''A celebration of the careers and 60th birthdays of Bill Unruh and Bob Wald'', held at the University of British Columbia, August 18-20, 2006
The meeting, with about 80 participants, had only four talks per day, with plenty of time for pleasant interactions. The birthday boys helped to pick their own speakers, and apparently felt they could be selective!
Matt Choptuik spoke about ``The influence of Unruh and Wald on numerical relativity'', a topic of personal interest to your correspondents. Both had long been interested in cosmic censorship. In 1991 Shapiro and Teukolsky famously claimed naked singularity formulation in prolate collapse of collisionless matter because singularities formed in the absence of an apparent horizon, and Wald and Iyer then showed that even slices through Schwarzschild need not have apparent horizons.
Choptuik then embarked on his own detailed study of spherical scalar field collapse, which led to the discovery of critical phenomena in gravitational collapse and a new, relatively ``natural'' way of creating naked singularities. As a consequence, Hawking conceded his bet with Preskill and Thorne that naked singularities could not form from smooth initial data (for reasonable matter).
In 1993, Gregory and Laflamme conjectured that black strings might become unstable and pinch off to form black holes; this would violate cosmic censorship. Motivated by Horowitz through Unruh, Choptuik, Lehner, Pretorius and Olabarrieta began investigating this numerically in 2003, but the jury is still out.
In 1987 Thornburg wrote up a suggestion of Unruh's now known as black hole excision (although he prefers singularity excision): no boundary conditions are required on a boundary which can be spacelike and stationary at once if it is inside a black hole. This was implemented in 1992 by Seidel and Suen.
Abhay Ashtekar spoke on ``The quantum nature of the big bang''.
This was treated in loop quantum cosmology: a minisuperspace version of
loop quantum gravity in which one restricts to a small number of
degrees of freedom:
in this case Friedmann spacetime with
a scalar field so that the degrees of freedom are the scalar field
and the scale factor 
. Instead of the Wheeler-de Witt equation
one obtains a difference equation for the wave function
. This peaks on a semiclassical trajectory which seems to go
through a bounce rather than to the big bang singularity. Thus loop quantum
cosmology resolves the big bang singularity. It will be interesting to
see whether this feature also holds in loop quantum gravity without
the minisuperspace approximation.
Jim Hartle spoke on ``What's wrong with your quantum mechanics?'' He imagined the objections that Bill and Bob, universally asknowledged as deep thinkers and fierce critics, might have to the Gell-Mann-Hartle consistent histories interpretation of quantum mechanics. For each objection, he then presented his response. His message was that his quantum mechanics is as applicapable to the whole universe as it is to any ordinary quantum mechanical system. In each case, we divide the possible outcomes for the system into ``approximately decohering coarse grained histories'' and use the quantum state and the rules of quantum mechanics to find the relative probability of each history.
Roger Penrose gave a public evening lecture ``What happened before the big bang?'' He pointed out the puzzle of the very special initial conditions of the universe and speculated that these might be connected to the final conditions of a system that has radiated away all of its degrees of freedom.
Kip Thorne gave a review talk on ``Quantum non-demolition'' which was both rich in historical detail, with much USA-USSR interaction and competition, and quite technical. The field started with the calculation by Braginsky in 1967 of the (standard) quantum limit for a gravitational wave detector, motivated by the experimental work of Weber. He stressed the crucial importance of two unpublished talks Unruh gave in Bad Bentheim in 1981 which changed the focus from the measurement of detector position to that of the classical force, which can be measured to arbitrary accuracy. One current focus of the field is to use LIGO to measure quantum mechanical effects on macroscopic objects such as the mirrors.
Wojciech Zurek gave a review on the ``Emergence of the classical world'' from quantum mechanics. He reviewed the Everett interpretation in the light of the key questions ``what is the preferred basis?'' and ``why are probabilities the amplitude squared of those states?''.
Ted Jacobson gave a talk on ``Black hole entropy.'' Wald and Parker showed in 1975 that Hawking radiation is in a thermal state. Unruh showed that the key ingredient is the horizon. Even an accelerated observer, who only has an ``acceleration horizon,'' sees thermal radiation. Bekenstein proposed that black hole entropy counts the ways in which a black hole could have formed. But how does thermodynamics know about this? Jacobson noted that the Hawking radiation has entropy of its own and therefore must contribute something to the black hole entropy. He then considered the possibility that the entropy of the radiation is in fact the entire black hole entropy. A calculation of the contribution of the radiation to the total entropy involves a cutoff and so the answer seems to hinge on the appropriate value of the cutoff to use. At present, it is not clear what that appropriate value of the cutoff is, so it is not clear whether the contribution of the Hawking radiation to the black hole entropy is negligible, as had been assumed, or dominant, as Jacobson seems to think likely.
Dick Bond gave a review talk on ``Inflation, gravitational waves and the cosmic microwave background (CMB).'' Currently all observations are compatible with cold dark matter and a simple cosmological constant, and large-scale structure seeded by scale-invariant Gaussian density fluctuations seeded by inflation. But much sophisticated observation, analysis and modelling is behind this simple result, and the limits on inflation history and the dark energy equation of state are getting better. One new big goal is to see primordial gravitational waves, through their interaction with CMB photons.
Ralf Schützhold spoke on ``Effective horizons in the laboratory.'' He noted that though black hole evaporation for stellar mass black holes is too small for us to hope to measure it, there should be analogs of the Hawking effect that are within reach of laboratory experiments. These take place in optical systems and in fluid systems where the medium is moving faster than the wave propagation speed. He presented the most promising such systems and for each system the most promising of the experimental techniques that might be used to detect the analog of the Hawking effect in that system.
Stefan Hollands talked on ``Quantum fields in curved spacetime.'' He emphasized that many of the ingredients used in specifying a quantum field theory in flat spacetime (spacetime symmetries, natural vacuum state, Euclidean methods, momentum space, S matrix, etc.) are simply absent in curved spacetime. One must therefore use completely different methods for quantum fields in curved spacetime, and Hollands and Wald advocate an algebraic approach that concentrates on the algebra of field operators and views quantum states as simply linear maps from the algebra to the complex numbers. It has been known for some time how to do this for free field theory; however it is only with the recent work of Hollands and Wald that the groundwork has been laid for treating perturbative interacting quantum fields in curved spacetime. In particular, this work allows one to make sense of products of field operators. These algebraic methods also allow one to formulate criteria for physically reasonable quantum states.
Gary Horowitz talked about ``Surprises in black hole evaporation.'' He noted that the standard picture of black hole evaporation within general relativity is that a black hole gives off thermal radiation until it reaches the Planck scale. However, string theory takes place in more than 4 spacetime dimensions and involves extended objects. This gives rise to new possibilities. There are higher dimensional analogs of black holes: black strings and black branes, which can be wrapped around extra spatial dimensions. The horizon can then contain a topologically nontrivial circle. Hawking radiation causes the size of this circle to decrease. When it becomes small enough, there is a “tachyon” instability. This instability is due to certain modes of the string and causes a change in topology. In the resulting spacetime the black hole is replaced by a ``bubble of nothing'' and simply disappears. This can occur when the curvature at the horizon is still small compared to the Planck scale.