[Trinity College]

Free Will and Retrocausality in the Quantum World

A conference held under the auspices of the JTF-funded project, New Agendas for the Study of Time

Venue: Winstanley Lecture Theatre, Trinity College, Cambridge
Dates: 1—4 July 2014

Programme [with links to videos of talks and discussion sessions]

Why retrocausality — and why free will?

The 'classic' motivation for retrocausal models in QM stems from Bell's Theorem, and the nonlocality it seems to entail. Nonlocality is often felt to be counterintuitive in itself, and the source of an unresolved tension between quantum theory and special relativity. As Bell himself described the implications of his famous result: “[I]t's a deep dilemma, and the resolution of it will not be trivial ... [T]he cheapest resolution is something like going back to relativity as it was before Einstein, when people like Lorentz and Poincaré thought that there was an aether — a preferred frame of reference — but that our measuring instruments were distorted by motion in such a way that we could not detect motion through the aether.''

As Bell was well aware, the dilemma can be avoided if the properties of quantum systems are allowed to depend on what happens to them in the future, as well as in the past. Like most researchers interested in these issues, however, Bell felt that the cure would be worse than the disease — he thought that this kind of “retrocausality” would conflict with free will, and with assumptions fundamental to the practice of science. (He said that when he tried to think about retrocausality, he “lapsed into fatalism”.)

If this objection to retrocausality in QM is well-founded, it raises interesting issues about the nature and origins of this "free will", that turns out to play such a surprising role in the foundations of physics. If the objection is not well-founded, then it is high time it is moved aside, so that the retrocausal approach can be given the attention it otherwise seems to deserve.

Moreover, there are other motivations for exploring retrocausal models in QM, some the focus of considerable current research. Examples include:

  • The proposed retrocausal explanation of the results of 'weak measurements' by Aharonov, Vaidman and others.
  • The relevance of retrocausality to the issue of the viability of an 'epistemic' interpretation of the quantum state, especially in the light of recent results such as the PBR Theorem.
  • Recent work throwing new light on the relation between retrocausality in QM, on the one hand, and time-symmetry and other symmetries, on the other.

For these reasons, too, there is a pressing need for a better understanding of notions of free will and causality, and of their relevance to the retrocausal approach to the quantum world. This conference brought together many of the leading writers and researchers on these topics, to discuss these issues.

Participants and invitees: John-Mark Allen, Jonathan Barrett, Howard Barnum, Stephen Bartlett, Dan Bedingham, Gordon Belot, David Braddon-Mitchell, Jeremy Butterfield, Adam Caulton, Eliahu Cohen, Karen Crowther, Phil Dowe, Fay Dowker, Avshalom Elitzur, Matt Farr, Nicolas Gisin, Berry Groisman, Joe Henson, Jenann Ismael, Ruth Kastner, Adrian Kent, James Ladyman, Ray Lal, Dustin Lazarovici, Ciaran Lee, Matt Leifer, Peter Lewis, Owen Maroney, Joseph Melia, Sonny Mendez, David Miller, Kristie Miller, Nick Murphy, Tim Palmer, Brian Pitts, Sandu Popescu, Huw Price, Matt Pusey, Dean Rickles, Sebastian Rivat, Bryan Roberts, Terry Rudolph, Rafael Sorkin, Nick Teh, Chris Timpson, Jeff Tollaksen, James Troupe, Lev Vaidman, Ken Wharton, Steve Weinstein, Lena Zuchowski. 

Organisers: Huw Price (Philosophy and Trinity College, Cambridge) and Matt Farr (Centre for Time, Sydney)