This isn’t the place to discuss “a genuine emergence of classical spacetime geometry from something more fundamental”. I wish you would persuade your colleague and mentor t’Hooft to put a review paper reviewing unorthodox work on the arXiv.org. Until there is a precedent by someone famous, they’ll delete everything submitted that looks crackpot.

…..maybe very theraputic, if someone could finally answer him?

Having said this, I should also say that my own view is that it is not likely that LQG, CDT or anything else now on the table will simply be the right thing. I believe these are all models, necessary steps from which we learn how to do non-trivial calculations in background independent, diffeomorphism invariant quantum field theories. As we gain control over them we are beginning to use the new language and tools gained to address not only quantum gravity but the other big problems such as unification and quantum cosmology. The right theory will solve all of these. And I believe it will do so by featuring a genuine emergence of classical spacetime geometry from something more fundamental.

best,

Moshe

Back to physics, very theraputic…

The comment of continuity was just an intuitive comment, not really necessarily an EFT argument. Basically the spectrum of the string is continuous as a function of the background fields (w/o changing the GSO projection), so string scale tachyon cannot just pop up when you turn on arbitrarily small background fields. In any event, I assure you there are also more complete calculations, that was just an argument why this had to be the case. Incidentally, even for static backgrounds there are non-SUSY classically stable cases, but they are quantum mechanically unstable in the sense that they don’t stay static after including quantum corrections.

As for the last paragraph, I mentioned that in the context of backgrounds which develope singularities, and then we don’t have a criteria for judging which background lift to the full quantum theory. My bet is that weakly curved backgrounds should be fine, but one cannot know for sure yet. (incidentally, is it always true that for time dependent background singularity thm. dictate there generaically exists future or past singulairty?)

best,

Moshe

Thanks very much for your #137. This is helpful, but can I query you on one point, where you say that on “…backgrounds that are almost flat, then just by continuity the only thing one has to worry about is the spectrum of modes that would have been zero modes in flat space. So my statement is that weakly curved solutions of string theory have tachyons if and only if the corresponding GR+matter solution is stable. So string theory at the classical level is as stable or unstable as the corresponding field theory it includes.”

Can you fill in a few details of this argument? It seems to me that given that the cancellation of the tachyon involves a projection to a smaller state space (the GSO projection) and given that the projection implies spacetime supersymmetry which is broken as soon as there is any time dependence, you have to show that you can continue to impose a projection that eliminates a tachyon for any small non-supersymmetric deformation of the background geometry. Has this been done? If not, I don’t think you can use an effective field theory argument that assumes that the tachyon is absent from the deformed theory.

On a related point, you say, “It is certainly not clear that a generic classical solution of any theory, especially containing gravity, should lift to a full solution of the quantum theory.” But for the classical theory to be recovered as the low energy limit, must it not be that every solution to the classical theory that is weakly curved on the Planck scale must lift to a coherent state in the quantum theory?

Thanks, Lee

Will string theorist’s burn in hell?

Some might use religion to distort the “empowerment” we all have? Place it outside of ourselves? While I say, it is quite feasible that such delusions, would have been sought to fool the public, by being anti-stringy?

Is it a plot un-becoming? If one could have a delusion about it’s inception, then why not the reverse of it’s recognition? Claim it to be a positive thing about our comprehension about it’s negative impact?

Have I followed the proper logic?