View source for Physics:List of unsolved problems in physics

{{Short description|None}}
{{Dynamic list}}
{{Use Oxford spelling|date=August 2016}}
The following is a list of notable unsolved problems grouped into broad areas of [[Physics:Physics|physics]].<ref>{{cite book|last=Ginzburg|first=Vitaly L.|title=The physics of a lifetime : reflections on the problems and personalities of 20th century physics|url=https://archive.org/details/physicslifetimer00ginz_439|url-access=limited|year=2001|publisher=Springer|location=Berlin|isbn=978-3-540-67534-1|pages=[https://archive.org/details/physicslifetimer00ginz_439/page/n14 3]–200}}</ref>

Some of the major unsolved problems in [[Physics:Physics|physics]] are theoretical, meaning that existing [[Theory|theories]] seem incapable of explaining a certain observed [[Philosophy:Phenomenon|phenomenon]] or experimental result. The others are experimental, meaning that there is a difficulty in creating an [[Experiment|experiment]] to test a proposed theory or investigate a phenomenon in greater detail.<!-- cleanup because of looks like Original Research. Needs objective criteria for what is or is not an unsolved problem. For example list of problems reported in abc or listed by xyz. Similar to Hilbert's list, or as listed by [[Organization:NASA|NASA]]. Suggest cite each one to one of the external links? What are the criteria for reputable source or authority? --><!-- agree with above, hopefully, new grouping will help -->

There are still some questions [[Physics:Physics beyond the Standard Model|beyond the Standard Model of physics]], such as the [[Physics:Strong CP problem|strong CP problem]], [[Physics:Neutrino#Mass|neutrino mass]], matter–antimatter asymmetry, and the nature of [[Astronomy:Dark matter|dark matter]] and [[Astronomy:Dark energy|dark energy]].<ref>{{Cite journal|title = The Unknown Universe: The Origin of the Universe, Quantum Gravity, Wormholes, and Other Things Science Still Can't Explain |journal = Proceedings of the Royal Society of London, Series A|volume = 456|issue = 1999|pages = 1685|last = Hammond|first = Richard|date = 1 May 2008}}</ref><ref name=sym-v2-feb-05>{{Cite web |last1=Womersley |first1=J. |date=February 2005 |title=Beyond the Standard Model |url=http://www.symmetrymagazine.org/pdfs/200502/beyond_the_standard_model.pdf |work=Symmetry Magazine |access-date=23 November 2010 |archive-url=https://web.archive.org/web/20071017160238/http://www.symmetrymagazine.org/pdfs/200502/beyond_the_standard_model.pdf |archive-date=17 October 2007 |url-status=dead }}</ref> Another problem lies within the [[Physics:Quantum field theory|mathematical framework]] of the [[Physics:Standard Model|Standard Model]] itself—the Standard Model is inconsistent with that of [[Physics:General relativity|general relativity]], to the point that one or both theories break down under certain conditions (for example within known [[Physics:Spacetime|spacetime]] [[Gravitational singularity|singularities]] like the [[Astronomy:Big Bang|Big Bang]] and the [[Astronomy:Black hole#Singularity|centres]] of [[Astronomy:Black hole|black hole]]s beyond the event horizon).<ref name="NYT-20230911">{{cite news |last=Overbye |first=Dennis |title=Don't Expect a 'Theory of Everything' to Explain It All - Not even the most advanced physics can reveal everything we want to know about the history and future of the cosmos, or about ourselves. |url=https://www.nytimes.com/2023/09/11/science/space/astronomy-universe-simulations.html |date=11 September 2023 |work={{wipe|The New York Times}} |url-status=live |archiveurl=https://archive.today/20230911043212/https://www.nytimes.com/2023/09/11/science/space/astronomy-universe-simulations.html |archivedate=11 September 2023 |accessdate=11 September 2023 }}</ref>

== General physics ==
* [[Physics:Theory of everything|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 [[Astronomy:Universe|universe]]?
* [[Physics:Dimensionless physical constant|Dimensionless physical constant]]s: At the present time, the values of various dimensionless [[Physics:Physical constant|physical constant]]s cannot be calculated; they can be determined only by physical measurement.<ref>{{cite web|title=Alcohol constrains physical constant in the early universe|url=http://phys.org/news/2012-12-alcohol-constrains-physical-constant-early.html|website=Phys Org|access-date=25 March 2015|date=13 December 2012|archive-date=2 April 2015|archive-url=https://web.archive.org/web/20150402122447/http://phys.org/news/2012-12-alcohol-constrains-physical-constant-early.html|url-status=live}}</ref><ref name="Effelsberg">{{cite journal|last1=Bagdonaite|first1=J.|last2=Jansen|first2=P.|last3=Henkel|first3=C.|last4=Bethlem|first4=H. L.|last5=Menten|first5=K. M.|last6=Ubachs|first6=W.|title=A Stringent Limit on a Drifting Proton-to-Electron Mass Ratio from Alcohol in the Early Universe|journal=Science|date=13 December 2012|volume=339|issue=6115|pages=46–48|doi=10.1126/science.1224898|bibcode=2013Sci...339...46B|pmid=23239626|hdl=1871/39591|s2cid=716087|url=https://research.vu.nl/en/publications/1d54a47c-d3cf-4eed-900d-6a035ac35d88|access-date=10 January 2020|archive-date=17 January 2023|archive-url=https://web.archive.org/web/20230117023340/https://research.vu.nl/en/publications/a-stringent-limit-on-a-drifting-proton-to-electron-mass-ratio-fro|url-status=live|doi-access=free}}</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?

== Quantum gravity ==
* [[Physics:Quantum gravity|Quantum gravity]]: Can [[Physics:Quantum mechanics|quantum mechanics]] and [[Physics:General relativity|general relativity]] be realized as a fully consistent theory (perhaps as a [[Physics:Quantum field theory|quantum field theory]])?<ref>{{Cite news |author=Sokal |first=Alan |date=22 July 1996 |title=Don't Pull the String Yet on Superstring Theory |newspaper=New York Times |url=https://query.nytimes.com/gst/fullpage.html?res=9D0DE7DB1639F931A15754C0A960958260 |url-status=live |access-date=17 February 2017 |archive-url=https://web.archive.org/web/20081207212917/https://query.nytimes.com/gst/fullpage.html?res=9D0DE7DB1639F931A15754C0A960958260 |archive-date=7 December 2008}}</ref> Is spacetime fundamentally continuous or discrete? Would a consistent theory involve a force mediated by a hypothetical [[Physics:Graviton|graviton]], or be a product of a discrete structure of spacetime itself (as in [[Physics:Loop quantum gravity|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?
* [[Astronomy:Black hole|Black hole]]s, [[Astronomy:Black hole information paradox|black hole information paradox]], and black hole radiation: Do black holes produce thermal radiation, as expected on theoretical grounds?<ref name=":2">{{Cite journal|last1=Peres|first1=Asher|last2=Terno|first2=Daniel R.|date=2004|title=Quantum information and relativity theory|journal=[[Physics:Reviews of Modern Physics|Reviews of Modern Physics]]|volume=76|issue=1|pages=93–123|arxiv=quant-ph/0212023|doi=10.1103/revmodphys.76.93|bibcode=2004RvMP...76...93P|s2cid=7481797}}</ref> Does this radiation contain information about their inner structure, as suggested by gauge–gravity duality, or not, as implied by [[Physics:Hawking radiation|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 [[Astronomy:Cosmic censorship hypothesis|cosmic censorship hypothesis]] and the [[Chronology protection conjecture|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 [[Biography:Roger Penrose|Roger Penrose]] which proposes that this is impossible?<ref>{{Cite news|last=Joshi|first=Pankaj S.|title=Do Naked Singularities Break the Rules of Physics?|magazine=Scientific American|date=January 2009|url=http://www.sciam.com/article.cfm?id=naked-singularities|archive-url=https://archive.today/20120525044855/http://www.sciam.com/article.cfm?id=naked-singularities|url-status=dead|archive-date=2012-05-25}}</ref> Similarly, will the [[Closed timelike curve|closed timelike curve]]s which arise in some solutions to the equations of general relativity (and which imply the possibility of backwards [[Physics:Time travel|time travel]]) be ruled out by a theory of [[Physics:Quantum gravity|quantum gravity]] which unites general relativity with quantum mechanics, as suggested by the "chronology protection conjecture" of [[Biography:Stephen Hawking|Stephen Hawking]]?
* [[Physics:Holographic principle|Holographic principle]]: Is it true that [[Physics:Quantum gravity|quantum gravity]] admits a lower-dimensional description that does not contain gravity? A well-understood example of holography is the AdS/CFT correspondence in [[Physics:String theory|string theory]]. Similarly, can quantum gravity in a [[De Sitter space|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 units#Planck scale|Planck scale]] very different from the [[Physics:Spacetime#Introduction to curved spacetime|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 [[Physics:Supersymmetry|supersymmetry]]), recent research building upon the [[Ryu–Takayanagi conjecture]] has taught that spacetime in string theory is emergent by using [[Quantum information|quantum information]] theoretic concepts such as [[Physics:Entropy of entanglement|entanglement entropy]] in the AdS/CFT correspondence.<ref>{{cite journal
	| last1 = Harlow
	| first1 = Daniel
	| date = 2018
	| title = TASI Lectures on the Emergence of Bulk Physics in AdS/CFT
	| url = https://pos.sissa.it/305/002/
	| journal = Proceedings of Science
	| volume = TASI2017
	| pages = 002
	| doi = 10.22323/1.305.0002
| doi-access = free
	| hdl = 1721.1/121453
	| hdl-access = free
	}}</ref> However, how exactly the familiar classical spacetime emerges within string theory or the AdS/CFT correspondence is still not well understood.
* [[Physics:Problem of time|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>{{Cite book|title=Integrable Systems, Quantum Groups, and Quantum Field Theories|last=Isham|first=C. J.|date=1993|publisher=Springer, Dordrecht|isbn=9789401048743|series=NATO ASI Series|pages=157–287|language=en|chapter=Canonical Quantum Gravity and the Problem of Time|arxiv=gr-qc/9210011|doi=10.1007/978-94-011-1980-1_6|s2cid=116947742|}}</ref>

== Quantum physics ==
* [[Yang–Mills theory]]: Given an arbitrary [[Compact space|compact]] gauge group, does a non-trivial quantum Yang–Mills theory with a finite [[Physics:Mass gap|mass gap]] exist? (This problem is also listed as one of the Millennium Prize Problems in mathematics.)<ref>{{Cite web|url=https://www.claymath.org/millennium-problems/yang%E2%80%93mills-and-mass-gap|title=Yang–Mills and Mass Gap|website=Clay Mathematics Institute|language=en|access-date=31 January 2018|archive-date=22 November 2015|archive-url=https://archive.today/20151122222730/http://www.claymath.org/millennium-problems/yang%E2%80%93mills-and-mass-gap|url-status=live}}</ref>
* [[Physics:Quantum field theory|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 [[Perturbation theory (quantum mechanics)|perturbative methods]]?
== Cosmology and general relativity ==
* [[Astronomy:Axis of evil (cosmology)|Axis of evil]]: Some large features of the microwave sky at distances of over 13&nbsp;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, an unexplained violation of the [[Astronomy:Copernican principle|Copernican principle]] and thus the concordance model, or are these features simply statistically insignificant?
* [[Astronomy:Fine-tuned universe|Fine-tuned universe]]: The values of the fundamental physical constants are in a narrow range necessary to support carbon-based life.<ref>{{Cite book |last=Rees |first=Martin |url=https://archive.org/details/justsixnumbersde00rees |title=Just Six Numbers: The Deep Forces That Shape The Universe |date=3 May 2001 |publisher=Basic Books; First American edition |isbn=9780465036721 |location=New York, New York |page=[https://archive.org/details/justsixnumbersde00rees/page/n14 4] |language=en-us |url-access=limited}}</ref><ref name=":1">Gribbin, J. and Rees, M., ''Cosmic Coincidences: Dark Matter, Mankind, and Anthropic Cosmology,'' pp. 7, 269. 1989, {{ISBN|0-553-34740-3}}</ref><ref>{{Cite book |last=Davis |first=Paul |url=https://archive.org/details/cosmicjackpotwhy0000davi/page/2 |title=Cosmic Jackpot: Why Our Universe Is Just Right for Life |publisher=Orion Publications |year=2007 |isbn=978-0618592265 |location=New York |page=[https://archive.org/details/cosmicjackpotwhy0000davi/page/2 2] |language=en-us}}</ref> Is this because there are an [[Physics:Multiverse|infinite amount of other universes]] with different constants, or are our universe's constants the result of chance, or some other factor or process? (See also the [[Astronomy:Anthropic principle|anthropic principle]].)
* [[Astronomy:Cosmic inflation|Cosmic inflation]]: Is the theory of cosmic inflation in the very early universe correct, and, if so, what are the details of this epoch? What is the hypothetical {{sic|[[Astronomy:Inflaton|inflaton]]|hide=y|expected=[[Finance:Inflation|inflation]]}} [[Scalar field|scalar field]] that gave rise to this cosmic 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?<ref name=podolsky-NEQNET>{{cite web|last=Podolsky|first=Dmitry|title=Top ten open problems in physics|url=http://www.nonequilibrium.net/225-top-ten-open-problems-physics/|publisher=NEQNET|access-date=24 January 2013|archive-url=https://web.archive.org/web/20121022112323/http://www.nonequilibrium.net/225-top-ten-open-problems-physics|archive-date=22 October 2012|url-status=dead}}</ref>
* [[Astronomy:Horizon problem|Horizon problem]]: Why is the distant universe so homogeneous when the Big Bang theory seems to predict larger measurable [[Anisotropy|anisotropies]] of the night sky than those observed? Cosmological [[Astronomy:Inflation (cosmology)|inflation]] is generally accepted as the solution, but are other possible explanations such as a [[Physics:Variable speed of light|variable speed of light]] more appropriate?<ref name=newscientist />
* Origin and [[Astronomy:Ultimate fate of the universe|future of the universe]]: [[Astronomy:Big Bang#Pre–Big Bang cosmology|How did the conditions for anything to exist arise?]] Is the universe heading towards a Big Freeze, a [[Astronomy:Big Rip|Big Rip]], a [[Astronomy:Big Crunch|Big Crunch]], or a [[Astronomy:Big Bounce|Big Bounce]]? Or is it part of an infinitely recurring [[Astronomy:Cyclic model|cyclic model]]?
* [[Astronomy:Universe#Size and regions|Size of universe]]: The diameter of the [[Astronomy:Observable universe|observable universe]] is about 93&nbsp;billion light-years, but what is the size of the whole universe? Is the universe infinite?
* [[Astronomy:Baryon asymmetry|Baryon asymmetry]]: Why is there far more [[Physics:Matter|matter]] than [[Physics:Antimatter|antimatter]] in the [[Astronomy:Observable universe|observable universe]]? (This may be solved due to the apparent asymmetry in neutrino-antineutrino oscillations.)<ref name="quantamagazine">{{cite web| url = https://www.quantamagazine.org/neutrino-asymmetry-passes-critical-threshold-20200415/| title = Quanta Magazine| access-date = 10 May 2020| archive-date = 27 April 2020| archive-url = https://web.archive.org/web/20200427235423/https://www.quantamagazine.org/neutrino-asymmetry-passes-critical-threshold-20200415/| url-status = live}}</ref>
* [[Astronomy:Cosmological principle|Cosmological principle]]: Is the universe homogeneous and isotropic at large enough scales, as claimed by the [[Astronomy:Cosmological principle|cosmological principle]] and assumed by all models that use the [[Astronomy:Friedmann–Lemaître–Robertson–Walker metric|Friedmann–Lemaître–Robertson–Walker metric]], including the current version of the [[Astronomy:Lambda-CDM model|ΛCDM model]], or is the universe [[Astronomy:Inhomogeneous cosmology|inhomogeneous]] or anisotropic?<ref name="Snowmass21">{{cite journal |last1=Abdalla |first1=Elcio |last2=Abellán |first2=Guillermo Franco |last3=Aboubrahim |first3=Amin |date=11 March 2022 |title=Cosmology Intertwined: A Review of the Particle Physics, Astrophysics, and Cosmology Associated with the Cosmological Tensions and Anomalies |journal=Journal of High Energy Astrophysics |volume=34 |page=49 |arxiv=2203.06142v1 |bibcode=2022JHEAp..34...49A |doi=10.1016/j.jheap.2022.04.002 |s2cid=247411131}}</ref> Is the CMB dipole purely kinematic, or does it signal anisotropy of the universe, resulting in the breakdown of the FLRW metric and the cosmological principle?<ref name="Snowmass21"/> Is the Hubble tension evidence that the cosmological principle is false?<ref name="Snowmass21"/> Even if the cosmological principle is correct, is the [[Astronomy:Friedmann–Lemaître–Robertson–Walker metric|Friedmann–Lemaître–Robertson–Walker metric]] the right metric to use for our universe?<ref name="FLRW breakdown">{{cite journal |last1=Krishnan |first1=Chethan |last2=Mohayaee |first2=Roya |last3=Colgáin |first3=Eoin Ó |last4=Sheikh-Jabbari |first4=M. M. |last5=Yin |first5=Lu |title=Does Hubble Tension Signal a Breakdown in FLRW Cosmology? |journal=Classical and Quantum Gravity |date=16 September 2021 |volume=38 |issue=18 |pages=184001 |doi=10.1088/1361-6382/ac1a81 |arxiv=2105.09790 |bibcode=2021CQGra..38r4001K |s2cid=234790314 |issn=0264-9381}}</ref><ref name="Snowmass21"/> Are the observations usually interpreted as the [[Astronomy:Accelerating expansion of the universe|accelerating expansion of the universe]] rightly interpreted, or are they instead evidence that the [[Astronomy:Cosmological principle|cosmological principle]] is false?<ref name="Ellis 2009">{{cite journal|last=Ellis|first=G. F. R.|title=Dark energy and inhomogeneity|journal=Journal of Physics: Conference Series|volume=189|doi=10.1088/1742-6596/189/1/012011|year=2009|issue=1|page=012011|bibcode=2009JPhCS.189a2011E|s2cid=250670331 |doi-access=free}}</ref><ref name="Colin et al">{{cite journal|title=Evidence for anisotropy of cosmic acceleration|first1=Jacques|last1=Colin|first2=Roya|last2=Mohayaee|first3=Mohamed|last3=Rameez|first4=Subir|last4=Sarkar|journal=Astronomy and Astrophysics|volume=631|doi=10.1051/0004-6361/201936373|arxiv=1808.04597|date=20 November 2019|pages=L13|bibcode=2019A&A...631L..13C|s2cid=208175643|access-date=25 March 2022|url=https://www.aanda.org/articles/aa/full_html/2019/11/aa36373-19/aa36373-19.html|archive-date=10 March 2022|archive-url=https://web.archive.org/web/20220310140316/https://www.aanda.org/articles/aa/full_html/2019/11/aa36373-19/aa36373-19.html|url-status=live}}</ref>
** [[Astronomy:Copernican principle|Copernican principle]]: Are cosmological observations made from Earth representative of observations from the average position in the universe?
* [[Astronomy:Cosmological constant problem#Cosmological constant problem|Cosmological constant problem]]: Why does the [[Physics:Zero-point energy|zero-point energy]] of the [[Physics:Vacuum|vacuum]] not cause a large [[Astronomy:Cosmological constant|cosmological constant]]? What cancels it out?<ref>{{cite journal |title=Why the Cosmological constant is so small and positive|last1=Steinhardt |first1=P. |last2=Turok |first2=N. |name-list-style=amp |year=2006 |journal=Science|volume=312 |issue=5777  |pages=1180–1183|doi=10.1126/science.1126231|arxiv=astro-ph/0605173 |bibcode = 2006Sci...312.1180S |pmid=16675662|s2cid=14178620 }}</ref><ref name=":3">
{{cite journal |last1=Wang|first1=Qingdi |last2=Zhu |first2=Zhen |last3=Unruh |first3=William G. |date=11 May 2017 |title=How the huge energy of quantum vacuum gravitates to drive the slow accelerating expansion of the Universe|journal=[[Physics:Physical Review D|Physical Review D]] |volume=95 |issue=10 |page=103504 |arxiv=1703.00543 |doi=10.1103/PhysRevD.95.103504 |bibcode=2017PhRvD..95j3504W |s2cid=119076077}}
</ref>{{efn|"This problem is widely regarded as one of the major obstacles to further progress in fundamental physics ... Its importance has been emphasized by various authors  from different aspects. For example, it has been described as a 'veritable crisis" ...] and even 'the mother of all physics problems' ... While it might be possible that people working on a particular problem tend to emphasize or even exaggerate its importance, those authors all agree that this is a problem that needs to be solved, although there is little agreement on what is the right direction to find the solution."<ref name=":3"/>
}}

[[File:DMPie 2013.svg|thumb|right|upright=1.1|Estimated distribution of dark matter and dark energy in the universe]]
* [[Astronomy:Dark matter|Dark matter]]: What is the identity of dark matter?<ref name="newscientist">{{cite news|title=13 things that do not make sense|first=Michael|last=Brooks|url=https://www.newscientist.com/article/mg18524911.600-13-things-that-do-not-make-sense.html|newspaper=New Scientist|id=Issue 2491|date=19 March 2005|access-date=7 March 2011|archive-date=23 June 2015|archive-url=https://web.archive.org/web/20150623175524/https://www.newscientist.com/article/mg18524911.600-13-things-that-do-not-make-sense.html|url-status=live}}</ref> Is it a [[Physics:Elementary particle|particle]]? If so, is it a [[Physics:Weakly interacting massive particles|WIMP]], [[Physics:Axion|axion]], the lightest [[Physics:Superpartner|superpartner]] (LSP), or some other particle? Or, do the [[Astronomy:Galaxy rotation curve|phenomena attributed to dark matter]] point not to some form of matter but actually to an extension of gravity?
* [[Astronomy:Dark energy|Dark energy]]: What is the cause of the observed [[Astronomy:Accelerating expansion of the universe|accelerating expansion of the universe]] (the [[Astronomy:De Sitter universe|de Sitter phase]])? Are the observations rightly interpreted as the [[Astronomy:Accelerating expansion of the universe|accelerating expansion of the universe]], or are they evidence that the [[Astronomy:Cosmological principle|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 [[Astronomy:Anthropic principle|right time]]? Is dark energy a pure cosmological constant or are models of [[Physics:Quintessence|quintessence]] such as [[Astronomy:Phantom energy|phantom energy]] applicable?
* [[Astronomy:Dark flow|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?
* [[Astronomy:Shape of the universe|Shape of the universe]]: What is the 3-[[Manifold|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 [[Astronomy:Cosmic inflation|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 [[Homology sphere#Cosmology|Poincaré dodecahedral space]]. Is the shape unmeasurable; the Poincaré space; or another 3-manifold?
* [[Astronomy:Extra dimensions|Extra dimensions]]: Does nature have more than four [[Physics:Spacetime|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?

== High-energy/particle physics ==
* [[Physics:Hierarchy problem|Hierarchy problem]]: Why is [[Physics:Gravity|gravity]] such a weak force? It becomes strong for particles only at the [[Physics:Planck mass|Planck scale]], around {{10^|19}}&nbsp;GeV, much above the [[Physics:Electroweak scale|electroweak scale]] (100&nbsp;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 [[Physics:Higgs boson|Higgs boson]] mass, from getting [[Physics:Renormalization|quantum corrections]] on the order of the Planck scale? Is the solution [[Physics:Supersymmetry|supersymmetry]], [[Astronomy:Extra dimensions|extra dimensions]], or just [[Astronomy:Anthropic principle|anthropic]] [[Astronomy:Fine-tuned universe|fine-tuning]]?
* [[Physics:Magnetic monopole|Magnetic monopole]]s: Did particles that carry "magnetic charge" exist in some past, higher-energy epoch? If so, do any remain today? ([[Biography:Paul Dirac|Paul Dirac]] showed the existence of some types of magnetic monopoles would explain [[Physics:Charge quantization|charge quantization]].)<ref>{{cite journal
 |last=Dirac
 |first=Paul
 |year=1931
 |title=Quantised singularities in the electromagnetic field
 |journal=Proceedings of the Royal Society A
 |volume=133
 |issue=821
 |page=60
 |doi=10.1098/rspa.1931.0130
 |bibcode=1931RSPSA.133...60D
 |url=http://users.physik.fu-berlin.de/~kleinert/files/dirac1931.pdf
 |access-date=25 December 2010
 |archive-date=20 May 2011
 |archive-url=https://web.archive.org/web/20110520041631/http://users.physik.fu-berlin.de/~kleinert/files/dirac1931.pdf
 |url-status=live
 }}</ref>
* [[Physics:Free neutron decay#Neutron lifetime puzzle|Neutron lifetime puzzle]]: While the neutron lifetime has been studied for decades, there currently exists a lack of [[Consilience|consilience]] on its exact value, due to different results from two experimental methods ("bottle" versus "beam").<ref name=Wolchover-20180213>{{cite magazine |last=Wolchover |first=Natalie |date=13 February 2018 |title=Neutron lifetime puzzle deepens, but no dark matter seen |magazine=Quanta Magazine |url=https://www.quantamagazine.org/neutron-lifetime-puzzle-deepens-but-no-dark-matter-seen-20180213/ |access-date=31 July 2018 |archive-date=30 July 2018 |archive-url=https://web.archive.org/web/20180730080707/https://www.quantamagazine.org/neutron-lifetime-puzzle-deepens-but-no-dark-matter-seen-20180213/ |url-status=live }}</ref>{{efn|
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&nbsp;minutes and 39&nbsp;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&nbsp;minutes and 48&nbsp;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.<ref name=Wolchover-20180213/>
}}
* [[Physics:Proton decay|Proton decay]] and [[Physics:Proton spin crisis|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>{{cite journal |last1=Li |first1=Tianjun |first2=Dimitri V. |last2=Nanopoulos |first3=Joel W. |last3=Walker |year=2011 |title=Elements of fast proton decay |doi=10.1016/j.nuclphysb.2010.12.014 |journal=Nuclear Physics B |volume=846 |issue=1 |pages=43–99 |arxiv=1003.2570 |bibcode=2011NuPhB.846...43L |s2cid=119246624}}</ref> How do the quarks and gluons carry the spin of protons?<ref name=hansson2010>{{cite journal |last=Hansson |first=Johan |year=2010 |title=The "proton spin crisis" – a quantum query |journal=Progress in Physics |volume=3 |page=23 |url=http://www.ptep-online.com/index_files/2010/PP-22-08.PDF |access-date=14 April 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120504134027/http://www.ptep-online.com/index_files/2010/PP-22-08.PDF |archive-date=4 May 2012 }}</ref>
* Grand Unification: Are the electromagnetic and nuclear forces different aspects of a [[Physics:Grand Unified Theory|Grand Unified Theory]]? If so, what [[Symmetry|symmetry]] governs this force and its behaviours?<ref>{{cite journal |doi=10.4249/scholarpedia.11419|title=Grand unification|journal=Scholarpedia|volume=7|issue=10|pages=11419|year=2012|last1=Langacker|first1=Paul|bibcode=2012SchpJ...711419L|doi-access=free}}</ref>
* [[Physics:Supersymmetry|Supersymmetry]]: Is spacetime supersymmetry realized at [[Physics:Electronvolt|TeV]] scale? If so, what is the mechanism of supersymmetry breaking? Does supersymmetry stabilize the electroweak scale, preventing high quantum corrections? Does the lightest [[Physics:Superpartner|supersymmetric particle]] ([[Astronomy:Lightest supersymmetric particle|LSP]]) comprise [[Astronomy:Dark matter|dark matter]]?
* [[Physics:Color confinement|Color confinement]]: The [[Physics:Quantum chromodynamics|quantum chromodynamics]] (QCD) color confinement conjecture is that [[Physics:Color charge|color-charged]] particles (such as quarks and gluons) cannot be separated from their parent [[Physics:Hadron|hadron]] without producing new hadrons.<ref>
{{cite book
 |last1=Wu     |first1=T.-Y.
 |year=1991
 |title=Relativistic Quantum Mechanics and Quantum Fields
 |publisher=[[Company:World Scientific|World Scientific]]
 |isbn=978-981-02-0608-6
}}
</ref> Is it possible to provide an analytic proof of color confinement in any non-abelian gauge theory?

[[File:Quark-gluon-plasma.jpg|thumb|right|Colour Confinement is the tendency of Quarks and Gluons to form colour neutral groups.]]

* The [[Physics:QCD vacuum|QCD vacuum]]: Many of the equations in non-perturbative [[Physics:QCD vacuum|QCD]] are currently unsolved. These energies are the energies sufficient for the formation of [[Physics:Atomic nucleus|atomic nuclei]]. How thus does low energy QCD give rise to the formation of complex nuclei and nuclear constituents?{{cn|date=December 2023}}
* [[Physics:Generation (particle physics)|Generations of matter]]: Why are there three generations of [[Physics:Quark|quark]]s and [[Physics:Lepton|lepton]]s? 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>
{{cite journal
 | author1=Blumhofer, A.
 | author2=Hutter, M. 
 | year = 1997
 | title = Family structure from periodic solutions of an improved gap equation
 | journal = Nuclear Physics
 | volume =  B484 | issue=1 | pages = 80–96
 | doi = 10.1016/S0550-3213(96)00644-X
 |bibcode = 1997NuPhB.484...80B | citeseerx=10.1.1.343.783
}}
</ref>
* Neutrino mass: What is the mass of neutrinos, whether they follow [[Physics:Fermi–Dirac statistics|Dirac]] or [[Physics:Majorana fermion|Majorana]] statistics? Is the mass hierarchy normal or inverted? Is the CP violating phase equal to 0?<ref name=ino-tifr>{{cite web |title=India-based Neutrino Observatory (INO) |publisher=Tata Institute of Fundamental Research |url=http://www.ino.tifr.res.in/ino/ |access-date=14 April 2012 |archive-date=26 April 2012 |archive-url=https://web.archive.org/web/20120426214134/http://www.ino.tifr.res.in/ino/ |url-status=live }}</ref><ref>{{cite journal |last1=Nakamura |first1=K. |display-authors=0 |collaboration=[[Physics:Particle Data Group|Particle Data Group]] |year=2010 |title=2011 Review of Particle Physics |journal=J. Phys. G |volume=37 |issue=7A |page=075021 |doi=10.1088/0954-3899/37/7A/075021 |doi-access=free |bibcode=2010JPhG...37g5021N |url=http://pdg.lbl.gov/2011/reviews/contents_sports.html |access-date=25 April 2012 |archive-date=23 April 2012 |archive-url=https://web.archive.org/web/20120423081050/http://pdg.lbl.gov/2011/reviews/contents_sports.html |url-status=live |hdl=10481/34593 |hdl-access=free }}</ref>
* Reactor antineutrino anomaly: There is an anomaly in the existing body of data regarding the [[Physics:Neutrino|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>{{cite journal |last1=Mention |first1=G. |last2=Fechner |first2=M. |last3=Lasserre |first3=Th. |last4=Mueller |first4=Th.A. |last5=Lhuillier |first5=D. |last6=Cribier |first6=M. |last7=Letourneau |first7=A. |date=2011-04-29 |title=Reactor antineutrino anomaly |journal=Physical Review D |volume=83 |issue=7 |page=073006 |doi=10.1103/PhysRevD.83.073006 |bibcode=2011PhRvD..83g3006M |s2cid=14401655 |arxiv=1101.2755 |url=https://link.aps.org/doi/10.1103/PhysRevD.83.073006 |access-date=2 October 2021 |archive-date=17 January 2023 |archive-url=https://web.archive.org/web/20230117023333/https://journals.aps.org/prd/abstract/10.1103/PhysRevD.83.073006 |url-status=live }}</ref> It is unknown whether this is due to unknown physics (such as [[Physics:Sterile neutrino|sterile neutrino]]s), experimental error in the measurements, or errors in the theoretical flux calculations.<ref>{{cite journal |last=Fallot |first=Muriel |date=2017-06-19 |title=Getting to the bottom of an antineutrino anomaly |journal=Physics |volume=10 |page=66 |lang=en |doi=10.1103/Physics.10.66 |bibcode=2017PhyOJ..10...66F |url=https://physics.aps.org/articles/v10/66 |access-date=2 October 2021 |archive-date=2 October 2021 |archive-url=https://web.archive.org/web/20211002140625/https://physics.aps.org/articles/v10/66 |url-status=live |doi-access=free }}</ref>
* [[Physics:Strong CP problem|Strong CP problem]] and [[Physics:Axion|axion]]s: Why is the strong nuclear interaction invariant to [[Physics:Parity|parity]] and [[Physics:Charge conjugation|charge conjugation]]? Is [[Physics:Peccei–Quinn theory|Peccei–Quinn theory]] the solution to this problem? Could axions be the main component of [[Astronomy:Dark matter|dark matter]]?
* [[Physics:Anomalous magnetic dipole moment|Anomalous magnetic dipole moment]]: Why is the experimentally measured value of the [[Physics:Muon|muon]]'s anomalous magnetic dipole moment ("muon {{nobr|{{mvar|g}} − 2}}") significantly different from the theoretically predicted value of that physical constant?<ref>{{cite arXiv |first1=Thomas |last1=Blum |first2=Achim |last2=Denig |first3=Ivan |last3=Logashenko |first4=Eduardo |last4=de Rafael |first5=B. Lee |last5=Roberts |first6=Thomas |last6=Teubner |first7=Graziano |last7=Venanzoni |year=2013 |title=The muon ({{nobr|{{mvar|g}} − 2}}) theory value: Present and future |class=hep-ph |eprint=1311.2198 }}</ref>
* [[Physics:Proton radius puzzle|Proton radius puzzle]]: What is the electric [[Physics:Charge radius|charge radius]] of the proton? How does it differ from a gluonic charge?
* [[Physics:Pentaquark|Pentaquark]]s and other [[Physics:Exotic hadron|exotic hadron]]s: What combinations of quarks are possible? Why were pentaquarks so difficult to discover?<ref name=NewScientist2003>{{cite web |author=Muir, H. |date=2 July 2003 |title=Pentaquark discovery confounds skeptics |magazine=New Scientist |url=https://www.newscientist.com/article/dn3903-pentaquark-discovery-confounds-sceptics.html |access-date=8 January 2010 |archive-date=10 October 2008 |archive-url=https://web.archive.org/web/20081010182322/http://www.newscientist.com/article/dn3903-pentaquark-discovery-confounds-sceptics.html |url-status=live }}</ref> Are they a tightly-bound system of five elementary particles, or a more weakly-bound pairing of a [[Physics:Baryon|baryon]] and a [[Physics:Meson|meson]]?<ref name=NewScientist2015>{{cite news |author=Amit, G. |date=14 July 2015 |title=Pentaquark discovery at LHC shows long-sought new form of matter |magazine=New Scientist |url=https://www.newscientist.com/article/dn27892-pentaquark-discovery-at-lhc-shows-long-sought-new-form-of-matter/ |access-date=14 July 2015 |archive-date=8 November 2020 |archive-url=https://web.archive.org/web/20201108163418/https://www.newscientist.com/article/dn27892-pentaquark-discovery-at-lhc-shows-long-sought-new-form-of-matter/ |url-status=live }}</ref>
* [[Physics:Mu problem|Mu problem]]: A problem in [[Physics:Supersymmetry|supersymmetric]] theories, concerned with understanding the reasons for parameter values of the theory.
* [[Physics:Koide formula|Koide formula]]: An aspect of the [[Physics:Generation (particle physics)|problem of particle generations]]. The sum of the masses of the three charged [[Physics:Lepton|lepton]]s, divided by the square of the sum of the roots of these masses, to within one standard deviation of observations, is {{Math|''Q'' {{=}} {{frac|2|3}}}}. It is unknown how such a simple value comes about, and why it is the exact arithmetic average of the possible extreme values of {{sfrac| 1 |3}} (equal masses) and 1 (one mass dominates).
* [[Physics:Strange matter|Strange Matter]]: Does Strange Matter exist? Is it stable? Can they form [[Astronomy:Strange star|Strange Stars]]? There have been various hypothesis and conjectures that it could form inside [[Astronomy:Exotic star|Exotic Stars]] such as [[Astronomy:Quark star|Quark Stars]] in which they would be stable. So stable that they would be the most stable thing in the [[Astronomy:Universe|Universe]] and even hypothetically outlive the Heat Death of the Universe.
* [[Physics:Glueball|Glueballs]]: Do they exist in nature? What configurations of them are stable?

== Astronomy and astrophysics ==
{{main|Astronomy:List of unsolved problems in astronomy}}
* [[Astronomy:Solar cycle|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>{{cite journal|arxiv=1406.4228|author1=Michael J. Thompson|title=Grand Challenges in the Physics of the Sun and Sun-like Stars|journal=Frontiers in Astronomy and Space Sciences|volume=1|pages=1|year=2014|bibcode=2014FrASS...1....1T|doi=10.3389/fspas.2014.00001|s2cid=1547625|doi-access=free}}</ref> What caused the [[Astronomy:Maunder Minimum|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 [[Astronomy:Magnetic reconnection|magnetic reconnection]] effect many orders of magnitude faster than predicted by standard models?
* [[Astronomy:Astrophysical jet|Astrophysical jet]]: Why do only certain accretion discs surrounding certain astronomical objects emit relativistic jets along their polar axes? Why are there [[Astronomy:Quasi-periodic oscillation|quasi-periodic oscillation]]s in many accretion discs?<ref>{{cite journal|last=Strohmayer|first=Tod E.|author2=Mushotzky, Richard F.|title=Discovery of X-Ray Quasi-periodic Oscillations from an Ultraluminous X-Ray Source in M82: Evidence against Beaming|journal=The Astrophysical Journal|date=20 March 2003|volume=586|issue=1|pages=L61–L64|doi=10.1086/374732|arxiv = astro-ph/0303665 |bibcode = 2003ApJ...586L..61S |s2cid=118992703}}</ref> Why does the period of these oscillations scale as the inverse of the mass of the central object?<ref>{{cite journal|last=Titarchuk|first=Lev|author2=Fiorito, Ralph|title=Spectral Index and Quasi-Periodic Oscillation Frequency Correlation in Black Hole Sources: Observational Evidence of Two Phases and Phase Transition in Black Holes|journal=The Astrophysical Journal|date=10 September 2004|volume=612|issue=2|pages=988–999|doi=10.1086/422573|url=http://159.226.72.19/share/reference/x-raybinary/Titarchuk-Fiorito04.pdf|access-date=25 January 2013|arxiv=astro-ph/0405360|bibcode=2004ApJ...612..988T|hdl=2060/20040182332|s2cid=4689535|url-status=dead|archive-url=https://web.archive.org/web/20140203131257/http://159.226.72.19/share/reference/x-raybinary/Titarchuk-Fiorito04.pdf|archive-date=3 February 2014}}</ref> Why are there sometimes overtones, and why do these appear at different frequency ratios in different objects?<ref>{{Cite journal|arxiv=1202.0121|author1=Shoji Kato|title=An Attempt to Describe Frequency Correlations among kHz QPOs and HBOs by Two-Armed Nearly Vertical Oscillations|journal=Publications of the Astronomical Society of Japan|volume=64|issue=3|pages=62|year=2012|doi=10.1093/pasj/64.3.62|bibcode=2012PASJ...64...62K|s2cid=118498018}}</ref>
* [[Astronomy:Diffuse interstellar bands|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?
* [[Astronomy:Supermassive black hole|Supermassive black hole]]s: What is the origin of the [[Astronomy:M–sigma relation|M–sigma relation]] between supermassive black hole mass and galaxy velocity dispersion?<ref>{{Cite journal|last1=Ferrarese|first1=Laura|last2=Merritt|first2=David|title=A Fundamental Relation between Supermassive Black Holes and their Host Galaxies|journal=The Astrophysical Journal|volume=539|issue=1|pages=L9–L12|year=2000|doi=10.1086/312838|arxiv=astro-ph/0006053|bibcode=2000ApJ...539L...9F |s2cid=6508110}}</ref> How did the most distant [[Astronomy:Quasar|quasar]]s grow their supermassive black holes up to 10{{sup|10}} solar masses so early in the history of the universe?
* Kuiper cliff: Why does the number of objects in the Solar System's [[Astronomy:Kuiper belt|Kuiper belt]] fall off rapidly and unexpectedly beyond a radius of 50 astronomical units?
* [[Physics:Flyby anomaly|Flyby anomaly]]: Why is the observed energy of satellites [[Astronomy:Gravity assist|flying by planetary bodies]] sometimes different by a minute amount from the value predicted by theory?
* Galaxy rotation problem: Is [[Astronomy:Dark matter|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.svg|frame|right|Rotation curve of a typical spiral galaxy: predicted ('''A''') and observed ('''B'''). Can the discrepancy between the curves be attributed to dark matter?]]
* [[Astronomy:Supernova|Supernova]]e: What is the exact mechanism by which an implosion of a dying star becomes an explosion?
* [[Physics:P-nuclei|p-nuclei]]: What astrophysical process is responsible for the nucleogenesis of these rare isotopes?
* [[Astronomy:Ultra-high-energy cosmic ray|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 [[Astronomy:Greisen–Zatsepin–Kuzmin limit|Greisen–Zatsepin–Kuzmin limit]]?<ref name="Open Questions">{{cite web|url=http://math.ucr.edu/home/baez/physics/General/open_questions.html|title=Open Questions in Physics|last=Baez|first=John C.|date=March 2006|work=Usenet Physics FAQ|publisher=[[Organization:University of California, Riverside|University of California, Riverside]]: Department of Mathematics|access-date=7 March 2011|archive-date=4 June 2011|archive-url=https://web.archive.org/web/20110604154302/http://math.ucr.edu/home/baez/physics/General/open_questions.html|url-status=live}}</ref><ref name="newscientist" />
* Rotation rate of [[Astronomy:Saturn|Saturn]]: Why does the [[Astronomy:Magnetosphere of Saturn|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>{{cite web|url=http://www.nasa.gov/mission_pages/cassini/media/cassini-062804.html|title=Scientists Find That Saturn's Rotation Period is a Puzzle|date=28 June 2004|publisher=NASA|access-date=22 March 2007|archive-date=29 August 2011|archive-url=https://web.archive.org/web/20110829082445/http://www.nasa.gov/mission_pages/cassini/media/cassini-062804.html|url-status=live}}</ref>
* Origin of [[Astronomy:Magnetar#Magnetic field|magnetar magnetic field]]: What is the origin of [[Astronomy:Magnetar|magnetar]] magnetic field?
* Large-scale anisotropy: Is the universe at very large scales [[Anisotropy|anisotropic]], making the [[Astronomy:Cosmological principle|cosmological principle]] an invalid assumption? The number count and intensity dipole anisotropy in radio, NRAO VLA Sky Survey (NVSS) catalogue<ref>{{Cite journal|bibcode=1998AJ....115.1693C|pages=1693–1716|title=The NRAO VLA Sky Survey|journal=The Astronomical Journal|volume=115|issue=5|last1=Condon|first1=J. J.|last2=Cotton|first2=W. D.|last3=Greisen|first3=E. W.|last4=Yin|first4=Q. F.|last5=Perley|first5=R. A.|last6=Taylor|first6=G. B.|last7=Broderick|first7=J. J.|year=1998|doi=10.1086/300337|s2cid=120464396 |doi-access=free}}</ref> is inconsistent with the local motion as derived from [[Astronomy:Cosmic microwave background|cosmic microwave background]]<ref>{{Cite journal|arxiv=1110.6260|last1= Singal|first1= Ashok K.|title= Large peculiar motion of the solar system from the dipole anisotropy in sky brightness due to distant radio sources|journal= The Astrophysical Journal|volume= 742|issue= 2|pages= L23–L27|year= 2011|doi= 10.1088/2041-8205/742/2/L23|bibcode= 2011ApJ...742L..23S|s2cid= 119117071}}</ref><ref>{{cite journal |doi=10.1016/j.astropartphys.2014.06.004 |title=Dipole anisotropy in sky brightness and source count distribution in radio NVSS data |journal=Astroparticle Physics |volume=61 |pages=1–11 |year=2015 |last1=Tiwari |first1=Prabhakar |last2=Kothari |first2=Rahul |last3=Naskar |first3=Abhishek |last4=Nadkarni-Ghosh |first4=Sharvari |last5=Jain |first5=Pankaj |arxiv=1307.1947 |bibcode=2015APh....61....1T |s2cid=119203300 }}</ref> and indicate an intrinsic dipole anisotropy. The same NVSS radio data also shows an intrinsic dipole in polarization density and degree of polarization<ref>{{cite journal|doi=10.1093/mnras/stu2535|title=Dipole anisotropy in integrated linearly polarized flux density in NVSS data|journal=Monthly Notices of the Royal Astronomical Society|volume=447|issue=3|pages=2658–2670|year=2015|last1=Tiwari|first1=P.|last2=Jain|first2=P.|arxiv=1308.3970|bibcode=2015MNRAS.447.2658T|s2cid=118610706}}</ref> in the same direction as in number count and intensity. There are several other observations revealing large-scale anisotropy. The optical polarization from quasars shows polarization alignment over a very large scale of Gpc.<ref>{{cite journal| bibcode=1998A&A...332..410H| pages=410–428| title=Evidence for very large-scale coherent orientations of quasar polarization vectors| journal=Astronomy and Astrophysics| volume=332| last1=Hutsemekers| first1=D.| year=1998| url=https://orbi.uliege.be/bitstream/2268/3811/1/1998A%2bA...332..410H.pdf}}</ref><ref>{{cite journal|arxiv=astro-ph/0012182|pages=381–387|doi=10.1051/0004-6361:20000443|title=Confirmation of the existence of coherent orientations of quasar polarization vectors on cosmological scales|journal=Astronomy & Astrophysics|volume=367|issue=2|year=2001|last1=Hutsemékers|first1=D.|last2=Lamy|first2=H.|bibcode=2001A&A...367..381H|s2cid=17157567}}</ref><ref>{{Cite journal|arxiv=astro-ph/0301530| pages=394–402|doi=  10.1111/j.1365-2966.2004.07169.x| title=Large-scale alignment of optical polarizations from distant QSOs using coordinate-invariant statistics| journal=Monthly Notices of the Royal Astronomical Society| volume=347| issue=2| year=2004| last1=Jain| first1=P.| last2=Narain| first2=G.| last3=Sarala| first3=S.| bibcode=2004MNRAS.347..394J| s2cid=14190653}}</ref> The cosmic-microwave-background data shows several features of anisotropy,<ref>{{Cite journal|arxiv=astro-ph/0307282|author1=Angelica de Oliveira-Costa|title=The significance of the largest scale CMB fluctuations in WMAP|journal=Physical Review D|volume=69|issue=6|pages=063516|last2=Tegmark|first2=Max|last3=Zaldarriaga|first3=Matias|last4=Hamilton|first4=Andrew|year=2004|doi=10.1103/PhysRevD.69.063516|bibcode=2004PhRvD..69f3516D|s2cid=119463060}}</ref><ref>{{Cite journal|bibcode=2004ApJ...605...14E|pages=14–20|title=Asymmetries in the Cosmic Microwave Background Anisotropy Field|journal=The Astrophysical Journal|volume=605|issue=1|last1=Eriksen|first1=H. K.|last2=Hansen|first2=F. K.|last3=Banday|first3=A. J.|last4=Górski|first4=K. M.|last5=Lilje|first5=P. B.|year=2004|doi=10.1086/382267|arxiv=astro-ph/0307507|s2cid=15696508}}</ref><ref>{{Cite journal|arxiv=0708.2816|author1=Pramoda Kumar Samal|title=Testing Isotropy of Cosmic Microwave Background Radiation|journal=Monthly Notices of the Royal Astronomical Society|volume=385|issue=4|pages=1718–1728|last2=Saha|first2=Rajib|last3=Jain|first3=Pankaj|last4= Ralston|first4=John P.|year=2008|doi=10.1111/j.1365-2966.2008.12960.x|bibcode=2008MNRAS.385.1718S|s2cid=988092}}</ref><ref>{{Cite journal|arxiv=0811.1639|author1=Pramoda Kumar Samal|title=Signals of Statistical Anisotropy in WMAP Foreground-Cleaned Maps|journal=Monthly Notices of the Royal Astronomical Society|volume=396|issue=511|pages=511–522|last2=Saha|first2=Rajib|last3=Jain|first3=Pankaj|last4= Ralston|first4=John P.|year=2009|doi=10.1111/j.1365-2966.2009.14728.x|bibcode=2009MNRAS.396..511S|s2cid=16250321}}</ref> which are not consistent with the [[Astronomy:Big Bang|Big Bang]] model.
* 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 [[Astronomy:Milky Way|Milky Way]] there is no evidence of a strong AMR,<ref>{{Cite journal|doi=10.1051/0004-6361/201016276|title=New constraints on the chemical evolution of the solar neighbourhood and Galactic disc(s)|journal=Astronomy & Astrophysics|volume=530|pages=A138|year=2011|last1=Casagrande|first1=L.|last2=Schönrich|first2=R.|last3=Asplund|first3=M.|last4=Cassisi|first4=S.|last5=Ramírez|first5=I.|last6=Meléndez|first6=J.|last7=Bensby|first7=T.|last8=Feltzing|first8=S.|author8-link= Sofia Feltzing |bibcode=2011A&A...530A.138C|arxiv=1103.4651|s2cid=56118016}}</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>{{cite journal|last=Bensby|first=T.|author2=Feltzing, S.|author3=Lundström, I.|title=A possible age–metallicity relation in the Galactic thick disk?|journal=Astronomy and Astrophysics|date=July 2004|volume=421|issue=3|pages=969–976|doi=10.1051/0004-6361:20035957|arxiv = astro-ph/0403591 |bibcode = 2004A&A...421..969B |s2cid=10469794}}</ref><ref>{{Cite journal|bibcode=2011sca..conf..280G|title=Open Issues in the Evolution of the Galactic Disks|journal=Stellar Clusters & Associations: A RIA Workshop on Gaia. Proceedings. Granada|pages=280|last1=Gilmore|first1=G.|last2=Asiri|first2=H. M.|year=2011}}</ref> Stellar ages from asteroseismology confirm the lack of any strong age–metallicity relation in the Galactic disc.<ref>{{Cite journal|bibcode=2016MNRAS.455..987C|doi=10.1093/mnras/stv2320|title=Measuring the vertical age structure of the Galactic disc using asteroseismology and SAGA|journal=Monthly Notices of the Royal Astronomical Society|volume=455|issue=1|pages=987–1007|year=2015|last1=Casagrande|first1=L.|last2=Silva Aguirre|first2=V.|last3=Schlesinger|first3=K. J.|last4=Stello|first4=D.|last5=Huber|first5=D.|last6=Serenelli|first6=A. M.|last7=Scho Nrich|first7=R.|last8=Cassisi|first8=S.|last9=Pietrinferni|first9=A.|last10=Hodgkin|first10=S.|last11=Milone|first11=A. P.|last12=Feltzing|first12=S.|author12-link= Sofia Feltzing |last13=Asplund|first13=M.|arxiv=1510.01376|s2cid=119113283}}</ref>
* [[Astronomy:Cosmological lithium problem|The lithium problem]]: Why is there a discrepancy between the amount of lithium-7 predicted to be produced in [[Astronomy:Big Bang nucleosynthesis|Big Bang nucleosynthesis]] and the amount observed in very old stars?<ref>{{Cite journal|arxiv=1203.3551|last1= Fields|first1= Brian D.|title= The Primordial Lithium Problem|journal= [[Physics:Annual Review of Nuclear and Particle Science|Annual Review of Nuclear and Particle Science]]|volume= 61|issue= 2011|pages= 47–68|year= 2012|doi= 10.1146/annurev-nucl-102010-130445| doi-access=free|bibcode= 2011ARNPS..61...47F|s2cid= 119265528}}</ref>
* [[Astronomy:Ultraluminous X-ray source|Ultraluminous X-ray source]]s (ULXs): What powers X-ray sources that are not associated with active galactic nuclei but exceed the Eddington limit of a [[Astronomy:Neutron star|neutron star]] or [[Astronomy:Stellar black hole|stellar black hole]]? Are they due to [[Astronomy:Intermediate-mass black hole|intermediate-mass black hole]]s? Some ULXs are periodic, suggesting non-isotropic emission from a neutron star. Does this apply to all ULXs? How could such a system form and remain stable?
* [[Astronomy:Fast radio burst|Fast radio burst]]s (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>{{cite journal|doi=10.1016/j.physrep.2019.06.003|title=A living theory catalogue for fast radio bursts|journal=Physics Reports|volume=821|pages=1–27|year=2019|last1=Platts|first1=E.|last2=Weltman|first2=A.|last3=Walters|first3=A.|last4=Tendulkar|first4=S.P.|last5=Gordin|first5=J.E.B.|last6=Kandhai|first6=S.|bibcode=2019PhR...821....1P|arxiv=1810.05836|s2cid=119091423}}</ref>
* 	Are voids in space empty or consist of transparent matter?<ref>{{cite journal |last1=Fong |first1=Richard |last2=Doroshkevich |first2=Andrei |last3=Turchaninov |first3=Victor |year=1995 |title=Dark Matter in Voids |journal=AIP Conference Proceedings |volume=336 |pages=429–432 |doi=10.1063/1.48369|bibcode=1995AIPC..336..429F }}</ref><ref>{{cite arXiv|last1=Doroshkevich|first1=Andrei|last2=Fong|first2=Richard|last3=Gottlober|first3=Stefan|last4=Mucket|first4=Jan|last5=Muller|first5=Volker|date=1995|title=The formation and evolution of large- and superlarge-scale structure in the universe - I: General Theory |eprint=astro-ph/9505088}}</ref><ref>{{cite journal |last1=Geller |first1=Margaret |last2=Hwang |first2=Ho Seong |year=2015 |title=Schwarzschild Lecture 2014: HectoMAPping the Universe |journal=Astronomische Nachrichten |volume=336 |pages=428 |doi=10.1002/asna.201512182 |arxiv=1507.06261|s2cid=11799339 }}</ref><ref>{{cite journal|last1=Ben-Amots|first1=N.|date=2021|title=Helium as a major portion of the dark matter and the cell structure of the universe|journal=[[Physics:Journal of Physics: Conference Series|Journal of Physics]]|volume=1956|issue=1 |pages=012006|doi=10.1088/1742-6596/1956/1/012006|bibcode=2021JPhCS1956a2006B |s2cid=235828320 |doi-access=free}}</ref>

== Nuclear physics ==
[[File:Island-of-Stability.png|thumb|right|upright=2|The "[[Physics:Island of stability|island of stability]]" in the proton vs. neutron number plot for heavy nuclei]]
* [[Physics:Quantum chromodynamics|Quantum chromodynamics]]: What are the phases of strongly interacting matter, and what roles do they play in the evolution of the [[Astronomy:Cosmos|cosmos]]? What is the detailed [[Physics:Parton (particle physics)|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 [[Physics:Gravity|gravity]] and [[Physics:Spacetime|spacetime]]? Does QCD truly lack [[Physics:CP violation|CP violation]]s?
* [[Physics:Quark–gluon plasma|Quark–gluon plasma]]: Where is the onset of [[Physics:Deconfinement|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 [[Physics:Baryon number|baryon-number]] stopping leading to creation of quark-gluon plasma in relativistic heavy-ion collisions? Why is sudden [[Physics:Hadronization|hadronization]] and the statistical-hadronization model a near-to-perfect description of [[Physics:Hadron|hadron]] production from quark–gluon plasma? Is quark flavor conserved in quark–gluon plasma? Are [[Physics:Strangeness and quark–gluon plasma|strangeness]] and [[Physics:Charm (quantum number)|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 [[Physics:Quark–gluon plasma#Flow|ideal flow]]?
* Specific models of quark–gluon plasma formation: Do [[Physics:Gluon|gluon]]s 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–[[Biography:Lev Lipatov|Lipatov]], Balitsky–Kovchegov, Catani–Ciafaloni–Fiorani–Marchesini evolution equations?
* [[Physics:Atomic nucleus|Nuclei]] and [[Physics:Nuclear astrophysics|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 [[Physics:Nuclear force|nuclear force]] that binds protons and neutrons into [[Physics:Stable isotope|stable nuclei]] and rare isotopes? What is the explanation for the [[Physics:EMC effect|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 [[Physics:Nuclear matter|nuclear matter]]? What is the origin of the elements in the [[Astronomy:Cosmos|cosmos]]? What are the nuclear reactions that drive stars and stellar explosions? What is the [[Chemistry:Extended periodic table|heaviest possible chemical element]]?

== Fluid dynamics ==
* Under what conditions do [[Navier–Stokes existence and smoothness|smooth solutions exist for the Navier–Stokes equations]], which are the equations that describe the flow of a [[Physics:Viscosity|viscous]] fluid? This problem, for an incompressible fluid in three dimensions, is also one of the Millennium Prize Problems in mathematics.<ref>{{cite web |url=https://www.claymath.org/sites/default/files/navierstokes.pdf |title=Existence and Uniqueness of the Navier-Stokes Equation |author=Charles Fefferman |publisher=Clay Mathematics Institute |access-date=29 April 2021 |archive-date=14 November 2020 |archive-url=https://web.archive.org/web/20201114191149/http://www.claymath.org/sites/default/files/navierstokes.pdf |url-status=live }}</ref>
*[[Physics:Turbulence|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" /> 
* [[Physics:Upstream contamination|Upstream contamination]]: When pouring water from a higher container to a lower one, particles floating in the latter can climb upstream into the upper container. A definitive explanation for this phenomenon is still lacking.
* [[Physics:Granular convection|Granular convection]]: why does a [[Chemistry:Granular material|granular material]] subjected to shaking or vibration exhibit circulation patterns similar to types of fluid [[Astronomy:Convection|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?

== Condensed matter physics ==
[[File:BI2223-piece3 001.jpg|thumb|right|upright=0.8|A sample of a [[Physics:Cuprate superconductor|cuprate superconductor]] (specifically BSCCO). The mechanism for superconductivity of these materials is unknown.]]
* Bose–Einstein condensation: How do we rigorously prove the existence of Bose–Einstein condensates for general interacting systems?<ref name="schlein-BEC">{{cite web |last=Schlein |first=Benjamin |title=Graduate Seminar on Partial Differential Equations in the Sciences – Energy and Dynamics of Boson Systems |url=http://www.hcm.uni-bonn.de/homepages/prof-dr-benjamin-schlein/teaching/graduate-seminar-on-pdes-in-the-sciences/ |url-status=live |archive-url=https://web.archive.org/web/20130504182433/http://www.hcm.uni-bonn.de/homepages/prof-dr-benjamin-schlein/teaching/graduate-seminar-on-pdes-in-the-sciences/ |archive-date=4 May 2013 |access-date=23 April 2012 |publisher=Hausdorff Center for Mathematics}}</ref>
* [[Physics:High-temperature superconductor|High-temperature superconductor]]s: What is the mechanism that causes certain materials to exhibit [[Physics:Superconductivity|superconductivity]] at temperatures much higher than around 25 [[Kelvin|kelvin]]s? Is it possible to make a material that is a [[Physics:Room-temperature superconductor|superconductor at room temperature and atmospheric pressure]]?<ref name="Open Questions" />
* [[Physics:Amorphous solid|Amorphous solid]]s: What is the nature of the [[Biology:Glass transition|glass transition]] between a fluid or regular solid and a glassy [[Physics:Phase (matter)|phase]]? What are the physical processes giving rise to the general properties of glasses and the glass transition?<ref>{{Cite news|url=https://www.nytimes.com/2008/07/29/science/29glass.html|title=The Nature of Glass Remains Anything but Clear|journal={{wipe|The New York Times}}|date=29 July 2008|author=Kenneth Chang|access-date=17 February 2017|archive-date=14 September 2017|archive-url=https://web.archive.org/web/20170914193149/https://www.nytimes.com/2008/07/29/science/29glass.html|url-status=live}}</ref><ref>{{Cite journal|author=P.W. Anderson|journal=Science|volume=267|year=1995|pages=1615–1616|title=Through the Glass Lightly|doi=10.1126/science.267.5204.1615-e|pmid=17808155|quote=The deepest and most interesting unsolved problem in solid state theory is probably the theory of the nature of glass and the glass transition.|issue=5204|s2cid=28052338}}</ref><ref name=":Z">{{Cite book |last1=Zaccone |first1=A. |title=Theory of Disordered Solids |series=Lecture Notes in Physics |publisher=Springer  |year=2023 |volume=1015 |doi=10.1007/978-3-031-24706-4|isbn=978-3-031-24705-7 |s2cid=259299183 |edition=1st}}</ref>
* [[Physics:Amorphous solid#Universal low-temperature properties of amorphous solids|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">{{Cite journal |last1=Pohl |first1=R.O. |last2=etc |first2=etc |title=Low-temperature thermal conductivity and acoustic attenuation in amorphous solids |year=2002 |journal=Rev. Mod Phys. |volume=74 |pages=991|doi=10.1080/14786437208229210}}</ref><ref name="Leggett1991">{{Cite journal |last1=Leggett |first1=A.J. |title=Amorphous materials at low temperatures: why are they so similar?|year=1991 |journal=Physica B |volume=169 |issue=1–4 |pages=322–327 |doi=10.1016/0921-4526(91)90246-B|bibcode=1991PhyB..169..322L }}</ref> This small ratio is observed for very large range of phonon frequencies.
* Cryogenic electron emission: Why does the electron emission in the absence of light increase as the temperature of a [[Physics:Photomultiplier|photomultiplier]] is decreased?<ref>[http://www.physorg.com/news187421719.html Cryogenic electron emission phenomenon has no known physics explanation] {{Webarchive|url=https://web.archive.org/web/20110605231503/http://www.physorg.com/news187421719.html |date=5 June 2011 }}. Physorg.com. Retrieved on 20 October 2011.</ref><ref>{{cite journal|last=Meyer|first=H. O.|title=Spontaneous electron emission from a cold surface|journal=Europhysics Letters|date=1 March 2010|volume=89|issue=5|pages=58001|doi=10.1209/0295-5075/89/58001|bibcode=2010EL.....8958001M|s2cid=122528463 |url=https://zenodo.org/record/896477|access-date=20 April 2018|archive-date=20 February 2020|archive-url=https://web.archive.org/web/20200220194241/https://zenodo.org/record/896477|url-status=live}}</ref>
* [[Physics:Sonoluminescence|Sonoluminescence]]: What causes the emission of short bursts of light from imploding bubbles in a liquid when excited by sound?<ref>{{cite journal|last1=Storey|first1=B. D.|last2=Szeri|first2=A. J.|title=Water vapour, sonoluminescence and sonochemistry|journal=Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences|date=8 July 2000|volume=456|issue=1999|pages=1685–1709|doi=10.1098/rspa.2000.0582|bibcode=2000RSPSA.456.1685S|s2cid=55030028}}</ref><ref>{{cite journal|last1=Wu|first1=C. C.|last2=Roberts|first2=P. H.|title=A Model of Sonoluminescence|journal=Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences|date=9 May 1994|volume=445|issue=1924|pages=323–349|doi=10.1098/rspa.1994.0064|bibcode=1994RSPSA.445..323W|s2cid=122823755}}</ref>
* [[Physics:Topological order|Topological order]]: Is topological order stable at non-zero [[Physics:Temperature|temperature]]? Equivalently, is it possible to have three-dimensional [[Stabilizer code|self-correcting]] [[Qubit|quantum memory]]?<ref name="Annals of Physics 2011 pp. 2566–2633">{{cite journal | title=Feasibility of self-correcting quantum memory and thermal stability of topological order | journal=[[Physics:Annals of Physics|Annals of Physics]] | volume=326 | issue=10 | date=1 October 2011 | issn=0003-4916 | doi=10.1016/j.aop.2011.06.001 | pages=2566–2633 |arxiv=1103.1885|bibcode=2011AnPhy.326.2566Y | last1=Yoshida | first1=Beni | s2cid=119611494 }}</ref>
* [[Engineering:Gauge block#Wringing|Gauge block wringing]]: What mechanism allows gauge blocks to be wrung together?
[[File:FQHE Hall.png|thumb|right|upright=1|Magnetoresistance in a {{nowrap|1=''u'' = 8/5}} fractional quantum Hall state]]
* Fractional Hall effect: What mechanism explains the existence of the {{nowrap|1=''u'' = 5/2}} state in the fractional [[Physics:Quantum Hall effect|quantum Hall effect]]? Does it describe quasiparticles with non-Abelian fractional statistics?<ref>{{Cite journal|last=Dean|first=Cory R.|date=2015|title=Even denominators in odd places|journal=[[Physics:Nature Physics|Nature Physics]]|language=En|volume=11|issue=4|pages=298–299|doi=10.1038/nphys3298|issn=1745-2481|bibcode=2015NatPh..11..298D|s2cid=123159205 }}</ref><!-- archive of a blog post; needs a better source to justify inclusion <ref name=podolsky-NEQNET-fqhe>{{cite web|last=Podolsky|first=Dmitry|title=Quantum Hall effect. One open question|url=http://www.nonequilibrium.net/203-quantum-hall-effect-open-question/|archive-url=https://archive.today/20130222015343/http://www.nonequilibrium.net/203-quantum-hall-effect-open-question/|url-status=dead|archive-date=22 February 2013|publisher=NEQNET|access-date=23 April 2012}}</ref> -->
* [[Physics:Liquid crystal|Liquid crystal]]s: Can the nematic to smectic (A) phase transition in liquid crystal states be characterized as a [[Physics:Background independence|universal]] phase transition?<ref name=mukherjee-liqcryst>{{cite journal|last=Mukherjee|first=Prabir K.|title=Landau Theory of Nematic-Smectic-A Transition in a Liquid Crystal Mixture|journal=Molecular Crystals & Liquid Crystals|year=1998|volume=312|issue=1 |pages=157–164|doi=10.1080/10587259808042438|bibcode=1998MCLCA.312..157M }}</ref><ref name=yethiraj-liqcryst>A. Yethiraj, [http://www.physics.mun.ca/~anand/pdf/NAReview.pdf "Recent Experimental Developments at the Nematic to Smectic-A Liquid Crystal Phase Transition"] {{Webarchive|url=https://web.archive.org/web/20130515221326/http://www.physics.mun.ca/~anand/pdf/NAReview.pdf |date=15 May 2013 }}, Thermotropic Liquid Crystals: Recent Advances, ed. A. Ramamoorthy, Springer 2007, chapter 8.</ref>
* Semiconductor nanocrystals: What is the cause of the nonparabolicity of the energy-size dependence for the lowest [[Physics:Absorption (electromagnetic radiation)|optical absorption transition]] of quantum dots?<ref name=norris-electronic>{{cite book|last=Norris|first=David J.|title=Electronic Structure in Semiconductors Nanocrystals: Optical Experiment (in ''Semiconductor and Metal Nanocrystals: Synthesis and Electronic and Optical Properties'')|chapter=The Problem Swept Under the Rug|page=97|editor-last=Klimov|editor-first=Victor|year=2003|publisher=CRC Press|isbn=978-0-203-91326-0|chapter-url=https://books.google.com/books?id=sarqqnaw-7oC&pg=PA97|access-date=18 October 2020|archive-date=27 April 2022|archive-url=https://web.archive.org/web/20220427065508/https://books.google.com/books?id=sarqqnaw-7oC&pg=PA97|url-status=live}}</ref>
* [[Engineering:Whisker (metallurgy)|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.
* [[Physics:Lambda point|Superfluid transition in helium-4]]: Explain the discrepancy between the experimental<ref>{{Cite journal|last1=Lipa|first1=J. A.|last2=Nissen|first2=J. A.|last3=Stricker|first3=D. A.|last4=Swanson|first4=D. R.|last5=Chui|first5=T. C. P.|date=14 November 2003|title=Specific heat of liquid helium in zero gravity very near the lambda point|journal=Physical Review B|volume=68|issue=17|pages=174518|doi=10.1103/PhysRevB.68.174518|bibcode=2003PhRvB..68q4518L|arxiv=cond-mat/0310163|s2cid=55646571}}</ref> and theoretical<ref>{{Cite journal|last1=Campostrini|first1=Massimo|last2=Hasenbusch|first2=Martin|last3=Pelissetto|first3=Andrea|last4=Vicari|first4=Ettore|date=6 October 2006|title=Theoretical estimates of the critical exponents of the superfluid transition in $^{4}\mathrm{He}$ by lattice methods|journal=Physical Review B|volume=74|issue=14|pages=144506|doi=10.1103/PhysRevB.74.144506|arxiv=cond-mat/0605083|s2cid=118924734}}</ref><ref>{{Cite journal|last=Hasenbusch|first=Martin|date=26 December 2019|title=Monte Carlo study of an improved clock model in three dimensions|arxiv=1910.05916|journal=Physical Review B|volume=100|issue=22|pages=224517|doi=10.1103/PhysRevB.100.224517|issn=2469-9950|bibcode=2019PhRvB.100v4517H|s2cid=204509042}}</ref><ref>{{cite journal|last1=Chester|first1=Shai M.|last2=Landry|first2=Walter|last3=Liu|first3=Junyu|last4=Poland|first4=David|last5=Simmons-Duffin|first5=David|last6=Su|first6=Ning|last7=Vichi|first7=Alessandro|title=Carving out OPE space and precise $O(2)$ model critical exponents|journal=Journal of High Energy Physics|year=2020|volume=2020|issue=6|page=142|doi=10.1007/JHEP06(2020)142|arxiv=1912.03324|bibcode=2020JHEP...06..142C|s2cid=208910721}}</ref> determinations of the heat capacity critical exponent {{math|''α''}}.<ref>{{Cite journal|last=Rychkov|first=Slava|date=31 January 2020|title=Conformal bootstrap and the λ-point specific heat experimental anomaly|url=https://www.condmatjclub.org/?p=4037|journal=Journal Club for Condensed Matter Physics|language=en|doi=10.36471/JCCM_January_2020_02|doi-access=free|access-date=8 February 2020|archive-date=9 June 2020|archive-url=https://web.archive.org/web/20200609002316/https://www.condmatjclub.org/?p=4037|url-status=live}}</ref>
* [[Physics:Scharnhorst effect|Scharnhorst effect]]: Can light signals travel slightly [[Physics:Faster-than-light|faster than ''c'']] between two closely spaced conducting plates, exploiting the [[Physics:Casimir effect|Casimir effect]]?<ref name="Scharnhorst">
{{cite journal |last1=Barton |first1=G. |last2=Scharnhorst |first2=K. |year=1993 |title=QED between parallel mirrors: light signals faster than ''c'', or amplified by the vacuum |journal=[[Physics:Journal of Physics A|Journal of Physics A]] |volume=26 |issue=8 |page=2037 |bibcode=1993JPhA...26.2037B |doi=10.1088/0305-4470/26/8/024}} A more recent follow-up paper is {{cite journal |last=Scharnhorst |first=K. |year=1998 |title=The velocities of light in modified QED vacua |journal=[[Physics:Annalen der Physik|Annalen der Physik]] |volume=7 |issue=7–8 |pages=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}}</ref>

==Quantum computing and quantum information==

* [[Threshold theorem|Threshold problem]]: Can we go beyond the [[Noisy intermediate-scale quantum era|noisy intermediate-scale quantum era]]? Can [[Quantum computing|quantum computers]] reach [[Fault tolerance|fault tolerance]]? Is it possible to have enough [[Qubit|qubit]] scalability to implement [[Quantum error correction|quantum error correction]]? What is the most promising candidate platforms to physically implement qubits?<ref name=":4" />
* Topological qubits: [[Topological quantum computer|Topological quantum computers]] are promising but can they be built? Can we demonstrate [[Physics:Majorana fermion|Majorana zero modes]] conclusively?<ref>{{Cite web |last=Ball |first=Phillip |date=2021 |title=Major Quantum Computing Strategy Suffers Serious Setbacks |url=https://www.quantamagazine.org/major-quantum-computing-strategy-suffers-serious-setbacks-20210929/ |access-date=2 September 2023 |website=Quanta Magazine}}</ref>
* Temperature: Can quantum computing be performed at non-cryogenic temperatures? Can we build room temperature quantum computers?<ref>{{cite web |last=Skyrme |first=Tess |date=2023-03-20 |title=The Status of Room-Temperature Quantum Computers |url=https://www.eetimes.eu/the-status-of-room-temperature-quantum-computers/ |access-date=2023-09-01 |website=EE Times Europe |language=en-US}}</ref>
* Complexity classes problems: What is the relation of [[BQP]] and [[BPP (complexity)|BPP]]? What is the relation between BQP and [[NP (complexity)|NP]]? Can 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" />
* [[Quantum capacity]]: The capacity of a quantum channel is in general not known.<ref>{{cite book |last=Shor |first=Peter |title=Visions in Mathematics, GAFA 2000 Special Volume: Part II |publisher=Birkhäuser Basel |year=2000 |isbn=978-3-0346-0425-3 |editor=Alon N. |series=Modern Birkhäuser Classics |pages=816–838 |chapter=Quantum Information Theory: Results and Open Problems |doi=10.1007/978-3-0346-0425-3_9 |editor2=Bourgain J. |editor3=Connes A. |editor4=Gromov M. |editor5=Milman V. |chapter-url=http://www-math.mit.edu/~shor/papers/GAFA.pdf}}</ref>

== Plasma physics ==
* [[Physics:Plasma|Plasma physics]] and [[Physics:Fusion power|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 [[Physics:Tokamak|confined]] long enough and at a high enough temperature to create fusion power? What is the physical origin of H-mode?<ref>{{Cite journal|author=F. Wagner |journal=Plasma Physics and Controlled Fusion|volume=49|issue=12B|year=2007|page=B1|title=A quarter-century of H-mode studies|bibcode = 2007PPCF...49....1W |doi = 10.1088/0741-3335/49/12B/S01 |s2cid=498401|url=http://pdfs.semanticscholar.org/4300/b370108edb8203ef135047b55b1510772227.pdf |archive-url=https://web.archive.org/web/20190223094141/http://pdfs.semanticscholar.org/4300/b370108edb8203ef135047b55b1510772227.pdf |url-status=dead |archive-date=2019-02-23 }}.</ref>
* [[Physics:Fermi acceleration#The injection problem|The injection problem]]: [[Physics:Fermi acceleration|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>{{Cite book |author1=André Balogh |author2=Rudolf A. Treumann |chapter-url=https://books.google.com/books?id=mR4_AAAAQBAJ |title=Physics of Collisionless Shocks: Space Plasma Shock Waves |year=2013 |chapter=Section 7.4 The Injection Problem |page=362 |publisher=Springer |isbn=978-1-4614-6099-2 |access-date=3 September 2015 |archive-date=17 January 2023 |archive-url=https://web.archive.org/web/20230117023443/https://books.google.com/books?id=mR4_AAAAQBAJ |url-status=live }}</ref>
* [[Physics:Magnetohydrodynamic turbulence|Alfvénic turbulence]]: In the solar wind and the turbulence in [[Astronomy:Solar flare|solar flare]]s, [[Astronomy:Coronal mass ejection|coronal mass ejection]]s, and [[Astronomy:Substorm|magnetospheric substorms]] are major unsolved problems in space plasma physics.<ref>{{cite journal|last=Goldstein|first=Melvyn L.|title=Major Unsolved Problems in Space Plasma Physics|journal=Astrophysics and Space Science|year=2001|volume=277|issue=1/2|pages=349–369|doi=10.1023/A:1012264131485|bibcode=2001Ap&SS.277..349G|s2cid=189821322}}</ref>

== Biophysics ==
* [[Stochastic process|Stochasticity]] and robustness to [[Physics:Signal-to-noise ratio|noise]] in [[Biology:Gene expression|gene expression]]: How do genes govern our body, withstanding different external pressures and internal stochasticity? [[Biology:Gene regulatory network|Certain models]] exist for genetic processes, but we are far from understanding the whole picture, in particular in [[Biology:Morphogenesis|development]] where gene expression must be tightly regulated.
* Quantitative study of the [[Biology:Immune system|immune system]]: What are the quantitative properties of [[Biology:Immune response|immune response]]s? What are the basic building blocks of [[Biology:Immune system|immune system]] networks?
* ''[[Physics:Homochirality|Homochirality]]'': What is the origin of the preponderance of specific enantiomers in biochemical systems?
* [[Physics:Magnetoreception|Magnetoreception]]: How do animals (e.g. migratory birds) sense the Earth's magnetic field?
* [[Biology:Protein structure prediction|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">{{cite journal |last1=Dill |first1=K. A. |last2=MacCallum |first2=J. L. |title=The Protein-Folding Problem, 50 Years On |journal=Science |volume=338|issue=6110 |year=2012 |pages=1042–1046 |issn=0036-8075 |doi=10.1126/science.1219021|pmid=23180855 |bibcode=2012Sci...338.1042D |s2cid=5756068 }}</ref>
* [[Biology:Quantum biology|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 persistance of coherence as a mechanism?

== Philosophy of physics ==
* [[Physics:Interpretation of quantum mechanics|Interpretation of quantum mechanics]]: How does the [[Physics:Quantum mechanics|quantum]] description of reality, which includes elements such as the [[Physics:Quantum superposition|superposition]] of states and wavefunction collapse or [[Physics:Quantum decoherence|quantum decoherence]], give rise to the reality we perceive?<ref name="Open Questions" /> Another way of stating this question regards the [[Physics:Measurement problem|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|quantum teleportation]] arising from [[Quantum entanglement|quantum entanglement]]) cannot be simultaneously "local", "causal", and "real", but it is not obvious which of these properties must be sacrificed,<ref name=":5">{{Cite book|title=What is Quantum Information?|last=Cabello|first=Adán|publisher=Cambridge University Press|year=2017|isbn=9781107142114|editor-last=Lombardi|editor-first=Olimpia|pages=138–143|chapter=Interpretations of quantum theory: A map of madness|arxiv=1509.04711|bibcode=2015arXiv150904711C|editor-last2=Fortin|editor-first2=Sebastian|editor-last3=Holik|editor-first3=Federico|editor-last4=López|editor-first4=Cristian|doi=10.1017/9781316494233.009|s2cid=118419619}}</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?
* [[Physics:Arrow of time|Arrow of time]] (e.g. [[Physics:Entropy (arrow of time)|entropy's arrow of time]]): Why does time have a direction? Why did the universe have such low [[Physics:Entropy|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 [[Physics:Second law of thermodynamics|second law of thermodynamics]]?<ref name="Open Questions" /> Why are [[Physics:CP violation|CP violation]]s 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 [[Physics:Causality|causality]]? Is there a single possible past? Is the [[Physics:Present#Philosophy of time|present]] moment physically distinct from the past and future, or is it merely an emergent property of [[Philosophy:Consciousness|consciousness]]? What links the quantum arrow of time to the thermodynamic arrow?
* [[Physics:Principle of locality|Locality]]: Are there non-local phenomena in quantum physics?<ref>{{Cite journal|last=Wiseman|first=Howard|date=2014|title=The Two Bell's Theorems of John Bell|journal=Journal of Physics A: Mathematical and Theoretical|language=en|volume=47|issue=42|pages=424001|doi=10.1088/1751-8113/47/42/424001|issn=1751-8121|arxiv=1402.0351|bibcode=2014JPhA...47P4001W|s2cid=119234957}}</ref><ref>{{Cite journal|last1=Fuchs|first1=Christopher A.|last2=Mermin|first2=N. David|author-link2=N. David Mermin|last3=Schack|first3=Rüdiger|date=2014|title=An introduction to QBism with an application to the locality of quantum mechanics|journal=[[Physics:American Journal of Physics|American Journal of Physics]]|volume=82|issue=8|pages=749|arxiv=1311.5253|doi=10.1119/1.4874855|bibcode=2014AmJPh..82..749F|title-link=QBism|s2cid=56387090}}</ref> If they exist, are non-local phenomena limited to the [[Quantum entanglement|entanglement]] revealed in the violations of the [[Bell's theorem|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?
* [[Physics:Quantum mind|Quantum mind]]: Does [[Physics:Quantum mechanical|quantum mechanical]] phenomena, such as [[Quantum entanglement|entanglement]] and [[Physics:Quantum superposition|superposition]], play an important part in the brain's function and can it explain critical aspects of [[Philosophy:Consciousness|consciousness]]?<ref>{{Citation |last=Atmanspacher |first=Harald |title=Quantum Approaches to Consciousness |date=2020 |url=https://plato.stanford.edu/archives/sum2020/entries/qt-consciousness/ |encyclopedia=The Stanford Encyclopedia of Philosophy |editor-last=Zalta |editor-first=Edward N. |access-date=2023-04-12 |edition=Summer 2020 |publisher=Metaphysics Research Lab, Stanford University}}</ref>

== Problems solved in the past 30 years ==
=== General physics/quantum physics ===

* Perform a [[Physics:Loopholes in Bell test experiments|loophole-free Bell test experiment]] (1970<ref name="Pearle-1970">{{citation|author=Philip M. Pearle|title=Hidden-Variable Example Based upon Data Rejection|journal=Phys. Rev. D|volume=2|issue=8|pages=1418–1425|year=1970|bibcode=1970PhRvD...2.1418P|doi=10.1103/PhysRevD.2.1418}}</ref>–2015): In October 2015, scientists from the [[Organization:Kavli Institute of Nanoscience|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">{{cite journal|author=Hensen, B.|display-authors=0|date=21 October 2015|title=Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres|journal=Nature|volume=526|issue=7575|pages=682–686|arxiv=1508.05949|bibcode=2015Natur.526..682H|doi=10.1038/nature15759|pmid=26503041|s2cid=205246446}}</ref><ref name="NYT-20151021">{{cite news|last=Markoff|first=Jack|date=21 October 2015|title=Sorry, Einstein. Quantum Study Suggests 'Spooky Action' Is Real|work={{wipe|New York Times}}|url=https://www.nytimes.com/2015/10/22/science/quantum-theory-experiment-said-to-prove-spooky-interactions.html|access-date=21 October 2015|archive-date=31 July 2019|archive-url=https://web.archive.org/web/20190731224443/https://www.nytimes.com/2015/10/22/science/quantum-theory-experiment-said-to-prove-spooky-interactions.html|url-status=live}}</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">{{cite journal|author=Giustina, M.|display-authors=0|date=16 December 2015|title=Significant-Loophole-Free Test of Bell's Theorem with Entangled Photons|journal=[[Physics:Physical Review Letters|Physical Review Letters]]|volume=115|issue=25|page=250401|arxiv=1511.03190|bibcode=2015PhRvL.115y0401G|doi=10.1103/PhysRevLett.115.250401|pmid=26722905|s2cid=13789503}}</ref><ref name="PRL115-250402">{{cite journal|author=Shalm, L. K.|display-authors=0|date=16 December 2015|title=Strong Loophole-Free Test of Local Realism|journal=[[Physics:Physical Review Letters|Physical Review Letters]]|volume=115|issue=25|page=250402|arxiv=1511.03189|bibcode=2015PhRvL.115y0402S|doi=10.1103/PhysRevLett.115.250402|pmc=5815856|pmid=26722906}}</ref>
* Create [[Physics:Bose–Einstein condensate|Bose–Einstein condensate]] (1924<ref>{{cite web|title=Einstein papers at the Instituut-Lorentz|url=http://www.lorentz.leidenuniv.nl/history/Einstein_archive/|access-date=30 April 2016|archive-date=19 May 2015|archive-url=https://web.archive.org/web/20150519023226/http://www.lorentz.leidenuniv.nl/history/Einstein_archive/|url-status=live}}</ref>–1995): Composite bosons in the form of dilute atomic vapours were cooled to quantum degeneracy using the techniques of [[Physics:Laser cooling|laser cooling]] and [[Physics:Evaporative cooling (atomic physics)|evaporative cooling]].{{fact|date=January 2022}}

=== Cosmology and general relativity ===

* Existence of [[Astronomy:Gravitational wave|gravitational wave]]s (1916–2016): On 11 February 2016, the [[Astronomy:LIGO|Advanced LIGO]] team announced that they had [[Physics:First observation of gravitational waves|directly detected gravitational waves]] from a [[Astronomy:Binary black hole|pair]] of black holes [[Astronomy:Stellar collision|merging]],<ref name="Discovery 2016">{{cite journal|last1=Castelvecchi|first1=Davide|last2=Witze|first2=Witze|date=11 February 2016|title=Einstein's gravitational waves found at last|url=http://www.nature.com/news/einstein-s-gravitational-waves-found-at-last-1.19361|journal=Nature News|doi=10.1038/nature.2016.19361|s2cid=182916902|access-date=11 February 2016|archive-date=24 December 2018|archive-url=https://web.archive.org/web/20181224230203/http://www.nature.com/news/einstein-s-gravitational-waves-found-at-last-1.19361|url-status=live}}</ref><ref name="Abbot">{{cite journal|author1=B. P. Abbott|year=2016|title=Observation of Gravitational Waves from a Binary Black Hole Merger|journal=Physical Review Letters|volume=116|issue=6|pages=061102|arxiv=1602.03837|bibcode=2016PhRvL.116f1102A|doi=10.1103/PhysRevLett.116.061102|pmid=26918975|s2cid=124959784|collaboration=LIGO Scientific Collaboration and Virgo Collaboration}}</ref><ref name="NSF">{{cite web|title=Gravitational waves detected 100 years after Einstein's prediction|url=https://www.nsf.gov/news/news_summ.jsp?cntn_id=137628|access-date=11 February 2016|website=www.nsf.gov|publisher=National Science Foundation|archive-date=19 June 2020|archive-url=https://web.archive.org/web/20200619020139/https://www.nsf.gov/news/news_summ.jsp?cntn_id=137628|url-status=live}}</ref> which was also the first detection of a stellar binary black hole.
* Numerical solution for [[Astronomy:Binary black hole|binary black hole]] (1960s–2005): The numerical solution of the two body problem in general relativity was achieved after four decades of research. Three groups devised the breakthrough techniques in 2005 ([[Physics:Annus mirabilis|annus mirabilis]] of [[Physics:Numerical relativity|numerical relativity]]).<ref name="breakthroughs">{{Cite journal|last1=Pretorius|first1=Frans|year=2005|title=Evolution of Binary Black-Hole Spacetimes|journal=Physical Review Letters|volume=95|issue=12|pages=121101|arxiv=gr-qc/0507014|bibcode=2005PhRvL..95l1101P|doi=10.1103/PhysRevLett.95.121101|pmid=16197061|s2cid=24225193}} {{Cite journal|last1=Campanelli|first1=M.|last2=Lousto|first2=C. O.|last3=Marronetti|first3=P.|last4=Zlochower|first4=Y.|year=2006|title=Accurate Evolutions of Orbiting Black-Hole Binaries without Excision|journal=Physical Review Letters|volume=96|issue=11|pages=111101|arxiv=gr-qc/0511048|bibcode=2006PhRvL..96k1101C|doi=10.1103/PhysRevLett.96.111101|pmid=16605808|s2cid=5954627}} {{Cite journal|last1=Baker|first1=John G.|last2=Centrella|first2=Joan|last3=Choi|first3=Dae-Il|last4=Koppitz|first4=Michael|last5=Van Meter|first5=James|year=2006|title=Gravitational-Wave Extraction from an Inspiraling Configuration of Merging Black Holes|journal=Physical Review Letters|volume=96|issue=11|pages=111102|arxiv=gr-qc/0511103|bibcode=2006PhRvL..96k1102B|doi=10.1103/PhysRevLett.96.111102|pmid=16605809|s2cid=23409406}}</ref>
*[[Astronomy:Cosmic age problem|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.{{fact|date=January 2022}}

=== High-energy physics/particle physics ===

* Existence of [[Physics:Pentaquark|pentaquark]]s (1964–2015): In July 2015, the LHCb collaboration at [[Organization:CERN|CERN]] identified pentaquarks in the {{nowrap|Λ{{su|p=0|b=b}}→J/ψK{{sup|−}}p}} channel, which represents the decay of the bottom [[Physics:Lambda baryon|lambda baryon]] {{nowrap|(Λ{{su|p=0|b=b}})}} into a J/ψ meson {{nowrap|(J/ψ)}}, a [[Physics:Kaon|kaon]] {{nowrap|(K{{su|p=−}})}} and a [[Physics:Proton|proton]] (p). The results showed that sometimes, instead of decaying directly into mesons and baryons, the {{nowrap|Λ{{su|p=0|b=b}}}} decayed via intermediate pentaquark states. The two states, named {{nowrap|P{{su|p=+|b=c}}(4380)}} and {{nowrap|P{{su|p=+|b=c}}(4450)}}, had individual [[Statistical significance|statistical significance]]s of 9&nbsp;σ and 12&nbsp;σ, respectively, and a combined significance of 15&nbsp;σ—enough to claim a formal discovery. The two pentaquark states were both observed decaying strongly to {{nowrap|J/ψp}}, hence must have a valence quark content of two [[Physics:Up quark|up quark]]s, a [[Physics:Down quark|down quark]], a [[Physics:Charm quark|charm quark]], and an anti-charm quark ({{Subatomic particle|up quark}}{{Subatomic particle|up quark}}{{Subatomic particle|down quark}}{{Subatomic particle|charm quark}}{{Subatomic particle|charm antiquark}}), making them charmonium-pentaquarks.<ref name="LHCb2015">{{cite journal|author=R. Aaij et al. (LHCb collaboration)|year=2015|title=Observation of J/ψp resonances consistent with pentaquark states in Λ{{su|p=0|b=b}}→J/ψK{{sup|−}}p decays|journal=[[Physics:Physical Review Letters|Physical Review Letters]]|volume=115|issue=7|pages=072001|arxiv=1507.03414|bibcode=2015PhRvL.115g2001A|doi=10.1103/PhysRevLett.115.072001|pmid=26317714|s2cid=119204136}}</ref>
* Existence of [[Physics:Quark–gluon plasma|quark-gluon plasma]], a new phase of matter was discovered and confirmed in experiments at [[Organization:CERN|CERN]]-[[Physics:Super Proton Synchrotron|SPS]] (2000), [[Organization:Brookhaven National Laboratory|BNL]]-[[Physics:Relativistic Heavy Ion Collider|RHIC]] (2005) and CERN-[[Physics:Large Hadron Collider|LHC]] (2010).<ref name=":0" />
*[[Physics:Higgs boson|Higgs boson]] and [[Physics:Electroweak symmetry breaking|electroweak symmetry breaking]] (1963<ref name="MyLifeAsABoson">{{cite web|last=Higgs|first=Peter|date=24 November 2010|title=My Life as a Boson|url=http://www.kcl.ac.uk/nms/depts/physics/news/events/MyLifeasaBoson.pdf|url-status=dead|archive-url=https://web.archive.org/web/20140501135924/http://www.kcl.ac.uk/nms/depts/physics/news/events/MyLifeasaBoson.pdf|archive-date=1 May 2014|access-date=17 January 2013|publisher=Talk given by Peter Higgs at King's College, London, 24 November 2010, expanding on a paper originally presented in 2001}} – the original 2001 paper can be found at: {{cite book|url=https://books.google.com/books?id=ONhnbpq00xIC&q=2001:+A+Space+Time+Odyssey++%22life+as+a+boson%22&pg=PR11|title=2001 A Spacetime Odyssey: Proceedings of the Inaugural Conference of the Michigan Center for Theoretical Physics, Michigan, USA, 21–25 May 2001|publisher=World Scientific|year=2003|isbn=978-9812382313|editor=Duff and Liu|pages=86–88|access-date=17 January 2013|orig-year=year of publication|archive-date=27 April 2022|archive-url=https://web.archive.org/web/20220427064048/https://books.google.com/books?id=ONhnbpq00xIC&q=2001:+A+Space+Time+Odyssey++%22life+as+a+boson%22&pg=PR11|url-status=live}}</ref>–2012): The mechanism responsible for breaking the electroweak gauge symmetry, giving mass to the [[Physics:W and Z bosons|W and Z bosons]], was solved with the discovery of the [[Physics:Higgs boson|Higgs boson]] of the [[Physics:Standard Model|Standard Model]], with the expected couplings to the weak bosons. No evidence of a strong dynamics solution, as proposed by [[Physics:Technicolor|technicolor]], has been observed.
* Origin of mass of most elementary particles: Solved with the discovery of the [[Physics:Higgs boson|Higgs boson]], which implies the existence of the [[Physics:Higgs field|Higgs field]] giving mass to these particles.

=== Astronomy and astrophysics ===

* Origin of short [[Astronomy:Gamma-ray burst|gamma-ray burst]] (1993<ref name="ReferenceA">{{cite journal|last1=Kouveliotou|first1=Chryssa|last2=Meegan|first2=Charles A.|last3=Fishman|first3=Gerald J.|last4=Bhat|first4=Narayana P.|last5=Briggs|first5=Michael S.|last6=Koshut|first6=Thomas M.|last7=Paciesas|first7=William S.|last8=Pendleton|first8=Geoffrey N.|year=1993|title=Identification of two classes of gamma-ray bursts|journal=The Astrophysical Journal|volume=413|pages=L101|bibcode=1993ApJ...413L.101K|doi=10.1086/186969}}</ref>–2017): From binary [[Astronomy:Neutron star|neutron star]]s merger, produce a [[Astronomy:Kilonova|kilonova]] explosion and short gamma-ray burst [[Astronomy:GRB 170817A|GRB 170817A]] was detected in both electromagnetic waves and [[Astronomy:Gravitational wave|gravitational wave]] [[Astronomy:GW170817|GW170817]].<ref name="SM-20171016">{{cite news|last=Cho|first=Adrian|date=16 October 2017|title=Merging neutron stars generate gravitational waves and a celestial light show|work=Science|url=https://www.science.org/content/article/merging-neutron-stars-generate-gravitational-waves-and-celestial-light-show|access-date=16 October 2017|archive-date=30 October 2021|archive-url=https://web.archive.org/web/20211030182044/https://www.science.org/content/article/merging-neutron-stars-generate-gravitational-waves-and-celestial-light-show|url-status=live}}</ref><ref name="NAT-20170825">{{cite journal|last=Casttelvecchi|first=Davide|date=25 August 2017|title=Rumours swell over new kind of gravitational-wave sighting|url=https://www.nature.com/news/rumours-swell-over-new-kind-of-gravitational-wave-sighting-1.22482|journal=Nature News|doi=10.1038/nature.2017.22482|access-date=27 August 2017|archive-date=16 October 2017|archive-url=https://web.archive.org/web/20171016192413/http://www.nature.com/news/rumours-swell-over-new-kind-of-gravitational-wave-sighting-1.22482|url-status=live}}</ref>
*[[Astronomy:Missing baryon problem|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>{{Cite news|title=Half the universe's missing matter has just been finally found|language=en-US|work=New Scientist|url=https://www.newscientist.com/article/2149742-half-the-universes-missing-matter-has-just-been-finally-found/|access-date=12 October 2017|archive-date=13 October 2017|archive-url=https://web.archive.org/web/20171013235848/https://www.newscientist.com/article/2149742-half-the-universes-missing-matter-has-just-been-finally-found/|url-status=live}}</ref><ref>{{cite journal|last1=Nicastro|first1=F.|last2=Kaastra|first2=J.|last3=Krongold|first3=Y.|last4=Borgani|first4=S.|last5=Branchini|first5=E.|last6=Cen|first6=R.|last7=Dadina|first7=M.|last8=Danforth|first8=C. W.|last9=Elvis|first9=M.|last10=Fiore|first10=F.|last11=Gupta|first11=A.|date=June 2018|title=Observations of the missing baryons in the warm–hot intergalactic medium|journal=Nature|language=En|volume=558|issue=7710|pages=406–409|arxiv=1806.08395|bibcode=2018Natur.558..406N|doi=10.1038/s41586-018-0204-1|issn=0028-0836|pmid=29925969|first16=D.|first15=L.|last12=Mathur|first12=S.|last13=Mayya|first13=D.|last14=Paerels|last15=Piro|last16=Rosa-Gonzalez|last17=Schaye|first21=L.|last21=Zappacosta|first20=N.|last20=Wijers|first19=J.|last19=Torres-Zafra|first18=J. M.|last18=Shull|first17=J.|first14=F.|s2cid=49347964}}</ref>
*Long-duration [[Astronomy:Gamma-ray burst|gamma-ray burst]]s (1993<ref name="ReferenceA" />–2003): Long-duration bursts are associated with the deaths of massive stars in a specific kind of [[Astronomy:Supernova|supernova]]-like event commonly referred to as a [[Astronomy:Hypernova|collapsar]]. However, there are also long-duration GRBs that show evidence against an associated supernova, such as the Swift event [[Astronomy:GRB 060614|GRB 060614]].
*[[Physics:Solar neutrino problem|Solar neutrino problem]] (1968<ref>{{Cite journal|last1=Cleveland|first1=Bruce T.|last2=Daily|first2=Timothy|last3=Davis, Jr.|first3=Raymond|last4=Distel|first4=James R.|last5=Lande|first5=Kenneth|last6=Lee|first6=C. K.|last7=Wildenhain|first7=Paul S.|last8=Ullman|first8=Jack|year=1998|title=Measurement of the Solar Electron Neutrino Flux with the Homestake Chlorine Detector|journal=The Astrophysical Journal|volume=496|issue=1|pages=505–526|bibcode=1998ApJ...496..505C|doi=10.1086/305343|doi-access=free}}</ref>–2001): Solved by a new understanding of [[Physics:Neutrino|neutrino]] physics, requiring a modification of the [[Physics:Standard Model|Standard Model]] of [[Physics:Particle physics|particle physics]]—specifically, [[Physics:Neutrino oscillation|neutrino oscillation]].
* 	Saturn's core spin was determined from its gravitational field.<ref>{{cite journal|last1=Helled|first1=Ravit|last2=Galanti|first2=Eli|last3=Kaspi|first3=Yohai|date=2015|title=Saturn's fast spin determined from its gravitational field and oblateness|journal=Nature|volume=520|issue=7546 |pages=202–204|doi=10.1038/nature14278|pmid=25807487 |arxiv=1504.02561|bibcode=2015Natur.520..202H |s2cid=4468877 }}</ref>

=== Nuclear physics ===

* Existence of [[Physics:Quark–gluon plasma|quark-gluon plasma]], a new phase of matter, was discovered and confirmed in experiments at [[Organization:CERN|CERN]]-[[Physics:Super Proton Synchrotron|SPS]] (2000), [[Organization:Brookhaven National Laboratory|BNL]]-[[Physics:Relativistic Heavy Ion Collider|RHIC]] (2005) and CERN-[[Physics:Large Hadron Collider|LHC]] (2010).<ref name=":0" />
* [[Physics:Hagedorn temperature|Hagedorn temperature]] recognized as phase transformation temperature between hadronic confined phase and deconfined phase of matter.

=== Rapidly solved problems ===
* Existence of [[Physics:Time crystal|time crystal]]s (2012–2016): The idea of a quantized time crystal was first theorized in 2012 by [[Biography:Frank Wilczek|Frank Wilczek]].<ref>{{cite journal|last1=Wilczek|first1=Frank|title=Quantum Time Crystals|journal=Physical Review Letters|volume=109|issue=16|pages=160401|year=2012|issn=0031-9007|doi=10.1103/PhysRevLett.109.160401|pmid=23215056|arxiv=1202.2539|bibcode=2012PhRvL.109p0401W|s2cid=1312256}}</ref><ref>{{cite journal|last1=Shapere|first1=Alfred|last2=Wilczek|first2=Frank|title=Classical Time Crystals|journal=Physical Review Letters|volume=109|issue=16|pages=160402|year=2012|issn=0031-9007|doi=10.1103/PhysRevLett.109.160402|pmid=23215057|bibcode=2012PhRvL.109p0402S|arxiv=1202.2537|s2cid=4506464}}</ref> In 2016, Khemani et al.<ref>{{cite journal|last1=Khemani|first1=Vedika|last2=Lazarides|first2=Achilleas|last3=Moessner|first3=Roderich|last4=Sondhi|first4=S. L.|title=Phase Structure of Driven Quantum Systems|journal=Physical Review Letters|date=21 June 2016|volume=116|issue=25|pages=250401|doi=10.1103/PhysRevLett.116.250401|pmid=27391704|bibcode=2016PhRvL.116y0401K|arxiv=1508.03344|s2cid=883197}}</ref> and Else et al.<ref>{{cite journal|last1=Else|first1=Dominic V.|last2=Bauer|first2=Bela|last3=Nayak|first3=Chetan|title=Floquet Time Crystals|journal=Physical Review Letters|date=25 August 2016|volume=117|issue=9|pages=090402|doi=10.1103/PhysRevLett.117.090402|pmid=27610834|bibcode=2016PhRvL.117i0402E|arxiv=1603.08001|s2cid=1652633}}</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">{{cite journal|last1=Yao|first1=N. Y.|last2=Potter|first2=A. C.|last3=Potirniche|first3=I.-D.|last4=Vishwanath|first4=A.|title=Discrete Time Crystals: Rigidity, Criticality, and Realizations|journal=Physical Review Letters|volume=118|issue=3|pages=030401|year=2017|url=https://physics.aps.org/featured-article-pdf/10.1103/PhysRevLett.118.030401|issn=0031-9007|doi=10.1103/PhysRevLett.118.030401|pmid=28157355|arxiv=1608.02589|bibcode=2017PhRvL.118c0401Y|s2cid=206284432|ref={{harvid|Yao et al.|2017}}|access-date=21 November 2021|archive-date=24 June 2021|archive-url=https://web.archive.org/web/20210624202827/https://physics.aps.org/featured-article-pdf/10.1103/PhysRevLett.118.030401|url-status=live}}</ref> This was then used by two teams, a group led by [[Biography:Christopher Monroe|Christopher Monroe]] at the University of Maryland and a group led by [[Biography:Mikhail Lukin|Mikhail Lukin]] at [[Organization:Harvard University|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">{{cite journal |title=Observation of a discrete time crystal |journal=Nature |date=8 March 2017 |author=Zhang, J. |display-authors=0 |doi=10.1038/nature21413  |pmid=28277505 |volume=543 |issue=7644 |pages=217–220|arxiv=1609.08684 |bibcode=2017Natur.543..217Z |s2cid=4450646 }}</ref><ref name="Lukin">{{cite journal |title=Observation of discrete time-crystalline order in a disordered dipolar many-body system |journal=Nature |date=8 March 2017 |author=Choi, S. |display-authors=0 |doi=10.1038/nature21426  |pmid=28277511 |pmc=5349499 |volume=543 |issue=7644 |pages=221–225|arxiv=1610.08057 |bibcode=2017Natur.543..221C }}</ref>
*[[Astronomy:Photon underproduction crisis|Photon underproduction crisis]] (2014–2015): This problem was resolved by Khaire and Srianand.<ref>{{Cite journal|doi=10.1093/mnrasl/slv060|title=Photon underproduction crisis: Are QSOs sufficient to resolve it?|journal=Monthly Notices of the Royal Astronomical Society: Letters|volume=451|pages=L30–L34|year=2015|last1=Khaire|first1=V.|last2=Srianand|first2=R.|arxiv=1503.07168|bibcode=2015MNRAS.451L..30K|s2cid=119263441}}</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.
* [[Astronomy:Hipparcos#The Pleiades distance controversy|Hipparcos anomaly]] (1997<ref>{{Cite journal|bibcode=1999A&A...341L..71V|title=HIPPARCOS distance calibrations for 9 open clusters|journal=Astronomy and Astrophysics|volume=341|pages=L71|last1=Van Leeuwen|first1=Floor|year=1999}}</ref>–2012): The High Precision Parallax Collecting Satellite (Hipparcos) measured the parallax of the [[Astronomy:Pleiades|Pleiades]] and determined a [[Astronomy:Pleiades#Distance|distance]] of 385 light years. This was significantly different from other measurements made by means of actual to apparent brightness measurement or [[Astronomy:Absolute magnitude|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>{{Cite journal|arxiv=1203.4945|title=XHIP-II: Clusters and associations|journal=Astronomy Letters|volume=38|issue=11|pages=681–693|year=2012|author1=Charles Francis|author2=Erik Anderson|doi=10.1134/S1063773712110023|bibcode=2012AstL...38..681F|s2cid=119285733}}</ref>
* [[Physics:Faster-than-light neutrino anomaly|Faster-than-light neutrino anomaly]] (2011–2012): In 2011, the [[Physics:OPERA experiment|OPERA experiment]] mistakenly observed [[Physics:Neutrino|neutrino]]s appearing to travel [[Physics:Faster-than-light|faster than light]]. On 12 July 2012 OPERA updated their paper after discovering an error in their previous flight time measurement. They found agreement of neutrino speed with the speed of light.<ref name=op4>{{Cite journal |author=OPERA collaboration |title=Measurement of the neutrino velocity with the OPERA detector in the CNGS beam |journal=Journal of High Energy Physics |volume=2012 |issue=10 |pages=93 |date=12 July 2012 |arxiv=1109.4897|doi=10.1007/JHEP10(2012)093 |bibcode=2012JHEP...10..093A|s2cid=17652398 }}</ref>
* [[Astronomy:Pioneer anomaly|Pioneer anomaly]] (1980–2012): There was a deviation in the predicted accelerations of the [[Astronomy:Pioneer program|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>{{Cite journal | last1 = Turyshev | first1 = S. | last2 = Toth | first2 = V. | last3 = Kinsella | first3 = G. | last4 = Lee | first4 = S. C. | last5 = Lok | first5 = S. | last6 = Ellis | first6 = J. | title = Support for the Thermal Origin of the Pioneer Anomaly | journal = Physical Review Letters | volume = 108 | issue = 24 | pages = 241101 | year = 2012 | arxiv = 1204.2507| doi = 10.1103/PhysRevLett.108.241101 | pmid = 23004253 |bibcode = 2012PhRvL.108x1101T | s2cid = 2368665 }}</ref><ref>{{cite news|last=Overbye|first=Dennis|title=Mystery Tug on Spacecraft Is Einstein's 'I Told You So'|url=https://www.nytimes.com/2012/07/24/science/mystery-tug-on-pioneer-10-and-11-probes-is-einsteins-i-told-you-so.html|access-date=24 January 2014|newspaper=The New York Times|date=23 July 2012|archive-date=27 August 2017|archive-url=https://web.archive.org/web/20170827000258/http://www.nytimes.com/2012/07/24/science/mystery-tug-on-pioneer-10-and-11-probes-is-einsteins-i-told-you-so.html|url-status=live}}</ref>

== See also ==
* [[Hilbert's sixth problem]]
* [[Unsolved:Lists of unsolved problems|Lists of unsolved problems]]
* [[Physics:Physical paradox|Physical paradox]]
* [[List of unsolved problems in mathematics]]
* [[Medicine:List of unsolved problems in neuroscience|List of unsolved problems in neuroscience]]

== Footnotes ==
{{notelist}}

== References ==
{{reflist|25em|refs=

<ref name=":0">{{cite journal |last=Rafelski |first=Johann |year=2020 |title=Discovery of Quark-Gluon Plasma: Strangeness Diaries |journal=The European Physical Journal Special Topics |language=en |volume=229 |issue=1 |pages=1–140 |doi=10.1140/epjst/e2019-900263-x |arxiv=1911.00831 |bibcode=2020EPJST.229....1R |issn=1951-6355 |doi-access=free}}</ref>

<ref name=":4">{{cite web |last=Aaronson |first=Scott |title=Ten Semi-Grand Challenges for Quantum Computing Theory |url=https://www.scottaaronson.com/writings/qchallenge.html |website=ScottAaronson.com |access-date=2023-09-01}}</ref>
}}

== External links ==
* [http://www.physics.rutgers.edu/colloq/ginzburg.pdf What problems of physics and astrophysics seem now to be especially important and interesting (thirty years later, already on the verge of XXI century)?] V. L. Ginzburg, Physics-Uspekhi 42 (4) 353–373, 1999
* [https://www.science.org/doi/10.1126/science.309.5731.75 What don't we know?] Science journal special project for its 125th anniversary: top 25 questions and 100 more.
* [http://www.openproblems.net/ List of links to unsolved problems in physics, prizes and research.]
* [http://www.nasa.gov/centers/glenn/technology/warp/ideachev.html Ideas Based On What We'd Like to Achieve]
* [http://www-conf.slac.stanford.edu/ssi/2004/Default.htm 2004 SLAC Summer Institute: Nature's Greatest Puzzles]
* [https://www.newscientist.com/article/dn14179-dual-personality-of-glass-explained-at-last/ Dual Personality of Glass Explained at Last]
* [https://www.nybooks.com/articles/2013/11/07/physics-what-we-do-and-dont-know/ What we do and don't know] Review on current state of physics by Steven Weinberg, November 2013
* [https://www.nybooks.com/articles/2012/05/10/crisis-big-science/ The crisis of big science] Steven Weinberg, May 2012

{{Unsolved problems}}

{{DEFAULTSORT:Unsolved problems in physics}}
[[Category:Physics-related lists]]
[[Category:Unsolved problems in physics| ]]

{{Sourceattribution|List of unsolved problems in physics}}