The arrow of time
This subject was in a confused state in the early 1970's. My bookThe 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 theeffect 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 becausenegative energyis flowing in. (See mypaperfor more about this topic).
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 mypaperfor 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 thiscosmic microwave backgroundradiation 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 thebig 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 mypaperfor 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 mypaperfor 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 bookThe Fifth Miraclefor 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.
Many astrobiologists are convinced that life arises readily on earthlike planets. If so, life should have begun many times on Earth, and microbial descendents of other genesis events could be all around us today, intermingled with standard life. How would we know? The answer is, we need to look very carefully at the microbial realm, most of which is still univestigated. In collaboration with several astrobiologists, I am devising search strategies for identifying these putative "aliens" under our noses.
Most molecular biologists and biochemists are happy with “ball-and-stick” models of molecules, but at some level quantum mechanics must play a role in life’s processes. Evolution has had over three billion years to exploit any worthwhile “quantum trickery,” and several recent lines of experimental inquiry suggest that non-trivial quantum effects could be vital to life. My research aims to elucidate the possibilities, especially in relation to the origin of life and the processing of biological information. See my book "Quantum Aspects of Life" for more information.
Physics and cancer
Recently, I have helped the US National Cancer Institute develop a new program of research which aims to understand cancer cells and tumors in terms of their physical properties, and investigate the relationship between gene expression and the physical forces and environment of the cells. The long-term goal of this research is to develop a radically new perspective on cancer’s role in biology, with a view to better clinical management.