Wikipedia:Reference desk/Archives/Science/2008 September 22

= September 22 =

Wheat straw
How much wheat straw (or straw in general) is produced in the US in a year? ike9898 (talk) 01:25, 22 September 2008 (UTC)


 * says that one acre of Winter Wheat produces 2 tons of straw. says 50 million acres are planted each year in the USA - so I guess around 100 million tons per year would be a good first guess.  I'm sure someone will come up with a better number. SteveBaker (talk) 01:53, 22 September 2008 (UTC)
 * Found it. According to DOE, currently 11 million metric tons/ year I'm curious about the big gap between this value and SteveBaker's estimate, but not enough to pursue it right now. ike9898 (talk) 17:12, 22 September 2008 (UTC)
 * Table B.4 in that last ref shows a breakdown of the total wheat straw biomass. The likely reasons for the 11 million/100 million gaps are: zero-tillage farming leaves the straw on the field, where it contributes to healthy soil; and in many areas, the straw is either plowed back in or burnt - it just costs too much to truck it to where someone might want to buy it. Franamax (talk) 20:56, 22 September 2008 (UTC)

Advantages of fertilisers
What are the real advantages of fertilisers besides enriching the soil ? —Preceding unsigned comment added by 130.217.76.77 (talk) 05:25, 22 September 2008 (UTC)
 * The Fertiliser page is a great place to start learning about fertiliser. DMacks (talk) 05:35, 22 September 2008 (UTC)

Propan-1,2-diol from acetaldehyde
Can anyone suggest a method by which propan-1,2-diol can be prepared starting at acetaldehyde?Leif edling (talk) 05:39, 22 September 2008 (UTC)
 * Adlehydes are oxidised alcohols, right? Can't you reduce it? —Preceding unsigned comment added by Schwarzes Nacht (talk • contribs) 05:55, 22 September 2008 (UTC)
 * "Yes." To be useful though, you gotta tell us the context for this question. Are you trying to do homework?--tell us what you think so far and we'll be happy to tell help push you in the right direction (make sure to tell us what academic level this is too). Are you trying to design an industrial process? Are you trying to make a small amount because you can't buy it? Do you care about optical purity? DMacks (talk) 06:01, 22 September 2008 (UTC)
 * For industry you would not start from acetaldehyde but from ethene, to first reduce on side and than oxidize the other is complicated. A possibel way might be oxidation to acetic acid, chlorination, reduction epoxidation and than hydrolysis.--Stone (talk) 06:21, 22 September 2008 (UTC)
 * For industry, I would start from propene. I misread the question the same way for a while...I had been thinking direct alpha-oxidation via Rubottom oxidation or related--or maybe even KMnO4 if you can get the pH right--then reduce. DMacks (talk) 06:35, 22 September 2008 (UTC)

Unknown plant, possibly rowan?
Good day, everybody. After the successful identification of a spider, I have another interesting species. The parents of my girlfriend got this cute plant (at right) as a gift, and want to know possibly what it is. We suspect it to be related to the rowan, but who can be sure with plants?

Again, this is northern Poland, out in the wild. The tree is about 2 m tall, and looking strong in the chilly autumn weather. Very probably, the exact identification will be difficult, but anyone with ideas is welcome to reply. Just for your information, my girlfriend's mum had eaten one of the berries, and she survived. Cheers and thanks in advance! --Ouro (blah blah) 08:11, 22 September 2008 (UTC)


 * I'm not sure at all it's a rowan. The rowans I know all have imparipinnate leaves. It can possibly be a hawthorn or something related, but I'm really not sure. Hope this helps. --Dr Dima (talk) 08:28, 22 September 2008 (UTC)


 * I'd bet my best socks that it is a rowan of some variety. The berries are unmistakeable, and quite unlike hawthorn berries that tend to be more oval in shape and a darker, more crimson colour. The leaves also do not correspond to any hawthorn leaves that I have encountered. Some members of the sorbus genus have simple leaves. As to the variety, well, without detailed examination it could be tricky. Of course the ultimate prize is that you have discovered a new variety but you will need some botanical heavyweights to help you with that. Richard Avery (talk) 14:00, 22 September 2008 (UTC)


 * I don't know, but the fruits remind me of dogwood, the leaves of alder, but the later doesn't have red fruits. 93.132.148.11 (talk) 18:33, 22 September 2008 (UTC)


 * I think it is a sorbus, but a whitebeam rather than a rowan - from the photo of the fruit, the edges of some of the leaves are curled back showing a silver underside, which is typical of whitebeam.88.107.50.211 (talk) 21:02, 22 September 2008 (UTC)


 * I think it's Lingonberry, they're edible. Though the leaves don't look right.Mile92 (talk) 17:53, 24 September 2008 (UTC)

Done : It's Sorbus aucuparia (Common name Jarzębina in Polish) it is common and even has a place in Polish culture (mostly stories, songs and poems for children) due to it's late ripening berries. They are edible but very bitter so in Poland they have no known culinary use. See more pics on pl wiki. The irregular shape of the leaves is probably caused by larva of the Gastropacha quercifolia moth, but look at one unchewed leaf at the top right of the "fruit picture". Mieciu K (talk) 23:54, 24 September 2008 (UTC)

All thiol groups in a particular protein
I would like to know the positions of all the thiol functional groups which exist within a particular protein. Is there an online database which can give me this information?


 * In a non-post-translationally modified protein, the only thiol groups will be the cysteine sidechains. You can determine the location of these groups by looking at the primary sequence of the protein, obtainable from a number of sites, including GenBank and Swiss-Prot. (Note that the protein you're looking at will need to have been sequenced.) If you are looking for the 3D location of the thiols, you would need to consult a crystal/NMR structure of the protein. This you would probably obtain from the Protein Data Bank (Again, the 3D structure for that protein would need to have been determined already - which hasn't happened for a number of proteins.) -- 128.104.112.147 (talk) 18:52, 22 September 2008 (UTC)


 * If "non-translationally modified" means that formation of disulfide bridges is prohibited, then there will be a 1:1 agreement between cysteine and thiol groups; however, the protein will most likely not be in its native conformation. One must choose between the prior simplification and a more "native" 3D structure.  In that case, some pairs of cysteines may have been converted to cystines, and this is probably closer to what the OP was seeking.  Thus, one simply needs to know which cysteine pairs participate in disulfide bonds, and the others should be functional thiols (neglecting any other post-translational modification). --Scray (talk) 02:01, 23 September 2008 (UTC)

Space exploration
 Fribbler (talk) 12:58, 22 September 2008 (UTC)

I am constantly hearing about space exploration missions being planned to either learn about our solar system and how it evolved or to learn about the nature of the universe and how it was formed. However I think I speak for many people when I say that what I wish to know about is extraterrestial life, and the ability to travel to other planets. So can some one please direct me to a link concerning the most current developments in space travel, more specifically what we are doing to develop near light speed travel or how we are planning to get to Solar systems many light years away. Or are we not working on this atall, and if not, why not. I understand that Einstein says that light speed travel is impossible, but we must be at least attempting to discover near light speed travel or worm hole technology. So, any links? Wiki links would be most appreciated as Wiki is great. Thanks people. P.S. feel free to move this to the Science desk if you feel its would be more appropriate there. —Preceding unsigned comment added by 193.115.175.247 (talk) 12:36, 22 September 2008 (UTC)


 * Well, this sort of thing is far enough out that it's largely the realm of science fiction authors and fringe research projects -- NASA isn't going to have a big budget pile for researching interstellar travel. However, efforts such as the US's current Vision for Space Exploration can be reasonably viewed as a stepping-stone towards interstellar travel.  Bear in mind that it's been 35 years since any human went over 300 miles from the Earth's surface.  We're not going to jump from the Shuttle to the Enterprise overnight. &mdash; Lomn 13:12, 22 September 2008 (UTC)


 * The most likely method of travelling to other stars is to do so fairly slowly (a few percent of the speed of light, say) and just take a long time. If you want to do that with manned spacecraft, that requires long stays in space. The International Space Station is doing research into that very topic (mainly on the scale of months with Mars trips in mind, but it's still useful information for planning a decades long interstellar trip). --Tango (talk) 13:29, 22 September 2008 (UTC)


 * There's no point in devoting resources to finding a method of rapid interstellar travel when there's no plausible theoretical basis for it. We might as well fund people to develop X-Men-like superpowers. How are they going to go about it? Probably they wouldn't do anything except write a paper every few months glossing over their lack of useful progress and trying to justify continued funding. I think that's what happened with NASA's Breakthrough Propulsion Physics Program. You can't change the laws of physics by throwing money at them. If we do find a warp drive it will be as a byproduct of basic research—so, if you like, the LHC is one part of our investment in space travel. But I see no reason to think we'll ever discover such a thing. We can't discover it unless it's there, and why would it be there? -- BenRG (talk) 13:33, 22 September 2008 (UTC)
 * That's certainly true for superluminal travel, but near-light speed travel is theoretically possible. When you take time dilation into account, that means you can travel any distance in arbitrarily small percieved time (from the point of view of the travellers). There are all kinds of engineering problems, and a few psychological ones (the travellers would return home after a few days exploration to a world that's experiences centuries - that's going to mess with anyone's head), but no laws of nature to get in the way. --Tango (talk) 14:30, 22 September 2008 (UTC)
 * No law per se, but the shielding required to protect the astronauts from the space dust-turned-hard radiation would be huge, and the amount of fuel required would increase exponentially in order to increase the speed, in addition to linearly with the payload (mostly said shielding). — DanielLC 15:48, 22 September 2008 (UTC)
 * Yeah, but those are just engineering problems. You throw enough money at them, they'll go away. The same cannot be said for the laws of physics. --Tango (talk) 15:54, 22 September 2008 (UTC)
 * What kind of solutions do you have in mind? There's nothing in known physics to suggest that an efficient enough shielding material or ramjet is physically possible. Hoping for a miraculous unobtainium alloy is no different from hoping for a warp drive.
 * I suppose there is one approach that wouldn't require new physics: send nanomachines that reconstruct you at the other end from locally available materials. A nanomachine that constructs a human being is physically possible, as the human egg demonstrates, and it seems plausible enough that one's adult memories could be encoded in a reasonable amount of space as well. So I could be persuaded that this is "only" an engineering problem, but it's a pretty difficult one. -- BenRG (talk) 16:21, 22 September 2008 (UTC)
 * I have to disagree with Tangos dismissal of the barriers preventing us from near light speed travel as mere 'engineering problems' that can be overcome with sufficient funding. Once something is knows to be theoretically possible, it is not just a mere matter of paying to do it. To give example, particle accelerators are from a scientific point of view, simple machines that just roll charge particles down potential gradients to make them go faster, we have known how to do that for at least an hundred years, but the LHC is absolutely cutting edge in some respects. In essence proving something is an outright possibility is one thing; but deciphering a to actually do it is a monumentally larger task. I think the original posters question is a humorous example of the rift between cutting edge science and the general public that exists today. The problem is science has become inaccessible to the public; leading to naïve questioning of science, and a failure to understand it answers; or worse them thinking they understand it because they have been given simplifications and anecdotes in order to try and get there heads around it. Near light speed travel is a horrifically long way away from being a possibility, and it is a near certainty that even with all the funding in the world it will not be developed in any of our lifetimes. 92.16.148.143 (talk) 17:10, 22 September 2008 (UTC)
 * I never said the "enough money" would be a realistic amount. With enough money you could just strap on to the back of Mars and use a massive amount of fuel to accelerate it up to near light speed (we're talking more money than actually exists in the world, so I should probably say "resources" instead - time would be a very valuable resource, it would probably take centuries to gather the fuel required). There is a fundamental difference between a scientific impossibility and an extremely difficult engineering problem. I oversimplified that difference to make a point, admittedly, but the difference is very real. That said, predicting the failure to achieve something a long time in advance is very dangerous - would anyone 60 years ago have predicted that we would be able to find prime numbers with 12 million digits within their lifetimes? --Tango (talk) 17:59, 22 September 2008 (UTC)
 * No, but would anyone 40 years ago have predicted we would still be quite firmly earthbound when holidaying, or that our cars would be rolling on mere wheels; or most shockingly, that we'd actually have an energy problem right now? I think they may well have predicted the rise in computing power, due to the quite well obeyed moores law. When suggesting that all you need to do to go to the speed of light, is keep accelerating, you negate the issues that are raised by high speed travel, such as avoiding other large objects, and surviving the collisions with minute objects. It is not merely an extension of what we already know, there will be significant challenges raised by merely moving at this speed, let alone obtaining it. And supposing you want to reach this speed in a reasonable time frame, how do you suppose passengers survive the acceleration? Do you see what I'm getting at. I agree that with the technology we have now we could possibly, given the resources, time and most importantly tenacity, move an object to a high speed, however this would not open the doors to 'travel' at this speed. Owing to the fact that there are more challenging problems raised. To return to my example of the LHC, the technology behind the accelerator itself has been known about for ages; but engineering a cooling system that would stop it all melting when powered up was a challenge. On this basis I continue to disagree that light speed travel is even possible with today's technology, or even knowledge base.  —Preceding unsigned comment added by 92.16.148.143 (talk) 18:24, 22 September 2008 (UTC)
 * Predicting that something will happen is no more reliable than predicting that it won't. Moore published his law only 43 years ago, 17 years too late for someone to be using it to predict things 60 years ago. In fact, the integrated circuit was only invented 50 years ago. I never claimed today's technology would do it, of course it won't, that would mean that we have a high-speed spacecraft already. I claimed the problems could all be overcome with enough resources, I never said it would be easy. Since many of the relevant resources are produced at a finite rate (engineer man-hours, for example), it would take a long time, but it could be done, whereas travelling faster than the speed of light can't be achieved regardless of how hard you try and how long for, unless it happens that we're wrong about that particular law, which is something we have no control of. --Tango (talk) 18:40, 22 September 2008 (UTC)
 * This is taking "throwing resources at the problem" to a level I'd never before imagined. :-) Anyway, let's look at what it would take to accelerate Mars to near light speed. I'll assume constant acceleration (for reasons of comfort). Then the ship's position is given by t(τ) = (1/a) sinh (aτ) and x(τ) = (1/a) cosh (aτ). The four-momentum is parallel to (t',x') and has length m(τ), so E(τ) = m(τ) cosh (aτ) and p(τ) = m(τ) sinh (aτ). The force is E'(τ) = m'(τ) cosh (aτ) + a m(τ) sinh (aτ) and p'(τ) = m'(τ) sinh (aτ) + a m(τ) cosh (aτ). I'll assume the exhaust speed of the rockets (with respect to the payload) is constant and equal to β, and since the exhaust four-velocity is parallel to the force (E',p'), that implies
 * $$\frac{(E E' - p p')^2}{(E^2-p^2)(E'^2-p'^2)} = \frac{1}{1 - \beta^2}$$.
 * Everything cancels out nicely and I get m'(τ) = −(a/β) m(τ), which has the solution m(τ) = mo e−aτ/β; in other words, the ratio of total mass to payload is eaτ/β. Now γ = cosh(aτ) ≈ (1/2) eaτ, so the ratio needed to accelerate to a given gamma factor is about e(1/β) ln(2γ) = (2γ)1/β, a rather interesting result which is independent of a. Taking γ=10 (high enough to travel 1000 light years in about 100 years of proper time) and β ≈ 10−5 (typical for rockets), we find that we need a launch weight of about 20100000 times the mass of Mars, or about 10130000 kg. As you probably know, the total mass of the observable universe (including dark energy) is about 1054 kg. How much better than this can we do while still staying within the bounds of known physical possibility? Obviously the mass of Mars is not a major contributing factor here. The big problem is β. Unfortunately I don't know anything about the fundamental physical limits of rocket engineering. Any rocket scientists care to comment? -- BenRG (talk) 00:57, 23 September 2008 (UTC)
 * Clearly you are working that backwards. What you really want to do is consume Mars while flying to your new home.  So a better question is how much of Mars' mass might be left if you were constantly expelling chunks of Mars through a rocket at speeds sufficient to travel interstellar distances in practical time.  Dragons flight (talk) 01:04, 23 September 2008 (UTC)
 * That's easy: 20−100000 times the mass of Mars, or about 10−130000 kg (fixing a stupid error in my math) . That's including the payload and the final rocket stage, of course. But I don't think that's what Tango was suggesting; (s)he meant that Mars should be used as a shield, since the other problem with high-gamma travel is that you need huge amounts of shielding. -- BenRG (talk) 01:20, 23 September 2008 (UTC)


 * Why bother with near-light speed when we've got a warp drive? We just need to bottle up some dark energy. Franamax (talk) 17:38, 22 September 2008 (UTC)
 * Here's the paper that article is referring to. The abstract begins "Certain classes of higher dimensional models..." so it's a non-starter as far as this thread goes. The introduction says "In a manner identical to the inflationary stage of the universe, the spacecraft would have a relative speed [...] faster than the speed of light," which is wrong (common misconception). They say an advanced civilization could alter the cosmological constant by altering the radius of a hidden dimension, but since it's no more obvious how to alter the radius of a hidden dimension than to alter the cosmological constant "directly," this is basically just restating the problem in different words. Don't hold your breath for this one. -- BenRG (talk) 00:57, 23 September 2008 (UTC)
 * Dang! I already had my uniform ordered and everything! :) Franamax (talk) 22:59, 23 September 2008 (UTC)


 * Just a little point: when people think about scientific funding they think about pouring a huge amount of money into a research program. But there are very different types of research programs. One example I like to use is that of the atomic bomb. It's true that the Manhattan Project became an immense, huge project, but it started quite modestly—a study here, an experiment there, lots of theory. The reason was not, as it is sometimes portrayed, because the people running the project at that stage (e.g. Lyman James Briggs) were totally incompetent or unaware of the possibilities. It was more because at that early stage it was not clear at all it would actually be feasible in any amount of time, much less feasible in time for actual practical use in war. So the initial investment was modest. When it became more clear that it would really be worth the gamble, then the huge amount of money and manpower was thrown at the problem—but only then (and even then, they were over-optimistic—it cost some 80 times more they predicted it would when they decided to go at it whole-hog). I don't think there's any evidence at the moment that interstellar travel is something that would obviously benefit from throwing a huge pot of resources at it. (And it would only do the scientists a disfavor if resources were thrown at it and came up with nothing—it would not be something anyone would be immediately interested in funding anytime soon after that.) --98.217.8.46 (talk) 22:19, 22 September 2008 (UTC)

I'm pretty comfortable with the speed that we're investigating the possibilities for extraterrestrial life. NASA are vigerously examining Mars and the moons of Saturn and Jupiter that seem likely locations for life. We're finding ever more suitable exo-planets and we're close to being able to look for bio-residues in their atmospheres that would clearly indicate the presence of life. We have people actively searching for life in meteorites and cometary tails - and of course there is SETI and SETI-at-home searching for radio signals. There isn't a whole lot more we could economically do.

Travel to the planets is less interesting than you might think. Really, Mars is just about the only place we'd remotely be able to thrive - we could probably engineer a way to live in various other places but it would be a really pointless uphill struggle.

Travel to the stars is going to be SLOW. It's simply not going to happen in the span of one human life. I can only see two ways it'll realistically happen:


 * 1) We may find that we can robotically mine asteroids and comets to make gigantic self-sustaining habitats.  If we could do that - and if humans happily spent their entire lives living and working on them - then one day, one of them might decided to nudge itself out of solar orbit and off to the nearest star.  It might take hundreds of generations to get there - but if the people out there are just as happy as they would be near the sun - then why not?
 * 2) I think it's only a matter of time until we can scan our brains into a computer and have the computer host our minds instead of all of that grey squishy stuff.  This will give people as much immortality as they can stand.  Then there are two ways to get to the stars within your lifetime.
 * 3) Firstly, you could download your brain into a small computer on the end of a rocket and shoot it (slowly) off to the nearest star.  Being stuck there with nothing to do for a few thousand years would be very boring - so the most likely option would be to radically slow down the clock on the computer - as far as you're concerned, the universe goes into fast-forward and you get to the nearest star in (say) a couple of days (subjective time).  If there was a problem along the way, you'd be able to speed your brain up again, fix it - then go back into fast-forward mode until you got where you were going.
 * 4) Secondly, you could send the rocket off to the stars with a bunch of "empty" computers on board - when they get there and signal that everything is OK - we take the bits and bytes that make up the software that runs your brain and send it as a high power radio signal (or maybe a laser or something) off to the rocket - and when the signal gets there, you're downloaded into the rocket and it'll seem like you got there in the blink of an eye.  You have to wait an enormous amount of time for the rocket to get there, signal back and then receive your signal - but you aren't stuck in a boring rocket while it does that.  With such a system, you could be in two or more places at once of course - but if you turn off the "you" that's on earth as you shoot yourself off to the stars as a burst of laser light - then it would be more or less indistinguishable from teleportation.

SteveBaker (talk) 23:18, 22 September 2008 (UTC)

It appears to be depressingly difficult to send an unmanned probe, much less a manned spacecraft, to a star outside the solar system with present technology. This was discussed extensively in the talk pages of Gliese 581 c at Talk:Gliese 581 c/Archive 1 and in the article Interstellar travel. NASA has looked into the possibility of laser powered interstellar sail ships which might be possible with 50 years of research, which could accelerate a craft to 1/10 the speed of light, allowing a flyby of a near star after a 43 year journey. To decelerate and stop would take a 100 year journey. Launching such a ship seems a bigger leap than that from the V-2 rocket of 1944 to the Saturn rockets used by the Apollo program to land men on the moon in 1969. That was actually an engineering problem more than a gap awaiting major scientific breakthroughs. I see this as at the level of tinkerers in the 1860's talking about television, before the invention of electronics. They knew what they wanted the thing to do, and had no technology which could be used to achieve it. It is hard to predict what the next generation is going to invent, or what those inventions will lead to. Edison (talk) 16:02, 23 September 2008 (UTC)
 * Well, 50 years research is likely to result in a bigger leap than something which took 25 years... I'm not sure I get your point. --Tango (talk) 17:04, 23 September 2008 (UTC)

C4
Can anybody please tell me what the gold cylinders are in the photo on the C4 page? Baked Bean Bob (talk) 16:56, 22 September 2008 (UTC)
 * This is not a question about investment banking, is it? (Sorry, couldn't help it) 93.132.148.11 (talk) 18:08, 22 September 2008 (UTC)


 * Possibly they are doing a throw test, measuring how far the (likely brass) cylinders fly after the ka-boom? Franamax (talk) 20:59, 22 September 2008 (UTC)


 * Just guessing: perhaps they are training how to destroy captured munitions, which are represented by the cylinders? --Sean 00:54, 23 September 2008 (UTC)


 * The cylinders look like really big shell casings. Note the circular indentation on the top. They also have the number 23 stamped on them. Could it be ammunition for a 3"/23 caliber gun? Apparently similar shells are used in M109 155mm SP Howitzers.  Plasticup  T / C
 * It strikes me also that placing weights on top of the explosive would tend to confine the blast, and thus increase the explosive yield. Why is SteveBaker not stepping in with the ultra-defintive answer? :) Franamax (talk) 12:17, 23 September 2008 (UTC)


 * Those are not shells. They are containers.  Notice, the lids are under the C4.  Also, the container for the blasting caps is in the upper-left (the green container with the light and switch on it). --  k a i n a w &trade; 12:20, 23 September 2008 (UTC)

clarification as to how time taken for light to cross the universe indicates "when" you are looking at?
in many astrophysics articles it often states "the further you look the further one is looking back in time" .... can someone explain this simply as i cannot get my head around it? in my simple understanding of an expanding universe - if the outer surface of such is moving away then the further you look the more you see the current position of an item - further - as one must always be looking at a moving object, the item's lightpaths must be forming an "arc" tending towards it's current position - appreciate any simple explanation/analogy Astromeastro (talk) 19:49, 22 September 2008 (UTC)


 * The statement isn't talking about the expansion of the universe; it's just talking about light having a finite speed. Even in a static universe, it takes a while for light to travel.  So when you see a solar flare, you're seeing something that happened eight minutes ago, when that light left the sun's surface.  When you see a star go nova, you're seeing something that happened ages ago, when the light from that star left it.  That's all the statement is saying, but it's worded in a way that makes it sound more mysterious.  --Allen (talk) 20:36, 22 September 2008 (UTC)

range of the pH scale
On the Wikipedia article about pH, it has the pH scale going down to -5, while another picture has it going down to -1. I thought that 0 was the absolute limit, but apparently I was wrong. Does anyone know what the most acidic and most basic compounds known to exist are, and what their pH is? Thank you. --Freiberg, Let's talk!, contribs 22:54, 22 September 2008 (UTC)
 * The strongest known acid is Fluoroantimonic acid. Unfortunately, pH is defined in terms of concentration of hydrogen atoms in an aqueous solution, and fluoroantimonic acid reacts explosively with water, so it doesn't have a pH value. Thus we need to invent new measures of acidity for such superacids. One such is the Hammett acidity function, which gives fluoroantimonic acid an acidity of -31.3, 20 quintillion times more acidic than pure sulphuric acid. Algebraist 23:01, 22 September 2008 (UTC)


 * To expand, pH is defined in terms of the logarithm of ion concentrations. (Or not? The ISO definition on the wikipedia article pH is odd, and unlike the one I'm familiar with.) Hence, in theory, it can go to plus or minus infinity. In practice, well, I leave that to the real chemists.--Fangz (talk) 23:37, 24 September 2008 (UTC)

Twin Paradox
I've spent a good few hours over the past couple of days trying to puzzle out the twin paradox. Basically all I've figured out is what it is. I have next to no idea how it actually works. I'm wondering if someone could explain how it works to me as though they were explaining it to a group of average people who had no knowledge of the concept, if that's at all possible. Thanks!  You're   dreaming   eh?   23:06, 22 September 2008 (UTC)
 * Our article twin paradox has what appears to be a full explanation. To avoid us just repeating what it says there, you should tell us what in that article you don't understand. Algebraist 23:14, 22 September 2008 (UTC)


 * Well, what I do understand is that when the traveling twin travels away from earth, he will appear to be going slower to the stationary twin than he perceives himself going. This is due to the light taking longer and longer to travel from the traveling twin to the twin on earth. The opposite happens on the way back; light takes less and less time to travel between the two, and the traveling twin appears to be going faster than he was on the outward trip (to the stationary twin), even though (to the traveling twin) he appears to the traveling at the same speed. According to this diagram, during the time the traveling twin turns around, a large amount of time passes on earth, while almost no time passes on the ship. From what I make of the description and diagram, the difference in perceived time is due to light traveling between the two twins, and also something during the turning-around. What I don't get is how just light traveling can alter time perception. It seems to me like the traveling twin would seem slow to the stationary twin on the way out, and fast on the way back, but would still end up at the same "time" as the stationary twin, and would be the same age. Hope that made sense.  You're   dreaming   eh?   23:39, 22 September 2008 (UTC)


 * Forget about that diagram. The red and blue lines are meaningless; you can draw any lines you want on a spacetime diagram and call it an "explanation". The only part that's real is the worldlines of the two twins. The time you experience is equal to the length of your worldline. The two twins end up different ages because their worldlines are different lengths (between their two meeting points). It's easy to see that the worldlines are different lengths; it's just the spacetime version of the triangle inequality. The only tricky part is that distances are measured differently in spacetime than in ordinary Euclidean geometry, with the result that the bent line comes out shorter than the straight line instead of longer (and, in general, a straight worldline is the longest distance between two points in spacetime). But in essence the difference in elapsed time is just a geometric difference of lengths. -- BenRG (talk) 01:10, 23 September 2008 (UTC)


 * Hmm... I think what I don't get in that explanation is why the total length of the line determines time, and not just the verticle component. Actually, I think I've found a pretty good explanation here. I havn't gone through and analized it all yet, but I think it may be what I'm looking for. Thanks!  You're   dreaming   eh?   02:10, 23 September 2008 (UTC)


 * You ask why the length of the line determines the elapsed time, and I don't think anyone has an answer for that. It's just the way things are. But then again, why shouldn't it be true? Your suggestion that the vertical distance determines the time is another hypothesis about the world, one which turns out to be wrong—and why should it be right? It does work approximately as long as all the worldlines you care about are nearly parallel (that is, all relative speeds are small). In that case you can choose your vertical axis to run in the same direction, and then the length of each worldline is roughly the same as its height. That's why Newtonian physics works in such cases. But there's no deeper truth than that to the Newtonian idea of time.[[Image:Euclidean Twin Paradox.svg|right|250px]]


 * If you find the explanation on that page helpful then maybe I shouldn't argue, but I want to stress again that the twin paradox is much simpler than most descriptions make it sound. This has come up many times and it's always hard for me to explain, so I drew the picture on the right. The blue-green triangle is not a spacetime diagram, it's just an ordinary triangle in the Euclidean plane, and what the captions attempt to explain is not a difference in the ages of two people, it's just a difference in two lengths. But I've deliberately used language similar to what you'll find in many special relativity texts. I hope it's clear that, while my explanation of the length difference isn't wrong—if you work everything out you will get a consistent answer, since Euclidean geometry is consistent—it is far more complicated than it needs to be, and not very enlightening. Constructing all those gray perpendicular lines is no help in solving the problem; in fact it creates a new "problem" which has to be resolved in the fourth panel. The analogy to typical explanations of the twin paradox is extremely close. In the twin paradox the other path is shorter by √1−v² instead of longer by √1+v² and the red part of the path is counted zero times instead of two; that's about the extent of the differences. People have an intuitive understanding of Euclidean geometry and know not to construct unnecessary lines. Unfortunately many people think that you have to construct perpendiculars to every worldline when you're solving problems in special relativity, and they turn even the simplest problems into a complicated mess. Stay away from those explanations if you can. As a rule of thumb, when you see someone writing about what somebody or other "observes," they're constructing unnecessary perpendiculars. -- BenRG (talk) 19:54, 23 September 2008 (UTC)

All right, thanks for the help! I think I now understand all I need to know; a little more research is in order if I plan explaining it to other people, however. I've got no more questions for here though. Thanks again, and cheers!  You're   dreaming   eh?   02:23, 27 September 2008 (UTC)

Solaire system
How come inner 4 planetys solid, but outer 4 gas giants? I do not want to play with pluto - he is not a real palnt —Preceding unsigned comment added by 79.76.252.28 (talk) 23:13, 22 September 2008 (UTC)


 * I'm not really sure why there is such a distinction, but you may find Formation and evolution of the Solar System interesting (the answer may well be in there somewhere, I haven't looked very carefully). --Tango (talk) 23:37, 22 September 2008 (UTC)
 * Ah, found it. The inner solar system was warmer, so more volatile molecules like water and methane couldn't condense out of the solar nebula, in the outer solar system (beyond the "frost line") those molecules could condense allowing the Jovian planets to grow massive enough to capture hydrogen and helium, the lightest and most abundant elements. --Tango (talk) 23:40, 22 September 2008 (UTC)


 * Some of the theories surrounding this phenomena are being rethought in the light of some of the wilder extra-solar planet discoveries. I think the jury is still out on this one. SteveBaker (talk) 01:24, 23 September 2008 (UTC)

submandibular lymph node
What is the function of the submandibular lymph node? —Preceding unsigned comment added by Leannfan 1 (talk • contribs) 23:24, 22 September 2008 (UTC)


 * Where did you hear about the "submandibular lymph node"? Just trying to find some context. -hydnjo talk 00:46, 23 September 2008 (UTC)


 * See Submandibular lymph nodes. Someguy1221 (talk) 01:02, 23 September 2008 (UTC)
 * Also see Lymph node. The fact that they are submandibular is more a matter of geography. Fribbler (talk) 10:59, 23 September 2008 (UTC)