Logic & Philosophy of Science


Hans Halvorson
Department of Philosophy, University of Pittsburgh

“Does Relativity Imply Quantum Nonlocality? Unpacking the Reeh-Schlieder Theorem”

A fundamental question of interest for philosophers of science in general, and philosophers of physics in particular, is: "Are relativity and quantum nonlocality consistent?" Some philosophers have ignored or made light of the fact that, at least on a purely formal level, the answer to this question has long been known: Yes, there is a mathematically consistent theory -- viz., algebraic relativistic quantum field theory (AQFT) -- that combines in a rigorous manner the fundamental assumptions of both quantum theory and the special theory of relativity. In fact, it appears that not only are relativity and quantum nonlocality consistently combined in AQFT; relativistic causality actually enforces quantum nonlocality in this context.

One sharp example of this phenomenon is the so-called Reeh-Schlieder (RS) theorem. In this presentation, I will show explicitly how the RS theorem uses the assumption of relativistic causality to derive a highly nonlocal conclusion. In particular, I will focus my discussion around explicating two claims that have been made about the RS theorem: (1) Simon Saunders claims that the RS theorem is a "purely relativistic result." I will show that while in one mathematically precise sense, Saunders' claim is true -- viz., the RS theorem fails when we pass to the non-relativistic limit (c -> infinity) -- in the strictest sense it is false, since a modified version of the RS theorem continues to hold in nonrelativistic, Galilei-invariant quantum field theory. (2) According to Irving Segal, the RS theorem (if its premises were true) would entail that, "the entire state vector space of the field could be obtained from measurements in an arbitrarily small region of space-time!" I will show the precise sense in which Segal's claim is true, and I will argue that the resulting nonlocality is not -- as Segal supposes -- "at variance with the spirit of relativistic causality."

Friday, May 26, 2000
SSPB 1208, 3 pm