# 24 Questions for Elementary Physics

This weekend at Caltech we had a small but very fun conference: the “Physics of the Universe Summit,” or POTUS for short. (The acronym is just an accident, I’m assured.) The subject matter was pretty conventional — particle physics, the LHC, dark matter — but the organization was a little more free-flowing and responsive than the usual parade of dusty talks.

One of the motivating ideas that was mentioned more than once was the famous list of important problems proposed by David Hilbert in 1900. These were Hilbert’s personal idea of what math problems were important but solvable over the next 100 years, and his ideas turned out to be relatively influential within twentieth-century mathematics. Our conference, 110 years later and in physics rather than math, was encouraged to think along similarly grandiose lines.

And indeed people had done exactly that, especially ten years ago when the century turned: see representative lists here and here. I asked the organizers if anyone was taking a swing at it this time, and was answered in the negative. I was scheduled to give one of the closing summaries, and this sounded more interesting than what I actually had planned, so naturally I had to step up.

Here are the slides from my presentation, where you can find some elaboration on my choices.

And here’s the actual list:

- What breaks electroweak symmetry?
- What is the ultraviolet extrapolation of the Standard Model?
- Why is there a large hierarchy between the Planck scale, the weak scale, and the vaccum energy?
- How do strongly-interacting degrees of freedom resolve into weakly-interacting ones?
- Is there a pattern/explanation behind the family structure and parameters of the Standard Model?
- What is the phenomenology of the dark sector?
- What symmetries appear in useful descriptions of nature?
- Are there surprises at low masses/energies?
- How does the observable universe evolve?
- How does gravity work on macroscopic scales?
- What is the topology and geometry of spacetime and dynamical degrees of freedom on small scales?
- How does quantum gravity work in the real world?
- Why was the early universe hot, dense, and very smooth but not perfectly smooth?
- What is beyond the observable universe?
- Why is there a low-entropy boundary condition in the past but not the future?
- Why aren’t we fluctuations in de Sitter space?
- How do we compare probabilities for different classes of observers?
- What rules govern the evolution of complex structures?
- Is quantum mechanics correct?
- What happens when wave functions collapse?
- How do we go from the quantum Hamiltonian to a quasiclassical configuration space?
- Is physics deterministic?
- How many bits are required to describe the universe?
- Will “elementary physics” ultimately be finished?

Clearly I cheated somewhat by squeezing multiple questions into single problems. But the real challenge was thinking sufficiently big to come up with problems that people a century from now would agree are interesting. And I stuck to “elementary physics” — particle physics, gravitation, cosmology — just because I’m not competent to pick out the important problems in any other fields. Twenty-four, of course, because Hilbert had 23, and we had to go one better. There was certainly no shortage of candidates; I was coming up with more good problems and throwing out old ones right up until the last minute. Any obvious ones I missed?