Wikipedia:Reference desk/Archives/Science/2007 January 4

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January 4

rodent shrew

Please can you describe to me the droppings of a shrew ? I have been finding around the edge of my lounge carpet droppings that look very simialar to thin grains of rice and knowing these do not fit the description of mice droppings I wondered of you could inform me of what these might be ? I urgently would be grateful of any suggestions.......... thankyou Verona Moore.

"Shrew droppings are similar to a rice grain in size, except for the short-tailed shrew, which may have droppings 1 inch long". From [1] . I don't find anything else...--cloviz 05:04, 4 January 2007 (UTC)

A shrew is pretty specific... Why do you believe it to be one rather than any other rodent? Russia Moore 01:51, 5 January 2007 (UTC)

Shrew droppings have, if sufficiently fresh, a characteristic musky odor. Nothing gross, more like funny (it's not a strong smell). BTW shrews are not rodents at all, more like mini spineless hedgehogs or moles. Dysmorodrepanis 02:45, 6 January 2007 (UTC)

 Please can you identify what this intruder might be and can you suggest how I rid my lounge of this problem also I have 2 small patches on the edge of my carpet that have been nibbled about 1/2 inch square. thankyou for any help

Lens Assembly - Collimating Beam

I want to convert the light from a single LED to a parallel beam so that it can travel further at a given intensity. There is talk of using more than one lens in series but I don't understand. If the light from the first lens is parallel and focused at infinity, the next lens wont make it any more parralel? What could be the point in a multiple lens assembly? --Username132 (talk) 01:11, 4 January 2007 (UTC)

Mmmm - perhaps extra lenses could be added to narrow the collimated beam. That could increase the intensity at a given distance. --Bmk 03:36, 4 January 2007 (UTC)

That's what I was going to say. One lens making the beam parallel will make a beam as wide as the lens which is only the amount of light which is hitting the lens from the LED in the first place, so it won't be any more intense. What I imagine is you use one convex lens to gather the light, then a concave lens somewhere close to the focus to make it parallel. Vespine 03:47, 4 January 2007 (UTC)

You mean take the diverging beam, make it converge to a point and then as its diverging, again use a lens of shorter focal length to make a narrower beam than the first lens produced? Wouldn't this be exactly the same as using the LED in conjunction with the last lens, leaving out the first one (or two)? --Username132 (talk) 01:18, 5 January 2007 (UTC)

On paper, the light 'lines' that you drew could indeed look identical, but, if you just used one lens, the light in the beam would be as wide as the lens and only as bright as the light going into the lens from the side of the LED in the first place. With two lenses, the same amount of light is converged to a much smaller area, then a second lens makes it focused at infinity, this time, the same amount of light that was in the first example's wide beam is in a much narrower beam giving it a much higher intensity.. Does that make sense? If I knew how to upload pictures i'd give you a drawn example;) Vespine 02:42, 5 January 2007 (UTC)

To upload an image go to www.imageshack.com, click on 'choose' and select image from local drive. Click 'host it' and wait for next screen. The last link it provides is the link you need. Then c&p the link into your post or use the image insertion button on forums (not Wikipedia).

Anyway I created a picture which attempts to display the original system, the system as I expect you are proposing and finally my idea. Two types of lens (blue), labelled A and B, B having shorter focal length than A. The LED is red and has a divergence of 10 degrees either side (half angle 7 degrees). Picture is here; http://img295.imageshack.us/img295/826/lensesir1.jpg --Username132 (talk) 20:32, 5 January 2007 (UTC)

Not quite, see the 1st and last image in your pic are exactly the same, you are just drawing lines at different points. Light doesn't travel from the LED as lines, we just use lines to represent what the line is doing at that point. In your first picture you are drawing the outer most lines of light, but you could draw an almost infinite amount of lines between them and all of them would be correct. Amongst that multidute of lines, some would look like the picture at the bottom. To compare lenses, you shouldn't change the lines that are coming out of the LED. What I was trying to explain is this http://img152.imageshack.us/my.php?image=lensdw6.jpg hope you can see it. The first lens focuses the LED light at infinity. You can see three lines into the lens and three lines out, see how widely spaced they are? That is a rough guide to intensity. In the second diagram, the 1st lens has a short focal length, can you see how the three beams converge and then are made parallel, in the second diagram, the light going in is the same, but the light coming out is much more closely spaced, this means the intensity of the beam is much higher.. Does that make more sense? Vespine 02:24, 6 January 2007 (UTC)

Note that when creating collimated beams, it's critical that you either use a small-diameter source, or include a pinhole aperture in the beam-forming optics. An ideal parallel beam (if we ignore diffraction effects) will focus to an infinitesimal point, and the reverse is true: to produce an ideal parallel beam, the source of light should be a pointsource and not an extended source such as an LED or incandescent filament. Try using a cheap diode laser rather than an LED. Diode lasers act like point source of light. The do still require lenses. --Wjbeaty 21:14, 8 January 2007 (UTC)

what is the relationship between a football's pressure and throwing accuracy

what is the relationship between a football's pressure and throwing accuracy —The preceding unsigned comment was added by 75.118.12.166 (talk) 04:47, 4 January 2007 (UTC).

A partly flat ball might be easier to grab and throw, but it might wobble around in the air a bit more. A harder (more fully inflated) ball might be able to "torpedo" a little easier. BenC7 08:24, 4 January 2007 (UTC)

huh? How do you torpedo a sphere? Grutness...wha? 12:44, 4 January 2007 (UTC)

An American football, presumably.

Atlant 13:22, 4 January 2007 (UTC)

(slaps forehead). of course. When he said football, I thought football, not gridiron. My mistook. Grutness...wha? 13:25, 4 January 2007 (UTC)

MythBusters did a segment on that last season, but I can't remember their conclusions. User:Zoe|(talk) 18:53, 4 January 2007 (UTC)

Mythbusters determined that at a lighter weight the ball travels less far (because it carries less momentum I'd guess) but I'm not sure they looked at inflation pressures. Lower inflation pressure might cause the same issue because there's less air crammed in there. They did start filling them with helium as well. Id be curious as to what a football (american) filled with Xenon would do though... Wintermut3 06:31, 6 January 2007 (UTC)

Fast light?

How come light is so fast?

From a physicist's perspective, light goes at the speed it goes, and a better question would be "why is everything else so slow?" There was a time when the universe was hot and most particles moved at speeds close to the speed of light, but the universe has gotten much colder since then and everything has slowed down accordingly. (See Timeline of the Big Bang for more information.) One might argue that, were things not moving slowly, no organized structures could form, and so there wouldn't be anybody to see things moving quickly; this is an argument related to the anthropic principle, and it is my own (educated) idea rather than a statement I've seen in a source. A professional cosmologist might be able to give a better answer, but I'm not sure there is an answer to your question that would be entirely satisfactory. -- SCZenz 06:46, 4 January 2007 (UTC)

I found another (related) answer to this question here. -- SCZenz 06:54, 4 January 2007 (UTC)

A related point is that light must move at the speed it does because it has no rest mass. Special relativity tells us that all massless particles must perpetually move at the speed of light, while any particle with mass will always be slower. Dragons flight 07:14, 4 January 2007 (UTC)

surely light must have mass to be affected by gravity: Gm1m2/d²

That's Newtonian gravitation. While it's accurate enough for most uses, general relativity makes things a little more complicated -- Consumed Crustacean (talk) 19:49, 4 January 2007 (UTC)

Light is so fast because it has no rest mass. That's also why it is the fastest thing. Imagine it like this: an infentismal amount of energy will propel a photon to the fastest speed possible, since there is nothing to push. That's why light is also the fastest speed possible (short of tachyons but ignore those). X [Mac Davis] (DESK|How's my driving?) 20:12, 4 January 2007 (UTC)

Why ignore them. And the theory of relativity proves things can go faster than light. Light also slows down in denser material, hence refraction.Hidden secret 7 21:22, 4 January 2007 (UTC)

Things can go faster than light, but they cannot go faster than c, the speed of light in a vacuum. Tachyons do not exist--they were a nonexistant particle predicted by an early version of String theory. — Dark Shikari ^talk/_contribs 16:24, 5 January 2007 (UTC)

Why is light so slow? If it has no mass, surely it would be able to move very fast with any amount of energy. Also wouldn't it move at the same speed however much energy there was, therefore suggesting it travels at an infinite speed, and we only see it traveling at c because we can't measure infinity.172.200.70.64 20:50, 6 January 2007 (UTC)

Bigger world?

In the future will the world be bigger cause if we keep on burying dead bodies, the decomposed bodies will turn to dirt. —The preceding unsigned comment was added by 74.102.217.142 (talk) 06:01, 4 January 2007 (UTC).

I don't think so, because human bodies are made from eating food, the food comes (directly or indirectly) from plant material, and the plant material is made out of nutrients from the dirt. There really isn't much material being added to, or leaving, the earth as a whole—it just changes forms sometimes. -- SCZenz 06:41, 4 January 2007 (UTC)

Well actually, something like 20,000 tons of material falls to Earth from space each year (most in the form of very fine dust). This isn't actually much from the point of view of making the Earth larger (about one centimeter thickness per million years), but it is still alot more than most people realize. Dragons flight 07:04, 4 January 2007 (UTC)

Still, the world isn't going to get bigger the way the questioner posed. - Mgm|^(talk) 10:12, 4 January 2007 (UTC)

A nice way to think about it is that the stuff you eat today is likely to contain atoms from many different dead animals and indeed dead, decomposed people, in their various stages of evolution. Bon apetit! --Username132 (talk) 13:47, 4 January 2007 (UTC)

Where did you expect the bodies come from? They COME FROM THE GROUND. Conservation of mass anyone?? X [Mac Davis] (DESK|How's my driving?) 20:09, 4 January 2007 (UTC)

Exactly. This is what David Bowie means when he says "Ashes to ashes, funk to funky..." --Username132 (talk) 23:22, 4 January 2007 (UTC)

Gas stove burners

When gas stove burners are ignited, why do they start on the highest setting, and not a lower setting? Thanks. - MSTCrow 09:16, 4 January 2007 (UTC)

I could come up with my own explanation, but I didn't find any solid references. I did find something neat, about all the troubles with a hydrogen barbeque [2] As for the question, just try to light a bbq with the gas on low! --Zeizmic 13:00, 4 January 2007 (UTC)

You need to ensure there's a sufficient volume of the proper fuel/air mixture passing by the ignition system (whether it's a spark or a pilot light). Running on "maximum" helps assure this. Also, I think you'll find there's a bit of human factors/safety going on there: a human can easily hear the high volume gas flow and might notice that ignition hasn't occurred, but if the burner started on "simmer" but failed to ignite, the human user might not notice that much-more-quiet flow of gas.

Atlant 13:31, 4 January 2007 (UTC)

I've seen many stoves that don't, they start on low. When I first came across what you are describing, it was on a bbq, and my guess was that when you are turning the bbq OFF, you turn the lever to the off position but it may not go all the way (especially if the knob has shhifted in its socket a little) and a tiny flame might remain burning that you might not notice, by having the high setting next to the off setting, you are much more likely to notice if the flame is still burning. Vespine 21:56, 4 January 2007 (UTC)

Mass spectrometry vs radio carbon dating

Is mass spectrometry better than radio carbon dating? I understand radio carbon dating destroys much of the material tested while mass spectrometry uses little material. So would this then be New School verses Old School? I am especially interested in this idea as related to artifacts of 2000 years old or younger. Is there scientific testing on such materials that basically do not harm or destroy the artifact at all? Perhaps some sort of electronic scanning? MRI or the like? I am thinking along the lines of old ancient manuscripts. --Doug 14:25, 4 January 2007 (UTC)

modern radio carbon dating uses mass spectrometry see Radiocarbon dating#Measurements and scales.87.102.8.102 19:39, 4 January 2007 (UTC)

Great. Apparently they work far back into thousands of years back. Can this testing only go back say just 500 years. Would the accuracy then still be around 1% (i.e. ink on an old mamuscript tests at 400 BP with accuracy of 4 +/- years; making it then the possible years of 1546 - 1554). Would that be correct thinking? --Doug 20:29, 4 January 2007 (UTC)
P.S. Is there scientific testing on such materials that basically do not harm or destroy the artifact at all? MRI or the like? --Doug 20:31, 4 January 2007 (UTC)

I can't answer your accuracy question properly - though obviously relatively recent sample will have smaller differences in C14 so the error in measuring the C14 will be greater. Also the atmospheric C14 levels may have changed over time - introducing more errors.See radiocarbon dating#calibration

I can however answer your 'PS' - yes there are lots of other methods for testing things that dont harm the sample - however I can think of one off hand that gives a date directly.. For example various types of spectrometry can give the chemical constitution of a sample (uv, ir) - this in turn can give an estimate of the date since different compounds have been used at different times through history eg ink in the 14th century is different from ink in the 20th century. There are methods for chemical analysis that are non destructive.

Some compounds will slowly isomerise or decay over time - this could be used to give an estimate of the age but these reactions are affected by temperature etc - so two manuscripts of the same age can be in different conditions according to how they were stored.87.102.8.102 21:07, 4 January 2007 (UTC)

For something like a 500 year old document being tested for authenticity, there are many methods that could be used:

• Ink: Is it a type of ink in use at that time and location ? Does the ink appear to be 500 years old (some inks fade with time) ?

• Paper: Is it a type of paper/material in use at that time and location ? Does it have a watermark or other identifying characteristic ? If so, is it from the proper time period ? Does the paper/material appear to be 500 years old (some papers yellow with time) ?

• Handwriting: Is that a style in use at the time ?

• Language: Are the words and spelling correct for the time ?

• Provenance: Is there a clear record of the document being created ? Is there a clear chain of custody between then and now ?

• The document may be photographed under different types of light (like UV), in order to verify that it appears the same as comparable docs of that period. This can also help to make faded ink more visible.

StuRat 21:08, 4 January 2007 (UTC)

could a book on a library shelf start turning to dust in a few decades?Hidden secret 7 21:20, 4 January 2007 (UTC)

A few decades, probably not, a few centuries, maybe, depending on how it was constructed, handled, and stored. StuRat 03:36, 6 January 2007 (UTC)

Very good answers. This is especially true pertaining to that of ink of the 14th Century and of a clear chain of custody between then and now. I believe StuRat has been following my other contributions related to Petrarch and Middle Ages questions on the Humanities Reference Desk and has seen my pet project on my User page of The Petrarch Code. A recent item that has steered up much controversy has been my contribution on Codex Vaticanus. I tend to believe this is a manuscript from the Fourteenth Century, not of the first few centuries. This statement alone has received much controversy. This is under the January 3 title: "Scientific testing of Codex Vaticanus". Now it turns out the chain of custody is only from 1475. I have several other reasons (explained later) to believe it is from the Fourteenth Century of 1372 +/- 8 years. Now this of the scientific testing (non destructive) above would prove my point. The other parameters of StuRat would then prove my point further. These other points follow palaeography, but the scientific testing I believe would be the most important to start with.
That of the question by Hidden Secret 7 is a very good question. Another reason I do not think Codex Vaticanus to be some 2000 years old. --Doug 22:35, 4 January 2007 (UTC)

Bioelectricity

Since it is possible to clone body parts is it possible to clone the cells from an electric eel to produce electricity from a nutrient solution? Barringa 16:50, 4 January 2007 (UTC)

Cells alone won't do the job, you'd need to duplicate the fishes' electric organ. - Nunh-huh 18:09, 4 January 2007 (UTC)

Is an electric organ similar to an electric guitar ? :-) StuRat 03:16, 6 January 2007 (UTC)

I beleive that hammond makes some excellent electric organs, though I prefer the Moog :p sorry. But theoretically, yes. On the other hand, the energy efficiency might be quite low given you'd have to include an electrical generation organ and a way to utilize food. Wintermut3 06:34, 6 January 2007 (UTC)

Equivalents to recreational mathematics

Are there some equivalent sciences to Recreational mathematics?Such as recreational biology, recreational physics...Mr.K. 18:37, 4 January 2007 (UTC)

Thinking laterally:

Chemistry - growing crystals

Biology - keeping pets, growing plants

Physics - meccano, messing around with sparks etc87.102.8.102 19:42, 4 January 2007 (UTC)

I think Meccano is closer to recreational engineering. :) — Kieff 20:04, 4 January 2007 (UTC)

When I was an undergrad we used the term "recreational biology" to mean sex. --Trovatore 19:51, 4 January 2007 (UTC)

Oh, and "recreational chemistry" meant drugs. Don't think we had one for rock 'n' roll. --Trovatore 04:39, 5 January 2007 (UTC)

I do a different sort of recreational biology. It is much more difficult.

Spore could be considered a recreational biology game, since it's a biological strategy\puzzle. The Incredible Machine, Armadillo Run or even Scorched Earth or Worms could be considered recreational physics (see Category:Physics-based games). So there are recreational instances of those sciences, but I don't think there's a general area of study based on them, like it happens with mathematics. — Kieff 19:59, 4 January 2007 (UTC)

Astronomy also attracts many amateurs. StuRat 20:59, 4 January 2007 (UTC)

Biology used to be a fairly recreational discipline in the 19th century, where you'd also get a lot of amateurs. Geology as well. But not so much in the 20th century. Astronomy is probably the only discipline of hard science that has a significant amateur community any more, excluding computer science (if you consider it a hard science; I don't). --140.247.251.72 21:04, 4 January 2007 (UTC)

Cooking is chemistry - but I don't know that I'd always describe it as recreational. Natgoo 18:01, 5 January 2007 (UTC)

ornithopters

Apparently They say it isn't possible for one to lift a human, but what is the maximum weight one could feasably lift, and how big would it have to be to do this? Is there any way to work out the required surface area easily if you know the weight? 172.189.77.244 19:03, 4 January 2007 (UTC)

Our article Ornithopter, under the heading "Manned flight", mentions several successful undertakings. What is problematic is human-powered flight: it is not totally impossible, but apparently we are just on the border between possible and impossible. If it is at all technologically possible to move away from that border well into the "possible" area, this cannot be a simple issue of size. Larger wings weigh more and require more force to flap them, and the human power output is a main limiting factor. Next to, perhaps, operators capable of more power output, the key is more likely to be increased efficiency through better designs and materials.  --Lambiam^Talk 19:25, 4 January 2007 (UTC)

Just because something isn't technologically possible, it doesn't mean it can't actually be done. I have never trusted technology, it always seems to go wrong somehow.172.189.77.244 20:21, 4 January 2007 (UTC)

If you can do it, then, by definition, it's technologically possible. StuRat 20:58, 4 January 2007 (UTC)

I never trusted technology either, as it's grounded in reality, and we all know reality has a well known liberal bias... --66.195.232.121 14:47, 5 January 2007 (UTC)

Technology isn't possible. (OP, but I have a name now)Hidden secret 7 21:18, 4 January 2007 (UTC)

Reading this comment on a website on the internet, by the use of a computer powered by electricity, I find that very hard to believe. — Kieff 22:42, 4 January 2007 (UTC)

How do you know you aren't dreaming, or that the internet isn't controlled by magic? But this isn't the point. What about the original question.Hidden secret 7 21:41, 5 January 2007 (UTC)

And how would you know this isn't real and true? Wouldn't everything be simpler if it was true and real? Assuming everything works by magic or is a dream doesn't solve anything, it just makes reality more complex than it seems to be, by all our accounts.

About your original question, it's actually quite difficult to answer. You should find someone who is an experienced ornithopter builder and ask them instead. The physics involved is too complex for back-of-the-envelope calculation, I'm a afraid. — Kieff 11:10, 6 January 2007 (UTC)

The upward force on the ornithopter would be the same as the force of air pushing against the wings, wouldn't it? Could this be considered an inelastic collision? Also would I have to take into account air resistance? If I knew the mass I wanted to lift, and the mass and area of the wingss, could I use this to calculate the speed required, and therefore the force needed at the pivot?Hidden secret 7 21:12, 6 January 2007 (UTC)

You would need to take air resistance into account. The answer to your other questions is "no". Read Bird flight to get some idea of the complexity involved.  --Lambiam^Talk 01:54, 7 January 2007 (UTC)

potatoe clock?

How does a potatoe clock work?

See the eHow Wiki[3]. X [Mac Davis] (DESK|How's my driving?) 19:30, 4 January 2007 (UTC)

That explains how to make one, not how it works. The article here on Lemon battery explains the basic principle. In a potato clock the potato is just acting as a battery. --140.247.251.72 21:00, 4 January 2007 (UTC)

tummy tucks and EKGs

Hi there- Can you please tell me if Tummy Tucks were available in the 1970's and also if EKGs were available in 1965? Thanks.

EKGs were certainly available in 1965. The Nobel Prize for its discovery was awarded in 1924. A more formal name for "tummy tuck" is abdominoplasty. Max Thorek wrote a treatise about abdominoplasty in 1924, so it was certainly available in the 1970s. - Nunh-huh 20:59, 4 January 2007 (UTC)

Faster than light?

Please sign your comments by appending ~~~~ to the end.

Is there anything faster than light?

nothing can go faster than the 'speed of light',c, as anything travelling at this speed would have an infinite mass. As light is affected by gravity it has a mass, but this is finite. Therefore light travels slower than the 'speed of light'. This means things can travel faster than light, but not faster than c.

Of course this is all probably nonsense. Light travels slower through denser materials, especially glass and plastic.Hidden secret 7 21:17, 4 January 2007 (UTC)

Hence how there is a reference to c. Nothing can travel faster than c. "Speed of light" usually refers to its speed in vacuum. - Mgm|^(talk) 12:46, 5 January 2007 (UTC)

And particles can exceed that speed of light through mediums. This causes Cherenkov radiation. X [Mac Davis] (DESK|How's my driving?) 21:34, 4 January 2007 (UTC)

Tachyons are hypothetical particles that travel faster than light. — Kieff 21:28, 4 January 2007 (UTC)

Hypothetical is probably too much. There is no tachyon hypothesis. X [Mac Davis] (DESK|How's my driving?) 21:34, 4 January 2007 (UTC)

Isn't there a theoretical partical particle that comes in pairs, and if one moves, the other does exactly the same thing immediatlley however far away it is?Hidden secret 7 21:44, 4 January 2007 (UTC)

You mean quantum entanglement? That's been experimentally verified. — Kieff 21:51, 4 January 2007 (UTC)

Heisenberg uncertainty suggests that a particles position is imprecise. This could be interpreted as a particle being everywhere in a quantum area at the same time.

Doesn't gravity travel faster than light?

No, general relativity predicts that gravity travels at exactly the speed of light. Experimental evidence bears that out, though within fairly large margins of error. -- Consumed Crustacean (talk) 22:20, 4 January 2007 (UTC)

If so, it doesn't matter anyway. You can't just 'delete' and 're-add' say, a 10-pound iron sphere in order to send information, which is what all of physics is really about. Vranak

Um light does travel at c and photons are massless particles.Light is affected by gravity not because it has mass but rather because gravity bends spacetime. If gravitons exist they would also travel at c, in entanglement you don't really get a real signal travelling faster than the speed of light. I mean you cant use it to send information faster than light. Theresa Knott | Taste the Korn 22:54, 4 January 2007 (UTC)

Tunneling of atomic particles was once said to be faster than light.--Stone 15:44, 5 January 2007 (UTC)

"If gravitons exist they would also travel at c" and lack mass. Dysmorodrepanis 02:56, 6 January 2007 (UTC)

Then how can a black hole's gravity have influence on it's surroundings if it's escape velocity is greater than c? If nothing can come out through the event horizon (excepting some Hawking radiation, but that's very small to have an effect on gravity) then how can gravity waves (or particles) come out? --V. Szabolcs 19:09, 7 January 2007 (UTC)

Quantum entanglement does have weired instantaneous signals however you cannot use them to transfer energy or information faster than light. It's freaky. Theresa Knott | Taste the Korn 01:09, 6 January 2007 (UTC)

The tunneling stuff was not really about entanglement. Basically it utilizes the fact that information is somewhat dislocated on the quantum level and the fact that by tunneling, you decrease the accuracy of measurement and increase the uncertainty of position. More like sleigh-of-hand, - tunneling is "faster" than light (not c however IIRC as it was not in a vacuum) because nothing actually moves; the information is more or less (re)created from scratch (which leads back to entanglement, which works with pairs of particles - determine the state of one and you automatically determine the state of the other no matter where it is in the universe) Dysmorodrepanis 02:56, 6 January 2007 (UTC)

Please, we do not know yet of gravity's speed. X [Mac Davis] (DESK|How's my driving?) 17:30, 5 January 2007 (UTC)

It's not been measured as far as I know. However it is expected to be c. It would be pretty big news if it were not.Theresa Knott | Taste the Korn 01:09, 6 January 2007 (UTC)

Yes we do —The preceding unsigned comment was added by 81.153.52.250 (talk) 23:07, 5 January 2007 (UTC).

Faster than 'light' is reletive to the medium travelled in. Certain things may move faster than the speed of light in a medium through that medium (the source of cherenkov radiation), but nothing has yet been found that exceeds C: the speed of light in a vaccum. That doesn't rule out the possibility of a tachyonic particle with imaginary mass moving faster than C, but in all honesty a tachyon is probably a function of mathematics and not physics, as in it satisfies the equations in a way that it can be superluminal (faster than light), but only because it's mass squared is negative (it's imaginary). Whether anything could exist that way is widely doubted, because it would also have a lot of other wierd properties, EG because mass is imaginary it would speed up as it lost energy (again because mass^2 is negative). It's an interesting thought experiment, but most reputable physicists I've ever read or heard of think that C is the absolute speed limit of 'real' particles. —The preceding unsigned comment was added by Wintermut3 (talk • contribs) 06:39, 6 January 2007 (UTC).

People say that if you shine a light coming from a craft going at lightspeed then the light will still go faster than the craft so maybe there is no speed limit to the universe and we are blinded from our perspective?68.120.229.114 07:03, 6 January 2007 (UTC)

Well a craft cannot go at lightspeed so the question is moot but if you shine a light from a craft going at near lightspeed then no, the light will go at c relative to all observers as per special relativity. Theresa Knott | Taste the Korn 09:49, 6 January 2007 (UTC)

However fast light travels, darkness always gets there first.Hidden secret 7 21:08, 6 January 2007 (UTC)

Travel of light

Is the travel of light simply the "leap frog" effect resulting from the alternation between the magnetic and an electric field of a photon? Barringa 22:28, 4 January 2007 (UTC)

An electromagnetic wave (e. g. light) can be described by the Maxwell equations. While the electric and magnetic field vectors are usually perpendicular, their maxima coincide (your description seems to imply that a maximum of the electric field creates a maximum of the magnetic field after a certain amount of time and vice versa). That may be counterintuitive because the minima also coincide and one might think that the wave would stop at the minimum, but the Maxwell equations contain not only the fields but also their time and space derivatives which are not necessarily zero when the fields are zero. Icek 14:00, 5 January 2007 (UTC)

Delta brainwave?

What is the delta brainwave mentioned in the futurama cartoon? —Preceding unsigned comment added by 74.102.217.142 (talk • contribs) 22:31, 4 January 2007

Assuming that Futurama is referring to the real phenomenon (which, knowing what the show is like, is a questionable assumption :-) ) see delta wave. -- AJR | Talk 00:30, 5 January 2007 (UTC)

Welcome to Wikipedia. You can easily look up this topic yourself. Please see Delta wave. For future questions, try using the search box at the top left of the screen. It's much quicker, and you will probably find a clearer answer. If you still don't understand, add a further question below by clicking the "edit" button to the right of your question title. X [Mac Davis] (DESK|How's my driving?) 17:31, 5 January 2007 (UTC)

Hard body bits

Just curious - apart from the teeth, nails and bones are there any parts of the body which are hard, or is the rest of it squishy stuff? Matt Eason ^{(Talk • Contribs)} 22:32, 4 January 2007 (UTC)

Kidney stones? — Kieff 22:38, 4 January 2007 (UTC)

I guess it depends on whether you call the other cartilage in the body "squishy". —AySz88\^-^ 23:21, 4 January 2007 (UTC)

Hmm. That's a hard on(e)

Hair is sometimes considered 'hard', it's one of the last substances to decompose and apparently along with teeth, it is all that remains if a corpse is devoured by pigs, they'll even munch down the bones. Vespine 00:56, 5 January 2007 (UTC)

Calluses, and the related foreign body response (like an internal callus coupled to a really bad allergic response) can become quite hard. One woman whose breast implants slid around a lot went on a daytime talk show to say that they ended up feeling like baseballs from all the collagen that built up around them. And, as a previous person mentioned, sometimes erectile tissue.--Joel 01:29, 5 January 2007 (UTC)

Isn't there an animal that keeps rocks in its stomach to aid digestion? Or am I imagining things? Melchoir 05:46, 5 January 2007 (UTC)

Several: see gastrolith. On the other hand, also see cow magnet. --Anonymous, January 5, 2007, 06:18 (UTC).

Awesome. Melchoir 07:10, 5 January 2007 (UTC)

Cheers guys, interesting stuff. I can't believe I've gone my whole life without knowing about cow magnets. Matt Eason ^{(Talk • Contribs)} 21:54, 5 January 2007 (UTC)

If you meant people only, then our fingernails and toenails aren't very hard, but they become hooves in some animals, which can be fairly hard. StuRat 02:59, 6 January 2007 (UTC)

Also see otoliths, the tiny stones in the vestibular apparatus of the ears. Also, if you're going to count kidney stones, you may as well count gallstones. --David Iberri (talk) 03:33, 6 January 2007 (UTC)

Resistor value

I want to obtain a controlled variable voltage from a 12 volt car battery to power a full current load which can be done with a variable resistor having a center tap. How many ohms and watts would the variable resistor need to be to span the battery terminals without allowing any (or absolute minimal) current to flow from one end of the resistor to the other but which would allow any amount of current from zero to full current to flow through the tap to power the load? 71.100.10.48 22:44, 4 January 2007 (UTC)

Please see our article on voltage dividers; a potentiometer is just a variable voltage divider. But I'm afraid what you want to do isn't practical with a potentiometer, at least if your load is going to draw any appreciable amount of power because a potentiometer of sufficiently low electrical resistance will waste a lot of power just "idling". A rheostat (a variable resistor without that pesky power-wasting connection to ground) won't waste any idling power, but the voltage drop across it will be sharply affected by the load current and a rheostat will also have trouble getting your load down to "zero volts", although you can get arbitrarily close, especially if the rheostat is "taper wound" (so it has proportionally much more resistance per degree of rotation near the "off" end than at, say, midpoint.

A variable voltage regulator sounds like a much more practical alternative. Think of it as an amplifier for the voltage produced by your potentiometer.

Atlant 23:29, 4 January 2007 (UTC)

For example, a buck converter built in order to allow for a variable tension at output would be a better choice, since you would waste no power in the potentiometer. However, it is a more expensive solution. —The preceding unsigned comment was added by 84.90.42.248 (talk) 17:05, 5 January 2007 (UTC).