Talk:Gravity

Guys... what's this about g being approximately pi^2 and that not being a coincidence? I can't figure out why this should be related to the choice of the length of the metre as approximately 1/4e7 of the circumference of the earth. It's got units m/s^2, and whilst the metre is based on the dimensions of the Earth the second is based on the Earth's rotational period - which is completely independent. Someone either explain it here or I'll delete that line.

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The next step will be to detect the graviton, the theorized quantum particle that carries the gravitational wave (much like the photon carries the electromagnetic wave).

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I removed this because I'm not sure how much sense this makes. Detecting gravitational waves is in a sense detecting gravitons, unless they are talking about discriminating individual gravitons. If the last is the case, I'm not sure is just a next step, but something further away in the future. If a professional physicist thinks the comment does belong in the main page, please put it back. --AN

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If I were 15 years old and simply did not know what gravity was, really, this article wouldn't be a whole lot of help...I'm not saying articles should be pitched at 15-year-olds, but that they should be blessed with simple explanations of complex concepts when helpful (as in this case, surely). My $0.02 as usual. --LMS

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Thanks, Larry, for pointing out that encyclopedias are supposed to eventually have readers. And readers need introductions.--MichaelTinkler

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The person who wrote the "complete overhaul" simply erased the previous article and wrote a new one. Information was omited, ad none of the previous content was kept. I hate it when someone does that. --AN

I actually tried to keep as much content as possible, and even used the old article as a guide for what content I should include. I did not try to incorporate everything, however, because some of it belonged in discussions about particle physics/quantum mechanics. If you fell I omitted something important, by all means reincorporate it (I tried to make the format extensible), or replace the new article with the old one. Which one is better for a general audience encyclopedia, though?

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Note that even though the masses of the individual objects are important, the distance between them is a term that is squared, so that it has a much greater effect. For instance the sun is many thousands of times more massive than the moon, but because the moon is closer, its gravity causes larger tides in the ocean than the sun's gravity.

This is simply untrue. I've just written tidal force which explains why the Moon affects the tides more than the Sun.

Is there anything useful to be gained by comparing the different terms in the equation? I don't think so. (After all, we wouldn't even be here if gravity were anything other than inverse square.) I've removed this paragraph from the main article.

On another point, the derivation of g in this article invokes the principle of equivalence, which I guess should be an article in its own right... (I don't have time right now to start it, or the knowledge to complete it!)

My notes on M-theory, which come from statements made by physicists working in superstring theory, mention the gaps Einstein left behind are finally resolved. I do not understand much of this, but am happy that a "20 year old problem" has been removed, for the people who started all this with the formulations of string theory. Perhaps include a ref to M-theory with this data at the end of your article. It may update your credibility.

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Did Galileo actually try dropping weights? My recollection is that he did an early, elegant thought experiment: he envisioned dropping two one-pound weights, each with a small chain atop it, simultaneously, and then linking the two chains and dropping the resulting paired weight.

I believe the weight dropping is legend, but he did experiment with inclined planes, where the effect can be demonstrated much easier. His thought experiment is not convincing though: it assumes that the force that an object feels does only depend on the object's mass and not on its shape. He forgot to state that assumption, and in fact the assumption is not justified in the presence of air resistance or in a non-homogeneous gravitational field (like the Earth's). --AxelBoldt

I don't recall if he physically dropped weights, but I am certain that he did pendulum experiments which are equivalent. His observation that the period of a pendulum only depends on it's length is equivalent to saying that falling objects accelerate at the same rate.

His inclined plane experiments may have been part of his observations, but their primary importance was in the idea of inertia. Galileo observed that when a ball rolled down an inclined plane and up a second, the ball will roll up the second plane until it has reached its original height, no matter the angle of the two planes. Galileo then imagined removing the second plane, and postulated that it would not stop since it would never re-achieve its original height.

Also, Galileo's assumption that force due to gravity does not depend on shape was a damn good one. How would he know that? Simple, people have been using balances for millenia. Balances measure the relative force due to gravity. Showing that two objects that have different shapes but the same mass experience the same gravitational force is trivial using a balance as long as their size is small compared to the radius of the Earth. The other nice thing about a balance is that it eliminates air resistance entirely.

You're point about Earth's gravitational field being non-homogenious is also false in a practical sense. IIRC, the variation to g (9.80665 m/s/s) is in the third decimal place as one travels over the surface of the Earth. Essentially, because the radius of the earth is so tremendous compared to human scales (on order of magnitude of thousands of kilometers IIRC) that treating the proportionality between mass and force due to gravity as a constant is perfectly fine for everyone but geophysicists. Essentially, all he needed to observe would be that traveling around he didn't change weight at all to prove that sufficiently for his purposes.

My whole point is that he didn't make these assumptions a priori, he made them based on experimental results. Whether or not people knew they were conducting experiments is another matter, but the results are the same.

BlackGriffen

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I didn’t change the article but note that Larson points out the fact that Newton did not assume action at a distance. Indeed he says Newton called it 'absurd' and refused to commit himself to any specific explanation of gravity. Maybe it would be more clear to say that 'Newton’s system necessarily involves the assumption of action at a distance.' But there is another problem as well because the way it is worded gives the impression that Einstein provided the way out of the dilemma of having to assume action at a distance to explain gravity, but this is just not so. He simply rearranged the problem. Larson quotes G. C. McVitte: 'To say instead that gravitation is a manifestation of the curvature of four-dimensional geometrical manifolds is to account for a mystery by means of an enigma…' and also Bridgman: 'I believe, however, that an analysis of the operations that are used in specifying what the field is will show that the conceptual dilemma…has by no means been successfully met, but has merely been smothered in a mass of neglected operational detail.' Larson sums up saying: 'As these observers indicate, what Einstein has actually accomplished, so far as the great dilemma of gravitation is concerned, is not to resolve it, but to push it farther into the background where its existence is less obvious.'

The dilemma of the origin of Newton’s force has been hidden by GR is all. Now, instead of asking '1) How does Newton’s force originate?' and 2) 'How does it work?' we have to ask 1) 'How does the deformation originate; that is what is there about the property of mass that deforms space or space-time?' and 2) 'What is the mechanism of the deformation (How does it work)?' which of course, is the same dilemma. (See Larson’s Beyond Newton, pages 13-19)

In view of these circumstances, I think the article could be revised considerably, not only to show the true status of gravitational theory, but also to eliminate the presentation of assumptions such as gravitational waves and black holes which are presented with the status of physical facts rather than the theoretical assumptions that they are.

Regards,

Doug

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I changed the line saying that a massless photon shouldn't be deflected in Newtonian gravity. On the basis that acceleration due to gravity is independent of mass you could argue that this is untrue, and on the other hand you could argue that it needs mass to respond to gravity. In essence this is a completely pointless argument because we can't get a massless particle in a Newtonian field to look at. I think it is much more revealing that Newtonian gravity still gets the answer wrong by a factor of two if you allow it to act on a photon.

Edd Edmondson

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Just added a remark on gravity and gravitation being different things. Gravity is what makes you fall and includes the centrifugal (pseudo-) force.

I think it would actually be a good idea to separate the physics of the gravity field from the study of the Earth's gravity field -- the subject of physical geodesy.

Martin Vermeer

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19 March 03. Someone should really edit the second paragraph after the subsection titled "History" that discusses how F=m*a together with Newton's law supposedly predicts Galileo's observation that unequal masses fall at the same rate. Someone should correct this and point out that Newton's theory of gravity together with F=m*a actually predicts the general violation of Galileo's observation for most typical cases, and that Galileo's empirical law is only approximately correct in Newtonian theory (to very high accuracy albeit) due to the unequal mutual attraction exerted between the Earth (fixed mass) and either of two unequal masses, that results in unequal rates of acceleration or fall (again the effect is very very closely the same as Galileo's result due to the huge diffrence in mass in most cases of practical importance for objects on or near Earth, or even for large satellites). But whatever the case, however tiny the effect, the reader should not be left with the impression that Newton's theory confirms Galileo to 100% precision! The bit where the acceleration of a mass is given as: a_1=G*m_2/r^2 should thus be qualified by noting that, a_2=G*m_1/r^2 (the acceleration of the Earth towards the mass m_1 that should be added vectorially to get the mutual falling rate) is insignificant in comparison with a_1, and so Galileo's rule holds approximately for all practical purposes, though not exactly theoretically.

In other words, one could say on the contrary to the current Wikipedia article, that Aristotle was techically correct in the absurd sense, i.e. in an ironic twisted sort of way only when one subtracts all the fallacious reasoning that Aristotle came up with!

B. Smith

LOL. Wow, here's a guy who knows how to keep a joke running for a while before getting to the punch line. -- Tim Starling 06:13 Mar 19, 2003 (UTC)

OK, I guess my amusing aside above was a tad overdone. Sorry if I caused you to split your cheeks laughing Tim! I'll try to do better next time I write to wiki-talk. -B. Smith

Well, to be fair, even to that degree of precision, Galileo's claim would still hold, since he was dropping two objects of different weight (mass) at the same time. So, the Earth's acceleration would be the sum of the acceleration caused by the two objects, but not different for the two objects. And both those objects would be accelerated at exactly the same rate. So, you'd get exactly the same measured acceleration no matter how many decimal places, as long as you dropped them both at the same time. ;-) -- JohnOwens 00:54 Mar 26, 2003 (UTC)

See also the three-body problem --Uncle Ed