ISSYP 2008

Put two physicists in a room and ask them to talk about the interpretation of quantum mechanics. This is a recipe for disagreement; the mysteries of quantum theory run so deep that it’s hard to find any interpretive claims that are immune to controversy. Therefore, when thinking about quantum theory, it is a useful tactic to first focus on the macroscopic facts it predicts while ignoring the formalism and what it might suggest about the constitution of reality. I will adopt this tactic in my talk to describe the strange features of sequences of Stern-Gerlach measurements. This background will be enough to formulate the principle of “no information gain without disturbance” and the technological possibilities it implies, such as money that can’t be counterfeit and secure ways of distributing secret keys for cryptography. I’ll then move on to explain one idea about what might be going on at a deeper level – the idea of hidden variables. Finally, I’ll present Bell’s theorem, a result that reveals an uncomfortable tension between quantum mechanics and the theory of relativity.

We will review the uncertainty principle of quantum mechanics, first formulated by Werner Heisenberg in 1927, and the role they played in the famous debate between Einstein and Bohr on the meaning of quantum theory. Along the way we will focus on questions like: what do we mean by "uncertainty", and how do we express that in the theory? What, in fact, is a physical property? Does a theory like quantum mechanics provide a description of physical reality? Interestingly, some of these questions do not have a unique answer.

One simple way to think about physics is in terms of information. We gain information about physical systems by observing them, and with luck this data allows us to predict what they will do next. Quantum mechanics doesn't just change the rules about how physical objects behave - it changes the rules about how information behaves. In this talk we explore what quantum information is, and how strangely it differs from our intuitions. In particular we see how information about quantum particles can become entangled, leading to seemingly impossibly coordinated behaviour for separate objects, and to phenomena such as quantum teleportation.

The reason cosmologists have a job is that the Universe as a whole -- the stuff between planets and stars and galaxies -- is, despite first appearances, a pretty interesting place. The strangest fact about it is that it\'s expanding, and always has been, as far as we know (and though Einstein\'s theory of gravity predicts this, Albert himself didn\'t much care for the idea, at least at first). After about seventy years -- it was discovered in 1929 -- this expansion was kind of old hat, but then new observations came around that shattered the old complacency. The old idea was that the Universe was expanding, but slowing down as it went -- since gravity, as far as anyone knew, could only cause attractive forces. What the new observations demonstrated is that the Universe\'s expansion is, in fact, accelerating -- getting faster with time. This is so shocking that most astronomers and cosmologists couldn\'t believe it at first, and some still don\'t. In this talk, I\'ll explain a bit about how we know this, why it\'s so shocking, and tell you something about the crazy ideas people at Perimeter have for what\'s going on.

In the 1980’s when Vera Rubin was analyzing how stars in galaxies revolve around the galactic core, she made an incredible discovery. The stars where moving much faster than anyone expected. This discovery helped open up a door in physics whose implications are far stranger than the best plot in any science fiction movie. Join us as we explore the mystery of dark matter.