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

Canadian Canadians (french: Canadiens) 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 ...

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 research university with campuses near Vancouver and in Kelowna, British Columbia. Established in 1908, it is British Columbia's oldest university. The university ranks among the top thre ...

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 ...

who described the hypothetical

Unruh effect The Unruh effect (also known as the Fulling–Davies–Unruh effect) is a kinematic prediction of quantum field theory that an accelerating observer will observe a thermal bath, like blackbody radiation, whereas an inertial observer would observe ...

in 1976.

Early life and education

Unruh was born into a

Mennonite Mennonites are groups of Anabaptist Christian church communities of denominations. The name is derived from the founder of the movement, Menno Simons (1496–1561) of Friesland. Through his writings about Reformed Christianity during the Radi ...

family in

Winnipeg Winnipeg () is the capital and largest city of the province of Manitoba in Canada. It is centred on the confluence of the Red and Assiniboine rivers, near the longitudinal centre of North America. , Winnipeg had a city population of 749, ...

Manitoba , image_map = Manitoba in Canada 2.svg , map_alt = Map showing Manitoba's location in the centre of Southern Canada , Label_map = yes , coordinates = , capital = Winn ...

. His parents were Benjamin Unruh, a refugee from

Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-eig ...

, 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 Canadian public research university in the province of Manitoba.Princeton University Princeton University is a private research university in Princeton, New Jersey. Founded in 1746 in Elizabeth as the College of New Jersey, Princeton is the fourth-oldest institution of higher education in the United States and one of the ...

New Jersey New Jersey is a state in the Mid-Atlantic and Northeastern regions of the United States. It is bordered on the north and east by the state of New York; on the east, southeast, and south by the Atlantic Ocean; on the west by the Delaware ...

, 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 in ...

Areas of research

Unruh has made seminal contributions to our understanding of

gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

, black holes,

cosmology Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe. The term ''cosmology'' was first used in English in 1656 in Thomas Blount's ''Glossographia'', and in 1731 taken up in Latin by German philosopher ...

, 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 a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistr ...

in areas such as

decoherence Quantum decoherence is the loss of quantum coherence. In quantum mechanics, particles such as electrons are described by a wave function, a mathematical representation of the quantum state of a system; a probabilistic interpretation of the wa ...

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 admit an interpretation in terms of a local realistic theory. Quantum nonlocality has been experime ...

does not follow from Bell's theorem and that ultimately quantum mechanics is a local theory. Unruh is also one of the main critics of the Afshar experiment. Unruh is also interested in music and teaches the Physics of Music.

The Unruh effect

The Unruh effect, described by Unruh in 1976, is the prediction that an accelerating observer will observe black-body radiation 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 provides a probability distribution for the outcomes of each possible measurement on a system. Knowledge of the quantum state together with the rules for the system's evolution i ...

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. The word zero-point field is sometimes used as ...

depends on the path of the observer through

spacetime In physics, spacetime is a mathematical model that combines the three dimensions of space and one dimension of time into a single four-dimensional manifold. Spacetime diagrams can be used to visualize relativistic effects, such as why differ ...

. 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 People from Winnipeg Princeton University alumni Relativity theorists University of British Columbia faculty University of Manitoba alumni