Holographic principle: Is it true that quantum gravity admits a lower-dimensional description that does not contain gravity? A well understood example of holography is the AdS/CFT correspondence in string theory.

The cosmic censorship hypothesis and the chronology protection conjecture: Can singularities not hidden behind an event horizon, known as ' naked singularities', arise from realistic initial conditions, or is it possible to prove some version of the 'cosmic censorship hypothesis' of Roger Penrose which proposes that this is impossible? Similarly, will the closed timelike curves which arise in some solutions to the equations of general relativity (and which imply the possibility of backwards time travel) be ruled out by a theory of quantum gravity which unites general relativity with quantum mechanics, as suggested by the 'chronology protection conjecture' of Stephen Hawking?.

Black holes, black hole information paradox, and black hole radiation: 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?.Vacuum catastrophe: Why does the predicted mass of the quantum vacuum have little effect on the expansion of the universe?.Quantum gravity: Can quantum mechanics and general relativity be realized as a fully consistent theory (perhaps as a quantum field theory)? 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)? Are there deviations from the predictions of general relativity at very small or very large scales or in other extreme circumstances that flow from a quantum gravity mechanism?.What is the minimum number of dimensionless physical constants from which all other dimensionless physical constants can be derived? Are dimensional physical constants necessary at all? Dimensionless physical constants: At the present time, the values of various dimensionless physical constants cannot be calculated they can be determined only by physical measurement.Theory of everything: Is there a singular, all-encompassing, coherent theoretical framework of physics that fully explains and links together all physical aspects of the universe?.Another problem lies within the mathematical framework of the Standard Model itself-the Standard Model is inconsistent with that of general relativity, to the point that one or both theories break down under certain conditions (for example within known spacetime singularities like the Big Bang and the centres of black holes beyond the event horizon). There are still some questions beyond the Standard Model of physics, such as the strong CP problem, neutrino mass, matter–antimatter asymmetry, and the nature of dark matter and dark energy. The others are experimental, meaning that there is a difficulty in creating an experiment to test a proposed theory or investigate a phenomenon in greater detail. Some of the major unsolved problems in physics are theoretical, meaning that existing theories seem incapable of explaining a certain observed phenomenon or experimental result.

The following is a list of notable unsolved problems grouped into broad areas of physics.

You can help by adding missing items with reliable sources. This is a dynamic list and may never be able to satisfy particular standards for completeness.

Physics phenomena