Wikipedia maintains a list of unsolved problems in physics.
From that list I picked the ones I consider as key questions. In the second section I remind readers about more fundamental questions that are not included in the Wikipedia list.
My selection of key questions from Wikipedia’s list
“Why are there three generations of quarks and leptons?”
“Is there a theory that can explain the masses of particular quarks and leptons in particular generations from first principles?”
“Why is there far more matter than antimatter in the observable universe?”
“Why is the experimentally measured value of the muon’s anomalous magnetic dipole moment (“muon g−2″) significantly different from the theoretically predicted value of that physical constant?”
“Are there non-local phenomena in quantum physics? If they exist, are non-local phenomena limited to the entanglement revealed in the violations of the Bell inequalities, or can information and conserved quantities also move in a non-local way? Under what circumstances are non-local phenomena observed? What does the existence or absence of non-local phenomena imply about the fundamental structure of spacetime? How does this relate to quantum entanglement? How does this elucidate the proper interpretation of the fundamental nature of quantum physics?”
“Is spacetime fundamentally continuous or discrete? Would a consistent theory involve a force mediated by a hypothetical graviton, or be a product of a discrete structure of spacetime itself (as in loop quantum gravity)?”
“Does nature have more than four spacetime dimensions? If so, what is their size? Are dimensions a fundamental property of the universe or an emergent result of other physical laws? Can we experimentally observe evidence of higher spatial dimensions?”
“Do black holes produce thermal radiation, as expected on theoretical grounds? Does this radiation contain information about their inner structure, as suggested by gauge–gravity duality, or not, as implied by Hawking’s original calculation? If not, and black holes can evaporate away, what happens to the information stored in them (since quantum mechanics does not provide for the destruction of information)? Or does the radiation stop at some point leaving black hole remnants? Is there another way to probe their internal structure somehow, if such a structure even exists?”
“Why does the number of objects in the Solar System’s Kuiper belt fall off rapidly and unexpectedly beyond a radius of 50 astronomical units?”
“Why is the observed energy of satellites flying by Earth sometimes different by a minute amount from the value predicted by theory?”
“Is dark matter responsible for differences in observed and theoretical speed of stars revolving around the centre of galaxies, or is it something else?”
Let’s not forget about these more fundamental questions
We still don’t know the answers to these questions:
- What is electrical charge?
- What is an electron?
- What is quantum-mechanical spin?
- Neutrinos are left-chiral. There are no right-chiral neutrinos. Why?
- Why is it that the weak-nuclear-force acts on the left-chiral fermions only?
In December 2011, I posted “Key Questions in Particle Physics.” Those questions are still open.
Here’s another list from UC Riverside.
Sergey Troitsky’s selection of unsolved problems in particle physics is explained in this arxiv paper .