All posts by Trinh Manh Do

Theory of relativity

The theory of relativity, or simply relativity in physics, usually encompasses two theories by Albert Einstein: special relativity and general relativity. (The word relativity can also be used in the context of an older theory, that of Galilean invariance.)

Concepts introduced by the theories of relativity include:

  • Measurements of various quantities are relative to the velocities of observers. In particular, space and time can dilate.
  • Spacetime: space and time should be considered together and in relation to each other.
  • The speed of light is nonetheless invariant, the same for all observers.

The term “theory of relativity” was based on the expression “relative theory” (German: Relativtheorie) used by Max Planckin 1906, who emphasized how the theory uses the principle of relativity. In the discussion section of the same paper Alfred Bucherer used for the first time the expression “theory of relativity” (German: Relativitätstheorie).


  • 1 Scope
    • 1.1 Two-theory view
  • 2 On the theory of relativity
  • 3 Special relativity
  • 4 General relativity
  • 5 Experimental evidence
    • 5.1 Tests of special relativity
    • 5.2 Tests of general relativity
  • 6 History
  • 7 Everyday Applications of the Theory of Relativity
  • 8 Minority views


The theory of relativity transformed theoretical physics and astronomy during the 20th century. When first published, relativity superseded a 200-year-old theory of mechanicscreated primarily by Isaac Newton.

In the field of physics, relativity catalyzed and added an essential depth of knowledge to the science of elementary particles and their fundamental interactions, along with ushering in the nuclear age. With relativity, cosmology and astrophysics predicted extraordinary astronomical phenomena such as neutron stars, black holes, and gravitational waves.

Two-theory view[edit]

The theory of relativity was representative of more than a single new physical theory. There are some explanations for this. First, special relativity was published in 1905, and the final form of general relativity was published in 1916.

Second, special relativity applies to elementary particles and their interactions, whereas general relativity applies to the cosmological and astrophysical realm, including astronomy.

Third, special relativity was accepted in the physics community by 1920. This theory rapidly became a significant and necessary tool for theorists and experimentalists in the new fields of atomic physics, nuclear physics, and quantum mechanics. Conversely, general relativity did not appear to be as useful. There appeared to be little applicability for experimentalists as most applications were for astronomical scales. It seemed limited to only making minor corrections to predictions of Newtonian gravitation theory.

Finally, the mathematics of general relativity appeared to be very difficult. Consequently, it was thought that a small number of people in the world, at that time, could fully understand the theory in detail, but this has been discredited by Richard Feynman. Then, at around 1960 a critical resurgence in interest occurred which has resulted in making general relativity central to physics and astronomy. New mathematical techniques applicable to the study of general relativity substantially streamlined calculations. From this, physically discernible concepts were isolated from the mathematical complexity. Also, the discovery of exotic astronomical phenomena in which general relativity was crucially relevant, helped to catalyze this resurgence. The astronomical phenomena included quasars (1963), the 3-kelvin microwave background radiation (1965), pulsars (1967), and the discovery of the first black hole candidates (1981).

On the theory of relativity

Einstein stated that the theory of relativity belongs to a class of “principle-theories”. As such it employs an analytic method. This means that the elements which comprise this theory are not based on hypothesis but on empirical discovery. The empirical discovery leads to understanding the general characteristics of natural processes. Mathematical models are then developed which separate the natural processes into theoretical-mathematical descriptions. Therefore, by analytical means the necessary conditions that have to be satisfied are deduced. Separate events must satisfy these conditions. Experience should then match the conclusions.

The special theory of relativity and the general theory of relativity are connected. As stated below, special theory of relativity applies to all physical phenomena except gravity. The general theory provides the law of gravitation, and its relation to other forces of nature.

Special relativity

USSR stamp dedicated to Albert Einstein

Special relativity is a theory of the structure of spacetime. It was introduced in Einstein’s 1905 paper “On the Electrodynamics of Moving Bodies” (for the contributions of many other physicists see History of special relativity). Special relativity is based on two postulates which are contradictory in classical mechanics:

  1. The laws of physics are the same for all observers in uniform motion relative to one another (principle of relativity).
  2. The speed of light in a vacuum is the same for all observers, regardless of their relative motion or of the motion of the light source.

The resultant theory copes with experiment better than classical mechanics, e.g. in the Michelson–Morley experiment that supports postulate 2, but also has many surprising consequences. Some of these are:

  • Relativity of simultaneity: Two events, simultaneous for one observer, may not be simultaneous for another observer if the observers are in relative motion.
  • Time dilation: Moving clocks are measured to tick more slowly than an observer’s “stationary” clock.
  • Relativistic mass
  • Length contraction: Objects are measured to be shortened in the direction that they are moving with respect to the observer.
  • Mass–energy equivalence: E = mc2, energy and mass are equivalent and transmutable.
  • Maximum speed is finite: No physical object, message or field line can travel faster than the speed of light in a vacuum.

The defining feature of special relativity is the replacement of the Galilean transformations of classical mechanics by the Lorentz transformations. (See Maxwell’s equations ofelectromagnetism).

General relativity

General relativity is a theory of gravitation developed by Einstein in the years 1907–1915. The development of general relativity began with the equivalence principle, under which the states of accelerated motion and being at rest in a gravitational field (for example when standing on the surface of the Earth) are physically identical. The upshot of this is that free fall is inertial motion: an object in free fall is falling because that is how objects move when there is no force being exerted on them, instead of this being due to the force of gravityas is the case in classical mechanics. This is incompatible with classical mechanics and special relativity because in those theories inertially moving objects cannot accelerate with respect to each other, but objects in free fall do so. To resolve this difficulty Einstein first proposed that spacetime is curved. In 1915, he devised the Einstein field equationswhich relate the curvature of spacetime with the mass, energy, and momentum within it.

Some of the consequences of general relativity are:

  • Clocks run more slowly in deeper gravitational wells.[8] This is called gravitational time dilation.
  • Orbits precess in a way unexpected in Newton’s theory of gravity. (This has been observed in the orbit of Mercury and in binary pulsars).
  • Rays of light bend in the presence of a gravitational field.
  • Rotating masses “drag along” the spacetime around them; a phenomenon termed “frame-dragging”.
  • The universe is expanding, and the far parts of it are moving away from us faster than the speed of light.

Technically, general relativity is a theory of gravitation whose defining feature is its use of the Einstein field equations. The solutions of the field equations are metric tensors which define the topology of the spacetime and how objects move inertially.

Experimental evidence

Tests of special relativity

A diagram of the Michelson–Morley experiment

Like all falsifiable scientific theories, relativity makes predictions that can be tested by experiment. In the case of special relativity, these include the principle of relativity, the constancy of the speed of light, and time dilation.[9] The predictions of special relativity have been confirmed in numerous tests since Einstein published his paper in 1905, but three experiments conducted between 1881 and 1938 were critical to its validation. These are the Michelson–Morley experiment, the Kennedy–Thorndike experiment, and the Ives–Stilwell experiment. Einstein derived the Lorentz transformations from first principles in 1905, but these three experiments allow the transformations to be induced from experimental evidence.

Maxwell’s equations – the foundation of classical electromagnetism – describe light as a wave which moves with a characteristic velocity. The modern view is that light needs no medium of transmission, but Maxwell and his contemporaries were convinced that light waves were propagated in a medium, analogous to sound propagating in air, and ripples propagating on the surface of a pond. This hypothetical medium was called the luminiferous aether, at rest relative to the “fixed stars” and through which the Earth moves. Fresnel’spartial ether dragging hypothesis ruled out the measurement of first-order (v/c) effects, and although observations of second-order effects (v2/c2) were possible in principle, Maxwell thought they were too small to be detected with then-current technology.

The Michelson–Morley experiment was designed to detect second order effects of the “aether wind” – the motion of the aether relative to the earth. Michelson designed an instrument called the Michelson interferometer to accomplish this. The apparatus was more than accurate enough to detect the expected effects, but he obtained a null result when the first experiment was conducted in 1881, and again in 1887. Although the failure to detect an aether wind was a disappointment, the results were accepted by the scientific community. In an attempt to salvage the aether paradigm, Fitzgerald and Lorentz independently created an ad hoc hypothesis in which the length of material bodies changes according to their motion through the aether. This was the origin of Fitzgerald-Lorentz contraction, and their hypothesis had no theoretical basis. The interpretation of the null result of the Michelson–Morley experiment is that the round-trip travel time for light is isotropic (independent of direction), but the result alone is not enough to discount the theory of the aether or validate the predictions of special relativity.

The Kennedy–Thorndike experimentshown with interference fringes.

While the Michelson–Morley experiment showed that the velocity of light is isotropic, it said nothing about how the magnitude of the velocity changed (if at all) in different inertial frames. The Kennedy–Thorndike experiment was designed to do that, and was first performed in 1932 by Roy Kennedy and Edward Thorndike. They obtained a null result, and concluded that “there is no effect … unless the velocity of the solar system in space is no more than about half that of the earth in its orbit”. That possibility was thought to be too coincidental to provide an acceptable explanation, so from the null result of their experiment it was concluded that the round-trip time for light is the same in all inertial reference frames.

The Ives–Stilwell experiment was carried out by Herbert Ives and G.R. Stilwell first in 1938 and with better accuracy in 1941. It was designed to test the transverse Doppler effect – the redshift of light from a moving source in a direction perpendicular to its velocity – which had been predicted by Einstein in 1905. The strategy was to compare observed Doppler shifts with what was predicted by classical theory, and look for a Lorentz factor correction. Such a correction was observed, from which was concluded that the frequency of a moving atomic clock is altered according to special relativity.

Those classic experiments have been repeated many times with increased precision. Other experiments include, for instance, relativistic energy and momentum increase at high velocities, time dilation of moving particles, and modern searches for Lorentz violations.

Tests of general relativity

General relativity has also been confirmed many times, the classic experiments being the perihelion precession of Mercury’s orbit, the deflection of light by the Sun, and thegravitational redshift of light. Other tests confirmed the equivalence principle and frame dragging.


The history of special relativity consists of many theoretical results and empirical findings obtained by Albert Michelson, Hendrik Lorentz, Henri Poincaré and others. It culminated in the theory of special relativity proposed by Albert Einstein, and subsequent work of Max Planck, Hermann Minkowski and others.

General relativity (GR) is a theory of gravitation that was developed by Albert Einstein between 1907 and 1915, with contributions by many others after 1915.

Currently, it can be said that far from being simply of theoretical scientific interest or requiring experimental verification, the analysis of relativistic effects on time measurement is an important practical engineering concern in the operation of the global positioning systems such as GPS, GLONASS, and the forthcoming Galileo, as well as in the high precision dissemination of time. Instruments ranging from electron microscopes to particle accelerators simply will not work if relativistic considerations are omitted.

Everyday Applications of the Theory of Relativity

The theory of Relativity is used in many of our modern electronics such as the Global Positioning System (GPS). GPS systems are made up of three components, the control component, the space component, and the user component. The space component consists of satellites that are placed in specific orbits. The control component consists of a station in which all of the data from the space component is sent to. Many relativistic effects occur in GPS systems. Since each of the components is in different reference frames, all of the relativistic effects need to be accounted for so that the GPS works with precision. The clocks used in the GPS systems need to be synchronized. In GPS systems, the gravitational field of the earth has to be accounted for. There are relativistic effects within the satellite that is in space that need to be accounted for too. GPS systems work with such precision because of the Theory of Relativity. 

Minority views

Einstein’s contemporaries did not all accept his new theories at once. However, the theory of relativity is now considered as a cornerstone of modern physics, see Criticism of relativity theory.

Although it is widely acknowledged that Einstein was the creator of relativity in its modern understanding, some believe that others deserve credit for it, see Relativity priority dispute.

Source: Wikipedia

List of particles

This is a list of the different types of particles found or believed to exist in the whole of the universe. For individual lists of the different particles, see the individual pages given below.

Elementary particles

Elementary particles are particles with no measurable internal structure; that is, they are not composed of other particles. They are the fundamental objects of quantum field theory. Many families and sub-families of elementary particles exist. Elementary particles are classified according to their spinFermions have half-integer spin while bosons have integer spin. All the particles of the Standard Model have been experimentally observed, recently including the Higgs boson.[1][2]


Fermions are one of the two fundamental classes of particles, the other being bosons. Fermion particles are described by Fermi–Dirac statistics and have quantum numbersdescribed by the Pauli exclusion principle. They include the quarks and leptons, as well as any composite particles consisting of an odd number of these, such as all baryons and many atoms and nuclei.

Fermions have half-integer spin; for all known elementary fermions this is 12. All known fermions are also Dirac fermions; that is, each known fermion has its own distinctantiparticle. It is not known whether the neutrino is a Dirac fermion or a Majorana fermion.[3] Fermions are the basic building blocks of all matter. They are classified according to whether they interact via the color force or not. In the Standard Model, there are 12 types of elementary fermions: six quarks and six leptons.


Quarks are the fundamental constituents of hadrons and interact via the strong interaction. Quarks are the only known carriers of fractional charge, but because they combine in groups of three (baryons) or in groups of two with antiquarks (mesons), only integer charge is observed in nature. Their respective antiparticles are the antiquarks which are identical except for the fact that they carry the opposite electric charge (for example the up quark carries charge +23, while the up antiquark carries charge −23), color charge, and baryon number. There are six flavors of quarks; the three positively charged quarks are called up-type quarks and the three negatively charged quarks are called down-type quarks.

Name Symbol Antiparticle Charge
Mass (MeV/c2)
up u u +23 1.5–3.3
down d d 13 3.5–6.0
charm c c +23 1,160–1,340
strange s s 13 70–130
top t t +23 169,100–173,300
bottom b b 13 4,130–4,370


Leptons do not interact via the strong interaction. Their respective antiparticles are the antileptons which are identical except for the fact that they carry the opposite electric charge and lepton number. The antiparticle of the electron is the antielectron, which is nearly always called positron for historical reasons. There are six leptons in total; the three charged leptons are called electron-like leptons, while the neutral leptons are called neutrinos. Neutrinos are known to oscillate, so that neutrinos of definite flavour do not have definite mass, rather they exist in a superposition of mass eigenstates. The hypothetical heavy right-handed neutrino, called a sterile neutrino, has been left off the list.

Name Symbol Antiparticle Charge
Mass (MeV/c2)
Electron e− e+ −1 0.511
Electron neutrino ν
0 < 0.000 0022
Muon μ− μ+ −1 105.7
Muon neutrino ν
0 < 0.170
Tau τ− τ+ −1 1,777
Tau neutrino ν
0 < 15.5


Bosons are one of the two fundamental classes of particles, the other being fermions. Bosons are characterized by Bose–Einstein statistics and all have integer spins. Bosons may be either elementary, like photons and gluons, or composite, like mesons.

The fundamental forces of nature are mediated by gauge bosons, and mass is believed to be created by the Higgs Field. According to the Standard Model (and to both linearizedgeneral relativity and string theory, in the case of the graviton) the elementary bosons are:

Name Symbol Antiparticle Charge (e) Spin Mass (GeV/c2) Interaction mediated Existence
Photon γ Self 0 1 0 Electromagnetism Confirmed
W boson W− W+ −1 1 80.4 Weak interaction Confirmed
Z boson Z Self 0 1 91.2 Weak interaction Confirmed
Gluon g Self 0 1 0 Strong interaction Confirmed
Higgs boson H0 Self 0 0 125.3 Mass Confirmed
Graviton G Self 0 2 0 Gravitation Unconfirmed

The graviton is added to the list[citation needed] although it is not predicted by the Standard Model, but by other theories in the framework of quantum field theory. Furthermore, gravity is non-renormalizable. There are a total of 8 independent gluons. The Higgs boson is postulated by the electroweak theory primarily to explain the origin of particle masses. In a process known as the Higgs mechanism, the Higgs boson and the other gauge bosons in the Standard Model acquire mass via spontaneous symmetry breaking of the SU(2) gauge symmetry. The Minimal Supersymmetric Standard Model (MSSM) predicts several Higgs bosons. A new particle expected to be the Higgs boson was observed at theCERN/LHC on March 14th of 2013 around the energy of 126.5GeV with the accuracy of close to five sigma (99.9999% that is accepted as definitive). The Higgs mechanism giving mass to other particles has not been observed yet.

Hypothetical particles

Supersymmetric theories predict the existence of more particles, none of which have been confirmed experimentally as of 2014:

Superpartners (Sparticles)
Superpartner Superpartner of Spin Notes
neutralino neutral bosons 12 The neutralinos are superpositions of the superpartners of neutral Standard Model bosons: neutral higgs bosonZ boson and photon.
The lightest neutralino is a leading candidate for dark matter.
The MSSM predicts 4 neutralinos.
chargino charged bosons 12 The charginos are superpositions of the superpartners of charged Standard Model bosons: charged higgs boson and W boson.
The MSSM predicts two pairs of charginos.
photino photon 12 Mixing with zino and neutral Higgsinos for neutralinos.
wino, zino W± and Z0 bosons 12 The charged wino mixing with the charged Higgsino for charginos, for the zino see line above.
Higgsino Higgs boson 12 For supersymmetry there is a need for several Higgs bosons, neutral and charged, according with their superpartners.
gluino gluon 12 Eight gluons and eight gluinos.
gravitino graviton 32 Predicted by Supergravity (SUGRA). The graviton is hypothetical, too – see next table.
sleptons leptons 0 The superpartners of the leptons (electron, muon, tau) and the neutrinos.
sneutrino neutrino 0 Introduced by many extensions of the Standard Supermodel, and may be needed to explain the LSND results.
A special role has the sterile sneutrino, the supersymmetric counterpart of the hypothetical right-handed neutrino, called sterile neutrino.
squarks quarks 0 The stop squark (superpartner of the top quark) is thought to have a low mass and is often the subject of experimental searches.

Note: Just as the photon, Z boson and W± bosons are superpositions of the B0, W0, W1, and W2 fields – the photino, zino, and wino± are superpositions of the bino0, wino0, wino1, and wino2 by definition.
No matter if one uses the original gauginos or this superpositions as a basis, the only predicted physical particles are neutralinos and charginos as a superposition of them together with the Higgsinos.

Other theories predict the existence of additional bosons:

Other hypothetical bosons and fermions
Name Spin Notes
graviton 2 Has been proposed to mediate gravity in theories of quantum gravity.
graviscalar 0 Also known as radion.
graviphoton 1 Also known as gravivector.
axion 0 A pseudoscalar particle introduced in Peccei–Quinn theory to solve the strong-CP problem.
axino 12 Superpartner of the axion. Forms, together with the saxion and axion, a supermultipletin supersymmetric extensions of Peccei–Quinn theory.
saxion 0
branon  ? Predicted in brane world models.
dilaton 0 Predicted in some string theories.
dilatino 12 Superpartner of the dilaton.
X and Y bosons 1 These leptoquarks are predicted by GUT theories to be heavier equivalents of the W and Z.
W’ and Z’ bosons 1
magnetic photon  ? A. Salam (1966). “Magnetic monopole and two photon theories of C-violation”. Physics Letters 22 (5): 683–684.
majoron 0 Predicted to understand neutrino masses by the seesaw mechanism.
majorana fermion 12 ; 32 ?… gluinoneutralino, or other – is its own antiparticle.
Chameleon particle 0 a possible candidate for dark energy and dark matter, and may contribute to cosmic inflation.

Mirror particles are predicted by theories that restore parity symmetry.

Magnetic monopole is a generic name for particles with non-zero magnetic charge. They are predicted by some GUTs.

Tachyon is a generic name for hypothetical particles that travel faster than the speed of light and have an imaginary rest mass.

Preons were suggested as subparticles of quarks and leptons, but modern collider experiments have all but ruled out their existence.

Kaluza–Klein towers of particles are predicted by some models of extra dimensions. The extra-dimensional momentum is manifested as extra mass in four-dimensional spacetime.

Composite particles


Hadrons are defined as strongly interacting composite particles. Hadrons are either:

Quark models, first proposed in 1964 independently by Murray Gell-Mann and George Zweig (who called quarks “aces”), describe the known hadrons as composed of valencequarks and/or antiquarks, tightly bound by the color force, which is mediated by gluons. A “sea” of virtual quark-antiquark pairs is also present in each hadron.


A combination of three u, d or s-quarks with a total spin of 32 form the so-calledbaryon decuplet.

Proton quark structure: 2 up quarks and 1 down quark.

Ordinary baryons (composite fermions) contain three valence quarks or three valence antiquarks each.

  • Nucleons are the fermionic constituents of normal atomic nuclei:
    • Protons, composed of two up and one down quark (uud)
    • Neutrons, composed of two down and one up quark (ddu)
  • Hyperons, such as the Λ, Σ, Ξ, and Ω particles, which contain one or more strange quarks, are short-lived and heavier than nucleons. Although not normally present in atomic nuclei, they can appear in short-lived hypernuclei.
  • A number of charmed and bottom baryons have also been observed.

Some hints at the existence of exotic baryons have been found recently; however, negative results have also been reported. Their existence is uncertain.

  • Pentaquarks consist of four valence quarks and one valence antiquark.


Mesons of spin 0 form a nonet

Ordinary mesons are made up of a valence quark and a valence antiquark. Because mesons have spin of 0 or 1 and are not themselves elementary particles, they are composite bosons. Examples of mesons include the pionkaon, the J/ψ. In quantum hydrodynamic models, mesons mediate the residual strong force between nucleons.

At one time or another, positive signatures have been reported for all of the following exotic mesons but their existence has yet to be confirmed.

  • tetraquark consists of two valence quarks and two valence antiquarks;
  • glueball is a bound state of gluons with no valence quarks;
  • Hybrid mesons consist of one or more valence quark-antiquark pairs and one or more real gluons.

Atomic nuclei

semi-accurate depiction of the helium atom. In the nucleus, the protons are in red and neutrons are in purple. In reality, the nucleus is also spherically symmetrical.

Atomic nuclei consist of protons and neutrons. Each type of nucleus contains a specific number of protons and a specific number ofneutrons, and is called a nuclide or isotopeNuclear reactions can change one nuclide into another. See table of nuclides for a complete list of isotopes.


Atoms are the smallest neutral particles into which matter can be divided by chemical reactions. An atom consists of a small, heavy nucleus surrounded by a relatively large, light cloud of electrons. Each type of atom corresponds to a specific chemical element. To date, 118 elements have been discovered, while only the elements 1-112,114, and 116 have received official names. Refer to the periodic table for an overview.

The atomic nucleus consists of protons and neutrons. Protons and neutrons are, in turn, made of quarks.


Molecules are the smallest particles into which a non-elemental substance can be divided while maintaining the physical properties of the substance. Each type of molecule corresponds to a specific chemical compound. Molecules are a composite of two or more atoms. See list of compounds for a list of molecules.

Condensed matter

The field equations of condensed matter physics are remarkably similar to those of high energy particle physics. As a result, much of the theory of particle physics applies to condensed matter physics as well; in particular, there are a selection of field excitations, called quasi-particles, that can be created and explored. These include:


  • An anyon is a generalization of fermion and boson in two-dimensional systems like sheets of graphene that obeysbraid statistics.
  • plekton is a theoretical kind of particle discussed as a generalization of the braid statistics of the anyon to dimension > 2.
  • WIMP (weakly interacting massive particle) is any one of a number of particles that might explain dark matter (such as the neutralino or the axion).
  • The pomeron, used to explain the elastic scattering of Hadrons and the location of Regge poles in Regge theory.
  • The skyrmion, a topological solution of the pion field, used to model the low-energy properties of the nucleon, such as the axial vector current coupling and the mass.
  • A genon is a particle existing in a closed timelike world line where spacetime is curled as in a Frank Tipler or Ronald Mallett time machine.
  • goldstone boson is a massless excitation of a field that has been spontaneously broken. The pions are quasi-Goldstone bosons (quasi- because they are not exactly massless) of the broken chiral isospin symmetry of quantum chromodynamics.
  • goldstino is a Goldstone fermion produced by the spontaneous breaking of supersymmetry.
  • An instanton is a field configuration which is a local minimum of the Euclidean action. Instantons are used in nonperturbative calculations of tunneling rates.
  • dyon is a hypothetical particle with both electric and magnetic charges.
  • geon is an electromagnetic or gravitational wave which is held together in a confined region by the gravitational attraction of its own field energy.
  • An inflaton is the generic name for an unidentified scalar particle responsible for the cosmic inflation.
  • spurion is the name given to a “particle” inserted mathematically into an isospin-violating decay in order to analyze it as though it conserved isospin.
  • What is called “true muonium”, a bound state of a muon and an antimuon, is a theoretical exotic atom which has never been observed.

Classification by speed

  • tardyon or bradyon travels slower than light and has a non-zero rest mass.
  • luxon travels at the speed of light and has no rest mass.
  • tachyon (mentioned above) is a hypothetical particle that travels faster than the speed of light and has an imaginary rest mass.

Source: Wikipedia

List of unsolved problems in physics

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

Unsolved problems by subfield

The following is a list of unsolved problems grouped into broad area of physics.

Cosmology, and general relativity

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?
Horizon problem
Why is the distant universe so homogeneous, when the Big Bang theory seems to predict larger measurable anisotropies of the night sky than those observed? Cosmologicalinflation is generally accepted as the solution, but are other possible explanations such as a variable speed of light more appropriate?
Future of the universe
Is the universe heading towards a Big Freeze, a Big Rip, a Big Crunch or a Big Bounce? Or is it part of an infinitely recurring cyclic model?
Gravitational wave
Can gravitational waves be detected experimentally?
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 the identity of dark matter? Is it a particle? Is it the lightest superpartner (LSP)? Do the phenomena attributed to dark matter point not to some form of matter but actually to an extension of gravity? The results obtained by the Large Underground Xenon (LUX) experiment that took place in 2013 at Sanford Underground Research Facility place a lower bound on the LSP mass; at this point light supersymmetric particles that are the main candidate for dark matter in the lower mass sector are excluded with 90% confidence.

The log-log plot of dark energy density rho_{*} and material density rho_m vs. scale factor a. The two straight lines intersect at current epoch.
Dark energy
What is the cause of the observed accelerated expansion (de Sitter phase) of the Universe? Why is the energy density of the dark energy component of the same magnitude as the density of matter at present when the two evolve quite differently over time; could it be simply that we are observing at exactly the right time? Is dark energy a pure cosmological constant, or are models of quintessence such as phantom energy applicable?
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?

Quantum mechanics

Quantum Mechanics
Why do point particles such as electrons behave according to wave mechanics ?
Pauli principle
What is the physics underlying the Pauli Exclusion Principle ?

Quantum gravity

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)? 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 observe evidence of higher spatial dimensions?
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?[8] Similarly, will the closed timelike curves which arise in some solutions to the equations of general relativity (and which imply the possibility of backwards time travel) be ruled out by a theory of quantum gravity which unites general relativity with quantum mechanics, as suggested by the “chronology protection conjecture” of Stephen Hawking?
Are there non-local phenomena in quantum physics? If they exist, are non-local phenomena limited to the entanglement revealed in the violations of the Bell Inequalities, or can information and conserved quantities also move in a non-local way? Under what circumstances are non-local phenomena observed? What does the existence or absence of non-local phenomena imply about the fundamental structure of spacetime? How does this relate to quantum entanglement? How does this elucidate the proper interpretation of the fundamental nature of quantum physics?

High energy physics/particle physics

A simulation of how a detection of the Higgs particle would appear in theCMS detector at CERN

Higgs mechanism
Are the branching ratios of the Higgs Boson consistent with the standard model? Is there only one type of Higgs Boson?
Hierarchy problem
Why is gravity such a weak force? It becomes strong for particles only at the Planck scale, around 1019 GeV, much above theelectroweak scale (100 GeV, the energy scale dominating physics at low energies). Why are these scales so different from each other? What prevents quantities at the electroweak scale, such as the Higgs boson mass, from getting quantum corrections on the order of the Planck scale? Is the solution supersymmetry, extra dimensions, or just anthropic fine-tuning?
Magnetic monopoles
Did particles that carry “magnetic charge” exist in some past, higher energy epoch? If so, do any remain today? (Paul Dirac showed the existence of some types of magnetic monopoles would explain charge quantization.)
Proton decay and spin crisis
Is the proton fundamentally stable? Or does it decay with a finite lifetime as predicted by some extensions to the standard model?How do the quarks and gluons carry the spin of protons?
Is spacetime supersymmetry realized at TeV scale? If so, what is the mechanism of supersymmetry breaking? Does supersymmetry stabilize the electroweak scale, preventing high quantum corrections? Does the lightest supersymmetric particle (LSP) comprise dark matter?
Generations of matter
Why are there three generations of quarks and leptons? Is there a theory that can explain the masses of particular quarks and leptons in particular generations from first principles (a theory of Yukawa couplings)?
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, or does nature make use of strong dynamics in breaking electroweak symmetry, as proposed by Technicolor?
Neutrino mass
What is the mass of neutrinos, whether they follow Dirac or Majorana statistics? Is mass hierarchy normal or inverted? Is the CP violating phase 0?
Asymptotic 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 fromQCD?
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?
Anomalous magnetic dipole moment
Why is the experimentally measured value of the muon’s anomalous magnetic dipole moment (“muon g-2”) significantly different from the theoretically predicted value of that physical constant?

Astronomy and astrophysics

Relativistic jet. The environment around the AGNwhere 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 reconnectioneffect many orders of magnitude faster than predicted by standard models?
Diffuse interstellar bands
What is responsible for the numerous interstellar absorption lines detected in astronomical spectra? Are they molecular in origin, and if so which molecules are responsible for them? How do they form?
Gamma ray bursts
How do these short-duration high-intensity bursts originate?
Supermassive black holes
What is the origin of the M-sigma relation between supermassive black hole mass and galaxy velocity dispersion? How did the most distant quasars grow their supermassive black holes up to 10^9 solar masses so early in the history of the Universe?
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?
Kuiper Cliff
Why does the number of objects in the Solar System’s Kuiper Belt fall off rapidly and unexpectedly beyond a radius of 50 astronomic units?
Flyby anomaly
Why is the observed energy of satellites flying by Earth sometimes 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?
What is the exact mechanism by which an implosion of a dying star becomes an explosion?
Ultra-high-energy cosmic ray
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?
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?
Origin of magnetar magnetic field
What is the origin of magnetar magnetic field?
Space roar
Why is space roar six times louder than expected? What is the source of space roar?
Age-metallicity relation in the Galactic disk
Is there a universal age-metallicity relation in the Galactic disks? A sample of 229 nearby thick disk stars has been used to investigate the existence of an age-metallicity relation (AMR) in the Galactic thickdisk. The results indicate that that there is indeed an age-metallicity relation present in the thick disk.

Nuclear physics

The “island of stability” in the proton vs. neutron number plot for heavy nuclei

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 andgluons into pions and nucleons? What is the role of gluons and gluon self-interactions in nucleons and nuclei? What determines the key features of QCD, and what is their relation to the nature of gravity and spacetime? Do glueballs exist? Do gluons acquire mass dynamically despite having a zero rest mass, within hadrons? Does QCD truly lack CP-violations?
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 exotic excitations in nuclei at the frontiers of stability and their role in stellar processes? What is the nature of neutron stars and dense nuclear matter? What is the origin of the elements in the cosmos? What are the nuclear reactions that drivestars and stellar explosions?

Atomic, molecular and optical physics

Hydrogen atom
What is the solution to the Schrödinger equation for the hydrogen atom in arbitrary electric and magnetic fields?
Helium atom
The helium atom is the simplest three-body problem in quantum mechanics; while approximations to a solution to the Schrödinger equation for He exist, can an exact solution be found?
Muonic hydrogen
Is the radius of muonic hydrogen inconsistent with the radius of ordinary hydrogen?

Condensed matter physics

A sample of a cupratesuperconductor (specificallyBSCCO). The mechanism for superconductivity of these materials is unknown.

High-temperature superconductors

What is the mechanism that causes certain materials to exhibit superconductivity at temperatures much higher than around 25 kelvin? Is it possible to make a material that is a superconductor at room temperature?
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 and the glass transition?
Cryogenic electron emission
Why does the electron emission in the absence of light increase as the temperature of a photomultiplier is decreased?
What causes the emission of short bursts of light from imploding bubbles in a liquid when excited by sound?
Is it possible to make a theoretical model to describe the statistics of a turbulent flow (in particular, its internal structures)? Also, under what conditions do smooth solutions to the Navier–Stokes equations exist? This problem is also listed as one of the Millennium Prize Problems in mathematics. Alfvénic turbulence in the solar wind and the turbulence in solar flares, coronal mass ejections, and magnetospheric substorms are major unsolved problems in space plasma physics.
Topological order
Is topological order stable at non-zero temperature? Equivalently, is it possible to have three-dimensional self-correcting quantum memory?
Ball lightning
What is this? What is its structure, mechanism of formation and destruction? Does it have an internal energy reserve or does its energy come from the outside?
Fractional Hall effect
What mechanism explains the existence of the nu=5/2 state in the fractional quantum Hall effect? Does it describe quasiparticles with non-Abelian fractional statistics?
Bose–Einstein condensation
How do we rigorously prove the existence of Bose–Einstein condensates for general interacting systems?
Liquid crystals
Can the nematic to smectic (A) phase transition in liquid crystal states be characterized as a universal phase transition?
Semiconductor nanocrystals
What is the cause of the nonparabolicity of the energy-size dependence for the lowest optical absorption transition of quantum dots?
Electronic band structure
Why can band gaps not accurately be calculated?


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 development where 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?
What is the origin of the preponderance of specific enantiomers in biochemical systems?

Other problems

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? Why are CP violationsobserved 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?
Quantum mechanics in the correspondence limit (sometimes called Quantum chaos)
Is there a preferred interpretation of quantum mechanics? How does the quantum description of reality, which includes elements such as the superposition of states andwavefunction collapse or quantum decoherence, give rise to the reality we perceive? Another way of stating this is the Measurement problem – what constitutes a “measurement” which causes the wave function to collapse into a definite state?
Theory of everything (“Grand Unification Theory”)
Is there a theory which explains the values of all fundamental physical constants? Is the theory string theory? Is there a theory which explains why the gauge groups of thestandard model are as they are, why observed space-time has 3 spatial dimensions and 1 dimension of time, and why all laws of physics are as they are? Do “fundamental physical constants” vary over time? Are any of the particles in the standard model of particle physics actually composite particles too tightly bound to observe as such at current experimental energies? Are there fundamental particles that have not yet been observed and if so which ones are they and what are their properties? Are there unobserved fundamental forces implied by a theory that explains other unsolved problems in physics?
Yang–Mills theory
Given an arbitrary compact gauge group, does a non-trivial quantum Yang–Mills theory with a finite mass gap exist? This problem is also listed as one of the Millennium Prize Problems in mathematics.
Physical information
Are there physical phenomena, such as wave function collapse or black holes, which irrevocably destroy information about their prior states? How is quantum information stored as a state of a quantum system?
Quantum Computation
Is David Deutsch’s notion of a universal quantum computer sufficient to efficiently simulate an arbitrary physical system?
Dimensionless physical constant
At the present time, the values of the dimensionless physical constants cannot be calculated; they are 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 dimensionful physical constants necessary at all?

Problems solved in recent decades

Hipparcos anomaly (2012)
The actual distance to the Pleiades – the High Precision Parallax Collecting Satellite (Hipparcos) measured the parallax of the Pleiades and determined a distance of 385 light years. This was significantly different from other measurements made by means of actual to apparent brightness measurement or absolute magnitude. The anomaly was due to a systematic bias in the Hipparcos data when it comes to star clusters; the Hipparcos results for clusters are consistently closer than they should be.
Pioneer anomaly (2012)
There was a deviation in the predicted accelerations of the Pioneer spacecraft as they left the Solar System. It is believed that this is a result of previously unaccounted-forthermal recoil force. Antonio Fernández-Rañada and Alfredo Tiemblo-Ramos propose “an explanation of the Pioneer anomaly that is a refinement of a previous one and is fully compatible with the cartography of the solar system. It is based on the non-equivalence of the atomic time and the astronomical time which happens to have the same observational fingerprint as the anomaly.”
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. 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.
                                                                           Source: Wikipedia

Sombrero Galaxy

The Sombrero Galaxy (also known as Messier Object 104M104 or NGC 4594) is an unbarred spiral galaxy in theconstellation Virgo located 28 megalight-years (8,600 kpc) from Earth. It has a bright nucleus, an unusually large central bulge, and a prominent dust lane in its inclined disk. The dark dust lane and the bulge give this galaxy the appearance of asombrero. Astronomers initially thought that the halo was small and light, indicative of a spiral galaxy, but Spitzer found that the halo around the Sombrero Galaxy is larger and more massive than previously thought, indicative of a giant elliptical galaxy. The galaxy has an apparent magnitude of +9.0, making it easily visible with amateur telescopes. The large bulge, the central supermassive black hole, and the dust lane all attract the attention of professional astronomers.

 Source: Wikipedia