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Two CHIMP meetings for Spring 2005

The CHIMP group studies Conceptual and Historical Issues in Modern Physics. The following meetings with distinguished overseas speakers were held in Spring 2005.

Thursday October 6, 2005, 5 - 6.30 pm

Armond Duwell

Philosophy, Probability and Modeling Group, University of Konstanz

Is quantum theory all about quantum information?

History Room, S223 (south-west corner), Main Quad, University of Sydney

Recently there have been several attempts to recast quantum theory as a fundamental physical theory of information. Perhaps one of the most influential attempts was made by Clifton, Bub, and Halvorson, who demonstrated that quantum theory can be recovered, more or less, from three information-theoretic constraints. Bub has recently commented on the foundational significance of the theorem. Using an analogy to special relativity, Bub argues that quantum information should be considered a new physical primitive. This talk offers an introduction to this debate for non-specialists, and raises some problems for Bub's approach. I criticize the analogy Bub draws to special relativity and generally de-emphasize the conception of information as a physical primitive. I argue that the analogy to special relativity instead favors a new conception of the properties of quantum mechanical systems, and explain the role of information with respect to these properties.


Thursday October 20, 2005, 5 - 6.30 pm

C. K. Raju

Centre for Computer Science, MCRP University, Bhopal, and Centre for Studies in Civilizations, New Delhi

Why time travel is possible, but time machines are not

History Room, S223 (south-west corner), Main Quad, University of Sydney

By time travel I mean a two-way interaction between future and present (or present and past). This is perfectly possible according to present-day physics, though its empirical consequences have not been much explored. I will explain why a key consequence of time travel 1 is the occurrence of spontaneous events - events that are not explicable or controllable from precise knowledge of the entire past or entire future (or most of both). A time machine would enable mechanical (rule-based) control of such spontaneous events, and is hence impossible. Further, interactions propagating into the past (advanced interactions) increase entropy towards the past, or, what is the same thing, will be seen to decrease entropy towards the future. Hence, a time machine would enable a systematic entropy decrease towards the future, and is hence impossible. For more details, see http://philsci-archive.pitt.edu/archive/00002416/01/Time_Travel_and_the_Reality_of_Spontaneity.pdf.


Thursday 3 November, 5 - 6.30 pm

Matthew Leifer

Perimeter Institute

What's wrong with these Quantum Computations?

venue to be announced

Despite tremendous progress in the theory of quantum computation, it raises many difficult conceptual issues at the intersection of physics, philosophy and computer science that have yet to be adequately resolved. Specifically, I will argue that our current models of quantum computation do not allow us to attach meaning to quantum computations in the same sense that models of classical computation allow us to attach meaning to classical computations. In the first half of this talk, I will give reasons why I think this is a major problem and outline the sort of progress that might be made in quantum computer science on resolving this issue. The skeptic might argue that the problem is just a subset of the difficulties inherent in interpreting quantum theory and is therefore unlikely to have a straightforward solution. However, I will argue that combining ideas from quantum logic, measurement based computing and (perhaps bizarrely) the Density Matrix Renormalization Group is the key to a solution.

In the second half of the talk, I will outline my attempt to construct a "logic of quantum computation" based on the Sequential Quantum Logic (SQL) that was developed in the 1970s. Whilst the attempt is not entirely successful, in that quantum computations cannot be thought of as merely tests of propositions in SQL, a method for deciding the truth of a proposition in SQL with nonzero probability of success on a quantum computer does exist. Thus, it may give useful insights into nondeterministic quantum computation and point the way to the "true" logic of quantum computation.



Last updated: 3 November 2005