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?
• Accelerating universe: Why is the expansion of the universe accelerating, as we have observed? Is our understanding of redshift complete? If it is, then what is the nature of the dark energy driving this acceleration? If it is due to a cosmological constant, why is the constant so small, yet non-zero? Why isn't it huge, as predicted by most quantum field theories, or zero, as predicted by supersymmetry? What is the ultimate fate of the universe?
• 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?
• Arrow of time: Why did the universe have such low entropy in the past, resulting in the distinction between past and future and the second law of thermodynamics?
• 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?
• Dark matter: What is it? How is it generated? Is it related to supersymmetry?
• Electroweak force: Given that the the theory behind the unification of the electromagnetic and weak nuclear forces is based in part on a mathematical trick called "renormalization" (which involves cancelling infinities), what is the reality behind this theory?
• Fusion power: Is it possible to construct a practical nuclear reactor that is powered by nuclear fusion rather than nuclear fission?
• Galaxy rotation problem: Why do galaxies rotate at speeds inconsistent with their apparent mass?
• 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?
• High-temperature superconductors: Why do certain materials exhibit superconductivity at temperatures much higher than 50 kelvins?
• 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?
• Proton decay: Do protons decay? If so, then what is its half-life?
• 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?
• 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? Why are there 3 classes of particles in the Standard Model? Is the Standard Model reality, a good approximation to reality or fatally flawed?
• String theory: Is string theory, or superstring theory, or M-theory, or some other variant on this theme, the final "theory of everything", a step on the road or a blind alley?
• 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)?

See also

• List of unsolved problems

External links

• APS
• Physics News Update A weekly physics news bulletin hosted by the American Institute of Physics.
• Science Frontiers Digests of reports that describe scientific anomalies.
• Open Questions in Physics " The following partial list of open questions is divided into five groups:

1. Condensed Matter and Nonlinear Dynamics
2. Quantum Mechanics
3. Cosmology and Astrophysics
4. Particle Physics
5. The Big Question"

• 13 things that do not make sense.