Hi All,

I read your essay with great interest. It is rather sobering from my point of view, since emergent spacetime is my own favorite way of thinking about quantum gravity. I have a few comments and questions.

1. Of the usual “discrete” FTS theories that you mention (LQG, CS, CDT), do you have one in mind more than the others in your research program?

2. On a related...

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Hi All,

I read your essay with great interest. It is rather sobering from my point of view, since emergent spacetime is my own favorite way of thinking about quantum gravity. I have a few comments and questions.

1. Of the usual “discrete” FTS theories that you mention (LQG, CS, CDT), do you have one in mind more than the others in your research program?

2. On a related note, I am not quite sure if you are advocating the emergence of spacetime and matter-energy together from a single fundamental structure in the strongest possible sense. On page 2, you express the opinion that one must recover matter along with gravitation from the FTS, and on page 6 you mention the intimate connection between spacetime probes and spacetime itself, but on the other hand you mention “fundamental matter degrees of freedom” on page 9. Are you suggesting that there must be a division of information between “spacetime” and “matter-energy” even at the fundamental scale?

3. Covariance (Lorentz invariance, etc.) plays a major role here. On page 5, you point out the problem of observer-dependence of black-hole formation associated with arbitrary boosts, which is related to DSR and similar noncommutative theories that have “Lorentz invariance issues,” and on page 6 you point out that a certain noncommutative structure associated with your CM calculations is Lorentz invariant. Now it seems to me that various types of “covariance breaking” that arise in QG might be best understood by simply interpreting covariance in a different way. So long as one insists that covariance is about symmetry, there will be problems with spacetime microstructure. But it one interprets covariance as being about something else, like order, these problems need not arise. I explain this further in my essay here On the Foundational Assumptions of Modern Physics.

4. In several places, you mention “correspondence principle-type” considerations involving classical limits, etc. From this point of view one regards “classical spacetime” as a suitable limit of a quantum theory. Feynman’s sum-over-histories method is a complementary viewpoint: here one views “quantum spacetime” as a superposition of classical universes. I am interested to know if you have worked much from this point of view, since that is the approach I prefer.

Thanks for the great read, and I’d be grateful for your responses. Take care,

Ben Dribus

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