This is a list of some of the great unsolved problems in physics.
During the long-term process of evolving theories according to the scientific method, there is an intermediary phase between two periods of stability where questions remain unanswered and more and more anomalies accumulate to cast doubt on the established theories in search of greater consistency with experiments.
Theory of Everything
Can there be a simple and unified physical theory that can explain all the phenomena of physics? How to put gravitation and general relativity within the framework of a quantum field theory?
Value of the free constants of the standard model
The Standard Model of particle physics contains about twenty constants, the value of which can only be determined by experiment. Some physicists consider that these values should be determined by a truly fundamental theory, and that the origin and reason for the values of these constants is an important unresolved question, prompting further developments in theoretical physics.
Black hole information paradox, evaporation of black holes
Do black holes produce radiation, as theory predicts? Does this radiation contain information about the internal structure of the black hole, as suggested by the gravity-gauge duality, or not, as Hawking’s work suggests? Otherwise, and if black holes can evaporate, what happens to the information they contain (quantum mechanics does not allow the loss of information)? Is there another way to probe the internal structure of black holes – provided that structure exists?
Does nature admit more than four spatio-temporal dimensions? If so, how many are there and what is their size? Are dimensions part of the fundamental properties of the universe or do they result from the laws of physics? Can we observe additional dimensions?
(Projection of a Calabi-Yau topological variety, one of the ways of compacting the additional dimensions proposed by string theory. )
Is the theory of inflation correct and, if so, what is the detailed description of the epoch of inflation? Is there a hypothetical inflation field giving rise to this process?
Are there any physical reasons to expect the existence of other universes, unobservable by nature? For example, are there “alternative stories” in quantum mechanics?
Cosmic censorship and the chronological protection conjecture
Can singularities not hidden behind an event horizon result from realistic initial conditions under which one can prove a version of conjecture called Roger Penrose’s cosmic censorship which proposes that such a naked singularity does not exist? Likewise, closed time curves which appear among the solutions of the equations of general relativity and which represent a possibility of time travel, could they be eliminated by a theory of quantum gravity (which would unite general relativity and mechanics quantum)?
Arrow of time
Why does time have a direction?
Unexplained experimental observations
Origin of ultra-high and extreme energy cosmic rays (ultra-high-energy cosmic ray (UHECR), extreme-energy cosmic ray (EECR))
The measured existence of cosmic rays of energy greater than 1020 electronvolts poses an unresolved problem, as the known mechanisms capable of generating particles of such energies would be very violent and recognizable, and would have to be relatively close to Earth. However, no notable and nearby star has been detected in the direction from which the particles originate.
The source of inertia
There is no single accepted theory that explains the source of inertia. Various notable efforts by physicists like Ernst Mach (Mach principle), Albert Einstein, and Dennis Sciama have all been criticized. Among recent treatments, we can cite the work of C. Johan Masreliez (2006-2009) and Vesselin Petkov (2009).
Curve of rotation of galaxies
Galaxies do not rotate as expected by Kepler’s laws. The main hypotheses to explain this phenomenon involve dark matter.
(Estimated distribution of dark matter and dark energy in the universe. )
Cosmological constant problem
The observed value of the cosmological constant is too low compared to that predicted by quantum field theory.