Unsolved problems in physics

The following 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 microscopic physics giving rise to the general properties of glasses?
Fusion power. Is it possible to construct a practical nuclear reactor that is powered by the nuclear fusion rather than nuclear fission?
Galaxy rotation problem. Why do galaxies rotate at speeds inconsistent with their apparent mass?
Gamma ray bursters. 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?
High-temperature superconductors. Why do certain materials exhibit superconductivity at temperatures much higher than 20K?
Magnetic monopoles. Are there any particles that carry "magnetic charge", and if so, why are they so difficult to detect?
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? Does our present understanding of the gravitational force remain correct at microscopic length scales?
Quantum mechanics in the correspondence limit. Is there a preferred interpretation of quantum mechanics? How does the quantum description of reality, which includes elements such as the superposition of states and wavefunction collapse, give rise to the reality we perceive?
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?
Supersymmetry. Is supersymmetry a symmetry of Nature? If so, how is supersymmetry broken, and why?
Theory of Everything -does it exist, how does it relate to everything, and how does it effect us?
Time Travel. Is it possible?
Turbulence. Is it possible to make a theoretical model to describe the behavior of a turbulent fluid (in particular, its internal structures)?
Why are we here?