William George Unruh (; born August 28, 1945) is a

Canadian Canadians () are people identified with the country of Canada. This connection may be residential, legal, historical or cultural. For most Canadians, many (or all) of these connections exist and are collectively the source of their being ''C ...

physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate cau ...

at the

University of British Columbia The University of British Columbia (UBC) is a Public university, public research university with campuses near University of British Columbia Vancouver, Vancouver and University of British Columbia Okanagan, Kelowna, in British Columbia, Canada ...

Vancouver Vancouver is a major city in Western Canada, located in the Lower Mainland region of British Columbia. As the List of cities in British Columbia, most populous city in the province, the 2021 Canadian census recorded 662,248 people in the cit ...

who described the hypothetical

Unruh effect The Unruh effect (also known as the Fulling–Davies–Unruh effect) is a theoretical prediction in quantum field theory that an observer who is uniformly accelerating through empty space will perceive a thermal bath. This means that even in the ...

in 1976.

Early life and education

Unruh was born into a

Mennonite Mennonites are a group of Anabaptism, Anabaptist Christianity, Christian communities tracing their roots to the epoch of the Radical Reformation. The name ''Mennonites'' is derived from the cleric Menno Simons (1496–1561) of Friesland, part of ...

family in

Winnipeg Winnipeg () is the capital and largest city of the Provinces and territories of Canada, Canadian province of Manitoba. It is centred on the confluence of the Red River of the North, Red and Assiniboine River, Assiniboine rivers. , Winnipeg h ...

Manitoba Manitoba is a Provinces and territories of Canada, province of Canada at the Centre of Canada, longitudinal centre of the country. It is Canada's Population of Canada by province and territory, fifth-most populous province, with a population ...

. His parents were Benjamin Unruh, a refugee from

Russia Russia, or the Russian Federation, is a country spanning Eastern Europe and North Asia. It is the list of countries and dependencies by area, largest country in the world, and extends across Time in Russia, eleven time zones, sharing Borders ...

, and Anna Janzen, who was born in Canada. He obtained his B.Sc. from the

University of Manitoba The University of Manitoba (U of M, UManitoba, or UM) is a public research university in Winnipeg, Manitoba, Canada. Founded in 1877, it is the first university of Western Canada. Both by total student enrolment and campus area, the University of ...

in 1967, followed by an M.A. (1969) and Ph.D. (1971) from

Princeton University Princeton University is a private university, private Ivy League research university in Princeton, New Jersey, United States. Founded in 1746 in Elizabeth, New Jersey, Elizabeth as the College of New Jersey, Princeton is the List of Colonial ...

New Jersey New Jersey is a U.S. state, state located in both the Mid-Atlantic States, Mid-Atlantic and Northeastern United States, Northeastern regions of the United States. Located at the geographic hub of the urban area, heavily urbanized Northeas ...

, under the direction of

John Archibald Wheeler John Archibald Wheeler (July 9, 1911April 13, 2008) was an American theoretical physicist. He was largely responsible for reviving interest in general relativity in the United States after World War II. Wheeler also worked with Niels Bohr to e ...

Areas of research

Unruh has made seminal contributions to our understanding of

gravity In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...

black holes A black hole is a massive, compact astronomical object so dense that its gravity prevents anything from escaping, even light. Albert Einstein's theory of general relativity predicts that a sufficiently compact mass will form a black hole. Th ...

cosmology Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe, the cosmos. The term ''cosmology'' was first used in English in 1656 in Thomas Blount's ''Glossographia'', with the meaning of "a speaking of the wo ...

, and quantum fields in curved spaces, including the discovery of what is now known as the

. Unruh has contributed to the foundations of

quantum mechanics Quantum mechanics is the fundamental physical Scientific theory, theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. Reprinted, Addison-Wesley, 1989, It is ...

in areas such as

decoherence Quantum decoherence is the loss of quantum coherence. It involves generally a loss of information of a system to its environment. Quantum decoherence has been studied to understand how quantum systems convert to systems that can be expla ...

and the question of time in quantum mechanics. He has helped to clarify the meaning of nonlocality in a quantum context, in particular that

quantum nonlocality In theoretical physics, quantum nonlocality refers to the phenomenon by which the measurement statistics of a multipartite quantum system do not allow an interpretation with local realism. Quantum nonlocality has been experimentally verified unde ...

does not follow from

Bell's theorem Bell's theorem is a term encompassing a number of closely related results in physics, all of which determine that quantum mechanics is incompatible with local hidden-variable theories, given some basic assumptions about the nature of measuremen ...

and that ultimately quantum mechanics is a local theory. Unruh is also one of the main critics of the

Afshar experiment The Afshar experiment is a variation of the double-slit experiment in quantum mechanics, devised and carried out by Shahriar Afshar in 2004. In the experiment, light generated by a laser passes through two closely spaced pinholes, and is refocused ...

. Unruh is also interested in music and teaches the Physics of Music.

Unruh effect

The Unruh effect, described by Unruh in 1976, is the prediction that an accelerating observer will observe

black-body radiation Black-body radiation is the thermal radiation, thermal electromagnetic radiation within, or surrounding, a body in thermodynamic equilibrium with its environment, emitted by a black body (an idealized opaque, non-reflective body). It has a specific ...

where an inertial observer would observe none. In other words, the accelerating observer will find itself in a warm background, the temperature of which is proportional to the acceleration. The same

quantum state In quantum physics, a quantum state is a mathematical entity that embodies the knowledge of a quantum system. Quantum mechanics specifies the construction, evolution, and measurement of a quantum state. The result is a prediction for the system ...

of a field, which is taken to be the ground state for observers in inertial systems, is seen as a thermal state for the uniformly accelerated observer. The Unruh effect therefore means that the very notion of the

quantum vacuum In quantum field theory, the quantum vacuum state (also called the quantum vacuum or vacuum state) is the quantum state with the lowest possible energy. Generally, it contains no physical particles. However, the quantum vacuum is not a simple ...

depends on the path of the observer through

spacetime In physics, spacetime, also called the space-time continuum, is a mathematical model that fuses the three dimensions of space and the one dimension of time into a single four-dimensional continuum. Spacetime diagrams are useful in visualiz ...

. The Unruh effect can be expressed in a simple equation giving the equivalent energy ''kT'' of a uniformly accelerating particle (with ''a'' being the constant acceleration), as: :

kT = \frac

References

External links

University of British Columbia Physics Dept. page

* ttp://www.theory.physics.ubc.ca/ UBC Theoretical Physics Homepage - a web server ran by Unruh {{DEFAULTSORT:Unruh, William George 1945 births Living people Canadian physicists Canadian Mennonites Fellows of the Royal Society Scientists from Winnipeg Princeton University alumni Relativity theorists Academic staff of the University of British Columbia University of Manitoba alumni Fellows of the American Physical Society