+ Brief History

I began work in the field of atomic astrophysics, and worked on the problem of di-electronic recombination in the solar corona. I then moved into cosmology, and the theory of black holes, especially their quantum and thermodynamic properties. During the 1970's and 1980's I helped develop the theory of quantum fields propagating in curved background spacetime (i.e. gravitational fields). This had immediate application to the creation of quantum particles by black holes (the Hawking effect), and in the very early universe as a result of the rapid cosmological expansion.

Early work included the investigation of particle creation by moving mirrors, the response of accelerating particle detectors in a variety of scenarios and the detailed study of quantum field theory in a background de Sitter space. These ideas found later application to topics as diverse as the Casimir effect, the inflationary universe scenario, the holographic principle, wormholes and time travel. Much of my work concerned the notorious divergences associated with the quantum vacuum, which become much more problematic when the spacetime is curved. Using point-splitting renormalization, which we developed in great detail, my colleagues and I were able extract meaningful finite answers for a range of physically interesting problems. The bulk of this work is contained in my book Quantum Fields in Curved Space, co-authored with my former PhD student Nicholas Birrell.

Throughout this time, I have maintained various enduring secondary interests. The most important of these is the nature and origin of time asymmetry in the universe (see my book for more on this topic). Others include the nature and origin of life, the measurement problem of quantum mechanics, the nature of complexity, the anthropic principle and the interface of science and religion. In the 1990's I began working seriously in astrobiology. Initially I was interested in whether life could propagate between planets in the impact ejecta. I then developed some ideas about the origin of life based on its informational aspect.

Highlights

The arrow of time

This subject was in a confused state in the early 1970's. My book The Physics of Time Asymmetry was an early attempt at a systematic approach.

Accelerating observers see heat!

In 1975 I used a simple quantum field theory model to argue that a highly accelerated observer would perceive a bath of thermal radiation even in empty space (i.e. the normal quantum vacuum). Independently, Bill Unruh discovered the same result with a mathematical model of a particle detector. The prediction that accelerated observers/detectors respond to empty space as if it is filled with heat is often termed the Davies-Unruh effect, and it has led to a considerable literature. Attempts have been made to detect the effect experimentally.

How do back holes radiate energy?

In 1975, Hawking famously predicted that black holes are not black, but radiate heat and slowly evaporate away. But how does the energy get out of the black hole? With my colleagues Steven Fulling and Bill Unruh, we were able to show, from a simple two-dimensional mathematical model, that the black hole shrinks, not because energy is coming out, but because negative energy is flowing in. (See my paper for more about this topic).

Conformal anomaly

Stephen Fulling and I discovered the first so-called conformal anomaly, a phenomenon in which a mathematical symmetry in the underlying theory is broken by subtle quantum field effects. (See my paper for more about this topic). Anomalies have proved to be crucial in the consistent formulation of quantum fields that interact with other fields.

Inflation and the cosmic ripples

Cosmologists have discovered that the fading afterglow of the big bang is distributed across the sky with almost perfect uniformity. However, superimposed on this cosmic microwave background radiation are tiny variations in temperature. These "ripples" track perturbations in the density of primordial matter, and are thought to be the beginnings of the large-scale structure in the universe - structure manifested today as galaxies. Mystery surrounds the origin of these all-important perturbations, but a popular school of thought is that they originated during inflation, when the universe suddenly jumped in size by an enormous factor during the first split second after the big bang. During inflation, the universe resembled de Sitter space, and the "ripples" could be quantum fluctuations generated at this time, writ large and frozen in the sky. In the 1970's my PhD student Tim Bunch and I worked on the theory of quantum vacuum states in de Sitter space. One of these states, known as the Bunch-Davies vacuum, later turned out to be the appropriate state to describe the "ripples" as quantum fluctuations. (See my paper for more about this topic).

Black hole specific heat

In 1977 I discovered an interesting fact about the thermodynamic properties of black holes. Static black holes radiate heat by the Hawking effect, and get hotter as a result. The process is therefore unstable. I showed that if the black hole spins faster than a certain rate, it undergoes some sort of abrupt transition (technically known as a phase transition), beyond which it can be stable in a surrounding heat bath, i.e. it cools as it radiates, after the fashion of a normal hot body. I found the same phenomenon occurs if the black hole carries a large enough electric charge. (See my paper for more about this topic).

Rocks and transpermia

In the early 1990s I proposed that life may have begun on Mars and spread to Earth (or vice versa) in rocks ejected from the planets by large comet impacts. (See my book The Fifth Miracle for more on this topic). This theory was discussed independently by Jay Melosh. After several years of scepticism, the basic idea of the theory has become generally accepted by astrobiologists.

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