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- Thread starter Cody Richeson
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cgk

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bhobba

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Check out:

http://arxiv.org/pdf/gr-qc/9512024v1.pdf

http://arxiv.org/pdf/1209.3511v1.pdf

'One can ﬁnd thousands of statements in the literature to the effect that “general relativity and quantum mechanics are incompatible”. These are completely outdated and no longer relevant. Effective ﬁeld theory shows that general relativity and quantum mechanics work together perfectly normally over a range of scales and curvatures, including those relevant for the world that we see around us. However, effective ﬁeld theories are only valid over some range of scales. General relativity certainly does have problematic issues at extreme scales. There are important problems which the effective ﬁeld theory does not solve because they are beyond its range of validity. However, this means that the issue of quantum gravity is not what we thought it to be. Rather than a fundamental incompatibility of quantum mechanics and gravity, we are in the more familiar situation of needing a more complete theory beyond the range of their combined applicability. The usual marriage of general relativity and quantum mechanics is ﬁne at ordinary energies, but we now seek to uncover the modiﬁcations that must be present in more extreme conditions. This is the modern view of the problem of quantum gravity, and it represents progress over the outdated view of the past.'

The short answer is about the plank scale.

The longer answer is gravity is not incompatible with QM - a perfectly valid quantum theory of gravity exists - it is just not renormalisable, which means a cutoff must be explicitly included, and the cutoff is about the plank scale. The difference between renormalisable and non-renormalisable theories is renormalisable theories have a magical property - what we observe does not depend on the cut-off. We know, QED for example, breaks down well before the plank scale so it really has a cutoff as well but it doesn't need to be explicitly included in the theory because of the magic property it has of renormalisability - the precise value of that cutoff doesn't matter. But the jig is up with gravity.

Thanks

Bill

http://arxiv.org/pdf/gr-qc/9512024v1.pdf

http://arxiv.org/pdf/1209.3511v1.pdf

'One can ﬁnd thousands of statements in the literature to the effect that “general relativity and quantum mechanics are incompatible”. These are completely outdated and no longer relevant. Effective ﬁeld theory shows that general relativity and quantum mechanics work together perfectly normally over a range of scales and curvatures, including those relevant for the world that we see around us. However, effective ﬁeld theories are only valid over some range of scales. General relativity certainly does have problematic issues at extreme scales. There are important problems which the effective ﬁeld theory does not solve because they are beyond its range of validity. However, this means that the issue of quantum gravity is not what we thought it to be. Rather than a fundamental incompatibility of quantum mechanics and gravity, we are in the more familiar situation of needing a more complete theory beyond the range of their combined applicability. The usual marriage of general relativity and quantum mechanics is ﬁne at ordinary energies, but we now seek to uncover the modiﬁcations that must be present in more extreme conditions. This is the modern view of the problem of quantum gravity, and it represents progress over the outdated view of the past.'

The short answer is about the plank scale.

The longer answer is gravity is not incompatible with QM - a perfectly valid quantum theory of gravity exists - it is just not renormalisable, which means a cutoff must be explicitly included, and the cutoff is about the plank scale. The difference between renormalisable and non-renormalisable theories is renormalisable theories have a magical property - what we observe does not depend on the cut-off. We know, QED for example, breaks down well before the plank scale so it really has a cutoff as well but it doesn't need to be explicitly included in the theory because of the magic property it has of renormalisability - the precise value of that cutoff doesn't matter. But the jig is up with gravity.

Thanks

Bill

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bhobba

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Quantum theory does not absolutely guarantee anything has to be quantized. However given a field Lagrangian the fact you can is very telling and allows theories to be developed and predictions made, which is what science is all about.

Feynman may be right, he may be wrong - exactly as it is with all speculation. What we need is a well developed alternate theory that makes predictions that can be tested. And that was pretty much Feynman all over - correspondence with experiment is his bottom line.

Thanks

Bill

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Quantum theorydoes nota[STRIKE]bsolutely[/STRIKE]guaranteeanything has to be quantized.

Bill

Righttttttt !

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DEvens

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http://en.wikipedia.org/wiki/Planck[snip]

The short answer is about the plank scale.

[snips]

the plank scale

Planck, not plank.

Dan

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