Talk:General relativity

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Yes check.svg Done Fixed it.Earthandmoon (talk) 10:22, 5 January 2014 (UTC)

Why is there a qualitative statement about the Wheeler-DeWitt equation in this article?[edit]

Hello, I've just read in section "Quantum Gravity" the following:

"Another approach starts with the canonical quantization procedures of quantum theory. Using the initial-value-formulation of general relativity (cf. evolution equations above), the result is the Wheeler–DeWitt equation (an analogue of the Schrödinger equation) which, regrettably, turns out to be ill-defined.[177] " The source given is the only one: Kuchař, Karel (1973), "Canonical Quantization of Gravity", in Israel, Werner, Relativity, Astrophysics and Cosmology, D. Reidel, pp. 237–288, ISBN 90-277-0369-8

Wheeler-DeWitt equation is pretty much out there influencing physics research, and it is extremely strange that here it is called "ill-defined" and only one reference dated 1973 is given.

There has been recent experimental research confirming the absence of time for an external observer of a universe: http://arxiv.org/pdf/1310.4691v1.pdf

It's highly ridiculous for a Wikipedia article to have qualitative statements about theories unless they have completely been dismissed by the entire scientific community through numerous failed attempts to verify them. — Preceding unsigned comment added by Whitely3000 (talkcontribs) 17:35, 24 February 2014 (UTC)

Expected domain of validity[edit]

I think there should be a (sub)section on the expected domain of validity of GR in the article. When reading about GR (whether popular or real thing), one is always fed the message that "most physicists expect..." and hence quantization is necessary. (The breakdown is just around the corner...) From the little I know myself, the circumstances are pretty extreme when one can surely expect GR to break down (giving incorrect predictions). Note: This is not something that can be disposed of by saying GR has already broken down because black holes exist and singularities don't. I think it would be interesting for the reader to get "pretty extreme" quantified in terms of, say, distance from the singularity of a really nasty black hole, or anything else suitable.

Also, provided GR is correct (within its expected domain of validity), can we ever expect to experimentally verify a quantum theory of gravity? Sure, such a theory might predict other things that can be detected, but I personally doubt that we will be able to produce graviton showers (instead of gravitational waves or whatever) at CERN. I understand I'm being vague here, but some skilled and knowledgeable editor might have an idea about good stuff to spice up the article with. YohanN7 (talk) 02:58, 4 July 2014 (UTC)

Objects do not follow the geodesic[edit]

Objects do not follow the geodesic because the gravitational field is in non-inertial motion. If there are two objects under their mutual gravitational field, both will be accelerated and follow a non-inertial motion. If one is a photon, the other will still follow non-inertial motion. Saying the object follow the geodesic is an approximation but not their true path. Shawn H Corey (talk) 14:05, 21 August 2014 (UTC)

Outstanding challenges[edit]

I feel it may be beneficial to include some discussion of the outstanding challenges encountered by the theory, and preferably in its own section. Certainly, the notion of singularities is one where most physicists are uncomfortable to the point where I really can't understand why it seems to be so universally espoused. Then of course there are the broader cosmological observations of dark matter and dark energy, which are almost beyond direct measurement despite comprising 95% of the universe. On top of that, we have inflation, and the elephant in the room - the fact the big bang could occur at all given that it should have by all rights gravitationally collapsed on itself.

As Lord Kelvin would no doubt attest, we still have a few clouds on the horizon. I fear I do not share his optimism, however. 150.203.179.56 (talk) 03:49, 8 September 2014 (UTC)

The statement about "the elephant in the room" is simply false whether or not the Big Bang should (have) collapse(d) back on it self is simply the old question of whether we live in an open or closed universe. Depending on the initial conditions both are valid outcomes.
Nonetheless, a section on "open challenges" is probably a good idea, as long as it is sourced by reliable sources. Given the upcoming centenary, I suspect we should be getting a few of those soon.TR 08:48, 8 September 2014 (UTC)
If the universe began with no matter or radiation (only the cosmological constant Λ), then the Friedmann equations which incorporate the Einstein field equations reduce to:
 \frac{\dot{a}^2 + kc^2}{a^2} = \frac{\Lambda c^2}{3}
\frac{\ddot{a}}{a} = \frac{\Lambda c^2}{3}
which can be satisfied by:
 a = \sinh \left( c \, t \, \sqrt{\frac{\Lambda}{3}} \right)
 k = - \frac{\Lambda}{3} \,.
At time t=0, the universe was already expanding so fast that it could not collapse on itself. JRSpriggs (talk) 15:26, 8 September 2014 (UTC)