Research

A hitherto sidelined sector of quantum mechanics is the application of weak measurement theory to ensembles of quantum systems. The theory provides for a range of experimental tests and potential technological applications. My main current interest is in the way quantum weak measurements provide novel insights into the nature of time, the far future of the universe and the role of information as a foundational concept at the heart of physics.

The application of quantum field theory to black holes shows that the horizon area is a measure of entropy. This idea may be generalized to cosmological horizons for a number of expanding universe models. I am generalizing the second law of thermodynamics to cosmological horizons as a means to place constraints on cosmological models, and to investigate the status of the so-called holographic principle. This is ongoing work that draws upon a string of earlier papers asking how far the generalized second law of thermodynamics can be generalized.

One application of horizon entropy is to de Sitter space, which describes the so-called inflationary phase of the very early universe. It is widely accepted that de Sitter space has essential thermal properties that help explain the well-known fluctuations in the cosmic background heat radiation from the big bang – the seeds of the large-scale structure of the universe. However, there are some oddities about the nature of de Sitter thermality. A current project involves the analysis of reflecting boundaries and a novel quantum vacuum state in de Sitter space. This is part of a broader project to investigate ways to ‘mine’ the quantum vacuum energy of de Sitter space using a form of Maxwell demon or Szilard engine to establish the basic principles.

The extraordinary properties of living systems are rarely well described by standard physical theory. I am interested in whether new physics lurks in living matter. One set of ideas comes under the broad category of ‘quantum biology.’ Another is the exploration of state-dependent dynamical systems. The latter involves the application of top-down causation formalized as non-local rules, with cellular automata as one particular ‘toy model.’

For much of my career I have worked on the theory of quantum fields in curved spacetime, i.e. background gravitational fields, an endeavor summarized in my book with Birrell, Quantum Fields in Curved Space. It is a very technical subject involving several branches of advanced mathematics and some subtle interpretational issues. It continues to find many applications to black holes, the Hawking effect, wormholes and the inflationary origin of the universe. One early result is that an exponentially expanding universe (usually referred to as de Sitter space), which seems to fit with current observations, has an intrinsic temperature and entropy associated with its event horizon. I am currently investigating whether this thermal background (quite distinct, and much cooler, that the heat from the big bang) can be screened out by a reflecting barrier. More generally, the assignment of a temperature and entropy to the cosmological horizon requires a generalization of the second law of thermodynamics, and I am investigating scenarios in which this generalization might fail, potentially leading to certain paradoxes in fundamental physics.

In a break with orthodoxy, I suggested that life may have begun many times, and that descendants from a different genesis may still be around us today, intermingled with microbial life. This idea led to a workshop and several papers, and continues to inform microbiology research. So far all life characterized has been the same life. But it remains possible that there is life on Earth not as we know it, a concept dubbed a shadow biosphere. If just a single ‘alien’ microbe were discovered, it would suggest that the pathway from non-life to life is not a difficult one and we could conclude that the universe is teeming with life.

In the early 1990s I suggested that microbes can hitch-hike between Earth and Mars on rocks blasted off the planets by asteroid and comet impacts. Initially greeted with derision, the suggestion is now widely accepted, since it is clear that, cocooned within a rock, microbes could easily survive the harsh conditions of space. If the planets aren’t quarantined, but can cross-contaminate each other, there is a possibility that terrestrial life originated on Mars and came here later in a meteorite. Many Mars meteorites are known. My current research focuses on whether early Mars offered a more favorable environment than the early Earth for life to get going.

SETI is the search for intelligent life beyond Earth, and I have been involved with it for much of my career. I have written several papers on ‘technosignatures’ (signs of alien technology) and suggested new search strategies. I am a Fellow of the International Academy of Astronautics, and I used to head their SETI Post-Detection Taskgroup – figuring out what to do next if and when ET calls. I am currently serving on the advisory board of Breakthrough Listen, a $100 million project funded by Yuri Milner primarily devoted to improving the search for alien radio signals. SETI was officially 50 years old in 2010, and I celebrated it with the publication of my book The Eerie Silence.

To a physicist life seems like magic matter. A major research theme I am involved with attempts to characterize life by the way it organizes information, what we sometimes call ‘the software of life.’ This involves branches of mathematics such as complexity theory and network theory. I am investigating whether there is ‘a new kind of physical law prevailing’ in living matter, as Erwin Schrödinger expressed it in his influential 1944 book What is Life? I have a hunch it has to do with how quantum effects operate in large organic molecules, but I am also persuaded by my colleague Sara Walker that nothing short of a revision in the notion of physical law will really give a convincing account of life.

Over the past ten years I have developed, with colleagues, a new interpretation of cancer that makes several specific predictions. The basis of the theory is that cancer is a type of throwback, or atavism – a reversion to an ancestral type. The idea has been confirmed using phylostratigraphy – dating the genes implicated in cancer. Many cancer genes cluster in age around the onset of multicellular life, about one billion years ago, suggesting that cancer cells revert to unicellular behavior. We discovered that some cancer genes are counterparts of the genes in bacteria that deliberately elevate their mutation rate when the cells are stressed. A high mutation rate is a key feature of cancer cells. The theory gives a natural account of this, and many of the other well-known hallmarks of cancer. It also has important implications for therapy.

Cancer or cancer-like phenomena are found in almost all mammals, as well as birds, fish, insects, plants, fungi, and corals. The pervasive nature of cancer implies that it has deep evolutionary roots, stretching back to the dawn of multicellularity over 1 billion years ago. Unicellular organisms are effectively immortal in that they just replicate whenever they can. However, multicellular life outsources a vestige of immortality to the germ line. The price paid by somatic cells is apoptosis. To ensure that somatic cells do not cheat, many layers of regulatory control have evolved. If something compromises cell and tissue management, cells may rebel and revert to quasi-unicellular behavior, such as turning off apoptosis and indulging in unrestrained proliferation. The result is cancer.

Many hallmarks of cancer recapitulate unicellular modalities, suggesting that cancer initiation and progression represent a systematic reversion to simpler ancestral phenotypes, an idea that dates back to Theodor Boveri. This so-called ‘atavism theory’ may be tested using phylostratigraphy, which can be used to assign ages to genes. Several research groups have confirmed that cancer cells tend to overexpress evolutionarily older genes, and rewire the architecture linking unicellular and multicellular gene networks. In addition, some of the elevated mutation rate (one of the hallmarks of cancer) is in fact self-inflicted, driven by genes found to be homologs of the ancient SOS genes activated in stressed bacteria, and used to evolve biological workarounds. The mutations arise from the switch to error-prone double-strand break DNA repair mechanisms that produce mathematically distinctive patterns of damage around the lesion. Cancer is an ancient deeply embedded, preprogrammed response to stress, such as chemical insults, poor tissue environment, hypoxia, or stroma damage. Rather like ‘safe mode’ on a computer that has suffered an insult, cancer is a deeply protected default state in which the cells run on their core functionality.

The arrow of time is often conflated with the popular but hopelessly muddled concept of the “flow” or “passage” of time. I have argued strenuously that the latter is a best an illusion with its roots in neuroscience, at worst a meaningless concept. However, what is beyond dispute is that physical states of the universe evolve in time with an objective and readily-observable directionality. The ultimate origin of this asymmetry in time, which is most famously captured by the second law of thermodynamics and the irreversible rise of entropy, rests with cosmology and the state of the universe at its origin. I have worked on the explanation for this since including it in my PhD thesis in 1970. My book The Physics of Time Asymmetry, published in 1975, set out the basic story. There I traced the various physical processes that contribute to the growth of entropy, and concluded that gravitation holds the key to providing a comprehensive explanation of the elusive arrow. Subsequent work over several decades by myself and others confirms this essential idea. Yet advances in cosmology, such as the possibility of a multiverse, have left several unanswered questions.

Since I was a student I have been fascinated with ‘the anthropic principle’ – the idea that had the laws of physics and the cosmological initial conditions been just slightly different the universe could not support life. To explain it, scientists are split between those who say it’s an insignificant fluke and those who posit a multiverse – an ensemble of universes in which all possible laws and conditions are instantiated somewhere, so that we are merely winners in a gigantic cosmic lottery. Theologians cite anthropic ‘fine-tuning’ as evidence for a Designer. I think they are all wrong, for reasons I set out in my book The Goldilocks Enigma: Why is the universe just right for life?

Most physicists assume the laws of physics were simply imprinted on the universe at the outset and are immutable and universal. This faith in the laws has withstood the test of time, but there is no logical reason why the laws could evolve with time, or with the state of the universe. There is also no way to test the usual assumption that the laws are infinitely precise mathematical relationships: might they be slightly ‘fuzzy’?. The source of the laws is normally regarded as beyond the scope of science. I have long wrestled with the possibility that we might give an explanation for the origin of the laws by altering the entire conceptual framework around which we organize facts about the world. This is ongoing research

Yes, into the future. But back in time? Maybe… To celebrate the new millennium, I was asked to deliver a lecture at The Royal Society in London on time travel. It developed into a book called How to Build a Time Machine. I set out what it would take to build a wormhole in space in order to visit the past, and I dealt with the paradoxes thereby unleashed. Currently I am interested in how quantum mechanics can be reformulated in a way that makes clear how events in the present can affect the nature of reality as it was in the past. (This is not the same as signaling the past or changing the past, but something more subtle.) It follows from work begun by John Wheeler and Yakir Aharonov, and relates to a result I published showing how, if the universe is constrained by both an initial and a final condition, there may be detectable consequences today.