PHIL DOWE A DEFENSEOF BACKWARDS IN TIME CAUSATION MODELS IN QUANTUM MECHANICS ABSTRACT. This paper offers a defense of backwards in time causation models in quantum mechanics. Particular attention is given to Cramer’s transactional account, which is shown to have the threefold virtue of solving the Bell problem, explaining the complex conjugate aspect of the quantum mechanical formalism, and explaining various quantum mysteries such as Schr¨ odinger’s cat. The question is therefore asked, why has this model not received more attention from physicists and philosophers? One objection given by physicists in assessing Cramer’s theory was that it is not testable. This paper seeks to answer this concern by utilizing an argument that backwards causation models entail a fork theory of causal direction. From the backwards causation model together with the fork theory one can deduce empirical predictions. Finally, the objection that this strategy is questionable because of its appeal to philosophy is deflected. 1. INTRODUCTION The concept of backwards in time causation has actually had quite a high profile in twentieth century physics. One has only to think of the interest aroused in tachyons, particles which travel faster than the speed of light, the “Feynman electron”, Feynman’s bold conjecture that positrons are really electrons travelling backwards in time, or the recent surge of interest in time travel (see Earman 1995, 268). It is quite surprising, therefore, that the backwards in time model of Bell phenomena, although it has a long tradition, 1 has received so little attention from theoretical physicists or from any of the numerous popularisers of the wonders of modern physics. 2 And this is all the more surprising when one considers the promise of this model to solve all the deep mysteries of quantum mechanics (see Cramer 1988, section IV). In this paper I want firstly (Section 2) to consider the backwards in time causation model of Bell phenomena, and in particular to sketch the transactional account due to Cramer, in order to highlight just why, from the point of view of physics, the theory deserves more attention than it has received. In particular I will highlight three putative achievements of the transactional account: its solution to the Bell problem, its explanation Synthese 112: 233–246, 1997. c 1997 Kluwer Academic Publishers. Printed in the Netherlands.