# Talk:Gravitation/Archive 2

## Mention the weakness of current theories of gravity?

Hello, I was thinking... You know how the religious folks say that evolution is "just a theory"? Even though they have theory and hypothesis confused, I'd like to indirectly help them understand that evolution is one of, if not the, strongest theory in science. I tend to say to religous folks "we also have the theory of gravity, and the theory of the atom, both of which are quite incomplete compared to the theory of evolution". I may be wrong with this statement, since i'm not a scientist. But should it be mentioned in the article that gravity really isnt very well understood at all, and is not one of science's strongest theories? The same could be said of time, I think....??

## Back from Holiday

(William M. Connolley 17:48, 24 Jul 2004 (UTC)) I'm back from holiday and rather than face the mess that Ed Poor has made of global warming I shall try here for a bit...

- Hi William, did you have a good time? Where have you been? Jim

So: I've stripped more stuff from the intro. Which will undoubtedly annoy Some People so I'll try to justify this:

*Later developments have shown that this force does not actually exist.*This can't really be justified. What anyway is a "force"? Don't forget, GR is not the end of gravity theory, its just another and better model (actually for all we know it may be the end but I don't think thats generally believed, what with QM and all)

- Whatever is
*believed*, this first sentence that you deleted presents the present state of the theory and we can't put here what we believe there should be according to our understanding of physics but what we*know*that there is in the theory that we describe. We are to inform readers and not to push our own ideas. Whoever wrote the sentence that you deleted (it was not me BTW) he/she expressed the truth about GR. Jim

- Whatever is

*Newton's formulas with their simple math are sufficiently accurate. For example, rocket ships, bullets, and peregrine falcons do not attain the high speeds nor involve the immense masses which necessitate the use of Einstein's math.*. I don't like "simple math" in this contexts since it suggests "simplistic". "high speed" is wrong, I think. Speed makes no difference that SR doesn't cover. "immense masses" is also wrong: its the concentration of the mass. After all, black holes are no more massive than the starts which made them.

- The same thing. The author (I believe the same) expressed very well why Newtonian stuff is OK (he/she was correcting my stuff and did it well). It is "simple" and surely not "simplistic". "High speed"
*makes*difference for Newtonian gravity, the same as "strong field". But "immense mass" make very good sortcut because the explanation of "strong field" that involves curvature of space has to be put for later. Surely can't be explained at the beginning. Jim

- The same thing. The author (I believe the same) expressed very well why Newtonian stuff is OK (he/she was correcting my stuff and did it well). It is "simple" and surely not "simplistic". "High speed"

*We present the description of gravitation starting with Newtonian math. It isn't physics, but it is simple and helpful for learning the quantitative part of the physics of gravitation. Then we proceed to Einstein's model to explain the true, as we believe now, and even simpler physics. It is simpler since there is no mysterious "gravitational attraction" in it that would require an explantion. We skip the exact math that is not very useful even for professonals and is generally considered the most difficult math that has ever existed in physics (Einstein once said about his theory something like "if you think that you have problems with math I may assure you they are nothing compared to mine".).*This isn't OK. You mean, it is no longer the currently accepted best-theory and has problems with action-at-a-distance. But that doesn't disqualify it from being physics. GR is very attractive but, I think, is properly regarded as a better model rather than true. "simpler" is a slippery concept. You and I are skipping the exact math for the excellent reason that neither of us understand it in detail. Suggesting that its not useflu for proffs is weird.

- (This is my part). There can be only one physics, and if there is a "better one" all others are disqualified since only one can be
**true**. Science is not like political theories. It is either true or not. If you don't like GR, too bad, but for the time being we can't prove it is wrong. We have to tell that it explains everything if it does, and we can't say "Oh yeah, it explain everything all right but we don't believe it anyway and actually we prefer the old theory, or we just wait for quantum gravity". You may express such an opinion privately only, because Wikipedia is here to inform people of what is there and not that we have better ideas.

- (This is my part). There can be only one physics, and if there is a "better one" all others are disqualified since only one can be

- I agree that "simpler" is the matter of opinion. It is simpler for me but it may be not simpler for you. "Objectively" it is simpler because it uses less assumptions and more facts that Newton didn't even know about. But if you don't like "simpler" let's dump it.

- It is really not usefull for proffs. There are no exact solutions for any practical problem and Newtonian math has to be used all over the place, even if its physics is wrong. But it is irrelevant. Introduction to GR's math is about 1000 pages long. We can't even start producing the GR's math, we just have to explain the (simpler than Newtonian) physics. Jim 22:53, 24 Jul 2004 (UTC)

*When we see Einstein's physics we might understand why Newton's math based on "gravitational attraction" is accurate despite that masses in the universe don't attract each other.*This actually skates round something the article doesn't mention - that GR has the newtonian limit built into it. GR *doesn't* (in my admittedly limited understanding) derive the laws ex nihilio. The basic equations come from the equiv prin but to actually get gravity you need to be aiming at something that looks like N's formulae. I'll try to dig out a ref to prove this.

- This is what I'm doing there: I'm showing how the most relavant part of GR is
*identical*to Newtonian math. The same equation different physics. I also show that whatever you may think about, Einstein's theory explains that too. What else do you need to show? And where did you get the idea about the "basic equation coming from the equivalence principle"? The basic equation is Einstein's field equation, that has very long way to this principle, which is just (simple) physics with very little value in proving anything. It is just another illustration that Einstein's physics explains whatever there is to be explained. I hope I'm explaining the stuff in a way that a 15 years old can understand, because if he/she can't we can just abandon the whole project. Jim 22:53, 24 Jul 2004 (UTC)

- This is what I'm doing there: I'm showing how the most relavant part of GR is

Bye for now...

## Mass equivalence

I find this puzzling: The sentence
"General relativity also explains the equivalence of gravitational and inertial mass, which has to be assumed in Newtonian theory."
is tucked away in the "Experimental" section of this article. Why? This a **very** important difference between Newton and GR and should be prominently displayed, even more so than the Mercury orbit precession, because it is a "qualitative" difference of the theories and **testable** for GR. Awolf002 18:06, 24 Jul 2004 (UTC)

- I explained in the end of section "Why Einsteins' gravity differs from Newton's" why quantitative differences are decisive and qualitative diffs are only a nice addition. It is an illustration of the old truth that only agreement with observation can deliver knowledge. It is because human logic can be too easily abused to be a reliable device in science (with a possible exception of math, but math without physics behind it is useless anyway). Jim 09:25, 25 Jul 2004 (UTC)

I completly disagree! Let's say theory A and B are said to explain why an experimenter measures the number 2.5 +- 0.01. This is a quantative check of both theories. But for something complex like real life, all theories must make decisions, what input to use to calculate the expected result, plus the systematic errors of the experiment (bias). For example the Mercury orbit precession: Do I try to add a not yet discovered planet (Vulcan) to explain its value or not? Does alpha Centauri have any influence or not? In contrast, if the foundation of theory B says that the number is fixed to 2.0, than there is no way to wiggle out of the conclusion that B is incorrect! That is a "quality" of theory B and can not be "tuned." Therefore I think that the equivalence principle in GR is one of the most important aspects, being a qualitative consequence of the geometric basis of the theory! I do not mean to say that "qualitative" means unmeasurable!! Awolf002 11:58, 25 Jul 2004 (UTC)

- You're right but I meant different kind of importance. Not a quality of a theory as a piece of art but simply whether we can trust the theory because the result it predicts are as observed (regardless of how the theory gets them). Of course if there is no flexibility in the way of predicting the results it is a "better" theory. However first of all it can't be a false theory since then no amount of beauty will save it. So testing predictions is always the first step and in this sense it is more important. Only then we analyze the means of achieving those results (and sometimes have to suffer in silence).

- Whatever Einstein's gravity predicts is achieved without any possibility of adjustments, which makes it a great theory, and maybe we should expose this fact. In my opinion it is because Einstein's theory is simply a true theory and so all the results have to come out necessarily true. But I think that very few people really appreciate this kind of beauty that you are talking about and the only thing that counts for them are "hard data". Especially in gravity physics which is too close to cosmology with its endless ad hoc adjustments any time a new discovery turns the old stuff upside down.

- Please note that in cosmology the leading scientists already abandoned even the principle of conservation of energy to be able to explain the observed results, while Einstein's theory requires only to abandon the idea of expansion. Yet it would be too tough on many leading scientists so we keep adding new epicycles and so our "hard data" still "confirm the theory". A theory without conservation of energy, with superluminal "speed of expansion" when it's needed to keep the theory alive, and now with unknown yet new forms of matter ("dark") and energy (also "dark") that is not even included in T
_{μν}tensor of Einstein's equation but mysteriously in Λ that suddenly became a "repulsive gravity" (in a supposedly Einsteinian type of theory in which there is not even "attractive" gravity!).

- However who cares if nothing else in it makes much sense anyway. Just let's see what other physics we have to give up to keep the expansion. Enjoy the spectacle while it lasts. Suddenly someone with access to a scientific journal may notice that Einstein's gravity doesn't need all that ... and it might end overnight as if it never happened. Jim 14:16, 25 Jul 2004 (UTC)

- BTW, in section "Why do we see an accelerating expansion of the universe?" I'm trying to explain why Einstein's gravity doesn't need all that ... I'd appreciate your opinion about it. Jim 14:37, 25 Jul 2004 (UTC)

I'm sorry, but I was under the impression we are trying to create a good encyclopedic page about Gravity, and have some information about its current explanation by science. Even though you're points are interesting and might lead to some entertaining discussion, how does what you say change the structure of this page? We should **not** do the work of the scientists trying to figure out how it works, but summarize what they have done so far, and if further work is being done. So, saying things like 'GR is true' is not helpful, since nobody knows that. It is an assumption, right? So, back to my point: I would suggest to add the "equivalence of masses" as an important quality of GR to a fitting section at the beginning, explaining that this follows from the geometry approach of its base equations and concepts. That's all I ask. Anything wrong with that? Awolf002 16:47, 25 Jul 2004 (UTC)

- Of course it is an assumption. What else one can have in science? But IMHO for the time being this asumption can't be falsified so we better treat it seriously. We don't need to try to figure out how it works since Einstein already did it for us, we just need to report it. And as for adding the "equivalence of masses" go ahead and add it. Then we see how it looks and if good then it stays. But there are people here who seem offended by suggestions that Newtonian stuff might be wrong. So they may remove it as they keep removing my messages that there is no universal gravitaional attraction in contemporary physics what Einsteinian stuff says and I'm just reporting it according to what is presently thought about the gravitation. Jim 06:37, 26 Jul 2004 (UTC)

## Creating a good encyclopedic page about Gravity

In "trying to create a good encyclopedic page about Gravity," special attention should be paid to the opening paragraphs. Currently, they read as follows:

*Gravitation is the tendency of masses to move toward each other.*

*The first mathematical formulation of the theory of gravitation was made by Isaac Newton and proved astonishingly accurate. He postulated the force of "universal gravitational attraction".*

*Newton's theory has now been replaced by Albert Einstein's theory of General relativity in which there is no such force as "gravitational attraction" ("universal" or otherwise) but for most purposes dealing with weak gravitational fields (for example, sending rockets to the moon or around the solar system) Newton's formulae are sufficiently accurate. For this reason Newtons law is often used and will be presented first.*

First of all, Newton did not formulate a "theory of gravitation."

- Someone had to and called it "Newton's theory of gravitation" because it is in the history of science. Is it known who did it? It's an important piece of info for a good page on gravitation. But it probably has to go to the page on history of science with a link in "Gravity" page. Jim

Second, he didn't postulate a "force of universal gravitational attraction."

- Someone had to and called it "Newton's force of universal gravitational attraction" because it is in the history of science as well. A doubt that it is known who's done it but link to history page should be sufficient. Jim

He formulated a mathematical law that described the observed universal attraction of matter. I know that this may seem nitpicky to some, but it's important to be accurate, especially in light of the fact that Newton stated that he refused to form a hypothesis (theory) concerning gravity, and indeed explained why he refused to do so.

- (William M. Connolley 08:31, 26 Jul 2004 (UTC)) I don't believe any of that, but I'll check in his papers when I get home. AFAIK Newton thought he was describing an universal law. The "no hypotheses" stuff is oft misquoted - all he meant was that he didn't like action at a distance.

- (William M. Connolley 21:38, 26 Jul 2004 (UTC)) OK, I am at home and do have my copy of principia with me (actually Hawkings collection "on the shoulders of giants"). Newton says (defn V and after):
*A centripetal force is... Of this sort is gravity, by which bodies are drawn to the centre of the earth magnestism, by which iron tends to the loadstone; and that force, whatever it is, by which the planets are perpetually drawn aside...*.

Then, in the Einstein section, the text of the article reads as follows:

*Newton's formulation of gravity is quite accurate for most practical purposes. There are a few problems with it though:*

*It assumes that changes in the gravitational force are transmitted instantaneously when positions of gravitating bodies change. However, this contradicts the fact that there exists a maximum velocity at which signals can be transmitted (speed of light in vacuum).**Assumption of absolute space and time contradicts Einstein's theory of special relativity.**It does not explain the small portion of precession of orbit of Mercury of order of one angular second per century that kept astronomers baffled for more then a century as to the reason for it.**It predicts that light is deflected by gravity only half as much as observed.*

Again, Newton's formulation was a mathematical expression of the force of gravity, not a formulation of (a theory of) gravity. So, this would read better if it read: "Newton's formulation of the law of gravity..." However, in the list of four problems that follows, there is more trouble. First, "it," that is, Newton's Law of Universal Gravity, doesn't "assume" that "changes in the gravitational force are transmitted instantaneously," this is an observed fact.

- You
*assume*that it is a fact, don't you? Jim

- (William M. Connolley 08:31, 26 Jul 2004 (UTC)) I agree with Jim, only more vehmenently (especially if I could spell it...): your so-called observed fact is nonsense. Nothing goes faster than light (or at least, nothing (carrying information) has ever been observed to, and it would destroy SR if it did).

Second, this observation doesn't "contradict the fact that there exists a maximum velocity at which signals can be transmitted (speed of light in vacuum);" The contradiction is with the *assumption* that gravity must operate through a transmission process that is so limited.

- (William M. Connolley 08:31, 26 Jul 2004 (UTC)) I think you're either in pet-theory territory or totally confused.

- I think it would be considered
*original research*if we did it this way. We have to stick closer to what is*known*. Jim

We need to keep the facts straight so as not to be misleading if at all possible.

- That's a good idea. But IMHO the "facts" which existence we
*assume*are at least a little misleading in view of present theory of gravitation (*Einstein's*). Jim 06:37, 26 Jul 2004 (UTC)

The second "problem," having to do with Newton's assumption of absolute space and time, does not pertain to his law of gravity specifically, and therefore to say that it contradicts "Einstein's theory of special relativity," and for this reason is a problem as a law of gravity is nonsense in this context. It doesn't make any sense because the fact that Einstein assumed relative values of space and time in order to explain the observed behavior of light is unrelated to law of gravity. This issue is only relevant to Einstein's *theory* of gravity, not to Newton's formulation of the law of gravity.

Likewise, the third and fourth "problems" could be stated better in order to reflect the true situation. The third one should simply state that Newton's law fails to correctly account for the gravitational force between bodies moving at relatively high speeds such as in the case of the planet mercury's high-speed orbit around the sun. The fourth one should state that it fails to correctly account for the effect of mass on the propagation of radiation, which of course was never contemplated in Newton's day.

Changing these paragraphs will necessitate a minor change in the text of the paragraph that follows them, but otherwise presents no real obstacle and would improve the accuracy of the article considerably in my opinion. Doug 21:11, 25 Jul 2004 (UTC)

- (William M. Connolley 21:36, 25 Jul 2004 (UTC)) I've now created equivalence principle and modified the gravity page text somewhat. Feel free to join in: *there are no experts here yet*.

- William, I happen to be an expert even if you aren't going to believe it. There is very little about Einstein's gravitation that I don't know or understand. If you don't want to use "my wisdom" and you have your own opinions on gravitation that's fine with me. We may discuss them. Just try not to remove stuff that you don't understand (didn't I tell you that before? Sorry if I'm repeating myself, but at my age, you know...). Ask about them before you remove them. It's really annoying when you remove stuff because it doesn't fit your ideas. Your ideas may be wrong too, you know, the same as mine. The only way to decide who is right is to discuss them using rational arguments. Jim 07:29, 26 Jul 2004 (UTC)

- (William M. Connolley 08:31, 26 Jul 2004 (UTC)) Fair enough. I'm very reluctant to trust you because of all that stuff about particles "wanting to" move around. And if you are trying to explain this to 15-y-olds, if I don't understand (never mind agree) with it, its pretty unlikely that 15-y-o's will. We shall see.

- The story about particles "wanting" move around gets a little bit into quantum mechanics (or perheps rather deeply but we, the intrepid discoverers of mysteries of the universe, don't mind).

- As you may recall, the basic thing in quantum mechanics is
*probability*that a particle is somewhere. Now each particle has its "wave function" that for our porpose is just "probability wave" with certain strange properties, which we don't need to get into because quantum physicists take care of them (and don't understand them neither, which is irrelevant here). We just need to know that those "probability waves" interfere either constructively or destructively with each other. Now, you can take it on faith (to save me long derivation) that when a paths along which the time has an extremum (either minimium or maximum) then all those waves add constructively otherwise the summation is destructive and results in*zero*. The same thing goes for "light waves" by the way, and so light moves only along paths with extremal time. If you remember this from physics it makes my lecture more credible and saves me derivation (simple derivation but tedious so why to bother if you believe me). So if those "waves of probability" add destructively (and so the probability of finding a particle on a "destructive path" is zero), quantum mechanics says: particle can't be there. And it just happens, for the form of the Newtonian math that is the same as Einsteinian math for "time dilation", that proper time along Newtonian orbits has a maximum. So all those "probability waves" add constructively only there. So one can find particles*only there*. Only on Newtonian orbits, without any "force" placing the particles on those orbits.

- As you may recall, the basic thing in quantum mechanics is

- So we are justified saying that they just "want" to be there "by themselves". It's of course realy not their "free will" but laws of quantum mechanics, but the final result is exactly the same, so we may advise 15 y. o. that those particles just want to be only there. When they got interested
*why*it happens, and start reading quantum mechanics stuff to figure it out, we don't need to worry any more because they'll learn pretty soon by themselves. And when their curiosity gets awaken maybe they find out also something that we don't know yet. Which would be a very good use of wikipedia.

- So we are justified saying that they just "want" to be there "by themselves". It's of course realy not their "free will" but laws of quantum mechanics, but the final result is exactly the same, so we may advise 15 y. o. that those particles just want to be only there. When they got interested

- Of course those Newtonian orbits are, because of extremal time, also geodesics in spacetime. But for 15 y.o. it might be safer to explain everything in 3D flat space only with which they may be more comfortable than with 4D curved spacetime. Besides it is a straightforward physics, even if quantum, rather than mysterious
*geometry in curved 4D spacetime*that for some people may be too mysterious to comprehend it (even if it is exactly the same thing). The advantage of physical explanation over mathematical.

- Of course those Newtonian orbits are, because of extremal time, also geodesics in spacetime. But for 15 y.o. it might be safer to explain everything in 3D flat space only with which they may be more comfortable than with 4D curved spacetime. Besides it is a straightforward physics, even if quantum, rather than mysterious

- Is the explanation sufficient or you have more questions why those particles take Newtonian orbits without any force holding them on those orbits? Of course with "strong fields" the picture is a little bit distorted by "curvature of space" but I hope you expected it anyway. So the exact math (with both, time dilation, and curvature of space, or as we rather say with
*curvatures of spacetime*) is rather complicated but we want only to have an idea how is it possible that something happens without a force forcing it to happen. And possibly what was Einstein thinking about when he proposed his theory of curvatures of spacetime replacing "gravitatinal attraction". Jim 17:23, 26 Jul 2004 (UTC)

- Is the explanation sufficient or you have more questions why those particles take Newtonian orbits without any force holding them on those orbits? Of course with "strong fields" the picture is a little bit distorted by "curvature of space" but I hope you expected it anyway. So the exact math (with both, time dilation, and curvature of space, or as we rather say with

## Faster than Speed of Light?

Can anybody tell me their opinions on these:

- [1]
*While the peak moves faster than light speed, the total energy of the pulse does not. This means Einstein's relativity is preserved* - [2] Probably junk
- [3]
*"This is anything but dramatic," said Happer. "If you look at the data, there’s essentially no evidence that [the beam] is going faster than the speed of light." ... What’s more, most scientists agree that even if such a beam can be proved speedier than light, it would probably not be able to carry any information.* - [4] Same chap as #2

So...what is the speed of gravity?

- According to Einstein's theory (of gravitation) the "speed of gravity" is c (the same as speed of light).

- (William M. Connolley 16:57, 26 Jul 2004 (UTC)) Agreed.

- There is an effect of "apparent speed", which is not really an effect of
*speed*but an effect of*dirction*from which the*interaction*comes that is a little bit tilted forward, pointing almost exactly toward the place where the actual position of the object that causes this interaction is, and that's why it looks like the*interaction*comes "*instantly*". Because it comes from the*direction*of actual position of the object and not from the*retarded position*as non physicists might expect it to come from. It is a well known effect in physics, especially in electromagnetism, and so real physicists simply ignore such problems. However those problems excite amateur astronomers (and even some professional astronomers who didn't have enough training in physics) a lot causing endless discussions. If you are interested in details of physics of this effect the subject to study is*Linard-Wiechert potential*or some variation of it.

- (William M. Connolley 16:57, 26 Jul 2004 (UTC)) I think I know what you mean here but it isn't really about the speed of gravity in particular.

- As for "super luminal speed of light" it is easy see how it can be simulated: If you set a long row of people and tell each of them to raise a hand at certain specific time, an observer might see a
*wave*of raising hands moving through this row with certain speed. This speed may be faster than light, but it is only a "wave moving" not any physical object moving, just a visual impression (what waves really are; we tend forgetting that in "wave motion" no substance really changes permanently position it just oscillates). So*waves*, also light*waves*(but only*waves*, not photons) can move with any speed imaginable because it is not a physical speed that is limited by relativity to c (in flat space, to be exact). Jim 16:24, 26 Jul 2004 (UTC)

- (William M. Connolley 16:57, 26 Jul 2004 (UTC)) Yes. Thats what the comments to the links above are saying.

## Why Einsteins' gravity differs from Newton's - Last paragraph

Some more issues with this article: The last paragraph of this particular section talks about the "cosmological constant" introduced by Einstein, I presume? If yes, than it is not correct (besides its also questionable style). The constant was not dropped because of "beauty," but because it was introduced by Einstein to accomodate the (then) held view of a static cosmos. That forced him to introduce a (mathematically natural) constant **and** at the same time **choosing its value** to counteract the attractive forces of the total mass in the cosmos, which by itself forces change to its curvature over time.

- Einstein's theory is based on lack of "attractive force" so the above is not quite exact: the theory doesn't need to counteract something that the theory sais doesn't exist. Jim

- I'm rather surprised you say that. I have the text book in front of me, which I used to study GR (by
*H. Stephani*) and it shows the "Einstein Universe" created in 1917 by Einstein as a solution to the GR equation for the cosmos. It is a specific setup of a "Friedman Cosmology," where all parts in the equation want to change the "curvature" of the cosmos in the**same**direction. I used the word "force" to describe its general effect on the Universe, even though the equation does not have the correct units for it, I admit. Even so, "Lambda" was explicitly added to have all such effects cancelled and make the Universe static and stable. Do you dispute that? Awolf002 11:05, 27 Jul 2004 (UTC)

- I'm rather surprised you say that. I have the text book in front of me, which I used to study GR (by

- Now I'm surprised that you are surprised. "Force" is measured in kgm/s/s (or N for short). If it is in other units that don't reduce to N it is not "force". If you call "force" something that is not force while you are talking about physics at least you have to mention what it is.

- Of course I don't dispute that "Λ was explicitly added to have all such effects cancelled and make the Universe static and stable". It was more than 20 years before anybody suspected that the universe may expand or contract (even now some doubt) so most people in Einstein's shoes would do it. What's strange about it? If you have a solution that is non physical in your opinion you set parameters in your equation in such way that the solution makes sense physically, which Einstein did according to his understanding of physics of the universe. Farthermore the value of Λ doesn't introduce any "force" since this variable only determines the evolution of geometry (if it has different than "Einstein's value", regardless of the fact that wordlines of the particles take geodesics in spacetime and therefore are imune to any forces). The burden of explaining why the universe is physically expanding without any force forcing it to do so (only with Freidman solutions showing that it is) is on you if you think that the universe is physically expanding. Einstein set Λ so that it doesn't show the expansion, so he is sitting pretty with nothing to prove (except possibly that his universe should show accelerating Hubble redshift but it is in his theory already if you look closely, as I tried to show in the last section). Jim

Einstein itself called it a "blunder," because he could have predicted that "Lambda" could be different than **that** value, and with it the expansion or contraction of the universe. This is important history, which predates discussions about the (second order) change of the speed of the curvature change, as it is now brought about by observation! This should not be mingled, IMO. I agree that setting it to zero and adjusting all other values to match observation was a particular choice, but it was done more because of the unknown physical meaning of "Lambda" than for any other reason, I believe.

- The
*physical reason*was known and it was of course*lack of expansion*assumed by Einstein. Whether it is a*true physical reason*is still the matter of debate among physicists. Jim

- I don't get it. Parts of this article describe the history of the theory of Gravity, so this
**is**an important item to describe correctly. What does any "not yet known*true reason*" have to do with that? I wonder if there is a problem with the general approach to NPOV in the current form of this article... Awolf002 11:05, 27 Jul 2004 (UTC)

- I don't get it. Parts of this article describe the history of the theory of Gravity, so this

- The problem that you missed somehow is that what you call "the (second order) change of the speed of the curvature change" has thrown a monkey wrench into cosmology. We don't know (surely I don't know), neither any astrophysicist knows (because none is volunteering any opinion), what is the true state of the universe. I've been attending seminars in cosmology for whole 2003/2004 academic year and it turns out that it is not known what is the true theory of gravity. Most people wait for quatum gravity. We have Einstein's theory, which we know exactly, and that so far didn't contradict any observations. Neither it explain existence of any "repulsive force" though, which some physicsists postulate to explain accelerating expansion ("dark energy" if it rings any bells). That's why quantum gravity got so promissing. But apparently no one knows how to reinstate the Newtonian "gravitational force" so that it would act as attractive force in one instance and as repulsive force in another (or work at all). And also to predict all things that Einstein's theory predicts, not to mention geodesic motion. That's the situation. So the best thing we can do is just to report all that we know and see if someone is able to make some sense out of it. Do you have a better idea? Jim 17:50, 27 Jul 2004 (UTC)

- Okay, I think I understand where you are coming from.
- I'm not sure about a "better idea", but I agree we should report what "we know," ie. what current science sees as our best theory matching (even if just barely) the observations. However, my "pet gripe" with some articles is that they have too much info crammed into one big page. I would like to see this article slightly changed, where the details of the theories have their own respective pages, and in here we have summaries of the theories of "Gravity" in their historic succession. This must include pre-Newtonian, Newtonian and GR theories.
- This would make this section nicely NPOV, since this is just reporting what happened. Keeping all speculations about possible extensions away from that "historic report" (maybe in another part of this article or making new ones) would help the readability and focus, IMHO. Awolf002 15:08, 31 Jul 2004 (UTC)

Does anybody have any real historic references to nail that down? Awolf002 23:19, 26 Jul 2004 (UTC)

- The story is described in "Gravitation" by Misner, Thorne, and Wheeler. Jim 06:28, 27 Jul 2004 (UTC)

### Rewrite the Article

In my opinion, the three subheadings "How Curvatures...", "Why Einstein's Gravity...", and "Why Do We See..." are out of scope for this article. Any in-depth discussion of GRT/SRT vs Newtonian concepts are not discussions of gravity, but discussions of theories. It would be better to stick to the facts of what gravity does, how it behaves, and what the experiments show, instead of getting into the theoretical concepts that can only lead to confusion here and POV disputes because they cannot possibly be clarified sufficiently within the scope of the article. Just state the the most basic facts:

- Newton's Law of Universal Gravitation requires "action at a distance."
- Einstein's GRT does not, but introduces a non-Euclidian reference system to escape it, which requires finite transmission of gravitational waves.

The gravitational issue is instaneous action vs non-instantaneous action. We shouldn't even get into the more esoteric theoretical aspects. These issues can be clarified in the articles on SRT, GRT, Newton's Law, Inertia, Speed of Light, Cosmology, etc.

Actually, the article could be written to disabuse the all too common perception that Einstein's theory was formulated to replace Newton's Law. This is not correct. The Michaelson - Morley experiment demolished the validity of the universal aspect of Newton's laws of motion. Their experiment produced a direct conflict between the observation of the constancy of the velocity of light and Newton's concept of motion. Obviously, something had to give and the issue was what should it be. The concept of sacrificing absolute magnitudes of space, time and motion started with Fitzgerald, continued with Lorentz, and culminated with Einstein, but this development lasted nearly a decade of excruciating embarassment because there seemed to be no answer to the problem.

Einstein's theories came to be accepted in spite of serious weaknesses due in large part to the fact that there was no viable alternative and the scientific community was desperate for a solution after so many years without a clue. However, the theoretical approach's vehement critics lost the battle because "you can't beat something with nothing." Nevertheless, that doesn't mean that some alternative other than the sacrifice of absolute magnitudes and Euclidian geometry won't be found eventually. In the meantime, we should strive to keep the facts straight and the issues clear, and not confuse the real issues of motion and force with issues of theoretical concepts such as choice of reference systems, assumptions of the nature of space and time, and political issues of "normal" science. Doug 15:31, 27 Jul 2004 (UTC)

- You might be right that we should limit ourselves to the math of gravity and whether it is based on "instaneous action vs non-instantaneous action". The problem though is that it wouldn't explain to readers "why things fall?" an answer to which they might be looking for in an article on gravity. For an article about gravity it would be probably even worse than venturing a little bit into various ideas responding to "why things fall?" Which is just a theory that you want to avoid discussing. Jim 17:50, 27 Jul 2004 (UTC)

- This is funny. You just made my point: WE DON'T KNOW WHY THINGS FALL! We have some theories, but those are best explained in their own articles. The only thing we know for sure is that things fall (move) inwards towards one another in proportion to their mass and in inverse proportion to their distance from each other. We also know that relative motion between them somehow affects the rate of their fall if it's magnitude is great enough. We also know that whatever causes the gravitational motion cannot be screened off nor modified in any way not involving mass.

- We do not know if space-time is a 4D metric that can be curved or warped or cause matter to follow a certain path. We do not know if time "dialates," if length "contracts," or if one mass instantly attracts another or not. These concepts belong to abstract ideas - speculations, hypotheses - proferred to explain how gravity operates, but they are not facts or laws. We should make this clear.

- It's not
*that*bad. We know that time gets dilated and how much (we don't know why). The time dilation has been measured. The same about curvature of space (gravitational lensing is an obvious astronomical observation). We know how much space has to be curved if time is dilated and energy is conserved. So*we know*that nothing in Einstein's theory is contradicted by observations. A lot of it is confirmed by measurments. What is not agreed upon is not proven either way yet so it still may agree with Einstein's. So*we know why*. So far there is no better theory. Don't we want to tell it the readers because it's**according to Einstein's theory**things fall*only a theory*? Jim 05:17, 28 Jul 2004 (UTC)

- It's not

- True, we know that motion in time occurs as well as motion in space, but we don't know that the time coeffiecent of SR (actually Lorentz), which is used in SR to make the speed of light "special," is the correct procedure or not. The sacrifice of absolute values of space and time is an expediency that works mathematically, but it doesn't mean that there is a corresponding physical reality behind it. That's why it is a theory, not a law. By the same token, gravitational lensing does not prove the existence of the curvature of space, only that the effect is similar, but what if there is no such thing as space, as all observations to date indicate, then what?

- " but what if there is no such thing as space [...]?" Then we are really in trouble because then there ain't such things as
*us*neither (we can't exist without space, can we?)

- " but what if there is no such thing as space [...]?" Then we are really in trouble because then there ain't such things as

- For space to be warped, curved or changed in any way by matter it has to exist as something, yet every observation, every experiment we know of has not been able to detect it. It only exists in theory. Current theory cannot account for gravity without a space continuum, a medium represented by a mathematical expression that can be changed by mass. Computer simulations always depict it as a grid, yet no grid exists as far as we know. There is no detectable physical phenomenon corresponding to a medium, or ether, or metric. This is a fact.

- We are supposed only to
*report what*according to actual theories, especially Einstein's whose theory so far has**gravity**is*no problems*with explanation of all*observed phenomena*. If the*space*as you suggest is not observed, Einstein's theory doesn't need*to explain*it, does it? We don't need to discuss here the deep philosophical problems of epistemology, only*gravity*. Jim

- We are supposed only to

- This is exactly why experiments like the Gravity-B Probe continue to be fielded at such great expense. We don't know if there is such a thing as "space" that can be curved, and we don't know what the meaning of "motion in time" really is.

- You mean
*you don't know*? A good GR course may fix that.

- You mean

- Look Jim, I'm not the one launching the Gravity-B Probe to test general relativity, if a good GR course can "fix that" for me, you better tell the NASA and Stanford scientists to enroll in the same class, so it can fix it for them too.

- What makes you think that they would listen to me? I tried but every one I try to talk to is already late for a very important date.

- You are approaching the subject like a kid, who doesn't know that science is a business and studying GR brings no money, only manufacturing problems does. That might be why you hear about problems a lot not much about the solutions though even if they already exist in Einstein's theory. If they do then someone will try to tell you that they are "controversial" because some VIP has a better theory than Einstein's and it fits Einstein's all right but not VIP's. Jim

- Here's what they say on their web site (http://einstein.stanford.edu/):

*General relativity is hard to reconcile with the rest of physics, and even within its own structure has weaknesses. Einstein himself was dissatisfied, and spent many years trying to broaden his theory and unify it with just one other branch of physics, electromagnetism . Modern physicists seeking wider unification meet worse perplexities. Above all, essential areas of general relativity have never been checked experimentally....*

*Moreover, deep theoretical problems -- some old and some new -- remain. Einstein himself remarked that the left-hand side of his field equation (describing the curvature of space-time) was granite, but that the right-hand side (connecting space-time to matter) was sand. The mathematical structures of general relativity and quantum mechanics, the two great theoretical achievements of 20th century physics, seem utterly incompatible. Some physicists, worried by this and by our continued inability to unite the four forces of nature -- gravitation, electromagnetism , and the strong and weak nuclear forces -- suspect that general relativity needs amendment.*

- "Deep theoretical problems" are their words, not mine. The fact is, general relativity is a theory with "weaknesses" in its structure and certainly one of these is the assumption that space is a medium.

- This is just a proof of what I already said: they list
*gravitation*as one of "*four fundamental forces of nature*". How could they miss the fact that GR is based on geodesic motion and so a "fundamental force of gravitation" doesn't exist, if they had a decent GR course, and consequently understood it?

- This is just a proof of what I already said: they list

- Since I happen to have a decent GR course, I might be able to explain any real problem that
*you*may have with it. Jim 13:08, 30 Jul 2004 (UTC)

- Since I happen to have a decent GR course, I might be able to explain any real problem that

- Indeed, leading string theorists like David Gross and Witten have concluded that space-time is emergent! The bottom-line is that the modern theories of physics are in trouble and it appears that a huge correction is in store. Hence, we need to stick to the facts and tell the story of gravity in terms of what we observe, not what we conclude theoretically. Besides, it makes the article easier to write and will link to the theoretical articles where these more esoteric points can be elaborated upon. Doug 19:03, 28 Jul 2004 (UTC)

- What trouble in modern theories of physics and esoteric points you are talking about? Einstein's theory is about a century old and there are no known problems with it. We didn't observe anything yet that wouldn't fit. Maybe we worry when we do? Have you noticed something that doesn't fit? Jim 20:05, 28 Jul 2004 (UTC)

- As I have shown above, scientists way above my pay grade say that "deep theoretical problems remain." These are mainstream relativists, not quacks. For example, in regards to gravitational radiation, a vital requirement of the theory, they say this, after pointing out that most scientists believe Einstein was on the "right track":

*Other more profound phenomena ...remain untested. Save for some indirect evidence from the binary pulsar, no data exist on gravitational radiation. Even less is known about a vitally important relativistic effect -- "frame-dragging."*

- Frame-dragging is a reference to the effect of mass on the "fabric" of space-time, a fabric that is totally a figment of the imagination. If matter/energy is the basic constituent of the universe and the fabric of space-time was generated and stretched out by the big bang, and this fabric is warped in the presence of matter/energy, then the next question is "How does matter warp space-time?" "What is the nature of space-time that makes it susceptible to the effects of matter?"
- Doug 04:37, 30 Jul 2004 (UTC)

- Do you really consider it a
*real problem*? Jim 13:08, 30 Jul 2004 (UTC)

- Do you really consider it a

- You're kidding, right? I mean if pursuing the answers isn't important, then why are we spending all the money looking for them, and why are you even trying to write this article? If asking "Why do things fall?" is answered by saying "Because matter warps space," then asking "Why does matter warp space?" is tantamount to saying that we've just begged the question by assuming that space exists as a medium that can be warped. We are not allowed to assume this, we must provide support that space is a medium that can be warped by mass. Thus, the great effort and expense to do so is underway. Doug 00:28, 31 Jul 2004 (UTC)

- Pursuing the answers is important, but the answers that you're looking for aren't about
*gravitation*any more (*why things fall*). Gravitation (*why things fall*) got explained once we found out*how*"warping the spacetime" causes gravitational effects: through geodesics in spacetime; explaining Newtonian orbits and all the rest of*known*gravitational phenomena. Now it is a problem of regular physics: "how (and possibly*why*) matter warps the spacetime". So far we have it as a fact with an unknow reason and not known the way*how*matter is doing it. I.e. the metric of spacetime is still not known. So far it is*assumed*by many cosmologists that it is a Riemannian FLRW with expanding space and not conserved energy, but it might be that energy is conserved and the effect of expansion is an illusion as predicted by (partly forgotten) Einstein's theory, which would predict a differently warped spacetime. Possibly with*non symmetric*metric, as Einstein proposed it in 1950. Also very interesting and worth pursuing because it explains the accelerating expansion that we observe as a total illusion in a totally stationary universe. Jim 16:46, 1 Aug 2004 (UTC)

- Pursuing the answers is important, but the answers that you're looking for aren't about

- Boy, it's hard to make this point stick, but I'll try again: The GR explanation of gravitation is nothing but conjecture, read theory, at this point. No one has ever "found out
*how*"warping the spacetime" causes gravitational effects." It's this confusion of facts, the elevating of theory to the status of facts, that is the point of this dialog. It's shameful and patently POV. Doug 09:39, 2 Aug 2004 (UTC)

- Boy, it's hard to make this point stick, but I'll try again: The GR explanation of gravitation is nothing but conjecture, read theory, at this point. No one has ever "found out

## Observations vs Conjecture

To simplify the discussion I'm removing all the previous stuff in this section (still available in "history") and the items that don't relate to observatons and cause only theoretical problems. Even if those problems are tremendous they shouldn't consider us since the phenomena that they refer to are not observed (e.g. quantum gravity, big bang, black holes, etc.) What's left is:

*"We need to be able to explain at the quantum level how gravitational mass arises and is always equivalent to inertial mass."*

- It was in Newton's theory that "(passive) gravitational mass [] is always [equal] to inertial mass". Einstein solved this problem almost a century ago by showing that there is
*no (passive) gravitational mass*.- Einstein did not "solve the problem." He merely postulated that gravitational mass is equivalent to a constantly accelerated mass. However, this does not answer the question as to
*why*they are the equivalent.- What's
*gravitational mass*? The mass that is accelerating is called*inertial mass*(or*relativistic mass*) (gamma)m, where m is*rest mass*The (active)*gravitational mass*is the one that causes gravitational effects around it. So what do you mean by "gravitational mass" while asking about "equivalent" to inertial mass? What equation does define this mass? Jim

- What's

- Einstein did not "solve the problem." He merely postulated that gravitational mass is equivalent to a constantly accelerated mass. However, this does not answer the question as to

- From Mass Wiki: Inertial mass is a measure of an object's inertia, which is its resistance to changing its state of motion when a force is applied. An object with small inertial mass changes its motion more readily, and an object with large inertial mass does so less readily.
- Gravitational mass is a measure of the strength of an object's interaction with the gravitational force. Within the same gravitational field, an object with a smaller gravitational mass experiences a smaller force than an object with a larger gravitational mass. (This quantity is sometimes confused with weight.)

*"how does it act instantaneously at a distance?"*

- You talking again about Newton's theory. There is no such a problem in GR. Do you want to know why? Jim 19:42, 8 Aug 2004 (UTC)
- Look, I know about the quadrapole moment and all, but, again, confusing the theory with the observaations is what we want to avoid. Observations of gravity show that it acts instantaneously from a distance, it cannot be screened or blocked off, and it cannot be modified. GR postulates that this is because a 4D space-time metric is somehow warped in the presence of matter. Such a hypothesis squares with some of these observations, but not all. For instance, the quadrapole moment must produce a gravitational wave, which has not been observed. Again, Newton had no theory of gravity. His Universal Law of gravity did assume instantaneous action at a distance, but he disliked the idea as much as anyone else. He simply did not know how to explain it, and refused to hypothesize ahout it, but that doesn't mean that it's not observed, because it is.
- So describe how it is observed, to be sure that it's really observed and it's not only a misinterpretation of the observation. Jim 11:36, 12 Aug 2004 (UTC)
- No, this is an endless loop conversation and is of no value in improving the article. I don't have time for it any more. I've made my point I think. Doug 14:14, 12 Aug 2004 (UTC)

- So describe how it is observed, to be sure that it's really observed and it's not only a misinterpretation of the observation. Jim 11:36, 12 Aug 2004 (UTC)

- Look, I know about the quadrapole moment and all, but, again, confusing the theory with the observaations is what we want to avoid. Observations of gravity show that it acts instantaneously from a distance, it cannot be screened or blocked off, and it cannot be modified. GR postulates that this is because a 4D space-time metric is somehow warped in the presence of matter. Such a hypothesis squares with some of these observations, but not all. For instance, the quadrapole moment must produce a gravitational wave, which has not been observed. Again, Newton had no theory of gravity. His Universal Law of gravity did assume instantaneous action at a distance, but he disliked the idea as much as anyone else. He simply did not know how to explain it, and refused to hypothesize ahout it, but that doesn't mean that it's not observed, because it is.

## Additions to Einstein's gravity section

I added a missing section about physics of conservation of energy. IMHO it might be interesting to lay people becuase of the lack of gravitational forces accelerating free falling objects while kinetic energy of those objects is increasing. It often puzzles lay people since the physics of the automatic conservation of energy that math of general relativity assures through the vanishing divergence of isn't obvious. So I showed the physics of this phenomenon.

There is also at the end of first section a new part about the true meaning of Newton's Law that makes Newton, for his refusal to interpret his equation as *action at distance*, the fortuitious discoverer of *geodesic motion in spacetime* (in this case in Euclidean spatial hypersurface) with Einstein showing how to transfer this law in a covariant form into pseudo Riemannian geometry or into any other suitable geometry.

If there is a NPOV concern about this piece, IMO it should be reformulated rather than deleted. Jim 19:15, 28 Jul 2004 (UTC)

- Jim, your work could be in the physics course in the wikibooks, would that be acceptable to you? One of us could take steps to move it there. Ancheta Wis 11:05, 10 May 2005 (UTC)

## Comparison with electromagnetic force

"Attractive force" (non existent in gravitation) has been replaced by "interaction" that seems good enough a term to fit gravitation. The same in the previous section. Jim 18:41, 1 Aug 2004 (UTC)

## noncovariant explanation

"It often puzzles students of Einstein's gravity that without any force acting at distance the kinetic energy of a free falling objects changes. The puzzling question is "where this kinetic energy is coming form where the object is moving down or going to when the object is moving up"? The old "gravitational field" of the "attractive force" that was considered to be a repository of this "gravitational energy" in Newton's gravity isn't any good any more since now, if "attractive force" is zero, so is the "gravitational field". We need to identify another repository for this energy.

As we know the total energy of an object is E = mc^{2}, where m is so called "relativistic mass", and c is speed of light. When object falls "down" its kinetic energy goes up. Energy has mass and so m goes up. However c^{2} drops down by the same amount since falling object gets into space where time is running slower (recall time dilation) and so the speed of light, as observed by the same observer who is seeing the increasing kinetic energy, is slower as well (that's why speed of light is not constant in gravitational field). If both m and c^{2} change in opposite direction by the same amount the product, the total energy of the object stays the same for a free falling object. That's how the conservatoin of energy works in Einstein's gravity.

There is one important result of Einstein's gravity: to keep the change of c^{2} the same as change of m there must be a relative increase in amount of space (space curvature) equal the relative time dilation. It might be said that the nature has to curve the space by the same amount as time gets dilated because of nature's inability to create energy from nothing."

- First of all, energy is only conserved in general relativity if spacetime possesses a timelike Killing vector field. Second of all, "relativistic mass" and "nonconstant speed on light" are outdated noncovariant concepts. Phys 14:11, 12 Aug 2004 (UTC)

- So? (Do you suggest that they should be reinstated as valid concepts? Jim 14:30, 19 Aug 2004 (UTC)

## I disagree with this edit.

I disagree with this edit.
*It should be noted that here, the word "cause" is not being used in the same sense as "cause and effect" or "the defendant caused the victim to die." Rather, when Newton uses the word "cause," he (apparently) is referring to what modern English-speakers call an "explanation." In other words, a phrase like "Newtonian gravity is the cause of planetary motion" means simply that Newtonian gravity explains the motion of the planets. It does not mean that Newtonian gravity (which is just an idea in someone's head) somehow sends planets telekinetically whirling through space. See Causality and Causality (physics).*

Newton was unhappy with *cause* in the sense of *why*. Part of his revolution was to relax the need for *why* and simply describe *what*, which we, as his intellectual descendants, call an *explanation*. However Newton was still (unsuccessfully) looking for *why*, and never found one. Ancheta Wis 21:55, 7 Sep 2004 (UTC)

As it stands, the paragraph is revisionist.

Let's see. *The Principa*, Translated by *Andrew Motte*, page 442, *General Scholium*. In here Newton says:

*Hitherto we have explained the phaenoma of the heavens and of our sea by the power of gravity, but have net yet assigned the cause of this power.*

So the first part of this edit looks okay, but I'm unsure of the latter! Obviously, if "Newtons Gravity" explains the motion, it is "on the surface" the cause for the planets to move in ellipses. However, "under the surface" the mathematical properties are not explained from deeper first principles, and so that "cause" is missing. So "Newtons Gravity" **does** somehow sends planets whirling, but it is unexplained through what mechanism. I would be in favor of changing the sentence. Awolf002 01:24, 8 Sep 2004 (UTC)

- Well, I don't know whether or not Newton was unhappy with "cause" in the sense of "why," but he does use it in the sense of "explanation." For example, "We are to admit no more causes of natural things than such as are both true and sufficient to explain their appearances." Objections and suggestions to sentences 3-4, however, seem reasonable to me, as their wording may be misleading. My edit does not claim that Newton cared about an underlying explanation. Nor does it claim that Newton didn't care. So I don't see where the "revisionist" label applies. If you think my wording doesn't convey what I just wrote here, go ahead and clarify it. As far as where Newton actually stood, it appears to be a mixed bag. On the one hand, he says, "Gravity explains the motions of the planets, but it cannot explain who set the planets in motion. God governs all things and knows all that is or can be done," but on the other hand, he also says, "For whatever is not deduced from the phenomena... have no place in experimental philosophy." Draw what conclusions you wish.

- By the way, does this section on "Newton's Reservations" really belong here in Gravity or there in Sir Isaac Newton? --author of edit 4.232.159.224 16:01, 8 Sep 2004 (UTC)

## Disambiguation

Rather than the first sentence about "seriousness" and the Anime picture, how about a disambiguation-page? :)

## A little disambiguation is in order

The sentence

*So for the majority of cases in the universe, and certainly for almost all cases in our solar system except two already specified as #3 and #4 at the beginning, we may treat the space as flat, as ordinary Euclidean space*

is missing the referents and really should refer to the cases by name, as in, for example

*#3 (Precession of perihelion of Mercury) and #4 (deflection of light rays by a gravitational field)*

When the intervening header markup was inserted, the reference to "#3 and #4" became ambiguous. If this is not what is meant, please add the prose to help the reader. Ancheta Wis 00:27, 18 Oct 2004 (UTC)

- Thanks for the sugesstion. I modified the text accrodingly and I dropped also the numbers since they might become outdated if someone added to the list a line or two in front of those numbers without necessarily updating the rest of the page. I changed the suggested
*"by a gravitational field"*to*"in vicinity of the sun"*to avoid the ambiguity of "gravitational field" meaning different things in Newtonian and Einsteinian theories. Jim 19:42, 3 Nov 2004 (UTC)

- Thanks for the sugesstion. I modified the text accrodingly and I dropped also the numbers since they might become outdated if someone added to the list a line or two in front of those numbers without necessarily updating the rest of the page. I changed the suggested

## The curvature of spacetime - image

New image was added to Commons:

During the Apollo 15 mission, David Scott gave a demonstration of Galileo's experiment. He dropped a hammer and a feather. With no friction to slow the feather, they fell at the same speed, thus proving Galileo was right and Aristotle was wrong about the effect of an object's mass under the pull of gravity.

## Newton & Calculus

I've heard that Newton couldn't convince himself that the law for point objects would also work for spherical heavenly bodies and hence didn't publish the theory until he invented the methods of calculus necessary to arrive at the total gravitational force of a sphere on a neighbouring point object. Should this be included in the article, e.g. under the "History" or "Newton's Reservations" sections? -- Paddu 22:39, 14 Jan 2005 (UTC)

- I learned the same thing. But I learned his work was for Earth specifically, as the celestial bodies are far away (hence could be treated as points), and Newton needed to prove that his theory applied to Earth, and to the Moon, and to all the other bodies. I learned that he spent years trying to include the Moon, and couldn't solve the 3-body problem exactly. (He got headaches from the effort) But that didn't stop his publication, because he included the System of the World as volume 3. Since he solved it for Earth, I imagine it wasn't a reservation any longer for him.

Now that I think about it, I believe it is a theorem in Principia. Total force from a sphere is same as from a point that would have been located at the center of the sphere. Ancheta Wis 22:56, 14 Jan 2005 (UTC)

- It's Proposition 75, Theorem 35 in Principia (I.Bernard Cohen/Anne Whitman 1999 translation). LXXV, XXXV in the 1729 Andrew Motte translation.

## Newton's law and Planck units

Does whoever wrote this section realize how large the planck mass is? I am not disagreeing with their mathematics at this point. What I would like them to realise is that the implication of the gravity not being transmited by a particle of less than the planck mass is that only large assemblies of particles can create a gravitational field while smaller particles can just react to a field that is already there.

Consider the though experiment of taking a particle that is less than the planck mass and collapseing it into a black hole. If you do this you get a Schwarzschild radius that is less than the planck lenth. What do you get? --Hfarmer 16:32, 11 Feb 2005 (UTC)

- See Mach's principle and equivalence principle . That is indeed the implication; that the totality of the fixed stars, and other large assemblages form our conception of gravitation. Ancheta Wis 18:10, 11 Feb 2005 (UTC)

I realize well how large the planck mass is. Indeed, it would be the "heaviest" and thereby smallest elemetary particle possible. But if the Planck mass would not exist in a "particle" per se but only as it's square (= hbar*c/G) you wouldn'n detect the "particle" when looking for a mass. And of course you hardly would detect it at all because it's so tiny. Consequently it appears questionable whether gravity really is transmitted by a particle or wave having whatever mass/energy.

From the mathematics I believe that the "particle" we need to look for is a "real" square. Taking into account the Planck charge I would visualise this "particle" by four Planck charges sitting at the corners of the square at a distance of a Planck length and moving (rotating square) with the speed of light. This moving could be the basis of Planck impulse. If you would illuminate that rotating square with "strobe light" of Planck frequency and look to it, the charges would appear static but change their signs with Planck frequency.

Because the charges get pretty much neutralised within the square the "particles" would not interact with their environment except transmitting impulses. Accordingly they would not appear in any instrument's reading. Therefore no wonder that the universe full of sqrt mP = hbar*c/G is largely considered to be pretty empty.

Only if you would "divide" the square into two dipoles with two Planck charges each at a distance of a Planck length within the dipole you would get the Planck mass. Guess that would be a difficult experiment and likely is the reason why we do not see Planck masses (their "weight" would be more than enough, in the microgram range, to be detected even as individual particles by simple instruments).

As far as black holes are concerned they are impossible if you accept the Planck length to be the smallest possible unit of length. Collapsing masses then cannot get a higher density than Planck density. If you do not accept the boundaries of Planck units (to save black hole theory) this means you cannot accept the natural constants to be constant either. As a consequence all calculations around a black hole or other "singularities" that are based on or contain the conventional values of the natural constants would then be forbidden or wrong.

What I therefore think really happens in a "black hole" is that the collapse results in Planck masses (dipoles mentioned) which then spontaneously associate (due to the huge attraction of the Planck charges) forming the squares as described. I.e. the masses therefore would disappear as such but "survive" as their squares (=hbar*c/G) at site. RABecker

- (William M. Connolley 18:38, 10 Mar 2005 (UTC)) I removed the section. It starts off with a fairly anodyne - if pointless - rewriting of the equations in terms of planck quantities, then goes off into bizarre stuff about pull and push. Where did it come from?

I re-built it because qualifiers like anodyne, pointless and bizarre I do not accept as scientific arguments. You cannot claim that your opinion is representative for the potential readers who are willing to look at the matter by a different perspective. Happy to modify any part if you show that it's wrong and how it should look like if right. Sorry, I don't have a web page. e-mail (infrequently checked) is rajmbecker@web.de After easter I'll do some modifications on the section showing even better that Newton's law expressed by Planck units is simply a geometrical/ statistical relationship applying to Planck Impulses. Also I will try to better visualise how the transmission of the impulses should work. It's a bit hard and time consuming for me as I'm not very familiar with Wiki formatting. RABecker

- This strikes me as
**original research**. Do you have any published references so we are able to get a grip on its current "notability" in the sciences? Awolf002 18:26, 24 Mar 2005 (UTC)

- (William M. Connolley 22:11, 24 Mar 2005 (UTC)) I've taken the section out again. If the ideas are your own, they don't belong in wiki: free web space, or the newsgroups, are readily available if you want to popularise your own ideas. OTOH, if they are someone elses, please reference them. Rewriting the equations in terms of new constants seems pointless: nothing is thereby gained. The pull/push stuff seems reminiscent of LeSage (gravity)... is this what you are thinking of? If not, I don't know what you are trying to say.

It will be in again with modifications (will take some while; comments on below are welcome to be considered). Rewritten equations I do not think belong to myself or anybody else but to the public. They are a matter of fact.

What I'm ultimately thinking of is the following re-written equation:

Gravitational force = Sum of gravitational interactions between the protons and neutrons of two masses, each interaction expressed as

[Planck Impulse * Planck Frequency * sqrt(Planck Length/wavelength of a neutron resp. proton) * sqrt(Planck Length/distance of interacting neutrons or protons)]

What is to be gained from the re-written equation are the following insights:

1. Transmission of Planck Impulses is the cause of gravity

2. Frequency of the impulse transmission is a fraction of Planck Frequency

3. The fraction of Planck Frequency is determined by two probabilities: sqrt(Planck Length/wavelength of a neutron or proton) and sqrt(Planck Length/distance of interacting neutrons resp. proptons)

4. The appearence of Planck Length within these probabilities suggests that the area of impulse transmission is of the size of sqrt of a Planck Length.

As gravitational force is always attractive whereas impulse transmission is repellant it is obvious to think about a push model, without the need to involve Le Sage. And Le Sage never considered Planck units. However, the "gas" Le Sage was thinking of exists in reality, being hbar*c/G.

Transmission of Planck Impulses could happen without generating heat as long as it occurs fully isotropic. Only if the impulse transmission gets anisotropic it would result in acceleration of a proton or neutron.

Transmission of Planck Impulses also seems to be the key mechanism of the inertial force as mentioned earlier. RABecker

- I do not believe we can accept the argument that "this belongs to the public" means it needs to be in WP. I only see some re-arranged equations that supposedly support your notion how gravity works, which is original research as far as I can tell. I object to having the "re-written" text in this article. Awolf002 15:33, 4 Apr 2005 (UTC)

What means "needs to be in WP"? I understand WP is to inform readers about facts and opinions they can verify and build on. Re-arranged equations they can verify via mathematics and then can build on them. Why do you want to suppress this? But taking your comments into account I will take care to stick on the things the reader can verify and limit opinion to things that are obvious from the equations (and therefore cannot be considered as original research).

As I am working in life sciences and Planck units just being a hobby I would appreciate some tips how to get the conclusions/models to be drawn from the equations discussed in the public domain. Thanks a lot, RABecker

- Here is the WP policy about Original Research. What you should avoid is to interpret your re-schaled equations in a way that you just discovered or invented. WP strives to be a secondary source of knowledge, not a primary one. Hope this clarifies my point: If you are searching for a "bullhorn" to announce your ideas, WP is not it. Awolf002 17:31, 7 Apr 2005 (UTC)

Have now put the re-worked chain of equations in. Considering your points, I tried to avoid any interpretation that is not obvious from the "target equation". Hope that's ok now. Thanks a lot for your input. RABecker

- Sorry. I reverted your changes again, since you still just seem to insert your own conclusions about a rewritten equation. There is no information in the rescaled force equation besides what you infer, and I do not find any primary sources for this. Awolf002 23:09, 12 Apr 2005 (UTC)

## gravitational entropy

Hi, Stephen Hawkings has a lecture on gravitational entropy, would someone incorporate it into the Wikipedia 'Gravity' entry? Thanks much, -Ben-

## General relativity

Briefly speaking, general relativity is a good theory which uses differential geometry and tensors to describe strong gravitational forces. But saying that gravity is weak is an oxymoron because according to the theory, gravity is not a force but a manifestation.

However what the theory fails to explain is how the gravitational field really works. What is spacetime?

I heard that the Higgs boson could be a next candidate. If not, could gravity be related to magnetic charges? If not could it be the opposite reaction of the CMB? -- Orionix 02:18, 6 Apr 2005 (UTC)

- I agree to the idea of charges being involved but as mentioned earlier I believe it's Planck Charges: 4 charges arranged as a square at distances of one Planck Length. RABecker

The question of why atoms attract one another is still a great mystery. Maybe a super-particle accelerator which will collide protons at energy of 10 PeV could answer this question. -- Orionix 02:37, 12 Apr 2005 (UTC)

- I have a more simple approach as outlined earlier in the discussion and I think there is no mystery.

It's just about disturbance of isotropicity of transmission of Planck impulses. This transmission occurs at any time and anywhere with high (Planck) frequency. But because it happens fully isotropic there is no force being detected despite the Planck force (= Planck impulse per Planck time)is applying everywhere and all the time. It's applying this way because the Planck Force is the speed of light to the power of four divided by the gravitational constant, both constants being present everywhere at any time.

If this isotropicity gets disturbed either by another mass (e.g. by "shielding" against transmission of Planck impulses) the excess (repellent) transmissions from the opposite side drive the particle towards the shielding mass. Also, if the isotropicity gets disturbed by applying a force to a particle, this force (= impulses per time) gets counteracted by the transmissions of impulses from the opposite side of the particle. This we experience as inertial force. - RABecker

## External link

Please '12.152.104.82', stop adding that link! It gives a FILE NOT FOUND error. Awolf002 14:10, 13 Apr 2005 (UTC)

## GR mistakes

I am not impressed with the GR part of this page, but am not in a good position to edit it.

First, the tendency of object to move towards a massive object in GR is due to geodesic deviation, (which I hope to write on soon). The change in time dilation, while closely associated with gravitaional potential, is **not** fundamentally the reason for gravitation.

Even if time dilation were known, noone would have known what to make of it or why it was associated with gravitation. The reason for the illusion of gravity is because gravity does exist in accelerated frames of reference as a D'Almbertian force: Since you are being accelerated, all objects in inertial motion must fall with respect to you. The result is what is perceived as a gravitational field.

Also, gravitational potential is nothing more than the difference in kinetic energy that an object would have as a result of its being accelerated with respect to the observer between two positions. (Yes. That is the Newtonian definition. Some things just don't change even in relativity.) There is no need to invoke changes in the speed of light, especially since such changes are in apparent contradiction to the constancy of the speed of light in SR. (Yes. I know that this is a real effect as perceived by an observer in accelerated frames even in SR. However, it is an effect of gravitation instead of a cause and presenting it in this context will cause confusion).

--EMS 03:10, 27 Apr 2005 (UTC)

- EMS, what would you think about a move of some of the material which you object to, to another article, simply reducing the prose to two sentences about the effect of curvature, etc. Then another sentence which leads to Einstein-Cartan theory which makes clear that an affine theory, not a metric theory, provides a better description of gravitation. In other words, there is a lot more down the road for this subject and GR may well be only a stage in the development of this subject. Might you have a suggestion for the name of the page which could hold the material? Ancheta Wis 10:14, 27 Apr 2005 (UTC)

## D'Alembertian force

a D'Almbertian force --EMS 03:10, 27 Apr 2005 (UTC)

- It took me some googling to find the meaning of
*D'alembert force*. I got 35 google hits (zero google hits for d'alembertian force). I am partial to enriching the lexicon of physics too, but is the expression D'Alembertian force different in content to 'inertial force'? (personally I think the expression 'inertial force' is an oxymoron, I prefer the neologism 'manifestation of inertia' for that.) -Cleon Teunissen | Talk 12:04, 27 Apr 2005 (UTC)

- I didn't think twice when I read the term; to me it would be a more polite and general term for
*fictitious force*; no problem for the usage on my end. Ancheta Wis 12:31, 27 Apr 2005 (UTC)

I strongly advise against calling gravity a "manifestation of inertia". My own preference is somewhat for "inertial force", but the technical term is "D'Albertian force". Basically D'Almbert came up with a beautiful formalism of Newton's third law. In the process, he showed that an observer being accelerated finds all inertially moving objects acting as if they are under the influence of a force which acts on the objects with a strength that is proportional to the mass of the object.

Look at it this way: Any observer, even an accelerated one, is always at rest with respect to themself. So there is a frame of reference for the accelerated observer which is at rest with respect to that observer, and in which objects which maintain their position with respect to the accelerated observer are considered to be at rest. However, inertially moving objects will not stay at rest in this frame of reference, but will instead accelerate with respect to it. There is not a real force involved since it is the observer and not the object that is being accelerated, but from the standpoint of the accelerated observer the effect really is that of a force acting on the object. That phenomenon is what Einstein refers to as a "gravitational field", and it exists in an accelerating rocketship in free space just the same as it does for us when we are standing on the surface of the Earth.

So the odd bottom line is that gravity is perfectly real even in GR. It's just that in GR, we are in an accelerated frame of reference, and what we perceive as acceleration due to gravity is really inertial motion as it exists due to gravitation. This automatically explains the equivalence of inertial and gravitaional mass: It is a required attribute of a "D'Almbert force". In essesnse the Equivalence Principle (or at least the weak version) is nothing more than a way of identifying a D'Almbert force and realizing that the observer is the one being accelerated instead of the objects in freefall.

Of course the problem with using the term "D'Almbert force" or "D'Almbertian force" is that noone will understand it, and if I attack the article itself I will seek to avoid the term (even though I like it). As I mentioned above, I would call gravity an "inertial force" since that is more easily definable. I hate to call it "fictitious" since that does not keep you from getting killed if you should fall off of a cliff.

Beyond that I strongly advise avoiding Cleon's suggestions in this matter. He is fairly smart and capable in dealing with classical phenomena, but in the GR realm he just does not get it. "Manifestation of inertia" is only half of the story. Instead, gravity is intertial motion *as it is perceived by an accelerated observer*. Without telling the reader how gravity is related to inertia, you are only creating confusion.

--EMS 02:57, 28 Apr 2005 (UTC)

- Ah, I notice you are getting desparate. You have moved towards personal attack. In short: you reject the modern interpretation of GR; it appears that you think that the vintage 1916 interpretation of GR is the one and only way to see GR. You have very often misinterpreted what I have written. External link: The differences between the modern and the Einstein interpretation of GR Anyway, this discussion does not belong here. -Cleon Teunissen | Talk 07:02, 28 Apr 2005 (UTC)

- Not desperate, just highly, highly annoyed. I apologize to you for publicly putting you down. I know that you are sincere about what you are doing here, but I assure you that you do not "get it" yet. Keep studying GR for a year or two and then look back on our conversations, and I think that you will be surprised by the extent to which your knowledge will have changed.

- "Manifestation of Inertia" is every bit as repugant to me and "fictitious force" is to both of us.
**How**does gravity manifest inertia???!!! That is what needs to be explained, and your pet phrase just does not do it.

- "Manifestation of Inertia" is every bit as repugant to me and "fictitious force" is to both of us.

- That link is quite useful BTW. It is quite correct that the understanding of GR has evolved beyond that of Einstein, but are you ready to have someone talking about Riemann tensors, Christoffel symbols, and the geodesics equation ad infinitum? I think that I would lose you quite quickly. Einstein's gravitational field permits a conceptual discussion without this "baggage" (although that the baggage is integral to GR theory).

- BTW - I just came upon the term "pseudo-force". Maybe that is what we need to use.

- --EMS 14:48, 28 Apr 2005 (UTC)

- (I switched the pdot term to the left to parallel the Lagrangian) So back to the story: The first term in both formulations is the geometrical part. The second term is the physical force part (inverse square, etc.). It looks like I can learn something here: We are taught in school that the Coriolis force etc are
*fictitious*but that is an unjust name, as it certainly explains a lot of our weather, etc. I am visualizing a cloud particle in a weather system, swirling above us. We are~~more~~subject to gravitation than the cloud particle, so as the Earth turns, we move, but the cloud swirls, apparently also subject to Coriolis force, as well as subject to gravitation. The difference for us, in D'Alembert's picture, is more complicated that the single terms of Lagrange, because D'Alembert's picture includes both us (Earth-bound) and the clouds swirling. True statements? Ancheta Wis 09:37, 28 Apr 2005 (UTC)

- True

- I just noticed that the Coriolis force article defines it as an inertial force as well. Ancheta Wis 10:12, 28 Apr 2005 (UTC)

- Of course it is. We are the ones being accelerated due to rotation.

- --EMS 14:48, 28 Apr 2005 (UTC)

## manifestation of inertia

Inertia is a rockbottom concept in physics. I use the expression 'manifestation of inertia' to express a dynamic process. When two objects are moving towards each other a force is required to reduce their relative speed to zero, and the necessity to exert a force in order to change velocity is called inertia. With inertia, the acceleration is proportional to the exerted force. Without inertia, objects would be instantly accelerated to lightspeed.

When an electric car designed to regain energy on decellerating is switched to braking then the manifestation of inertia is driving the generators, recharging the batteries. So the manifestation of inertia is doing work. Alternatively, you can do the accounting by stating that the batteries are doing negative work in the process of being recharged.

The way I like to do the accounting is to state that the manifestation of inertia is doing work. (It doesn't really matter, as long as you make sure that you keep track of conservation of energy.) One can also state this in the form that manifestation of inertia is involved in conversion of potential energy to kinetic energy and vice versa.

I think the expression 'inertial force' is an oxymoron because there is one thing that manifestation of inertia cannot do: accelerate an object with respect to the inertial frame of reference. Manifestation of inertia arises in response to a force being exerted. When the force stops, the manifestation of inertia stops immediately too.

Inertia and manifestation of inertia have unique properties, and I think this justifies giving them a name of their own.

The name 'fictitious force' is horrendous, I hate that expression so bad I can taste it.

--Cleon Teunissen | Talk 11:17, 28 Apr 2005 (UTC)

The expression 'fictitious force' is (I think) originally designed for the following type of situation: suppose you have a videocamera on a rotating platform, and it is recording a perfectly normal scene. Suppose you show the footage to some physicists, and ask them to *retrofit* force laws to what they see, on assumption that the camera is stationary. The force laws that they would retrofit do not represent anything real, so those retrofitted force laws describe *imaginary forces*. The formula's for those retrofitted force laws are a spitting image of for example the dynamic law for centripetal force. In fact, in the mathematical derivation often the fictitious force law is derived, and then a minus sign is added to the formula (or removed, I forget).

Anyway, manifestation of inertia is real physics, of course, it's doing work.

--Cleon Teunissen | Talk 11:42, 28 Apr 2005 (UTC)

## The physics of Gravitation and Inertia

Both in Special relativity and in General relativity the following is valid: objects follow geodesics. In order to make an object deviate from moving along a geodesic a force must be exerted. Whenever a force is exerted, inertia manifests itself. (Attempts have been made to identify a physical cause for inertia, but on the whole inertia seems to be regarded as something that needs to be assumed "as is" in order to frame a theory.)

Free-fall straight towards a center of gravitation is motion that follows a geodesic. (the usual example to illustrate what a geodesic can look like is orbital motion, but free-fall straight towards a center of graviation is geodesic motion just as well).

An example: an electrically powered elevator cabin designed to regain energy on descent. If the cabin's descent would not be a controlled one the cabin would free-fall, accelerating towards the center of gravitation. When there is a controlled descent, then a manifestation of inertia arises, directed towards the center of gravitation. This manifestation of inertia is driving the electric generator, recharging the battery system, analogous to a decelerating electric car recharging its batteries.

Metaphorically speaking: curvature of space-time geometry is "cheating". Space-time curvature changes the rules at the deepest level. In interstellar space, where the gravitation of surrounding stars evens out, space-time geometry is exceedingly close to Minkowski space, hence the geodesics there are very close to euclidean straight lines. A gravitating mass such as our planet Earth alters surrounding space-time geometry in such a way that inertial motion is motion towards the center of gravitation, and whenever a force prevents an object from moving towards the center of gravitation inertia manifests itself.

Because of the existence of inertia, acceleration with respect to the inertial frame of reference is proportional to the exerted force. Without the existence of inertia, gravitation wouldn't exist either.

Before general relativity, inertia and gravitation were seen as separate phenomena. General relativity reveals underlying physics principles, bringing many aspects of physics into a unified theory. General relativity unifies the descriptions of gravitation and inertia. --Cleon Teunissen | Talk 06:11, 29 Apr 2005 (UTC)

This "manifestation of inertia" business is so unique that I suspect that it can be described as "new research" under the rules of Wikipedia.

I am not contesting what you are saying, but the **how**. Free-fall is inertia in GR. That is a point that needs to be made. That the force of the engine on the elevator is pushing it out of inertial motion instead of keeping it from accelerating is the point. Instead you are declaring the interaction to be a "manifestation of inertia", and for someone who does not know what inertia is in GR, that term is useless, if not worse.

It is difficult not to inappropriately innovate in this forum. I have had my face rubbed in that with the Equivalence Principle page, and it makes me all the more careful now to be sure that I am trying to communicate real and known concepts (even if it is in my own way) instead of introducing new and novel concepts.

The term "manifestation of inertia" is wholely unique to you, and its conceptual orientation is also unique to you. As such, its use in Wikipedia is automatically questionable. Combine that with its not communicating what GR is about effectively, and the use of this phrase ends up being inappropriate. --EMS 18:59, 29 Apr 2005 (UTC)

- Actually, I have encountered the expression 'manifestation of inertia' in non-wikipedia related places that deal with physics. Inertia is a standard physics concept. Both in SR and in GR free moving objects move along geodesics. Moving along a geodesic is inertial motion. In order to make an object deviate from geodesic motion a force is required. None of that is a new conceptual orientation, that is all standard GR.

- Of course, no single expression can explain GR in one big swoop. It is how an assembly of concepts is lined up in a coherent picture that constitutes explaining.

- "Manifestation of inertia" may be a good physics concept, but it is not a good GR concept. However, the trouble with using it in GR is not in what it says, but in what it does not say.

Free-fall is inertia in GR. That is a point that needs to be made. --EMS 18:59, 29 Apr 2005 (UTC)

- In attempting to communicate GR concepts, we have the following 2 aspects of GR:
- (1) When an object is in free-fall in the curved space-time surrounding a gravitating mass, then it is accelerating towards that gravitating mass. In general: two masses in free motion will accelerate towards their common center of mass.
- (2) Free-fall is inertial motion. Special relativity defines inertial frames of reference as those frames in which the speed of light is the same in all directions, and during free-fall we have that measurement of the speed of light yields the same velocity in all directions.

- Any attempt to communicate GR must show how GR reconciles those two aspects into a coherent picture. Merely stating that "Free-fall is inertia", without further explanation, is worthless. Opinions may differ on exactly how the reconciliation can be shown, but any attempt to explain GR must in some form present the wonderful GR resolution to the above issue.

- The wikipedia Gravity article attempts to actually explain, and I appreciate that. --Cleon Teunissen | Talk 07:44, 1 May 2005 (UTC)

- I appreciate that too, which is one reason why I have not touched the aticle yet. I also agree that making the point about free-fall being inertia involves more than writing one sentence. However, that one sentence will be more valuable than taking about gravitation as a "manifestation...".

- I have too many Wikipedia project in my queue. What I want is for people to be aware the time dilation is an effect and not a cause of gravitation; that gravitational potential evergy even in GR is just as Newton described it: the potential to obtain kinetic energy through free-fall. In an accelerated frame of reference, that is a perfectly valid way of describing things. In fact, through the mass of an object rises as it is raised up.

- --EMS 20:54, 2 May 2005 (UTC)

## Movement of photons

According to the thread between EMS and Cleon above, one contrast between the inertial motion of a body in free-fall and a photon traversing space is that the photon is theoretically not subject to gravitation, as it has no mass (Newton's 2nd law). But a photon traversing a geodesic, which might be bent by the presence of a huge mass, is subject to that mass (GR + geodesic equations). But if I understand this thread, both a body in free-fall and the photon are in inertial motion. The question then arises: Is there a way to impart rotational acceleration to a photon?

In the presence of a black hole, as a star is getting squeezed and emits X-rays, spinning more and more rapidly, it appears that even those X-rays are not subject to rotational acceleration. Thus one formulation of *inertial frame* might be the *frame of motion of a photon*. Ancheta Wis 09:38, 4 May 2005 (UTC)

- The issue of whether a photon is subject to gravitation in Newton's physics is debatable. Surounding a massive object, everything is accelerated, and that can be interpreted as including photons. The Equivalence principle left no doubt that this is in fact the case, but at the cost of losing Newton's view that gravity is a force.

- In GR, massive objects follow timelike geodesics of spacetime, and photons follow lightlike geodesics of spacetime. Inertial motion is experienced by following a timelike geodesic. Photons are different in that since they are massless they automatically go as fast as they can (namely at
*c*as viewed by any observer they happen to be passing) and therefore cannot be accelerated. This is not to say that they are unaffected by the presense of massive objects or by whether the massive object is rotataing. Such things affect the spacetime itself. So a photon passing the Sun is deflected because the spacetime has become curved. Also, since the fabric of spacetime is somewhat pulled along by a rotataing object, a photon will have some rotation imparted to its motion by a rotataing massive object in its vicinity. (Note however that this is affecting where the photon goes. There is another interpretation to "rotataing a photon", but I really don't think that you are refering to polarized light.)

- The "frame of motion of a photon" is not useful. An observer on a photon experiences no proper time. When it comes to GR and gravity, I strongly council that people keep the discussion short and simple and use a minimum of GR itself. There are plenty of other places for GR in its full glory to be discussed (such as in the general relativity page itself). In GR, spacetime is curved as dictated by the Einstein field equations and the presense of mass,energy, and momentum in the spacetime. The paths that objects take are governed by geodesics of spacetime as described above. Given a lack of knowledge of curvature, it is easiest to treat gravitation as being due to a force (namely gravity). This article really does not need to say much more than that, although talking about the perihelion precession of Mercury and the bending of light by the Sun are not inappropriate. A mention of black holes is also not unreasonable.

- --EMS 15:33, 5 May 2005 (UTC)

## Gravitation and the propagation of light

- Thus one formulation of
*inertial frame*might be the*frame of motion of a photon*. Ancheta Wis 09:38, 4 May 2005 (UTC)

Indeed, in his explorations of theoretical physics, Einstein relied on the propagation of light to guide his intiutions.

Like all classical wave-mechanical models, Maxwell's equations describe that electromagnetic waves spread into space as a spherically symmetrical wave-front, so you get an equality like c^2t^2 = x^2 + y^2 + z^2. Einstein proposed for special relativity that *all* observers that move inertially observe a spherically symmetrical "expanding" wave-front. So instead of **one** ether you have in a sense a superposition of all ethers, which can be seen as a superposition of the symmetry-group of all inertial frames of reference of Minkowski space-time.

Special relativity unified two concepts of relativity of inertial motion. The kinematic concept, and the wave-mechanical concept. Special relativity says the two following things **coincide**: a mechanical accelerometer does not register acceleration, and (interferometric) measurement of the speed of light yields isotropy in all directions.

The online book *reflections on relativity* by the mathematician and astrophysicict Jonathan Vos Post is a rich source of ideas. From
section 1.4 The dilemma of light:

- (E2') The speed of light with respect to any system of inertial coordinates is isotropic and independent of the state of motion of the source.

- This form emphasizes the fact that this principle can be regarded as an assertion of wave-particle duality for light, because if light behaves like material particles its speed ought to be isotropic with respect to any inertial frame, and if light consists of a wave in a material medium its speed ought to be independent of the state of motion of the source, whereas the above principle asserts that light exhibits both of these characteristics. In this sense, we see that the wave-particle duality commonly associated with quantum mechanics is actually at the core of special relativity as well. (It's not surprising that Einstein was occupied with a paper on light quanta at the same time that he was formulating his ideas about relativity.)

- This is an interesting insight, but has nothing to do with inertial motion. Indeed, the whole point on inertial motion is lost in description. Inertial motion happens which there are no forces acting on you. Your being in an inertial frame of reference can be inferred by the lack of an apparent force that acts on all object in direct proportion to their mass.

- --EMS 15:57, 5 May 2005 (UTC)

In relativistic physics, the way gravitation affects photons cannot be understood in terms of force. The only way to understand gravitational bending of light is to consider the wave-mechanical properties of the propagation of light.

- That is a 1911 view, and it gives an answer that is off by a factor of 2 (being the same as the Newtonian one). Light follows lightlike geodesics through a curved spacetime. In essense, gravitation causes the path taken by light to appear to be bent as it is perceived by a distant observer.

- --EMS 15:57, 5 May 2005 (UTC)

In a Minkowski space-time diagram, a shift from one Lorentz frame to another can be represented as a rotation over a particular angle in space-time, involving the time-coordinate and at least one space-coordinate. The space-time surrounding a gravitating mass can be represented as rotated over an angle with respect to flat space-time; the closer to the gravitating mass the further away from the state of flat space-time geometry. (There is "rotation" with respect to multiple axes of rotation, hence curvature of space-time cannot be represented as a vector field.)

During acceleration in gravitationfree space the speed of light is not isotropic. If you are stationary (or in uniform motion) with respect to the center of gravitaiton of a gravitaing mass, then the speed of light is not isotropic. However, if you accelerate towards a center of gravitation, at exactly the gravitational acceleration, then the two anisotropies largely even out, and locally isotropy of space is restored.

- The question then arises: Is there a way to impart rotational acceleration to a photon? Ancheta Wis 09:38, 4 May 2005 (UTC)

When two atomic clocks are on a rotating disk, one halfway between the hub and the rime and the other at the rim, and they relay some of their photons to each other, then the recieved photons will not match the frequency of the "home-brew" photons. The difference in frequency will be consistent with the fact that the two clocks are in different Lorentz frames, with a correponding difference in rate of time. It would introduce inconsistency in the model to assume that something happens to the photons during transit. (Or worse: to assume that some kinematic momentum is imparted to the photons. Yukh!) Likewise: in the propagation of light in curved space-time it would introduce inconsistency to assume that something happens to the photons during transit. (Anyway, with photons, you can measure only once, so you can't compare.)

--Cleon Teunissen | Talk 11:22, 4 May 2005 (UTC)

- The idea that "something happens" to the photons is actually one way of viewing it. As for the photons having some momentum imparted to them due to the movement of their source: That does happen, affecting the direction in which the photons are perceived to go and even their energy (although the time dilation effect is a far simpler way of describing the red-shifting). So what your "yukh" is aimed at is actually good physics.

- --EMS 15:57, 5 May 2005 (UTC)

It is customary to understand doppler shift in terms of different velocities, it can also be understood in terms of difference in rate of time. It doesn't really matter, but thinking about doppler shift in terms of difference in rate of time can help to "blend" it into the rest of relativistic physics. --Cleon Teunissen | Talk 11:40, 4 May 2005 (UTC)

- Just remember that time dilation is a
*consequence*of gravitation and**not**the cause.

- Just remember that time dilation is a

- --EMS 15:57, 5 May 2005 (UTC)

## A quick question about the article...

*As we know the total energy of an object is E=mc², where m is the so-called "relativistic mass", and c is the speed of light. When an object falls "down" its kinetic energy goes up. Energy has mass and so m goes up. However c2 drops down by the same amount since the falling object gets into space where time is running slower (recall time dilation) and so the speed of light, as observed by the same distant observer who is seeing the increasing kinetic energy, is slower as well (that's why the speed of light is not constant in a gravitational field). If both m and c2 change in opposite directions by the same amount, the product (the total energy of the object) stays the same for a free falling object. That's how the conservation of energy works in Einstein's gravitation.*

Doesn't this mean that nothing actually ever drops into a black hole ie. reaches the event horizon? If the speed of light 'slows' near the event horizon to a near 0 wouldn't it mean the distance to such a *horizon* would be infinite?

- Dunno if this helps but I found it an interesting tangent; according to Andromeda series co-executive producer (and head writer) Robert Hewitt Wolfe, talking about the space ship 'stuck' at the event horizon: "If it were just the event horizon by itself, the point at which time really does slow down sufficiently is about 2 feet away from the actual event horizon. At that point the gravity tides would tear you apart molecule by molecule long before a time dilation could take place. At least we know we cheated." - I guess he's looked at the science???[5] SeanMack 14:51, 8 May 2005 (UTC)

The important fact about the event horizon is that is "easy to see" for an outside observer, but not for an object falling towards the black hole. If you imagine yourself surviving traveling towards it, there would be nothing holding you back from falling further in. For an outside observer, however, the time dilation becomes very large when the object (= you) nears the event horizon, so it seems to that observer that you never fall in. Awolf002 22:07, 8 May 2005 (UTC)

- Well yes and no to the issue of an object never entering a black hole as seen by an outside observer. As the next object comes in behind the previous one, the event horizon rises up towards it. It does not pull in the current object, but it does pull in the previous one. Or so I have been told.

- Also, an observer gets to and enters a black hole in finite proper time, which is considered to be more important than external/coordinate time.

- Beyond that, if the black hole business looks odd to you, you are not alone. I do not like it either. However, black holes are a theorem of general relativity (GR), so that concept will not be removed without modifying GR itself somehow. --EMS | Talk 04:59, 10 May 2005 (UTC)

- One more thing: In GR, the kinetic energy an object gains through free-fall is still offset by the loss of potential energy, just as is the case in classical mechanics. This business of light slowing down is
**bogus**in terms of its reducing the mass of the object. It should be noted that the retardation of light is an affectation of how measurements are made in relativity. Locally, the speed of light always remains*c*. It is as observed in a gravitational field that this retardation occurs. In fact, the loss of mass/energy in a gravitational field is proportional to the square of the time dilation factor, while the slowing down of light is proportional to the**square**of the time dilation factor. So not only is this the wrong concept, but it also produces the wrong answer!

- One more thing: In GR, the kinetic energy an object gains through free-fall is still offset by the loss of potential energy, just as is the case in classical mechanics. This business of light slowing down is

## Redirect to GR

- I am not yeat ready to redo the GR part of the gravity article, but it needs a good solid kick-in-the-pants. It should be much shorter and refer people to the real GR article. --EMS | Talk 04:59, 10 May 2005 (UTC)
- Done. Ancheta Wis 10:00, 10 May 2005 (UTC)

- I am not yeat ready to redo the GR part of the gravity article, but it needs a good solid kick-in-the-pants. It should be much shorter and refer people to the real GR article. --EMS | Talk 04:59, 10 May 2005 (UTC)

- Thanks. Even what you left needs some work, but this is a real improvement over the misleading stuff in the old write-up.

- You are still somewhat off-base in the write-up. I would not mention the expanding universe for instance. "Gravity" is about objects attracting each other and therefore tending to come together. The expanding universe is about their flying apart on cosmological scales. You also don't quite succeed in describing what gravity is in GR, although I will give you credit for a good try.

- If you will let me, I will start working on the GR section. I can easily improve on it as it now stands. It just needs to be redirected slightly and expanded back out a ways. (This should never go beyond 2 to 3 medium length paragraphs. There should be enough there for people to be able to grasp the basics without having to go to the GR article, but not so much there that it becomes a serious GR write-up itself.) --EMS | Talk 15:38, 10 May 2005 (UTC)
- Feel free, of course.
~~I am curious about what, if any, you are going to build atop of 137's rewritten paragraphs. Especially about photons. Ancheta Wis 17:21, 10 May 2005 (UTC)~~

- Feel free, of course.

- If you will let me, I will start working on the GR section. I can easily improve on it as it now stands. It just needs to be redirected slightly and expanded back out a ways. (This should never go beyond 2 to 3 medium length paragraphs. There should be enough there for people to be able to grasp the basics without having to go to the GR article, but not so much there that it becomes a serious GR write-up itself.) --EMS | Talk 15:38, 10 May 2005 (UTC)

I think that I have that part of the page is fairly good shape. About all that bugs me is a remaining reference to a geodesic in talking about how objects move. Geodesics are important to GR, but in this context it is a buzzword and if not needed.

As for "137"'s paragraphs: What are you talking about? I cannot find any references to photons in this article, nor Joke's being responsible for any edits in the history. What I can tell you is that he seems to know relativity at least as well as I do. I don't know that I need to build on what he has done. --EMS | Talk 20:45, 10 May 2005 (UTC)

- Oops! I got mixed-up of course. My thanks to you for your improvements! No offense meant. Ancheta Wis 23:47, 10 May 2005 (UTC)

## Gravity => Gravitation

Should this be moved to gravitation? Also, the distinction between them be made upfront: "The term gravity is commonly used synonymously with gravitation, but in correct usage a definite distinction is made. Whereas gravitation is the attractive force acting to draw any bodies together, gravity indicates that force in operation between the earth and other bodies, i.e., the force acting to draw bodies toward the earth." -Colmubia University Press. -SV|t 18:36, 21 May 2005 (UTC)

- I
~~concur. Ancheta Wis 21:03, 21 May 2005 (UTC)~~defer to EMS.

- I disagree. At least my understanding is that
**gravity**is an attractive*force*acting between objects, whereas**gravitation**refers to the tendency of objects to move towards each other. So Newton's theory describes gravity, whereas Einstein's describes gravitation. [In GR, no force is in action. Instead the way massive objects cause spacetime to curve results in inertial/geodesic motion in which objects undergo coordinate acceleration (as opposed to physical acceleration, which would involve a force) towards each other.]

- I disagree. At least my understanding is that

- My point is that these terms are not synonomous. I strongly counsel you to leave
**gravity**where it is. Let the GR pages handle gravitation. Let this article be primarily about Newton's theory. (Referencing GR from here is fine. In fact I worte the current GR section of this article.)

- My point is that these terms are not synonomous. I strongly counsel you to leave

- BTW -
**gravitation**is currently a redirect to gravity. This is OK for now, but either redirecting it to GR or letting it have a short article in its own right making the above point (with links to gravity and general relativity) are ideas worthy of consideration. --EMS | Talk 03:35, 22 May 2005 (UTC)

- BTW -

## aetherometry.com link

Please, do not add this link until you have shown us that this is not original research and can have its own description under "Alternative theories". Otherwise, it will always be removed. Awolf002 22:54, 8 Jun 2005 (UTC)

Having just reverted the article again to remove this link, I concur. ~~Whoever-this-is tried to cite Wikipedia's NPOV policy, but it turns out this is policy ~~
**explicity** states that:

~~If a viewpoint is held by an extremely small (or vastly limited) minority, it doesn't belong in Wikipedia (except perhaps in some ancillary article) regardless of whether it's true or not; and regardless of whether you can prove it or not.~~

~~Aetherometry (which I have never heard of before) most definitely qualifies as being held by a such a minority, assuming that is it not original research (which is even worse).~~

~~In addition, it is also noted that~~ the primary focus of this page is Newton's theory of gravity. As aetherometry is not Newtonian gravity, the link is inappropriate and misleading. It should not be here, *even if aetherometry is true*. --EMS | Talk 02:54, 9 Jun 2005 (UTC)

- Sometimes Wikipedia changes too fast. Between the time that I saw the NPOV note and compared the most recent versions, the link got re-inserted. So I assumed that the perpetrator was trying to justify his/her actions using the NPOV. So I rescind my NPOV remarks, but still refuse to admit this link. --EMS | Talk 03:09, 9 Jun 2005 (UTC)

## Gravity

We seem to observe conditions and then set mathimatical formulas around these observations. Gravity, we call an attraction. We know two or more objects tend to come together but how can we tell if these objects are being pulled together or pushed together? We can't see, touch, feel, or get our arms around it yet we have concluded that gravity is an attraction. If mass absorbs energy (the same energy we attribute to attraction) we would obviously see the same result. wmays@earthlink.net

## "Gravitational Field Strength"

I was just wondering whether the page Gravitational field strength (which currently doesn't exist) should be redirected here, because that is how it is referred to consistently in my Physics A Level textbook. I am aware that Gravitational field already redirects. Thanks. Will

- My advice is to redirect it to gravitational constant. That is the value that describes the gravitational field strength. --EMS | Talk 16:31, 15 Jun 2005 (UTC)

### Gauss's Law of Gravitation?

The electromagnetic form of Gauss's Law is an invaluable aid in developing equations for electric fields. While I realize that the gravitational field concept is not as well-developed as the electric field concept (In my gravitation course we dealt exclusively with forces, and used all the same line-after-line-of-integrals methods to solve gravitation problems as Newton himself probably did), the analogies that one can draw using Gauss's Law of electromagnetism suggests very powerfully to me that developing a fully-fledged usable Gauss's Law of gravitation would be an excellent pedagogical advance in teaching gravitation courses particularly at the college or university level.

Up until now the only treatment I have seen regarding Gauss's Law of gravitation has been to treat it as a theoretical curiosity in problem sets in electromagnetism texts amounting to "Develop from first principles Gauss's Law of gravitation and show that it is essentially identical to Gauss's Law of electromagnetism", whereupon if you crank through the formulas you get the familiar integral g dot n dA = 4*pi*G*M , which can be shown to be essentially equivalent to integral E dot n dA = 4*pi*k*Q.

But surely there is more use for it than just treating it as a theoretical curiosity. Would someone be kind enough to develop a section on the use of Gauss's Law in gravitation problems?

--24.84.203.193 28 June 2005 05:16 (UTC)

- I have added a link to Application to gravity of the divergence theorem. You are welcome to add more.--Patrick June 28, 2005 06:02 (UTC)

- http://www.physics.arizona.edu/~elliott/phys321/notes/week08_1.pdf <-- A small summary of the use of Gauss's Law and the gravitational potential (which is also, as far as I can tell, not a very well-developed concept in comparison to the electromagnetic potential).