Talk:Quantum gravity/Archive for 2007

Geometric Interpretation of General Relativity
Can someone explain to me (because the article doesn't!) what the geometric interpretation of general relativity is? It need only be a brief answer, simply because I can always reference the individual terms in the answer from there. --Susurrus 03:33, 15 May 2006 (UTC)


 * It is the only interpretation (there are no others). It says that worldline of every free falling particle follows a geodesic line in a curved spacetime and the spacetime is curved by presence of energy of the particles in space. The result of the above is that when a particle is not allowed for some reason to follow its free fall wordline (usually by electromagnetic forces, being therefore accelerated in relation to its free falling frame) an inertial force appears (that is proportional to the mass of the particle and this acceleration) and this inertial force is called "gravitational force". Before 1915 it had been considered a fundamental force by most physicists. Still is by those not familiar with General Relativity. Jim 11:25, 20 January 2007 (UTC)

Fixing the preamble
I added to the preamble a news that gravitational force is not a fundamental force of nature since 1915 and that this fact has been confirmed experimentally by Pound-Rebka experiment. Apparently the proponents of QG still don't know about it since no one dared to break out the news. If they ever read Wikipedia they may learn and then start doing something useful for our common good. Jim 12:09, 20 January 2007 (UTC)

I removed JimJast's edit, which was snarky and showed a poor understanding of the issues. There is nothing contradictory about describing gravity as curved spacetime and calling it a force; all theories of quantum gravity involve curved spacetime just as general relativity does. They also have propagating gravitons, which are something like quantum descendants of the classical gravitational waves predicted by Einstein's GR. These gravitons transmit forces and the curved spacetime background can be thought of as a sort of classical expectation value of the graviton field, roughly speaking. I assure you that no one working on QG has missed your "news" from 1915. -- MR, 23 January 2007


 * Dear MR, isn't a gravitational force a pseudoforce according to general relativity (the same as inertial, centrifugal, and Coriolis force none of which is acting at the distance)? So how come it is mediated by "gravitons"? Are those other forces mediated respectively by inertions, centrifugons, and Coriolisons? If yes then why don't we have Wikipedia pages dedicated to those particles? Please, be so kind and explain what I understand poorly since I have to understand gravitation for professional reasons (I teach the stuff) and I thought that since 1915 there wasn't anything left in it that wasn't explained. Was I wrong? Do you know something that wasn't explained? Jim 21:38, 10 March 2007 (UTC)

Jim - The problem with not calling gravity a force is that the other three (now unified to one) fundamental forces cannot explain it. The four (two) fundamental forces can, however, explain intertial "force", centrifugal "force", and the Coriolis "force". Does that clarify things for you? - Plato Demosthenes

Wording on last paragraph of incompatability section
The last paragraph state: There are two other points of tension between quantum mechanics and general relativity. First, general relativity predicts its own breakdown at singularities, and quantum mechanics becomes inconsistent with general relativity in a neighborhood of singularities (however, no one is certain that classical general relativity should necessarily be trusted near singularities in the first place). Second, it is not clear how to determine the gravitational field of a particle, if under the Heisenberg uncertainty principle of quantum mechanics its location and velocity cannot be known with certainty. The resolution of these points may come from a better understanding of general relativity [3].

However, when reviewing the cited reference, on page 8 of the PDF it says: To the extent that Theorem 1 applies generically, general relativity does not predict its own breakdown at singularities, and does not become inconsistent with quantum mechanics in the neighborhood of singularities.

I'm not an expert, so this is extremely confusing for me. I'm not sure if it's a matter of citing sources, or if it was a simple error.

--ShatterdRose 02:42, 18 April 2007 (UTC)

Quantum mechanics v. Quantum (Field) Theory
Hello - I notice that in the first paragraph, quantum gravity is described in terms of the effort to unify general relativity with quantum mechanics (with links for each of those topics). I propose that it would be more accurate to replace "quantum mechanics" with "quantum theory" or "quantum field theory," since strictly speaking, quantum mechanics refers to the quantization of systems with finitely many degrees of freedom (like a single simple harmonic oscillator) whereas general relativity describes gravitational fields, which have infinitely many degrees of freedom and hence must be treated with quantum field theory. Since I have not worked on the article, I will not attempt to make this change myself, especially since it will disrupt the current chain of wiki cross references, but I submit it to the consideration of the article's writers. Thanks. Idempotent 10:27, 12 October 2007 (UTC)


 * Seconded! - Saibod 22:01, 5 November 2007 (UTC)

Er
Hey! Don't you guys think this article could do with a slight dumbing-down? I really want to know what this entire quantum gravity sha-bang is all about. Thanks!  Amit @  Talk  17:40, 26 October 2007 (UTC)
 * :-(  Amit  @  Talk  15:28, 28 October 2007 (UTC)

link to GUT
The introduction gives the impression that TOE is synonymous to GUT. IMHO this is plain wrong, and this should be explained somewhere since it is a very common misconception. Please confirm. - Saibod 22:09, 5 November 2007 (UTC)
 * I fixed it :D I rock, btw.  Amit @  Talk  16:29, 21 December 2007 (UTC)
 * sorry but it was the wrong way round. A GUT is not supposed to describe gravity, while a Theory of Everything should do what its name says. - Saibod (talk) 11:47, 24 December 2007 (UTC)
 * Whoops! (hides face)  Amit @  Talk  15:53, 24 December 2007 (UTC)