- #1

MathematicalPhysicist

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does it imply faster than light speed? (i don't think so because lqg attempts to unite between GR and QM).

here's the link:http://www.wikipedia.org/wiki/Loop_quantum_gravity

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- #1

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does it imply faster than light speed? (i don't think so because lqg attempts to unite between GR and QM).

here's the link:http://www.wikipedia.org/wiki/Loop_quantum_gravity

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marcus

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Originally posted by loop quantum gravity

does it imply faster than light speed? (i don't think so because lqg attempts to unite between GR and QM).

here's the link:http://www.wikipedia.org/wiki/Loop_quantum_gravity

This is right. The ongoing experimental tests of LQG and some expected in the near-term future are discussed on pages 18-20

of Smolin's recent review article

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

This is a very interesting section of the survey called

"The near term experimental situation"

The predicted dispersion is so slight that it can only be tested over cosmological distances using very high energy photons so the most promising method is by observing gamma ray bursts (GRB). Smolin gives these references:

G. Amelino-Camelia, John Ellis, et al "Potential Sensitivity of Gamma-Ray Burster Observations to Wave Dispersion in Vacuo

http://arxiv.org/astro-ph/9712103 [Broken] (published in Nature in 1998)

J. P. Norris et al "GLAST, GRBs, and Quantum Gravity"

http://arxiv.org/astro-ph/9912136 [Broken]

John Ellis et al "Quantum Gravity Analysis of Gamma-Ray Bursts using Wavelets"

http://arxiv.org/astro-ph/0210124 [Broken]

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doesnt this dissobey the postulate in relativity that the speed of light is constant?

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marcus

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This is a fascinating area. It is not unreasonable to expect physics at Planck scale (e.g. very high energies, small distances) to diverge from the everyday and classical. But there is a scarcity of ways to test LQG because the Planck scale is so small and so high-energy.

One cannot just build an accelerator to get things up to Planck energy, it is too high!

But by being clever one can nevertheless find ways to test the theory. And already people are doing that! Here is what Smolin says on page 17,

"It turns out that this has consequences for the question of whether special relativity, and lorentz invariance, is exactly true in nature, or is only an approximation which holds on scales much longer than the Planck scale [28]-[40]. Several recent calculations...yield predictions for modifications to the energy momentum relations for elementary particles. These are of the form,

E

where predictions have been found for the leading coefficients α, which generally depend on spin and helicity [36]-[38]..."

The GRB observations are trying to find a bound on this α parameter.

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marcus

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Originally posted by loop quantum gravity

doesnt this dissobey the postulate in relativity that the speed of light is constant?

It certainly does. I would assume that only the low-energy limit is constant. This is true about several basic constant---there is divergence from them at very high energy or at very small scale.

The postulates of special relativity are not sacred and

even in general relativity already one finds that special relativity is just a very good local approximation.

so there are some very tiny divergences which it looks like it will be possible to test maybe even within the next 5 years

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marcus

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When I quoted from Smolin just now I left out some units, Planck length

"It turns out that this has consequences for the question of whether special relativity, and lorentz invariance, is exactly true in nature, or is only an approximation which holds on scales much longer than the Planck scale [28]-[40]. Several recent calculations...yield predictions for modifications to the energy momentum relations for elementary particles. These are of the form,

E

where predictions have been found for the leading coefficients α, which generally depend on spin and helicity [36]-[38]..."

I gather that the predicted values of alpha are no larger than order one. So since alpha gets multiplied by Planck length, in the energy momentum relation, and Planck length is around 10

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