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| {{main|List of unsolved problems}}
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| Some of the major [[List of unsolved problems|unsolved problems]] in [[physics]] are [[theory|theoretical]], meaning that existing theories seem incapable of explaining a certain observed [[phenomenon]] or experimental result. The others are [[experiment]]al, meaning that there is a difficulty in creating an experiment to test a proposed theory or investigate a phenomenon in greater detail.<!-- cleanup because 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 [[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 -->
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| ==Unsolved problems by subfield==
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| The following is a list of unsolved problems grouped into broad area of 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|year=2001|publisher=Springer|location=Berlin|isbn=9783540675341|pages=3–200}}</ref>
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| ===Cosmology, and general relativity===
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| ; [[Cosmic inflation]]: Is the theory of cosmic inflation correct, and if so, what are the details of this epoch? What is the hypothetical [[inflaton field]] giving rise to inflation? If inflation happened at one point, is it [[chaotic inflation|self-sustaining through inflation of quantum-mechanical fluctuations]], and thus ongoing in some impossibly 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|accessdate=24 January 2013|archiveurl=https://web.archive.org/web/20121022112323/http://www.nonequilibrium.net/225-top-ten-open-problems-physics|archivedate=22 October 2012 |deadurl=no}}</ref>
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| ; [[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 [[inflation (cosmology)|inflation]] is generally accepted as the solution, but are other possible explanations such as a [[variable speed of light]] more appropriate?<ref name=newscientist />
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| ; [[Ultimate fate of the universe|Future of the universe]]: Is the universe heading towards a [[Big Freeze]], a [[Big Rip]], a [[Big Crunch]] or a [[Big Bounce]]? Or is it part of an infinitely recurring [[cyclic model]]?
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| ; [[Gravitational wave]]: Can gravitational waves be detected experimentally?<ref name=nrc-gravity>{{cite book|last=National Research Council|title=Gravitation, Cosmology, and Cosmic-Ray Physics|year=1986|publisher=National Academies Press|location=Washington, D. C.|isbn=0-309-03579-1|url=http://books.google.com/?id=Hk1wj61PlocC&pg=PA36&lpg=PA36&dq=gravitational+wave+unsolved+problem+in+physics#v=onepage&q=unsolved&f=false}}</ref><ref name=seattle-pi>{{cite news|last=Paulson|first=Tom|title=Catching a cosmic wave of gravity|url=http://www.seattlepi.com/news/article/Catching-a-cosmic-wave-of-gravity-1088160.php#page-2|accessdate=10 April 2012|newspaper=Seattle Post-Intelligencer|date=May 27, 2002}}</ref>
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| ; [[Baryon asymmetry]]: Why is there far more [[matter]] than [[antimatter]] in the [[observable universe]]?
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| ; [[Cosmological constant problem#Cosmological constant problem|Cosmological constant problem]]: Why does the [[zero-point energy]] of the [[vacuum]] not cause a large [[cosmological constant]]? What cancels it out?
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| [[File:DMPie 2013.svg|thumb|right|250px|Estimated distribution of dark matter and dark energy in the universe]]
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| ; [[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=http://www.newscientist.com/article/mg18524911.600-13-things-that-do-not-make-sense.html |newspaper=[[New Scientist]] |id=Issue 2491|date=March 19, 2005 |accessdate=March 7, 2011}}</ref> Is it a [[Elementary particle|particle]]? Is it the lightest [[superpartner]] (LSP)? Do the phenomena attributed to dark matter point not to some form of matter but actually to an extension of gravity? The results obtained by the [[Large Underground Xenon]] (LUX) experiment that took place in 2013 at Sanford Underground Research Facility place a lower bound on the LSP mass; at this point light supersymmetric particles that are the main candidate for dark matter in the lower mass sector are excluded with 90% confidence.<ref>{{cite arXiv|eprint=1310.8214|title= First results from the LUX dark matter experiment at the Sanford Underground Research Facility|class= astro-ph.CO|year= 2013|version= |accessdate= |author1= LUX Collaboration |last2= Akerib |first2= D. S. |last3= Araujo |first3= H. M. |last4= Bai |first4= X. |last5= Bailey |first5= A. J. |last6= Balajthy |first6= J. |last7= Bedikian |first7= S. |last8= Bernard |first8= E. |last9= Bernstein |first9= A. |displayauthors= 30 |last10= Bolozdynya |first10= A. |last11= Bradley |last12= Byram |first12= D. |last13= Cahn |first13= S. B. |last14= Carmona-Benitez |first14= M. C. |last15= Chan |first15= C. |last16= Chapman |first16= J. J. |last17= Chiller |first17= A. A. |last18= Chiller |first18= C. |last19= Clark |first19= K. |last20= Coffey |first20= T. |last21= Currie |last22= Curioni |first22= A. |last23= Dazeley |first23= S. |last24= de Viveiros |first24= L. |last25= Dobi |first25= A. |last26= Dobson |first26= J. |last27= Dragowsky |first27= E. M. |last28= Druszkiewicz |first28= E. |last29= Edwards |first29= B. |last30= Faham |first30= C. H. }}</ref>
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| [[File:Cosmic Coincidence Problem.png|right|300px|thumb|The log-log plot of dark energy density <math>\rho_{*}</math> and material density <math>\rho_m</math> vs. scale factor <math>a</math>. The two straight lines intersect at current epoch.<ref name=steinardt>{{Cite book
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| | first = Paul
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| | last = Steinardt
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| | editor-last = Val Fitch et al.
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| | contribution = Cosmological Challenges For the 21st Century
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| | series = Critical problems in physics: proceedings of a conference celebrating the 250th anniversary of Princeton University
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| | year = 1997
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| | pages = 138–140
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| | place = Princeton, New Jersey
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| | publisher = Princeton University Press
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| | isbn =978-0-691-05784-2
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| | postscript = <!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}}
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| }}</ref>]]
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| ; [[Dark energy]]: What is the cause of the observed [[accelerated expansion]] ([[De Sitter universe|de Sitter phase]]) of the Universe? Why is the energy density of the dark energy component of the same magnitude as the density of matter at present when the two evolve quite differently over time; could it be simply that we are observing at exactly the [[anthropic principle|right time]]? Is dark energy a pure cosmological constant, or are models of [[Quintessence (physics)|quintessence]] such as [[phantom energy]] applicable?
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| ; [[Copernican principle#Ecliptic alignment of cosmic microwave background anisotropy|Ecliptic alignment of CMB anisotropy]]: Some large features of the microwave sky, at distances of over 13 billion light years, appear to be aligned with both the motion and orientation of the Solar System. Is this due to systematic errors in processing, contamination of results by local effects, or an unexplained violation of the [[Copernican principle]]?
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| ; [[Shape of the Universe]]: What is the 3-[[manifold]] of [[comoving coordinates|comoving space]], i.e., of a comoving spatial section of the Universe, informally called the "shape" of the Universe? Neither the curvature nor the topology is presently known, though the curvature is known to be "close" to zero on observable scales. The [[cosmic inflation]] hypothesis suggests that the shape of the Universe may be unmeasurable, but since 2003, [[Jean-Pierre Luminet]] et al. and other groups have suggested that the shape of the Universe may be the [[Homology sphere#Cosmology|Poincaré dodecahedral space]]. Is the shape unmeasurable; the Poincaré space; or another 3-manifold?
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| ===Quantum gravity===
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| ; [[Vacuum catastrophe]]: Why does the predicted mass of the [[Vacuum state|quantum vacuum]] have little effect on the expansion of the universe?
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| ; [[Quantum gravity]]: Can [[quantum mechanics]] and [[general relativity]] be realized as a fully consistent theory (perhaps as a [[quantum field theory]])?<ref>{{Cite news|url=http://query.nytimes.com/gst/fullpage.html?res=9D0DE7DB1639F931A15754C0A960958260|title=Don't Pull the String Yet on Superstring Theory|date=July 22, 1996|newspaper=New York Times|author=Alan Sokal|postscript=<!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}}}}</ref> Is spacetime fundamentally continuous or discrete? Would a consistent theory involve a force mediated by a hypothetical [[graviton]], or be a product of a discrete structure of spacetime itself (as in [[loop quantum gravity]])? Are there deviations from the predictions of general relativity at very small or very large scales or in other extreme circumstances that flow from a quantum gravity theory?
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| ; [[Black hole]]s, [[black hole information paradox]], and [[black hole radiation]]: Do black holes produce thermal radiation, as expected on theoretical grounds? Does this radiation contain information about their inner structure, as suggested by [[Gauge–gravity duality|Gauge-gravity duality]], or not, as implied by [[Hawking radiation|Hawking]]'s original calculation? If not, and black holes can evaporate away, what happens to the information stored in them (quantum mechanics does not provide for the destruction of information)? Or does the radiation stop at some point leaving black hole remnants? Is there another way to probe their internal structure somehow, if such a structure [[No hair theorem|even exists]]?
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| ; [[Extra dimensions]]: Does nature have more than four [[spacetime]] dimensions? If so, what is their size? Are dimensions a fundamental property of the universe or an emergent result of other physical laws? Can we experimentally observe evidence of higher spatial dimensions?
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| ; The [[cosmic censorship hypothesis]] and the [[chronology protection conjecture]]: Can singularities not hidden behind an event horizon, known as "[[naked singularities]]", arise from realistic initial conditions, or is it possible to prove some version of the "cosmic censorship hypothesis" of [[Roger Penrose]] which proposes that this is impossible?<ref>{{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 | postscript = <!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}} }}</ref> Similarly, will the [[closed timelike curve]]s which arise in some solutions to the equations of general relativity (and which imply the possibility of backwards [[time travel]]) be ruled out by a theory of [[quantum gravity]] which unites general relativity with quantum mechanics, as suggested by the "chronology protection conjecture" of [[Stephen Hawking]]?
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| ; [[Principle of locality|Locality]]: Are there non-local phenomena in quantum physics? If they exist, are non-local phenomena limited to the entanglement revealed in the violations of the Bell Inequalities, or can information and conserved quantities also move in a non-local way? Under what circumstances are non-local phenomena observed? What does the existence or absence of non-local phenomena imply about the fundamental structure of spacetime? How does this relate to [[quantum entanglement]]? How does this elucidate the proper interpretation of the fundamental nature of quantum physics?
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| ===High energy physics/particle physics===
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| {{see also|Beyond the Standard Model}}
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| [[File:CMS Higgs-event.jpg|thumb|right|200px|A [[simulation]] of how a detection of the Higgs particle would appear in the [[Compact Muon Solenoid|CMS]] detector at [[CERN]]]]
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| ; [[Higgs mechanism]]: Are the branching ratios of the [[Higgs Boson]] consistent with the standard model? Is there only one type of Higgs Boson?
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| ; [[Hierarchy problem]]: Why is [[gravity]] such a weak force? It becomes strong for particles only at the [[Planck scale]], around 10<sup>19</sup> [[GeV]], much above the [[electroweak scale]] (100 GeV, the energy scale dominating physics at low energies). Why are these scales so different from each other? What prevents quantities at the electroweak scale, such as the [[Higgs boson]] mass, from getting [[renormalization|quantum corrections]] on the order of the Planck scale? Is the solution [[supersymmetry]], [[extra dimensions]], or just [[anthropic principle|anthropic]] [[fine-tuning]]?
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| ; [[Magnetic monopole]]s: Did particles that carry "magnetic charge" exist in some past, higher energy epoch? If so, do any remain today? ([[Paul Dirac]] showed the existence of some types of magnetic monopoles would explain [[charge quantization]].)<ref>[[Paul Dirac|Dirac, Paul]], "[http://users.physik.fu-berlin.de/~kleinert/files/dirac1931.pdf Quantised Singularities in the Electromagnetic Field]". ''[[Proceedings of the Royal Society]]'' '''A 133''', 60 (1931).</ref>
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| ; [[Proton decay]] and [[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 | last=Li | first=Tianjun | coauthors=Dimitri V. Nanopoulos, Joel W. Walker | title=Elements of F-ast Proton Decay | year=2011 | doi=10.1016/j.nuclphysb.2010.12.014 | journal=Nuclear Physics B | volume=846 | pages=43–99 | arxiv=1003.2570}}</ref> How do the quarks and gluons carry the spin of protons?<ref name=hansson2010>{{cite journal | last=Hansson | first=Johan | title=The "Proton Spin Crisis" — a Quantum Query|journal=Progress in Physics | year=2010 | volume=3 | url=http://www.ptep-online.com/index_files/2010/PP-22-08.PDF | accessdate=14 April 2012}}</ref>
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| ; [[Supersymmetry]]: Is spacetime supersymmetry realized at TeV scale? If so, what is the mechanism of supersymmetry breaking? Does supersymmetry stabilize the electroweak scale, preventing high quantum corrections? Does the lightest [[Superpartner|supersymmetric particle]] (LSP) comprise [[dark matter]]?
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| ; [[Generation (particle physics)|Generations of matter]]: Why are there three generations of [[quark]]s and [[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 coupling]]s)?
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| ; [[Electroweak symmetry breaking]]: What is the mechanism responsible for breaking the electroweak gauge symmetry, giving mass to the [[W and Z bosons]]? Is it the simple [[Higgs mechanism]] of the [[Standard Model]],<ref name="Open Questions"/> or does nature make use of strong dynamics in breaking electroweak symmetry, as proposed by [[Technicolor (physics)|Technicolor]]?
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| ; [[Neutrino mass]]: What is the mass of neutrinos, whether they follow [[Fermi-Dirac statistics|Dirac]] or [[Majorana fermion|Majorana]] statistics? Is mass hierarchy normal or inverted? Is the CP violating phase 0?<ref name=ino-tifr>{{cite web | title=India-based Neutrino Observatory (INO) | url=http://www.ino.tifr.res.in/ino/ | publisher=Tata Institute of Fundamental Research | accessdate=14 April 2012}}</ref><ref name=christianto2007>{{cite journal | last=Smarandache | first=Vic | coauthors=Florentin Smarandache | title=Thirty Unsolved Problems in the Physics of Elementary Particles | journal=Progress in Physics | year=2007 | volume=4 | url=http://www.ptep-online.com/index_files/2007/PP-11-16.PDF | bibcode=2009APS..HAW.KD010C}}</ref><ref>{{cite journal|title=2011 Review of Particle Physics | last=Nakamura|coauthors=et al. (Particle Data Group) | journal=J. Phys. G | year=2010 | volume=37 | page=075021 | doi=10.1088/0954-3899/37/7A/075021 | url=http://pdg.lbl.gov/2011/reviews/contents_sports.html|bibcode = 2010JPhG...37g5021N|first1=K|issue=7A }}</ref>
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| ; [[Confinement|Asymptotic confinement]]: Why has there never been measured a free quark or gluon, but only objects that are built out of them, like [[meson]]s and [[baryon]]s? How does this phenomenon emerge from [[Quantum chromodynamics|QCD]]?
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| ; [[Strong CP problem]] and [[axion]]s: Why is the [[strong nuclear interaction]] invariant to [[parity (physics)|parity]] and [[charge conjugation]]? Is [[Peccei–Quinn theory]] the solution to this problem?
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| ; [[Anomalous magnetic dipole moment]]: Why is the experimentally measured value of the [[muon]]'s anomalous magnetic dipole moment ("muon g-2") significantly different from the theoretically predicted value of that physical constant?<ref>{{cite arXiv|eprint=1311.2198|title=The Muon (g-2) Theory Value: Present and Future|class= hep-ph|year=2013|author1=Thomas Blum|author2=Achim Denig|author3=Ivan Logashenko|author4=Eduardo de Rafael|last5= Lee Roberts|first5= B.|author6=Thomas Teubner|author7=Graziano Venanzoni}}</ref>
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| ; [[Proton Size Puzzle]]: What is the true [[charge radius]] of the proton?
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| ===Astronomy and astrophysics===
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| [[Image:Galaxies AGN Inner-Structure-of.jpg|right|270px|thumb|Relativistic jet. The environment around the [[Active galactic nucleus|AGN]] where the [[special relativity|relativistic]] [[plasma (physics)|plasma]] is collimated into jets which escape along the pole of the [[supermassive black hole]]]]
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| ; Accretion disc jets: Why do the [[accretion disc]]s surrounding certain astronomical objects, such as the nuclei of [[active galaxy|active galaxies]], emit [[relativistic jet]]s along their polar axes?<ref>{{cite arXiv|eprint=1311.1015|title=Systematic properties of decelerating relativistic jets in low-luminosity radio galaxies|class=astro-ph.CO|year=2013|last1=Laing|first1=R. A.|last2=Bridle|first2=A. H.}}</ref> Why are there [[quasi-periodic oscillation]]s in many accretion discs?<ref>{{cite journal|last=Strohmayer|first=Tod E.|coauthors=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}}</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|coauthors=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 |accessdate=25 January 2013}}</ref> Why are there sometimes overtones, and why do these appear at different frequency ratios in different objects?<ref>{{cite arxiv|eprint=1202.0121|author1=Shoji Kato|title=An Attempt to Describe Frequency Correlations among kHz QPOs and HBOs by Two-Armed Nearly Vertical Oscillations|class=astro-ph.HE|year=2012}}</ref>
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| ; [[Coronal heating problem]]: Why is the Sun's Corona (atmosphere layer) so much hotter than the Sun's surface? Why is the [[magnetic reconnection]] effect many orders of magnitude faster than predicted by standard models?
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| ;[[Diffuse interstellar band]]s: 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?
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| ; [[Gamma ray burst]]s: How do these short-duration high-intensity bursts originate?<ref name="Open Questions"/>
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| ; [[Supermassive black hole]]s: What is the origin of the [[M-sigma relation]] between supermassive black hole mass and galaxy velocity dispersion?<ref>{{Cite journal
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| | last = Ferrarese | first = Laura | last2 = Merritt | first2 = David | author2-link = David Merritt | title = A Fundamental Relation between Supermassive Black Holes and their Host Galaxies | journal = The Astrophysical Journal | volume = 539 | issue =
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| | pages = L9–L12 | date = | origyear = | year = 2000
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| | month =
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| | doi = 10.1086/312838| id = |arxiv = astro-ph/0006053 |bibcode = 2000ApJ...539L...9F
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| | postscript = <!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}} }}</ref> How did the most distant quasars grow their supermassive black holes up to 10^9 solar masses so early in the history of the Universe?
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| ; Observational anomalies:
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| [[Image: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?]]
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| ; [[Kuiper Cliff]]: Why does the number of objects in the Solar System's [[Kuiper Belt]] fall off rapidly and unexpectedly beyond a radius of 50 astronomic units?
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| ; [[Flyby anomaly]]: Why is the observed energy of satellites [[Gravity assist|flying by Earth]] sometimes different by a minute amount from the value predicted by theory?
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| ; [[Galaxy rotation problem]]: Is [[dark matter]] responsible for differences in observed and theoretical speed of stars revolving around the center of galaxies, or is it something else?
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| ; [[Supernova]]e: What is the exact mechanism by which an implosion of a dying star becomes an explosion?
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| ; [[Ultra-high-energy cosmic ray]]:<ref name="newscientist"/> Why is it that some cosmic rays appear to possess energies that are impossibly high (the so-called ''[[Oh-My-God particle|OMG particle]]''), given that there are no sufficiently energetic cosmic ray sources near the [[Earth]]? Why is it that (apparently) some cosmic rays emitted by distant sources have energies above the [[Greisen–Zatsepin–Kuzmin limit]]?<ref name="newscientist"/><ref name="Open Questions"/>
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| ; Rotation rate of [[Saturn]]: Why does the [[magnetosphere of Saturn]] exhibit a (slowly changing) periodicity close to that at which the planet's clouds rotate? What is the true rotation rate of Saturn's deep interior?<ref>{{cite web
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| |url = http://www.nasa.gov/mission_pages/cassini/media/cassini-062804.html
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| |title = Scientists Find That Saturn's Rotation Period is a Puzzle
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| |date = June 28, 2004
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| |publisher = NASA
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| |accessdate = 2007-03-22
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| }}</ref>
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| ; [[Magnetar#Magnetic field|Origin of magnetar magnetic field]]: What is the origin of magnetar magnetic field?
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| ; [[Space roar]]: Why is space roar six times louder than expected? What is the source of space roar?
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| ; Age-metallicity relation in the Galactic disk: Is there a universal age-metallicity relation in the Galactic disks? A sample of 229 nearby thick disk stars has been used to investigate the existence of an age-metallicity relation (AMR) in the Galactic thickdisk. The results indicate that that there is indeed an age-metallicity relation present in the thick disk.<ref>{{cite journal|last=Bensby|first=T.|coauthors=Feltzing, S.; 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}}</ref><ref>Gilmore, G.; Asiri, H.M. (00/2011). "[http://adsabs.harvard.edu/abs/2011sca..conf..280G Open Issues in the Evolution of the Galactic Disks]". Workshop on Gaia. Proceedings. Granada, ed. Navarro et al. 2011. Retrieved 2013-09-08.</ref>
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| ===Nuclear physics===
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| [[File:Island-of-Stability.png|thumb|right|230px|The "[[island of stability]]" in the proton vs. neutron number plot for heavy nuclei]]
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| ; [[Quantum chromodynamics]]: What are the phases of strongly interacting matter, and what roles do they play in the [[cosmos]]? What is the internal landscape of the [[nucleons]]? What does QCD predict for the properties of strongly interacting matter? What governs the transition of [[quarks]] and [[gluons]] into [[pions]] and nucleons? What is the role of [[gluons]] and gluon self-interactions in nucleons and nuclei? What determines the key features of QCD, and what is their relation to the nature of [[gravity]] and [[spacetime]]? Do [[glueball]]s exist? Do [[gluon]]s acquire mass dynamically despite having a zero [[rest mass]], within [[hadron]]s? Does QCD truly lack [[CP-violation]]s?
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| ; [[atomic nucleus|Nuclei]] and [[Nuclear astrophysics]]: What is the nature of the [[nuclear force]] that binds [[protons]] and [[neutrons]] into [[Stable isotope|stable nuclei]] and rare isotopes? What is the origin of simple patterns in complex nuclei? What is the nature of exotic excitations in nuclei at the frontiers of stability and their role in stellar processes? What is the nature of [[neutron stars]] and dense [[nuclear matter]]? What is the origin of the elements in the [[cosmos]]? What are the nuclear reactions that drive [[stars]] and stellar explosions?
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| ===Atomic, molecular and optical physics===
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| ; [[Hydrogen atom]]: What is the solution to the [[Schrödinger equation]] for the hydrogen atom in arbitrary electric and magnetic fields?<ref name=amo-nrc>{{cite book|last=Panel on Atomic, Molecular, and Optical Physics, Physics Survey Committee, Board on Physics and Astronomy, National Research Council|title=Atomic, Molecular, and Optical Physics|year=1986|publisher=National Academies Press|isbn=9780309594561|page=63|url=http://www.nap.edu/openbook.php?record_id=627&page=63}}</ref>
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| ; [[Helium atom]]: The helium atom is the simplest [[three-body problem]] in quantum mechanics; while approximations to a solution to the Schrödinger equation for He exist,<ref>[http://www.udel.edu/pchem/C444/spLectures/04222008b.pdf Helium Atom, Approximate Methods], University of Delaware, lecture notes for Physical Chemistry II, 22nd April 2008 (accessed 17 Sept. 2013)</ref> can an exact solution be found?<ref name=helium>{{cite book|last=McQuarrie|first=Donald A.|title=Quantum chemistry|year=2008|publisher=University Science Books|location=Sausalito, Calif.|isbn=9781891389504|pages=359–360|edition=2nd}}</ref>
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| ; [[Muonic hydrogen]]: Is the radius of [[muonic hydrogen]] inconsistent with the radius of ordinary hydrogen?
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| ===Condensed matter physics===
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| [[File:BI2223-piece3 001.jpg|thumb|right|150px|A sample of a [[cuprate]] superconductor (specifically [[BSCCO]]). The mechanism for superconductivity of these materials is unknown.]]
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| [[High-temperature superconductor|'''High-temperature superconductors''']]
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| : What is the mechanism that causes certain materials to exhibit [[superconductivity]] at temperatures much higher than around 25 [[kelvin]]? Is it possible to make a material that is a superconductor at room temperature?<ref name="Open Questions" />
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| ; [[Amorphous solid]]s: What is the nature of the [[glass transition]] between a fluid or regular solid and a glassy [[phase (matter)|phase]]? What are the physical processes giving rise to the general properties of [[glass]]es and the glass transition?<ref>{{Cite journal |url=http://www.nytimes.com/2008/07/29/science/29glass.html |title=The Nature of Glass Remains Anything but Clear |journal=[[The New York Times]] |date=July 29, 2008 |author=Kenneth Chang |postscript=<!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}}}}</ref><ref>{{Cite journal|author=P.W. Anderson| journal=Science|volume=267|year=1995|page=1615|title=Through the Glass Lightly|authorlink=Philip Warren Anderson|doi=10.1126/science.267.5204.1615-e|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|postscript=<!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}}}}</ref>
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| ; Cryogenic electron emission: Why does the electron emission in the absence of light increase as the temperature of a [[photomultiplier]] is decreased?<ref>[http://www.physorg.com/news187421719.html Cryogenic electron emission phenomenon has no known physics explanation]. Physorg.com. Retrieved on 2011-10-20.</ref><ref>{{cite journal|last=Meyer|first=H. O.|title=Spontaneous electron emission from a cold surface|journal=EPL (Europhysics Letters)|date=1 March 2010|volume=89|issue=5|pages=58001|doi=10.1209/0295-5075/89/58001}}</ref>
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| ; [[Sonoluminescence]]: What causes the emission of short bursts of light from imploding bubbles in a liquid when excited by sound?<ref>{{Cite journal |journal=[[Proceedings of the Royal Society A]] |volume=453 |publisher=[[Royal Society]] |year=1997 |quote=An unsolved problem in modern physics concerns the phenomenon of sonoluminescence |postscript=<!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}}}}</ref>{{update after|2012|4|reason=This may be out of date based on the age of the sources}}
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| ; [[Turbulence]]: 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"/> Also, under what conditions do [[Navier–Stokes existence and smoothness|smooth solutions to the Navier–Stokes equations]] exist? This problem is also listed as one of the [[Millennium Prize Problems]] in mathematics. [[Alfvénic turbulence]] in the solar wind and the turbulence in solar flares, coronal mass ejections, and magnetospheric substorms are major unsolved problems in space plasma physics.<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 }}</ref>
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| ; [[Topological order]]: Is topological order stable at non-zero [[temperature]]? Equivalently, is it possible to have three-dimensional [[Stabilizer code|self-correcting]] [[Qubit|quantum memory]]?<ref name=yoshida>{{cite journal|last=Yoshida|first=Beni|title=Feasibility of self-correcting quantum memory and thermal stability of topological order|journal=Annals of Physics|year=2011|volume=326|issue=10|doi=10.1016/j.aop.2011.06.001|url=http://www.sciencedirect.com/science/article/pii/S0003491611001023|accessdate=8 April 2012|arxiv = 1103.1885 |bibcode = 2011AnPhy.326.2566Y|pages=2566 }}</ref>
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| ; [[Fractional Hall effect]]: What mechanism explains the existence of the <math>\nu=5/2</math> state in the fractional [[quantum Hall effect]]? Does it describe quasiparticles with [[Fractional statistics#Non-abelian anyons|non-Abelian fractional statistics]]?<ref name=podolsky-NEQNET-fqhe>{{cite web | last=Podolsky | first=Dmitry | title=Quantum Hall effect. One open question | url=http://archive.is/W1hmB | publisher=NEQNET | accessdate=23 April 2012}}</ref>
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| ; [[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/ | publisher=Hausdorff Center for Mathematics | accessdate=23 April 2012}}</ref>
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| ; [[Liquid crystal]]s: Can the [[nematic]] to [[smectic|smectic (A)]] phase transition in liquid crystal states be characterized as a [[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 | pages=157–164 | url=http://www.tandfonline.com/doi/pdf/10.1080/10587259808042438 | accessdate=28 April 2012|doi=10.1080/10587259808042438}}</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"], Thermotropic Liquid Crystals: Recent Advances, ed. A. Ramamoorthy, Springer 2007, chapter 8.</ref>
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| ; [[Semiconductor nanocrystal]]s: What is the cause of the nonparabolicity of the energy-size dependence for the lowest [[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=9780203913260 | url=http://books.google.fr/books?id=sarqqnaw-7oC&lpg=PP1&pg=PA97#v=onepage&q&f=false}}</ref>
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| ; [[Electronic band structure]]: Why can band gaps not accurately be calculated?
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| ===Biophysics===
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| ; [[Stochastic process|Stochasticity]] and robustness to [[Signal-to-noise ratio|noise]] in [[gene expression]]: How do genes govern our body, withstanding different external pressures and internal stochasticity? [[Gene regulatory network|Certain models]] exist for genetic processes, but we are far from understanding the whole picture, in particular in [[Morphogenesis|development]] where gene expression must be tightly regulated.
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| ; Quantitative study of the [[immune system]]: What are the quantitative properties of immune responses? What are the basic building blocks of immune system networks? What roles are played by stochasticity?
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| ; [[Homochirality]]: What is the origin of the preponderance of specific [[enantiomers]] in [[biochemical system]]s?
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| ===Other problems===
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| ; [[Entropy (arrow of time)]]: Why did the universe have such low [[entropy]] in the past, resulting in the distinction between [[past]] and [[future]] and the [[second law of thermodynamics]]?<ref name="Open Questions"/> Why are [[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 [[causality (physics)|causality]]? Is there a single possible past? Is the [[present#Philosophy and science|present]] moment physically distinct from the past and future or is it merely an emergent property of consciousness?
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| ; [[Quantum mechanics]] in the [[correspondence principle|correspondence limit]] (sometimes called [[Quantum chaos]]): Is there a preferred [[interpretation of quantum mechanics]]? How does the quantum description of reality, which includes elements such as the [[quantum superposition|superposition]] of states and [[wavefunction collapse]] or [[quantum decoherence]], give rise to the reality we perceive? Another way of stating this is the [[Measurement problem]] – what constitutes a "measurement" which causes the wave function to collapse into a definite state?
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| ; [[Theory of everything]] ("[[Grand Unification Theory]]"): Is there a theory which explains the values of all [[fundamental physical constants]]?<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.|authorlink=John C. Baez |date=March 2006 |work=Usenet Physics FAQ |publisher=[[University of California, Riverside|University of California, Riverside: Department of Mathematics]] |accessdate=March 7, 2011}}</ref> Is the theory string theory? Is there a theory which explains why the [[gauge group]]s of the [[standard model]] are as they are, why observed [[space-time]] has 3 spatial dimensions and 1 dimension of time, and why all laws of physics are as they are? Do "fundamental physical constants" vary over time? Are any of the particles in the standard model of particle physics actually composite particles too tightly bound to observe as such at current experimental energies? Are there fundamental particles that have not yet been observed and if so which ones are they and what are their properties? Are there unobserved fundamental forces implied by a theory that explains other unsolved problems in physics?
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| ;[[Yang–Mills theory]]: Given an arbitrary [[compact space|compact]] [[gauge symmetry|gauge group]], does a non-trivial quantum [[Yang–Mills theory]] with a finite [[mass gap]] exist? This problem is also listed as one of the [[Millennium Prize Problems]] in mathematics.
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| ; [[Physical information]]: Are there physical phenomena, such as [[wave function collapse]] or [[black hole]]s, which irrevocably destroy information about their prior states? How is [[quantum information]] stored as a state of a quantum system?
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| ; [[Quantum Computation]]: Is [[David Deutsch]]'s notion of a [[universal quantum computer]] sufficient to [[Algorithmic efficiency|efficiently]] [[Dynamical simulation|simulate]] an arbitrary physical system?<ref name=nielson/chuang>{{cite book|last=Nielson|first=Michael|last2=Chuang|first2=Isaac|title=Quantum Computation and Quantum Information|year=2004|publisher=Cambridge University Press|isbn=978-0-521-63503-5}}</ref>
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| ; [[Dimensionless physical constant]]: At the present time, the values of the dimensionless physical constants cannot be calculated; they are determined only by physical measurement.<ref name="Effelsberg">{{cite web |url=http://www3.mpifr-bonn.mpg.de/public/pr/pr-methanol-dec2012-en.html |title=Alcohol constrains Physical Constant in the Early Universe |last= |first=|authorlink= |date=December 2012 |work= |publisher=[[Max Planck Institute for Radio Astronomy|Max-Planck-Institut für Radioastronomie]] |accessdate=December 15, 2012}}</ref> What is the minimum number of dimensionless physical constants from which all other dimensionless physical constants can be derived? Are dimensionful physical constants necessary at all?
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| ==Problems solved in recent decades==
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| ; [[Ball lightning]] (2014): In January 2014, scientists from [[Northwest Normal University]] in [[Lanzhou]], [[China]], published the results of recordings made in July 2012 of the optical spectrum of what was thought to be natural ball lightning made during the study of ordinary cloud–ground lightning on China's [[Qinghai Plateau]].<ref name="BLspectrum">{{cite journal|last1=Cen|first1=Jianyong| first2=Ping|last2= Yuan|first3=Simin|last3= Xue|date=17 January 2014|title=Observation of the Optical and Spectral Characteristics of Ball Lightning|journal=[[Physical Review Letters]]|publisher=[[American Physical Society]]|volume=112|issue= 35001|url=http://prl.aps.org/abstract/PRL/v112/i3/e035001|accessdate=19 January 2014}}</ref><ref name="BLspectrumvideo">{{cite journal|url=http://physics.aps.org/articles/v7/5|title=Focus: First Spectrum of Ball Lightning|last1=Ball|first1=Philip|authorlink1=Philip Ball|date=17 January 2014|work=Focus|publisher=[[American Physical Society]]|accessdate=19 January 2014|doi=10.1103/Physics.7.5}}</ref> At a distance of {{convert|900|m|ft|abbr=on}}, a total of 1.64 seconds of digital video of the ball lightning and its spectrum was made, from the formation of the ball lightning after the ordinary lightning struck the ground, up to the optical decay of the phenomenon. It is now believed that ball lightning is vaporized silicon in the soil that then rapidly oxidizes in the atmosphere.<ref name="BLspectrumvideo"/>
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| ;[[Hipparcos|Hipparcos anomaly]] (2012): The actual [[Pleiades#Distance|distance to the Pleiades]] - the High Precision Parallax Collecting Satellite (Hipparcos) measured the parallax of the Pleiades and determined a distance of 385 light years. This was significantly different from other measurements made by means of actual to apparent brightness measurement or [[absolute magnitude]]. The anomaly was due to a systematic bias in the Hipparcos data when it comes to star clusters; the Hipparcos results for clusters are consistently closer than they should be.<ref>{{cite arxiv|eprint=1203.4945|title= XHIP-II: Clusters and associations|class= astro-ph.GA|year= 2012 |author1= Charles Francis |author2= Erik Anderson}}</ref>{{not in reference|date=January 2014}}
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| ;[[Pioneer anomaly]] (2012):{{disputed-section|date=January 2014}}<!-- should include Fernández-Rañada and Tiemblo-Ramos counter-claim, deleted by Toth, co-author of one suggested resolution --> There was a deviation in the predicted accelerations of the [[Pioneer program|Pioneer]] spacecraft as they left the Solar System.<ref name="newscientist"/><ref name="Open Questions"/> It is believed that this is a result of previously unaccounted-for [[thermal recoil force]].<ref>{{cite doi|10.1103/PhysRevLett.108.241101}}</ref><ref>{{cite news|last=Overbye|first=Dennis|title=Mystery Tug on Spacecraft Is Einstein’s ‘I Told You So’|url=http://www.nytimes.com/2012/07/24/science/mystery-tug-on-pioneer-10-and-11-probes-is-einsteins-i-told-you-so.html?|accessdate=24 January 2014|newspaper=The New York Times|date=23 July 2012}}</ref>
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| ;Long-duration [[gamma ray bursts]] (2003): Long-duration bursts are associated with the deaths of massive stars in a specific kind of [[supernova]]-like event commonly referred to as a [[collapsar]]. However, there are also long-duration GRBs that show evidence against an associated supernova, such as the Swift event [[GRB 060614]].
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| ; [[Solar neutrino problem]] (2002): Solved by a new understanding of [[neutrino]] physics, requiring a modification of the [[Standard Model]] of [[particle physics]]—specifically, [[neutrino oscillation]].
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| ;[[Age Crisis]] (1990s): The estimated age of the universe was around 3 to 8 billion years younger than estimates of the ages of the oldest stars in our galaxy. Better estimates for the distances to the stars, and the recognition of the accelerating expansion of the universe, reconciled the age estimates.
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| ;[[Quasar]]s (1980s): The nature of quasars was not understood for decades.<ref name="quasar-jbo">{{cite web | url=http://www.jb.man.ac.uk/public/story/mk1quasars.html | title=The MKI and the discovery of Quasars | publisher=[[Jodrell Bank Observatory]] | accessdate=2006-11-23}}</ref> They are now accepted as a type of [[active galaxy]] where the enormous energy output results from matter falling into a massive [[black hole]] in the center of the galaxy.<ref name="quasar-hubble">[http://hubblesite.org/newscenter/archive/releases/1996/35/image/a/ Hubble Surveys the "Homes" of Quasars] Hubblesite News Archive, 1996-35</ref>
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| ==References==
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| {{Reflist|30em}}
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| ==External links==
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| * [http://www.sciencemag.org/sciext/125th/ What don't we know?] Science journal special project for its 125th anniversary: top 25 questions and 100 more.
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| * [http://www.openproblems.net/ List of links to unsolved problems in physics, prizes and research.]
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| * [http://www.nasa.gov/centers/glenn/technology/warp/ideachev.html Ideas Based On What We’d Like to Achieve]
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| * [http://www-conf.slac.stanford.edu/ssi/2004/Default.htm 2004 SLAC Summer Institute: Nature's Greatest Puzzles]
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| * [http://www.newscientist.com/article/dn14179-dual-personality-of-glass-explained-at-last.html#.UuLoQWTTkQM Dual Personality of Glass Explained at Last]
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| * [http://www.nybooks.com/articles/archives/2013/nov/07/physics-what-we-do-and-dont-know/ What we do and don't know] Review on current state of physics by Steven Weinberg, Nov 2013
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| * [http://www.nybooks.com/articles/archives/2012/may/10/crisis-big-science/ The crisis of big science] Steven Weinberg, May 2012
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| {{Unsolved problems}}
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| [[Category:Unsolved problems in physics| ]]
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| [[Category:Lists of unsolved problems|Physics]]
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| [[Category:Physics-related lists]]
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