Science desk
< January 22	<< Dec \| January \| Feb >>	January 24 >

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

January 23[edit]

Do grass grow?[edit]

nip this in the bud

The following discussion has been closed. Please do not modify it.

I mean, no matter how long I spend looking at grass in a lawn I have NEVER seen it grow with my own eyes. Yes there are time lapse photography but if you do not see it grow with your eyes, how do you know if the grass were teleported out and slight longer but identical grass were teleported in to replace it? 202.177.218.59 (talk) 02:33, 23 January 2013 (UTC)[reply]

Because teleportation is impossible, since it violates the Heisenberg uncertainty principle. 24.23.196.85 (talk) 02:36, 23 January 2013 (UTC)[reply]

Clearly, the replacement blades of grass are stored in Russell's teapot. TenOfAllTrades(talk) 02:44, 23 January 2013 (UTC)[reply]

Assuming good faith, you might be interested in epistomology. SemanticMantis (talk) 03:18, 23 January 2013 (UTC)[reply]

I understand that some types of bamboo, which is a grass, grow so quickly you can actually see and hear it (groaning sounds). See bamboo#Ecology. StuRat (talk) 08:46, 23 January 2013 (UTC)[reply]

Why doesn't the sun float away like a balloon?[edit]

If the sun is made of helium why doesn't it float away like a balloon? — Preceding unsigned comment added by 82.132.216.9 (talk) 09:05, 23 January 2013 (UTC)[reply]

Helium rises in the Earth's atmosphere because it is pushed up by the air around it. Basically, the Earth pulls harder on the air than it does on the helium, thus the air goes down, and the helium up. There is no such force in space, because there is no air, and because it is outside the Earth's gravitational reach. Also, the helium in the sun is much denser than the helium in a balloon, and there are other, heavier elements in the sun too.

For more information about how and why the sun and the planets move, see e.g. Planetary_orbit#Newton.27s_laws_of_motion and Solar_System#Structure_and_composition. - Lindert (talk) 09:25, 23 January 2013 (UTC)[reply]

It's not true that the sun is "outside the Earth's gravitational reach": the earth gravitationally attracts the sun with the same force as the sun attracts the earth, and the effect is the earth's orbit around the sun. AndrewWTaylor (talk) 12:02, 23 January 2013 (UTC)[reply]

You're right, I stand corrected. What I meant to say was that gasses near the sun are not very much affected by the Earth's field in comparison to other forces which are far stronger. - Lindert (talk) 17:14, 23 January 2013 (UTC)[reply]

I remember several children's science books that used to say that Saturn would "float on water." Here's one, Saturn at the BBC Solar System website. This fact is supposed to be surprising! Saturn is a gas giant; most of the planet's volume is comprised of low-pressure hydrogen, helium, methane, and ammonia - almost entirely in the gaseous state. For perspective, that portion of Earth that is in the gaseous state - what we call our atmosphere - also floats on water! Otherwise we'd have oceans in the sky!

When we talk about astronomical-scale objects, it's important to keep in mind that simple approximations about buoyancy break down, because the simplest approximations of gravity break down. If you ever took an elementary physics course, you may have studied fluid pressure and approximated it using the potential energy relationship, explained in our article as the "local approximation" equation. That equation totally doesn't work over large distances - like when we're measuring things the size and density of planets. For studying these objects on these length scales, the most important effect is gravity, not fluid pressure - so buoyancy is usually not even considered. If you get into technicalities, some scientists actually model the solar wind as a fluid (albeit, as an electromagnetically interacting charged plasma - "magnetohydrodynamics") - so some of the equations familiar to fluid dynamicists do show up, with a lot of adjustments.

And of course, if you study the sun's interior, it will be no surprise that the less dense parts of the sun "float to the surface" of the sun - in a constant cycle of swirling and electromagnetically-interacting convection. You can read about helioseismology to see how we study these enormous masses of constituent gas as they "float around" inside the Sun. If you want to start straying farther from common terminology, you could technically even call the solar wind a special case of "stellar" gas escape - with is a thermally-driven buoyant process: hot gas is floating away into space. Nimur (talk) 16:51, 23 January 2013 (UTC)[reply]

Almost the entirety of Saturn is liquid. The gas of Saturn is probably more like a layer of plastic wrap around a basketball or baseball. Sagittarian Milky Way (talk) 16:40, 24 January 2013 (UTC)[reply]

Well, it's a self-gravitating, cold, gas-like, liquid-like, fluid-y ball that orbits the sun. What's the difference between gas and liquid? Most people use the terminology to vaguely refer to something about the compressibility of the substance... I'm not sure that's really a legitimate or useful distinction in a giant self-gravitating ball of stuff. Nimur (talk) 22:39, 24 January 2013 (UTC)[reply]

The point is the density of the stuff it's made of is liquid-like, liquid hydrogen, while a number of times less dense thatn oil or gasoline, is 2 orders of magnitude denser than air, and in the planet the density is increased further by compressioon. It may be a supercritical fluid and liquidey, gassy, hydrogen metal or whatever but imagining it filled with gas like in a balloon is wrong. I remember hearing that plastic wrap around a basketball thing about Jupiter. Okay so even if that is a bit of an exaggeration and I forgot about the reduced gravity of Saturn making the atmosphere even thicker it's still a big mantle of 99.9% not gas by mass surrounded by a crust of gas. Sagittarian Milky Way (talk) 15:01, 26 January 2013 (UTC)[reply]

Passing out when hurt and evolution[edit]

I've just been wondering about how sometimes, when you get hurt badly (especially when you receive a blow to the head), the brain will essentially just shut off for a while and you will pass out. Now, I know from painful experience with the occasional stupid accident that there is a point where the level of pain becomes unbearable and where passing out and waking up a couple hours later in the hospital is a real blessing, but I'm wondering how this mechanism could have evolved - because it seems rather inefficient and dangerous to me. Let's say you're an early humanoid or an animal, you're happily walking around the steppes, then suddenly you're hit by a falling rock or tree branch or something - you pass out and you're just lying around motionless for a couple hours, being easy prey for any predator who happens to come along. I understand that depending on the type of injury you received you might hurt yourself even more while trying to get away, and in any case you could probably only stumble around slowly after an accident, but at least you could try to get to safety if you didn't pass out, instead of just presenting your edible and tasty body to predators on a silver plate - all of which could be avoided if our brains had some sort of limit for the level of pain we feel. So...has there ever been any research done into that? How could this mechanism have evolved, and what evolutionary advantage (or lack of disadvantage) can there be to passing out?

And just in case it's necessary, let me add that this is not one of those "if evolution is true, then how come we have eyes/giraffes/whatever" questions - I have no doubts whatsoever about evolution in general, I'm just idly curious in this instance. -- Ferkelparade π 11:06, 23 January 2013 (UTC)[reply]

Not every characteristic we have was evolved for some advantageous reason. Evolution often results in things that were NEVER an advantage. A well known example is the routing of the urethra through the prostate gland in male humans. It is an obvious consequence of having evolved from earlier life forms but has resulted in nothing but trouble for men. If humans were designed from the outset to walk upright while carrying things in our hands, instead of being a modification of a pattern for walking on all fours without a load, you can be sure our backs would look a lot different, and not have the current spine design that results in picking up a mere 30 kg weight, well within muscle strength, causing a load of a few tons on a bit of gristle below L4 about 7 or 8 cm across. I suspect that getting knocked out is one of those things - a disadvantage inherited from early life forms that needed a modicum of decision making capability at the mouth end. An obvious solution would be a standby brain somewhere else, or a brain inside the chest, but that is just not possible with the DNA we inherited. Floda 120.145.189.213 (talk) 12:09, 23 January 2013 (UTC)[reply]

A possible explanation:

http://cogprints.org/5046/1/2006_C.A.R_LETTER_Fear_response_of_Opossums.pdf

More info on it:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC506859/

--Guy Macon (talk) 12:23, 23 January 2013 (UTC)[reply]

It probably would have been possible for evolution to develop the enteric nervous system into the main nervous system if that was worthwhile - but having the brain just beside the eyes, ears, nose an mouth certainly looks more efficient to me. Dmcq (talk) 16:52, 23 January 2013 (UTC)[reply]

Most large predators (lions, bears, wolves, etc.) are more likely to notice a slowly-moving prey item than a completely motionless one. In other words, stumbling around slowly is more likely to draw a predator's attention. 24.23.196.85 (talk) 05:12, 24 January 2013 (UTC)[reply]

Another possible explanation is that it evolved in response to fights for dominance. That is, when one person was damaged enough to pass out, this made it clear that the other won the fight, and thus stopped the fight before either was killed. Being able to decide who is in control without actually killing anyone would be an advantage to a small group, which otherwise might have had trouble maintaining it's population. StuRat (talk) 07:01, 24 January 2013 (UTC)[reply]

I'm fairly certain sturat is completely wrong. To my knowledge, it is not the norm for animals in fights for dominance to knock each other into unconsciousness - rather, one side usually backs down or is killed. Anyway, I'd question the very premise of the question, that not passing out is beneficial. I would postulate that during that passed-out period, your body is carrying out some vital life-saying processes without which you would certainly die or suffer permanent brain damage. Someguy1221 (talk) 07:10, 24 January 2013 (UTC)[reply]

He's wrong alright. Note that the OP asked about passing out in response to a blow to the head. In such cases, for humans as with almost any animal, a blow hard enough to knock you out has a high risk of brain damage including permanent damage, and also a risk of death sometime later, eg from blood clots. If you are knocked out, there is an immediate risk of death from loss of control over the epiglotis. We often see in the news media that someone has died from the so-called one punch kill - where someone has punched someone else hard enough to render them unconscious while they are still falling - the loss of the reflex to tilt the haed up and use one's hand as shields means that the victim will strike his head on the ground from the fall. If the ground struck is in fact a small rock or concrete the result is often death. This is not an evolutionary result. Loss of consciousness occurs because the brain simply cannot remain functional under impact. Note that the links provided by Guy Macon, are, if you read them, about syncope, not a response from blows, and are to that extent irrelavent. Floda 121.215.130.102 (talk) 08:41, 24 January 2013 (UTC)[reply]

Evolution is about improving statistical outcomes. Suppose there was a genetic "choice" - have version 'A' of some gene that gives you a bigger brain with less "cushioning" or have a mutated version 'B' that gives less susceptibility to blackouts but the extra cushioning left less room in the skull for the bigger-brain benefit? Certainly, choosing 'A' will result in some people dying because they get knocked out and eaten by the predator rather than staying conscious and running away...but choosing 'B' might result in yet more people dying because they are too stupid to avoid being attacked by predators and so get attacked more often. The big question is: Does that mutation from the 'A' to the 'B' version of the gene result in statistically more children being born to people who have that mutation?

If that's the kind of trade-off we might have, then you could easily imagine that a genetic change that made us less susceptible to being knocked unconscious being selected against because it brings worse outcomes overall - and therefore disappearing from the gene pool...and that's all that evolution cares about. Is this better than that at making babies? This results in a bunch of ridiculous designs - but that's life...literally!

SteveBaker (talk) 15:58, 24 January 2013 (UTC)[reply]

I wonder why mice shake when afraid and scream when hurt (I panicked after being surprised at how much shaking force didn't unglue him (like the Achilles heel myth but with olive oil and fingers) and feared he might do anything to get off now that I've shaken him vigorously. The glue didn't fail till he broke bones on the floor. Poor mouse.) It wastes energy and screaming like a squeak but loud tells carnivores where a small injured food is. Though in it's credit it turned quiet on second ~4, while it was so hurt it didn't move. Mice are not social (I think), it's not for calling for help. He was still gimpy weeks later (though not as much) so that injury is survivable. Sagittarian Milky Way (talk) 15:54, 26 January 2013 (UTC)[reply]

Fascinating, multi-faceted question. Personally I think the OP is referring to issues of Evolutionary biology much of which seems to be highly theoretical, and cognitive neuroscience another field with many uncertainties, still. Ask five brain doctors and I bet you get five different answers. I don't pretend to know who's right or wrong, but I think it has something to do with how & why the brain shuts the body down in other circumstances. E.g. when drinking too much, you pass out to prevent drinking more. That may not always be the best solution to the problem, notwithstanding specific circumstances and further risks, but it's a biological judgement call based on how those panic-mode neurons evolved. I also think it's related to the pain threshold sub-question, again shutting everything else down to assess the internal situation. And by the way, the brain is still active while you're unconscious, so it's starting a diagnostic and recovery process (if possible), before you regain consciousness. I'd hope that my brain would be able to wake me up if i was being gnawed on by a hyena, even if it meant interrupting some hard-wired pit stop. My 2c. El duderino ^(abides) 21:01, 27 January 2013 (UTC)[reply]

Curricula/syllabuses for math & science education in secondary education[edit]

I'm looking for curricula/syllabuses for math & science subjects in secondary education that are generally regarded as well designed and modern. I'm hoping that the info would be available for free or at low cost. I'm trying to see how the curriculum of our local school district compare to elsewhere in the country (US) and in the world. If particular curricula/syllabuses have a reputation of being biased, unevenly covered, or faddish, I'd be interested to know when doing a comparison.

Any pointers to where I can find the info? Thanks. — Preceding unsigned comment added by 173.49.10.182 (talk) 14:15, 23 January 2013 (UTC)[reply]

Does this meet your needs? If so, you will find other syllabuses on the websites of the English exam boards. There's a list of them here, and the articles have links to the websites.

To explain some of the terminology, you will probably want to search for GCSE and A Level qualifications - GCSE being an exam taken at age 16, A Level at age 18. Students are free to sit an exam from any of the exam boards, though usually the school will choose which exam board to use for all their students in any particular subject. The standard of examinations is very similar between boards, although the exact subjects covered does vary. This is more obvious in the Humanities (one board might cover the Cold War whilst another focuses on the rise of Communism in Russia, for instance) than in Maths and Science, but some differences can be noted. - Cucumber Mike (talk) 15:27, 23 January 2013 (UTC)[reply]

Any teacher worth their salt knows which exam boards set less rigorous exams in a given subject, giving their students a better shot at a higher mark. And they know which ones will set more interesting exams, if achieving a good mark isn't an issue for their students. It's why some independent schools use the more rigorous International GCSEs. 86.163.209.18 (talk) 13:04, 25 January 2013 (UTC)[reply]

Oh, I meant to say that the National Curriculum for England for each subject is freely available: http://www.education.gov.uk/schools/teachingandlearning/curriculum/secondary You might find this pdf of the national curriculum for mathematics (from 1999) clearer: https://www.education.gov.uk/publications/standard/publicationDetail/Page1/QCA-99-460 Here's the same for science: http://dera.ioe.ac.uk/4402/ Key Stages 3 and 4 (and 5 if A-levels) are secondary school level. Maths especially is structured around the "National Curriculum levels" (although slightly less than it was few years ago), which means someone might be working on level 4 work when they're 10, or when they're 13, depending on their progress. 86.163.209.18 (talk) 13:24, 25 January 2013 (UTC)[reply]

The Australian standard curriculum is located at http://www.australiancurriculum.edu.au/. However, this curriculum and education standards are under considerable controversy, getting a lot of flak in the media. It has been established that Australian school students are not achieving educational standards that are achieved in other countries. This is OD, but having interviewed young people for jobs from time to time over several decades, and being a graduate of the Australian education system myself (to post grad level), I can tell you that kids today get nothing like as good an education that previous generations got. Young immigrants from Asian and European countries show up good in interviews. The main issue that I see is that Australian children do not get a good grounding in spelling, grammar, and writing good prose from personal research; their natural curiosity about "why is it so" is killed by teacher attitudes, and instead of getting a science education focused on starting with fundamentals and building up from there, they get an odd mix of unrelated topics mixed up with ethics. Our Prime Minister has expressed concern about the situation throughout her time in office - however her and her education minister hasn't figured out yet what to do about it. From what I see it seems that the USA and Britain are not much better off, for similar reasons. If you would like to know why and to what extent curricula and education standards have dropped over the years in the USA, Britain, and Australia, I suggest asking in separate questions. Ratbone 124.182.17.177 (talk) 11:50, 26 January 2013 (UTC)[reply]

The Chinese "Popcorn" Cannon[edit]

http://www.youtube.com/watch?v=lhTdAMWiYeo (demo of a U.S. franchisee of a Chinese popcorn chain)
http://www.youtube.com/watch?v=Ta5jh9VglDw (... their Chinese franchisor)
http://www.youtube.com/watch?v=SRHlEMlfArA (pop rice Gangnam Style)
http://www.youtube.com/watch?v=P0zgZ-m1J3o (an industrial pop rice cannon)
http://www.youtube.com/watch?v=NoMQm2WGM9Q (firing Battleship Yamato's 18" guns!!!)
http://www.youtube.com/watch?v=GBeTvvdBnOk (pop rice Vietnam Style, a very different way)
http://www.youtube.com/watch?v=_MrXFXFGTKo (handmade Indian popcorn, how lovely ...)
http://www.youtube.com/watch?v=B7UmUX68KtE (the correct way of making good ole popcorn)
http://www.youtube.com/watch?v=q1v52f1TrWg (John Madden and his Chinese franchisee)

Is it a good idea to use the Chinese popcorn cannon to pop corn?

Generally, I have only seen people make pop rice using this thing because thanks to today's international trade and advanced agriculture, Chinese people can obtain very cheap popcorn and pop them in the correct "Swedish way". People only use these cannons to pop rice and ordinary corn. Yes, Chinese people certainly know how to use a kettle. It's just milk in China is expensive. Many have to use soybean oil instead of butter.

However, I guess this cannon can be less efficient when it comes to make popcorn because corn is much larger than rice. Therefore, the former cooks much slower than the latter. It takes longer to cook corn using the cannon and it's more likely to burn the corn. I guess this Chinese cannon is good for making popped rice. It may not be a good way to make popcorn.

Is the Indian way of making popcorn a little better? I mean if you use hot salt to cook popcorn (Chinese people use hot sand and syrup to cook chestnuts), the contact surface is increased, cooking temperature is also higher, and popped seeds do not contact the salt (they float to the top) so they are less likely to become charcoal. And the very little salt on the popped corn's surface makes it taste good.

The Japanese video clip clearly shows the before and after samples. I measured the sizes and the popped rice seems to be about 20 times larger than rice. The pressure within the cannon shall be 20 times atmosphere. There must be a practical limit where rice can be popped without breaking apart.

If you put one grain of rice into the cannon. The very limited moisture inside the rice cannot generate so much steam to increase the cannon's pressure so it's not going to pop.

If you fill up the whole cannon with rice. The rice probably contain so much water and finally it may generate too much steam.

There must be a "sweet spot" where so much rice generates so much steam and make the pressure high enough to pop the rice correctly.

I also wonder how could the Vietnamese use hot sand to pop rice. I thought popping rice requires a pressure cooker. Anyway, they are not John Madden. -- Toytoy (talk) 16:14, 23 January 2013 (UTC)[reply]

The "cannon" or pressure vessel contains limited amount of air. Heating it only increase the air pressure a little bit.

To make it easier to analyze. Let's assume the cannon contains ideal gas (PV = nRT) and dried rice. Without water in the rice, if the temperature goes from 300K to 600K, the pressure shall be doubled. The rice is burnt and it's still not going to pop.

Now we add a drop of water in the vessel and restart the experiment. Water shall vaporize and greatly increase the vessel's pressure (forget about calculations). A drop of water generates much steam. Now "n" increases and "P" also increases as a result. I know water vapor is far from ideal gas.

If we add more water, certainly there will be more vapor. "n" and "P" shall be increased greatly. The rice may not pop because in this thought experiment, the rice contains no water. Vapor outside the rice cannot be used to pop the rice.

But you know what I mean anyway. It's just a simplified way to analyze the system.

If rice contains a little moisture. More rice inside the cannon means more water and more pressure when the cannon is heated. So this cannon is quite interesting. If you put to little rice, it's not going to pop. If you put too much rice (or the rice is too wet), it may generate too much pressure.

The only way to guarantee success seems to be watching the pressure gauge. When it reaches a predetermined pressure (probably depending on the type of grain, e.g., short grain rice, basmati rice, jasmine rice, winter red hard wheat, ... something like that), you must open it to pop the grains. I think this is very different from making popcorn the "traditional way".

If you put only a grain of popcorn inside the pan, it's going to pop anyway.

However, I think you must have a minimum amount of rice in the cannon to guarantee a successful pop. -- Toytoy (talk) 17:00, 23 January 2013 (UTC)[reply]

Voltage required to accelerate a proton?[edit]

Resolved

How much voltage would be required to constantly accelerate an initially stationary, free proton by say 17 meters/sec/sec for, say 23 seconds? Would a "voltage density" variable be required for the calculation? 75.220.14.192 (talk) 17:12, 23 January 2013 (UTC)[reply]

Well after 23 seconds it would be travelling at 391 m/s, which with KE = 1/2 m v^2 and a mass of 1.67262158 × 10-27 kilograms means it has 1.2785602988599 x 10^-22 Joules which is 0.0007991001867874375 electronvolts over 3325 m is 2.4*10^-7. Hope that helps--Gilderien Chat|List of good deeds 17:42, 23 January 2013 (UTC)[reply]

Yes, that helps immensely. Thanks!

75.220.14.192 (talk) 23:23, 23 January 2013 (UTC)[reply]

Hmmm... somehow I came up with a slightly different figure when I ran through. I think a voltage density is at least implied, though the above way avoids actually calculating it. 1 volt = 1 kg m²/C s². So (electron mass kg/electron charge C) * 17 m/s² = (9.1E-31 kg/1.6E-19 C)*17 m/s² = 9.7E-11 volt/m. To keep that up for 23 seconds, during which the particle moves 1/2(23*17 m/s)(23s) = 4496 m, requires a total voltage difference of 4496*9.7E-11 = 4.4E-7. Wnt (talk) 01:54, 24 January 2013 (UTC)[reply]

Plug in C=1.6E-19eV/V and the proton's mass to get 1.77E-7eV/m. -Modocc (talk) 07:06, 24 January 2013 (UTC)[reply]

The answer should be 0.000798eV/4496.5m = 1.77E-7 eV/m. -Modocc (talk) 07:06, 24 January 2013 (UTC) -Modocc (talk) 07:06, 24 January 2013 (UTC)[reply]

Dang it, we have three people and three answers? This is why people shouldn't use Wikipedia to do their homework! Wnt (talk) 16:39, 24 January 2013 (UTC)[reply]

I just ran through my figures again and yes, my distance was slightly out and it is actually 4496.5 metres and therefore 1.78 x 10 ^ -7 eV/m to 3 significant figures.--Gilderien Chat|List of good deeds 17:57, 24 January 2013 (UTC)[reply]

When converting from Joules to electronvolts using the online conversion site here: http://www.onlineconversion.com/energy.htm I got a slightly different result 1.2785602988599e-22 joule = 0.00079801423904 electronvolts to yours. Thus, 1.774745333125764e-7 eV/m which I truncated, but it could be rounded upward... with double rounding, but looking at the article on rounding, I see it should be probably be rounded only once and downward to 1.77 as I did, so I'll stand by my answer. --Modocc (talk) 18:33, 24 January 2013 (UTC)[reply]

Hmm I agree from your answer it should be rounded down, but with mine, which was slightly higher, it should have been rounded up. Probably the different accuracy of our conversions at each stage.--Gilderien Chat|List of good deeds 15:22, 26 January 2013 (UTC)[reply]

Lifespan of a blackhole[edit]

How long does a black hole lasts ? What happens to those clouds and ashes which revolves around the black hole ? Want to be Einstein (talk) 17:27, 23 January 2013 (UTC)[reply]

Hypothetically, all black holes will evaporate via Hawking radiation, so long as their rate of accretion is less than their rate of evaporation. However, there's a nice calculation in that article which shows that even for a black hole which is not adding mass to itself, it's lifespan is something like 40-50 orders of magnitude longer than the age of the entire universe. So, it is unlikely that any black hole will entirely evaporate before the heat death of the universe, which is as close to "forever" as there is. --Jayron32 17:31, 23 January 2013 (UTC)[reply]

So, then we would have a period with a universe containing just black holes, and nothing else of interest ? StuRat (talk) 18:31, 23 January 2013 (UTC)[reply]

Note that the article equivocates, with many "citations needed", about heat death of an open or flat universe, which we think we have. "Near" absolute zero is not "at" absolute zero, if one is willing to consider processes occurring over much longer time scales. I've been rather inspired by the concept of "neutrino nuggets", for example... [1] Wnt (talk) 18:56, 23 January 2013 (UTC)[reply]

Our article Hawking radiation says that "For a black hole of one solar mass (1.98892 × 10^30 kg), we get an evaporation time of 2.098 × 10^67 years — much longer than the current age of the universe at 13.73 ± 0.12 x 10^9 years." And that's based on the assumption that the temperature of the surrounding universe is absolute zero. In fact, for that evaporation process to begin, you'd have to wait for the cosmic microwave background temperature - currently 2.7K - to fall below the innate temperature of the black hole, which is about 100nK (about one thirty-millionth of the current temperature), which is also a process which takes an unspeakably long time.

Pretty much any conceivable matter in the vicinity of a black hole will have fallen into it in a period of time that, relative to these timescales, is effectively zero. AlexTiefling (talk) 17:42, 23 January 2013 (UTC)[reply]

Note that tiny black holes evaporate much quicker and the super-massive black holes in the center of galaxies much slower. StuRat (talk) 18:33, 23 January 2013 (UTC)[reply]

Yes, but with the exception of primordial black holes, which may not exist, no black holes are smaller than 1 solar mass. Black holes form after the supernovae of massive stars, and the star needs to be at least 8 solar masses to undergo this process, creating a black hole of 3 solar masses. --140.180.244.202 (talk) 00:26, 24 January 2013 (UTC)[reply]

There are other theoretical ways to form micro black holes. StuRat (talk) 06:54, 24 January 2013 (UTC)[reply]

...which would indeed evaporate via Hawking radiation in a short amount of time. SteveBaker (talk) 14:13, 24 January 2013 (UTC)[reply]

Sound[edit]

What is the speed of sound on the moon?--YanikB (talk) 19:37, 23 January 2013 (UTC)[reply]

The Apollo 17 Lunar Seismic Profiling Experiment Final Summary report yielded V_p = 8.3 ±0.4 km/s below the superficial layers of lunar rock. The atmosphere of the moon is too sparse to meaningfully discuss sound-speed. Nimur (talk) 19:43, 23 January 2013 (UTC)[reply]

There is no atmosphere on the moon - so sound doesn't travel at all. (Well, I suppose you could argue that it travels *though* the moon - as a siesmic wave - in which case the speed would be pretty similar to the speed of seismic waves on earth - which is around 8000ms^-1.) But assuming you mean the speed that sound travels - like here on earth - through the air - then it doesn't travel at all...there simply is no sound. If you are thinking about the speed that sound would travel inside a spacecraft (like the Apollo lunar module) - then it would be the same as the speed in a similarly pressurized vessel on earth. SteveBaker (talk) 19:45, 23 January 2013 (UTC)[reply]

Irrelevant to the question. Free feel to continue discussion inside hat.

The following discussion has been closed. Please do not modify it.

Considering just the earth, we know the speed of sound is faster in water and in solids than in air. Given that, is there a significant difference in the speed of sound, in air at sea level vs. in air high in the Himalayas, for example? ←Baseball Bugs ^{What's up, Doc?} carrots→ 19:49, 23 January 2013 (UTC)[reply]

Absolutely yes. That's why pilots of very fast, very-high-altitude airplanes must understand their Mach number very carefully. You may find this section of our sound-speed article helpful. Nimur (talk) 19:55, 23 January 2013 (UTC)[reply]

From Speed of sound:

In fact, assuming an ideal gas, the speed of sound c depends on temperature only, not on the pressure or density (since these change in lockstep for a given temperature and cancel out). Air is almost an ideal gas. The temperature of the air varies with altitude, giving the following variations in the speed of sound using the standard atmosphere - actual conditions may vary.

^{[citation needed]}

Effect of temperature on properties of air
Celsius temperature θ [°C]	Speed of sound c [m/s]	Density of air ρ [kg/m³]	Characteristic specific acoustic impedance z₀ [Pa⋅s/m]
35	351.88	1.1455	403.2
30	349.02	1.1644	406.5
25	346.13	1.1839	409.4
20	343.21	1.2041	413.3
15	340.27	1.2250	416.9
10	337.31	1.2466	420.5
5	334.32	1.2690	424.3
0	331.30	1.2922	428.0
−5	328.25	1.3163	432.1
−10	325.18	1.3413	436.1
−15	322.07	1.3673	440.3
−20	318.94	1.3943	444.6
−25	315.77	1.4224	449.1

Given normal atmospheric conditions, the temperature, and thus speed of sound, varies with altitude:

Altitude	Temperature	m·s⁻¹	km·h⁻¹	mph	knots
Sea level	15 °C (59 °F)	340	1225	761	661
11,000 m−20,000 m (Cruising altitude of commercial jets, and first supersonic flight)	−57 °C (−70 °F)	295	1062	660	573
29,000 m (Flight of X-43A)	−48 °C (−53 °F)	301	1083	673	585

--Guy Macon (talk) 20:01, 23 January 2013 (UTC)[reply]

Very interesting. There is a significant difference in the speed at different altitudes. That's one factor. But thinness affects sound also. Let's suppose I could magically ascend with a boom-box set at some particular volume. As I rise in the atmosphere, would the volume slowly become fainter until there was too little air to sustain sound waves? To put it another way, if the moon somehow magically had an atmosphere whose various altitudes matched those of the earth? Would sound operate the same way as on earth? (I'm thinking the answer is "Yes", but I'm just wanting to be certain.) And what I was really thinking about was not the moon, but Mars, which does have a thin atmosphere, which tends to be very cold. So, given the above info, would sound at the surface of Mars be both slower speed and lower volume? Again, I'm thinking "Yes", but I defer to the experts. 20:14, 23 January 2013 (UTC) — Preceding unsigned comment added by Baseball Bugs (talk • contribs)

Yes, there is a difference in the ability of the medium to efficiently transmit the sound, but this is irrespective of the speed the sound travels. Thinner air doesn't transmit the sound any slower, however thinner air has less molecules available to transmit information, so there is some greater loss of information, hence greater loss of volume at greater distances. --Jayron32 20:41, 23 January 2013 (UTC)[reply]

Sound waves need a medium to propagate - unlike (for example) radio waves. You may recall from High School science class the "Bell jar experiment" - (an electric bell in a glass jar) - As you pump-out the air in the jar, the bell gets quieter and quieter - until you reach a near-vacuum and you can't hear it at all, but you can still see the clapper banging. At altitude, the speed doesn't change, (disregarding temperature), just the volume (intensity) of the wave. ~:74.60.29.141 (talk) 20:43, 23 January 2013 (UTC)[reply]

Yes, I do recall that now. It's been a while. :) OK, so if I'm understanding correctly, for the OP's question about the speed of sound on the moon, assuming he's talking about atmospheric sound waves, would be that if the moon had a measurable atmosphere, then the speed of sound would be a function of the air temperature. As it stands, there is no defineable air temperature on the moon (to speak of) because there is no air (to speak of). ←Baseball Bugs ^{What's up, Doc?} carrots→ 20:54, 23 January 2013 (UTC)[reply]

And I failed to notice the earlier reference to Atmosphere of the Moon, which indicates that technically the moon has an atmosphere, but it's extremely thin. So, supposing that it could transmit sound waves, the speed would be a function of whatever micro-measurable temperature it has, hence it would be very slow; and the volume of any such sound waves would be extremely low. ←Baseball Bugs ^{What's up, Doc?} carrots→ 21:00, 23 January 2013 (UTC)[reply]

At the moment I can't find sources for this, but... essentially, you need enough air molecules so that they can "bang into each other" in order to propagate the sound energy. Warmer molecules, being in a more energetic state, (and occupy a larger volume of space) can more easily (and quickly) transfer the sound (motion) to other molecules. [There are equations somewhere, for those who like to do the math] ~:74.60.29.141 (talk) 21:32, 23 January 2013 (UTC)[reply]

As I understand it the speed of sound depends on the density of matter. 5000 m/s in solid, 1500 m/s in liquid and 340 m/s in the air. What is the density of matter at moon ground level?--YanikB (talk) 00:02, 24 January 2013 (UTC)[reply]

Did you read above? The speed of sound in a gas depends on the temperature of the gas primarily. Density has little to do with it. --Jayron32 02:00, 24 January 2013 (UTC)[reply]

I've hatted the majority of this discussion, because it's completely irrelevant to the OP's question. The OP didn't ask about the speed of sound at various altitudes on Earth; he asked about the speed of sound on the Moon. Nimur and SteveBaker already gave the answer, and there is no need to confuse the OP with irrelevant details that are clearly well above his level of understanding. --140.180.244.202 (talk) 05:23, 24 January 2013 (UTC)[reply]

Does this show us something new about African Grey Parrot behaviour?[edit]

Apparently, this guy has taught his parrot to drive. Seriously. As I'm sure that most of us know already, African Grey Parrots are one of the most intelligent of birds - but is the one here doing something, in terms of actually controlling and directing the buggy, that we didn't think that they could do already? --Kurt Shaped Box (talk) 23:47, 23 January 2013 (UTC)[reply]

Well, it would be nice to see a peer reviewed source ... this isn't an answer, but just looking at it, my feeling is that if the parrot can be persuaded to stay on that perch, it's bound to mess with the lever. I don't see any sure sign that it is directing the vehicle to a destination - it comes toward the camera in many of the clips, but there is likely a strong selection bias in what we're seeing. It seems like every time the vehicle jiggles, it stops, and the bird is liable to move the lever any which way when it starts again. But... I can't rule out that they really have something there, either. Wnt (talk) 01:35, 24 January 2013 (UTC)[reply]

Please check 2:09 of the video

Does the bird intentionally avoid driving over the pebbles? -- Toytoy (talk) 04:32, 24 January 2013 (UTC)[reply]

It looks that way, but then again as Wnt mentions, it could be selection bias on the part of the inventor. I'd love to know if the parrot definitely gets on the buggy and uses it with the intention of *going somewhere* - which I think would be evidence of something quite significant, as opposed to just (say) going backwards and forwards and spinning in circles at random. As for the bird stopping every few seconds - it's been suggested in the YouTube comments that he's stopping to check where he's going and look on the ground in front, which necessitates (due to limited binocular vision) turning his head sideways and craning his neck. This would be amazing if true. --Kurt Shaped Box (talk) 06:39, 24 January 2013 (UTC)[reply]

See 1:43, where the parrot fails to avoid driving over pebbles. --140.180.244.202 (talk) 08:19, 24 January 2013 (UTC)[reply]

That task doesn't seem beyond the capabilities of an African grey parrot to me. It's not particularly good at it, but may get better with practice. StuRat (talk) 04:46, 24 January 2013 (UTC)[reply]

Note that the inventor is Andrew Gray. Did he change his name to match the parrots ? :-) StuRat (talk) 04:44, 24 January 2013 (UTC) [reply]

This may not be as difficult or incredible as you might at first imagine. Here is a robot vehicle that is controlled (semi-successfully) by a cockroach. He mentions other robots controlled by guinea pigs and siamese fighting fish! SteveBaker (talk) 17:54, 24 January 2013 (UTC)[reply]

Math for astronomy degrees?[edit]

Do you have to be good at math to get undergrad and grad degrees in astronomy or astrophysics? Reflectionsinglass (talk) 23:53, 23 January 2013 (UTC)[reply]

Yes, or at least be able to pass advanced math courses. The BA of Astronomy degree from Boston University lists both Differential Equations and Linear Algebra, plus the necessary prerequisites, in their "Recommended" area (I have not attempted to determine their specific nomenclature for graduation requirements). — Lomn 00:10, 24 January 2013 (UTC)[reply]

It depends on what you mean by "good at math". To get an undergrad astrophysics degree, you will definitely need to be familiar with 3rd year physics, including Lagrangian mechanics, quantum mechanics, electromagnetism, and thermal physics. All of these courses require, at the very least, familiarity with calculus and some knowledge of linear algebra, along with the mathematical concepts from the courses themselves. That said, upper-level university math courses are usually proof based, and focus on rigorously proving some theorems at an abstract level. Science courses don't usually require as much rigor or as many proofs, because physical phenomena are more well-behaved than the crazy functions that mathematicians can think of. In this sense, it's not necessary to be good at math in order to do astro. Don't be fooled, however: if you were to take a random astro student and compare his math abilities to that of the average university student, he would be much better. He would certainly have counted as "good at math" in high school, unless said high school was exceptionally prestigious and focused on the sciences. --140.180.244.202 (talk) 00:21, 24 January 2013 (UTC)[reply]

I am not aware of any major accredited university that grants undergraduate degrees in astrophysics. If anyone knows of one, please feel free to post a link to that program; I would be interested to read about it. It is far more common to pursue an undergraduate degree in physics, or math (or even chemistry or biology), and then pursue graduate study in a physics department. Few graduate programs even have an "astrophysics" department; most graduate astrophysics Ph.D. students are rolled into the physics graduate program under the guidance of one or more specialized faculty. Nimur (talk) 00:31, 24 January 2013 (UTC)[reply]

It depends on what you mean by "major", but see [2]. I agree that what you said is a much more common route. --140.180.244.202 (talk) 00:37, 24 January 2013 (UTC)[reply]

Also, Earth and Space Exploration, from Arizona State University. It is worth emphasizing that there is a distinction between astronomy and astrophysics. Nimur (talk) 00:39, 24 January 2013 (UTC)[reply]

(ec) While I'm inclined to agree with your impression, there are at least a few schools that seem to have – or have recently had – an undergraduate program in astrophysics. There are a number of schools and individuals that refer to offering or acquiring an "undergraduate degree in astrophysics". The College of Charleston is one; McMaster University in Canada offers a physics degree with Astrophysics Specialization; UC Berkeley's Department of Astronomy has an astrophysics major; Princeton University's Department of Astrophysical Sciences offers an astrophysics major. (I'll be honest, I was a bit surprised at how many schools actually do have a distinct, degree-granting 'Department of Astronomy/Astrophysics' or similar.) TenOfAllTrades(talk) 00:53, 24 January 2013 (UTC)[reply]

Leicester University has this course, and you will see there is no mention of maths related subjects on that page. It may be lurking in the detail. --TammyMoet (talk) 10:39, 24 January 2013 (UTC)[reply]

Umm, actually, it's in virtually every sentence on that page.

"Astrophysics is the application of physics and mathematics..."

"You will study core topics in physics and mathematics"

"The core programme builds upon the First Year foundations and includes relativity and particles, waves and fields, condensed matter physics and electromagnetism." (as I mentioned in my previous post, all of these are typical courses for a physics student, and require mathematics)

"Core topics include quantum physics, atoms and nuclei, radiation and matter and plasma physics." (again, all typical physics courses)

"The core programme includes fluid dynamics, quantum solids, and statistical physics." (requires statistics) --140.180.242.224 (talk) 20:51, 24 January 2013 (UTC)[reply]

Astrophysics "is taught at Oxford as part of the undergraduate Physics degree", with an option for much of the third year (of the three-year course) being an astrophysics project. (Astrophysics is a sub-department of the Physics department.) "Everyone who applies to study Physics or Physics and Philosophy at Oxford must sit the Physics Aptitude Test. There are no exceptions". Try out a sample of that test, including its mathematics components, here. The main target audience for that test would be British 17 and 18 year olds, pretty much all of whom would be studying both Physics and Mathematics, and possibly Further Mathematics, at A-level in the UK equivalent of high school. Less aggressively selective universities may not be as concerned about mathematical aptitude. --Demiurge1000 (talk) 12:15, 24 January 2013 (UTC)[reply]