In physics, exotic matter is matter that somehow deviates from normal matter and has "exotic" properties. A broader definition of exotic matter is any kind of non-baryonic matter—that is not made of baryons, the subatomic particles (such as protons and neutrons) of which ordinary matter is composed. Exotic mass has been considered a colloquial term for matters such as dark matter, negative mass, or complex mass.
There are several proposed types of exotic matter:
Negative mass would possess some strange properties, such as accelerating in the direction opposite of applied force. Despite being inconsistent with the expected behavior of "normal" matter, negative mass is mathematically consistent and introduces no violation of conservation of momentum or energy. It is used in certain speculative theories, such as on the construction of artificial wormholes and the Alcubierre drive. The closest known real representative of such exotic matter is the region of pseudo-negative-pressure density produced by the Casimir effect.
According to mass–energy equivalence, mass is in proportion to energy and the coefficient of proportionality is . Actually, is still equivalent to although the coefficient is another constant such as . In this case, it is unnecessary to introduce a negative energy because the mass can be negative although the energy is positive. That is to say,
Under the circumstances，
where is invariant mass and invariant energy equals . The squared mass is still positive and the particle can be stable.
The negative momentum is applied to explain negative refraction, inverse Doppler effect and reverse Cherenkov effect observed in a negative index metamaterial. The radiation pressure in the metamaterial is also negative because the force is defined as . Negative pressure exists in dark energy too. Using these above equations, the energy-momentum relation should be
and range is from zero to infinity
Moreover, the kinetic energy is also negative
In fact, the negative kinetic energy exists in some models to describe dark energy (phantom energy) whose pressure is negative. In this way, the negative mass of exotic matter is now associated with negative momentum, negative pressure, negative kinetic energy and FTL (faster-than-light).
If the rest mass is complex this implies that the denominator is complex because the total energy is observable and thus must be real. Therefore, the quantity under the square root must be negative, which can only happen if v is greater than c. As noted by Gregory Benford et al., special relativity implies that tachyons, if they existed, could be used to communicate backwards in time (see tachyonic antitelephone). Because time travel is considered to be non-physical, tachyons are believed by physicists either not to exist, or else to be incapable of interacting with normal matter.
In quantum field theory, complex mass would induce tachyon condensation.
At high pressure, materials such as sodium chloride (NaCl) in the presence of an excess of either chlorine or sodium were transformed into compounds "forbidden" by classical chemistry, such as Na
3Cl and NaCl
3. Quantum mechanical calculations predict the possibility of other compounds, such as NaCl
2 and Na
2Cl. The materials are thermodynamically stable at high pressures. Such compounds may exist in natural environments that exist at high pressure, such as the deep ocean or inside planetary cores. The materials have potentially useful properties. For instance, Na
3Cl is a two-dimensional metal, made of layers of pure sodium and salt that can conduct electricity. The salt layers act as insulators while the sodium layers act as conductors.
The Alcubierre drive or Alcubierre warp drive (or Alcubierre metric, referring to metric tensor) is a speculative idea based on a solution of Einstein's field equations in general relativity as proposed by Mexican theoretical physicist Miguel Alcubierre, by which a spacecraft could achieve apparent faster-than-light travel if a configurable energy-density field lower than that of vacuum (that is, negative mass) could be created.
Rather than exceeding the speed of light within a local reference frame, a spacecraft would traverse distances by contracting space in front of it and expanding space behind it, resulting in effective faster-than-light travel. Objects cannot accelerate to the speed of light within normal spacetime; instead, the Alcubierre drive shifts space around an object so that the object would arrive at its destination faster than light would in normal space without breaking any physical laws.Although the metric proposed by Alcubierre is consistent with the Einstein field equations, it may not be physically meaningful, in which case a drive will not be possible. Even if it is physically meaningful, its possibility would not necessarily mean that a drive can be constructed. The proposed mechanism of the Alcubierre drive implies a negative energy density and therefore requires exotic matter. So if exotic matter with the correct properties cannot exist, then the drive could not be constructed. However, at the close of his original article Alcubierre argued (following an argument developed by physicists analyzing traversable wormholes) that the Casimir vacuum between parallel plates could fulfill the negative-energy requirement for the Alcubierre drive.
Another possible issue is that, although the Alcubierre metric is consistent with Einstein's equations, general relativity does not incorporate quantum mechanics. Some physicists have presented arguments to suggest that a theory of quantum gravity (which would incorporate both theories) would eliminate those solutions in general relativity that allow for backwards time travel (see the chronology protection conjecture) and thus make the Alcubierre drive invalid.Compact star
In astronomy, the term "compact star" (or "compact object") refers collectively to white dwarfs, neutron stars, and black holes. It would grow to include exotic stars if such hypothetical dense bodies are confirmed.
Most compact stars are the endpoints of stellar evolution, and thus often referred to as stellar remnants, the form of the remnant depending primarily on the mass of the star when it formed. All of these objects have a high mass relative to their radius, giving them a very high density. The term compact star is often used when the exact nature of the star is not known, but evidence suggests that it has a small radius, thus implying one of the above-mentioned categories. A compact star that is not a black hole may be called a degenerate star.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) velocity flowing in a common direction.
According to standard cosmological models, the motion of galaxy clusters with respect to the cosmic microwave background should be randomly distributed in all directions. However, analyzing the three-year Wilkinson Microwave Anisotropy Probe (WMAP) data using the kinematic Sunyaev-Zel'dovich effect, astronomers Alexander Kashlinsky, F. Atrio-Barandela, D. Kocevski and H. Ebeling found evidence of a "surprisingly coherent" 600–1000 km/s flow of clusters toward a 20-degree patch of sky between the constellations of Centaurus and Vela.
The researchers had suggested that the motion may be a remnant of the influence of no-longer-visible regions of the universe prior to inflation. Telescopes cannot see events earlier than about 380,000 years after the Big Bang, when the universe became transparent (the cosmic microwave background); this corresponds to the particle horizon at a distance of about 46 billion (4.6×1010) light years. Since the matter causing the net motion in this proposal is outside this range, it would in a certain sense be outside our visible universe; however, it would still be in our past light cone.
The results appeared in the October 20, 2008, issue of Astrophysical Journal Letters.In 2013, data from the Planck space telescope showed no evidence of "dark flow" on that sort of scale, discounting the claims of evidence for either gravitational effects reaching beyond the visible universe or existence of a multiverse. However, in 2015 Kashlinsky et al claim to have found support for its existence using both Planck and WMAP data.Degenerate matter
Degenerate matter is a highly dense state of fermionic matter in which particles must occupy high states of kinetic energy to satisfy the Pauli exclusion principle. The description applies to matter composed of electrons, protons, neutrons or other fermions. The term is mainly used in astrophysics to refer to dense stellar objects where gravitational pressure is so extreme that quantum mechanical effects are significant. This type of matter is naturally found in stars in their final evolutionary states, like white dwarfs and neutron stars, where thermal pressure alone is not enough to avoid gravitational collapse.
Degenerate matter is usually modelled as an ideal Fermi gas, an ensemble of non-interacting fermions. In a quantum mechanical description, particles limited to a finite volume may take only a discrete set of energies, called quantum states. The Pauli exclusion principle prevents identical fermions from occupying the same quantum state. At lowest total energy (when the thermal energy of the particles is negligible), all the lowest energy quantum states are filled. This state is referred to as full degeneracy. This degeneracy pressure remains non-zero even at absolute zero temperature. Adding particles or reducing the volume forces the particles into higher-energy quantum states. In this situation, a compression force is required, and is made manifest as a resisting pressure. The key feature is that this degeneracy pressure does not depend on the temperature but only on the density of the fermions. Degeneracy pressure keeps dense stars in equilibrium, independent of the thermal structure of the star.
A degenerate mass whose fermions have velocities close to the speed of light (particle energy larger than its rest mass energy) is called relativistic degenerate matter.
The concept of degenerate stars, stellar objects composed of degenerate matter, was originally developed in a joint effort between Arthur Eddington, Ralph Fowler and Arthur Milne. Eddington had suggested that the atoms in Sirius B were almost completely ionised and closely packed. Fowler described white dwarfs as composed of a gas of particles that became degenerate at low temperature. Milne proposed that degenerate matter is found in most of the nucleus of stars, not only in compact stars.Fermionic condensate
A fermionic condensate is a superfluid phase formed by fermionic particles at low temperatures. It is closely related to the Bose–Einstein condensate, a superfluid phase formed by bosonic atoms under similar conditions. The earliest recognized fermionic condensate described the state of electrons in a superconductor; the physics of other examples including recent work with fermionic atoms is analogous. The first atomic fermionic condensate was created by a team led by Deborah S. Jin in 2003.Glueball
In particle physics, a glueball (also gluonium, gluon-ball) is a hypothetical composite particle. It consists solely of gluon particles, without valence quarks. Such a state is possible because gluons carry color charge and experience the strong interaction between themselves. Glueballs are extremely difficult to identify in particle accelerators, because they mix with ordinary meson states.
Theoretical calculations show that glueballs should exist at energy ranges accessible with current collider technology. However, due to the aforementioned difficulty (among others), they have so far not been observed and identified with certainty, although phenomenological calculations have suggested that an experimentally identified glueball candidate, denoted , has properties consistent with those expected of a Standard Model glueball.
The prediction that glueballs exist is one of the most important predictions of the Standard Model of particle physics that has not yet been confirmed experimentally. Glueballs are the only particles predicted by the Standard Model with total angular momentum (J) (sometimes called "intrinsic spin") that could be either 2 or 3 in their ground states.Holeum
Holeums are hypothetical stable, quantized gravitational bound states of primordial or micro black holes. Holeums were proposed by L. K. Chavda and Abhijit Chavda in 2002. They have all the properties associated with cold dark matter. Holeums are not black holes, even though they are made up of black holes.Hypernucleus
A hypernucleus is a nucleus which contains at least one hyperon (a baryon carrying the strangeness quantum number) in addition to the normal protons and neutrons. The first was discovered by Marian Danysz and Jerzy Pniewski in 1952 using the nuclear emulsion technique.
The strangeness quantum number is conserved by the strong and electromagnetic interactions, a variety of reactions give access to depositing one or more units of strangeness in a nucleus. Hypernuclei containing the lightest hyperon, the Lambda, live long enough to have sharp nuclear energy levels. Therefore, they offer opportunities for nuclear spectroscopy, as well as reaction mechanism study and other types of nuclear physics (hypernuclear physics).
Hypernuclear physics differs from that of normal nuclei because a hyperon, having a non-zero strangeness quantum number, can share space and momentum coordinates with the usual four nucleon states that can differ from each other in spin and isospin. That is, they are not restricted by the Pauli Exclusion Principle from any single-particle state in the nucleus. The ground state of helium-5-Lambda, for example, must resemble helium-4 more than it does helium-5 or lithium-5 and must be stable, apart from the eventual weak decay of the Lambda. Sigma hypernuclei have been sought, as have doubly-strange nuclei containing Cascade baryons.
Hypernuclei can be made by a nucleus capturing a Lambda or K meson and boiling off neutrons in a compound nuclear reaction, or, perhaps most easily, by the direct strangeness exchange reaction.
K + nucleus → π + hypernucleusA generalized mass formula developed for both the non-strange normal nuclei and strange hypernuclei can estimate masses of hypernuclei containing Lambda, Lambda-Lambda, Sigma, Cascade and Theta+ hyperon(s). The neutron and proton driplines for hypernuclei are predicted and existence of some exotic hypernuclei beyond the normal neutron and proton driplines are suggested. This generalized mass formula was named as "Samanta Formula" by Botvina and Pochodzalla and used to predict relative yields of hypernuclei in multifragmentation of nuclear spectator matter.Hypernuclei were first observed by their energetic but delayed decay, but have also been studied by measuring the momenta of the K and pi mesons in the direct strangeness exchange reactions.Hyperon
In particle physics, a hyperon is any baryon containing one or more strange quarks, but no charm, bottom, or top quark. This form of matter may exist in a stable form within the core of some neutron stars.Hypertriton
A hypertriton is a type of hypernucleus, formed of a proton, a neutron and any hyperon. The name comes from hyperon, which refers to baryons containing strange quarks, and triton, which refers to the nucleus of tritium. Because low-mass hyperons are longer-lived and easier to create than high-mass hyperons, the most common hypertritons are those containing Lambda baryons.
Its antiparticle, the antihypertriton, is formed of an antiproton, an antineutron and any antihyperon. The first one was discovered in March 2010 by the STAR detector of the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory.Massive compact halo object
A massive astrophysical compact halo object (MACHO) is any kind of astronomical body that might explain the apparent presence of dark matter in galaxy halos. A MACHO is a body composed of normal baryonic matter that emits little or no radiation and drifts through interstellar space unassociated with any planetary system. Since MACHOs are not luminous, they are hard to detect. MACHOs include black holes or neutron stars as well as brown dwarfs and unassociated planets. White dwarfs and very faint red dwarfs have also been proposed as candidate MACHOs. The term was coined by astrophysicist Kim Griest.Negative mass
In theoretical physics, negative mass is matter whose mass is of opposite sign to the mass of normal matter, e.g. −1 kg. Such matter would violate one or more energy conditions and show some strange properties, stemming from the ambiguity as to whether attraction should refer to force or the oppositely oriented acceleration for negative mass. It is used in certain speculative hypotheses, such as on the construction of traversable wormholes and the Alcubierre drive. Initially, the closest known real representative of such exotic matter is a region of negative pressure density produced by the Casimir effect.
General relativity describes gravity and the laws of motion for both positive and negative energy particles, hence negative mass, but does not include the other fundamental forces. On the other hand, the Standard Model describes elementary particles and the other fundamental forces, but it does not include gravity. A unified theory that explicitly includes gravity along with the other fundamental forces may be needed for a better understanding of the concept of negative mass.
In December 2018, the astrophysicist Jamie Farnes from the University of Oxford proposed a "dark fluid" theory, related, in part, to notions of gravitationally repulsive negative masses, presented earlier by Albert Einstein, that may help better understand, in a testable manner, the considerable amounts of unknown dark matter and dark energy in the cosmos.Neutronium
Neutronium (sometimes shortened to neutrium, also referred to as neutrite) is a hypothetical substance composed purely of neutrons. The word was coined by scientist Andreas von Antropoff in 1926 (before the discovery of the neutron) for the conjectured "element of atomic number zero" that he placed at the head of the periodic table. However, the meaning of the term has changed over time, and from the last half of the 20th century onward it has been also used to refer to extremely dense substances resembling the neutron-degenerate matter theorized to exist in the cores of neutron stars; hereinafter "degenerate neutronium" will refer to this. Science fiction and popular literature frequently use the term "neutronium" to refer to a highly dense phase of matter composed primarily of neutrons.Quantum fluid
A quantum fluid refers to any system that exhibits quantum mechanical effects at the macroscopic level such as superfluids, superconductors, ultracold atoms, etc. Typically, quantum fluids arise in situations where both quantum mechanical effects and quantum statistical effects are significant.
Most matter is either solid or gaseous (at low densities) near absolute zero. However, for the cases of helium-4 and its isotope helium-3, there is a pressure range where they can remain liquid down to absolute zero because the amplitude of the quantum fluctuations experienced by the helium atoms is larger than the inter-atomic distances.
In the case of solid quantum fluids, it is only a fraction of its electrons or protons that behave like a “fluid”. One prominent example is that of superconductivity where quasi-particles made up of pairs of electrons and a phonon act as bosons which are then capable of collapsing into the ground state to establish a supercurrent with a resistivity near zero.Quark star
A quark star is a hypothetical type of compact exotic star, where extremely high temperature and pressure has forced nuclear particles to form a continuous state of matter that consists primarily of free quarks.
It is well known that massive stars can collapse to form neutron stars, under extreme temperatures and pressures. In simple terms, neutrons usually have space separating them due to degeneracy pressure keeping them apart. Under extreme conditions such as a neutron star, the pressure separating nucleons is overwhelmed by gravity, and the separation between them breaks down, causing them to be packed extremely densely and form an immensely hot and dense state known as neutron matter, where they are only held apart by the strong interaction. Because these neutrons are made of quarks, it is hypothesized that under even more extreme conditions, the degeneracy pressure keeping the quarks apart within the neutrons might break down in much the same way, creating an ultra-dense phase of degenerate matter based on densely packed quarks. This is seen as plausible, but is very hard to prove, as scientists cannot easily create the conditions needed to investigate the properties of quark matter, so it is unknown whether this actually occurs.
If quark stars can form, then the most likely place to find quark star matter would be inside neutron stars that exceed the internal pressure needed for quark degeneracy - the point at which neutrons (which are formed from quarks bound together) break down into a form of dense quark matter. They could also form if a massive star collapses at the end of its life, provided that it is possible for a star to be large enough to collapse beyond a neutron star but not large enough to form a black hole. However, as scientists are unable so far to explore most properties of quark matter, the exact conditions and nature of quark stars, and their existence, remain hypothetical and unproven. The question whether such stars exist and their exact structure and behavior is actively studied within astrophysics and particle physics.
If they exist, quark stars would resemble and be easily mistaken for neutron stars: they would form in the death of a massive star in a Type II supernova, they would be extremely dense and small, and possess a very high gravitational field. They would also lack some features of neutron stars, unless they also contained a shell of neutron matter, because free quarks are not expected to have properties matching degenerate neutron matter. For example, they might be radio-silent, or not have typical size, electromagnetic, or temperature measurements, compared to other neutron stars.
The hypothesis about quark stars was first proposed in 1965 by Soviet physicists D. D. Ivanenko and D. F. Kurdgelaidze. Their existence has not been confirmed. The equation of state of quark matter is uncertain, as is the transition point between neutron-degenerate matter and quark matter. Theoretical uncertainties have precluded making predictions from first principles. Experimentally, the behaviour of quark matter is being actively studied with particle colliders, but this can only produce very hot (above 1012 K) quark-gluon plasma blobs the size of atomic nuclei, which decay immediately after formation. The conditions inside compact stars with extremely high densities and temperatures well below 1012 K can not be recreated artificially, as there are no known methods to produce, store or study "cold" quark matter directly as it would be found inside quark stars. The theory predicts quark matter to possess some peculiar characteristics under these conditions.Rydberg polaron
A Rydberg polaron is an exotic state of matter, created at low temperatures, in which a very large atom contains other ordinary atoms in the space between the nucleus and the electrons. For the formation of this atom, scientists had to combine two fields of atomic physics: Bose-Einstein condensates and Rydberg atoms. Rydberg atoms are formed by exciting a single atom into a high-energy state, in which the electron is very far from the nucleus. Bose-Einstein condensates are a state of matter that is produced at temperatures close to absolute zero.
Polarons are induced by using a laser to excite Rydberg atoms contained as impurities in a Bose-Einstein condensate. In those Rydberg atoms, the average distance between the electron and its nucleus can be as large as several hundred nanometres, which is more than a thousand times the radius of a hydrogen atom. Under that circumstances, the distance between the nucleus and the electron of the excited Rydberg atoms is higher than the average distance of the atoms of the condensate. As a result, some atoms lie inside the orbit of the Rydberg atom's electron.
As the atoms don't have an electric charge, they only produce a minimal force on the electron. However, the electron is slightly scattered at the neutral atoms, without even leaving its orbit, and the weak bond that is generated between the Rydberg atom and the atoms inside of it, tying them together, is known as the Rydberg Polaron. The new state of matter was predicted by theorists at Harvard University in 2016 and confirmed in 2018 by spectroscopy in an experiment using a strontium Bose-Einstein condensate. Theoretically, up to 170 ordinary strontium atoms could fit closely inside the new orbital of the Rydberg atom, depending on the radius of the Rydberg atom and the density of the Bose-Einstein condensate. The theoretical work for the experiment was performed by theorists at Vienna University of Technology and Harvard University, while the actual experiment and observation took place at Rice University in Houston, Texas.Strange matter
Strange matter is a particular form of quark matter, usually thought of as a "liquid" of up, down and strange quarks. It is to be contrasted with nuclear matter, which is a liquid of neutrons and protons (which themselves are built out of up and down quarks), and with non-strange quark matter, which is a quark liquid containing only up and down quarks. At high enough density, strange matter is expected to be color superconducting. Strange matter is hypothesized to occur in the core of neutron stars, or, more speculatively, as isolated droplets that may vary in size from femtometers (strangelets) to kilometers (quark stars).Strange star
A strange star is a quark star made of strange quark matter. They form a subgroup under the quark star category.Strange stars might exist without regard to the Bodmer–Witten assumption of stability at near-zero temperatures and pressures, as strange quark matter might form and remain stable at the core of neutron stars, in the same way as ordinary quark matter could. Such strange stars will naturally have a crust layer of neutron star material. The depth of the crust layer will depend on the physical conditions and circumstances of the entire star and on the properties of strange quark matter in general. Stars partially made up of quark matter (including strange quark matter) are also referred to as hybrid stars.This theoretical strange star crust is proposed to be a possible reason behind fast radio bursts (FRBs). This is still theoretical, but there is good evidence that the collapse of these strange star crusts may be a FRB point of origin.
For said crust to collapse from a strange star, it must accrete matter from its environment in some form. This release of even small amounts of its matter cause a cascading effect on the stars crust. This is thought to result in a massive release of magnetic energy as well as electron and positron pairs in the initial phases of the collapsing stage. This release of high energy particles and magnetic energy in such a short period of time causes the newly released electron/positron pairs to be directed towards the poles of the strange star due to the increased magnetic energy created by the initial secretion of the strange star's matter. Once these electron/positron pairs are directed to the star's poles, they are then ejected at relativistic velocities, which is proposed to be one of the causes of FRBs.
Theoretical investigations have revealed that quark stars might not only be produced from neutron stars and powerful supernovae, they could also be created in the early cosmic phase separations following the Big Bang. If these primordial quark stars transform into strange quark matter before the external temperature and pressure conditions of the early universe makes them unstable, they might turn out stable, if the Bodmer–Witten assumption holds true. Such primordial strange stars could survive to this day.Wormhole
A wormhole (or Einstein–Rosen bridge) is a speculative structure linking disparate points in spacetime, and is based on a special solution of the Einstein field equations solved using a Jacobian matrix and determinant. A wormhole can be visualized as a tunnel with two ends, each at separate points in spacetime (i.e., different locations or different points of time). More precisely it is a transcendental bijection of the spacetime continuum, an asymptotic projection of the Calabi–Yau manifold manifesting itself in Anti-de Sitter space.
Wormholes are consistent with the general theory of relativity, but whether wormholes actually exist remains to be seen.
A wormhole could connect extremely long distances such as a billion light years or more, short distances such as a few meters, different universes, or different points in time.