List of unsolved problems in physics
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The following is a list of notable
unsolved problems List of unsolved problems may refer to several notable conjectures or open problems in various academic fields: Natural sciences, engineering and medicine * Unsolved problems in astronomy * Unsolved problems in biology * Unsolved problems in c ...
grouped into broad areas of
physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...
. Some of the major unsolved problems in
physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...
are theoretical, meaning that existing
theories A theory is a rational type of abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated with such processes as observational study or research. Theories may be s ...
seem incapable of explaining a certain observed
phenomenon A phenomenon ( : phenomena) is an observable event. The term came into its modern philosophical usage through Immanuel Kant, who contrasted it with the noumenon, which ''cannot'' be directly observed. Kant was heavily influenced by Gottfried W ...
or experimental result. The others are experimental, meaning that there is a difficulty in creating an
experiment An experiment is a procedure carried out to support or refute a hypothesis, or determine the efficacy or likelihood of something previously untried. Experiments provide insight into Causality, cause-and-effect by demonstrating what outcome oc ...
to test a proposed theory or investigate a phenomenon in greater detail. There are still some questions beyond the Standard Model of physics, such as the strong CP problem,
neutrino mass A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass i ...
, matter–antimatter asymmetry, and the nature of
dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ab ...
and
dark energy In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovas, which showed that the univer ...
. Another problem lies within the mathematical framework of the Standard Model itself—the Standard Model is inconsistent with that of
general relativity General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...
, to the point that one or both theories break down under certain conditions (for example within known
spacetime In physics, spacetime is a mathematical model that combines the three dimensions of space and one dimension of time into a single four-dimensional manifold. Spacetime diagrams can be used to visualize relativistic effects, such as why differen ...
singularities like the
Big Bang The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from the ...
and the centres of
black hole A black hole is a region of spacetime where gravitation, gravity is so strong that nothing, including light or other Electromagnetic radiation, electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts t ...
s beyond the
event horizon In astrophysics, an event horizon is a boundary beyond which events cannot affect an observer. Wolfgang Rindler coined the term in the 1950s. In 1784, John Michell proposed that gravity can be strong enough in the vicinity of massive compact obj ...
).


General physics

*
Theory of everything A theory of everything (TOE or TOE/ToE), final theory, ultimate theory, unified field theory or master theory is a hypothetical, singular, all-encompassing, coherent theoretical framework of physics that fully explains and links together all asp ...
: Is there a singular, all-encompassing, coherent theoretical framework of physics that fully explains and links together all physical aspects of the
universe The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. Acc ...
? *
Dimensionless physical constant In physics, a dimensionless physical constant is a physical constant that is dimensionless, i.e. a pure number having no units attached and having a numerical value that is independent of whatever system of units may be used. For example, if one c ...
s: At the present time, the values of various dimensionless physical constants cannot be calculated; they can be determined only by physical measurement. What is the minimum number of dimensionless physical constants from which all other dimensionless physical constants can be derived? Are dimensional physical constants necessary at all?


Quantum gravity

*
Quantum gravity Quantum gravity (QG) is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics; it deals with environments in which neither gravitational nor quantum effects can be ignored, such as in the vi ...
: Can
quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...
and
general relativity General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...
be realized as a fully consistent theory (perhaps as a
quantum field theory In theoretical physics, quantum field theory (QFT) is a theoretical framework that combines classical field theory, special relativity, and quantum mechanics. QFT is used in particle physics to construct physical models of subatomic particles and ...
)? Is spacetime fundamentally continuous or discrete? Would a consistent theory involve a force mediated by a hypothetical
graviton In theories of quantum gravity, the graviton is the hypothetical quantum of gravity, an elementary particle that mediates the force of gravitational interaction. There is no complete quantum field theory of gravitons due to an outstanding mathem ...
, or be a product of a discrete structure of spacetime itself (as in
loop quantum gravity Loop quantum gravity (LQG) is a theory of quantum gravity, which aims to merge quantum mechanics and general relativity, incorporating matter of the Standard Model into the framework established for the pure quantum gravity case. It is an attem ...
)? 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? *
Vacuum catastrophe In cosmology, the cosmological constant problem or vacuum catastrophe is the disagreement between the observed values of vacuum energy energy density, density (the small value of the cosmological constant) and theoretical large value of zero-poi ...
: Why does the predicted mass of the
quantum vacuum In quantum field theory, the quantum vacuum state (also called the quantum vacuum or vacuum state) is the quantum state with the lowest possible energy. Generally, it contains no physical particles. The word zero-point field is sometimes used as ...
have little effect on the expansion of the universe? *
Black hole A black hole is a region of spacetime where gravitation, gravity is so strong that nothing, including light or other Electromagnetic radiation, electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts t ...
s,
black hole information paradox The black hole information paradox is a puzzle that appears when the predictions of quantum mechanics and general relativity are combined. The theory of general relativity predicts the existence of black holes that are regions of spacetime from wh ...
, and
black hole radiation Hawking radiation is theoretical black body radiation that is theorized to be released outside a black hole's event horizon because of relativistic quantum effects. It is named after the physicist Stephen Hawking, who developed a theoretical arg ...
: 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 In physics, string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. String theory describes how these strings propagate through space and interac ...
, or not, as implied by
Hawking Hawking may refer to: People * Stephen Hawking (1942–2018), English theoretical physicist and cosmologist * Hawking (surname), a family name (including a list of other persons with the name) Film * ''Hawking'' (2004 film), about Stephen Ha ...
's original calculation? If not, and black holes can evaporate away, what happens to the information stored in them (since quantum mechanics does not provide for the destruction of information)? Or does the radiation stop at some point leaving black hole remnants? Is there another way to probe their internal structure somehow, if such a structure even exists? * The
cosmic censorship hypothesis The weak and the strong cosmic censorship hypotheses are two mathematical conjectures about the structure of gravitational singularities arising in general relativity. Singularities that arise in the solutions of Einstein's equations are typically ...
and the
chronology protection conjecture The chronology protection conjecture is a hypothesis first proposed by Stephen Hawking that laws of physics beyond those of standard general relativity prevent time travel on all but microscopic scales - even when the latter theory states that it ...
: Can singularities not hidden behind an event horizon, known as "
naked singularities In general relativity, a naked singularity is a hypothetical gravitational singularity without an event horizon. In a black hole, the singularity is completely enclosed by a boundary known as the event horizon, inside which the curvature of space ...
", arise from realistic initial conditions, or is it possible to prove some version of the "cosmic censorship hypothesis" of
Roger Penrose Sir Roger Penrose (born 8 August 1931) is an English mathematician, mathematical physicist, philosopher of science and Nobel Laureate in Physics. He is Emeritus Rouse Ball Professor of Mathematics in the University of Oxford, an emeritus fello ...
which proposes that this is impossible? Similarly, will the
closed timelike curve In mathematical physics, a closed timelike curve (CTC) is a world line in a Lorentzian manifold, of a material particle in spacetime, that is "closed", returning to its starting point. This possibility was first discovered by Willem Jacob van Sto ...
s which arise in some solutions to the equations of general relativity (and which imply the possibility of backwards
time travel Time travel is the concept of movement between certain points in time, analogous to movement between different points in space by an object or a person, typically with the use of a hypothetical device known as a time machine. Time travel is a w ...
) be ruled out by a theory of
quantum gravity Quantum gravity (QG) is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics; it deals with environments in which neither gravitational nor quantum effects can be ignored, such as in the vi ...
which unites general relativity with quantum mechanics, as suggested by the "chronology protection conjecture" of Stephen Hawking?


Quantum physics

*
Yang–Mills theory In mathematical physics, Yang–Mills theory is a gauge theory based on a special unitary group SU(''N''), or more generally any compact, reductive Lie algebra. Yang–Mills theory seeks to describe the behavior of elementary particles using th ...
: Given an arbitrary
compact Compact as used in politics may refer broadly to a pact or treaty; in more specific cases it may refer to: * Interstate compact * Blood compact, an ancient ritual of the Philippines * Compact government, a type of colonial rule utilized in British ...
gauge group In physics, a gauge theory is a type of field theory in which the Lagrangian (and hence the dynamics of the system itself) does not change (is invariant) under local transformations according to certain smooth families of operations ( Lie group ...
, does a non-trivial quantum Yang–Mills theory with a finite
mass gap In quantum field theory, the mass gap is the difference in energy between the lowest energy state, the vacuum, and the next lowest energy state. The energy of the vacuum is zero by definition, and assuming that all energy states can be thought of ...
exist? (This problem is also listed as one of the
Millennium Prize Problems The Millennium Prize Problems are seven well-known complex mathematical problems selected by the Clay Mathematics Institute in 2000. The Clay Institute has pledged a US$1 million prize for the first correct solution to each problem. According ...
in mathematics.) *
Quantum field theory In theoretical physics, quantum field theory (QFT) is a theoretical framework that combines classical field theory, special relativity, and quantum mechanics. QFT is used in particle physics to construct physical models of subatomic particles and ...
: Is it possible to construct, in the mathematically rigorous framework of algebraic QFT, a theory in 4-dimensional spacetime that includes interactions and does not resort to perturbative methods?


Cosmology and general relativity

*
Axis of evil The phrase "axis of evil" was first used by U.S. President George W. Bush and originally referred to Iran, Iraq, and North Korea. It was used in Bush's State of the Union address on January 29, 2002, less than five months after the 9/11 attac ...
: 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? *
Fine-tuned universe The characterization of the universe as finely tuned suggests that the occurrence of life in the universe is very sensitive to the values of certain fundamental physical constants and that the observed values are, for some reason, improbable. If ...
: The values of the fundamental physical constants are in a narrow range necessary to support carbon-based life.Gribbin. J and Rees. M, ''Cosmic Coincidences: Dark Matter, Mankind, and Anthropic Cosmology'' p. 7, 269, 1989, Is this because there exist other universes with different constants, or are our universe's constants the result of chance, or some other factor or process? (See also the
Anthropic Principle The anthropic principle, also known as the "observation selection effect", is the hypothesis, first proposed in 1957 by Robert Dicke, that there is a restrictive lower bound on how statistically probable our observations of the universe are, beca ...
.) *
Problem of time In theoretical physics, the problem of time is a conceptual conflict between general relativity and quantum mechanics in that quantum mechanics regards the flow of time as universal and absolute, whereas general relativity regards the flow of tim ...
: 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 In mathematical physics, Minkowski space (or Minkowski spacetime) () is a combination of three-dimensional Euclidean space and time into a four-dimensional manifold where the spacetime interval between any two events is independent of the inert ...
, 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? *
Cosmic inflation In physical cosmology, cosmic inflation, cosmological inflation, or just inflation, is a theory of exponential expansion of space in the early universe. The inflationary epoch lasted from  seconds after the conjectured Big Bang singularity ...
: 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
scalar field In mathematics and physics, a scalar field is a function (mathematics), function associating a single number to every point (geometry), point in a space (mathematics), space – possibly physical space. The scalar may either be a pure Scalar ( ...
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? *
Horizon problem The horizon problem (also known as the homogeneity problem) is a cosmological fine-tuning problem within the Big Bang model of the universe. It arises due to the difficulty in explaining the observed homogeneity of causally disconnected region ...
: Why is the distant universe so homogeneous when the
Big Bang theory The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from the ...
seems to predict larger measurable
anisotropies Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physic ...
of the night sky than those observed? Cosmological
inflation In economics, inflation is an increase in the general price level of goods and services in an economy. When the general price level rises, each unit of currency buys fewer goods and services; consequently, inflation corresponds to a reductio ...
is generally accepted as the solution, but are other possible explanations such as a
variable speed of light A variable speed of light (VSL) is a feature of a family of hypotheses stating that the speed of light may in some way not be constant, for example, that it varies in space or time, or depending on frequency. Accepted classical theories of physi ...
more appropriate? *
Origin Origin(s) or The Origin may refer to: Arts, entertainment, and media Comics and manga * Origin (comics), ''Origin'' (comics), a Wolverine comic book mini-series published by Marvel Comics in 2002 * The Origin (Buffy comic), ''The Origin'' (Bu ...
and future of the universe: How did the conditions for anything to exist arise? Is the universe heading towards a
Big Freeze The heat death of the universe (also known as the Big Chill or Big Freeze) is a hypothesis on the ultimate fate of the universe, which suggests the universe will evolve to a state of no thermodynamic free energy, and will therefore be unabl ...
, a
Big Rip In physical cosmology, the Big Rip is a hypothetical cosmological model concerning the ultimate fate of the universe, in which the matter of the universe, from stars and galaxies to atoms and subatomic particles, and even spacetime itself, is ...
, a Big Crunch, or a Big Bounce? Or is it part of an infinitely recurring
cyclic model A cyclic model (or oscillating model) is any of several cosmological models in which the universe follows infinite, or indefinite, self-sustaining cycles. For example, the oscillating universe theory briefly considered by Albert Einstein in 1930 t ...
? * Size of universe: The diameter of the
observable universe The observable universe is a ball-shaped region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because the electromagnetic radiation from these obj ...
is about 93 billion light-years, but what is the size of the whole universe? *
Baryon asymmetry In physical cosmology, the baryon asymmetry problem, also known as the matter asymmetry problem or the matter–antimatter asymmetry problem, is the observed imbalance in baryonic matter (the type of matter experienced in everyday life) and antib ...
: Why is there far more
matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic partic ...
than
antimatter In modern physics, antimatter is defined as matter composed of the antiparticles (or "partners") of the corresponding particles in "ordinary" matter. Antimatter occurs in natural processes like cosmic ray collisions and some types of radioac ...
in the
observable universe The observable universe is a ball-shaped region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because the electromagnetic radiation from these obj ...
? (This may be solved due to the apparent asymmetry in neutrino-antineutrino oscillations.) *
Cosmological principle In modern physical cosmology, the cosmological principle is the notion that the spatial distribution of matter in the universe is homogeneous and isotropic when viewed on a large enough scale, since the forces are expected to act uniformly throu ...
: Is the universe homogeneous and isotropic at large enough scales, as claimed by the
cosmological principle In modern physical cosmology, the cosmological principle is the notion that the spatial distribution of matter in the universe is homogeneous and isotropic when viewed on a large enough scale, since the forces are expected to act uniformly throu ...
and assumed by all models that use the
Friedmann–Lemaître–Robertson–Walker metric The Friedmann–Lemaître–Robertson–Walker (FLRW; ) metric is a metric based on the exact solution of Einstein's field equations of general relativity; it describes a homogeneous, isotropic, expanding (or otherwise, contracting) universe tha ...
, including the current version of the
ΛCDM model The ΛCDM (Lambda cold dark matter) or Lambda-CDM model is a Parameter#Modelization, parameterization of the Big Bang physical cosmology, cosmological model in which the universe contains three major components: first, a cosmological constant de ...
, or is the universe
inhomogeneous Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...
or anisotropic? Is the
CMB dipole In modern physical cosmology, the cosmological principle is the notion that the spatial distribution of matter in the universe is homogeneity (physics), homogeneous and Isotropy, isotropic when viewed on a large enough scale, since the forces ar ...
purely kinematic, or does it signal anisotropy of the universe, resulting in the breakdown of the FLRW metric and the cosmological principle? Is the
Hubble tension Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving ...
evidence that the cosmological principle is false? Even if the cosmological principle is correct, is the
Friedmann–Lemaître–Robertson–Walker metric The Friedmann–Lemaître–Robertson–Walker (FLRW; ) metric is a metric based on the exact solution of Einstein's field equations of general relativity; it describes a homogeneous, isotropic, expanding (or otherwise, contracting) universe tha ...
the right metric to use for our universe? Are the observations usually interpreted as the
accelerating expansion of the universe Observations show that the expansion of the universe is accelerating, such that the velocity at which a distant galaxy recedes from the observer is continuously increasing with time. The accelerated expansion of the universe was discovered durin ...
rightly interpreted, or are they instead evidence that the
cosmological principle In modern physical cosmology, the cosmological principle is the notion that the spatial distribution of matter in the universe is homogeneous and isotropic when viewed on a large enough scale, since the forces are expected to act uniformly throu ...
is false? ** Copernican principle: Are cosmological observations made from Earth representative of observations from the average position in the universe? *
Cosmological constant problem In cosmology, the cosmological constant problem or vacuum catastrophe is the disagreement between the observed values of vacuum energy density (the small value of the cosmological constant) and theoretical large value of zero-point energy sugges ...
: Why does the
zero-point energy Zero-point energy (ZPE) is the lowest possible energy that a quantum mechanical system may have. Unlike in classical mechanics, quantum systems constantly Quantum fluctuation, fluctuate in their lowest energy state as described by the Heisen ...
of the
vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...
not cause a large
cosmological constant In cosmology, the cosmological constant (usually denoted by the Greek capital letter lambda: ), alternatively called Einstein's cosmological constant, is the constant coefficient of a term that Albert Einstein temporarily added to his field equ ...
? What cancels it out? *
Dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ab ...
: What is the identity of dark matter? Is it a
particle In the Outline of physical science, physical sciences, a particle (or corpuscule in older texts) is a small wikt:local, localized physical body, object which can be described by several physical property, physical or chemical property, chemical ...
? If so, is it a
WIMP Wimp, WIMP, or Wimps may refer to: Science and technology * Weakly interacting massive particle, a hypothetical particle of dark matter * WIMP (computing), the "window, icon, menu, pointer" paradigm * WIMP (software bundle), the web stack of Wind ...
,
axion An axion () is a hypothetical elementary particle postulated by the Peccei–Quinn theory in 1977 to resolve the strong CP problem in quantum chromodynamics (QCD). If axions exist and have low mass within a specific range, they are of interes ...
, the lightest
superpartner In particle physics, a superpartner (also sparticle) is a class of hypothetical elementary particles predicted by supersymmetry, which, among other applications, is one of the well-studied ways to extend the standard model of high-energy physics. ...
(LSP), or some other particle? Or, do the phenomena attributed to dark matter point not to some form of matter but actually to an extension of gravity? *
Dark energy In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovas, which showed that the univer ...
: What is the cause of the observed
accelerating expansion of the universe Observations show that the expansion of the universe is accelerating, such that the velocity at which a distant galaxy recedes from the observer is continuously increasing with time. The accelerated expansion of the universe was discovered durin ...
(the de Sitter phase)? Are the observations rightly interpreted as the
accelerating expansion of the universe Observations show that the expansion of the universe is accelerating, such that the velocity at which a distant galaxy recedes from the observer is continuously increasing with time. The accelerated expansion of the universe was discovered durin ...
, or are they evidence that the
cosmological principle In modern physical cosmology, the cosmological principle is the notion that the spatial distribution of matter in the universe is homogeneous and isotropic when viewed on a large enough scale, since the forces are expected to act uniformly throu ...
is false? 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 ''Right Time'' is the 1976 studio album debut of influential reggae band the Mighty Diamonds. The album, released by Virgin Records after they signed the Mighty Diamonds following a search for talent in Jamaica, is critically regarded as a regg ...
? Is dark energy a pure cosmological constant or are models of quintessence such as
phantom energy Phantom energy is a hypothetical form of dark energy satisfying the equation of state with w < -1. It possesses negative kinetic energy, and predic ...
applicable? *
Dark flow In astrophysics, dark flow is a theoretical non-random component of the peculiar velocity of galaxy clusters. The actual measured velocity is the sum of the velocity predicted by Hubble's Law plus a possible small and unexplained (or ''dark'') v ...
: 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? *
Shape of the universe The shape of the universe, in physical cosmology, is the local and global geometry of the universe. The local features of the geometry of the universe are primarily described by its curvature, whereas the topology of the universe describes gen ...
: What is the 3-
manifold In mathematics, a manifold is a topological space that locally resembles Euclidean space near each point. More precisely, an n-dimensional manifold, or ''n-manifold'' for short, is a topological space with the property that each point has a n ...
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 In physical cosmology, cosmic inflation, cosmological inflation, or just inflation, is a theory of exponential expansion of space in the early universe. The inflationary epoch lasted from  seconds after the conjectured Big Bang singularity ...
hypothesis suggests that the shape of the universe may be unmeasurable, but, since 2003,
Jean-Pierre Luminet Jean-Pierre Luminet (born 3 June 1951) is a French astrophysicist, specializing in black holes and cosmology. He is an emeritus research director at the CNRS (Centre national de la recherche scientifique). Luminet is a member of the Laboratoir ...
, et al., and other groups have suggested that the shape of the universe may be the Poincaré dodecahedral space. Is the shape unmeasurable; the Poincaré space; or another 3-manifold? * The largest structures in the universe are larger than expected. Current cosmological models say there should be very little structure on scales larger than a few hundred million light-years across, due to the expansion of the universe trumping the effect of gravity. But the
Sloan Great Wall The Sloan Great Wall (SGW) is a cosmic structure formed by a giant wall of galaxies (a galaxy filament). Its discovery was announced from Princeton University on October 20, 2003, by J. Richard Gott III, Mario Jurić, and their colleagues, b ...
is 1.38 billion
light-years A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...
in length. And the largest structure currently known, the
Hercules–Corona Borealis Great Wall The Hercules–Corona Borealis Great Wall or simply the Great Wall is the largest known structure in the observable universe, measuring approximately 10 billion light-years in length (the observable universe is about 93 billion light-years in di ...
, is up to 10 billion light-years in length. Are these actual structures or random density fluctuations? If they are real structures, they contradict the '
End of Greatness The observable universe is a ball-shaped region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because the electromagnetic radiation from these ob ...
' hypothesis which asserts that at a scale of 300 million light-years, structures seen in smaller surveys are randomized to the extent that the smooth distribution of the universe is visually apparent. *
Extra dimensions In physics, extra dimensions are proposed additional space or time dimensions beyond the (3 + 1) typical of observed spacetime, such as the first attempts based on the Kaluza–Klein theory. Among theories proposing extra dimensions are: ...
: Does nature have more than four
spacetime In physics, spacetime is a mathematical model that combines the three dimensions of space and one dimension of time into a single four-dimensional manifold. Spacetime diagrams can be used to visualize relativistic effects, such as why differen ...
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 physics/particle physics

*
Hierarchy problem In theoretical physics, the hierarchy problem is the problem concerning the large discrepancy between aspects of the weak force and gravity. There is no scientific consensus on why, for example, the weak force is 1024 times stronger than gravit ...
: Why is
gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
such a weak force? It becomes strong for particles only at the Planck scale, around  
GeV GEV may refer to: * ''G.E.V.'' (board game), a tabletop game by Steve Jackson Games * Ashe County Airport, in North Carolina, United States * Gällivare Lapland Airport, in Sweden * Generalized extreme value distribution * Gev Sella, Israeli-Sou ...
, much above the
electroweak scale In particle physics, the electroweak scale, also known as the Fermi scale, is the energy scale around 246 GeV, a typical energy of processes described by the electroweak theory. The particular number 246 GeV is taken to be the vacuum expectation ...
(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 The Higgs boson, sometimes called the Higgs particle, is an elementary particle in the Standard Model of particle physics produced by the quantum excitation of the Higgs field, one of the fields in particle physics theory. In the Stand ...
mass, from getting quantum corrections on the order of the Planck scale? Is the solution
supersymmetry In a supersymmetric theory the equations for force and the equations for matter are identical. In theoretical and mathematical physics, any theory with this property has the principle of supersymmetry (SUSY). Dozens of supersymmetric theories e ...
,
extra dimensions In physics, extra dimensions are proposed additional space or time dimensions beyond the (3 + 1) typical of observed spacetime, such as the first attempts based on the Kaluza–Klein theory. Among theories proposing extra dimensions are: ...
, or just
anthropic {{Short pages monitor *
Liquid crystal Liquid crystal (LC) is a state of matter whose properties are between those of conventional liquids and those of solid crystals. For example, a liquid crystal may flow like a liquid, but its molecules may be oriented in a crystal-like way. T ...
s: Can the
nematic Liquid crystal (LC) is a state of matter whose properties are between those of conventional liquids and those of solid crystals. For example, a liquid crystal may flow like a liquid, but its molecules may be oriented in a crystal-like way. The ...
to
smectic Liquid crystal (LC) is a state of matter whose properties are between those of conventional liquids and those of solid crystals. For example, a liquid crystal may flow like a liquid, but its molecules may be oriented in a crystal-like way. The ...
(A) phase transition in liquid crystal states be characterized as a
universal Universal is the adjective for universe. Universal may also refer to: Companies * NBCUniversal, a media and entertainment company ** Universal Animation Studios, an American Animation studio, and a subsidiary of NBCUniversal ** Universal TV, a ...
phase transition?A. Yethiraj
"Recent Experimental Developments at the Nematic to Smectic-A Liquid Crystal Phase Transition"
Thermotropic Liquid Crystals: Recent Advances, ed. A. Ramamoorthy, Springer 2007, chapter 8.
* Semiconductor nanocrystals: What is the cause of the nonparabolicity of the energy-size dependence for the lowest optical absorption transition of
quantum dots Quantum dots (QDs) are semiconductor particles a few nanometres in size, having optical and electronic properties that differ from those of larger particles as a result of quantum mechanics. They are a central topic in nanotechnology. When the ...
? * Metal whiskering: In electrical devices, some metallic surfaces may spontaneously grow fine metallic whiskers, which can lead to equipment failures. While compressive mechanical stress is known to encourage whisker formation, the growth mechanism has yet to be determined. * Superfluid transition in helium-4: Explain the discrepancy between the experimental and theoretical determinations of the heat capacity critical exponent .


Plasma physics

*
Plasma physics Plasma ()πλάσμα
, Henry George Liddell, R ...
and
fusion power Fusion power is a proposed form of power generation that would generate electricity by using heat from nuclear fusion, nuclear fusion reactions. In a fusion process, two lighter atomic nucleus, atomic nuclei combine to form a heavier nucleus, whi ...
: 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 confined long enough and at a high enough temperature to create fusion power? What is the physical origin of H-mode? * The injection problem:
Fermi acceleration Fermi acceleration, sometimes referred to as ''diffusive shock acceleration'' (a subclass of Fermi accelerationOn the Origin of the Cosmic Radiation, E. Fermi, Physical Review 75, pp. 1169-1174, 1949), is the acceleration that charged particles u ...
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. * Alfvénic turbulence: In the solar wind and the turbulence in
solar flare A solar flare is an intense localized eruption of electromagnetic radiation in the Sun's atmosphere. Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar particle events, and other solar phe ...
s,
coronal mass ejection A coronal mass ejection (CME) is a significant release of plasma and accompanying magnetic field from the Sun's corona into the heliosphere. CMEs are often associated with solar flares and other forms of solar activity, but a broadly accepted ...
s, and magnetospheric substorms are major unsolved problems in space plasma physics.


Biophysics

*
Stochasticity Stochastic (, ) refers to the property of being well described by a random probability distribution. Although stochasticity and randomness are distinct in that the former refers to a modeling approach and the latter refers to phenomena themselve ...
and robustness to
noise Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference arise ...
in
gene expression Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, protein or non-coding RNA, and ultimately affect a phenotype, as the final effect. The ...
: How do genes govern our body, withstanding different external pressures and internal
stochasticity Stochastic (, ) refers to the property of being well described by a random probability distribution. Although stochasticity and randomness are distinct in that the former refers to a modeling approach and the latter refers to phenomena themselve ...
? Certain models exist for genetic processes, but we are far from understanding the whole picture, in particular in
development Development or developing may refer to: Arts *Development hell, when a project is stuck in development *Filmmaking, development phase, including finance and budgeting *Development (music), the process thematic material is reshaped * Photograph ...
where gene expression must be tightly regulated. * Quantitative study of the
immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinte ...
: What are the quantitative properties of
immune response An immune response is a reaction which occurs within an organism for the purpose of defending against foreign invaders. These invaders include a wide variety of different microorganisms including viruses, bacteria, parasites, and fungi which could ...
s? What are the basic building blocks of
immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinte ...
networks? * ''
Homochirality Homochirality is a uniformity of chirality, or handedness. Objects are chiral when they cannot be superposed on their mirror images. For example, the left and right hands of a human are approximately mirror images of each other but are not their ow ...
'': What is the origin of the preponderance of specific enantiomers in biochemical systems? *
Magnetoreception Magnetoreception is a sense which allows an organism to detect the Earth's magnetic field. Animals with this sense include some arthropods, molluscs, and vertebrates (fish, amphibians, reptiles, birds, and mammals, though not humans). The se ...
: How do animals (e.g. migratory birds) sense the Earth's magnetic field? *
Protein structure prediction Protein structure prediction is the inference of the three-dimensional structure of a protein from its amino acid sequence—that is, the prediction of its secondary and tertiary structure from primary structure. Structure prediction is different ...
: 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? *
Quantum biology Quantum biology is the study of applications of quantum mechanics and theoretical chemistry to aspects of biology that cannot be accurately described by the classical laws of physics. An understanding of fundamental quantum interactions is importan ...
: 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

*
Interpretation of quantum mechanics An interpretation of quantum mechanics is an attempt to explain how the mathematical theory of quantum mechanics might correspond to experienced reality. Although quantum mechanics has held up to rigorous and extremely precise tests in an extraord ...
: How does the
quantum In physics, a quantum (plural quanta) is the minimum amount of any physical entity (physical property) involved in an interaction. The fundamental notion that a physical property can be "quantized" is referred to as "the hypothesis of quantizati ...
description of reality, which includes elements such as the superposition of states and
wavefunction collapse In quantum mechanics, wave function collapse occurs when a wave function—initially in a superposition of several eigenstates—reduces to a single eigenstate due to interaction with the external world. This interaction is called an ''observa ...
or
quantum decoherence Quantum decoherence is the loss of quantum coherence. In quantum mechanics, particles such as electrons are described by a wave function, a mathematical representation of the quantum state of a system; a probabilistic interpretation of the wave ...
, give rise to the reality we perceive? Another way of stating this question regards the
measurement problem In quantum mechanics, the measurement problem is the problem of how, or whether, wave function collapse occurs. The inability to observe such a collapse directly has given rise to different interpretations of quantum mechanics and poses a key se ...
: What constitutes a "measurement" which apparently causes the wave function to collapse into a definite state? Unlike classical physical processes, some quantum mechanical processes (such as quantum teleportation arising from
quantum entanglement Quantum entanglement is the phenomenon that occurs when a group of particles are generated, interact, or share spatial proximity in a way such that the quantum state of each particle of the group cannot be described independently of the state of ...
) cannot be simultaneously "local", "causal", and "real", but it is not obvious which of these properties must be sacrificed, or if an attempt to describe quantum mechanical processes in these senses is a category error such that a proper understanding of quantum mechanics would render the question meaningless. Can a
multiverse The multiverse is a hypothetical group of multiple universes. Together, these universes comprise everything that exists: the entirety of space, time, matter, energy, information, and the physical laws and constants that describe them. The di ...
resolve it? *
Arrow of time The arrow of time, also called time's arrow, is the concept positing the "one-way direction" or " asymmetry" of time. It was developed in 1927 by the British astrophysicist Arthur Eddington, and is an unsolved general physics question. This ...
(e.g. entropy's arrow of time): Why does time have a direction? Why did the universe have such low
entropy Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynam ...
in the past, and time correlates with the universal (but not local) increase in entropy, from the past and to the future, according to the
second law of thermodynamics The second law of thermodynamics is a physical law based on universal experience concerning heat and Energy transformation, energy interconversions. One simple statement of the law is that heat always moves from hotter objects to colder objects ( ...
? Why are
CP violation In particle physics, CP violation is a violation of CP-symmetry (or charge conjugation parity symmetry): the combination of C-symmetry (charge symmetry) and P-symmetry ( parity symmetry). CP-symmetry states that the laws of physics should be the ...
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 Causality (also referred to as causation, or cause and effect) is influence by which one event, process, state, or object (''a'' ''cause'') contributes to the production of another event, process, state, or object (an ''effect'') where the cau ...
? Is there a single possible past? Is the
present The present (or here'' and ''now) is the time that is associated with the events perception, perceived directly and in the first time, not as a recollection (perceived more than once) or a speculation (predicted, hypothesis, uncertain). It is ...
moment physically distinct from the past and future, or is it merely an emergent property of
consciousness Consciousness, at its simplest, is sentience and awareness of internal and external existence. However, the lack of definitions has led to millennia of analyses, explanations and debates by philosophers, theologians, linguisticians, and scien ...
? What links the quantum arrow of time to the thermodynamic arrow? *
Locality Locality may refer to: * Locality (association), an association of community regeneration organizations in England * Locality (linguistics) * Locality (settlement) * Suburbs and localities (Australia), in which a locality is a geographic subdivis ...
: 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 elucidate the proper interpretation of the fundamental nature of quantum physics?


Problems solved since the 1990s


General physics/quantum physics

* Perform a loophole-free Bell test experiment (1970–2015): In October 2015, scientists from the 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. These results were confirmed by two studies with statistical significance over 5 standard deviations which were published in December 2015. * Existence of
ball lightning Ball lightning is a rare and unexplained phenomenon described as luminescent, spherical objects that vary from pea-sized to several meters in diameter. Though usually associated with thunderstorms, the observed phenomenon is reported to last c ...
(1638–2014): In January 2014, scientists from
Northwest Normal University Northwest Normal University, also translated into English as Northwestern Normal University (), is a comprehensive university in Lanzhou, Gansu Province of the People's Republic of China. The university is authorized by the State Council of the C ...
in
Lanzhou Lanzhou (, ; ) is the capital and largest city of Gansu Province in Northwest China. Located on the banks of the Yellow River, it is a key regional transportation hub, connecting areas further west by rail to the eastern half of the country. H ...
, China, published the results of recordings made in July 2012 of the optical spectrum of what was thought to be natural ball lightning made during the study of ordinary cloud–ground lightning on China's Qinghai Plateau. At a distance of , a total of 1.3 seconds of digital video of the ball lightning and its spectrum was made, from the formation of the ball lightning after the ordinary lightning struck the ground, up to the optical decay of the phenomenon. The recorded ball lightning is believed to be vaporized soil elements that then rapidly oxidize in the atmosphere. The validity of this hypothesis is still not clear. * Create
Bose–Einstein condensate In condensed matter physics, a Bose–Einstein condensate (BEC) is a state of matter that is typically formed when a gas of bosons at very low densities is cooled to temperatures very close to absolute zero (−273.15 °C or −459.67&n ...
(1924–1995): Composite bosons in the form of dilute atomic vapours were cooled to quantum degeneracy using the techniques of
laser cooling Laser cooling includes a number of techniques in which atoms, molecules, and small mechanical systems are cooled, often approaching temperatures near absolute zero. Laser cooling techniques rely on the fact that when an object (usually an atom) ...
and
evaporative cooling An evaporative cooler (also known as evaporative air conditioner, swamp cooler, swamp box, desert cooler and wet air cooler) is a device that cools air through the evaporation of water. Evaporative cooling differs from other air conditioning sy ...
.


Cosmology and general relativity

* Existence of
gravitational wave Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1 ...
s (1916–2016): On 11 February 2016, the
Advanced LIGO The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory designed to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool. Two large ...
team announced that they had directly detected gravitational waves from a
pair Pair or PAIR or Pairing may refer to: Government and politics * Pair (parliamentary convention), matching of members unable to attend, so as not to change the voting margin * ''Pair'', a member of the Prussian House of Lords * ''Pair'', the Frenc ...
of black holes
merging Merge, merging, or merger may refer to: Concepts * Merge (traffic), the reduction of the number of lanes on a road * Merge (linguistics), a basic syntactic operation in generative syntax in the Minimalist Program * Merger (politics), the comb ...
, which was also the first detection of a stellar binary black hole. * Numerical solution for
binary black hole A binary black hole (BBH) is a system consisting of two black holes in close orbit around each other. Like black holes themselves, binary black holes are often divided into stellar binary black holes, formed either as remnants of high-mass binary ...
(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 (
annus mirabilis ''Annus mirabilis'' (pl. ''anni mirabiles'') is a Latin phrase that means "marvelous year", "wonderful year", "miraculous year", or "amazing year". This term has been used to refer to several years during which events of major importance are rem ...
of
numerical relativity Numerical relativity is one of the branches of general relativity that uses numerical methods and algorithms to solve and analyze problems. To this end, supercomputers are often employed to study black holes, gravitational waves, neutron stars and ...
). * 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.


High-energy physics/particle physics

* Existence of
pentaquark A pentaquark is a human-made subatomic particle, consisting of four quarks and one antiquark bound together; they are not known to occur naturally, or exist outside of experiments specifically carried out to create them. As quarks have a baryon ...
s (1964–2015): In July 2015, the
LHCb The LHCb (Large Hadron Collider beauty) experiment is one of eight particle physics detector experiments collecting data at the Large Hadron Collider at CERN. LHCb is a specialized b-physics experiment, designed primarily to measure the paramet ...
collaboration at
CERN The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in a northwestern suburb of Gene ...
identified pentaquarks in the channel, which represents the decay of the bottom lambda baryon into a J/ψ meson , a
kaon KAON (Karlsruhe ontology) is an ontology infrastructure developed by the University of Karlsruhe and the Research Center for Information Technologies in Karlsruhe. Its first incarnation was developed in 2002 and supported an enhanced version of ...
and a
proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
(p). The results showed that sometimes, instead of decaying directly into mesons and baryons, the decayed via intermediate pentaquark states. The two states, named and , had individual
statistical significance In statistical hypothesis testing, a result has statistical significance when it is very unlikely to have occurred given the null hypothesis (simply by chance alone). More precisely, a study's defined significance level, denoted by \alpha, is the p ...
s of 9 σ and 12 σ, respectively, and a combined significance of 15 σ—enough to claim a formal discovery. The two pentaquark states were both observed decaying strongly to , hence must have a valence quark content of two
up quark The up quark or u quark (symbol: u) is the lightest of all quarks, a type of elementary particle, and a significant constituent of matter. It, along with the down quark, forms the neutrons (one up quark, two down quarks) and protons (two up quark ...
s, a
down quark The down quark or d quark (symbol: d) is the second-lightest of all quarks, a type of elementary particle, and a major constituent of matter. Together with the up quark, it forms the neutrons (one up quark, two down quarks) and protons (two up q ...
, a
charm quark The charm quark, charmed quark or c quark (from its symbol, c) is the third-most massive of all quarks, a type of elementary particle. Charm quarks are found in hadrons, which are subatomic particles made of quarks. Examples of hadrons containin ...
, and an anti-charm quark (), making them
charmonium In particle physics, quarkonium (from quark and -onium, pl. quarkonia) is a flavorless meson whose constituents are a heavy quark and its own antiquark, making it both a neutral particle and its own antiparticle. Light quarks Light quarks ( up ...
-pentaquarks. * Existence of quark-gluon plasma, a new phase of matter was discovered and confirmed in experiments at
CERN The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in a northwestern suburb of Gene ...
-
SPS SPS may refer to: Law and government * Agreement on the Application of Sanitary and Phytosanitary Measures of the WTO * NATO Science for Peace and Security * Single Payment Scheme, an EU agricultural subsidy * The Standard Procurement System, fo ...
(2000), BNL- RHIC (2005) and CERN- LHC (2010). *
Higgs boson The Higgs boson, sometimes called the Higgs particle, is an elementary particle in the Standard Model of particle physics produced by the quantum excitation of the Higgs field, one of the fields in particle physics theory. In the Stand ...
and
electroweak symmetry breaking In the Standard Model of particle physics, the Higgs mechanism is essential to explain the generation mechanism of the property "mass" for gauge bosons. Without the Higgs mechanism, all bosons (one of the two classes of particles, the other bein ...
(1963 – the original 2001 paper can be found at: –2012): The mechanism responsible for breaking the electroweak gauge symmetry, giving mass to the
W and Z bosons In particle physics, the W and Z bosons are vector bosons that are together known as the weak bosons or more generally as the intermediate vector bosons. These elementary particles mediate the weak interaction; the respective symbols are , , and ...
, was solved with the discovery of the
Higgs boson The Higgs boson, sometimes called the Higgs particle, is an elementary particle in the Standard Model of particle physics produced by the quantum excitation of the Higgs field, one of the fields in particle physics theory. In the Stand ...
of the
Standard Model The Standard Model of particle physics is the theory describing three of the four known fundamental forces (electromagnetism, electromagnetic, weak interaction, weak and strong interactions - excluding gravity) in the universe and classifying a ...
, with the expected couplings to the weak bosons. No evidence of a strong dynamics solution, as proposed by
technicolor Technicolor is a series of Color motion picture film, color motion picture processes, the first version dating back to 1916, and followed by improved versions over several decades. Definitive Technicolor movies using three black and white films ...
, has been observed. * Origin of mass of most elementary particles: Solved with the discovery of the
Higgs boson The Higgs boson, sometimes called the Higgs particle, is an elementary particle in the Standard Model of particle physics produced by the quantum excitation of the Higgs field, one of the fields in particle physics theory. In the Stand ...
, which implies the existence of the
Higgs field The Higgs boson, sometimes called the Higgs particle, is an elementary particle in the Standard Model of particle physics produced by the quantum excitation of the Higgs field, one of the fields in particle physics theory. In the Stand ...
giving mass to these particles.


Astronomy and astrophysics

* Origin of short
gamma-ray burst In gamma-ray astronomy, gamma-ray bursts (GRBs) are immensely energetic explosions that have been observed in distant galaxies. They are the most energetic and luminous electromagnetic events since the Big Bang. Bursts can last from ten milli ...
(1993–2017): From binary
neutron star A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. white ...
s merger, produce a
kilonova A kilonova (also called a macronova) is a transient astronomical event that occurs in a compact binary system when two neutron stars or a neutron star and a black hole merge. These mergers are thought to produce gamma-ray bursts and emit bright el ...
explosion and short gamma-ray burst
GRB 170817A GW 170817 was a gravitational wave (GW) signal observed by the LIGO and Virgo detectors on 17 August 2017, originating from the shell elliptical galaxy . The signal was produced by the last minutes of a binary pair of neutron stars' insp ...
was detected in both
electromagnetic wave In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visib ...
s and
gravitational wave Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1 ...
GW170817 GW 170817 was a gravitational wave (GW) signal observed by the LIGO and Virgo detectors on 17 August 2017, originating from the shell elliptical galaxy . The signal was produced by the last minutes of a binary pair of neutron stars' inspir ...
. * Missing baryon problem (1998–2017): proclaimed solved in October 2017, with the missing baryons located in hot intergalactic gas. *Long-duration
gamma-ray burst In gamma-ray astronomy, gamma-ray bursts (GRBs) are immensely energetic explosions that have been observed in distant galaxies. They are the most energetic and luminous electromagnetic events since the Big Bang. Bursts can last from ten milli ...
s (1993–2003): Long-duration bursts are associated with the deaths of massive stars in a specific kind of
supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...
-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 The solar neutrino problem concerned a large discrepancy between the flux of solar neutrinos as predicted from the Sun's luminosity and as measured directly. The discrepancy was first observed in the mid-1960s and was resolved around 2002. The fl ...
(1968–2001): Solved by a new understanding of
neutrino A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...
physics, requiring a modification of the
Standard Model The Standard Model of particle physics is the theory describing three of the four known fundamental forces (electromagnetism, electromagnetic, weak interaction, weak and strong interactions - excluding gravity) in the universe and classifying a ...
of
particle physics Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) an ...
—specifically, neutrino oscillation. * Nature of
quasar A quasar is an extremely Luminosity, luminous active galactic nucleus (AGN). It is pronounced , and sometimes known as a quasi-stellar object, abbreviated QSO. This emission from a galaxy nucleus is powered by a supermassive black hole with a m ...
s (1950s–1980s): The nature of quasars was not understood for decades. They are now accepted as a type of
active galaxy An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much-higher-than-normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not prod ...
where the enormous energy output results from matter falling into a massive
black hole A black hole is a region of spacetime where gravitation, gravity is so strong that nothing, including light or other Electromagnetic radiation, electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts t ...
in the centre of the galaxy. Quasars produce jets within the core, expel them at the opposite poles, and then the jets are collimated by radiation from the surrounding
accretion disk An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction, uneven irradiance, magnetohydrodynamic effects, and other fo ...
by the
hollow charge A shaped charge is an explosive charge shaped to form an explosively formed penetrator (EFP) to focus the effect of the explosive's energy. Different types of shaped charges are used for various purposes such as cutting and forming metal, init ...
effect. * Saturn’s core spin was determined from its gravitational field.


Nuclear physics

* Existence of quark-gluon plasma, a new phase of matter was discovered and confirmed in experiments at
CERN The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in a northwestern suburb of Gene ...
-
SPS SPS may refer to: Law and government * Agreement on the Application of Sanitary and Phytosanitary Measures of the WTO * NATO Science for Peace and Security * Single Payment Scheme, an EU agricultural subsidy * The Standard Procurement System, fo ...
(2000), BNL- RHIC (2005) and CERN- LHC (2010). *
Hagedorn Temperature The Hagedorn temperature, ''T''H, is the temperature in theoretical physics where hadronic matter (i.e. ordinary matter) is no longer stable, and must either "evaporate" or convert into quark matter; as such, it can be thought of as the "boiling p ...
recognized as phase transformation temperature between hadronic confined phase and deconfined phase of matter.


Rapidly solved problems

* Existence of
time crystal In condensed matter physics, a time crystal is a quantum system of particles whose lowest-energy state is one in which the particles are in repetitive motion. The system cannot lose energy to the environment and come to rest because it is alre ...
s (2012–2016): The idea of a quantized time crystal was first theorized in 2012 by
Frank Wilczek Frank Anthony Wilczek (; born May 15, 1951) is an American theoretical physicist, mathematician and Nobel laureate. He is currently the Herman Feshbach Professor of Physics at the Massachusetts Institute of Technology (MIT), Founding Direct ...
. In 2016, Khemani et al. and Else et al. 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. This was then used by two teams, a group led by Christopher Monroe at the
University of Maryland The University of Maryland, College Park (University of Maryland, UMD, or simply Maryland) is a public land-grant research university in College Park, Maryland. Founded in 1856, UMD is the flagship institution of the University System of M ...
and a group led by
Mikhail Lukin Mikhail Lukin (russian: Михаи́л Дми́триевич Луки́н); born 10 October 1971) is a Russian theoretical and experimental physicist and a professor at Harvard University. He was elected a member of the National Academy of Scien ...
at
Harvard University Harvard University is a private Ivy League research university in Cambridge, Massachusetts. Founded in 1636 as Harvard College and named for its first benefactor, the Puritan clergyman John Harvard, it is the oldest institution of higher le ...
, 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. *
Photon underproduction crisis The photon underproduction crisis is a cosmological discussion concerning the purported deficit between observed photons and predicted photons. The deficit, or underproduction crisis, is a theoretical problem, arising from comparing observations of ...
(2014–2015): This problem was resolved by Khaire and Srianand. They show that a factor 2 to 5 times large metagalactic photoionization rate can be easily obtained using updated quasar and galaxy observations. Recent observations of quasars indicate that the quasar contribution to ultraviolet photons is a factor of 2 larger than previous estimates. The revised galaxy contribution is a factor of 3 larger. These together solve the crisis. * Hipparcos anomaly (1997–2012): The High Precision Parallax Collecting Satellite (Hipparcos) measured the parallax of the
Pleiades The Pleiades (), also known as The Seven Sisters, Messier 45 and other names by different cultures, is an asterism and an open star cluster containing middle-aged, hot B-type stars in the north-west of the constellation Taurus. At a distance of ...
and determined a
distance Distance is a numerical or occasionally qualitative measurement of how far apart objects or points are. In physics or everyday usage, distance may refer to a physical length or an estimation based on other criteria (e.g. "two counties over"). ...
of 385 light years. This was significantly different from other measurements made by means of actual to apparent brightness measurement or
absolute magnitude Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse Logarithmic scale, logarithmic Magnitude (astronomy), astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent mag ...
. The anomaly was due to the use of a weighted mean when there is a correlation between distances and distance errors for stars in clusters. It is resolved by using an unweighted mean. There is no systematic bias in the Hipparcos data when it comes to star clusters. *
Faster-than-light neutrino anomaly In 2011, the OPERA experiment mistakenly observed neutrinos appearing to travel faster than light. Even before the source of the error was discovered, the result was considered anomalous because speeds higher than that of light in vacuum are g ...
(2011–2012): In 2011, the
OPERA experiment The Oscillation Project with Emulsion-tRacking Apparatus (OPERA) was an instrument used in a scientific experiment for detecting tau neutrinos from muon neutrino oscillations. The experiment is a collaboration between CERN in Geneva, Switzerla ...
mistakenly observed
neutrino A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...
s appearing to travel
faster than light Faster-than-light (also FTL, superluminal or supercausal) travel and communication are the conjectural propagation of matter or information faster than the speed of light (). The special theory of relativity implies that only particles with zero ...
. On 12 July 2012 OPERA updated their paper by including the new sources of errors in their calculations. They found agreement of neutrino speed with the speed of light. *
Pioneer anomaly The Pioneer anomaly, or Pioneer effect, was the observed deviation from predicted accelerations of the ''Pioneer 10'' and ''Pioneer 11'' spacecraft after they passed about on their trajectories out of the Solar System. The apparent anomaly was ...
(1980–2012): There was a deviation in the predicted accelerations of the
Pioneer Pioneer commonly refers to a settler who migrates to previously uninhabited or sparsely inhabited land. In the United States pioneer commonly refers to an American pioneer, a person in American history who migrated west to join in settling and de ...
10 and 11 spacecraft as they left the Solar System. It is believed that this is a result of previously unaccounted-for thermal recoil force.


See also

* Hilbert's sixth problem *
Lists of unsolved problems List of unsolved problems may refer to several notable conjectures or open problems in various academic fields: Natural sciences, engineering and medicine * Unsolved problems in astronomy * Unsolved problems in biology * Unsolved problems in c ...
*
Physical paradox A physical paradox is an apparent contradiction in physical descriptions of the universe. While many physical paradoxes have accepted resolutions, others defy resolution and may indicate flaws in theory. In physics as in all of science, contra ...


Footnotes


References


External links


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
What don't we know?
Science journal special project for its 125th anniversary: top 25 questions and 100 more.
List of links to unsolved problems in physics, prizes and research.





Dual Personality of Glass Explained at Last

What we do and don't know
Review on current state of physics by Steven Weinberg, November 2013
The crisis of big science
Steven Weinberg, May 2012 {{DEFAULTSORT:Unsolved problems in physics
Physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...
Physics-related lists