**Workshop on *Probability and Time Travel***

**Centre for Time, University of Sydney**

**Tue 18th & Wed 19th November 2014
Muniment Room, S401 Old Quad, University of Sydney**

**Speakers:**

Samuel Baron (UWA/Sydney)

John Bigelow (Monash)

Eric Cavalcanti (Sydney)

Phil Dowe (UQ)

Antony Eagle (Adelaide)

Nikk Effingham (Birmingham)

Katrina Elliott (UCLA)

All are welcome, but for catering purposes please confirm attendance to a.j.wilson@bham.ac.uk by 10 November. This workshop is supported by the New Agendas in the Study of Time project at the University of Sydney.

**Schedule:**

*Tue 18th Nov*

10.00am – 10.50am : Antony Eagle

“Chance, change, and time travel”

Abstract: I wish to discuss the following argument: Necessarily, the chance of A can be changed only if it can be changed whether A; whether past outcomes occur cannot be changed; therefore the chances of past outcomes cannot be changed. Since intuitive cases of backward time travel seem to involve our present activities changing the chance of various past outcomes, this argument poses a prima facie difficulty for backward time travel.

10.50am – 11.10am : Tea/coffee

11.10am – 12.00pm : Katrina Elliott

“Is Time Travel to the Local Past Improbable? A (Tentative) Defense of Horwich”

Abstract: What’s the point of time travel? Not to change the past. I wish I hadn’t bet all that money against the Ravens in last year’s Super Bowl. Unfortunately, there’s no fixing it now. I can travel back in time and try to talk my younger self out of placing the bet, or I can try to incapacitate my younger self until after the Super Bowl, or I can even try to kill my younger self, but nothing will make any difference. If it’s true that I bet all that money against the Ravens last year, it’s not true that I didn’t. What happens if I try to prevent my younger self from placing that bet? No matter how carefully I plan, no matter what extremes I go to, and no matter how many times I try, something goes wrong and I fail each time. Paul Horwich has argued that “it is implausible that such mishaps would occur so faithfully over and over again” (Horwich 1987, p. 121) and so it is unlikely that there will be time travelers who repeatedly attempt to change the local past. Since it isn’t particularly unlikely that people would repeatedly attempt to change the local past if they were to time travel, Horwich concludes that people will at most rarely time travel to the local past. Horwich’s argument has been influential, but it has its detractors. I consider two criticisms: that Horwich fallaciously projects regularities that we have only observed in contexts in which there is no backwards causation into contexts in which backwards causation is common (Smith 1997, Dowe 2003), and that the correlations that Horwich claims are improbable are actually analytic truths (Ismael 2003). Though I think these two criticisms fail, I empathize with the feeling that there is something intolerably strange about Horwich’s argument. I conclude by (merely!) speculating that these strange consequences might lead us to conclude that time travel is not possible after all.

12.00pm – 2.00pm : Lunch (own arrangements)

2.00pm – 2.50pm : Eric Cavalcanti

“Quantum models of time travel and the reality of quantum states”

2.50pm – 3.10pm : Tea/coffee

3.10pm – 4.00pm : Phil Dowe

“Closed Time-like Curves Require Revisions of Laws”

Abstract: Spacetimes containing closed timelike curves (CTC’s) require a Global Consistency Constraint on any dynamics in order to avoid paradoxical states. This is well known. However we will argue that global consistency also entails revisions of laws for the case of stochastic laws. We offer a general argument based on objective single case chance, and then we give essentially the same argument for irreversible quantum mechanical decay processes, except that in the second argument we do not rely on the notion of objective chance.

6.30pm : Dinner at Thanh Binh, King St.

*Wed 19th Nov:*

10.00am – 10.50am : John Bigelow

“Counterfactuals, probability-boosting, and timetravel”

Abstract: Lewis argued that time-travel was logically possible. He also argued that time-travel requires personal identity of an ‘older’ traveller, at an ‘objectively’ earlier time, with the ‘younger’ person who set out on the trip at the ‘objectively’ later time. And for Lewis, personal identity requires causal connectedness — which means either counterfactual dependence or probability-boosting interconnectedness or both. I will argue that time-travel requires a circular chain of such links, a snake that eats its own tail, and that this is in fact internally self-contradictory.

10.50am – 11.10am : Tea/coffee

11.10am – 12.00pm : Nikk Effingham

“Worries about the Ultimate Banana Peel”

Abstract: This paper investigates issues concerning the probability of various situations involving time travel. I divide the situations into two categories: negative causal loops and positive causal loops. These two types of situation require two different treatments. Having made some observations about those situations, I argue that (if time travel were possible) we would have an interesting response to the Fermi Paradox, as well as a more depressing result concerning species wide existential risk.

12.00pm – 12.50pm : Samuel Baron

“Time Enough for Explanation”

Abstract: The present paper advances an analogy between cases of extra mathematical explanation (mathematical explanations of physical facts) and cases of what might be termed `extra-logical explanation’: the explanation of a physical fact by a logical fact. A particular case of extra-logical explanation is identifed that arises in the philosophical literature on time travel. This instance of extra-logical explanation is subsequently shown to be of a piece with cases of extra-mathematical explanation. Using this analogy, we argue extra-mathematical explanation is part of a broader class of non-causal explanation. This has important implications for extra-mathematical explanation, for time travel and for theories of explanation more generally.

There is a problem with traveling back in time more significant than the usual paradoxes..

Suppose you ate a sandwich yesterday and its components are now part of your body. You travel back in time, say a week. The bread from the sandwich is part of the loaf that existed then. What happens to the material you ingested? Are there holes in your body? As you move further back in time you disintegrate?

I’m happy traveling in time into the future one second at a time…

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