Talk:Quantum gravity/Archive for 2017

Confusing wording

The second paragraph contains the phrase, “the (alleged) fact,” then goes on to say that it is false. I’m new here, and not an expert on semantics by any means, but is this intentional?? Like, if the textbook cited presented that idea as a fact (I don’t know if it did), and it was shown to be inaccurate post-publication, would this be the preferred wording?

kumowoon1025 (talk) 07:09, 7 August 2017 (UTC)

You are right, it is not good wording. I have tried to improve it and at the same time left the conclusion more open because it did not really reflect the concensus as it was. Weburbia (talk) 11:22, 7 August 2017 (UTC)

Um, thanks for that, and I apologize for pointing out problems I don’t know how to solve myself, but I feel like I forgot to mention that the what made it sound so weird to me was because both statements were made by the same person... Now it completely sounds like someone else made the claim on the incompatibility of quantum and classical models of gravity and Wald disagreed with him/her.

The thing is though, it turns out my physics teacher has a copy of General Relativity (unsurprisingly) . I read a couple cited pages, and I think the author’s words may have been mischaracterized. I assume whoever wrote that bit of the article was referring to the parts where Wald points out the difficulties in reconciliation the two seemingly disparate notions,…

Linearized gravity is just the theory of a massless, spin-2 field in Minkowski spacetime. Thus…it would be natural to expect the metric field operator ${\hat {g}}_{ab}$ to satisfy the commutation relation

$\left[{\hat {g}}_{ab}(x),{\hat {g}}_{cd}\left(x^{\prime }\right)\right]=0$ 14.1.2

for x and x' spacelike related. However, this equation makes no sense since we do not know if x and x' are spacelike related until we know the metric; and equation 14.1.2 is an operator equation which, if valid, must hold independently of…the metric. More generally, the entire notion of causality becomes ill-defined when the notion of a classical spacetime metric is abandoned…and the difficulties with causality that arise when the spacetime metric does not have a definite value suggest the possibility that perhaps the principles of quantum theory do not apply to gravity—that classical general relativity is correct at the fundamental level.

He goes on to explain the roadblocks that notion encounters when put up to the scrutiny of analysis, and that a quantum gravitational theory is the only way to get around said roadblocks.

The attempt to treat gravity classically leads to serious difficulties. These difficulties apparently can be avoided only by treating the spacetime metric in a probabilistic fashion—i.e. by quantizing the gravitational field

Then he pretty much ends the introduction with:

The issue of how to formulate a quantum theory of gravitation is presently under active investigation by many researchers. We shall confine our discussion here to a very brief mention of some of the main approaches that have been tried.

kumowoon1025 (talk) 01:51, 8 August 2017 (UTC)

Improving tone and generality

Greetings Wikians..

This is a subject which needs some attention, where I have some knowledge to offer. I have been a student of quantum gravity for some time, and I attended most of the lectures in that subject at GR21 - last July at Columbia in NYC - so my knowledge is fairly up to date. A modest re-write of the lead is in order. In the opening; the circumstances when quantum effects cannot be ignored deserve explication. Also; it is known the route to asymptotic safety in Quantum Einstein Gravity, by using a non-Gaussian fixed point in renormalization. And some other things seem a bit skewed or avoid the generalizations of categories within quantum gravity. So there is something left out or needing to be more fully explained - to get this article to better reflect the current consensus.

I will make only small changes, except where I know exactly what to add. But I am guided by something said at GR21. Beverly Berger, in her plenary panel talk, spoke about the need for quantum gravity theorists to compare notes more and build on each other's successes - in order to make quicker progress than strict competition would allow. This sentiment was echoed by Lee Smolin in the quantum gravity talks - that we could all make progress faster if we used the advances from another camp as a springboard to progress. So I would like to augment this, and I hope - long term - to make some attempt to give the different approaches a more equal basis or make a separation into categories that would allow the various styles of thinking about quantum gravity to be more clearly enumerated.

All the Best,

JonathanD (talk) 06:14, 13 October 2017 (UTC)