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 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.
Unsolved problems by subfield 
The following is a list of unsolved problems grouped by the broad area of physics into which they fall.
Cosmology, and general relativity 
- 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? If inflation happened at one point, is it self-sustaining through inflation of quantum-mechanical fluctuations, and thus ongoing in some impossibly distant place?
- Horizon problem
- Why is the distant universe so homogeneous, when the Big Bang theory seems to predict larger measurable anisotropies of the night sky than those observed? Cosmological inflation is generally accepted as the solution, but are other possible explanations such as a variable speed of light more appropriate?
- Future of the universe
- Is the universe heading towards a Big Freeze, a Big Rip, a Big Crunch or a Big Bounce? Or is it part of an infinitely recurring cyclic model?
- Gravitational wave
- Can gravitational waves be detected experimentally?
- Baryon asymmetry
- Why is there far more matter than antimatter in the observable universe?
- Cosmological constant problem
- Why does the zero-point energy of the vacuum not cause a large cosmological constant? What cancels it out?
Estimated distribution of dark matter and dark energy in the universe
- Dark matter
- What is dark matter? Is it related to supersymmetry? Is the mass that makes up the dark matter halos around galaxies made up of the lightest form of supersymmetric particle (LSP)? Do the phenomena attributed to dark matter point not to some form of matter but actually to an extension of gravity?
The log-log plot of dark energy density
and material density
Vs scale factor
. The two straight lines intersect at current epoch.
- Dark energy
- What is the cause of the observed accelerated expansion (de Sitter phase) of the Universe? Why is the energy density of the dark energy component of the same magnitude as the density of matter at present when the two evolve quite differently over time; could it be simply that we are observing at exactly the right time? Is dark energy a pure cosmological constant, or are models of quintessence such as phantom energy applicable?
- Dark flow
- What is the cause of a large swath of galaxy clusters all moving towards one part of the universe?
- Ecliptic alignment of CMB anisotropy
- Some large features of the microwave sky, at distances of over 13 billion light years, appear to be aligned with both the motion and orientation of the Solar System. Is this due to systematic errors in processing, contamination of results by local effects, or an unexplained violation of the Copernican principle?
- Shape of the Universe
- What is the 3-manifold of comoving space, i.e., of a comoving spatial section of the Universe, informally called the "shape" of the Universe? Neither the curvature nor the topology is presently known, though the curvature is known to be "close" to zero on observable scales. The cosmic inflation hypothesis suggests that the shape of the Universe may be unmeasurable, but since 2003, Jean-Pierre Luminet et al. and other groups have suggested that the shape of the Universe may be the Poincaré dodecahedral space. Is the shape unmeasurable, the Poincaré space, or another 3-manifold?
Quantum gravity 
- 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 theory?
- 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 (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?
- Extra dimensions
- Does nature have more than four spacetime dimensions? If so, what is their size? Are dimensions a fundamental property of the universe or an emergent result of other physical laws? Can we experimentally "see" evidence of higher spatial dimensions?
- 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?
- Are there non-local phenomena in quantum physics? If they exist, are non-local phenomena limited to the entanglement revealed in the violations of the Bell Inequalities, or can information and conserved quantities also move in a non-local way? Under what circumstances are non-local phenomena observed? What does the existence or absence of non-local phenomena imply about the fundamental structure of spacetime? How does this relate to quantum entanglement? How does this elucidate the proper interpretation of the fundamental nature of quantum physics?
High energy physics/particle physics 
of how a detection of the Higgs particle would appear in the CMS
detector at CERN
- Higgs mechanism
- Are the branching ratios of the Higgs Boson consistent with the standard model? Is there only one type of Higgs Boson?
- Hierarchy problem
- Why is gravity such a weak force? It becomes strong for particles only at the Planck scale, around 1019 GeV, much above the electroweak scale (100 GeV, the energy scale dominating physics at low energies). Why are these scales so different from each other? What prevents quantities at the electroweak scale, such as the Higgs boson mass, from getting quantum corrections on the order of the Planck scale? Is the solution supersymmetry, extra dimensions, or just anthropic fine-tuning?
- Magnetic monopoles
- Did particles that carry "magnetic charge" exist in some past, higher energy epoch? If so, do any remain today? (Paul Dirac showed the existence of some types of magnetic monopoles would explain charge quantization.)
- Proton decay and spin crisis
- Is the proton a fundamentally stable particle? Or does it have a finite lifetime as predicted by some extensions to the standard model? What explains the anomalous spin of protons?
- Is spacetime supersymmetry realized in nature? If so, what is the mechanism of supersymmetry breaking? Does supersymmetry stabilize the electroweak scale, preventing high quantum corrections? Does the lightest supersymmetric particle comprise dark matter?
- Generations of matter
- Are there more than three generations of quarks and leptons? Why are there generations at all? Is there a theory that can explain the masses of particular quarks and leptons in particular generations from first principles (a theory of Yukawa couplings)?
- Electroweak symmetry breaking
- What is the mechanism responsible for breaking the electroweak gauge symmetry, giving mass to the W and Z bosons? Is it the simple Higgs mechanism of the Standard Model, or does nature make use of strong dynamics in breaking electroweak symmetry, as proposed by Technicolor?
- Neutrino mass
- What is the mass of neutrinos, whether they follow Dirac or Majorana statistics? Is mass hierarchy normal or inverted? Is the CP violating phase 0?
- Why has there never been measured a free quark or gluon, but only objects that are built out of them, like mesons and baryons? How does this phenomenon emerge from QCD?
- Strong CP problem and axions
- Why is the strong nuclear interaction invariant to parity and charge conjugation? Is Peccei–Quinn theory the solution to this problem?
Astronomy and astrophysics 
- Accretion disc jets
- Why do the accretion discs surrounding certain astronomical objects, such as the nuclei of active galaxies, emit relativistic jets along their polar axes? Why are there quasi-periodic oscillations in many accretion discs? Why does the period of these oscillations scale as the inverse of the mass of the central object? Why are there sometimes overtones, and why do these appear at different frequency ratios in different objects?
- Coronal heating problem
- Why is the Sun's Corona (atmosphere layer) so much hotter than the Sun's surface? Why is the magnetic reconnection effect many orders of magnitude faster than predicted by standard models?
- Diffuse interstellar bands
- What is responsible for the numerous interstellar absorption lines detected in astronomical spectra? Are they molecular in origin, and if so which molecules are responsible for them? How do they form?
- Gamma ray bursts
- How do these short-duration high-intensity bursts originate?
- Supermassive black holes
- What is the origin of the M-sigma relation between supermassive black hole mass and galaxy velocity dispersion?
- Observational anomalies
Rotation curve of a typical spiral galaxy: predicted (A) and observed (B). Can the discrepancy between the curves be attributed to dark matter?
- Hipparcos anomaly: What is the actual distance to the Pleiades?
- Flyby anomaly: Why is the observed energy of satellites flying by Earth sometimes different by a minute amount from the value predicted by theory?
- Galaxy rotation problem: Is dark matter responsible for differences in observed and theoretical speed of stars revolving around the center of galaxies, or is it something else?
- What is the exact mechanism by which an implosion of a dying star becomes an explosion?
- Ultra-high-energy cosmic ray
-  Why is it that some cosmic rays appear to possess energies that are impossibly high (the so-called OMG particle), given that there are no sufficiently energetic cosmic ray sources near the Earth? Why is it that (apparently) some cosmic rays emitted by distant sources have energies above the Greisen–Zatsepin–Kuzmin limit?
- Rotation rate of Saturn
- Why does the magnetosphere of Saturn exhibit a (slowly changing) periodicity close to that at which the planet's clouds rotate? What is the true rotation rate of Saturn's deep interior?
- Origin of magnetar magnetic field
- What is the origin of magnetar magnetic field?
- Space roar
- Why is space roar six times louder than expected? What is the source of space roar?
Nuclear physics 
- Quantum chromodynamics
- What are the phases of strongly interacting matter, and what roles do they play in the cosmos? What is the internal landscape of the nucleons? What does QCD predict for the properties of strongly interacting matter? What governs the transition of quarks and gluons into pions and nucleons? What is the role of gluons and gluon self-interactions in nucleons and nuclei? What determines the key features of QCD, and what is their relation to the nature of gravity and spacetime?
- Nuclei and Nuclear astrophysics
- What is the nature of the nuclear force that binds protons and neutrons into stable nuclei and rare isotopes? What is the origin of simple patterns in complex nuclei? What is the nature of exotic excitations in nuclei at the frontiers of stability and their role in stellar processes? What is the nature of neutron stars and dense nuclear matter? What is the origin of the elements in the cosmos? What are the nuclear reactions that drive stars and stellar explosions?
- Island of stability
- What is the heaviest possible stable or metastable nucleus?.
Atomic, molecular and optical physics 
- Hydrogen atom
- What is the solution to the Schrodinger's equation for the Hydrogen atom in arbitrary electric and magnetic fields?
Condensed matter physics 
A sample of a cuprate
superconductor (specifically BSCCO
). The mechanism for superconductivity of these materials is unknown.
- Amorphous solids
- What is the nature of the glass transition between a fluid or regular solid and a glassy phase? What are the physical processes giving rise to the general properties of glasses and the glass transition?
- Cryogenic electron emission
- Why does the electron emission in the absence of light increase as the temperature of a photomultiplier is decreased?
- High-temperature superconductors
- What is the mechanism that causes certain materials to exhibit superconductivity at temperatures much higher than around 25 kelvin?
- What causes the emission of short bursts of light from imploding bubbles in a liquid when excited by sound?
- Is it possible to make a theoretical model to describe the statistics of a turbulent flow (in particular, its internal structures)? Also, under what conditions do smooth solutions to the Navier–Stokes equations exist? Alfvénic turbulence in the solar wind and the turbulence in solar flares, coronal mass ejections, and magnetospheric substorms are major unsolved problems in space plasma physics.
- Topological order
- Is topological order stable at non-zero temperature? Equivalently, is it possible to have three-dimensional self-correcting quantum memory?
- Fractional Hall effect
- What mechanism explains the existence of the state in the fractional quantum Hall effect? Does it describe quasiparticles with non-Abelian fractional statistics?
- Bose–Einstein condensation
- How do we rigorously prove the existence of Bose–Einstein condensates for general interacting systems?
- Liquid crystals
- Can the nematic to smectic (A) phase transition in liquid crystal states be characterized as a universal phase transition?
- Synaptic plasticity
- It is necessary for computational and physical models of the brain, but what causes it, and what role does it play in higher-order processing outside the hippocampus and visual cortex?
- Axon guidance
- How do axons branching out from neurons find their targets? This process is crucial to nervous system development, allowing the building up of the brain.
- Stochasticity and robustness to noise in gene expression
- How do genes govern our body, withstanding different external pressures and internal stochasticity? Certain models exist for genetic processes, but we are far from understanding the whole picture, in particular in development where gene expression must be tightly regulated.
- Quantitative study of the immune system
- What are the quantitative properties of immune responses? What are the basic building blocks of immune system networks? What roles are played by stochasticity?
Other problems 
- Entropy (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? Why are CP violations observed in certain weak force decays, but not elsewhere? Are CP violations somehow a product of the Second Law of Thermodynamics, or are they a separate arrow of time? Are there exceptions to the principle of causality? Is there a single possible past? Is the present moment physically distinct from the past and future or is it merely an emergent property of consciousness?
- Quantum mechanics in the correspondence limit (sometimes called Quantum chaos)
- 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 or quantum decoherence, give rise to the reality we perceive? Another way of stating this is the Measurement problem – what constitutes a "measurement" which causes the wave function to collapse into a definite state?
- Physical information
- Are there physical phenomena, such as black holes or wave function collapse, which irrevocably destroy information about their prior states?
- Theory of everything ("Grand Unification Theory")
- Is there a theory which explains the values of all fundamental physical constants? Is there a theory which explains why the gauge groups of the standard model are as they are, why observed space-time has 3 + 1 dimensions, and why all laws of physics are as they are? Do "fundamental physical constants" vary over time? Are any of the particles in the standard model of particle physics actually composite particles too tightly bound to observe as such at current experimental energies? Are there fundamental particles that have not yet been observed and if so which ones are they and what are their properties? Are there unobserved fundamental forces implied by a theory that explains other unsolved problems in physics?
- Yang–Mills theory
- Given an arbitrary compact gauge group, does a non-trivial quantum Yang–Mills theory with a finite mass gap exist? This problem is also listed as one of the Millennium Prize Problems in mathematics.
- Quantum Computation
- Is David Deutsch's notion of a universal quantum computer sufficient to efficiently simulate an arbitrary physical system?
- Dimensionless physical constant
- At the present time, the values of the dimensionless physical constants cannot be calculated; they are determined only by physical measurement.
Problems solved in recent decades 
- Pioneer anomaly (2012)
- There was a deviation in the predicted accelerations of the Pioneer spacecraft as they left the Solar System. It is believed that this is a result of previously unaccounted-for thermal recoil force.
- Long-duration gamma ray bursts (2003)
- Long-duration bursts are associated with the deaths of massive stars in a specific kind of supernova-like event commonly referred to as a collapsar. However, there are also long-duration GRBs that show evidence against an associated supernova, such as the Swift event GRB 060614.
- Solar neutrino problem (2002)
- Solved by a new understanding of neutrino physics, requiring a modification of the Standard Model of particle physics—specifically, neutrino oscillation.
- Age Crisis (1990s)
- The estimated age of the universe was around 3 to 8 billion years younger than estimates of the ages of the oldest stars in our galaxy. Better estimates for the distances to the stars, and the recognition of the accelerating expansion of the universe, reconciled the age estimates.
- Quasars (1980s)
- The nature of quasars was not understood for decades. They are now accepted as a type of active galaxy where the enormous energy output results from matter falling into a massive black hole in the center of the galaxy.
- ^ Podolsky, Dmitry. "Top ten open problems in physics". NEQNET. Retrieved 23 April 2012.
- ^ a b c d e Brooks, Michael (March 19, 2005). "13 Things That Do Not Make Sense". New Scientist. Issue 2491. Retrieved March 7, 2011.
- ^ National Research Council (1986). Gravitation, Cosmology, and Cosmic-Ray Physics. Washington, D. C.: National Academies Press. ISBN 0-309-03579-1.
- ^ Paulson, Tom (May 27, 2002). "Catching a cosmic wave of gravity". Seattle Post-Intelligencer. Retrieved 10 April 2012.
- ^ Steinardt, Paul (1997), "Cosmological Challenges For the 21st Century", in Val Fitch et. al., Critical problems in physics: proceedings of a conference celebrating the 250th anniversary of Princeton University, Princeton, New Jersey: Princeton University Press, pp. 138–140, ISBN 978-0-691-05784-2
- ^ "Dark Flow" Discovered at Edge of the Universe: Hundreds of Millions of Stars Racing Towards a Cosmic Hotspot. Dailygalaxy.com (2009-08-26). Retrieved on 2011-10-20.
- ^ Alan Sokal (July 22, 1996), "Don't Pull the String Yet on Superstring Theory", New York Times
- ^ Joshi, Pankaj S. (January 2009), "Do Naked Singularities Break the Rules of Physics?", Scientific American
- ^ Dirac, Paul, "Quantised Singularities in the Electromagnetic Field". Proceedings of the Royal Society A 133, 60 (1931).
- ^ Li, Tianjun; Dimitri V. Nanopoulos, Joel W. Walker (2011). "Elements of F-ast Proton Decay". ArXiv. Retrieved 14 April 2012.
- ^ Hansson, Johan (2010). "The "Proton Spin Crisis" — a Quantum Query". Progress in Physics 3. Retrieved 14 April 2012.
- ^ a b c d e f g h Baez, John C. (March 2006). "Open Questions in Physics". Usenet Physics FAQ. University of California, Riverside: Department of Mathematics. Retrieved March 7, 2011.
- ^ "India-based Neutrino Observatory (INO)". Tata Institute of Fundamental Research. Retrieved 14 April 2012.
- ^ Christianto, Vic; Florentin Smarandache (2007). "Thirty Unsolved Problems in the Physics of Elementary Particles". Progress in Physics 4.
- ^ Nakamura; et al (2010). J. Phys. G 37 (075021) http://pdg.lbl.gov/2011/reviews/contents_sports.html .
- ^ Ferrarese, Laura; Merritt, David (2000), "A Fundamental Relation between Supermassive Black Holes and their Host Galaxies", The Astrophysical Journal 539: L9–L12, arXiv:astro-ph/0006053, Bibcode:2000ApJ...539L...9F, doi:10.1086/312838
- ^ "Scientists Find That Saturn's Rotation Period is a Puzzle". NASA. June 28, 2004. Retrieved 2007-03-22.
- ^ Panel on Atomic, Molecular, and Optical Physics, Physics Survey Committee, Board on Physics and Astronomy, National Research Council (1986). Atomic, Molecular, and Optical Physics. National Academies Press. p. 63. ISBN 9780309594561.
- ^ Kenneth Chang (July 29, 2008), "The Nature of Glass Remains Anything but Clear", The New York Times
- ^ P.W. Anderson (1995), "Through the Glass Lightly", Science 267: 1615, doi:10.1126/science.267.5204.1615-e, "The deepest and most interesting unsolved problem in solid state theory is probably the theory of the nature of glass and the glass transition."
- ^ Cryogenic electron emission phenomenon has no known physics explanation. Physorg.com. Retrieved on 2011-10-20.
- ^ doi:10.1209/0295-5075/89/58001
- ^ Proceedings: Mathematical, physical, and engineering sciences (Royal Society) 453, 1997, "An unsolved problem in modern physics concerns the phenomenon of sonoluminescence"
- ^ Goldstein, Melvyn L. (2001). "Major Unsolved Problems in Space Plasma Physics". Astrophysics and Space Science 277 (1/2): 349–369. Bibcode:2001Ap&SS.277..349G. doi:10.1023/A:1012264131485.
- ^ Yoshida, Beni (2011). "Feasibility of self-correcting quantum memory and thermal stability of topological order". Annals of Physics 326 (10). arXiv:1103.1885. Bibcode:2011AnPhy.326.2566Y. doi:10.1016/j.aop.2011.06.001. Retrieved 8 April 2012.
- ^ Podolsky, Dmitry. "Quantum Hall effect. One open question". NEQNET. Retrieved 23 April 2012.
- ^ Schlein, Benjamin. "Graduate Seminar on Partial Differential Equations in the Sciences - Energy and Dynamics of Boson Systems". Hausdorff Center for Mathematics. Retrieved 23 April 2012.
- ^ Mukherjee, Prabir K. (1998). "Landau Theory of Nematic-Smectic-A Transition in a Liquid Crystal Mixture". Molecular Crystals & Liquid Crystals 312: 157–164. doi:10.1080/10587259808042438. Retrieved 28 April 2012.
- ^ A. Yethiraj, "Recent Experimental Developments at the Nematic to Smectic-A Liquid Crystal Phase Transition", Thermotropic Liquid Crystals: Recent Advances, ed. A. Ramamoorthy, Springer 2007, chapter 8.
- ^ Nielson, Micheal; Chuang, Isaac (2004). http://www.squint.org/qci/QINFO-book-nielsen-and-chuang-toc-and-chapter1-nov00.pdf. Cambridge University Press. ISBN 978-0-521-63503-5.
- ^ "Alcohol constrains Physical Constant in the Early Universe". Max-Planck-Institut für Radioastronomie. December 2012. Retrieved December 15, 2012.
- ^ Turyshev, S.; Toth, V.; Kinsella, G.; Lee, S. C.; Lok, S.; Ellis, J. (2012). "Support for the Thermal Origin of the Pioneer Anomaly". Physical Review Letters 108 (24). arXiv:1204.2507. doi:10.1103/PhysRevLett.108.241101.
- ^ http://www.nytimes.com/2012/07/24/science/mystery-tug-on-pioneer-10-and-11-probes-is-einsteins-i-told-you-so.html?
- ^ "The MKI and the discovery of Quasars". Jodrell Bank Observatory. Retrieved 2006-11-23.
- ^ Hubble Surveys the "Homes" of Quasars Hubblesite News Archive, 1996-35
External links