gravity “knows about” and encodes its own quantum states -> this meets the page curve and solves the black hole information paradox

(http://www.autoadmit.com/thread.php?thread_id=5859514&forum_id=2).#49837017)

When I look at Gravity the two things that stand out to me are the cosmological constant and the metric tensor. The cosmological constant is suspect because it was hand inserted then later removed then put back in again and it can be put on either the geometric side or mass-energy side of the equation. The thing thats interests me about the metric tensor is that it seems to be doing too much and assuming that "geometry" is the fundamental ontology and not just an effective representation. One thing especially suspect with the metric tensor is minimal coupling. It is hand inserted. It says matter fields couple to gravity only through the metric, no direct curvature coupling which is elegant but not derived. It is possible that the metric really is fundamental and gravity really is geomoetry. Or it's just a coincidence at the scale we can measure. I have more to say about this but don't feel like writing it all. A lot of it basically comes down to that there are a lot of points where geometry/smooth continuous manifolds describing "warped" space and time are suspect as fundamental vs. effective representation but basically at every leakage point we aren't quite there in terms of having the instruments to detect deviations that would prove it

(http://www.autoadmit.com/thread.php?thread_id=5859514&forum_id=2).#49842746)

I even e-mailed Stephen Benton (RIP) at the MIT Media Lab when I was 15. He e-mailed me back and gave me some pointers.

(http://www.autoadmit.com/thread.php?thread_id=5859514&forum_id=2).#49843053)

A few points are worth separating.

First, gravitational waves are fundamentally different from sound or electromagnetic waves in a material medium. In General Relativity, a gravitational wave is an oscillation of the spacetime metric itself. There is no underlying "stuff" whose density determines the propagation speed. The wave is a solution to Einstein's field equations, and in vacuum it propagates at the speed of light.

That naturally leads to your question: if there is no true vacuum in Quantum Field Theory, why doesn't the quantum vacuum behave like a medium?

The answer is subtle. The quantum vacuum is not "empty," but neither is it a classical medium like air or water. It is the lowest-energy state of quantum fields. It has vacuum fluctuations and can have measurable effects (for example, the Casimir effect), but it does not define a preferred rest frame. That Lorentz invariance is precisely what distinguishes it from the old luminiferous ether.

You can think of it this way: (i) Air has a velocity; (ii) Water has a velocity; (iii) The quantum vacuum has no velocity.

Every inertial observer describes themselves as being at rest relative to the vacuum, which is impossible for an ordinary material medium.

That removes the observational problems of the ether, but as you point out, it doesn't necessarily satisfy the philosophical desire for an underlying ontology.

Your criticism of holography is also one many physicists acknowledge.

(http://www.autoadmit.com/thread.php?thread_id=5859514&forum_id=2).#49963568)

I think a successful quantum gravity theory shouldn't merely say "geometry emerges from entanglement" or "holography", unless it can specify which degrees of freedom the entanglement are between, and why the resulting structure should have that specific Lorrentzian signature, rather than some other geometry.

(http://www.autoadmit.com/thread.php?thread_id=5859514&forum_id=2).#49963583)

General relativity is an extraordinarily successful model. It tells you that the dynamical variable is the metric tensor g_{\mu\nu}, and that gravitational waves are propagating perturbations of that metric. But the theory is deliberately silent on what the metric is. Einstein himself was fairly pragmatic about this: He largely abandoned attempts to assign a mechanical substrate to spacetime after the failure of the classical ether concept.

Your electromagnetic analogy is exactly right. Maxwell's equations describe an electromagnetic field F_{\mu\nu} (aka the Faraday tensor). Quantum electrodynamics says the field is quantized, and photons are its excitations. But if you ask what the electromagnetic field is actually made of, you will be disappointed: QED has no answer (the field is simply taken as fundamental) and GR is in the same position.

Your point is that a quantum gravity theory should do more than replace one primitive with another. My concern is ultimately ontological, not mathematical. General relativity tells us that gravitational waves are perturbations of the spacetime metric, just as electromagnetism describes waves in the electromagnetic field, but neither theory explains what the metric or the field actually are. Similarly, statements like "spacetime emerges from entanglement" or "the bulk is encoded on the boundary" seem to relocate the mystery rather than resolve it. A satisfactory theory of quantum gravity should identify the underlying degrees of freedom, explain why they exist, and show why they necessarily give rise to Lorentzian spacetime and the laws of physics we observe, rather than treating those structures as new unexplained primitives.

(http://www.autoadmit.com/thread.php?thread_id=5859514&forum_id=2).#49963601)

If you have his two-volume work on twistors, I'm surprised you'd say something like "reality is not all that complicated." The issue is that reality is different at that scale than that in which we live.

(http://www.autoadmit.com/thread.php?thread_id=5859514&forum_id=2).#49844875)