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The following is a list of notable unsolved problems grouped into broad areas of physics.[1]

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 questions beyond the Standard Model of physics, such as the strong CP problem, neutrino mass, matter–antimatter asymmetry, and the nature of dark matter and dark energy.[2][3] 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 centres of black holes beyond the event horizon).

Theories of Everything

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See also: Theory of everything

Is there a theory which explains the values of all fundamental physical constants, i.e., of all coupling constants, all elementary particle masses and all mixing angles of elementary particles?[4] 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? Are "fundamental physical constants" really fundamental or do they 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 elementary particles that have not yet been observed, and, if so, which ones are they and what are their properties? Are there unobserved fundamental forces?

Supersymmetry

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See also: Supersymmetry

Quantum Loop Gravity

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See also: Quantum loop gravity

String Theory

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See also: String theory

Physics beyond the Standard Model

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Simulated Large Hadron Collider CMS particle detector data depicting a Higgs boson produced by colliding protons decaying into hadron jets and electrons
See also: Beyond the Standard Model

The Standard Model of particle physics is the theory describing three of the four known fundamental forces (the electromagnetic, weak, and strong interactions, and not including the gravitational force) in the universe, as well as classifying all known elementary particles. It was developed in stages throughout the latter half of the 20th century, through the work of many scientists around the world,[5] with the current formulation being finalized in the mid-1970s upon experimental confirmation of the existence of quarks. Although the Standard Model is believed to be theoretically self-consistent, and has demonstrated huge successes in providing experimental predictions, it leaves some phenomena unexplained. Some of the most notable issues with the standard model include:

Quantum gravity

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See also: Quantum gravity

The standard model does not explain gravity. The approach of simply adding a graviton to the Standard Model does not recreate what is observed experimentally without other modifications, as yet undiscovered, to the Standard Model. Moreover, the Standard Model is widely considered to be incompatible with the most successful theory of gravity to date, general relativity.[6]

Dark matter

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See also: Dark matter

Cosmological observations tell us the standard model explains about 5% of the energy present in the universe. About 26% should be dark matter,[citation needed] which would behave just like other matter, but which only interacts weakly (if at all) with the Standard Model fields. Yet, the Standard Model does not supply any fundamental particles that are good dark matter candidates.

Dark energy

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See also: Dark energy

The remaining 69% of universe's energy should consist of the so-called dark energy, a constant energy density for the vacuum. Attempts to explain dark energy in terms of vacuum energy of the standard model lead to a mismatch of 120 orders of magnitude.[7]

Neutrino masses

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See also: neutrino oscillation

According to the standard model, neutrinos are massless particles. However, neutrino oscillation experiments have shown that neutrinos do have mass. Mass terms for the neutrinos can be added to the standard model by hand, but these lead to new theoretical problems. For example, the mass terms need to be extraordinarily small and it is not clear if the neutrino masses would arise in the same way that the masses of other fundamental particles do in the Standard Model.

Matter–antimatter asymmetry

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See also: Baryon asymmetry

The universe is made out of mostly matter. However, the standard model predicts that matter and antimatter should have been created in (almost) equal amounts if the initial conditions of the universe did not involve disproportionate matter relative to antimatter. Yet, there is no mechanism in the Standard Model to sufficiently explain this asymmetry.[citation needed]

Anomalous magnetic dipole moment of the muon

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See also: Anomalous magnetic dipole moment § Muon

The experimentally measured value of muon's anomalous magnetic dipole moment (muon "g − 2") is significantly different from the Standard Model prediction.[8]

Hierarchy problem

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See also: hierarchy problem

The standard model introduces particle masses through a process known as spontaneous symmetry breaking caused by the Higgs field. Within the standard model, the mass of the Higgs gets some very large quantum corrections due to the presence of virtual particles (mostly virtual top quarks). These corrections are much larger than the actual mass of the Higgs. This means that the bare mass parameter of the Higgs in the standard model must be fine tuned in such a way that almost completely cancels the quantum corrections.[9] This level of fine-tuning is deemed unnatural by many theorists.[who?]

Strong CP Problem

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See also: Strong CP problem

Theoretically it can be argued that the standard model should contain a term that breaks CP symmetry—relating matter to antimatter—in the strong interaction sector. Experimentally, however, no such violation has been found, implying that the coefficient of this term is very close to zero.[10]

Cosmology and Astrophysics

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Black Holes

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Origin of the Universe

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Structure of the Universe

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  • Black holes, black hole information paradox, and black hole radiation: Do black holes produce thermal radiation, as expected on theoretical grounds?[11] 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?[12] 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?
  • 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?[4] 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?
  • 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?[13]
  • 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: 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?
  • The largest structures in the universe are larger than expected. Current cosmological models say there should be very little structure on scales larger than a few hundred million light years across, due to the expansion of the universe trumping the effect of gravity.[16] But the Sloan Great Wall is 1.38 billion light-years in length. And the largest structure currently known, the Hercules–Corona Borealis Great Wall, is up to 10 billion light-years in length. Are these actual structures or random density fluctuations? If they are real structures, they contradict the 'End of Greatness' hypothesis which asserts that at a scale of 300 million light-years structures seen in smaller surveys are randomized to the extent that the smooth distribution of the universe is visually apparent.

Quantum Mechanics

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  • Locality: Are there non-local phenomena in quantum physics?[18][19] 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?

Quantum Field Theory

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  • Color confinement: The quantum chromodynamics (QCD) color confinement conjecture is that color charged particles (such as quarks and gluons) cannot be separated from their parent hadron without producing new hadrons.[21] Is it possible to provide an analytic proof of color confinement in any non-abelian gauge theory?
  • Quantum gravity: Can quantum mechanics and general relativity be realized as a fully consistent theory (perhaps as a quantum field theory)?[25] 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?

Nuclear and particle physics

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  • 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.)[26]
  • 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").[27]
  • 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)?[28]
  • 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?[29] Are they a tightly-bound system of five elementary particles, or a more weakly-bound pairing of a baryon and a meson?[30]
The "island of stability" in the proton vs. neutron number plot for heavy nuclei

Fluid dynamics

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Turbulence in the tip vortex from an airplane wing passing through coloured smoke

Condensed matter physics

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A sample of a cuprate superconductor (specifically BSCCO). The mechanism for superconductivity of these materials is unknown.
Magnetoresistance in a fractional quantum Hall state.
  • 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.
  • Superfluid transition in helium-4: Explain the discrepancy between the experimental[34] and theoretical[35][36][37] determinations of the heat capacity critical exponent α.[38]
  • Bose–Einstein condensation: How do we rigorously prove the existence of Bose–Einstein condensates for general interacting systems?[39]

Optics

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See also

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References

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  1. ^ 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.
  2. ^ Hammond, Richard (1 May 2008). "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. 456 (1999): 1685.
  3. ^ Womersley, J. (February 2005). "Beyond the Standard Model" (PDF). Symmetry Magazine. Archived from the original (PDF) on 17 October 2007. Retrieved 23 November 2010.
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  5. ^ R. Oerter (2006). The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern Physics (Kindle ed.). Penguin Group. p. 2. ISBN 978-0-13-236678-6.
  6. ^ Sushkov, A. O.; Kim, W. J.; Dalvit, D. A. R.; Lamoreaux, S. K. (2011). "New Experimental Limits on Non-Newtonian Forces in the Micrometer Range". Physical Review Letters. 107 (17): 171101. arXiv:1108.2547. Bibcode:2011PhRvL.107q1101S. doi:10.1103/PhysRevLett.107.171101. PMID 22107498. S2CID 46596924. It is remarkable that two of the greatest successes of 20th century physics, general relativity and the standard model, appear to be fundamentally incompatible. But see also Donoghue, John F. (2012). "The effective field theory treatment of quantum gravity". AIP Conference Proceedings. 1473 (1): 73. arXiv:1209.3511. Bibcode:2012AIPC.1483...73D. doi:10.1063/1.4756964. S2CID 119238707. One can find thousands of statements in the literature to the effect that "general relativity and quantum mechanics are incompatible". These are completely outdated and no longer relevant. Effective field theory shows that general relativity and quantum mechanics work together perfectly normally over a range of scales and curvatures, including those relevant for the world that we see around us. However, effective field theories are only valid over some range of scales. General relativity certainly does have problematic issues at extreme scales. There are important problems which the effective field theory does not solve because they are beyond its range of validity. However, this means that the issue of quantum gravity is not what we thought it to be. Rather than a fundamental incompatibility of quantum mechanics and gravity, we are in the more familiar situation of needing a more complete theory beyond the range of their combined applicability. The usual marriage of general relativity and quantum mechanics is fine at ordinary energies, but we now seek to uncover the modifications that must be present in more extreme conditions. This is the modern view of the problem of quantum gravity, and it represents progress over the outdated view of the past."
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  12. ^ Joshi, Pankaj S. (January 2009). "Do Naked Singularities Break the Rules of Physics?". Scientific American. Archived from the original on 25 May 2012.
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  19. ^ Fuchs, Christopher A.; Mermin, N. David; Schack, Rüdiger (2014). "An introduction to QBism with an application to the locality of quantum mechanics". American Journal of Physics. 82 (8): 749. arXiv:1311.5253. Bibcode:2014AmJPh..82..749F. doi:10.1119/1.4874855. S2CID 56387090.
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  27. ^ Wolchover, Natalie (13 February 2018). "Neutron Lifetime Puzzle Deepens, but No Dark Matter Seen". Quanta Magazine. Retrieved 31 July 2018. When physicists strip neutrons from atomic nuclei, put them in a bottle, then count how many remain there after some time, they infer that neutrons radioactively decay in 14 minutes and 39 seconds, on average. But when other physicists generate beams of neutrons and tally the emerging protons—the particles that free neutrons decay into—they peg the average neutron lifetime at around 14 minutes and 48 seconds. The discrepancy between the "bottle" and "beam" measurements has persisted since both methods of gauging the neutron's longevity began yielding results in the 1990s. At first, all the measurements were so imprecise that nobody worried. Gradually, though, both methods have improved, and still they disagree.
  28. ^ A. Blumhofer; M. Hutter (1997). "Family Structure from Periodic Solutions of an Improved Gap Equation". Nuclear Physics. B484 (1): 80–96. Bibcode:1997NuPhB.484...80B. CiteSeerX 10.1.1.343.783. doi:10.1016/S0550-3213(96)00644-X.
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  31. ^ Kenneth Chang (29 July 2008). "The Nature of Glass Remains Anything but Clear". The New York Times.
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  33. ^ Dean, Cory R. (2015). "Even denominators in odd places". Nature Physics. 11 (4): 298–299. Bibcode:2015NatPh..11..298D. doi:10.1038/nphys3298. ISSN 1745-2481.
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  35. ^ Campostrini, Massimo; Hasenbusch, Martin; Pelissetto, Andrea; Vicari, Ettore (6 October 2006). "Theoretical estimates of the critical exponents of the superfluid transition in $^{4}\mathrm{He}$ by lattice methods". Physical Review B. 74 (14): 144506. arXiv:cond-mat/0605083. doi:10.1103/PhysRevB.74.144506. S2CID 118924734.
  36. ^ Hasenbusch, Martin (26 December 2019). "Monte Carlo study of an improved clock model in three dimensions". Physical Review B. 100 (22): 224517. arXiv:1910.05916. Bibcode:2019PhRvB.100v4517H. doi:10.1103/PhysRevB.100.224517. ISSN 2469-9950. S2CID 204509042.
  37. ^ Chester, Shai M.; Landry, Walter; Liu, Junyu; Poland, David; Simmons-Duffin, David; Su, Ning; Vichi, Alessandro (2020). "Carving out OPE space and precise $O(2)$ model critical exponents". Journal of High Energy Physics. 2020 (6): 142. arXiv:1912.03324. Bibcode:2020JHEP...06..142C. doi:10.1007/JHEP06(2020)142. S2CID 208910721.
  38. ^ Rychkov, Slava (31 January 2020). "Conformal bootstrap and the λ-point specific heat experimental anomaly". Journal Club for Condensed Matter Physics. doi:10.36471/JCCM_January_2020_02.
  39. ^ 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.
  40. ^ Barton, G.; Scharnhorst, K. (1993). "QED between parallel mirrors: light signals faster than c, or amplified by the vacuum". Journal of Physics A. 26 (8): 2037. Bibcode:1993JPhA...26.2037B. doi:10.1088/0305-4470/26/8/024. A more recent follow-up paper is Scharnhorst, K. (1998). "The velocities of light in modified QED vacua". Annalen der Physik. 7 (7–8): 700–709. arXiv:hep-th/9810221. Bibcode:1998AnP...510..700S. doi:10.1002/(SICI)1521-3889(199812)7:7/8<700::AID-ANDP700>3.0.CO;2-K. S2CID 120489943.
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