Sheldon on unsolved problems in physics, based on a pop science article here:
1) are there more than 4 dimensions? (3 space, 1 time).
Why am I so sure?
Because there are only 3 families of neutrinos, leptons, quarks etc. Each additional dimension provides extra degrees of freedom, extra ways to wiggle. Each degree of freedom adds to the energy levels. When there are only 3 energy levels, it is telling you about the dimensionality of space.
A very similar argument can be made with gravity, which drops as the square of the distance, 1/r^2. Why the square? Because in a 3-dimensional space, the surface area of a sphere goes up with the square, and the density on the sphere goes down as the square. Gravity diminishes as the square because it is “filling” a 3-D volume.
So why is everybody trying to get 11 or 21 dimensions? Because of string theory, which is an attempt to unify particle physics by replacing particles with “strings” in 11 dimensions. It didn’t work 50 years ago, and it still doesn’t work.
2) Why isn’t there equal amounts of matter and antimatter in the universe? Because the Big Bang model is still a kludge. There are several non-conserved quantities that have never been properly incorporated into the Big Bang model.
3) How much does a neutrino weigh?
Well, its measured to be about 15eV right now, give or take 10 eV. But not enough to provide the dark matter. Which is why everyone is disappointed and asks questions they already know the answer to.
4) Is the proton immortal?
The Japanese built an enormous tank of water with phototubes all around it to detect the decay of just one proton. After 10 years of looking they haven’t seen it. They did detect neutrinos arriving from a supernovae however, so all wasn’t wasted. But the SuperKamiokande detector pretty much seals the fate of “proton-decay” theories. So why are we still asking?
Because if the proton decayed, it would breathe new life into “Steady-State” universe models which have no beginning, as well as “Super-Symmetry” models of dark matter. Theorists never lost faith in Plato’s eternal matter, or as Max Planck put it, “Physics progresses one funeral at a time.”
5) what causes sonoluminescence?
Again, the phenomenon was measured 20 years ago and explained soon after. Why is this question coming up again? Because it might fund cold fusion experiments–or so they said about 5 years ago. Did it cause cold fusion? No, and I’m surprised people are still asking about it.
6) “collapse of the wavefunction”
This is classified under “Foundations of QM”. The math is straightforward, its the metaphysics that is contested. As I understand it, this “collapse of the wavefunction” is a bad metaphor for the the coherence of measurement. One bad metaphor, and the metaphysical speculation runs rampant.
You heard it here first. In conclusion, we can categorize all our favorite “unsolved mysteries”, but what such lists actually show is our ideological preferences that prevent us from accepting “No” for an answer.
See also: The Science Fictions series at your fingertips (cosmology).