Wikipedia:Reference desk/Archives/Science/2008 March 27

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.

March 27

Turtles

When a turtle pulls its head into its shell, does its spine buckle or contract? 70.162.25.53 (talk) 01:11, 27 March 2008 (UTC)

Have you been reading Cat's Cradle? Anyway, the article you linked has the answer: it depends on the turtle. Pleurodira fold their neck to the side, while Cryptodira pull their neck straight back. Algebraist 01:51, 27 March 2008 (UTC)

I disagree. The article does not seem to answer his question. Now that 70.162.25.53 has mentioned it, I'm sort of curious too. How is the spine in the neck constructed so that it can change length so dramatically? APL (talk) 06:37, 27 March 2008 (UTC)

The "spine" of a tortoise is more like part of its shell. See this cutaway section of a tortoise and the mechanics will be obvious.--Shantavira|^{feed me} 15:07, 27 March 2008 (UTC)

Neat! APL (talk) 13:38, 28 March 2008 (UTC)

I think Algebraist is referring to "Turtles are broken down into two groups, according to how they evolved a solution to the problem of withdrawing their neck into their shell (something the ancestral Proganochelys could not do): the Cryptodira, which can draw their neck in while contracting it under their spine; and the Pleurodira, which contract their neck to the side." from the article, which was there at the time you posted from what I can tell [1] Nil Einne (talk) 11:50, 2 April 2008 (UTC)

Why a Horseshoe-shaped Gamut of Visible Colors?

The color space article describes visible colors in the RGB color model, the gamut, represented as a 2-D horseshoe shape. Are all 10 million visible colors found in this (idealized) 2-D image? Why a horseshoe shape? Is the shape a horseshoe on account of the underlying physiology of color reception in humans? Mark465 (talk) 14:52, 27 March 2008 (UTC)

The article on gamut wasn't wikilinked, see specifically Gamut#Representation_of_gamuts. It's really shaped more like a triangle with one rounded side and one rounded corner (the three corners being the three primary colors), than like a horseshoe. I too am interested in understanding the reasons why the range of possible chromaticities has a rounded blue-green side and straight blue-red side. ---Sluzzelin talk 15:11, 27 March 2008 (UTC)

See also CIE 1931 color space. ---Sluzzelin talk 15:17, 27 March 2008 (UTC)

There are three fixed points on these kinds of diagrams representing the three primary colours, which in turn are the wavelength of peak responses of the three kinds of cone detector in the human eye. These three points form the apexes of a triange. The straight line from the Red apex to the Green apex represents a fully saturated mix of the Red and Green wavelengths represented at the apexes. The colour along this line varies continuously from the red through orange and yellow to green. Similarly, the straight line between the Green and Blue apexes represents a fully saturated mix of green and blue light. However, not all light in nature is a mix of these particular three wavelengths, in fact most of it is not. Monachromatic yellow light, for instance, may appear as the same yellow as a mix of red and green light but it has a greater saturation. Since a mix of red and green light with no blue is considered to be 100% saturated, monochromatic yellow must be more than 100% saturated and is described as supersaturated. It must, therefore, lie outside the linear triangle thus accounting for the curves on the diagram. The Red-Blue axis is not curved because mixes of red and blue can only be achieved by mixing two wavelengths and so monochromatic supersaturation cannot occur and the line is therefore straight.

Anticipating the next question - why is monochromatic yellow supersaturated? The reason for this is that red/green mixed yellow obviously has wavelength component of green which is a shorter wavelength than the monochromatic yellow. At this wavelength there is some stimulation of the blue receptor also (mono yellow also stimulates the blue cone but to a much lesser degree). This stimulation of blue means that there is some component of red, green and blue in the eyes response. All three are interpreted as white, so the net result is yellow plus white or in other words desaturated yellow. Since this desaturated yellow has been predefined as 100% saturation, a more saturated yellow must consequently be supersaturated. Luckily for the television companies, supersaturated colours do not occur in nature much, so the fact that RGB television cannot reproduce them is of little consequence. SpinningSpark 16:13, 27 March 2008 (UTC)

It isn't that clear that there are three specified primaries which result from the maxima of cone receptors. The CIE color space isn't set up that way. How about magenta, cyan and yellow as alternative primaries? Edison (talk) 03:53, 28 March 2008 (UTC)

The CIE colour space uses three primaries (called the tristimulus in the article) which are arbitrarily chosen but close to the average human peak cone responses. The difficulty with humans is that they are all different so the human tristimulus is fuzzy, varying from person to person. My post above was really trying to describe the concept of supersaturation in general and was not particulary referring to the CIE colour space (although it does apply). Red-green-blue (RGB) colour spaces are used because light is detected in the human eye in terms of RGB. RGB is also most convenient where colours are being reproduced by an additive process of light such as in television or computer monitors. Where colours are reproduced by a subtractive process such as in printing then it becomes more convenient to use a cyan-magenta-yellow (CMY) colour space. In particular, printers favour the CMYK colour space. It is also worth noting that tristimulus based colour spaces only work for humans and other species with equally useless colour vision. Many species of birds, bees and moths have five colour receptors so RGB colour reproduction would not fool them for one minute. As for species like the pistol shrimp or mantis shrimp, their colour vision is so good that it would be necessary to reproduce a mix of wavelengths near identical to the original scene before they would be satisfied with television pictures. SpinningSpark 10:02, 28 March 2008 (UTC)

See Cone cell for information on the maxima of cone cells Nil Einne (talk) 06:46, 1 April 2008 (UTC)

Alternative medicine

Is there any branch of conventional (western) medicine that would at one time in the past have been considered alternative medicine, or is that by definition impossible?--Shantavira|^{feed me} 14:58, 27 March 2008 (UTC)

I don't know how far back the term "alternative medicine" was used, but there are examples of things that are mainstream thinking now and were not in the past. A great example is the good old germ theory of disease- now accepted by all but the quackiest of quacks, yet it was seen as quite outlandish by the medical establishment when first introduced. "Tiny little bugs made you sick?? Ridiculous. Come on by and I'll fix your humors for you, and you'll be all better." Friday (talk) 15:06, 27 March 2008 (UTC)

Right. Good question. My answer was going to be "Yes, practically all of it", for the reasons Friday mentions. I was thinking of Ambroise Paré and the treatment of battlefield wounds by ligation and clean compress when the rule was cauterize for bleeding and pour hot oil in any holes. (Say what you will about the French, they have produced some bright boys.) --Milkbreath (talk) 15:20, 27 March 2008 (UTC)

Germ Theory is a good answer, but I'm not sure "Practically all of it" works. It seems like there is a lot of modern medicine that never passed through a phase where it was practiced regularly, but not by mainstream doctors. (Though it depends a lot on how narrow you're allowed to define a "branch...of medicine") However, The first paragraph Wikipedia's article on alternative medicine offers a surprisingly broad definition of the term that probably covers even techniques undergoing legitimate clinical trials by mainstream researchers. By that definition "All of it" would be the right answer. APL (talk) 16:14, 27 March 2008 (UTC)

"The Web That Has No Weaver" is a fantastic resource on Chinese Traditional Medicine that may help untangle the philosophical kinks. Western Medicine is evidence-based, and has been for some time, and is largely based upon the principle that different people, with similar medical conditions and similar bodily make-up, should be treated in roughly the same way. TCM is also evidence based, in the sense that it works, but is based upon the principle that every individual must be treated individually based upon their unique energy patterns and fluid movements. That makes it virtually impossible to compare TCM and Western medicine; it is impossible to do a double blind study on a TCM treatment because the practitioner must necessarily know what they are doing. It is also impossible to do a rigorous statistical analysis with statistical significance on TCM treatment for a given condition, because in TCM every patient will be treated differently, and consistency cannot be assured. Western forms of naturopathic medicine follow similar philosophies. So while the treatments themselves may share homology, the philosophies are too divergent for appropriate comparison. Vance.mcpherson (talk) 16:37, 27 March 2008 (UTC)

I'm sorry to derail, but I can't let that pass. Statistical comparisons are ideal for comparing situations with inconsistencies if you use a large enough sample. Are you honestly saying you couldn't do a study that says "Of Ten thousand people with illness X who were treated with western medicine Y% recovered, and of ten thousand other people with illness X who were treated with traditional Chinese medicine Z% recovered." because there's a chance that one of the TCM patients might have gotten lucky and gotten a better treatment? Granted, It may not tell you the scientific validity of TCM,(or western medicine for that matter.) but it would give you an idea which of the two approaches are more likely to work, which is far more useful. APL (talk) 18:00, 27 March 2008 (UTC)

Continuing the derailment... yes. TCM is not disease based, but rather patient based. It is the patient, not the disease, that is treated. Therefore, for example, two people with acute angina, of the same height, weight, race, and sex, may be treated with a completely different set of medications, acupuncture and other regimens, based upon factors like Qi flow, etc. Every patient is different. Further, TCM focusses upon prevention; by the time the disease sets in, it's argued, it's for the most part too late. One could say, for example, that X people with angina were treated with TCM and Z% recovered, as you say, but there would be implied thereby X different treatments. Contrast this to Western Medicine statistics, where one would never say y people were treated for angina in a bunch of different ways, and z% recovered. We'd be insisting on a highly controlled experiment. Sorry to be unclear. Vance.mcpherson (talk) 18:30, 27 March 2008 (UTC)

I'd still say a comparison could be done with a suite a treatments, so "X% recovered using Western treatment A while Y% recovered using TCM treatments B-Z". However, a more useful approach might be to have all patients evaluated for TCM and determine which TCM treatment would be recommended. You could then split each group into two, and conclude "X1% recovered using Western treatment A while Y1% recovered using TCM treatment B", and "X2% recovered using Western treatment A while Y2% recovered using TCM treatment C", etc. You might thus find that some TCM treatments are useless while others are quite effective. As for the preventative aspect of TCM, western medicine also has preventative meds, like those that keep cholesterol or high blood pressure in check. Those could be compared with TCM methods and we could study which prevents a heart attack or other negative health effect the longest. Again, we could break down the study groups based on recommended TCM treatment. Of course, as far as prevention goes, I'd say a healthy diet and lifestyle, exercise, not smoking, avoiding stress, etc., will win every time. Those should ideally be also included in any study, so we don't end up with large numbers of people eating double bacon cheeseburgers every meal and taking drugs to try to prevent the damage, when the far better option is to just eat better. StuRat (talk) 18:59, 27 March 2008 (UTC)

I'm no statistician, but it seems to me that a look at world life expectancies constitutes a very good study of the relative efficacy of the two philosophies, and others, to boot. This is especially true if we're talking about prevention, and the numbers include unweighted infant mortality, which they usually do, I believe. The map I've linked to indicates that Western is better than Chinese, which is better than Indian, and African might just be worse than nothing. --Milkbreath (talk) 19:31, 27 March 2008 (UTC)

I don't think that approach is very useful because there are so many factors which vary from place to place. For example, one of the single most important factors is probably basic sanitation. Do they dump raw sewage into the same rivers from which they drink untreated water ? If so, take 20 years off their life expectancy. So, using such figures to determine which specific medical treatment is best is unlikely to work. StuRat (talk) 20:29, 27 March 2008 (UTC)

The comparison being made is between two schools of thought, two incompatible conceptions of the mechanism of disease. Civilizations that fail to pay due heed to the role of pathogenic organisms in disease will not practice adequate sanitation. That's what I meant by "prevention". --Milkbreath (talk) 21:56, 27 March 2008 (UTC)

The reason they don't have sewage and water treatment plants isn't that they don't believe in germs, it's because they lack the resources to do so. That map seems to correspond very well with per capita income, not philosophy. Haiti, for example, is the one spot of red in the Western Hemisphere. Why ? Not because they have a radical different approach to disease treatment and prevention, but because they are dirt poor. StuRat (talk) 00:05, 28 March 2008 (UTC)

I don't feel qualified to debate world health issues, and this is not the place for that. But I would add that however poor people are, they can choose to reject a religion that makes them ritualistically drink and bathe in some of the most polluted water on earth with dead bodies in it and buffalo dreck, etc. And perhaps per capita income correlates pretty well with the realistic world view and rejection of mysticism that goes along with Western medicine. I'm outta here. --Milkbreath (talk) 00:22, 28 March 2008 (UTC)

I should back up a bit. I was very interested in researching the scientific and medical basis of TCM a few years back, and ultimately in investigating a possibility of the synthesis of TCM and Western medicine. I was told vehemently by some in the TCM community that this could never be done. It's counterintuitive because we're used to Western approaches to thinking about scientific method. Take StuRat's excellent example. "Some TCM treatments are useless while others are quite effective"... this would make sense to the Western scientist, but not in TCM. TCM is predicated upon the idea that the treatment will work if administered in the right context... and if it doesn't work, then there were other factors not considered. For example, if I have anxiety prior to a test, I might take a liver tonic because liver heat results in anxiety. But I couldn't only do this; if I have a weak spleen, for example, (which may predispose me for example to autoimmune disease, allergy, etc., but also governs liver, if memory serves), then it may be more advantageous to achieve spleen yin-yang balance. This may be achieved through a combination of herbs and acupuncture, or medicinal soups, or even a simple diet change. For example, my TCM doctor says that my spleen is cold and damp, which means I should avoid grains except rice and drink warm liquids. This is especially necessary to control insomnia. See, it's really a very different, quite incomparable system. I can't just say "take ginseng to increase your energy" in quite the same way as I can say "take this antibiotic to clear this disease". I highly recommend looking at "The Web That Has No Weaver", ([[2]]) by Ted Kaptchuk for an excellent review of this system. The title of the book is a good analogy -- the human body is "the web that has no weaver" -- there's no beginning or end, and there's not exactly a cause-effect relationship between a pathogen and a disease. Anger could be caused by liver heat or disrupted qi... and on and on like that. Vance.mcpherson (talk) 19:41, 27 March 2008 (UTC)

You're spleen is damp ? LOL. I'd be worried if it wasn't. But seriously, beware that those with a questionable treatment method often come up with some reason to claim it can't possibly be tested, so it can never be shown to be ineffective. A trick chiropractors like to use is to push down on your extended arm before and after a treatment and amaze you by how much better you are able to resist their push, which they swear is just as hard as before the treatment. Of course, this could logically be solved simply be substituting a fixed weight to apply the same force both times, but they aren't interested in doing it that way, for obvious reasons. Another favorite of charlatans is to say "your negative energy (skepticism) is what made the treatment ineffective". Back to the TCM discussion, I'd ignore TCM treatments for "damp spleen" and such. Those people who are concerned that their spleens are damp can go right to the nearest TCM practitioner, as western meds has nothing for this "serious problem". However, I believe TCM does indeed tackle a number of issues which western meds also tackle, such as headaches. And, as I've noted previously, there are ways to design a scientific study to compare the two treatment approaches for such conditions. StuRat (talk) 20:41, 27 March 2008 (UTC)

Actually my chiropractor used an electronic force measuring device. Totally objective measurement. My left arm showed a pronounced difference, right arm was unchanged. There is no need for charlatanry in your example. ~Amatulić (talk) 20:49, 27 March 2008 (UTC)

An "electronic force measuring device" sounds like something that could be tampered with, possibly using a remote control, while a fixed weight could not (unless they can pull a switch on you when your back is turned). A needlessly complex device should be another warning sign that something weird is going on. More generally, the scientific method demands that you not use subjective methods of evaluating effectiveness, which is what TCM will suggest, but instead use objective measures. If they feel people's skin and tell them their "qi" is flowing better, this is of no use in evaluating effectiveness. If, on the other hand, the patient's bad cholesterol level is down in multiple blood tests, this indicates the treatment is doing some good. StuRat (talk) 20:54, 27 March 2008 (UTC)

(Sigh). Some folks just can't let go of preconceived notions. You could do it with a spring scale (which another chiropractor did, with similar result). Using an electronic device doesn't make the chiropractor dishonest; in my case it simply allowed him to record the data more easily in his computer. There was no calibration or adjustment between tests, just comparison of forces each time. If your chiropractor doesn't perform the measurement objectively, that doesn't mean that others don't. ~Amatulić (talk) 21:04, 27 March 2008 (UTC)

You mean you failed to observe any calibration or adjustment between tests. This doesn't mean it didn't happen, just that you didn't see it. It could be as simple as a lever that's pressed or dial that's turned when the device is handled to as complex as a remote control or timer. The only way to be certain it was not changed is to use a device which can't be changed, such as a fixed weight. StuRat (talk) 21:49, 27 March 2008 (UTC)

...or a fixed spring scale, as I already mentioned. Pay attention. ~Amatulić (talk) 22:12, 27 March 2008 (UTC)

I did, and you didn't say "fixed". I have a spring scale at home that is adjustable. Did you examine the scale thoroughly to determine that it can't be adjusted ? StuRat (talk) 23:57, 27 March 2008 (UTC)

Ok, but NONE of that would stop you from treating "traditional Chinese techniques as administered by a well trained doctor" as a single unit and comparing that unit against western medicine in general. To say that the medical techniques are not "disease based" is fine, but presumably, if someone has a disease, their doctor will attempt to either cure it or relieve the symptoms, correct? We could find ten thousand people from each of the two categories that all have disease X, wait a few months, and compare the results. This would provide useful information even if each of the TCM patients received different treatments, so long as those treatments were expertly chosen by whatever rules and criteria TCM doctors normally use.

Statistics could be used even within TCM. You could do a study that says something like "Of the 10,000 TCM patients with disease X, 159 of them recieved treatment Q. Of those 159, a large percentage dropped dead instantly. Treatment Q's effectiveness is therefore in question." I'm exaggerating for comedic effect, but you get my drift. APL (talk) 21:12, 27 March 2008 (UTC)

(outdent)

Derailing further: some aspects of TCM do lend themselves to western-style clinical testing. Regular intake of the herb jiaogulan, for example, has been shown consistently to reduce cholesterol. It's one of the few examples I know of that has solid test results behind it. That's one example where one can use TCM to treat a specific condition rather than adapting a different treatmen to a patient.

Getting back on topic: To answer the OP's question, I'd say one example of conventional medicine that may have once been considered "alternative" is physical therapy. And most herbal treatments are considered "alternative" until some drug company comes along and figures out how to synthesize the active ingredient, as was the case with ephedra, from which pseudoephedrine was derived. ~Amatulić (talk) 20:50, 27 March 2008 (UTC)

...not to mention willow bark. -- Coneslayer (talk) 01:42, 28 March 2008 (UTC)

re: jiaogulan -- yes, but from a TCM perspective, it is warm and it enhances yin while supporting yang, so you wouldn't want to take it if you had excess yin or excess warmth!Vance.mcpherson (talk) 14:26, 28 March 2008 (UTC)

Moon falling into Earth

How close does the moon have to be to fall into Earth's gravity? --Jonasmanohar (talk) 17:38, 27 March 2008 (UTC)

It's already in earth's gravity. We do have an article on the orbit of the Moon. Someguy1221 (talk) 17:46, 27 March 2008 (UTC)

Ok, then I meant crash into earth... --Jonasmanohar (talk) 17:50, 27 March 2008 (UTC)

How close does it have to be to crash? Very close indeed. ;-) As the article linked to above explained, the Moon is (very slowly) getting further from the Earth. Friday (talk) 17:53, 27 March 2008 (UTC)

(Edit conflict) Just to be clear, the moon is already significantly affected by the Earth's gravity. Otherwise it wouldn't orbit the earth. What I presume you really want to know is how close does the moon have to be to the earth such that it will no longer be in a stable orbit around the earth and will eventually crash into the earth. Unfortunately I don't know the answer except that as I expected, the moon isn't actually a truly stable orbit around the earth, see [3]. Not remembering physics well enough, I can't calculate what sort of distance change is necessary. But to bear in mind it's not that the moon is going to suddenly start 'falling' into the earth at high speed. Rather, there will be a distance where the moon is in a truly stable orbit, beyond which the moon will get closer over time. However at first, the distance change will be minimal such that even though the moon will eventually theoretically crash into earth, it will take billions and billions of years for this to actually happen such that the sun will become a red giant before then so presuming the earth and moon even last that long, it will never actually have happened in any case. Of course, the closer the moon gets to the earth, the less time it will take for it to collide with the earth so that at a certain distance, it will possibly have happened were the moon really the close Nil Einne (talk) 17:56, 27 March 2008 (UTC)

You also have to decide at what velocity the moon is traveling in whatever reference frame. You could keep it where it is but just slow it down enough, and it will hit the Earth right quick. But I also forget how to calculate the velocity/distance necessary for crash for an elliptic orbit. Someguy1221 (talk) 18:03, 27 March 2008 (UTC)

(EC again) BTW, do bear in mind this isn't a simple calculation. Both the earth and the moon have a momentum.[4] This momentum was gained during their formation, it's not as if someone simply dumped a planet and a moon in the middle of space and let them start moving. If these momentums had been different, then things would be different. [5] In other words, what I'm trying to say is you can't simply visualise it as a simple system as I have a moon with mass M and a planet with mass E, what will happen when they are distance ME from each other? (Answer I think, many different things could happen including the moon could simply crash into the earth without ever actually orbiting presuming both were simply completely static from the beginning and there was actually no other mass at all [NB this is obviously an impossible system]) Nil Einne (talk) 18:09, 27 March 2008 (UTC)

Final comment: The note on the Orbit of the Moon article about how no one is actually sure what will happen to the earth-moon system after the orbit of the moon reaches 47 days sums up quite well the fact that when you actually start to think about them, these are quite complex systems Nil Einne (talk) 18:16, 27 March 2008 (UTC)

Since we're clearly talking about different things, let's make it clear, there is a distinction between placing the moon in an unstable orbit such that it eventually hits the Earth, and placing the moon in an orbit that actually comes with a moon radius of the Earth's surface (i.e., they will hit during a single orbit). The latter is what could be calculate with relative ease, not that I remember how. Someguy1221 (talk) 18:20, 27 March 2008 (UTC)

I think it's safe to say that if you could place the Moon into any orbit you liked, you would not want to place it close enough that it could hit our atmosphere, because that would cause its orbit to decay and it would almost certainly crash eventually. According to wikipedia The Exosphere ends at about 10,000km from the surface. (for reference, the moon currently does not get close than 363,104km) APL (talk) 18:26, 27 March 2008 (UTC)

OK, placing the moon in an orbit that actually comes with a moon radius of the Earth's surface (i.e., they will hit during a single orbit). --Jonasmanohar (talk) 18:24, 27 March 2008 (UTC)

Actually, the better answer is if the Moon is within the Earth's atmosphere, or closer than 10,000 km, as APL said above. The Moon's radius is only 1,737 km. StuRat (talk) 18:40, 27 March 2008 (UTC)

Correct my if I'm wrong, but in order to maintain a stable orbit, distance is inversely proportional to velocity. That is, a closer orbit will need to be faster in order to be stable. Now, the moon isn't exactly stable, but it's pretty close. If we were to slow it down substantially or pull it closer to earth without speeding it up, the orbit would begin to decay. How long will it take? Not sure. --uǝʌǝsʎʇɹoɟʇs(st47) 19:50, 27 March 2008 (UTC)

Actually I think if you go faster, you fly away. A closer orbit is slower, but gets round the Earth faster. However, once the Moon comes within the Roche limit, wouldn't it break into pieces anyway? Franamax (talk) 20:10, 27 March 2008 (UTC)

If you can just satisfy yourself that orbital energy for a circular orbit is one half the kinetic energy at escape velocity, it's pretty clear that the minimum velocity for orbit increases as the radius of the orbit decreases. It's thetotal travel time around the Earth that decreases. Someguy1221 (talk) 20:17, 27 March 2008 (UTC)

Huh? The converse of your statement is that as the radius of orbit increases, the minimum velocity decreases, right? I don't think that's correct. The faster you move, the more energy you have, the farther away you fly. Isn't that how the shuttle does it? Franamax (talk) 20:31, 27 March 2008 (UTC)

I take that back. Brain is smoking and making funny noises. Franamax (talk) 20:39, 27 March 2008 (UTC)

Roche Limit. Objects only break up inside the Roche limit if there is nothing other than gravity holding them together. This is sometimes stated as the pile-of-sand model. Crystalline rock, on the other hand, is held together by forces a lot stronger than gravity. There are many examples of objects in the solar system quite happily existing inside the parents Roche limit. SpinningSpark 22:58, 27 March 2008 (UTC)

Yes, some moon-sized objects can exist within the Roche limit, but not by much. At some point, the tidal forces will rip it apart, and I am almost completely certain that this would be well before the moon would impact the atmosphere. Of course, all of this is assuming the moon's orbit is coming closer to the earth; it is actually receding at a few millimeters a year. -RunningOnBrains 23:20, 27 March 2008 (UTC)

True, but even if the Moon did break up, that wouldn't mean that the parts would crash into Earth. They might just form a ring. However, the lack of rings around the terrestrial planets seems to imply that those rings don't last for long. StuRat (talk) 23:52, 27 March 2008 (UTC)

(unindent) The Moon's gravitation produces tides which raise the water level over a meter at numerous locations. Would not a closer Moon produce higher tides, ultimately inundating most of the land mass, even before the Earth was rendered uninhabitable by movement of its crust resulting in earthquakes and volcanoes as the Moon drew nearer, long before there was any contact? Edison (talk) 03:49, 28 March 2008 (UTC)

Yes, the tides would be higher and earthquakes would also be produced. Towards the end the atmosphere would be heated by contact with the Moon, too. StuRat (talk) 14:42, 28 March 2008 (UTC)

A detailed description of this process is to be found at [6]. Edison (talk) 15:49, 28 March 2008 (UTC)

E=MC^2

What is the whole proof to the equation E=MC^2? —Preceding unsigned comment added by 68.252.70.175 (talk) 23:37, 27 March 2008 (UTC)

Mass-energy equivalence might be useful. Algebraist 23:44, 27 March 2008 (UTC)

If you mean the derivation, the original form of it is here. It's a little opaque if you aren't used to the notation and math Einstein is using but if I recall it relates to the energy a photon imparts on the inside of a box in space or something along those lines. --Captain Ref Desk (talk) 23:53, 27 March 2008 (UTC)

One way of checking the equation for reasonableness is by dimensional analysis. The units of energy (on the left side) are kilograms times meters ² per second², by definition. The units of the right side are kilograms times C², which is equivalent to kilograms times meters ² per second², just like the left side. The only thing missing is a constant to allow for the choice of units. The equation would not balance in dimensions if it were E=MC, or E=M²C, or E=MC³, etc. Edison (talk) 03:38, 28 March 2008 (UTC)

Which is fine, but that just says that if there is some kind of mass-energy equivalence, then the constant of proportionality will have dimensions of Length ^ 2 / Time ^ 2. Just through dimensional analysis you will be unable to tell whether that constant was c^2, or something involving the Hubble constant and Planck length. Of course, if you formulated a theory where the constant had dimensions of Length ^ 3 * Time / Mass ^ 4, then yes you might suspect you'd done something wrong. Confusing Manifestation(Say hi!) 15:11, 29 March 2008 (UTC)