Talk:General relativity

Possible incorrect equation?

Under "Total force in general relativity":

Applying $F(r)=-{\frac {\partial U}{\partial r}}$ to the given formula for potential energy gives ${\frac {GMm}{r^{2}}}-{\frac {L^{2}}{mr^{3}}}+{\frac {3GML^{2}}{mc^{2}r^{4}}}$. Is the force equation currently in the article incorrect? TeX seems reluctant to parse minus signs... — Preceding unsigned comment added by Compoteleon (talk • contribs) 12:57, 14 January 2021 (UTC)

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@Compoteleon: There currently is a problem with rendering minus signs in Chrome and Edge. Firefox is doing fine. See Talk:Fourier transform#Maybe a mistake in time shift property? - DVdm (talk) 14:45, 14 January 2021 (UTC)

Unclear sentence

The intended structure of the following sentence is unclear to me: “Reconciliation of general relativity with the laws of quantum physics remains a problem however, as there is a lack of a self-consistent theory of quantum gravity; and how gravity can be unified with the three non-gravitational forces—strong, weak, and electromagnetic forces.”

A minor issue is that a comma seems necessary before “however.” A more important issue is how the part after the semicolon is related to what comes before. Would the following rewrite be correct? Or would something else be better?

“Reconciliation of general relativity with the laws of quantum physics remains a problem, however, as there is a lack of a self-consistent theory of quantum gravity, and how gravity can be unified with the three non-gravitational forces—strong, weak, and electromagnetic forces—is not yet known.”

Tom Gally (talk) 13:27, 16 June 2022 (UTC)

Your suggested change would be an improvement. However, I personally would not want to suggest that gravity theory must change to accommodate those other theories. Perhaps they need to change instead. Or perhaps our four-dimensional space-time continuum should be embedded in a higher dimensional space with a more uniform structure, like Minkowski space? JRSpriggs (talk) 15:09, 16 June 2022 (UTC)

Thank you for the reply. I am afraid I don’t understand the issues well enough to feel confident about making any changes to the article (other than adding a comma before "however"). Can you, or someone else, use my suggestion above as a basis for fixing the structure of that sentence? As it is, it doesn’t make sense grammatically. Tom Gally (talk) 06:38, 18 June 2022 (UTC)

To me, the major issue is the forced mashing together of multiple sentences into a single sentence via the semicolon. How about this? “Reconciliation of general relativity with the laws of quantum physics remains a problem, however, as there is a lack of a self-consistent theory of quantum gravity. It not yet known how gravity can be unified with the three non-gravitational forces—strong, weak, and electromagnetic forces.” Prokaryotic Caspase Homolog (talk) 06:39, 19 June 2022 (UTC)

I like this, so I'll 'be bold' and put it in. David Spector (talk) 00:01, 24 June 2022 (UTC)

Many thanks to all of you for your replies and for fixing that sentence! I’ll try to be bolder next time. Tom Gally (talk) 12:19, 25 June 2022 (UTC)

"The relativistic effect"

In section Total force in general relativity, the two-body forces generated by GR are listed as Newton's force of gravity, centrifugal force, and the relativistic effect. What does "the relativistic effect" mean? Does it refer to SR or GR? What is it intuitively? Is it what happens in the gravitational lensing of galaxies? Was it described in previous sections? Could the wording be improved? David Spector (talk) 23:58, 23 June 2022 (UTC)

See Two-body problem in general relativity#Effective radial potential energy. According to that article, it is a GR effect. For particles traveling at the speed of light, it doubles the Newtonian bending. So it explains half of the lensing. JRSpriggs (talk) 05:24, 24 June 2022 (UTC)

A generalization of the Einstein-Maxwell equations

I think the solutions I have obtained will be of interest to the readers of this wikipedia article. Since I am the author, I cannot make any related edits. The article has been published open access CC BY 4.0. Cotton, F.W. A generalization of the Einstein–Maxwell equations. Eur. Phys. J. Plus 136, 162 (2021).

     https://doi.org/10.1140/epjp/s13360-021-01115-6 71.183.235.5 (talk) 19:32, 30 July 2022 (UTC)

Thanks for sharing. So far Google Scholar only produces this. When this is noticed, picked up and sufficiently cited in the relevant literature, it could be ready for being mentioned in Wikipedia. - DVdm (talk) 20:32, 30 July 2022 (UTC)

Wikipedia is written for narrow experts?

Wikipedia is written for narrow experts? What is the point of placing the forms of writing incomprehensible not to a narrow specialist? For example

$R_{\mu \nu }-{\textstyle 1 \over 2}R\,g_{\mu \nu }+\Lambda \ g_{\mu \nu }={\frac {8\pi G}{c^{4}}}\,T_{\mu \nu }$

is non-understandable form of writing. Why is the non-understandable form of writing is used unless an understandable form of writing for tensors? But the understandable form of writing is something like this:

$\mathop {\mu ,\nu } \limits _{\geq 0}^{n\leq }\left\{{R_{\mu \nu }-{\textstyle 1 \over 2}R\,g_{\mu \nu }+\Lambda \ g_{\mu \nu }={\frac {8\pi G}{c^{4}}}\,T_{\mu \nu }}\right.$

or

$\mathop {\mu } \limits _{\geq 0}^{n\leq }\left\{{\mathop {\nu } \limits _{\geq 0}^{n\leq }\left\{R_{\mu \nu }-{\textstyle 1 \over 2}R\,g_{\mu \nu }+\Lambda \ g_{\mu \nu }={\frac {8\pi G}{c^{4}}}\,T_{\mu \nu }\right.}\right.$

or

$\mathop {\left\{{}\right.} \limits _{\mu =0}^{n}\mathop {\left\{{}\right.} \limits _{\nu =0}^{n}R_{\mu \nu }-{\textstyle 1 \over 2}R\,g_{\mu \nu }+\Lambda \ g_{\mu \nu }={\frac {8\pi G}{c^{4}}}\,T_{\mu \nu }$

These forms of writing clearly indicate the system of equations and quantity of this equations. At the same time, this form is the same short. Unfortunately, it is difficult to find literature that would prove the importance of using such forms for better understanding by readers. At the same time, I do not understand why this problem is not obvious for writers. Voproshatel (talk) 08:18, 28 February 2023 (UTC)

Poincare

Lorentz thought that gravity was electromagnetic, but not Poincare. Poincare did have a relativistic gravity theory. See . So I do not think the last edit is correct. Roger (talk) 05:15, 24 July 2023 (UTC)

Weinstein translates a crucial passage from his 1905 note (which he expanded to a full paper in 1906) as follows: "If we were to admit the postulate of relativity, we would find the same number in the law of gravitation and the laws of electromagnetism – the speed of light – and we would find it again in all other forces of any origin whatsoever. This state of affairs may be explained in one of two ways: either everything in the universe would be of electromagnetic origin, or this aspect – shared, as it were, by all physical phenomena – would be a mere epiphenomenon, something due to our methods of measurement".

In other words, Poincare expressed a faith that either (1) gravitation would ultimately prove to be of electromagnetic origin, or (2) both gravitation and electromagnetism would be "epiphenomena" of some deeper theory.

In both 1905 note and his 1906 article, Poincare argued against an infinite speed of gravitation as representing a violation of the principle of relativity, which he considered absolute. Asserting that gravitational forces must obey the principle of relativity is, in my opinion, a far cry from being able to state that Poincare had anything resembling a relativistic theory of gravity. However, it would be correct to state that Poincare derived some results in what would today be called "relativistic gravitation". Prokaryotic Caspase Homolog (talk) 08:20, 24 July 2023 (UTC)

I guess you are agreeing that it is incorrect to say option (1) is Poincare's belief. He is favoring option (2). Option (2) does not say that gravitation and electromagnetism are from a deeper theory. It says that "this aspect" (propagating at the speed of light) is "due to our methods of measurement". That option is not much different from what many say today.

Weinstein says "when discussing his theory of gravitation, Poincaré extended his mathematical theory of groups from electrodynamics to gravitation." Damour says "Poincare’s June 5, 1905 Note announces important mathematical and physical advances in what we would call today, Special Relativity, relativistic electrodynamics and relativistic gravitation". Walter says "In July, 1905, Henri Poincaré (1854–1912) proposed two laws of gravitational attraction compatible with the principle of relativity and all astronomical observations explained by Newton’s law." So I think it is fair to say Poincare proposed a relativistic theory of gravity. Roger (talk) 17:22, 24 July 2023 (UTC)

I would no more state that than I would agree that Einstein proposed a relativistic theory of gravity in 1907 because, using relativistic arguments, he predicted gravitational time dilation and, in 1911, the bending of light. Einstein made a few predictions, but he had not yet tied everything together in a coherent theory. Neither did Poincare. Prokaryotic Caspase Homolog (talk) 20:54, 24 July 2023 (UTC)

Your edit is still wrong. You say Poincare made an "assumption that gravitation should ultimately have a common origin with electromagnetism". That is not true. Poincare said that Lorentz made such an assumption, and Poincare was pointedly not making that assumption. Then you say that he made a prediction as a consequence of that assumption. None of the primary or secondary sources say that. On the contrary, Poincare said that it was a consequence of his relativity theory that nothing goes faster than the speed of light. Before Poincare, it was thought that gravity went faster. Roger (talk) 01:59, 25 July 2023 (UTC)

References