Talk:Quantum gravity/Archive for 2005

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 [1]

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)

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"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."

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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. [2] --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)

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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

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.

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

First: There is the seemingly reasonable assertion:

1. If quantum gravity is a discrete, local quantum theory
2. 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)