Matter creation
Encyclopedia
Matter creation is the process inverse to particle annihilation
. It is the conversion of massless particles into one or more massive particles. This process is the time reversal
of annihilation. Since all known massless particles are boson
s and the most familiar massive particles are fermion
s, usually what is considered is the process which converts two bosons (e.g. photon
s) into two fermions (e.g., an electron
–positron
pair).
conservation laws, the creation of a pair of fermions (matter particles) out of a single photon cannot occur. However, matter creation is allowed by these laws when in the presence of another particle (another boson, or even a fermion) which can share the primary photon's momentum. Thus, matter can be created out of two photons.
The law of conservation of energy
sets a minimum photon energy required for creation of a pair of fermions: this threshold energy
must be greater than the total rest energy of the fermions created. To create an electron-positron pair the total energy of the photons must be at least 2mec2 = 2 × = (me is the mass of one electron and c is the speed of light
in vacuum), an energy value that corresponds to soft
gamma ray
photons
. The creation of a much more massive pair, like a proton
and antiproton
, requires photons with energy of more than (hard gamma ray photons).
First calculations of rate of e+–e− pair production in photon-photon collision was done by Lev Landau in 1934. It was predicted that the process of e+–e− pair creation (via collisions of photons) dominates in collision of ultra-relativistic charged particles—because those photons are radiated in narrow cones along the direction of motion of original particle greatly increasing photon flux.
In high-energy particle colliders, matter creation events have yielded a wide variety of exotic heavy particles precipitating out of colliding photon jets (see two-photon physics
). Currently, two-photon physics studies creation of various fermion pairs both theoretically and experimentally (using particle accelerator
s, air showers
, radioactive isotopes, etc).
As shown above, to produce ordinary baryonic matter out of a photon gas
, this gas must not only have a very high photon density, but also be very hot – the energy (temperature
) of photons must obviously exceed the rest mass energy of the given matter particle pair. The threshold temperature for production of electrons is about 1010 K
, 1013 K for proton
s and neutron
s, etc. According to the Big Bang
theory, in the early universe
, photons and fermions (massive particles of matter) would inter-convert freely. As photon gas expanded and cooled, some fermions would be left over (in extremely small amounts ~10−10) because low energy photons could no longer break them apart. Those left-over fermions would have become the matter we see today in the universe around us.
Annihilation
Annihilation is defined as "total destruction" or "complete obliteration" of an object; having its root in the Latin nihil . A literal translation is "to make into nothing"....
. It is the conversion of massless particles into one or more massive particles. This process is the time reversal
T-symmetry
T Symmetry is the symmetry of physical laws under a time reversal transformation: T: t \mapsto -t.Although in restricted contexts one may find this symmetry, the observable universe itself does not show symmetry under time reversal, primarily due to the second law of thermodynamics.Time asymmetries...
of annihilation. Since all known massless particles are boson
Boson
In particle physics, bosons are subatomic particles that obey Bose–Einstein statistics. Several bosons can occupy the same quantum state. The word boson derives from the name of Satyendra Nath Bose....
s and the most familiar massive particles are fermion
Fermion
In particle physics, a fermion is any particle which obeys the Fermi–Dirac statistics . Fermions contrast with bosons which obey Bose–Einstein statistics....
s, usually what is considered is the process which converts two bosons (e.g. photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s) into two fermions (e.g., an electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
–positron
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...
pair).
Photon pair production
Because of momentumMomentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
conservation laws, the creation of a pair of fermions (matter particles) out of a single photon cannot occur. However, matter creation is allowed by these laws when in the presence of another particle (another boson, or even a fermion) which can share the primary photon's momentum. Thus, matter can be created out of two photons.
The law of conservation of energy
Conservation of energy
The nineteenth century law of conservation of energy is a law of physics. It states that the total amount of energy in an isolated system remains constant over time. The total energy is said to be conserved over time...
sets a minimum photon energy required for creation of a pair of fermions: this threshold energy
Threshold energy
In particle physics, the threshold energy for production of a particle is the minimum kinetic energy a pair of traveling particles must have when they collide. The threshold energy is always greater than or equal to the rest energy of the desired particle...
must be greater than the total rest energy of the fermions created. To create an electron-positron pair the total energy of the photons must be at least 2mec2 = 2 × = (me is the mass of one electron and c is the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
in vacuum), an energy value that corresponds to soft
Soft photons
In particle physics, soft photons are photons with energies much smaller than the energies of the particles participating in a particular scattering process, and they are not energetic enough to be detected. Such photons can be emitted from the external lines of charged particles of the Feynman...
gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
photons
Soft photons
In particle physics, soft photons are photons with energies much smaller than the energies of the particles participating in a particular scattering process, and they are not energetic enough to be detected. Such photons can be emitted from the external lines of charged particles of the Feynman...
. The creation of a much more massive pair, like a proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
and antiproton
Antiproton
The antiproton is the antiparticle of the proton. Antiprotons are stable, but they are typically short-lived since any collision with a proton will cause both particles to be annihilated in a burst of energy....
, requires photons with energy of more than (hard gamma ray photons).
First calculations of rate of e+–e− pair production in photon-photon collision was done by Lev Landau in 1934. It was predicted that the process of e+–e− pair creation (via collisions of photons) dominates in collision of ultra-relativistic charged particles—because those photons are radiated in narrow cones along the direction of motion of original particle greatly increasing photon flux.
In high-energy particle colliders, matter creation events have yielded a wide variety of exotic heavy particles precipitating out of colliding photon jets (see two-photon physics
Two-photon physics
Two-photon physics, also called gamma-gamma physics, is a branch of particle physics for the interactions between two photons. If the energy in the center of mass system of the two photons is large enough, matter can be created.-Experiments:...
). Currently, two-photon physics studies creation of various fermion pairs both theoretically and experimentally (using particle accelerator
Particle accelerator
A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...
s, air showers
Air shower (physics)
An air shower is an extensive cascade of ionized particles and electromagnetic radiation produced in the atmosphere when a primary cosmic ray enters the atmosphere...
, radioactive isotopes, etc).
As shown above, to produce ordinary baryonic matter out of a photon gas
Photon gas
In physics, a photon gas is a gas-like collection of photons, which has many of the same properties of a conventional gas like hydrogen or neon - including pressure, temperature, and entropy...
, this gas must not only have a very high photon density, but also be very hot – the energy (temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
) of photons must obviously exceed the rest mass energy of the given matter particle pair. The threshold temperature for production of electrons is about 1010 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
, 1013 K for proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s and neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s, etc. According to the Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...
theory, in the early universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...
, photons and fermions (massive particles of matter) would inter-convert freely. As photon gas expanded and cooled, some fermions would be left over (in extremely small amounts ~10−10) because low energy photons could no longer break them apart. Those left-over fermions would have become the matter we see today in the universe around us.