Talk:Quantum gravity/Archive for 2008

Quantum Gravity.
Some time ago, I watched a television documentary in which scientists claimed the universe was created at the instant of the big bang by a collision between two pre-existing universes.

With this idea in mind, I have wondered if the reason why a theory of quantum gravity cannot be found is because our universe is a hybrid universe, represented by two mathematical theories, wholly alien to each other, which can never be unified.

This is just a thought from a layman. Derek R Crawford.

Half a thought? This is not an insult, it is a question.


 * Mathematics is all about consistency. Whatever theory should be appled to describe physics, if it isn't self-consistent then "anything goes", which is contrary to experience. — DAGwyn (talk) 23:39, 13 February 2008 (UTC)

Discussion of nonrenormalizability added
I was rather surprised to see that there was minimal discussion of the nonrenormalizability of gravity in this entry, since if gravity were renormalizable then quantizing it would not be such a difficult issue. I have added two paragraphs on this with mentions of two ways of getting around the problem (a nonperturbative UV fixed point, or string theory). I would appreciate it if someone familiar with loop quantum gravity can add an explanation of what principle (if any) in LQG might allow it to get around the nonrenormalizability argument. -- MR, 21 May 2006.

Someone edited this to claim that zeta regularization can solve the renormalization problem. This is not enough: we need not just to give finite values to divergent integrals, but to have a choice of finitely many such values (renormalization constants) that renders any amplitude calculable. This doesn't work in gravity. Any divergence in quantum field theory can be regularized, but not any quantum field theory can be renormalized. (If you think otherwise, please provide references.) -- MR, 2 January 2007.

The renormalization of gravity seems challenging in the sense that you can't assign all the parameters. As the article says, at low energies, you can't really do it (but that you don't "need to" and that the description converges to the classical treatment of gravity anyway) and that at high energies, there is no way to specify a unique solution. I don't know too much about the math at this level, but I was reading the article on symmetry breaking of a generalized unified force and it says that at high energies, symmetry is restored but it unstable, and essentially breaks randomly. So perhaps that's just the way it is: there is no way to uniquely specify all the parameters corresponding to the classical treatment, because by definition the symmetry is broken, and by definition some of the one-to-one correspondences are lost (or no longer uniquely assignable). There are many ways to break symmetry, but any particular way will always leave an unbalance that cannot be renormalized - that's why its broken. Perhaps its not an accident that we encounter this problem with the force that is most different from the others in strength and range and lack of extra parameters (no charge, color, flavor, and always attractive)  (??) -- AI, 10 Oct 2008  —Preceding unsigned comment added by 68.236.178.38 (talk) 14:00, 10 October 2008 (UTC)

Gravitational Field vs. Electric Force Field. Why?
In the last paragraph of section Quantum Mechanics and General Relativity:

"...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...".

I am just curious about why the electric force field for the central-force problem (finding the wave function for the electron circling the nucleus of a hydrogen atom) can be determined, but it's not clear how to determine the gravitational field of a particle?

To figure out the wave function $$\Psi$$ for the electron of the central-force problem, the potential energy field $$V$$ in the time-independent Schrödinger equation
 * $$E\Psi=-\frac{\hbar^2}{2m}\nabla^2\Psi + V\Psi$$

must be determined. But the potential energy field $$V$$ is known after the force field $$\mathbf{F}$$ exerting on the electron is determined.

In my text book of quantum mechanics, the force field $$\mathbf{F}$$ is just the central force caused by the charges of the nucleus and the electron from the hydrogen atom.

The nucleus, a proton, which is a particle should comply with?? the Heisenberg uncertainty principle with uncertain location and velocity. How can we say the electric force field between the proton and the electron is in the form of central force? Or, why can not we say the gravitational field between them IS in the form of central force just like classical mechanics?

p.s. I'm just new to quantum mechanics so the questions here may be ridiculous and stupid. Forgive me please if any.

Justin545 (talk) 11:01, 13 January 2008 (UTC)


 * Hi Justin - I'd be more than glad to answer some of your questions. The quote above sources from an article on arXiv that attempts to circumvent what's called a singularity. In general, this is nothing out-of-the-ordinary, as for example in electromagnetism (namely: Quantum electrodynamics) such a singularity exists as well, which would result in infinite polarization of the vacuum around an electric point charge - but (for some reason) a procedure called Renormalization happens to be able to resolve this successfully. For gravity, unfortunately, the singularity is more complex: Because the gravitational field itself becomes a source of gravity. Gravitational charges (e.g. point masses) and the resulting gravitational fields are in a dynamic balance, and cannot simply be separated anymore (contrary to electrodynamics, where one can separate the fields from test charges; the electromagnetic field can simply be added through superposition; no so for gravity). Naturally, the singularity of a point mass becomes more complex: In addition to a singularity at the origin, there is an additional singularity (though of a different quality) at the Schwarzschild radius: Space and time get quite weird and counterintuitive. So, the Schrödinger equation that you wrote above still holds as a good approximation for gravity if the field self-interaction could be neglected. Problem is: The gravitational force would, in this case, be so terribly weak that it is futile to even consider: Richard Feynman calculated one time (in Acta Physica Polonica, if I remember correctly) that the gravitational force of a proton in a hydrogen atom would have shifted the quantum mechanical phase of the electron in that same atom just a few docent arcseconds ... during 100 lifetimes of our universe! In order to get meaningfully close to anything that could possible ever be measured, for quantum gravity and to the best of today's knowledge, one would have to go to energies and length scales at which charges/masses and their resulting fields are tightly coupled. So, on first look, the uncertainty principle is just one out of a spectrum of problems (but nevertheless, surely is one of it). Hope this helps! Jens Koeplinger (talk) 00:00, 14 January 2008 (UTC)


 * I just know very little about the special/general relativity, and nothing about the quantum field theory. It seems they are required to truly understand your explanation. I started to study quantum mechanis because of my curiosity about knowing how quantum computer works, especially for entanglement. I found the more I learn the more qustions bother me. Your answer is a good guidance for me and it helps. Thank your for your patience and time to answer my qustions!
 * Justin545 (talk) 12:17, 14 January 2008 (UTC)


 * Ok ... I'm glad my 'sweep' that touches several points of interest seems helpful to you :) - Now, just re-reading what you wrote: "I found the more I learn the more questions bother me." Welcome to the club, you're in good company. You seem interested in Quantum information, Quantum computer, and also Quantum entanglement - from an engineering point of view maybe. If you're interested in the foundations of quantum mechanics, there's one thing I might want to recommend to you studying early on, which is Bell's theorem. Good luck! Jens Koeplinger (talk) 04:22, 17 January 2008 (UTC)

structure of GR
"In particular, contrary to the popular but erroneous[citation needed] claim that quantum mechanics and general relativity are fundamentally incompatible, one can in fact demonstrate that the structure of general relativity essentially follows inevitably from the quantum mechanics of interacting theoretical spin-2 massless particles"

Could someone please include a citation (scientific article) for this?

Thank you!

Zsolt —Preceding unsigned comment added by 85.238.79.126 (talk) 14:39, 13 March 2008 (UTC)
 * I believe the spin-2 / GRT consistency claim in the article goes back to the works of Kraichnan (beginning in the 1940's), Gupta (1950's), a few more, and Stanley Deser (1970's and 1980's). I don't have the exact references at hand, but you may do an internet seach for "Kraichnan Gupta Deser", or also check in the Misner/Thorne/Wheeler "Gravitation" under the spin-2 section. Note that the newer (1980's) publications aren't in "Gravitation", but in essence that's the trail I think that's referenced. Thanks, Jens Koeplinger (talk) 15:50, 13 March 2008 (UTC)
 * PS: Let me make a note to dig up the exact references. Thanks for pointing this out. Koeplinger (talk) 15:51, 13 March 2008 (UTC)
 * Here's what I got. Understandably, this is way too much and cryptic for the current article; still, let me write my view here with references:

Spin 2 / GRT: collection of articles
As for consistency between spin 2 and GRT, Kraichnan shows that such formalisms must necessarily be generally covariant field theories. Gupta ascertains that the only known physical quantity described by a symmetrical tensor which satisfies a vanishing divergence is the total energy-momentum tensor of a closed system. He suggests an infinite iteration series of terms in the Lagrangian to quantify self-coupling of the field. Thirring executes the first order correction from such a series in spherically symmetric static metric and finds agreement with the corresponding approximation from Schwarzschild coordinates in GRT. Weinberg shows that spin 2 GRT can also be obtained from an S-matrix ansatz. Deser then shows that the particular result of this iteration must be of a certain form to be consistent with self-coupling requirements on a generally covariant Lagrangian. A consistency condition is given explicitely by Wald . Deser then finds that one is not bound to the (unobservable) Minkowski flat space in order to derive generally covariant Einstein action through "consistent self-coupling requirements, from the linear graviton action".

As for the problems with this approach, one inevitably runs into problematic short wavelength divergences (see e.g. Feynman for an elaborate discussion, or Weinberg for a detailed overview). Because these problems are systematic, it appears impossible to build quantum gravity on the spin 2 ansatz.

Or in other words: It is believed that the spin 2 ansatz must necessarily lead to Einstein GRT (that part of it works), but when doing quantum gravitation, it yields unavoidable divergences when calculating field self-interaction (higher-order interactions; that part of it fails, or is incompatible).

Thank you very much! Zsolt —Preceding unsigned comment added by 152.66.104.6 (talk) 08:14, 14 March 2008 (UTC)

Er #2
Came across this article in some physics-surfing. I don't think the term "dumbing down" is necessarily true, but this article is pretty dense. It's clear that the author(s) know what they are talking about, it's equally clear that they aren't very good at conveying it to other people. I can understand some of it, but I certainly can't write about it, but if anybody can streamline this into something useful it sure would be nice. Pretty important article that's almost unintelligible, IMO. See "String Theory" for article that's deep and readable at same time. Jjdon (talk) 22:34, 5 May 2008 (UTC)

What about LIGO??
¿?--84.125.13.89 (talk) 14:27, 29 July 2008 (UTC)

Uhm... what about LIGO, then? The chance of finding macroscopic strings? Markus Poessel (talk) 21:39, 29 July 2008 (UTC)