Today was the second day of the Cosmology and Quantum Foundations conference, a symposium that forms part of the Establishing the Philosophy of Cosmology project at Cambridge and Oxford.
The workshop this morning started with a fascinating talk by Douglas Spolyar on a model of cosmic inflation that predicts that inflation could happen at relatively low energies. The big advantage of such models that they are testable at the TeV energies, i.e., at accelerators such as the LHC; I need to read the paper before I comment further, but all the talks will soon be available on the conference website.
Laura Mersini then gave a talk on evidence for the multiverse post-Planck. This was a discussion of her thesis that the multiverse should in principle be detectable in the cosmic microwave background because of the phenomena of quantum entanglement and decoherence. She then discussed how in her view the Planck data offers support for the model in terms of the cold spot, the dark flow and other effects. It was a good thorough lecture and I understood a lot more than I did at the Cambridge conference on the philosophy of cosmology last March. Of course, not all cosmologists agree with her thesis and there was plenty of lively discussion from the audience – as an experimentalist, I really like the way theoreticians constantly challenge each other during their talks, it’s very interactive!
In the afternoon , it was back to the conference proper for ‘Probability and the multiverse: an Everettian view’, the second installment of Simon Saunder’s discussion of the many-worlds interpretation of quantum theory. I found this a lot more challenging than Monday’s talk, I really need to brush up on my reading on many-worlds. Max Tegmark then gave a talk on ‘Thermodynamics, information and consciousness in a quantum multiverse’, a discussion that was full of interesting insights and provocative ideas. A central theme of his is that entropy does not always increase, but can in fact decrease on observation. I have heard this idea before but I’ve never been clear whether it is an argument that pertains to entropy as a state of information about a system, or whether it is literally true of physical entropy. I wanted to ask this at question time, and how one might test the hypothesis, but time ran out.
[Update: I asked Max this question over coffee. I think the answer is yes to physical entropy and he suggested an experiment that could test the idea; unfortunately, I understood about 5% of what he said, I need to read up on this!]
The last speaker of the day was Carlo Rovelli, who spoke on a new interpretation of quantum theory known as the relationary view, a hypothesis he put forward in the 1990s. This interpretation of qt imports a lot of ideas from special relativity, in particular applying the idea of the reference frame of the observer to the measurement problem. Thus, instead of talking about wavefunctions that collapse into one state or another, one has to consider that measurements of systems are made relative to another system – it is the relation between the systems that counts. It was fascinating to hear a description of this intriguing new idea from its creator, and tomorrow he will explain how the new theory gives a description of quantum gravity. [Writing this, I seem to remember that one of Schrodinger's own objections to the notion of collapsing wavefunctions involved the problem of observations of the same object from different reference frames, must look this up]
After all that, it was time for the conference dinner. I was lucky enough to be at the same table as Carlo, who is also the author of the highly regarded book ‘The First Scientist: Anaximander and His Legacy’ and we had a great discussion on the history of science. I have never met a physicist who is not interested in the history of our subject – how things were found out is almost as interesting as the things themselves!
As a bonus, the an after-dinner talk was given by Max Tegmark who posed an intriguing question; what if mathematics is a useful way of describing nature simply because nature *is* mathematics? This question was first raised by Pythagoras, and Max gave an extremely interesting talk on the subject. So much so that I finally realised who he reminds me of – Richard Feynman!
I had a quick walk under the Bridge of Sighs before dinner