The following are some of the unsolved problems in physics. This is an incomplete list of outstanding problems in physics. Some of these problems are theoretical, meaning that existing theories seem incapable of explaining some observed phenomenon or experimental result. 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.
Accretion disc jets: Why do the accretion discs surrounding certain astronomical objects, such as the nuclei of active galaxies, emit radiation jets along their polar axes?
Amorphous solids: What is the nature of the transition between a fluid or regular solid and a glassy phase? What are the physical processes giving rise to the general properties of glasses?
Cosmic inflation: Is the theory of cosmic inflation correct, and if so, what are the details of this epoch? What is the hypothetical inflaton field giving rise to inflation?
Gamma ray bursts: What is the nature of these extraordinarily energetic astronomical objects?
Gravitational waves: Is it possible to construct a device to detect the gravitational waves emitted by, for example, a pair of inspiralling neutron stars? Such a device would be invaluable for observational astronomy.
GZK paradox: Why is it that some cosmic rays appear to possess energies that are impossibly high, given that there are no sufficiently energetic cosmic ray sources near the Earth, and cosmic rays emitted by distant sources should have been absorbed by the cosmic microwave background radiation?
Magnetic monopoles: Are there any particles that carry "magnetic charge", and if so, why are they so difficult to detect?
Pioneer anomaly: What causes the apparent residual Sunward acceleration of the Pioneer spacecraft?
Quantum chromodynamics (QCD) in the non-perturbative regime: The equations of QCD remain unsolved at energy scales relevant for describing atomic nuclei. How does QCD give rise to the physics of nuclei and nuclear constituents?
Quantum computers: Is it possible to construct a practical computer that performs calculations on qubits (quantum bits)?
Quantum gravity: How can the theory of quantum mechanics be merged with the theory of general relativity to produce a so-called "theory of everything"? Does our present understanding of the gravitational force remain correct at microscopic length scales? What verifiable predictions does any theory of quantum gravity make?
Spintronics: Is it possible to construct a practical electronic device that operates on the spin of the electron, rather than its charge?
Standard Model parameters: What gives rise to the Standard Model of particle physics? Why do its particle masses and coupling constants possess the values we have measured? Does the Higgs boson predicted by the model really exist? Why are there 3 classes of particles in the Standard Model? Is the Standard Model reality, a good approximation to reality or fatally flawed?
Supersymmetry: Is supersymmetry a symmetry of Nature? If so, how is supersymmetry broken, and why? Can the new particles predicted by supersymmetry be detected?
Time travel: Is it theoretically possible? Is it possible in practice? If so, what are we to make of the time travel paradoxes (e.g. going back and killing your grandfather and so on)?
Turbulence: Is it possible to make a theoretical model to describe the behavior of a turbulent fluid (in particular, its internal structures)?
