Wikipedia:Reference desk/Archives/Science/2009 May 5

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May 5

meaning of computer

define in simple terms the meaning of the term computer —Preceding unsigned comment added by 79.75.105.45 (talk) 01:04, 5 May 2009 (UTC)

Please do your own homework.

Welcome to Wikipedia. Your question appears to be a homework question. I apologize if this is a misinterpretation, but it is our aim here not to do people's homework for them, but to merely aid them in doing it themselves. Letting someone else do your homework does not help you learn nearly as much as doing it yourself. Please attempt to solve the problem or answer the question yourself first. If you need help with a specific part of your homework, feel free to tell us where you are stuck and ask for help. If you need help grasping the concept of a problem, by all means let us know.

We're not allowed to give you the answers to homework questions - but we can give you ideas about where to look. In this case, it's pretty obvious: our article on Computer. --Scray (talk) 01:07, 5 May 2009 (UTC)

I'm not sure this is homework. I've been working with computers since the late 1960's - and the simplest, most elegant definition I have is: "A computer is a machine for following rules." - but our Computer article is the place to go. SteveBaker (talk) 03:32, 5 May 2009 (UTC)

Sadly, I can only claim late 1970's. However, I'd suggest the term defies a simple definition. For one thing, are you talking about humans or mechanical/electronic devices? Our computer article seems devoid of mention of those humans who were so vital to, among other things, several war efforts. Franamax (talk) 07:02, 5 May 2009 (UTC)

The etymology of the word compute: 1631, from Fr. computer, from L. computare "to count, sum up," from com- "with" + putare "to reckon," orig. "to prune." Computer used for person, 1646; mechanical calculating machine, 1897; and electronic machine, 1946 (ENIAC) or 1941. [1] Cuddlyable3 (talk) 10:01, 5 May 2009 (UTC)

Simple English Wikipedia says "A Computer is a machine that does exactly what a person tells it to."[2]. If only that were true! But seriously, Simple English Wikipedia is sometimes useful if the main Wikipedia article is incomprehensible. --Maltelauridsbrigge (talk) 12:33, 5 May 2009 (UTC)

I'm reminded of the rhyme:

"I never liked this damned machine,

I wish that they would sell it.

It never does quite what I want -

but only what I tell it."

Does anyone know who wrote that?

it reminds me of Nipsy Russell and his joking rhymes, but that's just a wild guess.Somebody or his brother (talk) 17:55, 8 May 2009 (UTC)

So the Simple English Wiki part about it doing "exactly what a person tells it to" is actually correct (with the emphasis on exactly!)...although that's a really poor description. I mean - a car is a machine that does what we tell it to do, is that a computer? As a computer programmer, I know that it always does exactly what I tell it (unless it's broken in some way) - but human frailty is such that I very often fail to tell it what I want. (Only yesterday, I told it to do this - albeit in a much more complicated manner:

 for ( unsigned int i = 6 ; i >= 0 ; i-- ) something() ; // Do something 7 times you stupid heap of...

Who said anything about doing something() 4,294,967,290 times?). Also, it might not be doing what a person tells it to do. It might be doing what another computer tells it to do...so I don't like the SE definition - but not for the reason Maltelauridsbrigge doesn't.

The opening sentence of our Computer article says:

"A computer is a machine that manipulates data according to a list of instructions."

...which is pretty simple too (actually, I may have written that line back when I was beating that article into shape). But I like "A machine for following rules" better because the line between data and instructions is vague - and the word "manipulate" is also a little unclear.

@Franamax - the use of the word "Computer" to mean a human who is doing arithmetic is hopelessly obsolete - nobody uses it in that way any more...not since the 1950's anyway. That's unnecessarily complicating a simple question.

SteveBaker (talk) 13:25, 5 May 2009 (UTC)

Actually, that loop loops forever, not just four-odd billion times. --Trovatore (talk) 01:28, 7 May 2009 (UTC)

Steve Baker

Why cannot I find an online pic of Steve Baker? Rarely a question goes unanswered that Mr Baker has an input into. Perhaps he is shy? Or perhaps he is a lady masquerading as a man? russ (talk) 01:13, 5 May 2009 (UTC)

I thought Steve Baker was just whatever you thought he was. Something of an antipode to Room 101. DMacks (talk) 01:21, 5 May 2009 (UTC)

I really don't think this is the place for this discussion. His Talk page would be a bit more appropriate. --Scray (talk) 01:33, 5 May 2009 (UTC)

You're really not trying very hard. I found one within a couple of minutes. Exxolon (talk) 01:58, 5 May 2009 (UTC)

They're all over the place. Looie496 (talk) 02:14, 5 May 2009 (UTC)

From my Wikipedia user page you can find a link to my web page - from there you can find my family photo gallery. Of course, whether ANY of that is true is impossible to know - anything could be faked. Signed photos will cost though! :-P SteveBaker (talk) 03:22, 5 May 2009 (UTC)

Here he is! Axl ¤ [Talk] 06:39, 5 May 2009 (UTC)

Wow, that's way different from the black & white version. Franamax (talk) 06:51, 5 May 2009 (UTC)

Yeah, but who's the old guy? Is that his dad or something? -- Captain Disdain (talk) 07:13, 5 May 2009 (UTC)

No - she's just masquerading as a man. :-) SteveBaker (talk) 14:32, 5 May 2009 (UTC)

Sorry you got the photos mixed up. That's User:Edison. Incidentally the other photo is quite old, this is a more recent pic [3] Nil Einne (talk) 16:36, 5 May 2009 (UTC)

Oooh! Nice cars! SteveBaker (talk) 23:10, 5 May 2009 (UTC)

You have Bill Clinton eye-bags!!! Were you also Rhodes scholar by chance? 79.122.45.107 (talk) 07:13, 5 May 2009 (UTC)

I'm most interested in the Temple of Tux he seems to have in his back yard. I wonder what sort of divine intervention can be granted by paying tribute there. APL (talk) 13:15, 6 May 2009 (UTC)

Overeating fish

The word on the street is that if you overfeed the fish in your fish tank, they can die. Is this actually true for any species of fish that might reasonably be expected in a fish tank in someone's house? What's the death mechanism, and how do such fish survive in the wild? Tempshill (talk) 06:06, 5 May 2009 (UTC)

My understanding is that it is not overeating that is the concern. Rather the worry is about the accumulation of uneaten food. If uneaten food accumulates it can be broken down by bacteria and algae into compounds like ammonia which can be toxic to fish if allowed to accumulate in the water. So the real trick is not to put more food in the tank than your fish are willing to eat. Dragons flight (talk) 06:19, 5 May 2009 (UTC)

My understanding is that the fish suffocate. Cuddlyable3 (talk) 09:48, 5 May 2009 (UTC)

Aerobic bacteria will consume oxygen whilst at the same time possibly producing noxious compounds, so Df and Ca3 are ilkely talking about the same phenomenon - excess nutrients in the water. In parts of the open ocean and in closed lakes like the Black Sea, anoxia dominates, but I have no idea how that translates into a small aquarium system. Wouldn't the bubbler thingy take care of the anoxic condition? Franamax (talk) 11:19, 5 May 2009 (UTC)

It was my understanding that some fish, goldfish in particular are prone to overeating but not all sources [4] seem to agree. I have to admit though, I don't really get the common suggestion you feed only as much as a fish can eat in 5 minutes unless overeating is a concern. While having food 6 hours later is likely to be a problem, surely having them take 20 minutes to eat is not a concern unless overeating is. Unless of course they would usually finish it within 5 minutes or else take hours to finish it Nil Einne (talk) 16:31, 5 May 2009 (UTC)

Help understanding a poem

Ralph Barton wrote a poem called "Twinkle, Twinkle, Little Star," reprinted in Fantasia Mathematica, which I was flipping through tonight (see Google Books). I'd like to know what the somewhat mangled formula is supposed to be. Is it something to do with parallax or the angular resolution of telescopes? —Bkell (talk) 07:46, 5 May 2009 (UTC)

1.73 arcsec is the deflection of a star's light caused by the gravitational field of the Sun (at the edge of a Sun), i.e. the difference between the star's positions with and without the Sun being in front of it. It was measured by Arthur Stanley Eddington and others during a solar eclipse in 1919 (can't find an english article that quotes the number; this German article does, although it gives 1.75 arcsec). This was one of the first observational tests of general relativity and caused quite a bit of excitement in the media at the time because it contradicted the value predicted by classical Newtonian mechanics (which gives half the value). The equation is some sort of metric (i.e. a description of the geometry of spacetime), used to derive the deflection value. It looks weird, though - giving it a bit of slack, one might assume that dO and d+ should actually be Greek, dφ and dθ, maybe. It should be the Schwarzschild metric, maybe in an unusual coordinate system. It might also be gibberish, loosely modelled after the Schwarzschild metric. --Wrongfilter (talk) 10:10, 5 May 2009 (UTC)

Thanks. —Bkell (talk) 01:26, 7 May 2009 (UTC)

sawtooth wave amplifire.

i'm using a sawtooth wave signal generator.(circuit diagram in http://www.electronic-circuits-diagrams.com/oscillatorsimages/2.gif). the output voltage and current of the circuit is too low for my purpose. where can i get a circuit diagram to amplify the output voltage & current of the sawtooth wave generator. i need peak output voltage of about 30-40 volts and current of 8 amp or above. —Preceding unsigned comment added by Shamiul (talk • contribs) 08:49, 5 May 2009 (UTC)

You will need to design a Class A amplifier with resistors to define gain. Power dissipation in the amplifier is 320 watts peak, 160 watts average. Supply voltage should be slightly above 40V. The voltage gain can be adjustable 2x to 8x. Some negative feedback will ensure the output sawtooth is linear. Cuddlyable3 (talk) 09:37, 5 May 2009 (UTC)

Since this is for a sawtooth wave, I suggest Class AB or B. Getting rid of the Class A's 160W is not trivial, and you need a monstrous power supply to match. - mako 05:37, 7 May 2009 (UTC)

We don't know the intended purpose of the 8 (or above) amp sawtooth but if space or weight are limited then I agree that a weighty heatsink with a fan are unacceptable features of a simple Class A amplifier. The coolest (pun intended) solution that avoids the big heatsink is a switching amplifier i.e. Class D. The power supply would still be half "monstrous".

If the power supply is not a battery, one might modulate its internal reference voltage so that it delivers the sawtooth directly. Cuddlyable3 (talk) 23:32, 8 May 2009 (UTC)

Indeed. I wonder what application the original poster has in mind. - mako 03:49, 9 May 2009 (UTC)

How to convert between micrograms per ml and parts per million?

Does the chemical or substance involved make any difference? 78.146.219.21 (talk) 09:28, 5 May 2009 (UTC)

According to our ppm article, parts per million is a unit ratio, so it's grams of one thing/grams it's contained in or voltage change/average voltage. So ppm is a dimensionless number. Is microgram/ml also dimensionless? Franamax (talk) 09:47, 5 May 2009 (UTC)

It might be added that 1 ppm is the same as 1 microgram per gram, so if we're talking about a relatively small amount of something dissolved in a relatively large amount of something that has about the same density as water (on account of, for instance, being water), then 1 ppm is pretty much the same as 1 microgram per ml—not exactly the same, but probably close enough. But that's assuming all that. —JAO • T • C 10:06, 5 May 2009 (UTC)

Ssshh! :) We're way across the homework line now. Franamax (talk) 10:46, 5 May 2009 (UTC)

I'm sick of every question I ask being considered homework! Just because I can put things in a concise and objective manner does not mean its a homework question. As I've mentioned before, I'm not a student, and there are very few students of my age. 78.144.240.92 (talk) 13:40, 5 May 2009 (UTC)

I sympathise. If this happens again, click on the 'discussion' tab at the top of this page - add a new section there explaining the situation - and "the right thing" will happen. It is tough for us to know with any degree of certainty that a question is homework - and we do make mistakes. The persons involved should have used the {{dyoh}} template - which is carefully apologetic under such circumstances. However, we are NOT allowed to answer homework questions - so we do have to make some effort to estimate the likelyhood that the question falls into that category - but it's always a judgement call. Sorry! (FWIW: It would help if you created a user account for yourself so we could more easily recognise you - that would definitely reduce the chances of this happening again!) SteveBaker (talk) 14:08, 5 May 2009 (UTC)

Sorry for the assumption I made 78.144! Homework questions quite typically look like the one you asked, as if they were copied off a piece of paper. You could help us (or me at least) by adding some context like "I'm trying to figure out comparisons of metal content in tea". Then we have some idea of why you are asking the question. Franamax (talk) 01:13, 6 May 2009 (UTC)

Microgram is a measure of mass, milliliter is a measure of volume. If the substances you're talking about have the same density as water - where (roughly) a milliliter weighs a milligram then one microgram per ml is one part per million by mass. However, if you were talking about dissolving silver into mercury then one microgram per ml would be nothing like one part per million. The term 'parts per million' is vague because it's unclear whether this is by mass, by volume, by moles or by some other arcane measure. However, parts per million is definitely a dimensionless quantity - and micrograms per ml isn't. So you absolutely cannot do the conversion at all without at least the following information:

• The density of at least one (and possibly both) of the two substances being mixed.
• A clear statement of what is being asked for. ("parts per million by XXX" where 'XXX' is...what?)

Without those two pieces of information - you're unable to perform the conversion. However, if you know both pieces of information then you can use the densities to convert micrograms of the first substance and milliliters of the second into whatever units you're doing the parts-per-million comparison in (mass, volume, etc). But you have to be VERY careful about the terminology used in the question. SteveBaker (talk) 12:56, 5 May 2009 (UTC)

A milliliter of water weighs a milligram? That's some light water. :) —JAO • T • C 13:05, 5 May 2009 (UTC)

Thank-you! My bad. Somebody forgot to refill the fridge with those little bottles of Starbucks Frappuccino here at work. <grumble> (I've fixed my post BTW). SteveBaker (talk) 13:50, 5 May 2009 (UTC)

The substances involved are aluminium/aluminum and water. I want to compare the numbers given here "The concentration of aluminum in tea infusions was 1.49-5.58 micrograms/ml in wulong tea, 0.90-4.92 micrograms/ml in green tea, and 0.64-4.35 micrograms/ml in black tea." http://www.ncbi.nlm.nih.gov/pubmed/8254994 and here http://www.vegsource.com/harris/brain_aging.htm 78.144.240.92 (talk) 13:45, 5 May 2009 (UTC)

Then, to a reasonable degree of approximation (I guess "tea" has a density close to 1 kg/liter) - you can just use those same numbers and call them "parts per million by weight". If you need "parts per million by volume" - then you have to figure out the volume of that mass of aluminium. Since aluminium is roughly 2.7 times denser than tea - the "parts per million by volume" would be roughly 2.7 times smaller than the "parts per million by weight" value. SteveBaker (talk) 13:57, 5 May 2009 (UTC)

As a side question, why is w/v often expressed as a percentage? That seems rather scientifically sloppy to me. SpinningSpark 21:07, 5 May 2009 (UTC)

As long as the ratio is consistent, does it matter how scientifically sloppy it is for the purpose of comparison? I can choose between 1%, 2% and 4.5% milk at the store, and I can choose between 3.5%, 5% and 7% beer at the other store. In those cases, the precise physical definition is unimportant. Similarly for grades of chemical purity, 95% "technical" grade, 98% "fine" grade, 99.5% "pure" grade - it's an internal comparison. Franamax (talk) 01:23, 6 May 2009 (UTC)

Extra-specific contagious yawn

May yawning be contagious between different species? I had sometimes the impression I was yawning after my dog or my cat did. I also tried the experiment of inducing them a yawn, pretending I was: it seemed it worked, especially with my cat. Is it possible/is there a documentation on this phenomenon? --131.114.72.215 (talk) 10:21, 5 May 2009 (UTC)

The last paragraph of Yawning#Contagiousness mentions a study. Also a quick googling for things like dog yawn contagious and monkey yawn contagious find mentions. Just thinking of your cat yawning makes me yawn... 88.114.222.252 (talk) 13:54, 5 May 2009 (UTC)

Stabbing the brain dead

If you stab someone who is brain dead, is that murder? You could always argue that it's impossible to kill an already dead person. --Mr.K. (talk) 12:32, 5 May 2009 (UTC)

No. The human body and the law by David W. Meyers[5] says that in the USA and UK death is taken as either brain death or the cessation of circulatory and respiratory function. After that point, removing someone from life support is not killing, and removing organs is not assault/battery. So you would not be committing murder; actually the analogy with organ removal is not too far-fetched, as without this legal definition it might be illegal to harvest someone's organs. Prior to this definition, it was possible to argue that if you attacked someone and left them brain-dead on a life support machine and the doctor turned the machine off, then you were not guilty of murder because the doctor killed them; but now you are assumed to have killed them if you render them brain-dead. On the other hand, there may be laws governing how dead bodies are treated, so your stabber may still be guilty of some crime. --Maltelauridsbrigge (talk) 12:45, 5 May 2009 (UTC)

But you should wait until the victim has been medically declared dead, and preferable be in possession of a signed death certificate, just to be on the safe side! Dbfirs 12:50, 5 May 2009 (UTC)

Indeed, you usually need a doctor to declare somebody dead in that situation. It would be difficult for the prosecution to prove they weren't dead, though, so you probably wouldn't get convicted. (Assuming you could provide some reasonable evidence that they were brain dead - if you cause biological processes to stop in someone whose heart was beating, I think that would be taken as a priori evidence of murder and the burden of proof would shift to you.) --Tango (talk) 12:59, 5 May 2009 (UTC)

If you are doing this without proper authorization there is no doubt some crime involved — thus doctors can't just unplug people just because they want to, they have to get family's permissions and things like that. Whether it would be "murder" is doubtful, but there are plenty of other possible crimes. --98.217.14.211 (talk) 13:02, 5 May 2009 (UTC)

If you did this "inappropriately", I'm sure they'd find something to convict you on. Certainly nobody is legally dead until pronounced dead by a doctor - so until that happens - you'd be on very shakey ground indeed. If you did this without evidence from the doctor, you could perhaps even be convicted of "intent to murder" on the grounds that you had no way to know the person was dead. But testing the outer boundaries of the legal system is dicey - to say the least - who knows what some judge might happen to decide? SteveBaker (talk) 14:03, 5 May 2009 (UTC)

I don't think it is strictly true that you always need a doctor's declaration of death. Most deaths are just handled by a coroner, no doctor is required. (It is usually fairly easy to tell when somebody is dead - they are cold and stiff and blue-grey lips and pale skin.) It's only in cases where it is difficult to know for sure that a doctor is need (obviously, brain death but still on life support is such a case). The case of Dora Kent is worth reading if you are interested in this subject. --Tango (talk) 14:20, 5 May 2009 (UTC)

Mistreating a corpse is a crime everywhere, I believe. Looie496 (talk) 14:49, 5 May 2009 (UTC)

Perhaps, but it isn't murder. As Steve says, they would probably find something to convict you with, but it may well not be murder. --Tango (talk) 15:31, 5 May 2009 (UTC)

"Intent to murder" is not the definition of a crime. The potential prosecution would be for attempted murder. Cuddlyable3 (talk) 23:55, 8 May 2009 (UTC)

_{Legal disclaimer: we do not give legal advice on Wikipedia. Please consult your lawyer before stabbing someone who is dead, brain-dead, or alive. Axl ¤ [Talk] 06:25, 7 May 2009 (UTC)}

Put another way, if you feel you want to stab a brain dead person then visit a lawyer.Cuddlyable3 (talk) 23:55, 8 May 2009 (UTC)

Mr.K why is you asking us respectable and perfectly law abiding Wikipedians this question? Do you have a job for Frankie, Guido or Scarface which is all legitimate businessmen whom we don't know? Cuddlyable3 (talk) 23:55, 8 May 2009 (UTC)

Act of (Human) Crying

Is there a website somewhere that decribes step-by-step what's happens when a human cries. I couldn't find a site nor is it in Wikipedia. Thanks!--Reticuli88 (talk) 13:04, 5 May 2009 (UTC)

It's on Wikipedia, in tears. We need some better wikilinking, I think. Looie496 (talk) 14:47, 5 May 2009 (UTC)

Standing on the world's axis

If I was standing exactly on the north (or south) geographical pole, would I be able to actually feel the Earth rotating ? Rosenknospe (talk) 14:39, 5 May 2009 (UTC)

It rotates at 1/2 the speed of the hour hand on a watch. Looie496 (talk) 14:45, 5 May 2009 (UTC)

I like your response, which I guess was lost on the OP until made more explicit below. I have a question though: if you had a 24-hour analog clock and you were standing right at the North pole (not magnetic but true north) and put it down on the ground, would it "keep" pointing at something, despite the Earth's rotation, and what would that something be?? Thanks! 79.122.21.123 (talk) 11:14, 6 May 2009 (UTC)

No, the rotation is far too slow to perceive it directly. When rotation is used on space stations to produce artificial gravity it might be perceivable (see Artificial_gravity#Rotation) but that is far faster rotation than the Earth's. --Tango (talk) 14:47, 5 May 2009 (UTC)

If you are standing in Germany, say, then the Earth is moving under you at something between 950 and 1150 km/h, and completing one turn of a circle every 24 hours. You don't feel that motion, do you? --Anonymous, 15:44 UTC, May 5, 2009.

No I don't, but if I was standing at the pole, I would be basically spinning, like at the center of a turnabout, wouldn't I ? Couldn't I even feel that ? Rosenknospe (talk) 16:15, 5 May 2009 (UTC)

Yes, you would be spinning, but only a rate of one revolution every 24 hours - that is very slow. It is 1/4 of a degree a minute. --Tango (talk) 16:18, 5 May 2009 (UTC)

Damn, it sounded like a good idea ;D Thank you very much for the quick answer, guys ! Rosenknospe (talk) 16:27, 5 May 2009 (UTC)

You're welcome! --Tango (talk) 18:27, 5 May 2009 (UTC)

If you happen to have a handy Focault pendulum hanging around - you can watch the effect of the earth spinning. The one at the science museum in London shows this in an extremely powerful way. SteveBaker (talk) 20:48, 5 May 2009 (UTC)

There's a nice one in the Boston Museum of Science as well. APL (talk) 13:07, 6 May 2009 (UTC)

During summer at a pole you would be able to see and feel the weak warmth of the sun rotating around you every 24 hours. To operate a Focault pendulum successfully you would need to build a big igloo to protect it from winds. Cuddlyable3 (talk) 00:09, 9 May 2009 (UTC)

Inspite of the tremendous speed of rotation of earth at the equators you still dont feel that you are rotating.This is because to perceive that you are rotating you would need some reference.For us there is no reference.The clouds ,the winds etc all rotate with the same speed.So there is no way of feeling that you rotate.To make things simpler consider a merry-go-round on which you stand.There you can sense the rotation because you many still objects around you and hence you feel you are moving.

Cross-breeding: human + something

Without considering legal or moral constrains: is it possible to cross-breed a human with some other form of life - like a bonobo or other chimp since they are more or less like us? Has it been done? If possible, but never done, how far are we from it?

See Humanzee. —JAO • T • C 15:13, 5 May 2009 (UTC)

I think there has been too much genetic drift between us and other species to allow any crossbreeding. Livewireo (talk) 18:02, 5 May 2009 (UTC)

There are examples of very different species interbreeding. There are even some instances of species in different families interbreeding, although I think that is more a sign of the inadequacies of our taxonomic definitions that anything else. --Tango (talk) 18:34, 5 May 2009 (UTC)

Certainly, we have things like geep, ligers and tigons as examples of how fairly different species can be crossed. But there are limits. Humans and chimps - for all of our genetic similarities - have different numbers of chromosomes - and that makes interbreeding extremely problematic - if not actually impossible. So it may well be simply impossible...which is probably a very good thing! SteveBaker (talk) 20:46, 5 May 2009 (UTC)

Horses and donkeys have difference numbers of chromosomes, but mules exist. It's not that simple. --Tango (talk) 20:52, 5 May 2009 (UTC)

But chromosome number does have something to do with it!!! Lions and tigers have the same number of chromosomes (19 pairs), which means that during every cell division the chromosomes get copied, pair up with the homologous partner, and probably segregate relatively faithfully. The Geep article refers to a fusion of embryos to form a chimera, whereas the actual Sheep-goat hybrid is typically stillborn due to the difference in chromosome complements (30 pairs in goat vs. 27 pairs in sheep) leading to mosaic aneuploidy in the developing fetus. Humans (23 pairs) and Chimps (24 pairs) might theoretically be able to produce an embryo but without knowing how the chromosomes will pair during mitosis, it's hard to say whether it would be live-born. With regard to the disparity in chromosome number between horses (32 pairs) and donkeys (31 pairs), again, you'd need to know how the chromosomes line up during mitosis and how faithfully they segregate (which I don't). The difference in chromosome number certainly does have a lot to do with the sterility of the hybrid. --- Medical geneticist (talk) 21:31, 5 May 2009 (UTC)

Indeed, the structure of the chromosomes (of which, number is a key part) is very relevant, but it is not as simple as Steve implied. --Tango (talk) 22:42, 5 May 2009 (UTC)

Sorry - I didn't mean to imply it was simply impossible - merely that the gene-count mismatch is a severe obstacle to interbreeding...which it clearly is in most cases. Incidentally, I just found Zonkey, Zebrinny, Zebrula, Zedonk, Zorse and Zony...I just love the names for these things - but I begin to suspect people attempt these crosses simply to have bragging rights to a new silly name! Sadly, I was unable to include Tijuana Zebra in the list! SteveBaker (talk) 23:06, 5 May 2009 (UTC)

I've long suspected something similar with telescopes. --Tango (talk) 00:04, 6 May 2009 (UTC)

op: no, not possible. no, hasn't been done. 94.27.168.220 (talk) 21:59, 5 May 2009 (UTC)

Do you have some kind of evidence or reference for your assertion that it is impossible? --Tango (talk) 22:42, 5 May 2009 (UTC)

tried it. 79.122.21.123 (talk) 10:56, 6 May 2009 (UTC)

Even if you have, you can't infer impossibility from lack of success. Perhaps it is just very difficult. --Tango (talk) 15:57, 6 May 2009 (UTC)

no, tried exhaustively. not just difficult: not possible. 79.122.21.123 (talk) 19:00, 6 May 2009 (UTC)

If the problem is the different number of chromosomes (23 pairs in humans, 24 pairs in chimps), could an interbreed between a man with Down syndrome and a female chimp be more successful? Of course, the experiment is ethically so appalling that it would never be approved. However, is it technically possible? Could the DNA be pre-processed before insemination to reduce chances of a miscarriage?--Mr.K. (talk) 11:20, 6 May 2009 (UTC)

No, it doesn't work like that. Someone with Downs has an extra copy (ie. 3 total) of chromosome 21, chimps only have two copies of that chromosome, so the chromosomes still wouldn't line up properly. It's not just the number that is important but rather which chromosomes each parent has - they have to be able to pair up properly. --Tango (talk) 11:56, 6 May 2009 (UTC)

OK, then let's forget the Down guy-chimp hybrid. But how about cutting and pasting a human DNA and injecting it into a chimp cell to create an embryo? According to DNA replication some processing can be done: "DNA replication can also be performed in vitro (outside a cell). DNA polymerases, isolated from cells, and artificial DNA primers are used to initiate DNA synthesis at known sequences in a template molecule." However, would the resulting DNA be in the correct shape?--Mr.K. (talk) 12:33, 6 May 2009 (UTC)

I'm not sure I understand the question. What human DNA are we injecting into what chimp cell? You could quite possibly inject the DNA (perhaps the entire nucleus) from a human cell into a chimp ovum (with the nucleus removed) and produce what would be mostly a clone of the human, but with chimp mitochondria and other organelles. (No-one has successfully cloned humans using human ova yet, but I think that's mostly for ethical reasons, rather than scientific ones.) If you took the DNA from a human gamete and injected it into a chimp gamete, it wouldn't be any different to regular insemination. --Tango (talk) 13:48, 6 May 2009 (UTC)

The idea was to take human DNA of a gamete (23 chromosomes, not 23 pairs) and complete it with an extra chromosome from a chimp. You would obtain an almost human gamete with the right number of chromosomes. Fuse it with a complete gamete of a chimp and try to inseminate a chimp with it. --Mr.K. (talk) 15:08, 6 May 2009 (UTC)

Nope, doesn't work. Humans aren't missing a chromosome, two of our chromosomes have merged. You could try and cut that merged chromosome in two, but I doubt that would work (it still contains some of the telomeres, etc., from before the merger but probably not enough to function as two separate chromosomes). Alternatively, you could remove the merged chromosome and replace it with the appropriate two chromosomes from a chimp. Or vice versa - remove the two chromosomes from a chimp gamete and replace them with the merged chromosome from a human. There are other differences that would make a hybrid difficult, so it wouldn't be a done deal even you if could manage such a transfer. I'm also not sure what genes are on that chromosome, so I'm not sure how big a difference it would make to the hybrid - it wouldn't be a 50/50 split, though. --Tango (talk) 15:56, 6 May 2009 (UTC)

With everything said above could i ask would it have been possible in the DISTANT past when we had not quite evolved into what we are today to be like other spieces either Chimp Horse Cat ECT leading to many of the ancient storys of cyclopse and half man half horse hybrids or even the sphinx? Chromagnum (talk) 05:40, 7 May 2009 (UTC)

It is theorised that proto-humans and proto-chimps interbred for about a million years after the lines first started to diverge, but that is only interesting because the lines didn't diverge all that long ago (so the proto-humans and proto-chimps weren't all that different to modern humans and chimps). If you go back to just after the human and horse, say, lines diverged then there might well have been interbreeding between them, but the two species would have been nothing like humans or horses. --Tango (talk) 13:26, 7 May 2009 (UTC)

Some people suspect that the cyclops myths arose from birth defects like holoprosencephaly in which the two sides of forebrain remain fused together, sometimes leading to a single eye (cyclopia) and a misplaced proboscis in the most severe form. There are also plants that contain an alkaloid compound named cyclopamine that, when ingested, inhibits signaling by sonic hedgehog and leads to holoprosencephaly, most notably in the sheep that ingest them. Other myths such as mermaids may also arise from rare birth defects like sirenomelia, where an infant is born with the legs fused together. Although rare, these types of birth defects have almost certainly occurred throughout human history, and one can imagine the ancients trying to explain them in terms of fantastical animal/human hybrids. I can't come up with any teratological explanations for centaurs or the sphinx. --- Medical geneticist (talk) 13:31, 7 May 2009 (UTC)

Digital audio player with resume-from-playlist function

moved to Computing Ref Desk

Best sources

I grew up using Wikipedia so I'm very used to looking for information here, but where do you go if you want to know whether or not some claim is pseudoscience other than Wikipedia? Where would I go if I want to know e.g. whether there is any truth in Homoeopathy or how reliable Evolution or Cosmological models like the Big Bang are? I know peer-reviewed journals and articles are supposed to be some of the best sources you can use but I don't even know where I can read peer-reviewed articles; and aren't there very few peer-reviewed articles since the reviewing process is so time-consuming? And what do you do if an peer-reviewed article about the subject at hand does not exist? --BiT (talk) 17:13, 5 May 2009 (UTC)

Most articles published in authoritative journals such as Nature are peer-reviewed. You should try searching PubMed for articles on, say, homeopathy, and click the "Review" tab for meta-analyses. Inside these reviews/meta-analyses, there will be hundreds of references for you to look through and you can make your own mind up on the data. --Mark PEA (talk) 17:40, 5 May 2009 (UTC)

I can't seem to find the articles themselves on PubMed, only the 'review' part.. --BiT (talk) 23:39, 5 May 2009 (UTC)

You may find Google Scholar a useful way of finding academic studies into things like homoeopathy, etc. --Tango (talk) 18:29, 5 May 2009 (UTC)

How about a library or book store? I bet if you went to the reference librarian, he or she would be thrilled to help you find whatever information you were looking for. A school teacher would also be able to help you find the right resources for a given subject. I'm not trying to be (too) cheeky or patronizing here, but I find that people these days with such easy access to "information" on the internet tend to forget about books or even good general audience magazines. I used to read Discover Magazine and Scientific American faithfully when I was a teenager. Master the information in your basic textbooks, formulate a specific question, and then drill down to more specific details based on the references within. Read, read, read, read, and read some more. Ask people questions along the way. You'll eventually a) find the answer to your question and have to come up with a new question, or b) find out that no-one knows the answer to your question (which would make it a great question).

@ Mark PEA and Tango: I'd disagree with you slightly about referring the OP straight to the primary literature... although PubMed is generally my first stop in any literature search (as I'm sure it is yours as well), it takes quite a bit of experience to be able to use the primary literature effectively. I'm guessing that going straight to the primary scientific literature may be slightly beyond the OP's level at this point, given that he/she is asking about some pretty basic stuff and may not be quite ready to read an academic study or meta-analysis and come away with any meaningful interpretation. Forgive me if I'm wrong.

--- Medical geneticist (talk) 18:45, 5 May 2009 (UTC)

No you are quite right, I've just started studying at an University and I've fallen in love science, the scientific methods and mathematics so I decided to ask here. But although I'm not a 'real' scientist I have been reading scientific text for quite some time so I hope I am in fact capable of reading and inferring some information from the more complex articles and journals. You are correct though that I shouldn't only seek information online, but sometimes I want a quick answer and in that case searching the Internet is very handy. What is PubMed anyway, does it only include medial and biological journals (it seems you have to pay for access to most of the articles there)? Google Scholar works great. --BiT (talk) 19:48, 5 May 2009 (UTC)

If you don't want to go directly to primary literature, though, I would recommend Wikipedia, and the OP was asking for alternatives. Review articles, like Mark PEA was suggesting, are a good secondary source between the studies themselves and Wikipedia. I'm not sure popular science books will be any better than Wikipedia, and less popular science books will be very much like the review articles already mentioned. --Tango (talk) 19:37, 5 May 2009 (UTC)

You can get a good fix on whether a topic is widely considered pseudoscience by looking at the Skeptical Inquirer or the CSICOP web site. But really it is better to come to your own conclusions based on looking at as much material as you have time to, than to depend on any one authority to tell you THE TRUTH. Looie496 (talk) 19:43, 5 May 2009 (UTC)

If you are an american university student and are interested in law or journalism, LexisNexis is easily the best reference tool of all time. Most college libraries have access to it and it is a great tool for print resources. Livewireo (talk) 20:16, 5 May 2009 (UTC)

I'm neither an American university student nor studying law or journalism, but thanks anyway =] --BiT (talk) 20:35, 5 May 2009 (UTC)

There are of course websites which are entirely dedicated to examining claims and identifying them as pseudo/junk science or otherwise, for example Science Based Medicine, Bad Astronomy, Snopes, The James Randi Educational Foundation, etc. etc. Of course, it's up to you to decide whether these sites are trustworthy or not. One thing that can help is if the authors cite an original, peer-reviewed source for their information, either by directly linking on on their site or by request. Truthforitsownsake (talk) 12:59, 6 May 2009 (UTC)

My main purpose wasn't to debunk pseudo-scientific claims but rather to know about a good reference if someone says "Wikipedia isn't a valid source", or just for personal use- i.e. a good reference if two different parties say different things. --BiT (talk) 18:21, 8 May 2009 (UTC)

If two people tell you different things and you need to decide, check what their sources of information are. WP:RS is the Wikipedia guideline. Cuddlyable3 (talk) 18:01, 11 May 2009 (UTC)

Buoyant plume

I am trying to design an outdoor chimney for bonfires to utilise some of the waste heat. If I want to display the buoyant plume from the middle of the bonfire to an edge (from whence up a chimney) does anyone have an idea whether a chimney (from the bottom of the fire) would be more or less effective than an open channel relying on the Trench effect (or its non combustive version). If it makes much odds everything is in cast iron. --BozMo talk 17:25, 5 May 2009 (UTC)

spinning

why the earth is spinning around the sun , why coul'nt it just stay still

i know my question seem so dump , but i'am asking because i coud'nt figure

the energy transform in this process , sun is'nt lossing any kind of energy

in it , ignouring other forms , and so the earth ..

is'nt it a form of unlimited energy , ,,,???

so that if we could generate enough pressure to produce a very dence object while other ligther

objects would spin around , then we have created a non limited energy source ...????

wild thougts huh ... thank you for answering ....

If the Earth stood still for even a day (say, that'd make a good movie title), it wouldn't for long. The Sun's gravity would pull us in, and then we'd really see global warming. The Earth is actually trying to move away from the Sun, and the Sun's gravity is just enough to bend its path so it stays in the neighborhood. Clarityfiend (talk) 19:08, 5 May 2009 (UTC)

See uniform circular motion (the earth's orbit is an ellipse, but in this discussion UCM is a rational assumption). The earth is constantly moving at a right angle relative to its position from the sun. The force on it from the sun (gravity) is perpendicular to its path, which is why its patch ends up turning around the sun. The work (energy) that is exerted on the earth ($W=Fd\cos \theta \,\!$) by the sun is exactly zero for two reasons: 1) the force is perpendicular to any infinitesimal dispacement, so cos θ is zero, 2) the net displacement is zero, since the path is a closed loop. Thus, the sun is not a mechanical energy source to the earth. —Akrabbim^talk 19:27, 5 May 2009 (UTC)

The Day the Earth Stood Still is a film name. Sifaka ^talk 20:27, 5 May 2009 (UTC)

Call me crazy, but I think Akrabbim might have been aware of that. -- Captain Disdain (talk) 23:25, 5 May 2009 (UTC)

You're crazy. --Tango (talk) 00:02, 6 May 2009 (UTC)

Especially since that was my observation. Clarityfiend (talk) 04:44, 6 May 2009 (UTC)

Uhh... yeah, I knew that. Of course I did! I was just checking to see if you were paying attention. Because my girlfriend asked me to. You wouldn't know her, 'cause she's... Canadian... no, wait! Aw, crap. -- Captain Disdain (talk) 01:08, 7 May 2009 (UTC)

But is "If the Earth stood still for even a day..." a film name? It sounds to me like a chick flick. I can imagine the trailer now..."He looked into my eyes and the Earth stood still. Forever? For even a day?"...Bleaugh! SteveBaker (talk) 02:48, 6 May 2009 (UTC)

Ahh, too many songs in my head right now. The original question brought to mind The Lucky old Sun, and also something from the original Sound of Music that I don't have the title for at the top of my mind.

"No Way to Stop It".... - Nunh-huh 04:35, 7 May 2009 (UTC)

But your response reminds me of that haunting air from Susannah, you know, come back, O summer, come back, blue flame.... I'd really like to see Susannah performed some day, and I'd even go see it in movie version if they made one, but I doubt they will. --Trovatore (talk) 18:03, 6 May 2009 (UTC)

You are right of course but also (although I am sure you know it) wrong....since the sun is a source of mechanical energy whilst the rotation of the earth around the sun does not match the rotation of the earth around the earth's own axis. Or you could equally call the earth's spin the energy source. But the difference can be taped via tidal power etc. --BozMo talk 20:14, 5 May 2009 (UTC)

Because the earth is in orbit - the 'centrifugal force' balances the sun's gravity exactly. If we stopped moving - then *poof* no more centrifugal force and we'd fall like a very large rock into the sun! But your misconception here (and it's an amazingly common one) is that you are confusing 'force' with 'energy'. This is such a spectacularly common mistake that you can say that a solid 99% of the world's crackpots who come up with "free energy" machines are making this exact same error. (Not that I'm accusing you of being a crackpot - you have the sense to come and ask rather than starting a new religion based around your ideas!) When a refrigerator magnet is stuck to your fridge - the magnet exerts a force against the metal. But because a force doesn't require any energy to maintain it - the magnet can stay there forever without 'running down'. Similarly, if you put a book onto a table - the table exerts an upward force on the book that counteracts the gravitational force pulling it down. The upward force comes about because the atoms of the book and the table repel each other. Again, the book will stay on the table from now until doomsday - the forces are maintained with zero energy expenditure. And the same is true of the earth in orbit. Gravity is a force - it doesn't take energy to keep it operating. Hence, there is no 'extra' energy to be exploited here. The earth can orbit the sun (as it has for a solid few billion years) without any energy being consumed. SteveBaker (talk) 20:38, 5 May 2009 (UTC)

As usual, Steve summarized it well. The force is continuously exerted, but there is no energy exchange. If you want to drill down to insignificant decimal points, there are probably tidal forces, orbit gravitational perturbations, and net flows of mass involved. Even the elliptical orbit means that technically, there is a cyclic, periodic exchange of energy between kinetic and potential energy (but the net energy exchange between Sun and Earth is still zero). For most reasonable, practical considerations, the system is "at equilibrium", but if you felt like exploring the long-term, billion-year timescales for cosmological and stellar-formation/evolution sort of stuff, you would need to worry about very slow processes like mass exchange and orbit stability. Nimur (talk) 00:15, 6 May 2009 (UTC)

if you lie down and put a hefty slab of concrete on your chest you will feel pressure. If you spend a day like that you will feel it, even if you spend a year like that the force is still there. So that force that you feel, it's being exerted EVEN THOUGH THE SLAB ISN'T LOSING ENERGY. You could put it on a kitchen scale, and the same as you feel the weight on your chest, however long you lie there, the scale would show the weight, even if it were there for a year. So does that then mean that we could "harness" the weight of a concrete slab, by putting it on a device that takes the feeling of pressure, and extracts energy from it? NO. The slab is exerting a force, but it isn't doing any work. Now, the Earth-Sun system is similar. The sun isn't doing WORK to keep the Earth revolving around it, just as the slab isn't doing WORK to push down on you (or the scale). Since there's no work being done, there isn't any energy to harness. Of course, you could create a system of pulleys the slab could pull as it falls down -- but then it could fall only once. You could also imagine a system to attach the Earth to that would use up the motion it is in around the sun -- but then it could only fall into it just once. So, to recap: yes, there IS some energy in the system that could potentially be harnessed (simple example: put something in Earth's orbit so Earth crashes into it, losing inertia in the process) however the system itself, just a slab lying on you, is not exerting work and there is no source of energy keeping the forces up.... Hope these analogies help, though I'm not a physicist... 94.27.168.220 (talk) 21:52, 5 May 2009 (UTC)

We usually say that the earth orbits around the sun. That avoids confusion with the fact that the earth is also spinning about its axis. A system of a body in orbit around another neither gains nor loses energy, similar to a spinning flywheel. Therefore the orbit can in principle continue indefinitely.

The motion of the orbiting body (earth) can be described as the sum of these two vectors: 1) falling towards the orbited body (sun), and 2) the momentum of its continuing motion in what would be a straight line tangential to the orbit curve, if not for 1).

Energy is needed to set up an orbital system. The system stores the energy. As has been pointed out, the energy could potentially be recovered but at no point has a source of new energy been created.

Incidentally while the earth is in orbit around the sun, the sun is also in orbit around the earth. Since the sun is so massive its motions due to orbiting planets are very small. Cuddlyable3 (talk) 08:57, 6 May 2009 (UTC)

Actually, I'd say the Earth orbits around the Sun. Saying the earth orbits around the sun brings to mind a clump of dirt making wheelies around a bright spot between the trees. --Trovatore (talk) 17:57, 6 May 2009 (UTC)

You would be right to say that. Against creeping capitalism I could better say that all the earth orbits around the nearest sun. Cuddlyable3 (talk) 19:59, 7 May 2009 (UTC)

if we attach some kind of mechanical device like acrank or some arm ... after creating a earth sun system like machine..

can we transfere the spining to other kind of motion ... so we move acar maybe ....???? —Preceding unsigned comment added by Mjaafreh2008 (talk • contribs) 18:39, 7 May 2009 (UTC)

In principle yes, a solid crank joining the earth to the sun would make a big flywheel. Just make sure to balance it on a lubricated bearing. I have heard that a spinning flywheel has been used to store motive power for a bus. Cuddlyable3 (talk) 19:59, 7 May 2009 (UTC)

Group velocity of a monochromatic wave

When reading about the group velocity, I always find the discussion framed in terms of wave packets. But what about a pure monochromatic wave traveling through a dispersive medium? The group velocity is defined as:

$v_{g}={\frac {c}{d(n\omega )/d\omega }}$

where $n$ is the refractive index of the medium (when the refractive index is independent of frequency the group velocity equals the phase velocity). Clearly the group velocity can be calculated for a monochromatic wave in a dispersive medium and it will be different than the phase velocity. But what does this velocity signify? What travels at the group velocity for a monochromatic wave? Is it the energy of the wave? If so why doesn't it travel with the phase? Thanks in advance, I am working through L&L's ECM right now and can't quite figure this issue out. mislih 20:45, 5 May 2009 (UTC)

If the wave is truly monochromatic, it is infinite in extent (in spatial location and duration). However, if there is a "turn on" or and "turn off" transient (meaning that the monochromatic wave is "pulse like" and has finite spatial extent), then it must have spectral content outside its single frequency (in the simplest case, it is a tone modulated by a step-function, which has LOTS of frequency content!) The dispersion would cause the turn-on and turn-off transients to creep to other parts of the pure tone region.

"What travels at the group velocity?" Well, the phase front of the turn-on transient, of course! Both the turn-on time, and the spatial extent dispersion, will propagate at this frequency. Clearly this is only relevant to "pulse-like" waveforms, and cannot apply to a "monochromatic" wave.

If you could construct a truly monochromatic wave, with infinite extent in space and time, you'd have a lot of other unphysical issues to worry about before you need to worry about phase-coherence drifting independently of the group velocity. Nimur (talk) 00:19, 6 May 2009 (UTC)