Once per year, I spend a morning visiting a local high school to discuss physics with a class of talented seniors. This is a program organized by a wonderful teacher - Ranald Bleakley - who I met through the Saturday Morning Physics program that I have run at Syracuse for the last five years. Ranald spends the entire school year teaching physics to these students, covering the classic subjects - mechanics, electricity and magnetism, etc. - and trying to connect these ideas to frontier topics in modern physics. Because these are all bright, motivated and college-bound students, they generate a large number of questions, many of which Ranald handles. Nevertheless, a pile of unanswered questions inevitably accumulates, and it is my job to try to get through some of these during my visit. Although I didn't attend high school in the U.S., there are enough similarities to the British system that I always find it strangely nostalgic to walk the corridors again after all these years. There is a certain smell, and a certain atmosphere of restrained mayhem that one can just sense as soon as one walks through the doors. The students in Ranald's class are always a great deal of fun - smart and curious and easy to interact with after the first few minutes when they're first getting used to speaking to me. So, what was the number one question they had? "We've heard a lot about this thing called string theory - what's it all about?" It doesn't matter where you go - people are talking about it! I was able to give them a little review of the regimes of validity of General Relativity (GR) and Quantum Field Theory (QFT), talk about physical situations where both theories seem to be needed, and hence discuss the problems that their incompatibilities present. I then sketched out what string theory is, and why we think it is attractive - namely that it seems to provide a consistent theory of quantum gravity. This discussion also allowed me to make some useful distinctions between Theories (with a capital "T", if you like) such as GR and QFT, which have made numerous verified predictions, and theories (with a small "t"), such as string theory, which many scientists find extremely attractive, for good reasons, but which have yet to confront experiment. A second question was a standard one I get in public lectures, although these students seemed to have more background than the general public, - "We know that the universe is expanding; do we know if it is infinite in size or not, and whether it will expand forever or eventually recollapse?" This is an interesting question, to which cosmologists for a while often gave a set of technically wrong answers. What we think we measure, through combining results from a number of experiments such as the WMAP satellite and the Hubble Space Telescope, is the local spatial geometry of the universe (the geometry of spatial slices). This is determined by the local energy density in the universe. If the matter in the universe consisted only of regular matter (dark matter, baryons, and radiation) for all time, then one could indeed infer the ultimate destiny of the universe from such measurements, since positive spatial curvature implies more than a critical density and hence ultimate collapse, while flat (which seems to be what we measure) or negatively curved spatial geometries imply eternal expansion. However, the possibility of a cosmological constant (which may be causing cosmic acceleration) ruins this connection, meaning that one could, in fact, live in a positively curved universe that expands forever. Furthermore, our measurements of the local spatial geometry tell us nothing about the topology of the universe - i.e. its connectedness, and whether it is finite. For example, there actually exist so-called compact hyperbolic manifolds, which are homogeneous and everywhere negatively curved, but in fact are of finite volume. One can construct these in analogous ways to making a torus from an infinite flat plane (and this means also that if the universe is flat, we also don't know if it is finite or infinite). One can, of course, perform measurements to see if the universe is finite on a given scale (because if so there would be correlations in light coming from beyond that distance on very different parts of the sky. The furthest away light we have is the CMB, and current tests have not revealed the telltale signs of cosmic topology in it. Therefore, our best knowledge of the universe is that, even if it were negatively curved, it could be finite or infinite, but if finite, then only on so-far unobserved scales. Later questions included
"I understand that when some stars die they end up as neutron stars, and others end up as black holes. What's the difference?" This allowed me to discuss some quantum mechanics - the Pauli exclusion principle and degeneracy pressure.
"How do astronomers measure distances?" This gave me a chance to talk about the cosmological distance ladder, parallax, cepheid variables, type Ia supernovae, and much more.
"Why do you spend your time studying these things?"
This last question led into a discussion of what an academic's life is like, and then a chat about what kinds of other careers open up to you when you have a physics degree, with the associated critical thinking and problem solving skills. This type of public outreach is extremely rewarding and requires basically no preparation. Most cosmologists and particle physicists can provide coherent answers to the questions above straight off the tops of their heads. The students seem to enjoy the time, get their questions answered, and provide good donuts by way of thanks. And another thing - it's worth commenting on the makeup of the class this year. I counted around fifteen students, only three of whom were men. I don't know whether this was anomalous, and I certainly don't want boys to be discouraged from taking physics, but it was wonderful to see so many young women enthusiastic about science.
