Axion

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Axion

Axion

Interactions	Gravitational, electromagnetic, strong nuclear, weak nuclear
Status	Hypothetical
Symbol	A0, a, θ
Theorized	1978, Wilczek and Weinberg
Mass	10−5 to 1 eV/c2
Electric charge	0
Spin	0

An axion (/ˈæksiɒn/) is a hypothetical elementary particle originally theorized in 1978 independently by Frank Wilczek and Steven Weinberg as the Goldstone boson of Peccei–Quinn theory, which had been proposed in 1977 to solve the strong CP problem in quantum chromodynamics (QCD). If axions exist and have low mass within a specific range, they are of interest as a possible component of cold dark matter.

History

Strong CP problem

As shown by Gerard 't Hooft, strong interactions of the standard model, QCD, possess a non-trivial vacuum structure that in principle permits violation of the combined symmetries of charge conjugation and parity, collectively known as CP. Together with effects generated by weak interactions, the effective periodic strong CP-violating term, Θ, appears as a Standard Model input – its value is not predicted by the theory, but must be measured. However, large CP-violating interactions originating from QCD would induce a large electric dipole moment (EDM) for the neutron. Experimental constraints on the unobserved EDM implies CP violation from QCD must be extremely tiny and thus Θ must itself be extremely small. Since Θ could have any value between 0 and 2π, this presents a "naturalness" problem for the standard model. Why should this parameter find itself so close to zero? (Or, why should QCD find itself CP-preserving?) This question constitutes what is known as the strong CP problem.

Prediction

In 1977, Roberto Peccei and Helen Quinn postulated a more elegant solution to the strong CP problem, the Peccei–Quinn mechanism. The idea is to effectively promote Θ to a field. This is accomplished by adding a new global symmetry (called a Peccei–Quinn (PQ) symmetry) that becomes spontaneously broken. This results in a new particle, as shown independently by Frank Wilczek and Steven Weinberg, that fills the role of Θ, naturally relaxing the CP-violation parameter to zero. Wilczek named this new hypothesized particle the "axion" after a brand of laundry detergent because it "cleaned up" a problem, while Weinberg called it "the higglet". Weinberg later agreed to adopt Wilczek's name for the particle. Because it has a non-zero mass, the axion is a pseudo-Nambu–Goldstone boson.

Axion dark matter

QCD effects produce an effective periodic potential in which the axion field moves.  Expanding the potential about one of its minima, one finds that the product of the axion mass with the axion decay constant is determined by the topological susceptibility of the QCD vacuum. An axion with mass much less than 60 keV is long-lived and weakly interacting: A perfect dark matter candidate.

The oscillations of the axion field about the minimum of the effective potential, the so-called misalignment mechanism, generate a cosmological population of cold axions with an abundance depending on the mass of the axion. With a mass above 5 μeV/c² (10⁻¹¹ times the electron mass) axions could account for dark matter, and thus be both a dark-matter candidate and a solution to the strong CP problem. If inflation occurs at a low scale and lasts sufficiently long, the axion mass can be as low as 1 peV/c².

There are two distinct scenarios in which the axion field begins its evolution, depending on the following two conditions:

(a)	The PQ symmetry is spontaneously broken during inflation. This condition is realized whenever the axion energy scale is larger than the Hubble rate at the end of inflation
(b)	The PQ symmetry is never restored after its spontaneous breaking occurs. This condition is realized whenever the axion energy scale is larger than the maximum temperature reached in the post-inflationary Universe.

Broadly speaking, one of the two possible scenarios outlined in the two following subsections occurs:

Pre-inflationary scenario

If both (a) and (b) are satisfied, cosmic inflation selects one patch of the Universe within which the spontaneous breaking of the PQ symmetry leads to a homogeneous value of the initial value of the axion field. In this "pre-inflationary" scenario, topological defects are inflated away and do not contribute to the axion energy density. However, other bounds that come from isocurvature modes severely constrain this scenario, which require a relatively low-energy scale of inflation to be viable.

Post-inflationary scenario

If at least one of the conditions (a) or (b) is violated, the axion field takes different values within patches that are initially out of causal contact, but that today populate the volume enclosed by our Hubble horizon. In this scenario, isocurvature fluctuations in the PQ field randomise the axion field, with no preferred value in the power spectrum.

The proper treatment in this scenario is to solve numerically the equation of motion of the PQ field in an expanding Universe, in order to capture all features coming from the misalignment mechanism, including the contribution from topological defects like "axionic" strings and domain walls. An axion mass estimate between 0.05 and 1.50 meV was reported by Borsanyi et al. (2016). The result was calculated by simulating the formation of axions during the post-inflation period on a supercomputer.

Progress in the late 2010s in determining the present abundance of a KSVZ-type axion using numerical simulations lead to values between 0.02 and 0.1 meV, although these results have been challenged by the details on the power spectrum of emitted axions from strings.

Phenomenology of the axion field

Searches

The axion models originally proposed by Wilczek and by Weinberg chose axion coupling strengths that were so strong that they would have already been detected in prior experiments. It had been thought that the Peccei-Quinn mechanism for solving the strong CP problem required such large couplings. However, it was soon realized that "invisible axions" with much smaller couplings also work. Two such classes of models are known in the literature as KSVZ (Kim–Shifman–Vainshtein–Zakharov) and DFSZ (Dine–Fischler–Srednicki–Zhitnitsky).

The very weakly coupled axion is also very light, because axion couplings and mass are proportional. Satisfaction with "invisible axions" changed when it was shown that any very light axion would have been overproduced in the early universe and therefore must be excluded.

Maxwell's equations with axion modifications

Pierre Sikivie computed how Maxwell's equations are modified in the presence of an axion in 1983. He showed that these axions could be detected on Earth by converting them to photons, using a strong magnetic field, motivating a number of experiments. For example, the Axion Dark Matter Experiment converts axion dark matter to microwave photons, the CERN Axion Solar Telescope converts axions produced in the Sun's core to X-rays, and other experiments search for axions produced in laser light. As of the early 2020s, there are dozens of proposed or ongoing experiments searching for axion dark matter.

The equations of axion electrodynamics are typically written in "natural units", where the reduced Planck constant ${\displaystyle \hslash }$, speed of light ${\displaystyle c}$, and permittivity of free space ${\displaystyle {\epsilon }_0}$ all reduce to 1 when expressed in these "natural units". In this unit system, the electrodynamic equations are:

Name	Equations
Gauss's law	${\displaystyle \mathrm{\nabla }\cdot \mathbf{E}=\rho -g_{a\gamma \gamma }\mathbf{B}\cdot \mathrm{\nabla }a}$
Gauss's law for magnetism	${\displaystyle \mathrm{\nabla }\cdot \mathbf{B}=0}$
Faraday's law	${\displaystyle \mathrm{\nabla }\times \mathbf{E}=-\dot{\mathbf{B}}}$
Ampère–Maxwell law	${\displaystyle \mathrm{\nabla }\times \mathbf{B}=\dot{\mathbf{E}}+\mathbf{J}+g_{a\gamma \gamma }\left(\dot{a}\mathbf{B}-\mathbf{E}\times \mathrm{\nabla }a\right)}$
Axion field's equation of motion	${\displaystyle {\ddot{a}}^2-{\mathrm{\nabla }}^{2}a+m_a^{2}a=-g_{a\gamma \gamma }\mathbf{E}\cdot \mathbf{B}}$

Above, a dot above a variable denotes its time derivative; the dot spaced between variables is the vector dot product; the factor ${\displaystyle g_{a\gamma \gamma }}$ is the axion-to-photon coupling constant rendered in "natural units".

Alternative forms of these equations have been proposed, which imply completely different physical signatures. For example, Visinelli wrote a set of equations that imposed duality symmetry, assuming the existence of magnetic monopoles. However, these alternative formulations are less theoretically motivated, and in many cases cannot even be derived from an action.

Analogous effect for topological insulators

A term analogous to the one that would be added to Maxwell's equations to account for axions also appears in recent (2008) theoretical models for topological insulators giving an effective axion description of the electrodynamics of these materials.

This term leads to several interesting predicted properties including a quantized magnetoelectric effect. Evidence for this effect has been given in THz spectroscopy experiments performed at the Johns Hopkins University on quantum regime thin film topological insulators developed at Rutgers University.

In 2019, a team at the Max Planck Institute for Chemical Physics of Solids published their detection of an axion insulator phase of a Weyl semimetal material. In the axion insulator phase, the material has an axion-like quasiparticle – an excitation of electrons that behave together as an axion – and its discovery demonstrates the consistency of axion electrodynamics as a description of the interaction of axion-like particles with electromagnetic fields. In this way, the discovery of axion-like quasiparticles in axion insulators provides motivation to use axion electrodynamics to search for the axion itself.

Experiments

Despite not yet having been found, the axion has been well studied for over 40 years, giving time for physicists to develop insight into axion effects that might be detected. Several experimental searches for axions are presently underway; most exploit axions' expected slight interaction with photons in strong magnetic fields. Axions are also one of the few remaining plausible candidates for dark matter particles, and might be discovered in some dark matter experiments.

Constraints on the axion's coupling to the photon

Constraints on the axion's dimensionless coupling to electrons

Direct conversion in a magnetic field

Several experiments search for astrophysical axions by the Primakoff effect, which converts axions to photons and vice versa in electromagnetic fields.

The Axion Dark Matter Experiment (ADMX) at the University of Washington uses a strong magnetic field to detect the possible weak conversion of axions to microwaves. ADMX searches the galactic dark matter halo^[39] for axions resonant with a cold microwave cavity. ADMX has excluded optimistic axion models in the 1.9–3.53 μeV range. From 2013 to 2018 a series of upgrades were done and it is taking new data, including at 4.9–6.2 μeV. In December 2021 it excluded the 3.3–4.2 μeV range for the KSVZ model.

Other experiments of this type include DMRadio, HAYSTAC, CULTASK, and ORGAN. HAYSTAC completed the first scanning run of a haloscope above 20 μeV in the late 2010s.

Polarized light in a magnetic field

The Italian PVLAS experiment searches for polarization changes of light propagating in a magnetic field. The concept was first put forward in 1986 by Luciano Maiani, Roberto Petronzio and Emilio Zavattini. A rotation claim in 2006 was excluded by an upgraded setup. An optimized search began in 2014.

Light shining through walls

Another technique is so called "light shining through walls", where light passes through an intense magnetic field to convert photons into axions, which then pass through metal and are reconstituted as photons by another magnetic field on the other side of the barrier. Experiments by BFRS and a team led by Rizzo ruled out an axion cause. GammeV saw no events, reported in a 2008 Physics Review Letter. ALPS I conducted similar runs, setting new constraints in 2010; ALPS II is being built in 2022. OSQAR found no signal, limiting coupling and will continue.

Astrophysical axion searches

Axion-like bosons could have a signature in astrophysical settings. In particular, several works have proposed axion-like particles as a solution to the apparent transparency of the Universe to TeV photons. It has also been demonstrated that, in the large magnetic fields threading the atmospheres of compact astrophysical objects (e.g., magnetars), photons will convert much more efficiently. This would in turn give rise to distinct absorption-like features in the spectra detectable by early 21st century telescopes. A new (2009) promising means is looking for quasi-particle refraction in systems with strong magnetic gradients. In particular, the refraction will lead to beam splitting in the radio light curves of highly magnetized pulsars and allow much greater sensitivities than currently achievable. The International Axion Observatory (IAXO) is a proposed fourth generation helioscope.

Axions can resonantly convert into photons in the magnetospheres of neutron stars. The emerging photons lie in the GHz frequency range and can be potentially picked up in radio detectors, leading to a sensitive probe of the axion parameter space. This strategy has been used to constrain the axion–photon coupling in the 5–11 μeV mass range, by re-analyzing existing data from the Green Bank Telescope and the Effelsberg 100 m Telescope. A novel, alternative strategy consists in detecting the transient signal from the encounter between a neutron star and an axion minicluster in the Milky Way.

Axions can be produced in the Sun's core when X-rays scatter in strong electric fields. The CAST solar telescope is underway, and has set limits on coupling to photons and electrons. Axions may be produced within neutron stars, by nucleon–nucleon bremsstrahlung. The subsequent decay of axions to gamma rays allows constraints on the axion mass to be placed from observations of neutron stars in gamma-rays using the Fermi LAT. From an analysis of four neutron stars, Berenji et al. (2016) obtained a 95% confidence interval upper limit on the axion mass of 0.079 eV. In 2021 it has been also suggested that a reported excess of hard X-ray emission from a system of neutron stars known as the magnificent seven could be explained as axion emission.

In 2016, a theoretical team from Massachusetts Institute of Technology devised a possible way of detecting axions using a strong magnetic field that need be no stronger than that produced in an MRI scanning machine. It would show variation, a slight wavering, that is linked to the mass of the axion. As of 2019, the experiment is being implemented by experimentalists at the university.

In 2022 the polarized light measurements of Messier 87* by the EHT were used to constrain the mass of the axion assuming that hypothetical clouds of axions could form around a black hole, rejecting the approximate 10⁻²¹ eV/c² – 10⁻²⁰ eV/c² range of mass values.

Searches for resonance effects

Resonance effects may be evident in Josephson junctions from a supposed high flux of axions from the galactic halo with mass of 110 μeV and density 0.05 GeV/cm³ compared to the implied dark matter density 0.3±0.1 GeV/cm³, indicating said axions would not have enough mass to be the sole component of dark matter. The ORGAN experiment plans to conduct a direct test of this result via the haloscope method.

Dark matter recoil searches

Dark matter cryogenic detectors have searched for electron recoils that would indicate axions. CDMS published in 2009 and EDELWEISS set coupling and mass limits in 2013. UORE and XMASS also set limits on solar axions in 2013. XENON100 used a 225-day run to set the best coupling limits to date and exclude some parameters.

Nuclear spin precession

While Schiff's theorem states that a static nuclear electric dipole moment (EDM) does not produce atomic and molecular EDMs, the axion induces an oscillating nuclear EDM that oscillates at the Larmor frequency. If this nuclear EDM oscillation frequency is in resonance with an external electric field, a precession in the nuclear spin rotation occurs. This precession can be measured using precession magnetometry and if detected, would be evidence for Axions.

An experiment using this technique is the Cosmic Axion Spin Precession Experiment (CASPEr).

Searches at particle colliders

Axions may also be produced at colliders, in particular in electron positron collisions as well as in ultra-peripheral heavy ion collisions at the Large Hadron Collider at CERN, reinterpreting the light-by-light scattering process. Those searches are sensitive for rather large axion masses between 100 MeV/c² and hundreds of GeV/c². Assuming a coupling of axions to the Higgs Boson, searches for anomalous Higgs boson decays into two axions can theoretically provide even stronger limits.

Disputed detections

It was reported in 2014 that evidence for axions may have been detected as a seasonal variation in observed X-ray emission that would be expected from conversion in the Earth's magnetic field of axions streaming from the Sun. Studying 15 years of data by the European Space Agency's XMM-Newton observatory, a research group at Leicester University noticed a seasonal variation for which no conventional explanation could be found. One potential explanation for the variation, described as "plausible" by the senior author of the paper, is the known seasonal variation in visibility to XMM-Newton of the sunward magnetosphere in which X-rays may be produced by axions from the Sun's core.

This interpretation of the seasonal variation is disputed by two Italian researchers, who identify flaws in the arguments of the Leicester group that are said to rule out an interpretation in terms of axions. Most importantly, the scattering in angle assumed by the Leicester group to be caused by magnetic field gradients during the photon production, necessary to allow the X-rays to enter the detector that cannot point directly at the sun, would dissipate the flux so much that the probability of detection would be negligible.

In 2013, Christian Beck suggested that axions might be detectable in Josephson junctions; and in 2014, he argued that a signature, consistent with a mass ≈110 μeV, had in fact been observed in several preexisting experiments.

In 2020, the XENON1T experiment at the Gran Sasso National Laboratory in Italy reported a result suggesting the discovery of solar axions. The results are not yet significant at the 5-sigma level required for confirmation, and other explanations of the data are possible though less likely. New observations made in July 2022, after the observatory upgrade to XENONnT, discarded the excess thus ending the possibility of new particle discovery.

Properties

Predictions

One theory of axions relevant to cosmology had predicted that they would have no electric charge, a very small mass in the range from 1 μeV/c² to 1 eV/c², and very low interaction cross-sections for strong and weak forces. Because of their properties, axions would interact only minimally with ordinary matter. Axions would also change to and from photons in magnetic fields.

Cosmological implications

The properties of the axion, such as the axion mass, decay constant, and abundance, all have implications for cosmology.

Inflation suggests that if they exist, axions would be created abundantly during the Big Bang. Because of a unique coupling to the instanton field of the primordial universe (the "misalignment mechanism"), an effective dynamical friction is created during the acquisition of mass, following cosmic inflation. This robs all such primordial axions of their kinetic energy.

Ultralight axion (ULA) with m ~ 10⁻²² eV/c² is a kind of scalar field dark matter that seems to solve the small scale problems of CDM. A single ULA with a GUT scale decay constant provides the correct relic density without fine-tuning.

Axions would also have stopped interaction with normal matter at a different moment after the Big Bang than other more massive dark particles. The lingering effects of this difference could perhaps be calculated and observed astronomically.

If axions have low mass, thus preventing other decay modes (since there are no lighter particles to decay into), theories predict that the universe would be filled with a very cold Bose–Einstein condensate of primordial axions. Hence, axions could plausibly explain the dark matter problem of physical cosmology. Observational studies are underway, but they are not yet sufficiently sensitive to probe the mass regions if they are the solution to the dark matter problem with the fuzzy dark matter region starting to be probed via superradiance. High mass axions of the kind searched for by Jain and Singh (2007) would not persist in the modern universe. Moreover, if axions exist, scatterings with other particles in the thermal bath of the early universe unavoidably produce a population of hot axions.

Low mass axions could have additional structure at the galactic scale. If they continuously fall into galaxies from the intergalactic medium, they would be denser in "caustic" rings, just as the stream of water in a continuously flowing fountain is thicker at its peak. The gravitational effects of these rings on galactic structure and rotation might then be observable. Other cold dark matter theoretical candidates, such as WIMPs and MACHOs, could also form such rings, but because such candidates are fermionic and thus experience friction or scattering among themselves, the rings would be less sharply defined.

João G. Rosa and Thomas W. Kephart suggested that axion clouds formed around unstable primordial black holes might initiate a chain of reactions that radiate electromagnetic waves, allowing their detection. When adjusting the mass of the axions to explain dark matter, the pair discovered that the value would also explain the luminosity and wavelength of fast radio bursts, being a possible origin for both phenomena. In 2022 a similar hypothesis was used to constrain the mass of the axion from data of M87*.

In 2020, it was proposed that the axion field might actually have influenced the evolution of early Universe by creating more imbalance between the amounts of matter and antimatter – which possibly resolves the baryon asymmetry problem.

Supersymmetry

In supersymmetric theories the axion has both a scalar and a fermionic superpartner. The fermionic superpartner of the axion is called the axino, the scalar superpartner is called the saxion or dilaton. They are all bundled in a chiral superfield.

The axino has been predicted to be the lightest supersymmetric particle in such a model. In part due to this property, it is considered a candidate for dark matter.

Electromagnetism

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Electromagnetism

Electromagnetic interactions are responsible for the glowing filaments in this plasma globe.

In physics, electromagnetism is an interaction that occurs between particles with electric charge via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interactions of atoms and molecules. Electromagnetism can be thought of as a combination of electrostatics and magnetism, which are distinct but closely intertwined phenomena. Electromagnetic forces occur between any two charged particles. Electric forces cause an attraction between particles with opposite charges and repulsion between particles with the same charge, while magnetism is an interaction that occurs between charged particles in relative motion. These two forces are described in terms of electromagnetic fields. Macroscopic charged objects are described in terms of Coulomb's law for electricity and Ampère's force law for magnetism; the Lorentz force describes microscopic charged particles.

The electromagnetic force is responsible for many of the chemical and physical phenomena observed in daily life. The electrostatic attraction between atomic nuclei and their electrons holds atoms together. Electric forces also allow different atoms to combine into molecules, including the macromolecules such as proteins that form the basis of life. Meanwhile, magnetic interactions between the spin and angular momentum magnetic moments of electrons also play a role in chemical reactivity; such relationships are studied in spin chemistry. Electromagnetism also plays several crucial roles in modern technology: electrical energy production, transformation and distribution; light, heat, and sound production and detection; fiber optic and wireless communication; sensors; computation; electrolysis; electroplating; and mechanical motors and actuators.

Electromagnetism has been studied since ancient times. Many ancient civilizations, including the Greeks and the Mayans, created wide-ranging theories to explain lightning, static electricity, and the attraction between magnetized pieces of iron ore. However, it was not until the late 18th century that scientists began to develop a mathematical basis for understanding the nature of electromagnetic interactions. In the 18th and 19th centuries, prominent scientists and mathematicians such as Coulomb, Gauss and Faraday developed namesake laws which helped to explain the formation and interaction of electromagnetic fields. This process culminated in the 1860s with the discovery of Maxwell's equations, a set of four partial differential equations which provide a complete description of classical electromagnetic fields. Maxwell's equations provided a sound mathematical basis for the relationships between electricity and magnetism that scientists had been exploring for centuries, and predicted the existence of self-sustaining electromagnetic waves. Maxwell postulated that such waves make up visible light, which was later shown to be true. Gamma-rays, x-rays, ultraviolet, visible, infrared radiation, microwaves and radio waves were all determined to be electromagnetic radiation differing only in their range of frequencies.

In the modern era, scientists have continued to refine the theorem of electromagnetism to take into account the effects of modern physics, including quantum mechanics and relativity. The theoretical implications of electromagnetism, particularly the establishment of the speed of light based on properties of the "medium" of propagation (permeability and permittivity), helped inspire Einstein's theory of special relativity in 1905. Meanwhile, the field of quantum electrodynamics (QED) has modified Maxwell's equations to be consistent with the quantized nature of matter. In QED, the changes in the electromagnetic field is expressed in terms of discrete excitations, particles known as photons, the quanta of light.

History

Main article: History of electromagnetic theory

Ancient world

Investigation into electromagnetic phenomena began about 5,000 years ago. There is evidence that the ancient Chinese, Mayan, and potentially even Egyptian civilizations knew that the naturally magnetic mineral magnetite had attractive properties, and many incorporated it into their art and architecture. Ancient people were also aware of lightning and static electricity, although they had no idea of the mechanisms behind these phenomena. The Greek philosopher Thales of Miletus discovered around 600 B.C.E. that amber could acquire an electric charge when it was rubbed with cloth, which allowed it to pick up light objects such as pieces of straw. Thales also experimented with the ability of magnetic rocks to attract one other, and hypothesized that this phenomenon might be connected to the attractive power of amber, foreshadowing the deep connections between electricity and magnetism that would be discovered over 2,000 years later. Despite all this investigation, ancient civilizations had no understanding of the mathematical basis of electromagnetism, and often analyzed its impacts through the lens of religion rather than science (lightning, for instance, was considered to be a creation of the gods in many cultures).

19th century

Cover of A Treatise on Electricity and Magnetism

Electricity and magnetism were originally considered to be two separate forces. This view changed with the publication of James Clerk Maxwell's 1873 A Treatise on Electricity and Magnetism in which the interactions of positive and negative charges were shown to be mediated by one force. There are four main effects resulting from these interactions, all of which have been clearly demonstrated by experiments:

1. Electric charges attract or repel one another with a force inversely proportional to the square of the distance between them: unlike charges attract, like ones repel.
2. Magnetic poles (or states of polarization at individual points) attract or repel one another in a manner similar to positive and negative charges and always exist as pairs: every north pole is yoked to a south pole.
3. An electric current inside a wire creates a corresponding circumferential magnetic field outside the wire. Its direction (clockwise or counter-clockwise) depends on the direction of the current in the wire.
4. A current is induced in a loop of wire when it is moved toward or away from a magnetic field, or a magnet is moved towards or away from it; the direction of current depends on that of the movement.

In April 1820, Hans Christian Ørsted observed that an electrical current in a wire caused a nearby compass needle to move. At the time of discovery, Ørsted did not suggest any satisfactory explanation of the phenomenon, nor did he try to represent the phenomenon in a mathematical framework. However, three months later he began more intensive investigations. Soon thereafter he published his findings, proving that an electric current produces a magnetic field as it flows through a wire. The CGS unit of magnetic induction (oersted) is named in honor of his contributions to the field of electromagnetism.

His findings resulted in intensive research throughout the scientific community in electrodynamics. They influenced French physicist André-Marie Ampère's developments of a single mathematical form to represent the magnetic forces between current-carrying conductors. Ørsted's discovery also represented a major step toward a unified concept of energy.

This unification, which was observed by Michael Faraday, extended by James Clerk Maxwell, and partially reformulated by Oliver Heaviside and Heinrich Hertz, is one of the key accomplishments of 19th-century mathematical physics. It has had far-reaching consequences, one of which was the understanding of the nature of light. Unlike what was proposed by the electromagnetic theory of that time, light and other electromagnetic waves are at present seen as taking the form of quantized, self-propagating oscillatory electromagnetic field disturbances called photons. Different frequencies of oscillation give rise to the different forms of electromagnetic radiation, from radio waves at the lowest frequencies, to visible light at intermediate frequencies, to gamma rays at the highest frequencies.

Ørsted was not the only person to examine the relationship between electricity and magnetism. In 1802, Gian Domenico Romagnosi, an Italian legal scholar, deflected a magnetic needle using a Voltaic pile. The factual setup of the experiment is not completely clear, nor if current flowed across the needle or not. An account of the discovery was published in 1802 in an Italian newspaper, but it was largely overlooked by the contemporary scientific community, because Romagnosi seemingly did not belong to this community.

An earlier (1735), and often neglected, connection between electricity and magnetism was reported by a Dr. Cookson. The account stated:

A tradesman at Wakefield in Yorkshire, having put up a great number of knives and forks in a large box ... and having placed the box in the corner of a large room, there happened a sudden storm of thunder, lightning, &c. ... The owner emptying the box on a counter where some nails lay, the persons who took up the knives, that lay on the nails, observed that the knives took up the nails. On this the whole number was tried, and found to do the same, and that, to such a degree as to take up large nails, packing needles, and other iron things of considerable weight ...

E. T. Whittaker suggested in 1910 that this particular event was responsible for lightning to be "credited with the power of magnetizing steel; and it was doubtless this which led Franklin in 1751 to attempt to magnetize a sewing-needle by means of the discharge of Leyden jars."

A fundamental force

Representation of the electric field vector of a wave of circularly polarized electromagnetic radiation

The electromagnetic force is the second strongest of the four known fundamental forces. It operates with infinite range. All other forces (e.g., friction, contact forces) are derived from these four fundamental forces and they are known as non-fundamental forces. At high energy, the weak force and electromagnetic force are unified as a single interaction called the electroweak interaction.

Roughly speaking, all the forces involved in interactions between atoms can be explained by the electromagnetic force acting between the electrically charged atomic nuclei and electrons of the atoms. Electromagnetic forces also explain how these particles carry momentum by their movement. This includes the forces we experience in "pushing" or "pulling" ordinary material objects, which result from the intermolecular forces that act between the individual molecules in our bodies and those in the objects. The electromagnetic force is also involved in all forms of chemical phenomena.

A necessary part of understanding the intra-atomic and intermolecular forces is the effective force generated by the momentum of the electrons' movement, such that as electrons move between interacting atoms they carry momentum with them. As a collection of electrons becomes more confined, their minimum momentum necessarily increases due to the Pauli exclusion principle. The behaviour of matter at the molecular scale including its density is determined by the balance between the electromagnetic force and the force generated by the exchange of momentum carried by the electrons themselves.

Classical electrodynamics

Main article: Classical electrodynamics

In 1600, William Gilbert proposed, in his De Magnete, that electricity and magnetism, while both capable of causing attraction and repulsion of objects, were distinct effects. Mariners had noticed that lightning strikes had the ability to disturb a compass needle. The link between lightning and electricity was not confirmed until Benjamin Franklin's proposed experiments in 1752 were conducted on 10 May 1752 by Thomas-François Dalibard of France using a 40-foot-tall (12 m) iron rod instead of a kite and he successfully extracted electrical sparks from a cloud.

One of the first to discover and publish a link between human-made electric current and magnetism was Gian Romagnosi, who in 1802 noticed that connecting a wire across a voltaic pile deflected a nearby compass needle. However, the effect did not become widely known until 1820, when Ørsted performed a similar experiment. Ørsted's work influenced Ampère to conduct further experiments, which eventually gave rise to a new area of physics: electrodynamics. By determining a force law for the interaction between elements of electric current, Ampère placed the subject on a solid mathematical foundation.

A theory of electromagnetism, known as classical electromagnetism, was developed by several physicists during the period between 1820 and 1873, when James Clerk Maxwell's treatise was published, which unified previous developments into a single theory, proposing that light was an electromagnetic wave propagating in the luminiferous ether. In classical electromagnetism, the behavior of the electromagnetic field is described by a set of equations known as Maxwell's equations, and the electromagnetic force is given by the Lorentz force law.

One of the peculiarities of classical electromagnetism is that it is difficult to reconcile with classical mechanics, but it is compatible with special relativity. According to Maxwell's equations, the speed of light in vacuum is a universal constant that is dependent only on the electrical permittivity and magnetic permeability of free space. This violates Galilean invariance, a long-standing cornerstone of classical mechanics. One way to reconcile the two theories (electromagnetism and classical mechanics) is to assume the existence of a luminiferous aether through which the light propagates. However, subsequent experimental efforts failed to detect the presence of the aether. After important contributions of Hendrik Lorentz and Henri Poincaré, in 1905, Albert Einstein solved the problem with the introduction of special relativity, which replaced classical kinematics with a new theory of kinematics compatible with classical electromagnetism. (For more information, see History of special relativity.)

In addition, relativity theory implies that in moving frames of reference, a magnetic field transforms to a field with a nonzero electric component and conversely, a moving electric field transforms to a nonzero magnetic component, thus firmly showing that the phenomena are two sides of the same coin. Hence the term "electromagnetism". (For more information, see Classical electromagnetism and special relativity and Covariant formulation of classical electromagnetism.)

Today few problems in electromagnetism remain unsolved. These include: the lack of magnetic monopoles, Abraham–Minkowski controversy, and the mechanism by which some organisms can sense electric and magnetic fields.

Extension to nonlinear phenomena

The Maxwell equations are linear, in that a change in the sources (the charges and currents) results in a proportional change of the fields. Nonlinear dynamics can occur when electromagnetic fields couple to matter that follows nonlinear dynamical laws. This is studied, for example, in the subject of magnetohydrodynamics, which combines Maxwell theory with the Navier–Stokes equations. Another branch of electromagnetism dealing with nonlinearity is nonlinear optics.

Quantities and units

See also: List of physical quantities and List of electromagnetism equations

Here is a list of common units related to electromagnetism:

• ampere (electric current)
• coulomb (electric charge)
• farad (capacitance)
• henry (inductance)
• ohm (resistance)
• siemens (conductance)
• tesla (magnetic flux density)
• volt (electric potential)
• watt (power)
• weber (magnetic flux)

In the electromagnetic CGS system, electric current is a fundamental quantity defined via Ampère's law and takes the permeability as a dimensionless quantity (relative permeability) whose value in vacuum is unity. As a consequence, the square of the speed of light appears explicitly in some of the equations interrelating quantities in this system.

SI electromagnetism units

Symbol	Name of quantity	Unit name	Symbol	Base units
E	energy	joule	J = C⋅V = W⋅s	kg⋅m2⋅s−2
Q	electric charge	coulomb	C	A⋅s
I	electric current	ampere	A = C/s = W/V	A
J	electric current density	ampere per square metre	A/m2	A⋅m−2
U, ΔV; Δϕ; E, ξ	potential difference; voltage; electromotive force	volt	V = J/C	kg⋅m2⋅s−3⋅A−1
R; Z; X	electric resistance; impedance; reactance	ohm	Ω = V/A	kg⋅m2⋅s−3⋅A−2
ρ	resistivity	ohm metre	Ω⋅m	kg⋅m3⋅s−3⋅A−2
P	electric power	watt	W = V⋅A	kg⋅m2⋅s−3
C	capacitance	farad	F = C/V	kg−1⋅m−2⋅A2⋅s4
ΦE	electric flux	volt metre	V⋅m	kg⋅m3⋅s−3⋅A−1
E	electric field strength	volt per metre	V/m = N/C	kg⋅m⋅A−1⋅s−3
D	electric displacement field	coulomb per square metre	C/m2	A⋅s⋅m−2
ε	permittivity	farad per metre	F/m	kg−1⋅m−3⋅A2⋅s4
χe	electric susceptibility	(dimensionless)	1	1
p	electric dipole moment	coulomb metre	C⋅m	A⋅s⋅m
G; Y; B	conductance; admittance; susceptance	siemens	S = Ω−1	kg−1⋅m−2⋅s3⋅A2
κ, γ, σ	conductivity	siemens per metre	S/m	kg−1⋅m−3⋅s3⋅A2
B	magnetic flux density, magnetic induction	tesla	T = Wb/m2 = N⋅A−1⋅m−1	kg⋅s−2⋅A−1
Φ, ΦM, ΦB	magnetic flux	weber	Wb = V⋅s	kg⋅m2⋅s−2⋅A−1
H	magnetic field strength	ampere per metre	A/m	A⋅m−1
F	magnetomotive force	ampere	A = Wb/H	A
R	magnetic reluctance	inverse henry	H−1 = A/Wb	kg−1⋅m−2⋅s2⋅A2
P	magnetic permeance	henry	H = Wb/A	kg⋅m2⋅s-2⋅A-2
L, M	inductance	henry	H = Wb/A = V⋅s/A	kg⋅m2⋅s−2⋅A−2
μ	permeability	henry per metre	H/m	kg⋅m⋅s−2⋅A−2
χ	magnetic susceptibility	(dimensionless)	1	1
m	magnetic dipole moment	ampere square meter	A⋅m2 = J⋅T−1	A⋅m2
σ	mass magnetization	ampere square meter per kilogram	A⋅m2/kg	A⋅m2⋅kg−1

Formulas for physical laws of electromagnetism (such as Maxwell's equations) need to be adjusted depending on what system of units one uses. This is because there is no one-to-one correspondence between electromagnetic units in SI and those in CGS, as is the case for mechanical units. Furthermore, within CGS, there are several plausible choices of electromagnetic units, leading to different unit "sub-systems", including Gaussian, "ESU", "EMU", and Heaviside–Lorentz. Among these choices, Gaussian units are the most common today, and in fact the phrase "CGS units" is often used to refer specifically to CGS-Gaussian units.

Applications

The study of electromagnetism informs electric circuits, magnetic circuits, and semiconductor devices' construction.

Electromagnetic theories of consciousness

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Electromagnetic_theories_of_consciousness

 

Electromagnetic theories of consciousness propose that consciousness can be understood as an electromagnetic phenomenon.

Overview

Theorists differ in how they relate consciousness to electromagnetism. Electromagnetic field theories (or "EM field theories") of consciousness propose that consciousness results when a brain produces an electromagnetic field with specific characteristics. Susan Pockett and Johnjoe McFadden have proposed EM field theories; William Uttal has criticized McFadden's and other field theories.

In general, quantum mind theories do not treat consciousness as an electromagnetic phenomenon, with a few exceptions.

AR Liboff has proposed that "incorporating EM field-mediated communication into models of brain function has the potential to reframe discussions surrounding consciousness".

Also related are E. Roy John's work and Andrew and Alexander Fingelkurts theory "Operational Architectonics framework of brain-mind functioning".

Cemi theory

The starting point for McFadden and Pockett's theory is the fact that every time a neuron fires to generate an action potential, and a postsynaptic potential in the next neuron down the line, it also generates a disturbance in the surrounding electromagnetic field. McFadden has proposed that the brain's electromagnetic field creates a representation of the information in the neurons. Studies undertaken towards the end of the 20th century are argued to have shown that conscious experience correlates not with the number of neurons firing, but with the synchrony of that firing. McFadden views the brain's electromagnetic field as arising from the induced EM field of neurons. The synchronous firing of neurons is, in this theory, argued to amplify the influence of the brain's EM field fluctuations to a much greater extent than would be possible with the unsynchronized firing of neurons.

McFadden thinks that the EM field could influence the brain in a number of ways. Redistribution of ions could modulate neuronal activity, given that voltage-gated ion channels are a key element in the progress of axon spikes. Neuronal firing is argued to be sensitive to the variation of as little as one millivolt across the cell membrane, or the involvement of a single extra ion channel. Transcranial magnetic stimulation is similarly argued to have demonstrated that weak EM fields can influence brain activity.

McFadden proposes that the digital information from neurons is integrated to form a conscious electromagnetic information (cemi) field in the brain. Consciousness is suggested to be the component of this field that is transmitted back to neurons, and communicates its state externally. Thoughts are viewed as electromagnetic representations of neuronal information, and the experience of free will in our choice of actions is argued to be our subjective experience of the cemi field acting on our neurons.

McFadden's view of free will is deterministic. Neurons generate patterns in the EM field, which in turn modulate the firing of particular neurons. There is only conscious agency in the sense that the field or its download to neurons is conscious, but the processes of the brain themselves are driven by deterministic electromagnetic interactions. The feel of subjective experience or qualia corresponds to a particular configuration of the cemi field. This field representation is in this theory argued to integrate parts into a whole that has meaning, so a face is not seen as a random collection of features, but as somebody's face. The integration of information in the field is also suggested to resolve the binding/combination problem.

In 2013, McFadden published two updates to the theory. In the first, 'The CEMI Field Theory: Closing the Loop' McFadden cites recent experiments in the laboratories of Christof Koch and David McCormick which demonstrate that external EM fields, that simulate the brain's endogenous EM fields, influence neuronal firing patterns within brain slices. The findings are consistent with a prediction of the cemi field theory that the brain's endogenous EM field - consciousness - influences brain function. In the second, 'The CEMI Field Theory Gestalt Information and the Meaning of Meaning', McFadden claims that the cemi field theory provides a solution to the binding problem of how complex information is unified within ideas to provide meaning: the brain's EM field unifies the information encoded in millions of disparate neurons.

Susan Pockett has advanced a theory, which has a similar physical basis to McFadden's, with consciousness seen as identical to certain spatiotemporal patterns of the EM field. However, whereas McFadden argues that his deterministic interpretation of the EM field is not out-of-line with mainstream thinking, Pockett suggests that the EM field comprises a universal consciousness that experiences the sensations, perceptions, thoughts and emotions of every conscious being in the universe. However, while McFadden thinks that the field is causal for actions, albeit deterministically, Pockett does not see the field as causal for our actions.

Quantum brain dynamics

Main article: Quantum brain dynamics

The concepts underlying this theory derive from the physicists, Hiroomi Umezawa and Herbert Fröhlich in the 1960s. More recently, their ideas have been elaborated by Mari Jibu and Kunio Yasue. Water comprises 70% of the brain, and quantum brain dynamics (QBD) proposes that the electric dipoles of the water molecules constitute a quantum field, referred to as the cortical field, with corticons as the quanta of the field. This cortical field is postulated to interact with quantum coherent waves generated by the biomolecules in neurons, which are suggested to propagate along the neuronal network. The idea of quantum coherent waves in the neuronal network derives from Fröhlich. He viewed these waves as a means by which order could be maintained in living systems, and argued that the neuronal network could support long-range correlation of dipoles. This theory suggests that the cortical field not only interacts with the neuronal network, but also to a good extent controls it.

The proponents of QBD differ somewhat as to the way in which consciousness arises in this system. Jibu and Yasue suggest that the interaction between the energy quanta (corticons) of the quantum field and the biomolecular waves of the neuronal network produces consciousness. However, another theorist, Giuseppe Vitiello, proposes that the quantum states produce two poles, a subjective representation of the external world and also the internal self.

Advantages

Locating consciousness in the brain's EM field, rather than the neurons, has the advantage of neatly accounting for how information located in millions of neurons scattered through the brain can be unified into a single conscious experience (called the binding problem): the information is unified in the EM field. In this way, EM field consciousness can be considered to be "joined-up information". This theory accounts for several otherwise puzzling facts, such as the finding that attention and awareness tend to be correlated with the synchronous firing of multiple neurons rather than the firing of individual neurons. When neurons fire together, their EM fields generate stronger EM field disturbances; so synchronous neuron firing will tend to have a larger impact on the brain's EM field (and thereby consciousness) than the firing of individual neurons. However their generation by synchronous firing is not the only important characteristic of conscious electromagnetic fields—in Pockett's original theory, spatial pattern is the defining feature of a conscious (as opposed to a non-conscious) field.

Objections

In a circa-2002 publication of The Journal of Consciousness Studies, the electromagnetic theory of consciousness faced an uphill battle for acceptance among cognitive scientists.

"No serious researcher I know believes in an electromagnetic theory of consciousness", Bernard Baars wrote in an e-mail. Baars is a neurobiologist and co-editor of Consciousness and Cognition, another scientific journal in the field. "It's not really worth talking about scientifically", he was quoted as saying.

McFadden acknowledges that his theory, which he calls the "cemi field theory", is far from proven but he argues that it is certainly a legitimate line of scientific inquiry. His article underwent peer review before publication.

The field theories of consciousness do not appear to have been as widely discussed as other quantum consciousness theories, such as those of Penrose, Stapp or Bohm. However, David Chalmers argues against quantum consciousness. He instead discusses how quantum mechanics may relate to dualistic consciousness. Chalmers is skeptical that any new physics can resolve the hard problem of consciousness. He argues that quantum theories of consciousness suffer from the same weakness as more conventional theories. Just as he argues that there is no particular reason why particular macroscopic physical features in the brain should give rise to consciousness, he also thinks that there is no particular reason why a particular quantum feature, such as the EM field in the brain, should give rise to consciousness either. Despite the existence of transcranial magnetic stimulation with medical purposes, Y. H. Sohn, A. Kaelin-Lang and M. Hallett have denied it, and later Jeffrey Gray states in his book Consciousness: Creeping up on the Hard Problem, that tests looking for the influence of electromagnetic fields on brain function have been universally negative in their result. However, a number of studies have found clear neural effects from EM stimulation.

• Dobson, et al. (2000): 1.8 millitesla = 18,000 mG
• Thomas, et al. (2007): 400 microtesla = 4000 milligauss
• Huesser, et al. (1997): 0.1 millitesla = 1000 mG
• Bell, et al. (2007) 0.78 Gauss = 780 mG
• Marino, et al. (2004): 1 Gauss = 1000 mG
• Carrubba, et al. (2008): 1 Gauss = 1000 mG
• Jacobson (1994): 5 picotesla = 0.00005 mG
• Sandyk (1999): Picotesla range

In April 2022, the results of two related experiments at the University of Alberta and Princeton University were announced at The Science of Consciousness conference, providing further evidence to support quantum processes operating within microtubules. In a study Stuart Hameroff was part of, Jack Tuszyński of the University of Alberta demonstrated that anesthetics hasten the duration of a process called delayed luminescence, in which microtubules and tubulins re-emit trapped light. Tuszyński suspects that the phenomenon has a quantum origin, with superradiance being investigated as one possibility. In the second experiment, Gregory D. Scholes and Aarat Kalra of Princeton University used lasers to excite molecules within tubulins, causing a prolonged excitation to diffuse through microtubules further than expected, which did not occur when repeated under anesthesia. However, diffusion results have to be interpreted carefully, since even classical diffusion can be very complex due to the wide range of length scales in the fluid filled extracellular space. Nevertheless, University of Oxford quantum physicist Vlatko Vedral told that this connection with consciousness is a really long shot.

Also in 2022, a group of Italian physicists conducted several experiments that failed to provide evidence in support of a gravity-related quantum collapse model of consciousness, weakening the possibility of a quantum explanation for consciousness.

Influence on brain function

The different EM field theories disagree as to the role of the proposed conscious EM field on brain function. In McFadden's cemi field theory, as well as in Drs Fingelkurts' Brain-Mind Operational Architectonics theory, the brain's global EM field modifies the electric charges across neural membranes, and thereby influences the probability that particular neurons will fire, providing a feed-back loop that drives free will. However, in the theories of Susan Pockett and E. Roy John, there is no necessary causal link between the conscious EM field and our consciously willed actions.

References to "Mag Lag" also known as the subtle effect on cognitive processes of MRI machine operators who sometimes have to go into the scanner room to check the patients and deal with issues that occur during the scan could suggest a link between magnetic fields and consciousness. Memory loss and delays in information processing have been reported, in some cases several hours after exposure.

One hypothesis is that magnetic fields in the 0.5-9 Tesla range can affect the ion permeability of neural membranes, in fact this could account for a lot of the issues seen as this would affect many different brain functions.

Implications for artificial intelligence

If true, the theory has major implications for efforts to design consciousness into artificial intelligence machines; current microprocessor technology is designed to transmit information linearly along electrical channels, and more general electromagnetic effects are seen as a nuisance and damped out; if this theory is right, however, this is directly counterproductive to creating an artificially conscious computer, which on some versions of the theory would instead have electromagnetic fields that synchronized its outputs—or in the original version of the theory would have spatially patterned electromagnetic fields.