Gerardus 't Hooft
Encyclopedia
Gerardus 't Hooft (ˌɣɪːrɑrt ət ˈɦoːft) (born July 5, 1946, Den Helder
, Netherlands
) is a Dutch
theoretical physicist and professor at Utrecht University
, the Netherlands
. He shared the 1999 Nobel Prize in Physics
with his thesis advisor Martinus J. G. Veltman
"for elucidating the quantum structure of electroweak interaction
s".
His work concentrates on gauge theory
, black hole
s, quantum gravity
and fundamental aspects of quantum mechanics. His contributions to physics include a proof that gauge theories are renormalizible
, dimensional regularization
, and the holographic principle
.
on July 5, 1946, but grew up in The Hague
, the seat of government of the Netherlands
. He was the middle child of a family of three. He comes from a family of scholars. His grandmother was a sister of Nobel prize laureate Fritz Zernike, and was married to Pieter Nicolaas van Kampen, who was a well-known professor of zoology
at Leiden university
. His uncle Nico van Kampen is an (emeritus) professor of theoretical physics at Utrecht University, and while his mother did not opt for a scientific career because she was a girl, she did marry a maritime engineer. Following his family's footsteps, he showed interest in science at an early age. When his primary school teacher asked him what he wanted to be when he grew up, he boldly declared, "a man who knew everything."
After primary school Gerard attended the Dalton Lyceum, a school that applied the ideas of the Dalton Plan
, an educational method that suited him well. He easily passed his science and mathematics courses, but struggled with his language courses. Nonetheless, he passed his classes in English, French, German, classical Greek and Latin
. At the age of sixteen he earned a silver medal in the second Dutch Math Olympiad.
with their rowing club "Triton" and organized a national congress for science students with their science discussion club "Christiaan Huygens".
In the course of his studies he decided he wanted to go into what he perceived as the heart of theoretical physics, elementary particles. His uncle had grown to dislike the subject and in particular its practitioners, so when it became time to write his 'doctoraalscriptie' (Dutch equivalent of a master's thesis) in 1968, 't Hooft turned to the newly appointed professor Martinus Veltman, who specialized in Yang–Mills theory, a relative fringe subject at the time because it was thought that these could not be renormalized
. His assignment was to study the Adler–Bell–Jackiw anomaly, a mismatch in the theory of the decay of neutral pion
s; formal arguments forbid the decay into photon
s, whereas practical calculations and experiments showed that this was the primary form of decay. The resolution of the problem was completely unknown at the time, and 't Hooft was unable to provide one.
In 1969, 't Hooft started on his PhD with Martinus Veltman as his advisor. He would work on the same subject Veltman was working on, the renormalization of Yang–Mills theories. In 1971 his first paper was published. In it he showed how to renormalize massless Yang–Mills fields, and was able to derive relations between amplitudes, which would be generalized by Andrei Slavnov and J.C. Taylor, and become known as the Slavnov–Taylor identities.
The world took little notice, but Veltman was excited because he saw that the problem he had been working on was solved. A period of intense collaboration followed in which they developed the technique of dimensional regularization
. Soon 't Hooft's second paper was ready to be published, in which he showed that Yang–Mills theories with massive fields due to spontaneous symmetry breaking could be renormalized. This paper earned them world wide recognition, and would ultimately earn the pair the 1999 Nobel Prize in Physics.
These two papers formed the basis of 't Hooft's dissertation, The Renormalization procedure for Yang-Mills Fields, and he obtained in his PhD in 1972. In the same year he married his wife, Albertha A. Schik, a student of medicine
in Utrecht.
in Geneva, where he had a fellowship. He further refined his methods for Yang-Mills theories with Veltman (who went back to Geneva). In this time he became interested in the possibility that the strong interaction
could be described as a massless Yang-Mills theory, i.e. one of a type that he had just proved to be renormalizable and hence be susceptible to detailed calculation and comparison with experiment. According to his calculations, this type of theory possessed just the right kind of scaling properties (asymptotic freedom
) that this theory should have according to deep inelastic scattering
experiments. This was contrary to popular perception of Yang-Mills theories at the time, that like gravitation and electrodynamics, their intensity should decrease with increasing distance between the interacting particles; such conventional behaviour with distance was unable to explain the results of deep inelastic scattering, whereas 't Hooft's calculations could. When he mentioned his results at a small conference at Marseilles in 1972, Kurt Symanzik
urged him to publish this result. He did not, and the result was eventually rediscovered and published by Hugh David Politzer
, David Gross
, and Frank Wilczek
in 1973, which led to them earning the 2004 Nobel Prize in Physics.
In 1974, 't Hooft returned to Utrecht where he became assistant professor. In 1976, he was invited for a guest position at Stanford and a position at Harvard as Morris Loeb lecturer. His eldest daughter, Saskia Anne, was born in Boston
, while his second daughter, Ellen Marga, was born in 1978 after he returned to Utrecht, where he was made full professor. He will officially retire from this position — becoming an emeritus professor — on his 65th birthday, July 5, 2011.
, awarded every four years in recognition of the most important contributions in theoretical physics. In 1995, he was one of the first recipients of the Spinozapremie, the highest award available to scientists in the Netherlands. In the same year he was also honoured with a Franklin Medal
Since his Nobel Prize, 't Hooft has received a slew of awards, honorary doctorates and honorary professorships. He was knighted commander in the Order of the Netherlands Lion, and officer in the French Legion of Honor. The asteroid 9491 Thooft
has been named in his honor, and he has written a constitution for its future inhabitants.
He is a member of the Royal Netherlands Academy of Arts and Sciences
(KNAW), where he was made academy professor in 2003. He is also a foreign member of many other science academies, including the French Académie des Sciences, the American National Academy of Sciences
and American Academy of Arts and Sciences
and the English Institute of Physics
.
After his PhD, he became interested in the role of gauge theories in the strong interaction, the leading theory of which is called quantum chromodynamics
or QCD. Much of his research focused on the problem of colour confinement
in QCD, i.e. the observational fact that only colour neutral particles are observed at low energies. This lead led him to the discovery that SU(N) gauge theories simplify in the large N limit
, a fact which has proved important in the examination of the conjectured correspondence
between string theories
in an Anti de Sitter space
and conformal field theories
in one lower dimension. By solving the theory in one space and one time dimension, 't Hooft was able to derive a formula for the masses of mesons.
He also studied the role of so called instanton
contributions in QCD. His calculation showed that these contributions lead to an interaction between light quark
s at low energies not present in the normal theory. Studying instanton solutions of Yang-Mills theories, 't Hooft discovered that spontaneously breaking
a theory with SU(N) symmetry to a U(1) symmetry will lead to the existence of magnetic monopole
s. These monopoles are called 't Hooft–Polyakov monopoles, after Alexander Polyakov
, who independently obtained the same result.
As another piece in the colour confinement puzzle 't Hooft introduced 't Hooft operator
s, which are the magnetic dual
of Wilson loop
s. Using these operators he was able to classify different phases
of QCD, which form the basis of the QCD phase diagram
.
In 1986, he was finally able to show that instanton contributions solve the Adler–Bell–Jackiw anomaly, the topic of his doctoral thesis.
was not completely renormalizible, they felt important lessons were to be learned by studying the formal renormalization of the theory order by order. This work would be continued by Stanley Deser
and another PhD student of Veltman, Peter van Nieuwenhuizen
, who later found patterns in the renormalization counter terms, which led to the discovery of supergravity
.
In the 1980s, 't Hooft's attention was drawn to the subject of gravity in 3 spacetime dimensions. Together with Deser and Jackiw he published an article in 1984 describing the dynamics of flat space where the only local degrees of freedom were propagating point defects. His attention returned to this model at various points in time, showning that Gott pairs would not cause causality
violating timelike loops, and showing how the model could be quantized. More recently he proposed generalizing this piecewise flat model of gravity to 4 spacetime dimensions.
With Stephen Hawking
's discovery of Hawking radiation
of black hole
s, it appeared that the evaporation of these objects violated a fundamental property of quantum mechanics, unitarity
. 't Hooft refused to accept this problem, known as the black hole information paradox
, and assumed that this must be the result of the semi-classical treatment of Hawking, and that it should not appear in a full theory quantum gravity. He proposed that it might be possible to study some of the properties of such a theory, by assuming that such a theory was unitary.
Using this approach he was able to show that near a black hole, quantum fields could be described by a theory in a lower dimension. This led to introduction of the holographic principle
by him and Leonard Susskind
.
of quantum theory
". He believes that there should be a deterministic theory underlying quantum mechanics. Using a toy model he has shown how such a theory could avoid the usual Bell inequality arguments that disallow such a hidden variable theory
.
Den Helder
Den Helder is a municipality and a city in the Netherlands, in the province of North Holland. Den Helder occupies the northernmost point of the North Holland peninsula...
, Netherlands
Netherlands
The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...
) is a Dutch
Netherlands
The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...
theoretical physicist and professor at Utrecht University
Utrecht University
Utrecht University is a university in Utrecht, Netherlands. It is one of the oldest universities in the Netherlands and one of the largest in Europe. Established March 26, 1636, it had an enrollment of 29,082 students in 2008, and employed 8,614 faculty and staff, 570 of which are full professors....
, the Netherlands
Netherlands
The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...
. He shared the 1999 Nobel Prize in Physics
Nobel Prize in Physics
The Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and...
with his thesis advisor Martinus J. G. Veltman
Martinus J. G. Veltman
Martinus Justinus Godefriedus Veltman is a Dutch theoretical physicist. He shared the 1999 Nobel Prize in physics with his former student Gerardus 't Hooft for their work on particle theory.-Biography:...
"for elucidating the quantum structure of electroweak interaction
Electroweak interaction
In particle physics, the electroweak interaction is the unified description of two of the four known fundamental interactions of nature: electromagnetism and the weak interaction. Although these two forces appear very different at everyday low energies, the theory models them as two different...
s".
His work concentrates on gauge theory
Gauge theory
In physics, gauge invariance is the property of a field theory in which different configurations of the underlying fundamental but unobservable fields result in identical observable quantities. A theory with such a property is called a gauge theory...
, black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
s, quantum gravity
Quantum gravity
Quantum gravity is the field of theoretical physics which attempts to develop scientific models that unify quantum mechanics with general relativity...
and fundamental aspects of quantum mechanics. His contributions to physics include a proof that gauge theories are renormalizible
Renormalization
In quantum field theory, the statistical mechanics of fields, and the theory of self-similar geometric structures, renormalization is any of a collection of techniques used to treat infinities arising in calculated quantities....
, dimensional regularization
Dimensional regularization
In theoretical physics, dimensional regularization is a method introduced by Giambiagi and Bollini for regularizing integrals in the evaluation of Feynman diagrams; in other words, assigning values to them that are meromorphic functions of an auxiliary complex parameter d, called the...
, and the holographic principle
Holographic principle
The holographic principle is a property of quantum gravity and string theories which states that the description of a volume of space can be thought of as encoded on a boundary to the region—preferably a light-like boundary like a gravitational horizon...
.
Personal life
He is married to Albertha Schik (Betteke) and has two daughters, Saskia and Ellen. Saskia has translated one of her father's popular Dutch fiction books 'Planetenbiljart' into English. The book's title is 'Playing with Planets' and was launched in Singapore in November 2008.Early life
Gerard 't Hooft was born in Den HelderDen Helder
Den Helder is a municipality and a city in the Netherlands, in the province of North Holland. Den Helder occupies the northernmost point of the North Holland peninsula...
on July 5, 1946, but grew up in The Hague
The Hague
The Hague is the capital city of the province of South Holland in the Netherlands. With a population of 500,000 inhabitants , it is the third largest city of the Netherlands, after Amsterdam and Rotterdam...
, the seat of government of the Netherlands
Netherlands
The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...
. He was the middle child of a family of three. He comes from a family of scholars. His grandmother was a sister of Nobel prize laureate Fritz Zernike, and was married to Pieter Nicolaas van Kampen, who was a well-known professor of zoology
Zoology
Zoology |zoölogy]]), is the branch of biology that relates to the animal kingdom, including the structure, embryology, evolution, classification, habits, and distribution of all animals, both living and extinct...
at Leiden university
Leiden University
Leiden University , located in the city of Leiden, is the oldest university in the Netherlands. The university was founded in 1575 by William, Prince of Orange, leader of the Dutch Revolt in the Eighty Years' War. The royal Dutch House of Orange-Nassau and Leiden University still have a close...
. His uncle Nico van Kampen is an (emeritus) professor of theoretical physics at Utrecht University, and while his mother did not opt for a scientific career because she was a girl, she did marry a maritime engineer. Following his family's footsteps, he showed interest in science at an early age. When his primary school teacher asked him what he wanted to be when he grew up, he boldly declared, "a man who knew everything."
After primary school Gerard attended the Dalton Lyceum, a school that applied the ideas of the Dalton Plan
Dalton Plan
The Dalton Plan is an educational concept created by Helen Parkhurst.Inspired by the intellectual ferment at the turn of the 19th century, educational thinkers such as Maria Montessori and John Dewey began to cast a bold vision of a new progressive approach to education...
, an educational method that suited him well. He easily passed his science and mathematics courses, but struggled with his language courses. Nonetheless, he passed his classes in English, French, German, classical Greek and Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
. At the age of sixteen he earned a silver medal in the second Dutch Math Olympiad.
Education
After Gerard 't Hooft passed his high school exams in 1964, he enrolled in the physics program at Utrecht University. He opted for Utrecht instead of the much closer Leiden, because his uncle was a professor there and he wanted to attend his lectures. Because he was so focused on science, his father insisted that he join the Utrechtsch Studenten Corps, an elite student association, in the hope that he would do something else besides studying. This worked to some extent, during his studies he was a coxswainCoxswain
The coxswain is the person in charge of a boat, particularly its navigation and steering. The etymology of the word gives us a literal meaning of "boat servant" since it comes from cox, a coxboat or other small vessel kept aboard a ship, and swain, which can be rendered as boy, in authority. ...
with their rowing club "Triton" and organized a national congress for science students with their science discussion club "Christiaan Huygens".
In the course of his studies he decided he wanted to go into what he perceived as the heart of theoretical physics, elementary particles. His uncle had grown to dislike the subject and in particular its practitioners, so when it became time to write his 'doctoraalscriptie' (Dutch equivalent of a master's thesis) in 1968, 't Hooft turned to the newly appointed professor Martinus Veltman, who specialized in Yang–Mills theory, a relative fringe subject at the time because it was thought that these could not be renormalized
Renormalization
In quantum field theory, the statistical mechanics of fields, and the theory of self-similar geometric structures, renormalization is any of a collection of techniques used to treat infinities arising in calculated quantities....
. His assignment was to study the Adler–Bell–Jackiw anomaly, a mismatch in the theory of the decay of neutral pion
Pion
In particle physics, a pion is any of three subatomic particles: , , and . Pions are the lightest mesons and they play an important role in explaining the low-energy properties of the strong nuclear force....
s; formal arguments forbid the decay into photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s, whereas practical calculations and experiments showed that this was the primary form of decay. The resolution of the problem was completely unknown at the time, and 't Hooft was unable to provide one.
In 1969, 't Hooft started on his PhD with Martinus Veltman as his advisor. He would work on the same subject Veltman was working on, the renormalization of Yang–Mills theories. In 1971 his first paper was published. In it he showed how to renormalize massless Yang–Mills fields, and was able to derive relations between amplitudes, which would be generalized by Andrei Slavnov and J.C. Taylor, and become known as the Slavnov–Taylor identities.
The world took little notice, but Veltman was excited because he saw that the problem he had been working on was solved. A period of intense collaboration followed in which they developed the technique of dimensional regularization
Dimensional regularization
In theoretical physics, dimensional regularization is a method introduced by Giambiagi and Bollini for regularizing integrals in the evaluation of Feynman diagrams; in other words, assigning values to them that are meromorphic functions of an auxiliary complex parameter d, called the...
. Soon 't Hooft's second paper was ready to be published, in which he showed that Yang–Mills theories with massive fields due to spontaneous symmetry breaking could be renormalized. This paper earned them world wide recognition, and would ultimately earn the pair the 1999 Nobel Prize in Physics.
These two papers formed the basis of 't Hooft's dissertation, The Renormalization procedure for Yang-Mills Fields, and he obtained in his PhD in 1972. In the same year he married his wife, Albertha A. Schik, a student of medicine
Medicine
Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....
in Utrecht.
Career
After obtaining his doctorate 't Hooft went to CERNCERN
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...
in Geneva, where he had a fellowship. He further refined his methods for Yang-Mills theories with Veltman (who went back to Geneva). In this time he became interested in the possibility that the strong interaction
Strong interaction
In particle physics, the strong interaction is one of the four fundamental interactions of nature, the others being electromagnetism, the weak interaction and gravitation. As with the other fundamental interactions, it is a non-contact force...
could be described as a massless Yang-Mills theory, i.e. one of a type that he had just proved to be renormalizable and hence be susceptible to detailed calculation and comparison with experiment. According to his calculations, this type of theory possessed just the right kind of scaling properties (asymptotic freedom
Asymptotic freedom
In physics, asymptotic freedom is a property of some gauge theories that causes interactions between particles to become arbitrarily weak at energy scales that become arbitrarily large, or, equivalently, at length scales that become arbitrarily small .Asymptotic freedom is a feature of quantum...
) that this theory should have according to deep inelastic scattering
Deep Inelastic Scattering
Deep inelastic scattering is the name given to a process used to probe the insides of hadrons , using electrons, muons and neutrinos. It provided the first convincing evidence of the reality of quarks, which up until that point had been considered by many to be a purely mathematical phenomenon...
experiments. This was contrary to popular perception of Yang-Mills theories at the time, that like gravitation and electrodynamics, their intensity should decrease with increasing distance between the interacting particles; such conventional behaviour with distance was unable to explain the results of deep inelastic scattering, whereas 't Hooft's calculations could. When he mentioned his results at a small conference at Marseilles in 1972, Kurt Symanzik
Kurt Symanzik
Kurt Symanzik was a German physicist working in quantum field theory.- Life :Symanzik was born in Lyck , East Prussia, and spent his childhood in Königsberg. He started studying physics in 1946 at Universität München but after a short time moved to Werner Heisenberg at Göttingen...
urged him to publish this result. He did not, and the result was eventually rediscovered and published by Hugh David Politzer
Hugh David Politzer
Hugh David Politzer is an American theoretical physicist. He shared the 2004 Nobel Prize in Physics with David Gross and Frank Wilczek for their discovery of asymptotic freedom in quantum chromodynamics.-Life and career:...
, David Gross
David Gross
David Jonathan Gross is an American particle physicist and string theorist. Along with Frank Wilczek and David Politzer, he was awarded the 2004 Nobel Prize in Physics for their discovery of asymptotic freedom. He is currently the director and holder of the Frederick W...
, and Frank Wilczek
Frank Wilczek
Frank Anthony Wilczek is a theoretical physicist from the United States and a Nobel laureate. He is currently the Herman Feshbach Professor of Physics at the Massachusetts Institute of Technology ....
in 1973, which led to them earning the 2004 Nobel Prize in Physics.
In 1974, 't Hooft returned to Utrecht where he became assistant professor. In 1976, he was invited for a guest position at Stanford and a position at Harvard as Morris Loeb lecturer. His eldest daughter, Saskia Anne, was born in Boston
Boston
Boston is the capital of and largest city in Massachusetts, and is one of the oldest cities in the United States. The largest city in New England, Boston is regarded as the unofficial "Capital of New England" for its economic and cultural impact on the entire New England region. The city proper had...
, while his second daughter, Ellen Marga, was born in 1978 after he returned to Utrecht, where he was made full professor. He will officially retire from this position — becoming an emeritus professor — on his 65th birthday, July 5, 2011.
Honors
In 1999 't Hooft shared the Nobel prize in Physics with his thesis adviser Veltman for "elucidating the quantum structure of the electroweak interactions in physics". Before that time his work had already been recognized by other notable awards. In 1981, he was awarded the Wolf Prize, possibly the most prestigious prize in physics after the Nobel prize. Five years later he received the Lorentz MedalLorentz Medal
Lorentz Medal is a prize awarded every four years by the Royal Netherlands Academy of Arts and Sciences. It was established in 1925 on the occasion of the 50th anniversary of the doctorate of Hendrik Lorentz. This solid gold medal is given for important contributions to theoretical physics, though...
, awarded every four years in recognition of the most important contributions in theoretical physics. In 1995, he was one of the first recipients of the Spinozapremie, the highest award available to scientists in the Netherlands. In the same year he was also honoured with a Franklin Medal
Franklin Medal
The Franklin Medal was a science and engineering award presented by the Franklin Institute, of Philadelphia, PA, USA.-Laureates:*1915 - Thomas Alva Edison *1915 - Heike Kamerlingh Onnes *1916 - John J...
Since his Nobel Prize, 't Hooft has received a slew of awards, honorary doctorates and honorary professorships. He was knighted commander in the Order of the Netherlands Lion, and officer in the French Legion of Honor. The asteroid 9491 Thooft
9491 Thooft
Asteroid 9491 Thooft is named for 1999 Nobel physics laureate Gerardus 't Hooft. He has drawn up a constitution for future inhabitants of the asteroid .-External links:*...
has been named in his honor, and he has written a constitution for its future inhabitants.
He is a member of the Royal Netherlands Academy of Arts and Sciences
Royal Netherlands Academy of Arts and Sciences
The Royal Netherlands Academy of Arts and Sciences is an organisation dedicated to the advancement of science and literature in the Netherlands...
(KNAW), where he was made academy professor in 2003. He is also a foreign member of many other science academies, including the French Académie des Sciences, the American National Academy of Sciences
United States National Academy of Sciences
The National Academy of Sciences is a corporation in the United States whose members serve pro bono as "advisers to the nation on science, engineering, and medicine." As a national academy, new members of the organization are elected annually by current members, based on their distinguished and...
and American Academy of Arts and Sciences
American Academy of Arts and Sciences
The American Academy of Arts and Sciences is an independent policy research center that conducts multidisciplinary studies of complex and emerging problems. The Academy’s elected members are leaders in the academic disciplines, the arts, business, and public affairs.James Bowdoin, John Adams, and...
and the English Institute of Physics
Institute of Physics
The Institute of Physics is a scientific charity devoted to increasing the practice, understanding and application of physics. It has a worldwide membership of around 40,000....
.
Research
't Hooft's research interest can be divided in three main directions: 'gauge theories in elementary particle physics', 'quantum gravity and black holes', and 'foundational aspects of quantum mechanics'.Gauge theories in elementary particle physics
't Hooft is most famous for his contributions to the development of gauge theories in particle physics. The best known of these is the proof in his PhD thesis that Yang–Mills theories are renormalizable, for which he shared the 1999 Nobel Prize in Physics. For this proof he introduced (with his adviser Veltman) the technique of dimensional regularization.After his PhD, he became interested in the role of gauge theories in the strong interaction, the leading theory of which is called quantum chromodynamics
Quantum chromodynamics
In theoretical physics, quantum chromodynamics is a theory of the strong interaction , a fundamental force describing the interactions of the quarks and gluons making up hadrons . It is the study of the SU Yang–Mills theory of color-charged fermions...
or QCD. Much of his research focused on the problem of colour confinement
Colour confinement
Color confinement, often simply called confinement, is the physics phenomenon that color charged particles cannot be isolated singularly, and therefore cannot be directly observed. Quarks, by default, clump together to form groups, or hadrons. The two types of hadrons are the mesons and the baryons...
in QCD, i.e. the observational fact that only colour neutral particles are observed at low energies. This lead led him to the discovery that SU(N) gauge theories simplify in the large N limit
1/N expansion
In quantum field theory and statistical mechanics, the 1/N expansion is a particular perturbative analysis of quantum field theories with an internal symmetry group such as SO or SU...
, a fact which has proved important in the examination of the conjectured correspondence
AdS/CFT correspondence
In physics, the AdS/CFT correspondence , sometimes called the Maldacena duality, is the conjectured equivalence between a string theory and gravity defined on one space, and a quantum field theory without gravity defined on the conformal boundary of this space, whose dimension is lower by one or more...
between string theories
String theory
String theory is an active research framework in particle physics that attempts to reconcile quantum mechanics and general relativity. It is a contender for a theory of everything , a manner of describing the known fundamental forces and matter in a mathematically complete system...
in an Anti de Sitter space
Anti de Sitter space
In mathematics and physics, n-dimensional anti de Sitter space, sometimes written AdS_n, is a maximally symmetric Lorentzian manifold with constant negative scalar curvature...
and conformal field theories
Conformal field theory
A conformal field theory is a quantum field theory that is invariant under conformal transformations...
in one lower dimension. By solving the theory in one space and one time dimension, 't Hooft was able to derive a formula for the masses of mesons.
He also studied the role of so called instanton
Instanton
An instanton is a notion appearing in theoretical and mathematical physics. Mathematically, a Yang–Mills instanton is a self-dual or anti-self-dual connection in a principal bundle over a four-dimensional Riemannian manifold that plays the role of physical space-time in non-abelian gauge theory...
contributions in QCD. His calculation showed that these contributions lead to an interaction between light quark
Quark
A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never directly...
s at low energies not present in the normal theory. Studying instanton solutions of Yang-Mills theories, 't Hooft discovered that spontaneously breaking
Spontaneous symmetry breaking
Spontaneous symmetry breaking is the process by which a system described in a theoretically symmetrical way ends up in an apparently asymmetric state....
a theory with SU(N) symmetry to a U(1) symmetry will lead to the existence of magnetic monopole
Magnetic monopole
A magnetic monopole is a hypothetical particle in particle physics that is a magnet with only one magnetic pole . In more technical terms, a magnetic monopole would have a net "magnetic charge". Modern interest in the concept stems from particle theories, notably the grand unified and superstring...
s. These monopoles are called 't Hooft–Polyakov monopoles, after Alexander Polyakov
Alexander Polyakov
Alexander Markovich Polyakov is a theoretical physicist, formerly at the Landau Institute in Moscow, at Princeton University.-Important discoveries:...
, who independently obtained the same result.
As another piece in the colour confinement puzzle 't Hooft introduced 't Hooft operator
't Hooft operator
In theoretical physics, a 't Hooft operator, introduced by Gerard 't Hooft in the 1978 paper is a dual version of the Wilson loop in which the electromagnetic potential A is replaced by its electromagnetic dual Amag where the exterior derivative of A is equal to the Hodge dual of the exterior...
s, which are the magnetic dual
S-duality
In theoretical physics, S-duality is an equivalence of two quantum field theories or string theories. An S-duality transformation maps states and vacua with coupling constant g in one theory to states and vacua with coupling constant 1/g in the dual theory...
of Wilson loop
Wilson loop
In gauge theory, a Wilson loop is a gauge-invariant observable obtained from the holonomy of the gauge connection around a given loop...
s. Using these operators he was able to classify different phases
Phase (matter)
In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...
of QCD, which form the basis of the QCD phase diagram
QCD matter
Quark matter or QCD matter refers to any of a number of theorized phases of matter whose degrees of freedom include quarks and gluons. These theoretical phases would occur at extremely high temperatures and densities, billions of times higher than can be produced in equilibrium in laboratories...
.
In 1986, he was finally able to show that instanton contributions solve the Adler–Bell–Jackiw anomaly, the topic of his doctoral thesis.
Quantum gravity and black holes
When Veltman and 't Hooft moved to CERN after 't Hooft obtained his PhD, Veltman's attention was drawn to the possibility of using their dimensional regularization techniques to the problem of quantizing gravity. Although it was known that perturbative quantum gravityQuantum gravity
Quantum gravity is the field of theoretical physics which attempts to develop scientific models that unify quantum mechanics with general relativity...
was not completely renormalizible, they felt important lessons were to be learned by studying the formal renormalization of the theory order by order. This work would be continued by Stanley Deser
Stanley Deser
Stanley Deser is an American physicist known for his contributions to general relativity. Currently, he is the Ancell Professor of Physics at Brandeis University in Waltham, Massachusetts....
and another PhD student of Veltman, Peter van Nieuwenhuizen
Peter van Nieuwenhuizen
Peter van Nieuwenhuizen is a Dutch physicist. He is now a distinguished Professor at Stony Brook University in the United States. Van Nieuwenhuizen is best-known for his discovery of supergravity with Sergio Ferrara and Daniel Z...
, who later found patterns in the renormalization counter terms, which led to the discovery of supergravity
Supergravity
In theoretical physics, supergravity is a field theory that combines the principles of supersymmetry and general relativity. Together, these imply that, in supergravity, the supersymmetry is a local symmetry...
.
In the 1980s, 't Hooft's attention was drawn to the subject of gravity in 3 spacetime dimensions. Together with Deser and Jackiw he published an article in 1984 describing the dynamics of flat space where the only local degrees of freedom were propagating point defects. His attention returned to this model at various points in time, showning that Gott pairs would not cause causality
Causality (physics)
Causality is the relationship between causes and effects. It is considered to be fundamental to all natural science, especially physics. Causality is also a topic studied from the perspectives of philosophy and statistics....
violating timelike loops, and showing how the model could be quantized. More recently he proposed generalizing this piecewise flat model of gravity to 4 spacetime dimensions.
With Stephen Hawking
Stephen Hawking
Stephen William Hawking, CH, CBE, FRS, FRSA is an English theoretical physicist and cosmologist, whose scientific books and public appearances have made him an academic celebrity...
's discovery of Hawking radiation
Hawking radiation
Hawking radiation is a thermal radiation with a black body spectrum predicted to be emitted by black holes due to quantum effects. It is named after the physicist Stephen Hawking, who provided a theoretical argument for its existence in 1974, and sometimes also after the physicist Jacob Bekenstein...
of black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
s, it appeared that the evaporation of these objects violated a fundamental property of quantum mechanics, unitarity
Unitarity (physics)
In quantum physics, unitarity is a restriction on the allowed evolution of quantum systems that insures the sum of probabilities of all possible outcomes of any event is always 1....
. 't Hooft refused to accept this problem, known as the black hole information paradox
Black hole information paradox
The black hole information paradox results from the combination of quantum mechanics and general relativity. It suggests that physical information could disappear in a black hole, allowing many physical states to evolve into the same state...
, and assumed that this must be the result of the semi-classical treatment of Hawking, and that it should not appear in a full theory quantum gravity. He proposed that it might be possible to study some of the properties of such a theory, by assuming that such a theory was unitary.
Using this approach he was able to show that near a black hole, quantum fields could be described by a theory in a lower dimension. This led to introduction of the holographic principle
Holographic principle
The holographic principle is a property of quantum gravity and string theories which states that the description of a volume of space can be thought of as encoded on a boundary to the region—preferably a light-like boundary like a gravitational horizon...
by him and Leonard Susskind
Leonard Susskind
Leonard Susskind is the Felix Bloch Professor of Theoretical Physics at Stanford University. His research interests include string theory, quantum field theory, quantum statistical mechanics and quantum cosmology...
.
Fundamental aspects of quantum mechanics
't Hooft has "deviating views on the physical interpretationInterpretation of quantum mechanics
An interpretation of quantum mechanics is a set of statements which attempt to explain how quantum mechanics informs our understanding of nature. Although quantum mechanics has held up to rigorous and thorough experimental testing, many of these experiments are open to different interpretations...
of quantum theory
Quantum theory
Quantum theory may mean:In science:*Quantum mechanics: a subset of quantum physics explaining the physical behaviours at atomic and sub-atomic levels*Old quantum theory under the Bohr model...
". He believes that there should be a deterministic theory underlying quantum mechanics. Using a toy model he has shown how such a theory could avoid the usual Bell inequality arguments that disallow such a hidden variable theory
Hidden variable theory
Historically, in physics, hidden variable theories were espoused by some physicists who argued that quantum mechanics is incomplete. These theories argue against the orthodox interpretation of quantum mechanics, which is the Copenhagen Interpretation...
.
See also
- Anomaly matching conditionAnomaly matching conditionIn quantum field theory, the anomaly matching condition by Gerard 't Hooft states that the calculation of any chiral anomaly by using the degrees of freedom of the theory at some energy scale, must not depend on what scale is chosen for the calculation...
- Feynman–'t Hooft gauge
- Minimal subtraction schemeMinimal subtraction schemeIn quantum field theory, the minimal subtraction scheme, or MS scheme is a particular renormalization scheme used to absorb the infinities that arise in perturbative calculations beyond leading order, introduced independently by and...
- Naturalness (physics)Naturalness (physics)Naturalness is the property that all parameters appearing in a theory take values of order 1...
- RenormalonRenormalonIn quantum field theory, renormalon is a term suggested by 't Hooft to signify a certain type of singularities and contributions. The renormalon singularity is a possible type of singularities arising in the complex Borel plane, as counterpart of instanton singularities...
- 't Hooft symbol't Hooft symbolThe t Hooft η symbol is a symbol which allows one to express the generators of the SU Lie algebra in terms of the generators of Lorentz algebra. The symbol is a blend between the Kronecker delta and the Levi-Civita symbol. It was introduced by Gerard 't Hooft...
External links
- Gerard 't Hooft (homepage)
- How To Become a Good Theoretical Physicist
- The Nobel Prize in Physics 1999
- A Confrontation with Infinity, Nobel Lecture
- Publications from Google ScholarGoogle ScholarGoogle Scholar is a freely accessible web search engine that indexes the full text of scholarly literature across an array of publishing formats and disciplines. Released in beta in November 2004, the Google Scholar index includes most peer-reviewed online journals of Europe and America's largest...
- Publications on the ArxivArXivThe arXiv |Chi]], χ) is an archive for electronic preprints of scientific papers in the fields of mathematics, physics, astronomy, computer science, quantitative biology, statistics, and quantitative finance which can be accessed online. In many fields of mathematics and physics, almost all...
- TVO.org video - Gerard t'Hooft lectures on Science Fiction and Reality Lecture delivered at the Perimeter Institute in Waterloo, Ontario, Canada on May 7, 2008