< Gravity

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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).

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

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

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

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?

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.

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.