, keep in mind that accepting a wrong answer on this might result in your microwave bursting into flame. You may still want to contact the manufacturer, especially if no one can give you a referenced answer. Someguy1221 (talk) 22:40, 17 August 2012 (UTC)


 * It's difficult to say as it depends greatly on exactly what the composition of the ring is. Perhaps it's actually glass-ceramic like white casserole dishes? BigNate37(T) 23:02, 17 August 2012 (UTC)


 * I'd play it safe and take the ring out. Does it even try to rotate while in grill mode ? StuRat (talk) 02:01, 18 August 2012 (UTC)


 * I have seen combination-Microwave ovens that rotate a grill in convection mode that sits atop the glass plate, and whose instructions specifically state to allow the grill to rotate. The design of the grill may make it obvious what was intended by the manufacturer. Someguy1221 (talk) 02:57, 18 August 2012 (UTC)


 * I don't have a source, but I do have a microwave, and the plastic ring is of heat-resistant plastic, and it only touches the glass plate at three very small rolling points that will not conduct enough heat to melt the plastic even if I run the oven 20 minutes, which I never do. μηδείς (talk) 06:26, 18 August 2012 (UTC)


 * Right, and that's why they don't melt in a normal microwave, but a 400° F microwave/grill is a different story. Presumably the plastic will eventually heat to 400 degrees to match the air temperature.  The question is whether it can take that heat. StuRat (talk) 06:31, 18 August 2012 (UTC)
 * Well, contact the m'f'er is still the only valid response. μηδείς (talk) 06:36, 18 August 2012 (UTC)


 * If the manual doesn't say to remove the ring, it's probably fine. Manufacturers tend to over-warn users about things that may damage the equipment. For reference, note that the melting point of teflon is 620°F. Just because the ring is plastic, doesn't mean that it will melt at 400°F.--Srleffler (talk) 17:04, 20 August 2012 (UTC)