Here are the details of the next Q+ hangout.

Date/time: Tuesday 23rd October 2pm BST

Speaker: Markus Mueller (Perimeter Institute)

Title: Three-dimensionality of space and the quantum bit: an information-theoretic approach

Absract: It is sometimes pointed out as a curiosity that the state space of quantum two-level systems, i.e. the qubit, and actual physical space are both three-dimensional and Euclidean. In this talk, I report on joint work with Lluis Masanes, where we attempt an information-theoretic analysis of this relationship, by proving a particular mathematical result: suppose that physics takes place in d spatial dimensions, and that some events happen probabilistically (not assuming quantum theory in any way). Furthermore, suppose there are systems that behave in some sense as “units of direction information”, interacting via some continuous reversible time evolution. We prove that this uniquely determines spatial dimension d=3 and quantum theory on two qubits. Hence, abstractly postulating the “nice” behavior of a Stern-Gerlach device in information-theoretic terms determines already some important aspects of physics as we know it.

This talk is based on http://arxiv.org/abs/1206.0630

To watch the talk live go to http://qplus.to/qplus at the appointed hour.

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The Q+ hangout series looks really good, but it is a shame that in either Seattle or Sydney the time difference makes it very inconvenient to attend. 2pm BST is 6am in Seattle (or Berkeley or Caltech, etc.) and it’s midnight in Sydney.

When we first started the Q+ hangouts we had interest from the east coast of US and Canada and from the far east, so we chose a time that would make it possible for those people to watch live. We know we will get our regular audience if we schedule the hangouts at that sort of time. Obviously, it is impossible to choose a time that is convenient in every timezone, but we would be willing to experiment with the time if it seems like we would get a decent audience and of course we would have to do so if we get a speaker from the west coast or Australia.

I have a question that’s off-topic.

The Kochen-Specker theorem shows, essentially, that a quantum system does not possess some properties before observation. One of the properties a quantum system does not possess before observation in the appropriate double-slit experiment is location. (The dimension of the Hilbert space is infinite: greater than 2.) As a result, the system does not exist in “space” as realized by the double-slit apparatus, until observation.

So, how does the quantum system know to collapse onto the screen at all? Is it only because of superselection rules? If so, how do they work? How does the rule lead to the screen’s knowing when/where to collapse the quantum system?