List of unsolved problems in physics
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.
There are still some deficiencies in the Standard Model of physics, such as the origin of mass, the strong CP problem, neutrino oscillations, matter–antimatter asymmetry, and the nature of dark matter and dark energy. Another problem lies within the mathematical framework of the Standard Model itself—the Standard Model is inconsistent with that of general relativity, to the point that one or both theories break down under certain conditions (for example within known spacetime singularities like the Big Bang and the centers of black holes beyond the event horizon).
Unsolved problems by subfieldEdit
The following is a list of unsolved problems grouped into broad areas of physics.
General physics/quantum physicsEdit
- 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? 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?
- 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 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, 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.
- Grand Unification Theory/Theory of everything: 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, and why observed spacetime has 3 spatial dimensions and 1 temporal dimension? Do "fundamental physical constants" vary over time? Are any of the fundamental 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?
- 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?
- Dimensionless physical constant: At the present time, the values of the dimensionless physical constants cannot be calculated; they are determined only by physical measurement. 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?
- Fine-tuned Universe: The values of the fundamental physical constants are in a narrow range necessary to support carbon-based life. Is this because there exist other universes with different constants, or are our universe’s constants the result of chance, or some other factor or process?
Cosmology and general relativityEdit
- Problem of time: How can time be reconciled with 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?
- 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?
- Origin and future of the universe: How did the conditions for anything to exist arise? 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?
- Size of universe: The diameter of the observable universe is about 93 billion light-years, but what is the size of the whole universe? Does a multiverse exist?
- 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?
- Dark matter/Galaxy rotation curve: What is the identity of dark matter? Is it a particle? Is it the lightest superpartner (LSP)? [Or] 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?
- Axis of evil (cosmology): 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?
- 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 (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?
- 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? 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 physicsEdit
- 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?
- Planck particle: The Planck mass plays an important role in parts of mathematical physics. A series of researchers have suggested the existence of a fundamental particle with mass equal to or close to that of the Planck mass. The Planck mass is however enormous compared to any detected particle even compared to the Higgs particle. While working at the Rutherford Laboratory, Lloyd Motz suggested that such a particle with Planck mass likely had existed but that most of its mass had radiated away. Others have suggested particles with close to the Planck mass are micro black holes. It is still an unsolved problem if there exist or even have existed a particle with close to the Planck mass. This is indirectly related to the hierarchy problem.
- 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 fundamentally stable? Or does it decay with a finite lifetime as predicted by some extensions to the standard model? How do the quarks and gluons carry the spin of protons?
- 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 or Lightest Supersymmetric Particle) 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)?
- 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?
- Colour confinement: Why has there never been measured a free quark or gluon, but only objects that are built out of them, such as 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? Could axions be the main component of dark matter?
- 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?
- Proton radius puzzle: What is the electric charge radius of the proton? How does it differ from gluonic charge?
- Pentaquarks and other exotic hadrons: What combinations of quarks are possible? Why were pentaquarks so difficult to discover? Are they a tightly-bound system of five elementary particles, or a more weakly-bound pairing of a baryon and a meson?
- Mu problem: problem of supersymmetric theories, concerned with understanding the parameters 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 is , to within one standard deviation of observations. It is unknown how such a simple value comes about, and why it is the exact arithmetic average of the possible extreme values of 1/3 (equal masses) and 1 (one mass dominates).
Astronomy and astrophysicsEdit
- Astrophysical jet: Why do only certain accretion discs surrounding certain astronomical objects 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?
- 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?
- Supermassive black holes: What is the origin of the M-sigma relation between supermassive black hole mass and galaxy velocity dispersion? How did the most distant quasars grow their supermassive black holes up to 1010 solar masses so early in the history of the universe?
- 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 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 centre of galaxies, or is it something else?
- 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: 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?
- 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?
- Large-scale anisotropy: Is the universe at very large scales anisotropic, making the cosmological principle an invalid assumption? The number count and intensity dipole anisotropy in radio, NRAO VLA Sky Survey (NVSS) catalogue is inconsistent with the local motion as derived from cosmic microwave background and indicate an intrinsic dipole anisotropy. The same NVSS radio data also shows an intrinsic dipole in polarization density and degree of polarization in the same direction as in number count and intensity. There are other several observation revealing large-scale anisotropy. The optical polarization from quasars shows polarization alignment over a very large scale of Gpc. The cosmic-microwave-background data shows several features of anisotropy, which are not consistent with the Big Bang model.
- 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 (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, 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. Stellar ages from asteroseismology confirm the lack of any strong age-metallicity relation in the Galactic disc.
- 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?
- 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.
- Fast radio bursts: Transient radio pulses lasting only a few milliseconds, from emission regions thought to be no larger than a few hundred kilometres, and estimated to occur several hundred times a day. While several theories have been proposed, there is no generally accepted explanation for them. They may come from cosmological distances, but there is no consensus on this, either.
- Nature of KIC 8462852, commonly known as Tabby's Star: What is the origin of unusual luminosity changes of this star?
- Fermi paradox: Do extraterrestrial civilizations exist? If so, why do we not see them?
- Nature of Wow! signal: Was that a real signal and, if so, what is the origin of it?
- Planetary systems: How does accretion form planetary systems? Where did Earth's water come from?
- Quantum chromodynamics: What are the phases of strongly interacting matter, and what roles do they play in the evolution of 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? Do glueballs exist? Do gluons acquire mass dynamically despite having a zero rest mass, within hadrons? Does QCD truly lack CP-violations? Do gluons saturate when their occupation number is large? Do gluons form a dense system called Colour Glass Condensate? What are the signatures and evidences for the Balitsky-Fadin-Kuarev-Lipatov, Balitsky-Kovchegov, Catani-Ciafaloni-Fiorani-Marchesini evolution equations?
- 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 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?
Atomic, molecular and optical physicsEdit
- Abraham–Minkowski controversy: What is the momentum of light in optical media?
- Bose–Einstein condensation: How do we rigorously prove the existence of Bose–Einstein condensates for general interacting systems?
Condensed matter physicsEdit
- 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?
- 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?
- Sonoluminescence: What causes the emission of short bursts of light from imploding bubbles in a liquid when excited by sound?
- Turbulence: 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? 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.
- 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?
- Liquid crystals: Can the nematic to smectic (A) phase transition in liquid crystal states be characterized as a universal phase transition?
- Semiconductor nanocrystals: What is the cause of the nonparabolicity of the energy-size dependence for the lowest optical absorption transition of quantum dots?
- Whisker (metallurgy): 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.
- 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 is the physical origin of H-mode?
- 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? 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?
- 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.
- 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.
- 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?
- Unified brain processing theory: How to unify physics and neuroscience?
- Homochirality: What is the origin of the preponderance of specific enantiomers in biochemical systems?
Problems solved in recent decadesEdit
- Origin of short gamma-ray burst (1993-2017): From binary neutron stars merger, produce a kilonova explosion and short gamma-ray burst GRB 170817A was detected in both electromagnetic waves and gravitational wave GW170817.
- Missing baryon problem (1998-2017): proclaimed solved in October 2017, with the missing baryons located in hot intergalactic gas.
- Existence of time crystals (2012–2016): In 2016, the idea of time-crystals was proposed by two groups independently Khemani et al. and Else et al. Both of these groups showed that in small systems which are disordered and periodic in time, one can observe the phenomenon of time crystals. Norman Yao et al. extended the calculations for a model (which has the same qualitative features) in the laboratory environment. 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 lab-setting, showing that for short times the systems exhibited the dynamics similar to the predicted one.
- Existence of gravitational waves (1916–2016): On 11 February 2016, the Advanced LIGO team announced that they had directly detected gravitational waves from a pair of black holes merging, which was also the first detection of a stellar binary black hole.
- Perform a loophole-free Bell test experiment (1970–2015): In October 2015, scientists from the Kavli Institute of Nanoscience reported that the quantum nonlocality phenomenon is supported at the 96% confidence level based on a "loophole-free Bell test" study. These results were confirmed by two studies with statistical significance over 5 standard deviations which were published in December 2015.
- Existence of pentaquarks (1964–2015): In July 2015, the LHCb collaboration at CERN identified pentaquarks in the Λ0
b→J/ψK−p channel, which represents the decay of the bottom lambda baryon (Λ0
b) into a J/ψ meson (J/ψ), a kaon (K−
) and a proton (p). The results showed that sometimes, instead of decaying directly into mesons and baryons, the Λ0
b decayed via intermediate pentaquark states. The two states, named P+
c(4380) and P+
c(4450), had individual statistical significances of 9 σ and 12 σ, respectively, and a combined significance of 15 σ — enough to claim a formal discovery. The two pentaquark states were both observed decaying strongly to J/ψp, hence must have a valence quark content of two up quarks, a down quark, a charm quark, and an anti-charm quark (
), making them charmonium-pentaquarks.
- Photon underproduction crisis (2014–2015): This problem was resolved by Khaire and Srianand. 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.
- Existence of ball lightning (1638–2014): 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. At a distance of 900 m (3,000 ft), 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 oxidize in the atmosphere. The nature of the true theory is still not clear.
- Higgs boson and electroweak symmetry breaking (1963–2012): The mechanism responsible for breaking the electroweak gauge symmetry, giving mass to the W and Z bosons, was solved with the discovery of the Higgs boson of the Standard Model, with the expected couplings to the weak bosons. No evidence of a strong dynamics solution, as proposed by technicolor, has been observed.
- Hipparcos anomaly (1997–2012): 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.
- Faster-than-light neutrino anomaly (2011–2012): In 2011, the OPERA experiment mistakenly observed neutrinos appearing to travel faster than light. On July 12, 2012 OPERA updated their paper by including the new sources of errors in their calculations. They found agreement of neutrino speed with the speed of light.
- Pioneer anomaly (1980–2012): 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.
- Numerical solution for binary black hole (1960s–2005): The numerical solution of the two body problem in general relativity was achieved after four decades of research. In 2005 (annus mirabilis of numerical relativity) when three groups devised the breakthrough techniques.
- Long-duration gamma ray bursts (1993–2003): 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 (1968–2001): Solved by a new understanding of neutrino physics, requiring a modification of the Standard Model of particle physics—specifically, neutrino oscillation.
- Create Bose–Einstein condensate (1924-1995): Composite bosons in the form of dilute atomic vapours were cooled to quantum degeneracy using the techniques of laser cooling and evaporative cooling.
- 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.
- Nature of quasars (1950s-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 centre of the galaxy.
- Hammond, Richard (May 1, 2008). The Unknown Universe: The Origin of the Universe, Quantum Gravity, Wormholes, and Other Things Science Still Can't Explain. Franklin Lakes, NJ: Career Press.
- Womersley, J. (February 2005). "Beyond the Standard Model" (PDF). Symmetry Magazine. Retrieved 2010-11-23.
- Ginzburg, Vitaly L. (2001). The physics of a lifetime : reflections on the problems and personalities of 20th century physics. Berlin: Springer. pp. 3–200. ISBN 978-3-540-67534-1.
- Baez, John C. (March 2006). "Open Questions in Physics". Usenet Physics FAQ. University of California, Riverside: Department of Mathematics. Retrieved March 7, 2011.
- "Alcohol constrains physical constant in the early universe". Phys Org. December 13, 2012. Retrieved 25 March 2015.
- Bagdonaite, J.; Jansen, P.; Henkel, C.; Bethlem, H. L.; Menten, K. M.; Ubachs, W. (13 December 2012). "A Stringent Limit on a Drifting Proton-to-Electron Mass Ratio from Alcohol in the Early Universe". Science. 339 (6115): 46–48. Bibcode:2013Sci...339...46B. doi:10.1126/science.1224898. PMID 23239626.
- Rees, Martin (May 3, 2001). Just Six Numbers: The Deep Forces That Shape The Universe. New York, NY: Basic Books; First American edition. p. 4.
- Gribbin. J and Rees. M, Cosmic Coincidences: Dark Matter, Mankind, and Anthropic Cosmology p. 7, 269, 1989, ISBN 0-553-34740-3
- Davis, Paul (2007). Cosmic Jackpot: Why Our Universe Is Just Right for Life. New York, NY: Orion Publications. p. 2. ISBN 0618592261.
- Podolsky, Dmitry. "Top ten open problems in physics". NEQNET. Archived from the original on 22 October 2012. Retrieved 24 January 2013.
- Brooks, Michael (March 19, 2005). "13 Things That Do Not Make Sense". New Scientist. Issue 2491. Retrieved March 7, 2011.
- Steinhardt, P. & Turok, N. (2006). "Why the Cosmological constant is so small and positive". Science. 312: 1180–1183. arXiv: . Bibcode:2006Sci...312.1180S. doi:10.1126/science.1126231. PMID 16675662.
- 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". Nuclear Physics B. 846: 43–99. arXiv: . Bibcode:2011NuPhB.846...43L. doi:10.1016/j.nuclphysb.2010.12.014.
- Hansson, Johan (2010). "The "Proton Spin Crisis" — a Quantum Query" (PDF). Progress in Physics. 3. Archived from the original (PDF) on 4 May 2012. Retrieved 14 April 2012.
- A. Blumhofer; M. Hutter (1997). "Family Structure from Periodic Solutions of an Improved Gap Equation". Nuclear Physics. B484: 80–96. Bibcode:1997NuPhB.484...80B. doi:10.1016/S0550-3213(96)00644-X.
- "India-based Neutrino Observatory (INO)". Tata Institute of Fundamental Research. Retrieved 14 April 2012.
- Smarandache, Vic; Florentin Smarandache (2007). "Thirty Unsolved Problems in the Physics of Elementary Particles" (PDF). Progress in Physics. 4. Bibcode:2009APS..HAW.KD010C. Archived (PDF) from the original on 2008-09-10.
- Nakamura (Particle Data Group), K; et al. (2010). "2011 Review of Particle Physics". J. Phys. G. 37 (7A): 075021. Bibcode:2010JPhG...37g5021N. doi:10.1088/0954-3899/37/7A/075021.
- Thomas Blum; Achim Denig; Ivan Logashenko; Eduardo de Rafael; Lee Roberts, B.; Thomas Teubner; Graziano Venanzoni (2013). "The Muon (g-2) Theory Value: Present and Future". arXiv: [hep-ph].
- H. Muir (2 July 2003). "Pentaquark discovery confounds sceptics". New Scientist. Retrieved 2010-01-08.
- G. Amit (14 July 2015). "Pentaquark discovery at LHC shows long-sought new form of matter". New Scientist. Retrieved 2015-07-14.
- Strohmayer, Tod E.; Mushotzky, Richard F. (20 March 2003). "Discovery of X-Ray Quasi-periodic Oscillations from an Ultraluminous X-Ray Source in M82: Evidence against Beaming". The Astrophysical Journal. 586 (1): L61–L64. arXiv: . Bibcode:2003ApJ...586L..61S. doi:10.1086/374732.
- Titarchuk, Lev; Fiorito, Ralph (10 September 2004). "Spectral Index and Quasi‐Periodic Oscillation Frequency Correlation in Black Hole Sources: Observational Evidence of Two Phases and Phase Transition in Black Holes" (PDF). The Astrophysical Journal. 612 (2): 988–999. arXiv: . Bibcode:2004ApJ...612..988T. doi:10.1086/422573. Archived from the original (PDF) on 3 February 2014. Retrieved 25 January 2013.
- Shoji Kato (2012). "An Attempt to Describe Frequency Correlations among kHz QPOs and HBOs by Two-Armed Nearly Vertical Oscillations". Publications of the Astronomical Society of Japan. 64 (3): 62. arXiv: . Bibcode:2012PASJ...64...62K. doi:10.1093/pasj/64.3.62.
- Ferrarese, Laura; Merritt, David (2000). "A Fundamental Relation between Supermassive Black Holes and their Host Galaxies". The Astrophysical Journal. 539: L9–L12. arXiv: . 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.
- Condon, J. J.; Cotton, W. D.; Greisen, E. W.; Yin, Q. F.; Perley, R. A.; Taylor, G. B.; Broderick, J. J. (1998). "The NRAO VLA Sky Survey". The Astronomical Journal. 115 (5): 1693–1716. Bibcode:1998AJ....115.1693C. doi:10.1086/300337.
- Singal, Ashok K. (2011). "Large peculiar motion of the solar system from the dipole anisotropy in sky brightness due to distant radio sources". The Astrophysical Journal. 742 (2): L23–L27. arXiv: . Bibcode:2011ApJ...742L..23S. doi:10.1088/2041-8205/742/2/L23.
- Tiwari, Prabhakar; Kothari, Rahul; Naskar, Abhishek; Nadkarni-Ghosh, Sharvari; Jain, Pankaj (2015). "Dipole anisotropy in sky brightness and source count distribution in radio NVSS data". Astroparticle Physics. 61: 1–11. arXiv: . Bibcode:2015APh....61....1T. doi:10.1016/j.astropartphys.2014.06.004.
- Tiwari, P.; Jain, P. (2015). "Dipole anisotropy in integrated linearly polarized flux density in NVSS data". Monthly Notices of the Royal Astronomical Society. 447 (3): 2658–2670. arXiv: . Bibcode:2015MNRAS.447.2658T. doi:10.1093/mnras/stu2535.
- Hutsemekers, D. (1998). "Evidence for very large-scale coherent orientations of quasar polarization vectors". Astronomy and Astrophysics. 332: 410–428. Bibcode:1998A&A...332..410H.
- Hutsemékers, D.; Lamy, H. (2001). "Confirmation of the existence of coherent orientations of quasar polarization vectors on cosmological scales". Astronomy & Astrophysics. 367 (2): 381–387. arXiv: . Bibcode:2001A&A...367..381H. doi:10.1051/0004-6361:20000443.
- Jain, P.; Narain, G.; Sarala, S. (2004). "Large-scale alignment of optical polarizations from distant QSOs using coordinate-invariant statistics". Monthly Notices of the Royal Astronomical Society. 347 (2): 394–402. arXiv: . Bibcode:2004MNRAS.347..394J. doi:10.1111/j.1365-2966.2004.07169.x.
- Angelica de Oliveira-Costa; Tegmark, Max; Zaldarriaga, Matias; Hamilton, Andrew (2003). "The significance of the largest scale CMB fluctuations in WMAP". Physical Review D. 69 (6). arXiv: . Bibcode:2004PhRvD..69f3516D. doi:10.1103/PhysRevD.69.063516.
- Eriksen, H. K.; Hansen, F. K.; Banday, A. J.; Górski, K. M.; Lilje, P. B. (2004). "Asymmetries in the Cosmic Microwave Background Anisotropy Field". The Astrophysical Journal. 605: 14–20. arXiv: . Bibcode:2004ApJ...605...14E. doi:10.1086/382267.
- Pramoda Kumar Samal; Saha, Rajib; Jain, Pankaj; Ralston, John P. (2007). "Testing Isotropy of Cosmic Microwave Background Radiation". Monthly Notices of the Royal Astronomical Society. 385 (4): 1718–1728. arXiv: . Bibcode:2008MNRAS.385.1718S. doi:10.1111/j.1365-2966.2008.12960.x.
- Pramoda Kumar Samal; Saha, Rajib; Jain, Pankaj; Ralston, John P. (2008). "Signals of Statistical Anisotropy in WMAP Foreground-Cleaned Maps". Monthly Notices of the Royal Astronomical Society. 396 (511): 511–522. arXiv: . Bibcode:2009MNRAS.396..511S. doi:10.1111/j.1365-2966.2009.14728.x.
- Casagrande, L.; Schönrich, R.; Asplund, M.; Cassisi, S.; Ramírez, I.; Meléndez, J.; Bensby, T.; Feltzing, S. (2011). "New constraints on the chemical evolution of the solar neighbourhood and Galactic disc(s)". Astronomy & Astrophysics. 530: A138. arXiv: . Bibcode:2011A&A...530A.138C. doi:10.1051/0004-6361/201016276.
- Bensby, T.; Feltzing, S.; Lundström, I. (July 2004). "A possible age-metallicity relation in the Galactic thick disk?". Astronomy and Astrophysics. 421 (3): 969–976. arXiv: . Bibcode:2004A&A...421..969B. doi:10.1051/0004-6361:20035957.
- Gilmore, G.; Asiri, H. M. (2011). "Open Issues in the Evolution of the Galactic Disks". Stellar Clusters & Associations: A RIA Workshop on Gaia. Proceedings. Granada: 280. Bibcode:2011sca..conf..280G.
- Casagrande, L.; Silva Aguirre, V.; Schlesinger, K. J.; Stello, D.; Huber, D.; Serenelli, A. M.; Scho Nrich, R.; Cassisi, S.; Pietrinferni, A.; Hodgkin, S.; Milone, A. P.; Feltzing, S.; Asplund, M. (2015). "Measuring the vertical age structure of the Galactic disc using asteroseismology and SAGA". Monthly Notices of the Royal Astronomical Society. 455: 987–1007. arXiv: . Bibcode:2016MNRAS.455..987C. doi:10.1093/mnras/stv2320.
- Fields, Brian D. (2012). "The Primordial Lithium Problem". Annual Review of Nuclear and Particle Science. 61 (2011): 47–68. arXiv: . Bibcode:2011ARNPS..61...47F. doi:10.1146/annurev-nucl-102010-130445.
- Futurism.com: "Tentatively finding even the most faint sign of extraterrestrial life would be the single most important discovery in the history of mankind, it could possibly help us find answers to the most existential mysteries of science(…)"
- Carnegie Institution (16 June 2014). "Making Earth-Like Planets: Five Great Mysteries". YouTube.
- 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.
- 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 (5204): 1615–1616. 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.
- Meyer, H. O. (1 March 2010). "Spontaneous electron emission from a cold surface". EPL. 89 (5): 58001. Bibcode:2010EL.....8958001M. doi:10.1209/0295-5075/89/58001.
- Storey, B. D.; Szeri, A. J. (8 July 2000). "Water vapour, sonoluminescence and sonochemistry". Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences. 456 (1999): 1685–1709. Bibcode:2000RSPSA.456.1685D. doi:10.1098/rspa.2000.0582.
- Wu, C. C.; Roberts, P. H. (9 May 1994). "A Model of Sonoluminescence". Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences. 445 (1924): 323–349. Bibcode:1994RSPSA.445..323W. doi:10.1098/rspa.1994.0064.
- 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): 2566–2633. arXiv: . Bibcode:2011AnPhy.326.2566Y. doi:10.1016/j.aop.2011.06.001. Retrieved 8 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.
- Norris, David J. (2003). "The Problem Swept Under the Rug". In Klimov, Victor. Electronic Structure in Semiconductors Nanocrystals: Optical Experiment (in Semiconductor and Metal Nanocrystals: Synthesis and Electronic and Optical Properties). CRC Press. p. 97. ISBN 978-0-203-91326-0.
- F. Wagner (2007). "A quarter-century of H-mode studies". Plasma Physics and Controlled Fusion. 49: B1. Bibcode:2007PPCF...49....1W. doi:10.1088/0741-3335/49/12B/S01..
- Michael J. Thompson (2014). "Grand Challenges in the Physics of the Sun and Sun-like Stars". arXiv: [astro-ph.SR].
- André Balogh; Rudolf A. Treumann (2013). "Section 7.4 The Injection Problem". Physics of Collisionless Shocks: Space Plasma Shock Waves. p. 362. ISBN 978-1-4614-6099-2.
- Van den Noort M, Lim S, Bosch P (2016). "On the need to unify neuroscience and physics". Neuroimmunology and Neuroinflammation. 3: 271–273. doi:10.20517/2347-8659.2016.55.
- Kouveliotou, Chryssa; Meegan, Charles A.; Fishman, Gerald J.; Bhat, Narayana P.; Briggs, Michael S.; Koshut, Thomas M.; Paciesas, William S.; Pendleton, Geoffrey N. (1993). "Identification of two classes of gamma-ray bursts". The Astrophysical Journal. 413: L101. Bibcode:1993ApJ...413L.101K. doi:10.1086/186969.
- Cho, Adrian (16 October 2017). "Merging neutron stars generate gravitational waves and a celestial light show". Science. Retrieved 16 October 2017.
- Casttelvecchi, Davide (25 August 2017). "Rumours swell over new kind of gravitational-wave sighting". Nature News. doi:10.1038/nature.2017.22482. Retrieved 27 August 2017.
- 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.
- "Half the universe's missing matter has just been finally found". New Scientist. Retrieved 2017-10-12.
- Khemani, Vedika; Lazarides, Achilleas; Moessner, Roderich; Sondhi, S. L. (21 June 2016). "Phase Structure of Driven Quantum Systems". Physical Review Letters. 116 (25). arXiv: . Bibcode:2016PhRvL.116y0401K. doi:10.1103/PhysRevLett.116.250401. Retrieved 30 May 2017.
- Else, Dominic V.; Bauer, Bela; Nayak, Chetan (25 August 2016). "Floquet Time Crystals". Physical Review Letters. 117 (9): 090402. arXiv: . Bibcode:2016PhRvL.117i0402E. doi:10.1103/PhysRevLett.117.090402. Retrieved 30 May 2017.
- Zhang, J.; et al. (March 8, 2017). "Observation of a discrete time crystal". Nature. 543: 217–220. arXiv: . Bibcode:2017Natur.543..217Z. doi:10.1038/nature21413. Retrieved 2017-03-10.
- Choi, S.; et al. (March 8, 2017). "Observation of discrete time-crystalline order in a disordered dipolar many-body system". Nature. 543: 221–225. arXiv: . Bibcode:2017Natur.543..221C. doi:10.1038/nature21426. Retrieved 2017-03-10.
- Castelvecchi, Davide; Witze, Witze (February 11, 2016). "Einstein's gravitational waves found at last". Nature News. doi:10.1038/nature.2016.19361. Retrieved 2016-02-11.
- B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration) (2016). "Observation of Gravitational Waves from a Binary Black Hole Merger". Physical Review Letters. 116 (6): 061102. arXiv: . Bibcode:2016PhRvL.116f1102A. doi:10.1103/PhysRevLett.116.061102. PMID 26918975.
- "Gravitational waves detected 100 years after Einstein's prediction | NSF – National Science Foundation". www.nsf.gov. Retrieved 2016-02-11.
- Philip M. Pearle (1970), "Hidden-Variable Example Based upon Data Rejection", Phys. Rev. D, 2 (8): 1418–25, Bibcode:1970PhRvD...2.1418P, doi:10.1103/PhysRevD.2.1418
- Hensen, B.; et al. (21 October 2015). "Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres". Nature. 526 (7575): 682–686. Bibcode:2015Natur.526..682H. doi:10.1038/nature15759. PMID 26503041.
- Markoff, Jack (21 October 2015). "Sorry, Einstein. Quantum Study Suggests 'Spooky Action' Is Real". New York Times. Retrieved 21 October 2015.
- Giustina, M.; et al. (16 December 2015). "Significant-Loophole-Free Test of Bell's Theorem with Entangled Photons". Physical Review Letters. 115 (25): 250401. arXiv: . Bibcode:2015PhRvL.115y0401G. doi:10.1103/PhysRevLett.115.250401. PMID 26722905.
- Shalm, L. K.; et al. (16 December 2015). "Strong Loophole-Free Test of Local Realism". Physical Review Letters. 115 (25): 250402. arXiv: . Bibcode:2015PhRvL.115y0402S. doi:10.1103/PhysRevLett.115.250402. PMID 26722906.
- R. Aaij et al. (LHCb collaboration) (2015). "Observation of J/ψp resonances consistent with pentaquark states in Λ0
b→J/ψK−p decays". Physical Review Letters. 115 (7): 072001. arXiv: . Bibcode:2015PhRvL.115g2001A. doi:10.1103/PhysRevLett.115.072001.
- Khaire, V.; Srianand, R. (2015). "Photon underproduction crisis: Are QSOs sufficient to resolve it?". Monthly Notices of the Royal Astronomical Society: Letters. 451: L30. arXiv: . Bibcode:2015MNRAS.451L..30K. doi:10.1093/mnrasl/slv060.
- Girvan, Ray. "Devon History Society: Widecombe Great Storm, 1638". Archived from the original on 2016-04-13.
- Cen, Jianyong; Yuan, Ping; Xue, Simin (17 January 2014). "Observation of the Optical and Spectral Characteristics of Ball Lightning". Physical Review Letters. American Physical Society. 112 (35001): 035001. Bibcode:2014PhRvL.112c5001C. doi:10.1103/PhysRevLett.112.035001. PMID 24484145. Archived from the original on 18 January 2014. Retrieved 19 January 2014.
- Ball, Philip (17 January 2014). "Focus: First Spectrum of Ball Lightning". Focus. American Physical Society. 7: 5. Bibcode:2014PhyOJ...7....5B. doi:10.1103/Physics.7.5. Retrieved 19 January 2014.
- Higgs, Peter (2010-11-24). "My Life as a Boson" (PDF). Talk given by Peter Higgs at Kings College, London, Nov 24 2010, expanding on a paper originally presented in 2001. Archived from the original (PDF) on 1 May 2014. Retrieved 17 January 2013. – the original 2001 paper can be found at: Duff and Liu, ed. (2003) [year of publication]. 2001 A Spacetime Odyssey: Proceedings of the Inaugural Conference of the Michigan Center for Theoretical Physics, Michigan, USA, 21–25 May 2001. World Scientific. pp. 86–88. ISBN 9812382313. Retrieved 17 January 2013.
- Van Leeuwen, Floor (1999). "HIPPARCOS distance calibrations for 9 open clusters". Astronomy and Astrophysics. 341: L71. Bibcode:1999A&A...341L..71V.
- Charles Francis; Erik Anderson (2012). "XHIP-II: Clusters and associations". Astronomy Letters. 38 (11): 681–693. arXiv: . Bibcode:2012AstL...38..681F. doi:10.1134/S1063773712110023.
- OPERA collaboration (July 12, 2012). "Measurement of the neutrino velocity with the OPERA detector in the CNGS beam". Journal of High Energy Physics. 2012 (10). arXiv: . Bibcode:2012JHEP...10..093A. doi:10.1007/JHEP10(2012)093.
- 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): 241101. arXiv: . Bibcode:2012PhRvL.108x1101T. doi:10.1103/PhysRevLett.108.241101. PMID 23004253.
- Overbye, Dennis (23 July 2012). "Mystery Tug on Spacecraft Is Einstein's 'I Told You So'". The New York Times. Retrieved 24 January 2014.
- Pretorius, Frans (2005). "Evolution of Binary Black-Hole Spacetimes". Physical Review Letters. 95 (12): 121101. arXiv: . Bibcode:2005PhRvL..95l1101P. doi:10.1103/PhysRevLett.95.121101. PMID 16197061. Campanelli, M.; Lousto, C. O.; Marronetti, P.; Zlochower, Y. (2006). "Accurate Evolutions of Orbiting Black-Hole Binaries without Excision". Physical Review Letters. 96 (11): 111101. arXiv: . Bibcode:2006PhRvL..96k1101C. doi:10.1103/PhysRevLett.96.111101. PMID 16605808. Baker, John G.; Centrella, Joan; Choi, Dae-Il; Koppitz, Michael; Van Meter, James (2006). "Gravitational-Wave Extraction from an Inspiraling Configuration of Merging Black Holes". Physical Review Letters. 96 (11): 111102. arXiv: . Bibcode:2006PhRvL..96k1102B. doi:10.1103/PhysRevLett.96.111102. PMID 16605809.
- Cleveland, Bruce T.; Daily, Timothy; Davis, Jr., Raymond; Distel, James R.; Lande, Kenneth; Lee, C. K.; Wildenhain, Paul S.; Ullman, Jack (1998). "Measurement of the Solar Electron Neutrino Flux with the Homestake Chlorine Detector". The Astrophysical Journal. 496: 505–526. Bibcode:1998ApJ...496..505C. doi:10.1086/305343.
- "Einstein papers at the Instituut-Lorentz".
- "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
- What don't we know? Science journal special project for its 125th anniversary: top 25 questions and 100 more.
- List of links to unsolved problems in physics, prizes and research.
- Ideas Based On What We’d Like to Achieve
- 2004 SLAC Summer Institute: Nature's Greatest Puzzles
- Dual Personality of Glass Explained at Last
- What we do and don't know Review on current state of physics by Steven Weinberg, November 2013
- The crisis of big science Steven Weinberg, May 2012
- The 10 Biggest Unsolved Problems in Physics Johan Hansson, March 2015