# List of unsolved problems in physics

{{#invoke:main|main}} 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 into broad area of physics.[1]

### 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 extremely distant place?[2]
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?[3]
Electroweak horizon problem
Why aren't there obvious large-scale discontinuities in the electroweak vacuum if distant parts of the observable universe were causally separate when the electroweak epoch ended? Standard cosmological inflation models have inflation cease well before electroweak symmetry breaking occurs, so it is not at all clear how inflation could prevent such discontinuities.[4]
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 directly detected?[5][6]
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 the identity of dark matter?[3] Is it a particle? Is it the lightest superpartner (LSP)? Do the phenomena attributed to dark matter point not to some form of matter but actually to an extension of gravity?
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
Is a non-spherically symmetric gravitational pull from outside the observable Universe responsible for some of the observed motion of large objects such as galactic clusters in 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)?[7] 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 observe 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?[8] 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?
Locality
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

A simulation 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.)[9]
Proton decay and spin crisis
Is the proton fundamentally stable? Or does it decay with a finite lifetime as predicted by some extensions to the standard model?[10] How do the quarks and gluons carry the spin of protons?[11]
Supersymmetry
Is spacetime supersymmetry realized at TeV scale? If so, what is the mechanism of supersymmetry breaking? Does supersymmetry stabilize the electroweak scale, preventing high quantum corrections? Does the lightest supersymmetric particle (LSP) comprise dark matter?
Generations of matter
Why are there three generations of quarks and leptons? 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,[12] 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?[13][14][15]
Asymptotic freedom
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?
Anomalous magnetic dipole moment
Why is the experimentally measured value of the muon's anomalous magnetic dipole moment ("muon g-2") significantly different from the theoretically predicted value of that physical constant?[16]
Proton size puzzle
What is the true charge radius of the proton?

### Astronomy and astrophysics

File:Galaxies AGN Inner-Structure-of.jpg
Relativistic jet. The environment around the AGN where the relativistic plasma is collimated into jets which escape along the pole of the supermassive black hole
Astrophysical jet
Why do the accretion discs surrounding certain astronomical objects, such as the nuclei of active galaxies, emit relativistic jets along their polar axes?[17] Why are there quasi-periodic oscillations in many accretion discs?[18] Why does the period of these oscillations scale as the inverse of the mass of the central object?[19] Why are there sometimes overtones, and why do these appear at different frequency ratios in different objects?[20]
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?[12]
Supermassive black holes
What is the origin of the M-sigma relation between supermassive black hole mass and galaxy velocity dispersion?[21] How did the most distant quasars grow their supermassive black holes up to 109 solar masses so early in the history of the Universe?
Rotation curve of a typical spiral galaxy: predicted (A) and observed (B). Can the discrepancy between the curves be attributed to dark matter?
Kuiper cliff
Why does the number of objects in the Solar System's Kuiper belt fall off rapidly and unexpectedly beyond a radius of 50 astronomic units?
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?
Supernovae
What is the exact mechanism by which an implosion of a dying star becomes an explosion?
Ultra-high-energy cosmic ray
[3] 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?[3][12]
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?[22]
Origin of magnetar magnetic field
What is the origin of magnetar magnetic field?
Photon underproduction crisis
Why do galaxies and quasars produce about 5 times less ultraviolet light than expected in the low-redshift universe?
Space roar
Why is space roar six times louder than expected? What is the source of space roar?
Age-metallicity relation in the Galactic disk
Is there a universal age-metallicity relation (AMR) in the Galactic disk (thin and thick disk)? While in the local (primarily thin) disk there is no evidence of a strong AMR,[23] a sample of 229 nearby thick disk stars has been used to investigate the existence of an age-metallicity relation in the Galactic thick disk. The results indicate that there is indeed an age-metallicity relation present in the thick disk.[24][25]
The lithium problem
Why is there a discrepancy between the amount of lithium-7 predicted to be produced in Big Bang nucleosynthesis and the amount observed in very old stars?[26]
Solar wind interaction with comets
In 2007 the Ulysses spacecraft passed through the tail of comet C/2006 P1 (McNaught) and found surprising results concerning the interaction of the solar wind with the tail.
Ultraluminous pulsar
The ultraluminous X-ray source M82 X-2 was thought to be a black hole, but in October 2014 data from NASA's space-based X-ray telescope NuStar indicated that M82 X-2 is a pulsar many times brighter than the Eddington limit.

### Nuclear physics

The "island of stability" in the proton vs. neutron number plot for heavy nuclei
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 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? Do glueballs exist? Do gluons acquire mass dynamically despite having a zero rest mass, within hadrons? Does QCD truly lack CP-violations?
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 patternsTemplate:Which 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?
Plasma Physics and Fusion Power
Fusion energy may potentially provide power from abundant resource (e.g. Hydrogen) without the type of radioactive waste that fission energy currently produces. However, can ionized gases (plasma) be confined long enough and at a high enough temperature to create fusion power? What kinds of advances in material science must be made?

### Atomic, molecular and optical physics

Hydrogen atom
What is the solution to the Schrödinger equation for the hydrogen atom in arbitrary electric and magnetic fields?[27]
Muonic hydrogen
Is the radius of muonic hydrogen inconsistent with the radius of ordinary hydrogen?
Laser cooling
Can molecules be cooled down by laser cooling as which has been done to atoms?
Abraham–Minkowski controversy
What's the momentum of photons in optical media?

### Condensed matter physics

A sample of a cuprate superconductor (specifically BSCCO). The mechanism for superconductivity of these materials is unknown.
High-temperature superconductors
What is the mechanism that causes certain materials to exhibit superconductivity at temperatures much higher than around 25 kelvin? Is it possible to make a material that is a superconductor at room temperature?[12]
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?[28][29]
Cryogenic electron emission
Why does the electron emission in the absence of light increase as the temperature of a photomultiplier is decreased?[30][31]
Sonoluminescence
What causes the emission of short bursts of light from imploding bubbles in a liquid when excited by sound?[32]Template:Update after
Turbulence
Is it possible to make a theoretical model to describe the statistics of a turbulent flow (in particular, its internal structures)?[12] Also, under what conditions do smooth solutions to the Navier–Stokes equations exist? This problem is also listed as one of the Millennium Prize Problems in mathematics. 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.[33]
Topological order
Is topological order stable at non-zero temperature? Equivalently, is it possible to have three-dimensional self-correcting quantum memory?[34]
Fractional Hall effect
What mechanism explains the existence of the ${\displaystyle \nu =5/2}$ state in the fractional quantum Hall effect? Does it describe quasiparticles with non-Abelian fractional statistics?[35]
Bose–Einstein condensation
How do we rigorously prove the existence of Bose–Einstein condensates for general interacting systems?[36]
Magnetoresistance in a ${\displaystyle \nu =8/5}$ fractional quantum Hall state.
Liquid crystals
Can the nematic to smectic (A) phase transition in liquid crystal states be characterized as a universal phase transition?[37][38]
Semiconductor nanocrystals
What is the cause of the nonparabolicity of the energy-size dependence for the lowest optical absorption transition of quantum dots?[39]
Electronic band structure
Why can band gaps not accurately be calculated?

### Biophysics

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?
Homochirality
What is the origin of the preponderance of specific enantiomers in biochemical systems?

### 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?[12] 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? Why does time have a direction?
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? Unlike classical physical processes, some quantum mechanical processes (such as quantum teleportation arising from quantum entanglement) cannot be simultaneously "local", "causal" and "real", but it is not obvious which of these properties must be sacrificed or if an attempt to describe quantum mechanical processes in these senses is a category error that doesn't even make sense to talk about if one properly understands quantum mechanics.
Theory of everything ("Grand Unification Theory")
Is there a theory which explains the values of all fundamental physical constants?[12] Is the theory string theory? Is there a theory which explains why the gauge groups of the standard model are as they are, why observed space-time has 3 spatial dimensions and 1 dimension of time, 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.
Physical information
Are there physical phenomena, such as wave function collapse or black holes, which irrevocably destroy information about their prior states? How is quantum information stored as a state of a quantum system?
Quantum Computation
Is David Deutsch's notion of a universal quantum computer sufficient to efficiently simulate an arbitrary physical system?[40]
Dimensionless physical constant
At the present time, the values of the dimensionless physical constants cannot be calculated; they are determined only by physical measurement.[41] What is the minimum number of dimensionless physical constants from which all other dimensionless physical constants can be derived? Are dimensionful physical constants necessary at all?

## Problems solved in recent decades

Ball lightning (2014)
In January 2014, scientists from Northwest Normal University in Lanzhou, China, published the results of recordings made in July 2012 of the optical spectrum of what was thought to be natural ball lightning made during the study of ordinary cloud–ground lightning on China's Qinghai Plateau.[42][43] At a distance of Template:Convert, a total of 1.3 seconds of digital video of the ball lightning and its spectrum was made, from the formation of the ball lightning after the ordinary lightning struck the ground, up to the optical decay of the phenomenon. The recorded ball lightning is believed to be vaporized soil elements that then rapidly oxidizes in the atmosphere. The nature of the true theory is still not clear.[43]
Hipparcos anomaly (2012)
The actual distance to the Pleiades - the High Precision Parallax Collecting Satellite (Hipparcos) measured the parallax of the Pleiades and determined a distance of 385 light years. This was significantly different from other measurements made by means of actual to apparent brightness measurement or absolute magnitude. The anomaly was due to the use of a weighted mean when there is a correlation between distances and distance errors for stars in clusters. It is resolved by using an unweighted mean. There is no systematic bias in the Hipparcos data when it comes to star clusters.[44]
Pioneer anomaly (2012)
There was a deviation in the predicted accelerations of the Pioneer spacecraft as they left the Solar System.[3][12] It is believed that this is a result of previously unaccounted-for thermal recoil force.[45][46]
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. (This assertion is being contested. Justification has been posted on Age Crisis page).
Quasars (1980s)
The nature of quasars was not understood for decades.[47] 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.[48]

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