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p/s: I don't have any facts to support my question, so this is possibly one of my illogical thought

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p/s: I don't have any facts to support my question, so this is possibly one of my illogical thought

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Some approaches to quantum gravity, such as loop quantum gravity, postulate that spacetime itself is quantized, this tentative theory being background independent (string theories for example are background dependent, that is say nothing about the structure of spacetime itself other that it is continuous). Sure the work is still in progress and some still argue that LQG is a dead end, yet I see no good reason for such a strong claim. We can never be sure of what the future has in store for us. The most prominent figure pursuing this research programm is Lee Smolin, if you are interested you can find a review of the actual 'state of affairs' on the field at:

http://arxiv.org/PS_cache/hep-th/pdf/0303/0303185.pdf [Broken]

and Carlo Rovelli's account at:

http://relativity.livingreviews.org/Articles/lrr-1998-1/ [Broken]

http://arxiv.org/PS_cache/hep-th/pdf/0303/0303185.pdf [Broken]

and Carlo Rovelli's account at:

http://relativity.livingreviews.org/Articles/lrr-1998-1/ [Broken]

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That's the area of Loop Quantum Gravity : the quantization of space time. i wrote a nice introduction on LQG here : https://www.physicsforums.com/showthread.php?t=54167scilover89 said:

p/s: I don't have any facts to support my question, so this is possibly one of my illogical thought

I suggest you read and try to understand it before starting on the Smolin-paper. You should also check out the website of Carlo Rovelli. Just google for his name.

marlon

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You are mixing this here : LQG is fundamentally different from String Theory. In LQG you hold on to general relativity and try to rewrite QFT so that space time becomes quantized. In String Theory you hold on to QFT and you rewrite general relativity. It is up to you what the best choice is...jackle said:

Gravitons arise in QFT actually. You know that particles are the excitations (vibrations) of fields (which are continuous). If you take a massive spin 2 field and you distrube it by a massive object (a source and a sink, if you will) this field will start to vibrate and therefore yield a spin two particle called the graviton. The disturbance is expressed by the stress energy tensor from general relativity.

The reason that this field must be spin two has to do with the fact that it has 5 degrees of freedom. The stress energy tensor is a symmetric Lorentz tensor with ten degrees of freedom but this field theory will eliminate 5 degrees of freedom so that only five remain...

regards

marlon

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Hurkyl

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Yes, it does...though we need to be clear on what is meant here :Hurkyl said:

I refer to page 31 of Rovelli's book : http://www.cpt.univ-mrs.fr/~rovelli/book.pdf

Quantization of space-time means that starting from the volume-operator you can prove that it's spectrum is discrete, expressing these little grains of space time. Space time (the overall manifold that is) itself is no continuum anymore.

Besides, the most common misconception of LQG is the fact that many think this theory is an attempt for a TOE, which is NOT the case.

marlon

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Hurkyl

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Well, this is the case, each 'quantumnumber' denotes an infinitesimal grain of space volume. as another reference i urge you to read page five of this marvellous article :Hurkyl said:

http://cgpg.gravity.psu.edu/people/Ashtekar/articles/rovelli03.pdf

Or

https://www.physicsforums.com/showthread.php?p=505486#post505486

marlon

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In really simple terms then, does this mean that if LQG is correct then gravitons are not likely to exist, but if string theory is correct then gravity can not always be treated as the curvature of space-time?marlon said:You are mixing this here : LQG is fundamentally different from String Theory. In LQG you hold on to general relativity and try to rewrite QFT so that space time becomes quantized. In String Theory you hold on to QFT and you rewrite general relativity. It is up to you what the best choice is...

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No, gravity can always be treated as curvature of spacetime. Both string theory and LQG do not state the general relativity is wrong. It is NOT.jackle said:In really simple terms then, does this mean that if LQG is correct then gravitons are not likely to exist, but if string theory is correct then gravity can not always be treated as the curvature of space-time?

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marlon

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Well, first of all you need to understand very well what virtual particles are. I have explained this many times so i refer to the 'virtual particles'-entry in my journal...jackle said:

Also, the curvature of space time is NOT related to virtual particles because the latter arise from QFT and the former arises from general relativity. Both theories are fundamentally different in nature and even very much contradictory, though BOTH correct in their physical regime

regards

marlon

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Sorry, i disagree,selfAdjoint said:I think marlon overspoke a bit.

I specifically talked about physical regime.

I find this hard to believe though i am not denying that Weinberg actually said that (i don't know).And some physicists have certainly denied the truth of gravity from curved spacetime, for instance Nobel winner Steven Weinberg, who called it a metaphor that was out of date.

What exactly does he means by these words.

I mean, curved spacetime produces correct experimental data so let us not start 'semi-philosophical' discussions as to whether general relativity is right or wrong. We have seen these discussions many times in the QM-section and they never brought anything worthy. Why ? Well, because QM is NOT wrong. Same thing here...

regards

marlon