This is a list of some of the major unsolved problems in physics. Some of these problems are theoretical, meaning that existing theories seem incapable of explaining a certain observed phenomenon or experimental result. The others are experimental, meaning that there is a difficulty in creating an experiment to test a proposed theory or investigate a phenomenon in greater detail.

Theoretical problems

The following problems are either fundamental theoretical problems, or theoretical ideas which lack experimental evidence and are in search of one, or both, as most of them are. Some of these problems are strongly interrelated. For example, extra dimensions or supersymmetry may solve the hierarchy problem. It is thought that a full theory of quantum gravity should be capable of answering most of these problems (other than the Island of stability problem).

Quantum gravity, cosmology, and general relativity

Vacuum catastrophe: Why does the predicted mass of the quantum vacuum have little effect on the expansion of the universe?
Quantum gravity: Can quantum mechanics and general relativity be realized as a fully consistent theory (perhaps as a quantum field theory)?^[1] Is spacetime fundamentally continuous or discrete? Would a consistent theory involve a force mediated by a hypothetical graviton, or be a product of a discrete structure of spacetime itself (as in loop quantum gravity)? Are there deviations from the predictions of general relativity at very small or very large scales or in other extreme circumstances that flow from a quantum gravity theory?
Black holes, black hole information paradox, and black hole radiation: Do black holes produce thermal radiation, as expected on theoretical grounds? Does this radiation contain information about their inner structure, as suggested by Gauge-gravity duality, or not, as implied by Hawking's original calculation? If not, and black holes can evaporate away, what happens to the information stored in them (quantum mechanics does not provide for the destruction of information)? Or does the radiation stop at some point leaving black hole remnants? Is there another way to probe their internal structure somehow, if such a structure even exists?
Extra dimensions: Does nature have more than four spacetime dimensions? If so, what is their size? Are dimensions a fundamental property of the universe or an emergent result of other physical laws? Can we experimentally "see" evidence of higher spatial dimensions?
Cosmic inflation: Is the theory of cosmic inflation correct, and if so, what are the details of this epoch? What is the hypothetical inflaton field giving rise to inflation? If inflation happened at one point, is it self-sustaining through inflation of quantum-mechanical fluctuations, and thus ongoing in some impossibly distant place?
Multiverses: Are there physical reasons to expect other universes that are fundamentally non-observable? For instance: Are there quantum mechanical "alternative histories" or "many worlds"? Are there "other" universes with physical laws resulting from alternate ways of breaking the apparent symmetries of physical forces at high energies, possibly incredibly far away due to cosmic inflation? Is the use of the anthropic principle to resolve global cosmological dilemmas justified?
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?^[2] Similarly, will the closed timelike curves which arise in some solutions to the equations of general relativity (and which imply the possibility of backwards time travel) be ruled out by a theory ofquantum gravity which unites general relativity with quantum mechanics, as suggested by the "chronology protection conjecture" of Stephen Hawking?
Arrow of time: What do the phenomena that differ going forward and backwards in time tell us about the nature of time? How does time differ from space? Why are CP violations observed in certain weak force decays, but not elsewhere? Are CP violations somehow a product of the Second Law of Thermodynamics, or are they a separate arrow of time? Are there exceptions to the principle of causality? Is there a single possible past? Is the present moment physically distinct from the past and future or is it merely an emergent property of consciousness? Why do people appear to agree on what the present moment is? (See also Entropy (arrow of time) below)
Locality: Are there non-local phenomena in quantum physics? If they exist, are non-local phenomena limited to transfers of information, or can energy and matter 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?
Future of the universe: Is the universe heading towards a Big Freeze, a Big Rip, a Big Crunch or a Big Bounce? Is our universe part of an infinitely recurring cyclic model?

High energy physics/Particle physics

Nuclear physics

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 gravityand spacetime?
Nuclei and Nuclear astrophysics: What is the nature of the nuclear force that binds protons and neutrons into stable nuclei and rare isotopes? What is the origin of simple patterns in complex nuclei? 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?
Island of stability: What is the heaviest possible stable or metastable nucleus?

Empirical phenomena lacking clear scientific explanation

Cosmology

Existence of the Universe: What is the origin of matter, energy, spacetime and the fundamental forces that form the universe / multiverse?
Baryon asymmetry: Why is there far more matter than antimatter in the observable universe?
Cosmological constant problem: Why does the zero-point energy of the vacuum not cause a large cosmological constant? What cancels it out?

Estimated distribution of dark matter and dark energy in the universe

Dark matter: What is dark matter?^[5] Is it related to supersymmetry? Do the phenomena attributed to dark matter point not to some form of matter but actually to an extension of gravity?

The log-log plot of dark energy density

ρ *

and material density

ρ m

Vs scale factor

a

. The two straight lines intersect at current epoch.^[6]

Dark energy: What is the cause of the observed accelerated expansion (deSitter phase) of the Universe? Why is the energy density of the dark energy component of the same magnitude as the density of matter at present when the two evolve quite differently over time; could it be simply that we are observing at exactly the right time? Is dark energy a pure cosmological constant, or are models of quintessence such as phantom energy applicable?
Dark flow: What is the cause of a large swath of galaxy clusters all moving towards one part of the universe?^[7]
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?^[4]
Horizon problem: Why is the distant universe so homogeneous, when the Big Bang theory seems to predict measurable anisotropies of the night sky larger than those observed? Possible approaches to a solution are inflation and the variable speed of light hypothesis.
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?
Shape of the Universe: What is the 3-manifold of comoving space, i.e. of a comoving spatial section of the Universe, informally called the "shape" of the Universe? Neither the curvature nor the topology is presently known, though the curvature is known to be "close" to zero on observable scales. The cosmic inflation hypothesis suggests that the shape of the Universe may be unmeasurable, but since 2003, Jean-Pierre Luminet et al. and other groups have suggested that the shape of the Universe may be the Poincaré dodecahedral space. Is the shape unmeasurable, the Poincaré space, or another 3-manifold?

High energy physics/Particle physics

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,^[4] or does nature make use of strong dynamics in breaking electroweak symmetry, as proposed by Technicolor?
Neutrino mass: What is the mechanism responsible for generating neutrino masses? Is the neutrino its own antiparticle? Or could it be an antiparticle that simply cannot join and annihilate with a normal particle because of its irregular state?
Inertial mass/gravitational mass ratio of elementary particles: According to the equivalence principle of general relativity, the ratio of inertial mass to gravitational mass of all elementary particles is unity. However, there is no experimental confirmation for many particles. In particular, we do not know what the weight of a macroscopic lump of antimatter of known mass would be.
Proton spin crisis: As initially measured by the European Muon Collaboration, the three main ("valence") quarks of the proton account for about 12% of its total spin. Can the gluons that bind the quarks together, as well as the "sea" of quark pairs that are continually being created and annihilated, properly account for the rest of it?
Quantum chromodynamics (QCD) in the non-perturbative regime: The equations of QCD remain unsolved at energy scales relevant for describing atomic nuclei, and, among others, mainly numerical approaches seem to begin to give answers at this limit. How does QCD give rise to the physics of nuclei and nuclear constituents?
Confinement: Why has there never been measured a free quark or gluon, but only objects that are built out of them, like mesons and baryons? How does this phenomenon emerge from QCD?^{[citation needed]}
Strong CP problem and axions: Why is the strong nuclear interaction invariant to parity and charge conjugation? Is Peccei-Quinn theory the solution to this problem?
Hypothetical particles: Which of the hypothetical particles predicted by supersymmetric theories and other fairly well known theories actually occur in nature?

[edit] Astronomy and astrophysics

Relativistic jet. The environment around theAGN where the relativistic plasma is collimated into jets which escape along the pole of the supermassive black hole

Accretion disc jets: Why do the accretion discs surrounding certain astronomical objects, such as the nuclei of active galaxies, emit relativistic jets along their polar axes? Why are there quasi-periodic oscillations in many accretion discs? Why does the period of these oscillations scale as the inverse of the mass of the central object? Why are there sometimes overtones, and why do these appear at different frequency ratios in different objects?
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?
Gamma ray bursts: How do these short-duration high-intensity bursts originate?^[4]
Supermassive black holes: What is the origin of the M-sigma relation between supermassive black hole mass and galaxy velocity dispersion?^[8]
Observational anomalies

Rotation curve of a typical spiral galaxy: predicted (A) and observed (B). Can the discrepancy between the curves be attributed to dark matter?

Hipparcos anomaly: What is the actual distance to the Pleiades?^{[citation needed]}; Pioneer anomaly^[5]: What causes the small additional sunward acceleration of the Pioneer spacecraft?^[4]^[5]; Flyby anomaly: Why is the observed energy of satellites flying by earth different by a minute amount from the value predicted by theory?; Galaxy rotation problem: Is dark matter responsible for differences in observed and theoretical speed of stars revolving around the center of galaxies, or is it something else?
Supernovae: What is the exact mechanism by which an implosion of a dying star becomes an explosion?
Ultra-high-energy cosmic ray^[5]: Why is it that some cosmic rays appear to possess energies that are impossibly high (the so called 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?^[4]^[5]
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? ^[9]

[edit] Condensed matter physics

Amorphous solids: What is the nature of the glass transition between a fluid or regular solid and a glassy phase? What are the physical processes giving rise to the general properties of glasses?^[10]^[11]
Cold fusion: What is the explanation for the controversial reports of excess heat, radiation and transmutations?^[5]^[12]^[13]
Cryogenic electron emission: Why does the electron emission in the absence of light increase as the temperature of a photomultiplier is decreased?^[14]^[15]
High-temperature superconductors: What is the mechanism that causes certain materials to exhibit superconductivity at temperatures much higher than around 50 kelvins?^[4]
Sonoluminescence: What causes the emission of short bursts of light from imploding bubbles in a liquid when excited by sound?^[16]
Turbulence: Is it possible to make a theoretical model to describe the statistics of a turbulent flow (in particular, its internal structures)?^[4] Also, under what conditions do smooth solutions to the Navier-Stokes equations exist? This is probably the last unsolved problem in Classical or Newtonian Physics.

Biological physics

These fields of research normally belong to biology, and traditionally were not included in physics but are included here because increasingly it is physicists who are researching them using methods and tools more popular in physics research than biology.^[17]^[18]

Synaptic plasticity: It is necessary for computational and physical models of the brain, but what causes it, and what role does it play in higher-order processing outside the hippocampus and visual cortex?
Axon guidance: How do axons branching out from neurons find their targets? This process is crucial to nervous system development, allowing the building up of the brain.
Stochasticity and robustness to noise in gene expression: How do genes govern our body, withstanding different external pressures and internal stochasticity? Certain models exist for genetic processes, but we are far from understanding the whole picture, in particular in developmentwhere gene expression must be tightly regulated.
Quantitative study of the immune system: What are the quantitative properties of immune responses? What are the basic building blocks of immune system networks? What roles are played by stochasticity?
Consciousness: What mechanism causes sentience within of arrangements of otherwise non-sentient particles (as with the human brain, for example)? Is this mechanism reproducible?

Problems solved in recent decades

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.
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.
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.
Quasars (1980s): The nature of quasars was not understood for decades.^[19] 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.^[20]

References

^ Alan Sokal (July 22, 1996), "Don't Pull the String Yet on Superstring Theory", New York Times
^ Joshi, Pankaj S. (January 2009), "Do Naked Singularities Break the Rules of Physics?", Scientific American
^ Dirac, Paul, "Quantised Singularities in the Electromagnetic Field".Proceedings of the Royal Society A 133, 60 (1931).
^ ^a ^b ^c ^d ^e ^f ^g ^h Baez, John C. (March 2006). "Open Questions in Physics". Usenet Physics FAQ. University of California, Riverside: Department of Mathematics. Retrieved March 7, 2011.
^ ^a ^b ^c ^d ^e ^f Brooks, Michael (March 19, 2005). "13 Things That Do Not Make Sense". New Scientist. Issue 2491. Retrieved March 7, 2011.
^ Steinardt, Paul (1997), "Cosmological Challenges For the 21st Century", in Val Fitch et. al., Critical problems in physics: proceedings of a conference celebrating the 250th anniversary of Princeton University, Princeton, New Jersey: Princeton University Press, pp. 138–140,ISBN 9780691057842
^ http://www.dailygalaxy.com/my_weblog/2009/08/dark-flow-discovered-at-edge-of-the-universe-hundreds-of-millions-of-stars-racing-toward-an-cosmic-h.html
^ Ferrarese, Laura; Merritt, David (2000), "A Fundamental Relation between Supermassive Black Holes and their Host Galaxies", The Astrophysical Journal 539: L9–L12, arXiv:astro-ph/0006053, Bibcode 2000ApJ...539L...9F, doi:10.1086/312838
^ "Scientists Find That Saturn's Rotation Period is a Puzzle". NASA. June 28, 2004. Retrieved 2007-03-22.
^ Kenneth Chang (July 29, 2008), "The Nature of Glass Remains Anything but Clear", The New York Times
^ "The deepest and most interesting unsolved problem in solid state theory is probably the theory of the nature of glass and the glass transition." P.W. Anderson (1995), "Through the Glass Lightly", Science267: 1615
^ John R. Vacca (2004), The World's 20 Greatest Unsolved Problems, Prentice Hall, ISBN 9780131426436
^ Feder, T.; John, O. (2004), "Cold Fusion Gets Chilly Encore",Physics Today 58: 27, Bibcode 2005PhT....58a..31F,doi:10.1063/1.1881896
^ http://www.physorg.com/news187421719.html
^ doi:10.1209/0295-5075/89/58001
^ Proceedings: Mathematical, physical, and engineering sciences (Royal Society) 453, 1997, "An unsolved problem in modern physics concerns the phenomenon of sonoluminescence"
^ The Nobel Prizes in Physics 1901-2000
^ The Office of Science - What is Physics?
^ "The MKI and the discovery of Quasars". Jodrell Bank Observatory. Retrieved 2006-11-23.
^ Hubble Surveys the "Homes" of Quasars Hubblesite News Archive, 1996-35

External links

What don't we know? Science journal special project for its 125th anniversary: top 25 questions and 100 more.
Physics News Update A weekly physics news bulletin hosted by the American Institute of Physics.
List of links to unsolved problems in physics, prizes and research.
Ideas Based On What We’d Like to Achieve
2004 SLAC Summer Institute: Nature's Greatest Puzzles
Dual Personality of Glass Explained at Last

From Wikipedia

2 comments:

Jérôme Chauvet said...: Hi Steve,

I have got ideas on some of these items, not only in a philosophical way but also in a technical way.

Is it what your think tank could be about?; October 5, 2011 at 4:57 AM
Steven Colyer said...: Well yes Philosophy is called for. George Musser, Jr. at Sci Am and lee Smolin at Perimeter and the Foundational Questions Institute strongly believe MORE not less philosophers are needed to question the assumptions, in order to promote better science.

Problem is, not all philosophers understand the mathematics, and none of them I imagine understand String theory maths. Tim Maudlin is one of the best, and we have him. So yes, it's right up your alley which is why I nominated you.; October 5, 2011 at 3:33 PM

Multiplication by Infinity

Tuesday, October 4, 2011

List of Unsolved Problems in Physics

Contents

Theoretical problems

Quantum gravity, cosmology, and general relativity

High energy physics/Particle physics

Nuclear physics

Other problems

Empirical phenomena lacking clear scientific explanation

Cosmology

High energy physics/Particle physics

[edit] Astronomy and astrophysics

[edit] Condensed matter physics

Biological physics

Problems solved in recent decades

References

External links

2 comments: