Gravitational interaction of antimatter
Encyclopedia
The gravitational interaction of antimatter with matter
or antimatter
has not been conclusively observed by physicists. While the overwhelming consensus among physicists is that antimatter will attract both matter and antimatter at the same rate that matter attracts matter, there is a strong desire to confirm this experimentally, given that consensus in science is for this to be true, but the hypothesis still open to falsification.
Antimatter's rarity and tendency to annihilate
when brought into contact with matter makes its study a technically demanding task. Most methods for the creation of antimatter (specifically antihydrogen
) result in high energy atoms unsuitable for gravity-related study. In recent years, the ATHENA
and ATRAP consortia have successfully created low-energy antihydrogen, but observations have thus far been methodically limited to annihilation events that yield little-to-no gravitational data.
asserts that antimatter should attract antimatter in the same way that matter attracts matter. However, there are several theories about how antimatter gravitationally interacts with normal matter:
in the Large Magellanic Cloud
. Although the supernova happened about 164,000 light years
away, both neutrinos and antineutrinos may have been detected virtually simultaneously. If both were actually observed, then any difference in the gravitational interaction would have to be very small. However, neutrino detectors cannot distinguish perfectly between neutrinos and antineutrinos. Some physicists conservatively estimate that there is less than a 10% chance that no regular neutrinos were observed at all. Others estimate even lower probabilities, some as low as 1%. Unfortunately, this accuracy is unlikely to be improved by duplicating the experiment any time soon. The last known supernova
to occur at such a close range prior to Supernova 1987A was around 1867.
s and positron
s. However, their charge-to-mass ratio is so large that electromagnetic effects overwhelmed the experiment.
It is difficult to directly observe gravitational forces at the particle level. At these small distances, electric forces tend to overwhelm the much weaker gravitational interaction. Furthermore, antiparticles must be kept separate from their normal counterparts or they will quickly annihilate. Worse still, production methods typically result in high-energy antimatter particles which are unsuitable for observation of gravitational effects in a laboratory environment. Understandably, this has made it difficult to directly measure the gravitational reaction of antimatter.
has become possible at the ATHENA and ATRAP experiments at CERN
. Antihydrogen, which is electrically neutral, should make it possible to directly measure the gravitational attraction of antimatter particles to the matter Earth. CERN is aiming to do this.
, result in vacuum instability, and result in CP violation
. It was also theorized that it would be inconsistent with the results of the Eötvös
test of the weak equivalence principle. Many of these early theoretical objections were later overturned.
argued that antigravity would violate conservation of energy
. If matter and antimatter responded oppositely to a gravitational field, then it would take no energy to change the height of a particle-antiparticle pair. However, when moving through a gravitational potential, the frequency and energy of light is shifted. Morrison argued that energy would be created by producing
matter and antimatter at one height and then annihilating it higher up, since the photons used in production would have less energy than the photons yielded from annihilation. However, it was later found that antigravity would still not violate the second law of thermodynamics
.
s are their own antiparticles, and in all respects behave exactly symmetrically with respect to matter and antimatter particles. In a large number of laboratory and astronomical tests, (gravitational redshift
and gravitational lensing, for example) photons are observed to be attracted to matter, exactly in accordance with the theory of General Relativity. It is possible to find atoms and nuclei whose elementary particle contents are the same, but whose masses are different. For example, Helium-4 weighs less than 2 atoms of deuterium due to binding-energy differences. The gravitational force constant is observed to be the same, up to the limits of experimental precision, for all such different materials, suggesting that "binding energy"—which, like the photon, has no distinction between matter and antimatter—experiences the same gravitational forces as matter. This is again in accordance with the theory of General Relativity, and difficult to reconcile with any theory predicting that matter and antimatter repel.
used quantum field theory to argue that antigravity would be inconsistent with the results of the Eötvös experiment. However, the renormalization technique used in Schiff's analysis is heavily criticized, and his work is seen as inconclusive.
argued that antigravity would result in the observation of an unacceptably high amount of CP violation
in the anomalous regeneration of kaon
s. At the time, CP violation had not yet been observed. However, Good's argument is criticized for being expressed in terms of absolute potentials. By rephrasing the argument in terms of relative potentials, Gabriel Chardin found that it resulted in an amount of Kaon regeneration which agrees with observation. He argues that antigravity is in fact a potential explanation for CP violation.
"). They also observe that exactly half of the ordinary energy of the photon appears as the electron, and, due to the energy conservation law, the other half of the photon's ordinary energy must become the anti-electron. Similar observations hold for all other particles of anti-matter. This means that all particles of anti-matter must consist of ordinary energy, and strongly implies that they should interact gravitationally just like particles of ordinary matter. It is remotely possible that some other aspect of anti-particles, besides consisting of ordinary energy, might cause them to behave differently in an ordinary gravitational field, but there are very few candidates for what might be that "other" aspect of anti-particles. But, it should be noted, that photons are considered their own antiparticle.
This same galactic repulsion is also endorsed as a potential explanation to the observation of a flatly accelerating universe
. If gravity was always attractive, the expansion of the universe might be expected to decelerate and eventually contract into a big crunch
. Using redshift
observations, astronomers and physicists estimate that instead, the size of the universe is expanding and the rate of expansion is accelerating at an approximately constant rate. Several theories have been proposed to explain this observation within the context of an always-attractive gravity. On the other hand, supporters of antigravity argue that if mutually repulsive, equal amounts of matter and antimatter would precisely offset any attraction.
CERN
physicist Dragan Slavkov Haidukovic has proposed an explanation for the problem of galaxy rotational speeds (currently explained by dark matter
models) based on antimatter antigravity. Assuming that a particle and its antiparticle have the gravitational charge of the opposite sign, the physical vacuum may be considered as a fluid of virtual gravitational dipoles. Following this hypothesis, he presents indications that dark matter may not exist at all and that the phenomena for which it was invoked might be explained by the gravitational polarization of the quantum vacuum by the known baryonic matter.
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...
or antimatter
Antimatter
In particle physics, antimatter is the extension of the concept of the antiparticle to matter, where antimatter is composed of antiparticles in the same way that normal matter is composed of particles...
has not been conclusively observed by physicists. While the overwhelming consensus among physicists is that antimatter will attract both matter and antimatter at the same rate that matter attracts matter, there is a strong desire to confirm this experimentally, given that consensus in science is for this to be true, but the hypothesis still open to falsification.
Antimatter's rarity and tendency to annihilate
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"....
when brought into contact with matter makes its study a technically demanding task. Most methods for the creation of antimatter (specifically antihydrogen
Antihydrogen
Antihydrogen is the antimatter counterpart of hydrogen. Whereas the common hydrogen atom is composed of an electron and proton, the antihydrogen atom is made up of a positron and antiproton...
) result in high energy atoms unsuitable for gravity-related study. In recent years, the ATHENA
Athena
In Greek mythology, Athena, Athenê, or Athene , also referred to as Pallas Athena/Athene , is the goddess of wisdom, courage, inspiration, civilization, warfare, strength, strategy, the arts, crafts, justice, and skill. Minerva, Athena's Roman incarnation, embodies similar attributes. Athena is...
and ATRAP consortia have successfully created low-energy antihydrogen, but observations have thus far been methodically limited to annihilation events that yield little-to-no gravitational data.
Three theories
The CPT theoremCPT theorem
In quantum field theory the CPT theorem states that any canonical quantum field theory is invariant under the CPT operation, which is a combination of three discrete transformations: charge conjugation C, parity transformation P, and time reversal T...
asserts that antimatter should attract antimatter in the same way that matter attracts matter. However, there are several theories about how antimatter gravitationally interacts with normal matter:
- Normal gravity: Standard theory asserts that antimatter should fall in exactly the same manner as normal matter.
- Antigravity: The theoretical analysis also focused on whether antimatter might instead repel with the same magnitude. This should not be confused with the many other speculative phenomena which may also be called 'anti-gravityAnti-gravityAnti-gravity is the idea of creating a place or object that is free from the force of gravity. It does not refer to the lack of weight under gravity experienced in free fall or orbit, or to balancing the force of gravity with some other force, such as electromagnetism or aerodynamic lift...
'. - Gravivector & GraviscalarGraviscalarIn theoretical physics, a graviscalar is a hypothetical particle that emerges as an excitation of the metric tensor but whose physical properties are virtually indistinguishable from a scalar in four dimensions, as shown in Kaluza-Klein theory...
: Later difficulties in creating quantum gravityQuantum gravityQuantum gravity is the field of theoretical physics which attempts to develop scientific models that unify quantum mechanics with general relativity...
theories have led to the idea that antimatter may react with a slightly different magnitude.
Supernova 1987A
Many scientists consider the best experimental evidence in favor of normal gravity to come from the observations of neutrinos from Supernova 1987A. In this landmark event, three neutrino detectors around the world simultaneously observed a cascade of neutrinos emanating from a supernovaSupernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
in the Large Magellanic Cloud
Large Magellanic Cloud
The Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...
. Although the supernova happened about 164,000 light years
Light Years
Light Years is the seventh studio album by Australian recording artist Kylie Minogue. It was released on 25 September 2000 by Parlophone and Mushroom Records. The album's style was indicative of her return to "mainstream pop dance tunes"....
away, both neutrinos and antineutrinos may have been detected virtually simultaneously. If both were actually observed, then any difference in the gravitational interaction would have to be very small. However, neutrino detectors cannot distinguish perfectly between neutrinos and antineutrinos. Some physicists conservatively estimate that there is less than a 10% chance that no regular neutrinos were observed at all. Others estimate even lower probabilities, some as low as 1%. Unfortunately, this accuracy is unlikely to be improved by duplicating the experiment any time soon. The last known supernova
Supernova remnant G1.9+0.3
Supernova remnant G1.9+0.3 is the youngest known supernova remnant in the Milky Way Galaxy. The remnant's young age was established by combining data from NASA's Chandra X-ray Observatory and the VLA radio observatory, and is believed to have exploded about 25,000 years ago, and the signal began...
to occur at such a close range prior to Supernova 1987A was around 1867.
Fairbank's experiments
Physicist William Fairbank attempted a laboratory experiment to directly measure the gravitational acceleration of both electronElectron
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...
s and 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...
s. However, their charge-to-mass ratio is so large that electromagnetic effects overwhelmed the experiment.
It is difficult to directly observe gravitational forces at the particle level. At these small distances, electric forces tend to overwhelm the much weaker gravitational interaction. Furthermore, antiparticles must be kept separate from their normal counterparts or they will quickly annihilate. Worse still, production methods typically result in high-energy antimatter particles which are unsuitable for observation of gravitational effects in a laboratory environment. Understandably, this has made it difficult to directly measure the gravitational reaction of antimatter.
Cold neutral antihydrogen experiments
In recent years, the production of cold antihydrogenAntihydrogen
Antihydrogen is the antimatter counterpart of hydrogen. Whereas the common hydrogen atom is composed of an electron and proton, the antihydrogen atom is made up of a positron and antiproton...
has become possible at the ATHENA and ATRAP experiments at CERN
CERN
The European Organization for Nuclear Research , known as CERN , is an international organization whose purpose is to operate the world's largest particle physics laboratory, which is situated in the northwest suburbs of Geneva on the Franco–Swiss border...
. Antihydrogen, which is electrically neutral, should make it possible to directly measure the gravitational attraction of antimatter particles to the matter Earth. CERN is aiming to do this.
Antimatter gravity debate
When antimatter was first discovered in 1932, physicists wondered about how it would react to gravity. Initial analysis focused on whether antimatter should react the same as matter or react oppositely. Several theoretical arguments arose which convinced physicists that antimatter would react exactly the same as normal matter. They inferred that a gravitational repulsion between matter and antimatter was implausible as it would violate CPT invariance, conservation of energyConservation 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...
, result in vacuum instability, and result in CP violation
CP violation
In particle physics, CP violation is a violation of the postulated CP-symmetry: the combination of C-symmetry and P-symmetry . CP-symmetry states that the laws of physics should be the same if a particle were interchanged with its antiparticle , and left and right were swapped...
. It was also theorized that it would be inconsistent with the results of the Eötvös
Loránd Eötvös
Baron Loránd Eötvös de Vásárosnamény , more commonly called Baron Roland von Eötvös in English literature, was a Hungarian physicist. He is remembered today largely for his work on gravitation and surface tension.-Life:...
test of the weak equivalence principle. Many of these early theoretical objections were later overturned.
Morrison's argument
In 1958, Philip MorrisonPhilip Morrison
Philip Morrison, was Institute Professor Emeritus and Professor of Physics Emeritus at the Massachusetts Institute of Technology .-Early life and education:...
argued that antigravity would violate 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...
. If matter and antimatter responded oppositely to a gravitational field, then it would take no energy to change the height of a particle-antiparticle pair. However, when moving through a gravitational potential, the frequency and energy of light is shifted. Morrison argued that energy would be created by producing
Pair production
Pair production refers to the creation of an elementary particle and its antiparticle, usually from a photon . For example an electron and its antiparticle, the positron, may be created...
matter and antimatter at one height and then annihilating it higher up, since the photons used in production would have less energy than the photons yielded from annihilation. However, it was later found that antigravity would still not violate the second law of thermodynamics
Second law of thermodynamics
The second law of thermodynamics is an expression of the tendency that over time, differences in temperature, pressure, and chemical potential equilibrate in an isolated physical system. From the state of thermodynamic equilibrium, the law deduced the principle of the increase of entropy and...
.
The equivalence principle
If one can invent a theory in which matter and antimatter repel one another, what does it predict for things which are neither matter nor antimatter? PhotonPhoton
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 are their own antiparticles, and in all respects behave exactly symmetrically with respect to matter and antimatter particles. In a large number of laboratory and astronomical tests, (gravitational redshift
Gravitational redshift
In astrophysics, gravitational redshift or Einstein shift describes light or other forms of electromagnetic radiation of certain wavelengths that originate from a source that is in a region of a stronger gravitational field that appear to be of longer wavelength, or redshifted, when seen or...
and gravitational lensing, for example) photons are observed to be attracted to matter, exactly in accordance with the theory of General Relativity. It is possible to find atoms and nuclei whose elementary particle contents are the same, but whose masses are different. For example, Helium-4 weighs less than 2 atoms of deuterium due to binding-energy differences. The gravitational force constant is observed to be the same, up to the limits of experimental precision, for all such different materials, suggesting that "binding energy"—which, like the photon, has no distinction between matter and antimatter—experiences the same gravitational forces as matter. This is again in accordance with the theory of General Relativity, and difficult to reconcile with any theory predicting that matter and antimatter repel.
Schiff's argument
Later in 1958, L. SchiffLeonard I. Schiff
Leonard Isaac Schiff was born in Fall River, Massachusetts on March 29, 1915 and died on Jan 21, 1971.He was a physicist best known for his book Quantum Mechanics.-Education:...
used quantum field theory to argue that antigravity would be inconsistent with the results of the Eötvös experiment. However, the renormalization technique used in Schiff's analysis is heavily criticized, and his work is seen as inconclusive.
Good's argument
In 1961, Myron L. GoodMyron L. Good
Myron Lindsay Good was an American physicist, a professor of physics at the University of Wisconsin–Madison and Stony Brook University.Good's research interests spanned a broad range of topics in particle physics...
argued that antigravity would result in the observation of an unacceptably high amount of CP violation
CP violation
In particle physics, CP violation is a violation of the postulated CP-symmetry: the combination of C-symmetry and P-symmetry . CP-symmetry states that the laws of physics should be the same if a particle were interchanged with its antiparticle , and left and right were swapped...
in the anomalous regeneration of kaon
Kaon
In particle physics, a kaon is any one of a group of four mesons distinguished by the fact that they carry a quantum number called strangeness...
s. At the time, CP violation had not yet been observed. However, Good's argument is criticized for being expressed in terms of absolute potentials. By rephrasing the argument in terms of relative potentials, Gabriel Chardin found that it resulted in an amount of Kaon regeneration which agrees with observation. He argues that antigravity is in fact a potential explanation for CP violation.
The E=mc² argument
Physicists routinely observe that ordinary energy such as an appropriate gamma photon can be converted into an electron and an anti-electron, in accordance with Einstein's famous equation ("pair productionPair production
Pair production refers to the creation of an elementary particle and its antiparticle, usually from a photon . For example an electron and its antiparticle, the positron, may be created...
"). They also observe that exactly half of the ordinary energy of the photon appears as the electron, and, due to the energy conservation law, the other half of the photon's ordinary energy must become the anti-electron. Similar observations hold for all other particles of anti-matter. This means that all particles of anti-matter must consist of ordinary energy, and strongly implies that they should interact gravitationally just like particles of ordinary matter. It is remotely possible that some other aspect of anti-particles, besides consisting of ordinary energy, might cause them to behave differently in an ordinary gravitational field, but there are very few candidates for what might be that "other" aspect of anti-particles. But, it should be noted, that photons are considered their own antiparticle.
Motivations for antigravity
Supporters argue that antimatter antigravity would solve several important problems in physics. Besides the already mentioned prediction of CP violation, they argue that it explains two cosmological paradoxes. The first is the apparent local lack of antimatter: by theory antimatter and matter would repel each other gravitationally, forming separate matter and antimatter galaxies. These galaxies would also tend to repel one another, thereby preventing possible collisions and annihilations.This same galactic repulsion is also endorsed as a potential explanation to the observation of a flatly accelerating universe
Accelerating universe
The accelerating universe is the observation that the universe appears to be expanding at an increasing rate, which in formal terms means that the cosmic scale factor a has a positive second derivative, implying that the velocity at which a given galaxy is receding from us should be continually...
. If gravity was always attractive, the expansion of the universe might be expected to decelerate and eventually contract into a big crunch
Big Crunch
In physical cosmology, the Big Crunch is one possible scenario for the ultimate fate of the universe, in which the metric expansion of space eventually reverses and the universe recollapses, ultimately ending as a black hole singularity.- Overview :...
. Using redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
observations, astronomers and physicists estimate that instead, the size of the universe is expanding and the rate of expansion is accelerating at an approximately constant rate. Several theories have been proposed to explain this observation within the context of an always-attractive gravity. On the other hand, supporters of antigravity argue that if mutually repulsive, equal amounts of matter and antimatter would precisely offset any attraction.
CERN
CERN
The European Organization for Nuclear Research , known as CERN , is an international organization whose purpose is to operate the world's largest particle physics laboratory, which is situated in the northwest suburbs of Geneva on the Franco–Swiss border...
physicist Dragan Slavkov Haidukovic has proposed an explanation for the problem of galaxy rotational speeds (currently explained by dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
models) based on antimatter antigravity. Assuming that a particle and its antiparticle have the gravitational charge of the opposite sign, the physical vacuum may be considered as a fluid of virtual gravitational dipoles. Following this hypothesis, he presents indications that dark matter may not exist at all and that the phenomena for which it was invoked might be explained by the gravitational polarization of the quantum vacuum by the known baryonic matter.