Please find below this term’s programme of the Sigma Club. As always, further information is available at:

http://www.lse.ac.uk/CPNSS/events/SigmaClub/Home.aspx

7 October

Oswaldo Zapata, LSE

Is there still any hope for string theory?

Abstract: In the first part of the talk we will cover how string theory is related to supersymmetry, and what we can learn from the experiments at the LHC. We will also touch upon how supersymmetry extends our notion of the standard relativistic four-dimensional space-time. A short discussion of supergravity, that is, local supersymmetry, follows. We will see that supergravity is in fact a special limit of superstring theory. Next, we will explore the string theory proposal stating that the universe is a sort of hologram. According to this idea, the universe we live win is in one to one correspondence with a lower dimensional world where all the information of our higher dimensional universe is stored. Some phenomenological consequences of this correspondence, such as the understanding and conception of new materials with technological applications, will be discussed at the end.

21 October

Joseph Henson, Imperial College London

Locality Reinstated?

The assumptions of Bell’s theorem were meant by its author to to characterise a lack of any physical superluminal influences. The “Quantum non-locality” he discovered does violence not only to our intuitions, but arguably also to any thoroughgoing attempts to apply causal explanation in QM.

It is natural to wonder if there is a better way to characterise locality (= lack of superluminal influence) that is consistent with QM, thereby resolving an apparent contradiction by discarding some conceptual “excess baggage.” However, in order to do this we must be careful to preserve what is essential in the idea of locality. In this talk, I will describe a particular view of Bell’s condition, and point out a (hopefully uncontroversial) list of conditions that any reinstated “locality” should obey. I will then apply these conditions to a number of candidates in the literature, including some treatments of locality in many-worlds QM, Howard’s non-separability and “super-determinism,” and find them to be violated. Finally I will speculate about the extent to which a useful concept of locality \textit{can} be preserved in QM.

4 November

Samuel Fletcher, Logic and Philosophy of Science, University of California Irvine

On the Reduction of General Relativity to Newtonian Gravitation

Abstract: Accounts of the reduction of general relativity (GR) to Newtonian gravitation (NG) usually take one of two approaches. One considers the limit as the speed of light c → ∞, while the other focuses on the limit of formulae (e.g., three-momentum) in the low-velocity limit, i.e., as v/c ≈ 0. Although the first approach treats the reduction of relativistic spacetimes globally, many have argued that ‘c → ∞’ can at best be interpreted counterfactually, which is of limited value in explaining the past empirical success of NG. The second, on the other hand, while more applicable to explaining this success, only treats a small fragment of GR. Further, it usually applies only locally, hence is unable to account for the reduction of global structure. Building on work by Ehlers, I propose a different account of the reduction relation that offers the global applicability of the c → ∞ limit while maintaining the explanatory utility of the v/c ≈ 0 approximation. In doing so, I highlight the role that a topology on the collection of all spacetimes plays in defining the relation, and how the choice of topology corresponds with broader or narrower classes of observables that one demands be well-approximated in the limit