- How can the apparent disagreement between general relativity and quantum gravity be resolved? Does it require to quantize gravity?
(Still 1. Haven't changed my mind about that.)
- Can we understand quantization?
(Up from 9. The more I think about it, the more I believe our problem with quantizing gravity is in the quantum part, not in the gravity part.)
Do black holes destroy information?What happens to the matter that collapses to a black hole?
(I think we spent enough time on the black hole information loss problem. It would be fruitful to instead think about what happens to matter at Planckian densities. Down from 2.)
- Are the electroweak and strong interaction unified at high energies? If so, are they also unified with gravity?
(That is, is there a theory of everything? Up from 8. I'm undecided whether or not unification is helpful to the problem of quantizing gravity.)
- Are the currently known particles of the standard model (SM) elementary? Are there more so far unobserved particles? Why are the parameters of the SM what they are and are they in yet unknown ways related to each other? Why are the gauge groups of the SM what they are? Is it even possible to uniquely answer this question?
(Formerly 8, minus the question for unification plus the question whether there's a unique answer.)
- Did the universe start with a big bang, a big bounce or something else entirely?
(This is a reformulation of the earlier question number 4 which focused on singularities specifically. Down from 3.)
- Why do we experience 3+1 dimensions? Are there extra dimensions? Does the effective dimension of space-time decrease at short distances?
(This is an extension of the earlier question 7, taking into account that dimensional reduction to 2 dimensions has received some attention recently.)
- Why is gravity so much weaker than the other interactions?
(Up from 10.)
- Does dark energy exist? If so, what is it? Is the coincidence problem more than a coincidence?
(Down from 4. I think that the dark energy puzzle is possibly a relevant hint for quantum gravity. But then, maybe not.)
- How do we correctly assign an entropy to gravitational degrees of freedom? Is this testable at all?
Thursday, May 10, 2012
This is a repost and update of a six year old post in which I listed what I think are the most interesting and pressing open problems in theoretical physics, or at least the area that I work in, quantum gravity. I thought it might be worthwhile to revisit. This list doesn't even pretend to be objective - it omits entire areas of theoretical physics - it is mainly a reflection of my personal interests; a summary of puzzles I find promising to spend brain time on.