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.
Theory of everything: Is there a singular, all-encompassing, coherent theoretical framework of physics that fully explains and links together all physical aspects of the universe?<ref>"The Unknown Universe: The Origin of the Universe, Quantum Gravity, Wormholes, and Other Things Science Still Can't Explain." Proceedings of the Royal Society of London, Series A, 1 May 2008.</ref>
Dimensionless physical constants: At the present time, the values of various dimensionless physical constants cannot be calculated; they can be determined only by physical measurement.<ref>Template:Cite web</ref><ref name="Effelsberg">Template:Cite journal</ref> What is the minimum number of dimensionless physical constants from which all other dimensionless physical constants can be derived? Are dimensional physical constants necessary at all?<ref>Alcohol constrains physical constant in the early universe." Science, 13 December 2012.</ref>
Quantum gravity: Can quantum mechanics and general relativity be realized as a fully consistent theory (perhaps as a quantum field theory)?<ref>Template:Cite news</ref> 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 mechanism?
Black holes, black hole information paradox, and black hole radiation: Do black holes produce thermal radiation, as expected on theoretical grounds?<ref name=":2">Template:Cite journal</ref> 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 (since 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?
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?<ref>Template:Cite news</ref> 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?
Holographic principle: Is it true that quantum gravity admits a lower-dimensional description that does not contain gravity? A well-understood example of holography is the AdS/CFT correspondence in string theory. Similarly, can quantum gravity in a de Sitter space be understood using dS/CFT correspondence? Can the AdS/CFT correspondence be vastly generalized to the gauge–gravity duality for arbitrary asymptotic spacetime backgrounds? Are there other theories of quantum gravity other than string theory that admit a holographic description?
Quantum spacetime or the emergence of spacetime: Is the nature of spacetime at the Planck scale very different from the continuous classical dynamical spacetime that exists in General relativity? In loop quantum gravity, the spacetime is postulated to be discrete from the beginning. In string theory, although originally spacetime was considered just like in General relativity (with the only difference being supersymmetry), recent research building upon the Ryu–Takayanagi conjecture has taught that spacetime in string theory is emergent by using quantum information theoretic concepts such as entanglement entropy in the AdS/CFT correspondence.<ref>Template:Cite journal</ref> However, how exactly the familiar classical spacetime emerges within string theory or the AdS/CFT correspondence is still not well understood.
Problem of time: In quantum mechanics, time is a classical background parameter, and the flow of time is universal and absolute. In general relativity, time is one component of four-dimensional spacetime, and the flow of time changes depending on the curvature of spacetime and the spacetime trajectory of the observer. How can these two concepts of time be reconciled?<ref>Template:Cite book</ref>
Quantum field theory (this is a generalization of the previous problem): Is it possible to construct, in a mathematically rigorous way, a quantum field theory in 4-dimensional spacetime that includes interactions and does not resort to perturbative methods?
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?<ref name=newscientist />
Size of universe: The diameter of the observable universe is about 93 billion light-years, but what is the size of the whole universe? Is the universe infinite?
Matter–antimatter asymmetry Theoretical models suggest that the early universe should have produced equal amounts of matter and antimatter. However, observations indicate no significant primordial antimatter. Understanding the mechanisms that led to this asymmetry is a major unsolved problem in physics.<ref name=PDG-2024>Template:Cite journal</ref>Template:Rp
File:DMPie 2013.svgEstimated distribution of dark matter and dark energy in the universe
Dark matter: What is the identity of dark matter?<ref name="newscientist">Template:Cite news</ref> Is it a particle? If so, is it a WIMP, axion, the lightest superpartner (LSP), or some other particle? Or, are the phenomena attributed to dark matter the result of an alternate theory of gravity separate from general relativity altogether? Despite extensive research, the exact composition of dark matter remains unknown. It is inferred from gravitational effects on visible matter, radiation, and the universe's large-scale structure. Understanding its properties is crucial for a comprehensive understanding of the universe.
Dark energy: What is the cause of the observed accelerating expansion of the universe (the de Sitter phase)? Are the observations rightly interpreted as the accelerating expansion of the universe, or are they evidence that the cosmological principle is false?<ref name="Ellis 2009"/><ref name="Colin et al"/> 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?
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. Is the shape unmeasurable; the Poincaré space; or another 3-manifold?Template:Cn
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?Template:Cn
Neutron lifetime puzzle: While the neutron lifetime has been studied for decades, there currently exists a lack of consilience on its exact value, due to different results from two experimental methods ("bottle" versus "beam").<ref name=Wolchover-20180213>Template:Cite magazine</ref>
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?<ref name=li-proton2011>Template:Cite journal</ref> How do the quarks and gluons carry the spin of protons?<ref name=hansson2010>Template:Cite journal</ref>
Grand Unification: Are the electromagnetic and nuclear forces different aspects of a Grand Unified Theory? If so, what symmetry governs this force and its behaviours?<ref>Template:Cite journal</ref>
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?
File:Quark-gluon-plasma.jpgColour Confinement is the observed phenomenon that colored particles (quarks and gluons) cannot be isolated and are always bound to color neutral groups (at low energies). Such bound states are generally called hadrons.
The QCD vacuum: Many of the equations in non-perturbative QCD are currently unsolved. These energies are the energies sufficient for the formation and description of atomic nuclei. How thus does low energy /non-pertubative QCD give rise to the formation of complex nuclei and nuclear constituents?Template:Citation needed
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)?<ref name=BlumhoferHutter1997>
Reactor antineutrino anomaly: There is an anomaly in the existing body of data regarding the antineutrino flux from nuclear reactors around the world. Measured values of this flux appears to be only 94% of the value expected from theory.<ref>Template:Cite journal</ref> It is unknown whether this is due to unknown physics (such as sterile neutrinos), experimental error in the measurements, or errors in the theoretical flux calculations.<ref>Template:Cite journal</ref>
Pentaquarks and other exotic hadrons: What combinations of quarks are possible? Why were pentaquarks so difficult to discover?<ref name=NewScientist2003>Template:Cite web</ref> Are they a tightly bound system of five elementary particles, or a more weakly-bound pairing of a baryon and a meson?<ref name=NewScientist2015>Template:Cite news</ref>
Mu problem: A problem in supersymmetric theories, concerned with understanding the reasons for parameter values of the theory.
Koide formula: An aspect of the problem of particle generations. The sum of the masses of the three charged leptons, divided by the square of the sum of the roots of these masses, to within one standard deviation of observations, is Template:Math. It is unknown how such a simple value comes about, and why it is the exact arithmetic average of the possible extreme values of Template:Sfrac (equal masses) and 1 (one mass dominates).
Strange Matter: Does Strange Matter exist? Is it stable? Can they form Strange Stars? Is strange matter stable at 0 pressure (i.e in the vacuum)?
The gallium anomaly: The measurements of the charged-current capture rate of neutrinos on Ga from strong radioactive sources have yielded results below those expected, based on the known strength of the principal transition supplemented by theory.<ref>Template:Cite journal</ref>
Solar cycle: How does the Sun generate its periodically reversing large-scale magnetic field? How do other solar-like stars generate their magnetic fields, and what are the similarities and differences between stellar activity cycles and that of the Sun?<ref>Template:Cite journal</ref> What caused the Maunder Minimum and other grand minima, and how does the solar cycle recover from a minima state?
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?<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
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 astronomical units?
Flyby anomaly: Why is the observed energy of satellites flying by planetary bodies 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 centre of galaxies, or is it something else? File:GalacticRotation2.svgRotation curve of a typical spiral galaxy: predicted (A) and observed (B). Can the discrepancy between the curves be attributed to dark matter?
Supernovae: What is the exact mechanism by which an implosion of a dying star becomes an explosion?
p-nuclei: What astrophysical process is responsible for the nucleogenesis of these rare isotopes?
Ultra-high-energy cosmic ray:<ref name="newscientist" /> 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? Why is it that (apparently) some cosmic rays emitted by distant sources have energies above the Greisen–Zatsepin–Kuzmin limit?<ref name="Open Questions">Template:Cite web</ref><ref name="newscientist" />
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?<ref>Template:Cite web</ref>
Age–metallicity relation in the Galactic disk: Is there a universal age–metallicity relation (AMR) in the Galactic disk (both "thin" and "thick" parts of the disk)? Although in the local (primarily thin) disk of the Milky Way there is no evidence of a strong AMR,<ref>Template:Cite journal</ref> 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, and indicate that there is an age–metallicity relation present in the thick disk.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Stellar ages from asteroseismology confirm the lack of any strong age–metallicity relation in the Galactic disc.<ref>Template:Cite journal</ref>
Fast radio bursts (FRBs): What causes these transient radio pulses from distant galaxies, lasting only a few milliseconds each? Why do some FRBs repeat at unpredictable intervals, but most do not? Dozens of models have been proposed, but none have been widely accepted.<ref>Template:Cite journal</ref>
Origin of Cosmic Magnetic Fields
Observations reveal that magnetic fields are present throughout the universe, from galaxies to galaxy clusters. However, the mechanisms that generated these large-scale cosmic magnetic fields remain unclear. Understanding their origin is a significant unsolved problem in astrophysics.<ref>"Origin of Cosmic Magnetic Fields." *Nature Astronomy*, https://www.nature.com/articles/s41550-019-0862-0.</ref>
Quantum chromodynamics: What are the phases of strongly interacting matter, and what roles do they play in the evolution of the cosmos? What is the detailed partonic structure of the nucleons? What does QCD predict for the properties of strongly interacting matter? What determines the key features of QCD, and what is their relation to the nature of gravity and spacetime? Does QCD truly lack CP violations?
Quark–gluon plasma: Where is the onset of deconfinement: 1) as a function of temperature and chemical potentials? 2) as a function of relativistic heavy-ion collision energy and system size? What is the mechanism of energy and baryon-number stopping leading to creation of quark-gluon plasma in relativistic heavy-ion collisions? Why is sudden hadronization and the statistical-hadronization model a near-to-perfect description of hadron production from quark–gluon plasma? Is quark flavor conserved in quark–gluon plasma? Are strangeness and charm in chemical equilibrium in quark–gluon plasma? Does strangeness in quark–gluon plasma flow at the same speed as up and down quark flavours? Why does deconfined matter show ideal flow?
Nuclei and nuclear astrophysics: Why is there a lack of convergence in estimates of the mean lifetime of a free neutron based on two separate—and increasingly precise—experimental methods? What is the nature of the nuclear force that binds protons and neutrons into stable nuclei and rare isotopes? What is the explanation for the EMC effect? 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? What is the heaviest possible chemical element?
Turbulent flow: Is it possible to make a theoretical model to describe the statistics of a turbulent flow (in particular, its internal structures)?<ref name="Open Questions" />
Granular convection: why does a granular material subjected to shaking or vibration exhibit circulation patterns similar to types of fluid convection? Why do the largest particles end up on the surface of a granular material containing a mixture of variously sized objects when subjected to a vibration/shaking?<ref name="AransonTsimring2006Review">Template:Cite journal</ref>
Bose–Einstein condensation: How do we rigorously prove the existence of Bose–Einstein condensates for general interacting systems?<ref name="schlein-BEC">Template:Cite web</ref>
Universality of low-temperature amorphous solids: why is the small dimensionless ratio of the phonon wavelength to its mean free path nearly the same for a very large family of disordered solids?<ref name="Pohl2002">Template:Cite journal</ref><ref name="Leggett1991">Template:Cite journal</ref> This small ratio is observed for very large range of phonon frequencies.
Metal whiskering: In electrical devices, some metallic surfaces may spontaneously grow fine metallic whiskers, which can lead to equipment failures. While compressive mechanical stress is known to encourage whisker formation, the growth mechanism has yet to be determined.
Temperature: Can quantum computing be performed at non-cryogenic temperatures? Can we build room temperature quantum computers?<ref>Template:Cite web</ref>
Complexity classes problems: What is the relation of BQP and BPP? What is the relation between BQP and NP? Can computation in plausible physical theories (quantum algorithms) go beyond BQP?<ref name=":4" />
Post-quantum cryptography: Can we prove that some cryptographic protocols are safe against quantum computers?<ref name=":4" />
Plasma physics and fusion power: Fusion energy may potentially provide power from an 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 is the physical origin of H-mode?<ref>Template:Cite journal.</ref>
The injection problem: Fermi acceleration is thought to be the primary mechanism that accelerates astrophysical particles to high energy. However, it is unclear what mechanism causes those particles to initially have energies high enough for Fermi acceleration to work on them.<ref>Template:Cite book</ref>
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.
Magnetoreception: How do animals (e.g. migratory birds) sense the Earth's magnetic field?
Protein structure prediction: How is the three-dimensional structure of proteins determined by the one-dimensional amino acid sequence? How can proteins fold on microsecond to second timescales when the number of possible conformations is astronomical and conformational transitions occur on the picosecond to microsecond timescale? Can algorithms be written to predict a protein's three-dimensional structure from its sequence? Do the native structures of most naturally occurring proteins coincide with the global minimum of the free energy in conformational space? Or are most native conformations thermodynamically unstable, but kinetically trapped in metastable states? What keeps the high density of proteins present inside cells from precipitating?<ref name="DillMacCallum2012">Template:Cite journal</ref>
Quantum biology: Can coherence be maintained in biological systems at timeframes long enough to be functionally important? Are there non-trivial aspects of biology or biochemistry that can only be explained by the persistence of coherence as a mechanism?
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?<ref name="Open Questions" /> Another way of stating this question regards the measurement problem: What constitutes a "measurement" which apparently 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,<ref name=":5">Template:Cite book</ref> or if an attempt to describe quantum mechanical processes in these senses is a category error such that a proper understanding of quantum mechanics would render the question meaningless. Can the many worlds interpretation resolve it?
Arrow of time (e.g. entropy's arrow of time): Why does time have a direction? Why did the universe have such low entropy in the past, and time correlates with the universal (but not local) increase in entropy, from the past and to the future, according to the second law of thermodynamics?<ref name="Open Questions" /> 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? What links the quantum arrow of time to the thermodynamic arrow?
Locality: Are there non-local phenomena in quantum physics?<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> 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 elucidate the proper interpretation of the fundamental nature of quantum physics?
Perform a loophole-free Bell test experiment (1970<ref name="Pearle-1970">Template:Citation</ref>–2015): In October 2015, scientists from the Kavli Institute of Nanoscience reported that the failure of the local hidden-variable hypothesis is supported at the 96% confidence level based on a "loophole-free Bell test" study.<ref name="NTR-20151021">Template:Cite journal</ref><ref name="NYT-20151021">Template:Cite news</ref> These results were confirmed by two studies with statistical significance over 5 standard deviations which were published in December 2015.<ref name="PRL115-250401">Template:Cite journal</ref><ref name="PRL115-250402">Template:Cite journal</ref>
Cosmic age problem (1920s–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 the Milky Way. Better estimates for the distances to the stars, and the recognition of the accelerating expansion of the universe, reconciled the age estimates.Template:Citation needed
Existence of quark-gluon plasma, a new phase of matter was discovered and confirmed in experiments at CERN-SPS (2000), BNL-RHIC (2005) and CERN-LHC (2010).<ref name=":0" />
Origin of mass of most elementary particles: Solved with the discovery of the Higgs boson, which implies the existence of the Higgs field giving mass to these particles.
Missing baryon problem (1998<ref>Shull, J. Michael, Britton D. Smith, and Charles W. Danforth. "The baryon census in a multiphase intergalactic medium: 30% of the baryons may still be missing." The Astrophysical Journal 759.1 (2012): 23.</ref>–2017): proclaimed solved in October 2017, with the missing baryons located in hot intergalactic gas.<ref>Template:Cite news</ref><ref>Template:Cite journal</ref>
Long-duration gamma-ray bursts (1993<ref name="ReferenceA" />–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.
Existence of time crystals (2012–2016): The idea of a quantized time crystal was first theorized in 2012 by Frank Wilczek.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> In 2016, Khemani et al.<ref>Template:Cite journal</ref> and Else et al.<ref>Template:Cite journal</ref> independently of each other suggested that periodically driven quantum spin systems could show similar behaviour. Also in 2016, Norman Yao at Berkeley and colleagues proposed a different way to create discrete time crystals in spin systems.<ref name="yao et al 2017">Template:Cite journal</ref> This was then used by two teams, a group led by Christopher Monroe at the University of Maryland and a group led by Mikhail Lukin at Harvard University, who were both able to show evidence for time crystals in the laboratory setting, showing that for short times the systems exhibited the dynamics similar to the predicted one.<ref name="Monroe">Template:Cite journal</ref><ref name="Lukin">Template:Cite journal</ref>
Photon underproduction crisis (2014–2015): This problem was resolved by Khaire and Srianand.<ref>Template:Cite journal</ref> They show that a factor 2 to 5 times large metagalactic photoionization rate can be easily obtained using updated quasar and galaxy observations. Recent observations of quasars indicate that the quasar contribution to ultraviolet photons is a factor of 2 larger than previous estimates. The revised galaxy contribution is a factor of 3 larger. These together solve the crisis.
Hipparcos anomaly (1997<ref>Template:Cite journal</ref>–2012): 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.<ref>Template:Cite journal</ref>
Pioneer anomaly (1980–2012): There was a deviation in the predicted accelerations of the Pioneer 10 and 11 spacecraft as they left the Solar System.<ref name="Open Questions" /><ref name="newscientist" /> It is believed that this is a result of previously unaccounted-for thermal recoil force.<ref>Template:Cite journal</ref><ref>Template:Cite news</ref>
