Most Interesting Open Physics Questions

I make no pretense of originality in the following. This list of what I consider the most interesting and important unanswered physics questions borrows heavily from Warren Siegel, John Baez and Lee Smolin. However, this compilation does introduce one or two new ideas not contained in any of their writings. It is worth noting that nine questions (with multiple subparts) is almost certainly excessive. A greater mind than mine could probably collapse these into fewer questions, and it is likely that identifying the right relations between these questions would in and of itself lead to some sort of breakthrough.

Question 1 – The Standard Model
(a) Are all 61 fundamental particles in the Standard Model truly elementary, or do they have some substructure?  Can the various particles be explained as manifestations of a more fundamental entity? Do quarks or leptons have any substructure, or are they truly elementary?
(b) What is the mechanism of CP violation? Is it explicable entirely within the Standard Model, or is some new force or mechanism required? Related to this, is there more matter than antimatter? If so, why?
(c) Are the measurable dimensionless parameters that characterize Nature calculable in principle or are they merely determined by historical or quantum mechanical accident and incalculable?
(d) Are there exactly three generations of leptons and quarks? Why do the generations have the structure they do?
(e) Why do these particles have the precise masses they do? Or is this an unanswerable question? How do we understand neutrino mass?
(f) Is it possible to calculate masses of hadrons from QCD?

Question 2 – String Theory
(a) Does string theory actually work? Do problems fixed at the perturbative level return with the non-perturbative 11th dimension described by non-renormalizable membrane theory? Is a 10-dimensional perturbation expansion reasonable for an 11-dimensional theory?
(b) Are there any other strings than the D=10(11) and 26 ones, not counting dual theories?
(c) Do the four forces really become unified at sufficiently high energy?
(d) What are the fundamental degrees of freedom of M-theory? Does M-theory describe Nature? Does M-theory give specific predictions about elementary particles? If so, are they correct?

Question 3 - Gravity
(a) What is gravity? Can we merge quantum theory and general relativity to create a quantum gravity?
(b) What, if anything, do gravity waves (now that they have been discovered) teach us about Nature?
(c) Does the graviton exist? If so, is it fundamental?
(d) Can quantum gravity help explain the origin of the Universe?

Question 4 – Cosmology
(a) What happened at or before the Big Bang? Was there really an initial singularity? Does the history of the Universe go back in time forever or only a finite amount? Will the future of the Universe go on forever or not?
(b) Is the Universe infinite in spatial extent? More generally, what is the topology of space?
(c) Why is there an arrow of time? Why is the future so much different from the past?
(d) How can we understand the cosmological horizon problem? Why is the Universe almost, but not quite, homogeneous, on the very largest distance scales? Is this the result of an inflationary epoch? If so, what caused this inflation?
(e) Why does the cosmological constant have the value that it has? Is it zero? How do we reconcile the requirements of cosmology with the predictions of quantum field theory or string theory? Is the cosmological constant actually constant?
(f) What is the real solution to the "dark matter" and "dark energy" problems? Do they actually exist? If so, what are they? If not, how and why is gravity modified on large scales?
(g) Do we live in a false vacuum (i.e., not the lowest possible energy state)?

Question 5 – Black Holes
(a) Do black holes exist? What happens inside a black hole? What do you do with the singularities? Doesn't a singularity signify a breakdown of the theory? Do naked singularities exist, or is the Cosmic Censorship Hypothesis true?
(b) Do black holes evaporate through Hawking radiation? If so, what happens when they radiate completely away?
(c) Has the information paradox really been resolved? Was Hawking right in 1975, or was Hawking right in 2005?

Question 6 - Confinement & the Mass Gap
(a) Does confinement work? Can we calculate the observed linear Regge trajectories and see what happens to the bound states as their excitation energy increases?
(b) Is there a mathematically rigorous formulation of a relativistic quantum field theory describing interacting fields in four dimensions?
(c) Can we rigorously solve the SU(2) Yang-Mills theory in four dimensions so that we can quantitatively predict quark and gluon confinement as well as the existence of a mass gap between the Planck scale and the electroweak unification scale? (This is actually one of the seven Millenium mathematics problems.)
(d) Does QCD predict that quarks and gluons become deconfined and form plasma at high temperature? If so, what are the characteristics of the deconfinement phase transition? Does this really happen in Nature?

Question 7 – Nature’s Dimensionality
(a) Is Nature four-dimensional? If not, then why does Nature appear to have one time and three space dimensions?
(b) Does compactification work? What forces the extra dimensions to hide and prevents them from reappearing? Does compactification destroy predictability? Do the extra dimensions really do anything we couldn't reproduce without them?

Question 8 - Supersymmetry
(a) Is Nature supersymmetric? If so, how is supersymmetry broken? If not, is supersymmetry still useful?
(b) Is fine tuning really that much worse than any other kind of tuning? Are superpartners any worse than non-minimal Higgs?

Question 9 – Quantum Mechanics
How should we think about the foundations of quantum mechanics? What is meant by a "measurement" in quantum mechanics? Does "wavefunction collapse" actually happen as a physical process? If so, how and under what conditions? If not, what happens instead? Can we make sense of the theory as it stands today? If not, can we invent a new theory that does make sense?