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William G. Unruh


  • Associate Fellow
  • Gravity & the Extreme Universe


William Unruh was founding director of CIFAR’s Cosmology & Gravity program (1985–96).

A theoretical physicist, he is renowned for his pivotal contributions to the understanding of a range of subtle problems arising at the crossroads of quantum physics, gravitational theory and cosmology. He has developed concepts and tools that now bear his name, including the Unruh particle detector, Unruh vacuum and Unruh temperature.

In 1976, he discovered that what is defined as the vacuum by inertial observers is perceived by an accelerated observer as a thermal bath at a characteristic temperature proportional to his acceleration. This was instrumental (along with Hawking’s parallel discovery of black hole evaporation) in opening new vistas in fundamental physics.

Unruh is concerned with understanding the key physical principles that are important in explaining the very earliest stages in the formation of the universe – in particular, the way in which gravity and quantum mechanics interact in those extreme early stages. He is also interested in the gravitational structures of cosmic strings. These big questions have impact on our understanding of the world around us. Cosmic strings, for example, have close theoretical ties with the magnetic flux tubes found in many types of superconductors. To understand the role of time in quantum gravity, we must understand ordinary quantum theory much better than we do at present.


  • Steacie Prize, National Research Council
  • Herzberg Medal, Canadian Association of Physicists
  • Rutherford Medal, Royal Society of Canada
  • Fellow, Royal Societies of Canada and London

Relevant Publications

  • Unruh, W.G., and R.M. Wald. "Time and the interpretation of canonical quantum gravity." Phys. Rev. D 40 (October 1989): 2598.

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