Finally have some time to blog. I’ve been travelling around going to conferences and visits, and I’ll try to leave some of my impressions here. The first stop was the Vienna Symposium on the Foundations of Modern Physics, from June 11 to June 14. Yes, I know, that is so last month. But hey, I’m travelling, ok? Give me a break.
It was organised by the Institute of Quantum Optics and Quantum Information (IQOQI) at the Austrian Academy of Sciences and by the Faculty of Physics at the University of Vienna. Daniel Greenberger and Helmut Rauch were honoured guests, in the occasion of their 75th and 70th birthdays, respectively.
The list of speakers was impressive, and the talks delivered were on par. Some of the highlights, in my opinion:
Bill Wootters opened the conference with an interesting model of Quantum Mechanics on real vector spaces. His motivation was to give an answer to the commonly held misconception that complex numbers are essential for quantum theory. It is to be expected that any theory that reproduces quantum mechanics has got to be weird, and Wooters’ model isn’t an exception. He models a qubit as being composed of two parts, both represented in real vector spaces. It has to have two components if he is to have sufficient parameters to compensate for the lack of complex numbers. Besides the actual qubit there is an “ubit” — an “universal bit”. There’s a rule that one cannot learn the state of the ubit (so that you have something of the taste of an uncertainty principle), and furthermore, all particles in the universe share the same ubit (and here enters quantum nonlocality). I asked (yay! the first question of the workshop!) if he thought there was an analogy between his postulate that the ubit is unknowable and the epistemic restriction in Robert Spekkens’ toy model (hey, just found there is a Wikipedia entry for it!). He said it was a very interesting question (thank you!) but he didn’t think about it. In hindsight, there is an analogy but also an important difference. In Spekkens’ model the epistemic restriction is about the ontic state of an individual system, whereas in Wootters model it is about an entity shared between all systems. This is why Wootters model can actually reproduce quantum mechanics, whereas Spekkens’ toy model can’t reproduce some features like violations of Bell inequalities.
Simon Kochen (from Kochen-Specker fame) followed with a talk titled “A Reconstruction of Quantum Mechanics”. He started by essentially assuming a Hilbert space structure for the experimental outcomes (although phrased in a quantum logic parlance: lattices and so on) and derived the rest of the formalism from it (with some extra assumptions he tried to justify with an appeal to experimental facts). However, it is well-known that if you assume the Hilbert space structure you can derive the quantum probabilities via Gleason’s theorem. When I asked him about how he justified that assumption (yay! the first question on the second talk of the conference!), he said something like “I am not trying to reconstruct quantum mechanics”.
After the coffee break, it was Stig Steinholm’s talk “Quantum Theory and reality”. He tried to find a way to ground the information encoded by a quantum state in an objective observer-independent manner. Anton Zeilinger added a comment near the end of the question time: “The concept of a reality beyond empirical evidence is not part of Science”. Steinholm’s answer was brilliant: “What’s the empirical evidence for that statement?”
Basil Hiley’s talk on how to derive the basic equations of Bohmian Mechanics for the Dirac equation using Clifford Algebras was very entertaining. Although he seemed to want to distant himself from being called a “Bohmian”. “All I’m saying is that it’s all there in the mathematics, you can interpret it as you want” he replied to a skeptical Zeilinger.
On the second day, Markus Arndt detailed the state-of-the-art on some of the very interesting experiments on matter-wave interferometry. They can interfere objects of up to 2934 atomic mass units and 5600 vibrational modes! One of these is called the “Vienna quantum man”, as the molecule has a shape that looks a bit like a stick man.
Raymond Chiao wants to test Heinsenberg’s Uncertainty Principle against Einstein’s equivalence. He claimed that the clash between the two could be tested with accelerated superconductors. His analysis seemed highly controversial, but hey, there was a clear experimental test proposed, so we can just leave it to experiment to decide.
Nicolas Gisin gave an interesting talk detailing some very nice Bell nonlocality experiments, including of a concept called “Bi-locality”, where the correlations between three particles are modeled by independent local correlations between a central particle and each of the other two. This being a weaker assumption than full locality, the experiments are easier to make, and can be more rigorous as far as loopholes are concerned. Needless to say, quantum mechanics was still upheld.
Abner Shimony’s (from CHSH fame) talk “Quantum Mechanics and Mind” — where he proposed an experimental test for the hypothesis that a definite reduction of a superposition only occurs after a conscious observer learns the outcome — caused clear discomfort in the audience. He was followed by his colleague Michael Horne (the second H in CHSH), who talked about a neat result on the shifting of fringes in an interferometer due to an applied external force.
It was inspiring to see Daniel Greenberger, (from GHZ fame) detail his “Tic-tac-toe theory of gravity”. He starts from a simple assumption that there are three types of mass which can repeal or attract each other in different ways, and derives from very simple non-mathematical arguments things that look like dark matter and the accelerated expansion of the universe. Interestingly, he derived those results before the existence of these phenomena was known (!) but the manuscript was rejected by an angry referee who replied that “we don’t need any new ideas on this field”(!!!). This got to be one of the best referee quotes ever! Surely, there are many problems that would need to be worked in his theory, like why don’t we observe clusters of the other types of matter, to which he gives only tentative partial answers, but he was the first to say he doesn’t take this theory seriously. His main aim was to present it as a motivational talk for the students and postdocs in the audience, so that we not believe that our elders know everything. Thank you, Danny!
I can’t comment on Nobel-prize winner Gerard t’Hooft’s talk. All I can say in my defense is that they shouldn’t have put him first in the morning after all the wine they served at the conference dinner! (ahem!) Well, ok, it was about trying to reproduce quantum mechanics using cellular automata, and the little I saw of it sounded very very interesting! Damn it!
Next it was the turn of Reinhard Werner (from Werner state fame) to remind us that quantum states cannot be thought of as being attributed to each individual system, as this amounts to a local hidden variable theory (alluded to in his title “The most popular hidden variable theory ever”). Actually, that is a fact “well-known by those who know things well”, as a colleague likes to say: it amounts to a quantum separable model, the violation of which is a demonstration of entanglement. But some things need to be repeated until they sink in, I guess.
Bill Unruh (from the Unruh effect fame) cautioned us that looking at the reduced density matrix for signs of decoherence can lead to wrong conclusions. Sometimes it is possible to have large entropy in the reduced density matrices but almost perfect coherence in interference experiments. Thanks for the heads up Bill!
There were many other very interesting speakers and posters, but I wouldn’t have the space to comment on them all, and if you have read this far you will agree with me. As usual, Zeilinger gathered a very nice group of physicists! Looking forward to the Vienna Symposium of 2011!