Wikipedia:Reference desk/Archives/Science/2008 October 10

= October 10 =

Particle Physics
What is the difference between the Higg's boson and the graviton? They both are hypothesized to explain gravity, right? So how are they different? Ζρς ι'β' ¡hábleme! 00:44, 10 October 2008 (UTC)


 * The Higgs boson is the unobserved member of the Standard Model that is supposed to explain inertial mass. The Standard Model does not deal with gravity at all.  The graviton is the hypothetical force carrier for gravity within various theories of quantum gravity. Dragons flight (talk) 01:29, 10 October 2008 (UTC)


 * That's right. Sorry, I got the ideas cross for a second.  Higgs deals with mass and the standard model, and graviton deals with gravity and hasn't anything to do with the standard model, really.  Thanks, Ζρς ι'β' ¡hábleme! 02:19, 10 October 2008 (UTC)

Turning off all electronic equipment during take-off and landing
Why are airline passengers instructed to turn off all electronic equipment during take-off and landing, even equipment that does not contain radio transmitters or receivers? I overheard a conversation recently, in which a fellow passenger claimed that it is done to ensure that people pay attention to what is being said over the loudspeakers, in case of emergencies during the most critical parts of a flight. Can anyone confirm this, or suggest other reasons for this requirement? --NorwegianBluetalk 11:07, 10 October 2008 (UTC)


 * I've heard the same reason (on numerous ocassions) as you suggest. LIke you say it ensures people are not distracted if there is a need to make an annoucement/emergency decisions. I have been told to stop reading my book before so I would suggest it is more about paying attention than it is about anything else. 194.221.133.226 (talk) 11:19, 10 October 2008 (UTC)
 * In the past it could have been do to with interference (even without transmitters any electronic equipment will emit some EM, I believe), but I'm pretty sure all critical systems on planes are shielded these days. As such, it is probably just to make sure people pay attention and, if not, at least don't make too much noise stopping other people from hearing announcements. On a related note, the reason you aren't allowed to use mobile phones in hospitals is simply because it annoys people, it's been a long time since medical equipment was sensitive to such things. --Tango (talk) 11:21, 10 October 2008 (UTC)


 * In general it's both. EM interference is a legitimate risk (though a much smaller one than when the rules were written in the 60s and 70s), and it is easier to swtich off all electronics than have flight attendents try to figure out which ones actually need to be disabled.  At the same time, the FAA also cites the "possibility of missing important safety announcements during these important phases of flight"  as an additional reason to turn off electronics during takeoff and landing.  Dragons flight (talk) 11:32, 10 October 2008 (UTC)


 * Note as well that handheld electronics represent dangerous projectiles in the cabin in the event of a crash. Headphone cables can present a tripping hazard.  On takeoff and landing, the cabin crew want you to stow everything securely, not just electronics. TenOfAllTrades(talk) 13:30, 10 October 2008 (UTC)


 * The turning off electronics thing is just to "make sure", but realistically there's no point. If turning on an electronic device could really interfere with the cockpit's electronics, then terrorists would have a field day. 98.221.85.188 (talk) 14:41, 10 October 2008 (UTC)


 * The initial justification, Crossair Flight 498, was pretty lame since there were other confounding factors involved. That said, I can hear my speakers making odd noises when I point my cell phone at them the right way, and if I were talking to a control tower to avoid smacking into somebody at 400 knots, I think I'd rather the pilot have a clear signal.  SDY (talk) 14:51, 10 October 2008 (UTC)
 * Your speakers (and the cables attached to them) aren't shielded from EM interference, I would hope the flight deck radio is. --Tango (talk) 15:10, 10 October 2008 (UTC)
 * How does that work with wireless communication, though? Then again, I'd imagine that the cell phone bands are all quite separate from the bands that aircraft use.  SDY (talk) 15:18, 10 October 2008 (UTC)
 * Is anything in planes wireless? The computers they use for duty free transactions might be, but that's hardly a critical system! --Tango (talk) 15:27, 10 October 2008 (UTC)
 * Many planes have satellite radios, satellite TV, etc. for the passengers. Not to mention all of their telemetry equipment that is used to monitor where the plane is, how it is flying, etc. by flight control. --98.217.8.46 (talk) 15:49, 10 October 2008 (UTC)

The thinking is that if some of the electronic equipment onboard had been stripped of shielding (say, by shoddy maintenance) then your electronics could interfere. Of course, the plane has a high-voltage radio of its own, which would produce a thousand times more interference than your iPod. It is a dumb rule, but lots of these FAA rules are. They are rituals meant to make you feel safe, not actual safety measures. The lifejackets are a great example. How long do they spend teaching you how to put on a lifejacket? "Your life jacket is located under your seat, or under the arm rest between the seats. Pull the life jacket over your head and attach the strap. Infant life jackets will be distributed, if required. Do not inflate your jacket until you leave the aircraft. Pull the strap until the jacket is properly adjusted. If the life jacket does not inflate or needs more air, blow through the rubber tube." It's a nice image, you bobbing safely in the water with a bright yellow life jacket on. How many people have they actually saved? Zero. Meanwhile hundreds of people die from smoke inhalation which can be prevented by a lightweight mask. There is no rhyme or reason.  Plasticup  T / C  16:05, 10 October 2008 (UTC)


 * Are you sure of that number? I'm aware of several water landings where there were survivors; are you saying that in none of the cases were life vests used? --Carnildo (talk) 22:32, 10 October 2008 (UTC)
 * They shouldn't have been used if the evacuation went as planned since everyone would be in inflatable life rafts. Of course, if you're making a water landing, things aren't exactly going to plan, so... --Tango (talk) 23:14, 10 October 2008 (UTC)
 * Carnildo, for my interest, could you point to a water landing where there were survivors? My impression is that no commercial (large) jet passengers have ever survived a water impact. Skidding off runways, yes, but not "crashes". I'd be interested in the details. Franamax (talk) 00:58, 11 October 2008 (UTC)


 * See Ditching. From the article this crash] had 52 survivors. - Akamad (talk) 02:19, 11 October 2008 (UTC)


 * And more specifically, Ethiopian Airlines Flight 961, although I'm under the impression that life jackets actually killed more people than they saved in that particular incident. --antilivedT 05:12, 11 October 2008 (UTC)


 * Seen another way, improper use of life jackets caused loss of life, because people inflated them prior to exiting the plane, which is directly contrary to standard instruction. Maybe the relatively protracted training reflects the complexity of using these devices properly.  Perhaps they should spend more time on when to inflate than how to inflate. --Scray (talk) 15:02, 12 October 2008 (UTC)


 * They usually say the standard, 'pull one just before you leave the plane, pull the second one after you leave' whenever I've been in a plane, that I recall anyway. Also, I think your summation is more accurate. We don't actually know whether it costs more people their lives then it saved. It's possible many of those who survived would have died without lifejackets and many of those who died would have died anyway. Nil Einne (talk) 13:16, 13 October 2008 (UTC)


 * coincidentally, yesterday:
 * Safety investigators will now ask passengers if they were using any electronic equipment at the time of this latest incident. "Certainly in our discussions with passengers that is exactly the sort of question we will be asking - 'Were you using a computer?'," The Courier Mail quoted an Australian Transport Safety Bureau (ATSB) spokesman as saying. The ATSB said the pilots received messages about "some irregularity with the aircraft's elevator control system", before the plane climbed 300 feet and then nosedived. but apparently they've decided laptops were innocent.
 * that article does contain the following surprising (to me) sentence, though: In July, a passenger clicking on a wireless mouse mid-flight was blamed for causing a Qantas jet to be thrown off course, according to the Australian Transport Safety Bureau's monthly report. Gzuckier (talk) 05:33, 11 October 2008 (UTC)


 * Thanks, everyone, for your responses! --NorwegianBluetalk 12:52, 11 October 2008 (UTC)


 * This one on the same incident also mentions modems and previous cases Nil Einne (talk) 13:12, 13 October 2008 (UTC)


 * Now seems that in the specific case that brought all this to light, it wasn't interference

problem sum
a constant retarding force of 50 newtons is applied to a body of mass 20 kilograms moving initially with speed of 15 metres per second. how lomg does the body take to stop —Preceding unsigned comment added by 77.31.137.209 (talk) 16:22, 10 October 2008 (UTC)


 * Not that we should solve homework problems for you, but consider that a newton is a kilogram meter per second squared (kg*m/s2) and simple factor label cancelling (i.e. do the algebra with the units to figure out how to multiply and divide the numbers) should give you the answer. The article I linked shows the basic framework for solving problems like this. --Jayron32. talk . contribs  16:51, 10 October 2008 (UTC)
 * While I would probably do the same as you, the more standard approach is the learn the constant acceleration formulae. Pick the appropriate one of them along with F=ma and substitute in the numbers. --Tango (talk) 16:58, 10 October 2008 (UTC)
 * True, but that requires one to either memorize a list of formlas, or to be able to work the calculus on one formula to derive the rest. The nice thing about the factor-label method is that it requires learning a single method that is broadly applicable accross MANY fields.  It will get you the right answer, for example, in any high school or introductory collegiate chemistry and/or physics class for, quite literally, 90% of the homework problems you will get.  One method, 90% of the problems.  The "learn every formula method" also works, but is, IMHO, more labor intensive and time consuming. --Jayron32. talk . contribs  17:33, 10 October 2008 (UTC)
 * Couldn't agree more, but it's generally best to help someone get to grips with the method they're being taught (which is almost certainly the memorise formulae method) rather than teaching them a whole new method. --Tango (talk) 19:01, 10 October 2008 (UTC)
 * You've all forgotten that the dimensional analysis, while certainly very useful, is no substitute for knowing why you're multiplying things in the first place--it's simply a way to check that you haven't combined units in a nonsensical way. For example, for energy stored in a spring, you may be tempted to use U = kx^2, when the formula is actually (1/2)kx^2. If you were to merely use dimensional analysis you would arrive at the wrong answer. Actually, the best way is to learn everything, including the calculus behind it. Then you'll never have any doubt as to whether the formula has a 1/2 or not; just do the integral. --M1ss1ontomars2k4 (talk) 21:03, 11 October 2008 (UTC)
 * Actually, you don't even need calculus to get the constant acceleration formulae, you can work them out geometrically from distance/time and velocity/time graphs (well, I guess you could say it's calculus because you need to know how to interpret the area under the curve, etc., but you don't actually need to integrate or differentiate anything). But you are correct that the method does mean you are occasionally out by a factor of two (it's always two...), but I generally ignore factors of two, they're rarely important! ;) --Tango (talk) 22:56, 11 October 2008 (UTC)

threshold logic synthesis
can anybody help me to know how to find the false vertices for threshold logic synthesis....... —Preceding unsigned comment added by Sveta rathi (talk • contribs) 19:04, 10 October 2008 (UTC)


 * Wikipedia has an article on Logic synthesis that also has a long list of referenes and other links at the end. This may be a good place to start.  --Jayron32. talk . contribs  19:20, 10 October 2008 (UTC)

"Virgin birth" in a shark
http://ap.google.com/article/ALeqM5gV-UePymWuPU7HFxNgUXRUrakU1wD93NPTM80

How did this exactly happen? Can the same thing happen to humans? --Emyn ned (talk) 20:07, 10 October 2008 (UTC)


 * The process is called parthenogenesis. (Our article discusses the matter in some detail.)  Briefly, there's never been a substantiated case of parthenogenesis in any mammal (including humans) in the wild.  Induced parthenogenesis has apparently been demonstrated in rabbits and mice, producing viable offspring.  Human parthenogenesis has been demonstrated to the extent of creating human embryonic stem cells from unfertilized eggs, though no human beings have been born via this method.  TenOfAllTrades(talk) 20:22, 10 October 2008 (UTC)


 * it's not too hard to make an egg cell of any animal start to divide, even if not fertilized; it's all caused by a bunch of calcium entering, calcium being a handy ion in organisms for signals like that, it's not too scarce like magnesium and not too abundant like sodium. The chromosomes provided by the sperm actually don't have anything to do with triggering the egg's development, but the whole thing is set up that the arrival of the sperm triggers that calcium influx which starts the division, and the arrival of the chromosomes in the same package is basically a happy accident. but you can trigger the calcium influx via drugs and things in eggs of many species and away they go. once the cell starts the process of dividing and then duplicating chromosomes and dividing again, the fact that it only has one set of chromosomes instead of two gets fixed as that set gets duplicated after a cell division. (the other product of the division has no sets of chromosomes, and just sort of fades away, but that's not a problem; cells early in the division process aren't specialized yet, each one can produce a complete embryo if they're separated, that's where identical twins come from.) so at that point you've got a dividing egg cell with two sets of chromosomes, and you're on your way. the fact that both sets are identical isn't critical, but of course any undesirable recessive genes will pop up.


 * in sharks? well we're seeing a lot of it now, because we didn't look for it before, so it's likely that some mechanism has evolved which can trigger the egg reasonably frequently without the need for sperm. there are other species which don't have males at all, of course, so this is just a less emphatic version of that. might happen occasionally in other animals too; people for instance. it would be hard to prove, particularly if it was pretty rare. there is this one story about a virgin, a couple of thousand years ago.... Gzuckier (talk) 20:29, 10 October 2008 (UTC)


 * It could happen in humans, I believe. Unfortunately or fortunately, only females would be produced in such cases, due to the XY sex chromosome thing. Imagine Reason (talk) 04:15, 11 October 2008 (UTC)

i really should know this but
how do volumes add in a liquid solution? i know all about partial pressures, etc. but if 10 ml of alcohol is added to 90 ml of water, is the result 100 ml? how about when dissolving solids in a liquid? thanks. Gzuckier (talk) 20:18, 10 October 2008 (UTC)


 * Unfortunately, there's not a simple answer to this one. In general, the volume of a solution will not be equal to the sum of the separate volumes of its components.  In other words, if you add 10 mL of ethanol to 90 mL of water, the final volume will come out to be slightly less than 100 mL (about 99.5 mL, actually).  This discrepancy will depend on the compounds being mixed, and on their proportions.  (If you add 40 mL of ethanol to 60 mL of water, the final solution will be a shade less than 98 mL volume.)  Dissolving solids in liquids has similar problems.


 * Conceptually, you can think of the molecules of solute being able to at least partially occupy gaps left between the loosely-packed solvent molecules, but that's an awfully hand-waving description. TenOfAllTrades(talk) 20:44, 10 October 2008 (UTC)


 * As a more detailed description, consider that there is in the individual pure liquids, the macroscopic "volume" property is determined by a microscopic property we can call "intermolecular distance" that the molecules seperate themselves by. So, there is a water-water intermolecular distance and a ethanol-ethanol intermolecular distance.  When you mix the two, you create a new interaction, the ethanol-water interaction, which is a shorter distance than either the ethanol-ethanol or water-water distance.  This makes sense if you consider that in order to for two substances be miscable, the molecules of each substance must be more attracted to each other than to themselves.(if the water and ethanol were more attracted to themselves than to each other, then the two would merely aggregate seperately, and would not mix).  More attractive force means shorter distance between them.  So a solution of two substances should always occupy a smaller volume than the sum of their pre-mixed volumes.  --Jayron32. talk . contribs  23:13, 10 October 2008 (UTC)


 * That's a completely different description. The first one is wrong, and just happens to partially explain a little. — DanielLC 16:22, 11 October 2008 (UTC)


 * Wow. I mean, really? Come on guys, let's keep our feet on the ground. Yes intermolecular forces in the solution will increase with the presence of a solute (except when combining immiscible solutions), but the resulting change in volume is unnoticeable unless you have spectacular equipment or a tremendous amount of material. When you add salt to water the melting point may drop but the volume does not, even though the packing of molecules in the liquid changes significantly. For all intents and purposes the OP's original intuitions are correct: 10ml EtOH + 90 ml H2O = 100ml. The only real concerns with respect to change in volume when adding liquids is whether or not they'll react, causing product to evolve out of solution or causing the temperature of solution to change. --Shaggorama (talk) 05:02, 13 October 2008 (UTC)
 * Er no, really really, the OP asked whether volumes add, and gave an example. The answer is no, they almost never exactly add, and in particular it's "close but not quite" for the example given. The why is a different story, but volume of mixing is a real phenomenon (and can be greater or less than the sum of the parts). Mixing a solute into a solvent and looking for change in solution (vs solvent) volume is indeed often a small deviation. But still, that's just a question of temperature whether the solute is solid or liquid, not something about the solution. You can dissolve a heck of a lot of sugar in water and only get a marginal increase in volume, which just proves that volumes often don't add, even approximately. DMacks (talk) 05:11, 13 October 2008 (UTC)
 * There seems to be a pretty clear consensus here so maybe I should just sit down and shut up, but I've taken chemistry through college level orgo and the phenomenon at hand never reared its ugly head. I maintain that for all intents and purposes it can be ignored. Unless the op is in a high level course (in which case they certainly wouldn't be asking this question) then the phenomenon can certainly be ignored for the purposes of performing ceteris peribus chemistry math on paper. Furthermore, the quantities of solution and necessary significant figures they will be using in lab preclude concerning about it there. I'm not denying that the phenomenon exists, I'm arguing that :
 * it's not even close to as significant/noticeable as has been suggested, and
 * we should therefore be advising the op not to worry about it and do the math intuitively since that's what they'll be observing in the real world.
 * Dmacks, I don't know where you're going with the sugar example. I introduced the salt example to illustrate that intermolecular forces do not cause significant decrease in volume, as had been suggested earlier. That adding solid solute offers effectively no change to volume, as you pointed out in the sugar case, lends support to my stance. I hadn't suggested that solid + liquid adds the same way as liquid + liquid. Feel free to elaborate if I misunderstood. --Shaggorama (talk) 06:16, 13 October 2008 (UTC)
 * I think you did understand but maybe are seeing a false dichotomy. There are indeed two issues: does solvent volume change when a solute is added and does the total volume change add when two volumes are mixed. Common experience says "little if any" to the first (salt or sugar in water) and "approximately yes" to the second (alcohol plus water). But wait...those are contradictory positions! If adding solute doesn't much affect the solution volume, the solute acts as if it has no volume of its own. Otherwise, if adding a volume of solute increases the solution volume by its volume, then the whole intermolecular-attraction and fills-in-the-spaces idea is wrong. Now here's where I think you misunderstood. There's no intrinsic difference between "chemicals that are solids" and "chemicals that are liquids", and there's no memory once in solution whether the solute happened to have been a solid or liquid. Solid+liquid vs liquid+liquid is just a matter of whether the experiment is done at a high enough temperature that the solute melts. I picked sugar just as a parallel example to salt, but as something that can be melted easily to ponder whether "physical state" matters. DMacks (talk) 06:44, 13 October 2008 (UTC)
 * You make a good point and I'm inclined to agree with your reasoning, but that doesn't make the phenomenon itself any stronger. You could never actually add liquid sugar to liquid water because sugar melts above water's boiling temperature. Once you dissolve sugar to add it to pure water as an aqueous solution, you're end result is still mostly water and the volumes will add intuitively. Perhaps I should have qualified my rule-of-thumb for aqueous solutions, but I'm fairly certain it holds true in hydrophobic solutions as well. I'm inclined to agree with you on the "false dichotomy" you pointed out, but in empirical terms solids that dissolve in water do not significantly change the volume of solution, and liquids do (maybe because they are solutions in water). --Shaggorama (talk) 07:16, 13 October 2008 (UTC)
 * How about trying to mix some liquid water with alcohol at just above °C  and mixing that same amount of water as ice with the same amount of alcohol at just below  °C until they dissolve, then compare the volumes. DMacks (talk) 19:10, 13 October 2008 (UTC)

What is the movement of a submarine called?
Sailing? Driving? —Preceding unsigned comment added by 87.165.220.170 (talk) 20:55, 10 October 2008 (UTC)


 * 'Sailing' appears to be a widely-used, widely-accepted term. (Google submarine sailed or submarine sailing to see many, many examples of usage.)  'Driving' is definitely not.  TenOfAllTrades(talk) 23:30, 10 October 2008 (UTC)


 * It's odd that, for ships, at one time "sailing" was updated to "steaming" but now seems to have reverted to "sailing" even though the technology has gone somewhere else entirely. I guess dieseling, electricking or nuclear reacting just don't roll off the tongue.  Sp in ni  ng  Spark  12:41, 11 October 2008 (UTC)


 * So we sailed up to the sun . . . in our yellow submarine - Lennon-McCartney

Sudden moon
I am writing a story set on a planet that has suddenly acquired a moon. For the purposes of this question, just assume it suddenly appeared and that both the planet and moon are the same size as ours.

I'm trying to work out exactly the nature and scale of the disasters this would cause. All I've got so far are extreme tidal waves and flooding, but would there also be earthquakes? I think it would probably result in massive unbalancing and possibly death among nocturnal species, not to mention the probable extinction of a lot of tidal zone species. What else, though?

While I'm at it, what would a world be like that did not have a moon? It would have very small tides thanks to the sun, and would always be very dark at night... One of my friends claims that it wouldn't have seasons, but I find that dubious. Any thoughts? --Masamage ♫ 21:12, 10 October 2008 (UTC)


 * I don't think you would get anything more drastic that we get every day, since as the Earth rotates different parts of the Earth are affected by the moon's gravity (you might get some problems immeadiately after the arrival since you would have the effect of 6 hours worth of tidal change in an instant [although the magnitude of the tides would be the same as for the Earth], but that wouldn't last long, although the damage from it might). The fact that life wouldn't be used to the tides would be a problem, certainly, but I think that's about it (and remember, it's just increased tides, not new tides, because of the sun). As for a planet without a moon, it would probably affect the seasons, but it wouldn't preclude having them. Seasons are caused by the rotational axis being tilted with respect to the orbit, the moon may well have affected our axial tilt, but having a moon isn't a requirement to have one. The day would also be shorter, since the same tidal forces which mean the moon always shows the same face to Earth are gradually slowing the Earth's rotation, without the moon that wouldn't have happened so the day would be a few hours shorter (I'm not sure how many, but I believe it's been measured by looking at fossilised coral). I read somewhere that the moon has helped stabilise the Earth's rotational axis, but I'm not sure how, so the seasons may be more variable without a moon (although probably on the scale of centuries at least). --Tango (talk) 21:31, 10 October 2008 (UTC)


 * Instantaneous appearance? You'd get at least the following:
 * A sudden shock as the planet shifts from following a simple orbital path around its sun to the sine-wave pattern the Earth follows. (The center of gravity of the Earth-Moon system follows an elliptical path around the Sun; the Earth and Moon orbit that center of gravity with a period of one Lunar month).
 * Gradually-increasing tidal heights, with the final tidal range being about three times what it used to be. Timing of the high tides will also change.  You won't get tidal waves because the water has a long way to flow to adapt to the new gravity patterns, and it doesn't move very fast.  At a guess, it'll take a month or two for the tides to reach their final heights.
 * More earthquakes. They won't be stronger, and they might be weaker, because of increased tidal flexing of the crustal plates.
 * Disruption of activity for many species: some night-active species will have trouble being active during full moons; some day-active species will stay active at night during full moons.
 * Increased predation of day-active prey species. The increased night-time lighting means that camoflage patterns and sleeping habits are no longer adequate for protection.  It'll take about five years for the resulting boom-and-bust of night-active predator species to settle out.
 * Over the long term, you'd see the following:
 * More reliable seasons. Adding a large moon will stabilize the planet's rotation axis, so the strength of the seasons won't change over time (millions to billion of years)
 * An increase in the number of night-active species. More light means it's easier to move around at night. (hundreds to tens of thousands of years)
 * A reduction in meteor impacts: the Moon provides some protection against meteors by variously blocking them, causing them to break up, or throwing them out of Earth-crossing orbits.
 * An increase in volcanism: the increase in tidal flexing will increase the temperature of the planet. (Hundreds of thousands to millions of years)
 * Hope this helps. --Carnildo (talk) 22:58, 10 October 2008 (UTC)


 * If you are interested in this subject, and want to see how another author has treated a similar situation (though not exactly the same), may I recommend Jack McDevitt's book Deepsix, it has some interesting descriptions of weird tidal effects caused by two large planets on a collision course. Its part of his "Priscilla Hutchins" series, and is an enjoyable read.  --Jayron32. talk . contribs  23:05, 10 October 2008 (UTC)


 * Why would it take months for the tides to reach their maximum? The water doesn't need to move any faster than it does on Earth and it can go all the way round the Earth in 24 hours (well, individual bits of water don't, but you know what I mean). --Tango (talk) 23:13, 10 October 2008 (UTC)


 * On Earth, the tides have had billions of years to build up momentum. The Moon's gravity isn't very strong, so the water doesn't accelerate very fast. --Carnildo (talk) 23:29, 10 October 2008 (UTC)


 * That's utter nonsense.


 * The water oscillates back and forth twice per day. So the momentum builds up slowly over about six hours then back the other way over the next six. It doesn't "build up" over millions of years.  The tides would settle into their regular pattern in about a day or two.  I think there could be no earthquakes or anything because the earth too squeezes and stretches in a cycle over 12 hours and we don't see particular problems because of that.  I agree though that the SUDDEN arrive of a few gigatons of stuff in orbit would very abruptly jerk the planet in it's orbit - and that would be utterly disasterous - all of the oceans and atmosphere would slosh violently - possibly flying off the planet completely...it's hard to imagine any life surviving that.  But if the moon somehow slowly spiralled into position over decades - then I think it would have fairly benign effects (well, crazy weather - tides where they'd never been before - flooding, rivers running backwards...but definitely something you could survive).  You'd also have to consider the consequences of the moon suddenly being dumped into the much stronger gravity well of the planet.  Since we have no knowledge of the "magic" that makes the moon suddenly teleport into place - we can't guess what forces that entails - but it might well break up, ending up as a truly spectacular ring system...or possibly raining death and destruction onto the surface. SteveBaker (talk) 01:12, 11 October 2008 (UTC)


 * The change in orbital motion would not cause a "shock"; each planet is in free fall, and would continue falling freely even if in a different direction. The only "sudden shock" would be from tides: expect a lot of earthquakes at first.   —Tamfang (talk) 01:39, 14 October 2008 (UTC)

hydrogen reaction
Will a balloon filled with pure hydrogen and pierced with a needle made of palladium explode/combust? —Preceding unsigned comment added by Kaufmann1 (talk • contribs) 21:57, 10 October 2008 (UTC)
 * Not unless the palladium is particularly hot. Palladium can act as a catalyst for certain reactions involving hydrogen, however as far as I am aware, it does not lower the activation energy of the combustion reaction enough to cause it to become spontaneous.  --Jayron32. talk . contribs  22:56, 10 October 2008 (UTC)


 * Pure hydrogen? What would it react with? --Carnildo (talk) 22:58, 10 October 2008 (UTC)
 * The oxygen in the atmosphere outside the balloon, presumably. Algebraist 23:05, 10 October 2008 (UTC)


 * Our article on palladium says it can absorb large amounts of hydrogen. I'm not sure if that reaction releases energy, but I doubt it would do so to the extent of causing an explosion. --Tango (talk) 23:10, 10 October 2008 (UTC)


 * Where can I get a palladium needle, to try the experiment? Edison (talk) 19:30, 11 October 2008 (UTC)
 * You would probably have to turn it into a needle yourself, but you could buy a palladium coin here (for about $200, although they're out of stock apparently...). --Tango (talk) 22:51, 11 October 2008 (UTC)
 * However, a needle or other solid form is just about the worst you can do if you're trying to get a rapid reaction. The reaction between a gas and a metal (or "chemical adsorbed or absorbed onto/into the metal" happens at the metal surface, so the bulk of the solid metal below the surface is useless. Much better to use powdered Pd (or Pd coated on some other powder) or a porous matrix. The large surface area allows much more rapid reaction. I've seen active metals like Pd and Pt supported on powdered charcoal--the form commonly used for lab-scale catalytic hydrogenation reactions--lead to combustion with atmospheric oxygen even without substantial hydrogen present (I'm sure helped by the charcoal being flammable). DMacks (talk) 18:46, 12 October 2008 (UTC)

Mars climate
How does climates on Mars work?Does temperats range by latitudes, seasons or night-and day. Ithouhgt Mars is a very cold planet, often colder than a freezer, and the average planet temp is around minus 67 F. Is that the mid-latitude average surface temp? I thought only tropical zones of Mars or low latitudes get temp range from +10 to +69 F.-- Freeway 9  1  22:15, 10 October 2008 (UTC)
 * Interestingly, we have an article on that: Climate of Mars. --Jayron32. talk . contribs  22:58, 10 October 2008 (UTC)


 * The variation of temperature with latitude, seasons and day/night don't really depend on the planet so much as how it moves around the sun. Pretty much all of the planets have all of those things (with the sole exception of Mercury which keeps the same face pointing towards the sun all the time - so it doesn't have day/night cycles).
 * Variation by latitude is because the planet is round and the sun's rays spread out more at the poles than at the equator.
 * Variation by season is because most planets are doing their daily rotation about an axis that's tipped over somewhat. This means that the suns rays are more spread out at some times of the year than others.  For planets with very elliptical orbits, there is a variation due to distance from the sun too.
 * Variation between night and day is because the sun isn't shining on the surface at night.
 * So all of those things vary on all planets that are round, have an axial tilt and rotate on their axis...and that includes Mars.
 * SteveBaker (talk) 00:47, 11 October 2008 (UTC)
 * Actually, Mercury isn't tidally locked (although astronomers did think it was at one time). See Mercury (planet). --Tango (talk) 00:55, 11 October 2008 (UTC)
 * Seconded. Apparently Steve is stuck in 1964;-) --Stephan Schulz (talk) 01:01, 11 October 2008 (UTC)
 * In more ways than you can imagine! SteveBaker (talk) 01:20, 11 October 2008 (UTC)

So isn't temperate zone on Mars lattide of 30+ always or often below 0, and lattitude of 50+ alwas colder than Greenland? Generally, Mars I thought is very cold.-- Freeway 9  1  01:04, 11 October 2008 (UTC)
 * Because of its thin atmosphere it doesn't retain heat well, so at night it is going to be extremely cold regardless of your latitude. During the day, it will be warmer the nearer the equator you are (well, not quite the equator due to the axial tilt). I don't know any numbers off the top of my head, but they shouldn't be too difficult to find with a bit of googling. --Tango (talk) 13:53, 11 October 2008 (UTC)