Colloquium

Viscosity is a very old concept which was introduced to physics by Navier in the 19th century. However, in strongly coupled systems, the viscosity is usually difficult to compute. In this talk I will describe how gauge/gravity duality, a by-product of string theory, allows one to compute the viscosity for a class of strongly interacting fluids not too dissimilar to the quark gluon plasma. I will also describe efforts to measure the viscosity and other physical properties of the quark gluon plasma at the Relativistic Heavy Ion Collider.

We will review the definitions of spin foam models for quantum gravity and the recent advances in this field, such as the "graviton propagator", the definition of coherent states of geometry and the derivation of non-commutative field theories as describing the effective dynamics of matter coupled to quantum gravity. I will insist on the role of group field theories as providing a non-perturbative definition of spinfoams and their intricate relation with non-commutative geometry and matrix models.

Last May, NASA astronauts performed a challenging and flawless Space Shuttle servicing mission to the orbiting Hubble Space Telescope. With science instruments repaired on board and incredible new ones installed, the observatory is more powerful now than ever before. I will show the dramatic highlights of the mission, and present some of the first results from the refurbished telescope. The sensitivity and multi-wavelength capabilities are revealing the highest redshift galaxies ever seen as well as details of the cosmic web of intergalactic medium, large scale structure formation, and stellar evolution. Studies of dark matter, dark energy, and exoplanet atmospheres add to the profound contributions to astrophysics that Hubble is making, at the new apex of its capabilities.

Ever since there's been money, there have been people trying to counterfeit it, and governments trying to stop them. In 1969, the physicist Stephen Wiesner raised the remarkable possibility of money whose authenticity would be guaranteed by the laws of quantum mechanics. However, the question of whether one can have secure quantum money that anyone (not only the bank) can verify has remained open for forty years. In this talk, I'll tell you about progress on the question over the last two years. (1) I'll show that no publicly-verifiable quantum money scheme can have security based on quantum physics alone: like in most cryptography, one also needs a computational hardness assumption. (2) I'll show that one can have quantum money that remains hard to counterfeit, even if a counterfeiter gains access to a "black box" for verifying the money. (3) I'll describe a candidate quantum money scheme I proposed last spring, and how that scheme was recently broken by Lutomirski et al. I'll also discuss a new class of schemes that might evade the existing attacks -- schemes with the bizarre property that not even the bank can prepare the same bill twice. The talk is designed to be accessible to those without a quantum information background. Reference for (1)-(2): S. Aaronson, "Quantum copy-protection and quantum money," in Proceedings of CCC'2009, http://www.scottaaronson.com/papers/noclone-ccc.pdf. Reference for (3): A. Lutomirski, S. Aaronson, E. Farhi, D. Gosset, A. Hassidim, J. Kelner, and P. Shor. Breaking and making quantum money: toward a new quantum cryptographic protocol, Proceedings of Innovations in Computer Science (ICS), 2010. http://arxiv.org/abs/0912.3825.

The graph isomorphism (GI) problem plays a central role in the theory of computational complexity and has importance in physics and chemistry as well. While no general efficient algorithm for solving GI is known, it is unlikely to be NP-complete; in this regard it is similar to the factoring problem, for which Shor has developed an efficient quantum algorithm. In this talk I will discuss our investigations of quantum particles walking on graphs and their implications for possible algorithms for GI. We find that single-particle quantum random walks fail to distinguish some nonequivalent graphs that can be distinguished by random walks with two interacting particles. The implications of these observations for classical and quantum algorithms for GI will be discussed.

I will describe a new connection between supersymmetry, geometry and computer science. An exploration of the equations of supersymmetry has revealed a geometrical sub-structure whose classification depends on self-dual error correcting codes.

Of all four forces only the weak interaction has experimentally exhibited parity violation. At the same time observations suggest that general relativity may require modification to account for dark matter and dark energy. Could it be that this modification involves gravitational parity violation? Many of the dominant approaches to quantum gravity, such as string theory and loop quantum gravity, point to an effective parity violating extension to general relativity known as Chern-Simons General Relativity (CSGR). In this colloquium I will discuss the uniqueness and phenomenological implications of parity violation in CSGR. In particular, I will discuss how CSGR can work together with inflation to generate the cosmic baryon asymmetry in a most economical fashion, through gravitational waves; and its predictions for upcoming CMB polarization experiments. I will also discuss current predictions of CSGR on binary pulsars, neutron stars and prospects for the LISA/LIGO gravitational wave detectors. While we focus on a specific theory for concreteness, some of the results presented in this colloquium can be seen as model independent.

Is there a theory yet to be discovered that underlies quantum theory and explains its structure? If there is such a theory, one of the features it will have to explain is the central role of complex numbers as probability amplitudes. In this talk I explore the physical meaning of the statement “probability amplitudes are complex” by comparing ordinary complex-vector- space quantum theory with the real-vector-space theory having the same basic structure. Specifically, I discuss three questions that bring out qualitative differences between the two theories: (i) Is information about a preparation expressed optimally in the outcomes of a measurement? (ii) Are multipartite states locally accessible? (iii) Is entanglement “monogamous”?

In 1981 Bill Unruh showed that the equation of motion for sound waves in a convergent fluid flows is given by a wave equation in an acoustic metric geometry. More importantly it is possible to set up sonic horizons in transsonic flows, and thus in principle to mimic experimentally the black-hole evaporation process. Almost 30 years later we have set up an experiment at the University of British Columbia to find out if indeed it is possible to detect (traces of) black hole radiation. We will discuss the necessary theoretical milestones, such as the effective field theory description for water to explore the robustness of Hawking radiation, and the avoidance of shock waves using surface waves rather then sound waves. In theory we should be able to detect the classical component of the thermal emission of black holes in our system and answer the ultimate question: How much do we trust our effective quantum field theory in curved spacetimes?

Gravitational waves provide a unique way to study the Universe. From 2005 to 2007, the Laser Interferometer Gravitational-wave Observatory (LIGO) took data at design sensitivity. After describing gravitational waves and how LIGO works, I will discuss the status of searches for those waves and current astronomical constraints imposed by those searches. Data taking resumed in summer 2009 with enhanced LIGO detectors and the European Virgo detectors. I will discuss plans for combined electromagnetic and gravitational observing campaigns. Finally, I will highlight the prospects for gravitational-wave astronomy with Advanced LIGO over the next decade.

Physicists have been working for banks and hedge funds on applied problems in finance for more than two decades, and recently have doing academic research as well. This talk will survey academic research by physicists and contrast it with mainstream economics. I will argue that the difference comes not from the application of alternative techniques or new mathematics, but rather from fundamental differences in what questions are considered interesting and how one should go about solving them. This will be illustrated with a simple model for how systemic risks and extreme price movements are generated by the use of leverage (buying with credit). The current financial crisis illustrates that the economy is indeed a complex system, and that new approaches are needed that properly take this into account.