Talk:Gravity/Archive 1

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graviton

"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

target audience

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

editing

... Information was omitted ... I hate it when someone does that. --AN

... If you fell I omitted something important, by all means reincorporate it (I tried to make the format extensible) ...

tidal force

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.

perhaps it would be better to say something like
Note that even though the masses of the individual objects are important, the distance between them is a term that is squared, so that changing the distance has a much greater effect than changing the mass. For instance the sun is many thousands of times more massive than the earth, but because the earth is closer, the force of earth's gravity is much larger than the force of the sun's gravity on most artificial satellites.

Galileo legends

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

we *still* don't know; let's not pretend that we do

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

photons and gravity

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.

EddEdmondson

"gravity" vs "gravitation" vs "geodesy"

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

close but not exact

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)

dark energy

Removed this

Interestingly, recent experiments are leading many physicists to the conclusion that there is another force in the universe, a repulsive force, that opposes gravity. Currently dubbed Dark Energy, the source of this force is also a mystery to science, but it may be connected to the same phenomenon responsible for the gravitational force. If so, Sir Isaac's insight into the nature of the problem may yet prove prophetic, and his reservations prudent. His desire was to be able to circumscribe physics into one great whole by what he called 'reasoning from mechanical principles.'

There's no particular reason to think that dark energy has anything at all to do with gravity.

Or that there is such a thing as "dark energy".
IMHO the "dark energy" is only an attempt to modify Einstein's Equation by making the cosmological constant a part of the source in order to fix another lame modification of Einstein's theory that demanded zero cosmological constant (to fix "Einstein's greatest blunder") and so it predicted a decelerating expansion. After this prediction turned out to be wrong (and Einstein not as inept as assumed), the failure had to be blamed on something that was not possible to guess rather than on poor understanding of Einsteinian physics. But since Einsteinian physics works quite well then due to Occam's razor we may safely forget the "dark energy". Jim 16:23, 2004 Jun 27 (UTC)

waves vs fields

How does this statement supports General Relativity?

  • More recent experimental confirmations of General Relativity were gravitational waves from orbiting binary stars and existence of neutron stars and black holes.

The concept of waves implies a field effect rather than a time-space curvature. This was Nikola Tesla's major reason to objecting to Einstein's theory and proposing his concept of a dynamic theory of gravity that uses a field of force. Was Tesla right and Einstein wrong? Can anyone explain how we get waves out of curved space-time? -- kiwiinapanic 04:49, 3 Aug 2003 (UTC)

No I can't explain it, but I can use google:
-- Tim Starling 11:20, Aug 3, 2003 (UTC)

G rather than gamma

Have reverted to the use of G rather than gamma. NIST amongst others, quoting CODATA use G. I believe this is standard. -- EddEdmondson

EM v. gravity

The subparagraph concerning the relative strengths of EM and gravity substitutes unfounded theory for tested fact. It is stated that one or both masses must be neutral to a very high degree, but there is no confirmation of this. In fact there is a huge amount of common sense data that conflicts with it. It is highly unlikely the all the gravitating masses we know of could be EM neutral to the degree stated. It is felt that they must be, to make QED's current conception of gravity consistent, but no one has ever measured the overall electromagnetism of the earth, as a discrete body. Likewise, the statement that a simple magnet overwhelms gravity is only one explanation of the phenomenon, an explanation that has no data to back it up. The truth, as admitted earlier in the article, is that no one, not Newton, not Einstein, and not current theorists, know what causes gravity. All the analyses, no matter how mathematically complex, are very short on basic concepts. It is not even known that gravity is in fact a field. The graviton is just an hypothesis at this point. No scientist should pretend to understand a phenomenon he or she does not understand. The magnet sentence, as well as the EM neutral sentence should be saved for much later, lest they end up making the writer look foolish.

Miles Mathis

Is it g, gee, ge, g0, or what ?

In the specific impulse and gravity articles "the acceleration at Earth's surface (9.8 m/s2)." was once variously indicated as

  • ge
  • go
  • g0
  • g

I'm glad someone fixed the inconsistency in specific impulse. But it's still inconsistent with the gravity article.

Is there some reason to pick one over the other ? I hear it pronounced like "gee" ("most people black out in a 11-gee acceleration"), but how is that spelled ?

I like to use ge for Earth and gm for Mars and gL for Luna. -- DavidCary 18:06, 1 Jun 2004 (UTC)

Well, I would recommend a lone g, and put in a qualification to say that acceleration at the Earth's surface is intended in this context. I'd make it an italic g, too, since physical constants are typically italics. That distinguishes it from g (grams), but it's a pretty weak distinction. I see that NIST calls it [1] gn. I suppose the n is for "normalized" or something. Happy editing, Wile E. Heresiarch 05:05, 2 Jun 2004 (UTC)

Any reason to center the table of planetary surface gravities? It messes up the alignment.--wwoods 20:10, 3 Jun 2004 (UTC)

Well, I centered the table since tables in books and papers are centered on the page. I guess I don't know which alignment is messed up. Do you mean the words and numbers within the columns should be right or left justified? I guess that seems reasonable. Maybe we can play with the table formatting parameters. Regards, Wile E. Heresiarch 06:04, 4 Jun 2004 (UTC)

Is this really a problem with Newtonian gravity ?

The article currently says

5. Under Newtonian gravity (with instantaneous transmission of gravitational force), if the universe is Euclidean, static, infinite, and of uniform, average, positive density, then the total gravitational force on a point is a divergent series. In other words, newtonian gravity is inconsistent with a universe which is Euclidean, static, infinite, and of uniform, average, positive density.

I fail to see why this is a "problem". All kinds of other bizzare things happen in a infinite, uniform universe (for example, Olbers' paradox). I've been told that even Einstein's general theory of relativity can't handle an infinite, uniform universe either. -- DavidCary 16:51, 26 Jun 2004 (UTC)

the acceleration due to gravity

Is "the acceleration due to gravity" really a helpful concept, or is it just confusing ? Raindrops and parachutists are clearly pulled to the ground by gravity, but over most of the fall, their acceleration is zero. -- DavidCary 20:49, 28 Jun 2004 (UTC)

See WikiBooks:Gravity (Physics Study Guide)

In the case of raindrops and parachutists, the acceleration due to gravity is still present, but at a certain point, the air resistance pushing against the fall becomes great enough to stop any further acceleration - this is what we call terminal velocity. --Darkstone 19:55, 29 Jun 2004 (UTC)

Yes, that's true. Some textbooks say that "the acceleration due to gravity is 9.81 m/s^2". Is that a good thing to say, or will it be less confusing if we emphasize that gravity is a force, not an acceleration ? Raindrops *never* accelerate at 9.81 m/s^2. -- DavidCary 05:42, 30 Jun 2004 (UTC)

Gravity is a force, not an acceleration: "the acceleration due to gravity", there's a distinction. Raindrops don't accelerate at that speed, and I'm willing to bet that parachutists don't either, because of air resistance. The acceleration due to gravity on Earth would only be 9.81ms^-2 if one was falling through a vacuum

Rename law of universal grav to Newtons...

The section "law of universal graviation" needs renaming (I have) to "newtons law of..." to emphasise that this is the old theory. Its not true under GR, of course.

Various other changes that will probably annoy someone.

I think it's largely fine. "Newtons" needs to be "Newton's" - I think I've changed all of those. I'm also changing the bit about very dense objects - because the field is then only strong very close to such an object (unless it's already massive). EddEdmondson 22:28, 9 Jul 2004 (UTC)
OK so far (WMC)
Hello. If I may jump in -- perhaps giving each theory the most descriptive name will be most felicitous. Naming theories after people is slightly problematic in that it doesn't tell anything about the content of the theory, and sometimes introduces irrelevant controversies about credit and precedence. Can we say relativistic theory instead of Einstein's theory? I'd prefer classical to Newtonian or Newton's too, although I must admit "classical" is not very descriptive. Comments? -- Wile E. Heresiarch 02:36, 14 Jul 2004 (UTC)
(William M. Connolley 09:14, 14 Jul 2004 (UTC)) I renamed it because the previous title - universal law or whatever - was quite misleading (for the N section). classical graviation is a possibility, but I would prefer N's, because: *everyone* knows its N's law of gravitation. If you called it classical, many would mistake that for pre-N laws (ok, they would be swiftly corrected when they read it). As for the GR version: again, I would say E's name should be there. Perhaps "E's GR theory of G"?

Theories of gravity in chronological order

WMC is right. People's names are much better to identify theories since they prevent ambiguity. E.g. "classical" may be presently understood as Einstein's general relativity, a physical theory of gravitation based on curvatures of spacetime, not necessarily adopting any particular metric. Unlike standard model general relativity that is based on particular form of Einstein's Equation and on FRW metric as its solution, a particular mathematical theory, which also, unlike Einstein's universe, includes expanding space.

We have in chronological order:
  1. Newtonian gravitation characterised by its fundamental force of universal gravitational attraction if not necessarily invented by Newton then by someone who couldn't imagine anything else
  2. since 1915, Einstein's general relativity with the fundamental gravitational attractive force replaced by pseudoforce and cosmological constant fitting "static" universe since it Hubble redshift hasn't been discovered yet
  3. since 1929 standard general relativity without cosmological constant (named then "Einstein's biggest blunder") and with FRW metric predicting expanding universe with expansion slowing down due to the "attractive gravitational force" (mysteriously recovered) since Hubble redshift has been discovered by Edwin Hubble and soon after for lack of any sensible ideas why the universe would look expanding nut really wasn't it was declared that the universe must be really expanding, an idea lasting till today (2004)
  4. since 1998, the present general relativity (non standard yet, since cosmologists are not sure any more) with restored cosmological constant in form of dark energy represetning now a "repulsive force" (presumably fundamental too, "overcoming" the mysteriously recoverd attractive force) and very unpleasant accelerating expansion of the universe (unfortunately observed by astronomers to the great dismay of cosmologists)
  5. nonexistent yet "quantum gravity" that everybody and his brother is talking about but makes no sense as Einstein's type theory since it is supposed to restore the "fundamental force of gravity", this time "repulsive", in form of action of "gravitons" so nobody needs to learn general relativity any more (in which I see certain cultural regress and a possible return of the new middle ages but I hope I'm wrong). Jim 05:40, 17 Jul 2004 (UTC)

removing outdated stuff to save space (and time)

pulling the present discussion out of the archive. Jim 07:10, 26 Jul 2004 (UTC)


My dear,

Actually we should know if we read "The Universal Theory Of Pressure" by Zony Said. The thesis proved that gravity is actually caused by pressure and not by mass.


Simple Question that May Not Be Answerable

When I read this discourse on gravity, it seems to me this is all a presentation of our observations and measurements of gravity, its effects and so on, but it leaves me unfulfilled.

Is there any way to tell readers exactly What Is Gravity?

What is it? We have explanations for photons, for waves, for whatever phenomenom. But why isn't there an explanation for what gravity really IS.

Maybe it's like magnetism. We don't know yet what it is.

I think it's better to change its concept rather than its name.

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