After a ‘traditional’ Ph.D. in solid-state physics, random chance led me to a newly-formed research group studying – and defining – the properties of the “soot particle.” Neither boson nor lepton, this pollutant, once considered an obsolete relic of the Coal Age, turns out to be the number-2 driver of global climate change; the number-1 driver of Arctic and Himalayan melting; and the number-1 indicator of the adverse health effects of combustion exhaust.  In addition to killing babies and submerging Florida, black particles also soil artwork, can trace the penetrability of buildings to biological attack, and can defeat directed-energy laser weapons.  The development of real-time techniques to measure ‘Black Carbon’ led to a great increase in research in these areas; to a niche business; and to field projects from Siberia to Calcutta to the South Pole.  This talk will use the above points as illustrations of how the principles of a Physics education can be applied to a “dirt” problem, with real-world consequences.

The Ancient Greeks regarded geometry and algebra as two quite separate mathematical subjects. It was principally Fermat and Descartes who combined the two into coordinate-geometry, which since then has played an enormous role in the development of physics. I will discuss to what extent one should think that the physical structure of the world is geometric or algebraic.

Many ideas and techniques developed by physicists turn out to have applications outside the traditional realm of physics. In recent years physicists have made major contributions in computer science, economics, biology, and other fields. In this talk I will describe a number of projects I have worked on that fall in the general area known as "complex systems", including work on computer models of social networks, new methods for making maps based on the physics of diffusion, and a simple physical proof that could explain why we've never heard from any extraterrestrials---and why we never will.

slides (pdf, 4 Mb)

We can all tell when a movie of some everyday event, such as a kettle boiling or a glass shattering is run backwards. Similarly, we all feel that we can remember the past and affect the future, not vice versa. So there is a very clear "arrow" (direction) of time built into our interpretation of our everyday experience. Yet the fundamental microscopic laws of physics, be they classical or quantum-mechanical, look exactly the same if the direction of time is reversed. So what is the origin of the "arrow" of time? This is one of the deepest questions in physics; I will review some relevant considerations, but do not pretend to give a complete answer.

On April 10, 2009, the world’s first hard X-ray free-electron laser was brought to lasing. Producing an X-ray beam with more than a billion times higher peak brightness than the most powerful existing synchrotron sources, it marked the beginning of a new era of science. The Linac Coherent Light Source’s (LCLS) pulses arrive at a rate of 60-120 Hz in an energy range from 480 eV to 10 keV, with pulse lengths as short as a few to about 300 femtoseconds. Since October 2009, users have been performing experiments at the LCLS. This talk will describe the LCLS and its unique new capabilities, followed by some examples of the first experiments, and finish with an outlook of future plans in the short as well as long term.

A quantum magician can play tricks that are completely impossible for any classical magician. For example, two dice rolled at an arbitrary distance will show the same number, or balls hidden under a cup can show colors impossible in any classical scenario. These are just two examples of consequences of the challenges to classical reality in the quantum world. I will show in a very instructive way how such features and others follow from the basic features of quantum physics and what they teach us about reality and free will.

Human desire to communicate secretly is at least as old as writing itself and goes back to the beginnings of our civilisation. Over the centuries many ingenious methods of secret communication have been developed, only to be matched by the ingenuity of code-breakers. As the result, the quest for a perfect, unbreakable, cipher, had been declared a futile pursuit. That is, until recently! Surprisingly, a combination of quantum physics and cryptography promises to dash the hopes of would-be eavesdroppers, perhaps for good. Code-makers, it seems, may have beaten code-breakers at last. In my talk I will focus on the quest for perfect secrecy. I will describe how people tried to protect communication in the past, how it is done today, and I will speculate how it may be done in the future. Physics plays increasingly more important role in this field simply because the process of sending and storing of information is always carried out by physical means. In particular, eavesdropping can be viewed as a measurement on a physical object, in this case the carrier of the information. What an eavesdropper can measure, and how, depends exclusively on the laws of physics. I will explain how, using quantum phenomena, physicists managed to design and to implement a system which is regarded to be unbreakable. Moreover, recent research shows that security of communication can be guaranteed by peculiar "non-local" correlations, no matter whether they are of quantum origin or not. Bell’s inequality alone makes seemingly insane scenario possible---devices of unknown or dubious provenance, even those that are manufactured by our enemies, can be safely used for secure communication!  I will provide a brief overview of the intriguing connections between Bell's inequality and cryptography.

The recent banking crises have made it clear that increasingly complex strategies for managing risk in individual banks and investment funds (pension funds, etc.) has not been matched by corresponding attention to overall systemic risks. Simple mathematical caricatures of “banking ecosystems”, which capture some of the essential dynamics and which have some parallels (along with significant differences) with earlier work on stability and complexity in ecological food webs, have interesting implications. In particular, strategies that tend to minimise risk for individual banks can – under certain circumstances – maximise the probability of systemic failure. This talk will first sketch these models and then discuss some of the ensuing conclusions.

slides (pdf, 14.6Mb)