Talk:Quantum gravity

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Help with this template Comments: edit – history – watch – refresh Putting statements such as "they are so astronomically small" in the overview leaves this reader dazed and confused, he finds it in stiff competition with such concepts as "a worldwide bottleneck". I know it is being pedantic and in this case nobody is deceived but there are very many disputed matters in wiki where real confusion arises because the matters are explained in a similarly confused way. In this case leaving out "astronomically" would reinforce the idea to be conveyed--Damorbel 06:57, 13 April 2007 (UTC)

This sentence fragment sounds wrong to me:

"The energies and conditions at which quantum gravity are likely to be important are..."

I assume it should be something like "quantum gravity effects are...", but unfortunately I don't understand enough about the subject to feel confident in changing it.

Marsvin 19:54, 2004 Jul 13 (UTC)

I heve moved the section "the incompatibility between QM and GR" from LQG. Miguel 16:25, 25 Jul 2004 (UTC)

~~Some of the criticisms of LQG from the ST corner are distinctly NPOV. Weeding it a bit.~~ On second thought, the section makes it clear that it's one persons list of criticisms. Rebuttals from the ST side of the coin seem desirable. Perhaps it's even more desirable for most of this to be drastically shortened or summarized, lest the article turns into a soapbox platform for various QG proponents. Nobody yet knows what the right solution looks like, OK? Give it a rest! :-) —JRM 11:50, 2004 Sep 14 (UTC)

[edit] Less Emphasis on ST/LQG Debate

While the String Theory vs. Loop Quantum Gravity debate is obviously relevent here, the ST objections to LQG take up what, to me, seems an inordiante amount of space on this page--especially with the counter argument confined to a single external link.

At least untill the LQG guys get their "merge" worked out, wouldn't it make more sense to link to the Loop_gravity page from here, since that presents a more thourough treatment of the ST criticizms? (And maybe, then, organize the two links under a single "Ongoing Debate" section on this page?)

I'd make the change myself but I'm a total Wikipida n00b and don't want to step on anybody's toes... --SMQ 66.84.200.34 19:44, 7 Oct 2004 (UTC) (edited stupid link typo 66.84.200.34 19:47, 7 Oct 2004 (UTC))

In any case the discussion is too big, so I suggest moving it to another article. I can do it by myself, but let's first decide on title. I suggest:

Loop quantum gravity versus string theory

217.26.0.121 10:03, 10 Oct 2004 (UTC)

[edit] bibliography

Hi, can we decide on bibliography? I like some books from loop quantum gravity's bib but I don't want to put them here since in the light on ST/LQG debate this can be considered POV. By the way, as we are far from QG in both approaches this article should IMHO concentrate on principal problems with QM+GR=QG ant not on ST and LQG --- we are moving to it. 193.124.225.253 16:26, 13 Oct 2004 (UTC)

Hi, is everybody sure that [1] is an appropriate quotation? It is the abstract of a more detailed document [2] which contains many statements which, at least in my opinion, are really far from being neutral. For example: Scientists do not analyze temporal energy in all its quantic details because they study space and time through Mathematics, which has... a limited capacity to carry information. 82.84.211.213 15:47, 12 January 2007 (UTC)

I must agree that article seems to be written more by a philosopher than a physicist. I read through most of it and can not find any mathematical basis for anything proposed there. —Preceding unsigned comment added by 65.162.109.77 (talk) 14:15, 13 February 2008 (UTC)

[edit] The page promotes loop quantum gravity and downplays string theory

Neutrality would ideally mean that it's hard to infer the author's opinion, but I could tell immediately that much of this page is written by advocates of loop quantum gravity.

As I said on the other page, I am neither a string theorist nor a loop-quantum-gravity theorist. I respect loop quantum gravity papers as mathematical physics and their authors as mathematical physicists. But I also see some loop quantum gravity people as engaged in a Naderite quest to compete with string theory. This quest is not physics. For better or worse, it should be presented on the LQG page. It does not belong on this second page on quantum gravity in general; this page should only have a reference to it.

Greg Kuperberg - 24.59.196.30 14:09, 4 Nov 2004 (UTC)

I agree with Greg, especially if he meant the section about "theories and prototheories". As a string theorist, I must say that this section reminds me Feynman's comments about the cargo cult sciences: the primitive tribes choose a guy with wooden earphones who expects the airplanes to land, much like they did in the Second World War. They don't land. Something is wrong with their science. It's hard to explain them what's exactly wrong - it would be easier to point out how they should change the shape of the wooden earphones.

In a similar way, this paragraph about "theories and prototheories" asks a lot of irrelevant technical questions about the shape of various brackets, constraints, and the redefinitions of the fields, without asking whether the "direct" way of quantizing "pure gravity" is the right approach. Of course that this is not the right approach, according to virtually all particle physicists and string theorists. General relativity, as a quantum theory, is just an effective theory that works at long distances, but breaks down at short distances, and no field redefinition or shaping of the wooden earphones is able to change the fact that quantized GR is "incomplete in the UV". This makes the whole paragraph irrelevant.

A question for Wikipedia is whether the viewpoint - that gravity should still be quantized "directly" - should be given a lot of attention. It is certainly a viewpoint that does not seem to lead anywhere in science. Most scientists in the field "know" that it can't work, even though it is hard to state it as a theorem. Yes, the more you move from the actual scientists via science fans to the laymen, the more they find it plausible that it should be possible to quantize pure gravity directly after all. Well, I still think that Wikipedia should try to prefer the "professional" viewpoint over the misunderstandings of outsiders. If it's true, this paragraph should not assume that "quantizing gravity directly (without any new physics)" is a right approach. --Lumidek 14:45, 4 Nov 2004 (UTC)

Well, Lubos, you may agree with me more than I agree with you. I have no reason to doubt your physics; from the larger context I am inclined to believe it. But at a human level, your explanation is terrible. I said that I respect LQG papers as mathematical physics and their authors as mathematical physicists, but that is not consistent with your narrative at all. Feynman's famous essay refers to sheer crackpots, and not to trained scientists who trip badly in their work. It is not easy to understand string theory, much less to believe it for the proper internal reasons. I believe it largely for external reasons, e.g., that I have never heard of Witten making a big mistake.

In any case you are better at explaining why people don't properly understand string theory than what is really wrong with loop quantum gravity. I have met string theorists who aren't at all adamant that LQG is worthless, much less that it will always be worthless. The most that they will say is that they haven't learned from it. Possibly they are being polite and they really do think that it's worthless. Since popular accounts of string theory are so fashionable, I think that they should do more to counter this apparently Naderite alternative. But it should be more adroit than your severe polemics.

Greg Kuperberg - 24.59.196.30 16:31, 4 Nov 2004 (UTC)

[edit] A new answer to Greg from Lubos

Hey Greg, obviously, I am not gonna agree with anything you wrote above. ;-) Which string theorists are you exactly talking about? Be sure that string theorists such as Witten, Strominger, Vafa, Gross, Polchinski, Susskind, Banks, etc., but also particle physicists like Nima Arkani-Hamed, ... I could continue for a long time, all of them are convinced that loop quantum gravity is rubbish - and they will tell you about it, even though (sometimes) with a more diplomatic language (but sometimes tougher language). Sure, you can find a string theorist - especially if she or he is a young one - who will tell you that (s)he is open-minded about LQG. But sorry, this just proves a lack of experience with the subject.

Concerning Feynman, he had DEFINITELY said a lot of these explicit comments about the "general relativity" community. (Well, he was slightly critical of string theory, too, but it was not as emotional as the LQG-like people.) If you read his books, you would know that Feynman was really angry about these relativists at the conferences, and he asked his wife to remind him that he should never visit another conference about general relativity - exactly because they often like to discuss the religious rubbish about "background independence" and "special role of gravity" WITHOUT TRYING TO MAKE A QUANTITATIVE CONTACT WITH DOABLE EXPERIMENTS, which is something that Feynman could not stand. Feynman definitely thought that these people were lousy scientists, and he was never hiding it.

There are thousands of pages, even on the web, where you can learn about it. For example, open [3]

Feynman has given an amusing account of attending the conference on general relativity and gravitation, in Warsaw in 1962. In a letter to his wife, he said.

I am not getting anything out of the meeting. I am learning nothing. Because there are no experiments, this field is not an active one, so few of the best men are doing work in it. The result is that there are hosts of dopes here (126) and it is not good for my blood pressure. Remind me not to come to any more gravity conferences!

Once again, your statement that Feynman did not say that these people "admiring the exceptional beauty and role of GR" were stupid is not true, and can easily be shown incorrect. And if you had doubts that the way of thinking of the dopes on the 1962 conference was essentially identical to the LQG community today (and there are no real new discoveries either), I can give you references for it, too. --Lumidek 15:23, 7 Nov 2004 (UTC)

"On the other hand," Feynman wrote in The Character of Physical Law, "I believe that the theory that space is continuous is wrong, because we get these infinities and other difficulties, and we are left with questions on what determines the size of all the particles. I rather suspect that the simple ideas of geometry, extended down into infinitely small spaces, are wrong."

A brief comment:

"Both have been highly successful and there are no known phenomena that contradict the two" is incorrect. Quantum entanglement contradicts quantum mechanics in that information is supposedly not able to travel faster than the speed of light, yet this is exactly what happens.

Whoever wrote this nonsense, has not signed his or her contribution, so I hope that others will know that it is irrelevant. Quantum entanglement is a successfully verified prediction of quantum mechanics, and saying that they "contradict each other" is simply a stupidity. No real information is propagating in these experiments. The outcomes are correlated, but it does not require any propagation of real information superluminally. [4] --Lumidek 15:37, 7 Nov 2004 (UTC)

Although I don't really understand string theory, I do study quantum information theory and I can comment on this. If you attempt to stick to the non-quantum model of information, it is not only that it can travel faster than light in quantum mechanics. Rather, quantum mechanics overthrows the old notion of information entirely. So you have to properly redefine information in quantum mechanics. Once you do that, the plain answer is that quantum entanglement does not send any quantum information faster than light. It does allow classically impossible things (like quantum secrecy and quantum computation), but faster-than-light communication is not one of those things.

- Greg Kuperberg - 24.59.196.30 22:13, 8 Nov 2004 (UTC)

As a young scientist there is one thing I know for certain about theoretical physics. I know that theories that are not falsifiable are not science. As far as I know String theory and loop gravity are very weak in that regard.

The above is an example of a neutral statement. Study it.

--HFarmer 03:21, 11 Jan 2005 (UTC)

If memory serves, there were a few experiments that placed bounds on variations of string theory/m theory and LQG that _could_ be correct. For string theory, there were searches for dimensions that had curled to a size large enough to be detected (as opposed to planck-scale), and for LQG someone had searched for the effects of space quantization on the propagation of photons from very distant sources. Both searches turned up empty, but the point is that at least a _few_ experiments were done.

That having been said, I'm staying away from touching any of the articles until it starts looking less like the plasma cosmology flame war. --Christopher Thomas 00:49, 28 Feb 2005 (UTC)

Yes, let us think about this: Science and flame war. Both sides' objectives are the same--explaining how the world works. As far as I can tell as an aerospace engineering outsider (particle physics make my brain hurt), my suggestion is that we just have one section each on LQG and ST ideas on this and leave the rebuttals to a minimum. I very much doubt the average Wiki person wanting to learn more about quantum gravity -really- wants his head blown off by petty factions. That being said, I've removed the apparently aggrivating suggestion of "proto-theories" and completely removed the latter section of the LQG/ST debate, as that was already mentioned in the history. --The Centipede, 12 Apr 2005

[edit] 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)

[edit] Quantum Gravity from Quantum Computation?

Can anyone make sense of this? I found a scientific paper online that describes how quantum gravity can arise out of quantum information and quantum computation... but the physics is a little over my head. Here is the link:

http://arxiv.org/abs/quant-ph/0501135

I can't even begin to comprehend how you could show that the unification of the four forces of nature arises out of quantum computation. That's like saying that when my computer calculates something it causes gravitation.

Having just read the opening of the cited paper here's what I think is being suggested.

First: There is the seemingly reasonable assertion:

If quantum gravity is a discrete, local quantum theory
then it can also be expressed as a quantum computation.

This is essentially asserting Turing equivalence between quantum processes and quantum computations.

Alan Turing's primary concern was with what it means to "compute" something. He was able to demonstrate that anything that can be computed in finite time can be computed on his hypothetical machine in finite time, and visa versa. Thus, all computers and programming languages are equivalent in the sense that they can perform any finite computation in finite time. This is not to say they are equally efficient or effective.

Second: The author, Seth Lloyd, is asserting (implicity) if he can demonstrate a quantum computation that verifiably models gravity, then this program/computation in itself is a demonstration of quantum gravity.

Third: Lloyd asserts he has a model that satisfies the Einstein-Regge equations and make appropriate predictions about known phenomina such as black holes.

I have neither the mathematics not physics to understand if he has even provided a good start on the theory. However, as a computer scientist familiar with finite state machines I recognize Lloyd's method and it's inherant validity

There is a standard technique for deriving a deterministic finite-state machine from a non-deterministic machine. The technique is like branching universes. One performs the computation one step at a time generating additional parallel comutations each time indeterminacy is encountered. For a finite computation at each point in the computation there are at most a finite number of posible states. The discrete machine is is created by constructing a state corresponding to each collection of states. Again, the number of these states might be huge, but it is finite. The computation is now remapped onto the new collection of states.

One way to view the universe is as an indeterminate finite state machine. That is, there is a smallest unit of time and the state of the universe at any point in time is finite. The standard method given above can be ammended to account for the statistical distubtion of the possible subsequent states. The result is still a system that can be represented as a deterministic finite state machine. The composite states represent either future posibilities fanning out from a given state as well as the past histories converging on the current state.

Physicist Steven Wolfram has explored this world view extensively in his controversial book A New Kind of Science. The book is a comprehensive study of how complexity equivalent to the universe can arise from extremely simply finite-state systems. Steven Lloyd's attempt to model/explain quantum gravity as a quantum computation seems an obvious consequence of the work of Turing and Wolfram. So long as his model's predictions can be scientifically tested, this method of zeroing in on quantum gravity is no more or less valid than other technique.

--TheWolfGirl 05:54, 16 November 2005 (UTC)

[edit] Gravity

My perception is that gravity is simply the following.

I treat the whole universe as a closed system consisting of two basic Quantum’s that facilitate Gravity.

One Quantum is all the Mass or Energy The other is Time.

If work or change is necessary and the system is to remain at an equilibrium state, one quantum must divide and avail itself to the other quantum “Time”

Time must therefore accrue.

Now if all Mass sheds to Time and Time accrues this quantum of Mass, it’s increasing quantum applies its force of quantum onto all mass including its own quantum of mass and all respective fields or surfaces, it is important to keep in mind Times Quantum or mass is subject to times original quantum of force.

If two fields of charges or mass are close to each other they are forced together because the force is not as effective between them by Times quantum.

Why don’t we see this shrinking? Well we can if we look beyond our local area of mass to where the shedding to time is at a different rate via Redshift and the fact the universe seems to be expanding and at an increasing rate as it should.

of course I could be wrong about Time being the increasing quantum and Dark matter could be the quantum?

== Theories == I understand that HEIM THEORY (hereinafter "HT") is not considered mainstream, but it at least makes as much sense as "process theory." In addition HT has a formula derived from first principles that predicts particle masses. Like LQG, it is a canonical theory that quantizes space time and winds up with a minimum slice of space-time. It just does the quantization in a different way. It has a Wikipedia article, so i propose it should be added under theories. Take Care.--Will314159 21:56, 30 March 2006 (UTC)

[edit] Mendel Sachs' work

I would like to request that an expert discuss Mendel Sachs' (physicist at SUNY) work -- that is, discuss his quaternion formulation of GR that reduces to QM in the linear, flat-space approximation. From the point of view of this perplexed graduate student of physics, Sachs' theory shows remarkable promise, and yet his work is by and large overlooked by the physics community. He has written a number of books on his theory, and as far as I can tell is a highly regarded physicist.

[edit] quantum gravity as causal force of big bang

somehow, this point seems to be missed.

the very definition of quantum gravity here seems to have missed the point. Quantum gravity is that force which was very similar to gravity which drove the initial expansion of the universe. Most theorists believe Quantum gravity quit being an active force in the universe somewhere in the plank epoch (This last i am not sure of.)

http://arstechnica.com/journals/science.ars/2006/4/15/3602

http://prola.aps.org/abstract/PRD/v65/i4/e043508

http://arxiv.org/abs/gr-qc/9510063

This turns out to be almost, but not quite, what the situation is. Quantum gravity refers to the behavior of gravity at length and energy scales where quantum effects are expected to play a major role in its behavior (point masses close to the Planck mass, temperatures where particles have energy close to the Planck energy, and so forth). In the early universe, this is expected to have occurred during the Planck epoch (when the universe was compact and dense enough to be close to the Planck temperature). Nowadays, it's expected that quantum gravity effects will be significant in any complete description of black holes. Some proposed versions of quantum gravity also quantize spacetime in a way that is expected to have noticeable effects on light that propagates an extremely long distance (such as light from galaxies formed very early in the universe's history).

As far as the Big Bang is concerned, what theorists actually tend to say is that our models of what happens are only valid after quantum gravity ceases being important, and also after gravity ceases to be unified with the other fundamental forces. This is expected to be after the Planck epoch, though some unification theories propose that gravity unifies with the other forces at lower temperatures. What effect quantum gravity had on how the universe behaved prior to the threshold time is simply unknown, though most physicists believe that it prevents singularities from occurring in the description. --Christopher Thomas 21:21, 20 April 2006 (UTC)

[edit] 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)

[edit] Tad misleading use of "vacuum"

I don't expect most highschool freshmen that try reading this page to understand the actual meaning of the word "vacuum" when used in this manner -> "In particular, the vacuum, when it exists, is shown to depend on the path of the observer through space-time (see Unruh effect)." Of course, further studying the Unruh effect will explain the matter conveniently, but my personal opinion is that including the modern definition of vacuum in this paragraph shouldn't be too difficult. As I don't want to mess with the article and cause any outrage by simply doing as I like, I'll leave it for others to judge convenient or not. :) 213.161.190.228 07:07, 15 May 2006 (UTC) Henning

[edit] 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.

[edit] Leprechaun???

In Historical Perspective section there is a phrase:

(P.S. and a leprechaun).

What is it? Is it a joke?

--213.142.209.184 11:07, 29 August 2006 (UTC)

[edit] 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

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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 $Ψ$ 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 $\bold{F}$ exerting on the electron is determined.

In my text book of quantum mechanics, the force field $\bold{F}$ 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)

[edit] 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:

[edit] Spin 2 / GRT: collection of articles

As for consistency between spin 2 and GRT, Kraichnan ^[1] shows that such formalisms must necessarily be generally covariant field theories. Gupta ^[2] ^[3] ^[4] 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 ^[5] 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 ^[6] shows that spin 2 GRT can also be obtained from an S-matrix ansatz. Deser ^[7] 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 ^[8]. Deser then finds ^[9] 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 ^[10] for an elaborate discussion, or Weinberg ^[11] 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).

^ Kraichnan, R. H. (1955). "Special-Relativistic Derivation of Generally Covariant Gravitation Theory". Phys. Rev. 98: 1118-1122.
^ Gupta, S. N. (1954). "Gravitation and Electromagnetism". Phys. Rev. 96: 1683-1685.
^ Gupta, S. N. (1957). "Einstein's and Other Theories of Gravitation". Rev. Mod. Phys. 29: 334-336.
^ Gupta, S. N. (1962), “Quantum Theory of Gravitation”, Recent Developments in General Relativity, Pergamon Press, pp. 251-258
^ Thirring, W. E. (1961). "An Alternative Approach to the Theory of Gravitation". Ann. Phys. (NY) 16: 96-117.
^ Weinberg, S. (1965). "Photons and Gravitons in Perturbation Theory: Derivation of Maxwell's and Einstein's Equations". Phys. Rev. 138: B988-B1002.
^ Deser, S. (1970). "Self-Interaction and Gauge Invariance". Gen. Relativ. Gravit. 1: 9-18.
^ Wald, R. (1986). "Spin-two fields and general covariance". Phys. Rev. D 33: 3613-3625.
^ Deser, S. (1987). "Gravity from self-interaction in a curved background". Class. Quantum Grav. 4: L99-L105.
^ Feynman, R. P. (1963). "Quantum Theory of Gravitation". Acta Phys. Pol. 24: 697-722.
^ Weinberg, S. (1979), “Ultraviolet divergences in quantum theories of gravitation”, in Hawking, S. W. & Israel, W., General Relativity: An Einstein Centenary Survey, Cambridge University Press, pp. 790-831

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

[edit] 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)