# Time Travel and Backwards Causation

Over at Tapped Matthew Yglesias links (tounge-in-cheek) to the SEP’s entry on backwards causation, with some sarcastic suggestions for how Bush’s defence of the war in Iraq might look better if backwards causation were plausible. I don’t want to enter into the politics of this, but there is something odd about the Stanford piece.

As Matt says, in the debate about backwards causation as such, the main defender of the coherence of the view is Michael Dummett. But surely there are many more defenders of this view, namely all the defenders of the coherence of time travel. (Of which, as the SEP also notes, there are quite a few.)

Well, maybe not. Here’s the entry on backwards causation again.

The idea of backward causation should not be confused with that of time travel. These two notions are related to the extent that both agree that it is possible to causally affect the past. The difference, however, is that time travel involves a causal loop whereas backward causation does not. Causal loops for their part can only occur in a universe in which one has closed time-like curves. In contrast, backward causation may take place in a world where there are no such closed time-like curves. In other words, an ordinary system S taking part in time travel would preserve the temporal order of its proper time during its travel, it would keep the same time sense during its entire flight (a watch measuring S’s proper time would keep moving clockwise); but if the same system S were to become involved in a process of backward causation, the order of its proper time would have to reverse in the sense that the time sense of the system would become opposite of what it was before its back-in-time travel (the watch will start to move counter-clockwise). So neither backward causation nor time travel logically entails each other and time travel is distinct from back-in-time travel.

This seems to me mistaken on several fronts. First, time travel does not logically require closed time-like curves. Plenty of time travel defenders (e.g. Lewis) defend the coherence of sci-fi style ‘jumps’ from one time to an earlier time. This is not scientifically plausible, for several reasons, but then neither is backwards causation. (Except, I should add, in quantum cases where backwards causation might explain the Bell inequality.) Second, there’s no reason to say that backwards causation need be a process. Dummett’s original example was the rain dance on Wednesday causing the rain on Tuesday (or something like this – I don’t have the book in front of me). Backwards causation might be event causation, not process causation. Third, we might have backwards causation along one kind of process, even while we have forward causation in others. Imagine a case where some coffee ‘despills’ from the floor to the coffee cup, but all the while cools down as coffee does at room temperature. It doesn’t have its ‘personal’ time reversed, but there is something we’d generally call backwards causation going on. So I don’t really see why these cases should be treated apart, and these debates kept separate.

## 2 Replies to “Time Travel and Backwards Causation”

1. Kenny Easwaran says:

Wouldn’t most sci-fi style time travel jumps create closed time-like curves of a sort? Or at least, couldn’t they be seen as such?

If I go back in time to shoot at my grandfather (and miss), then there’s a closed time-like curve going from me a few minutes ago, through me at the time of the jump, to the bullet near my grandfather, to his perception of the bullet, to his children, to their children, including me a few minutes ago. The fact that there’s one object (namely me) that has parts right now causally affecting parts in the past suggests that it is reasonable to think of there being a continuous time-like curve going through those parts in that order.

With the rain dance however, the causation is all at a distance of some sort, and no time-like curves are necessary.

2. A Scott Crawford says:

I think there’s a logical case to be made for something akin to what SEP calls “backwards causation” that doesn’t require time travel or obnoxious fallacy. A better term might be “20/20 hindsight causation”.

The case goes something like this: A broad conclusion is supported by a large number of inference chains, some inter-related, some independent. The variety and nature of the supporting propositions and/or premises taken with all of their various unstated or implied inferences make the evaluation of the argument as a whole one of measuring inductive strength rather than determining necessary validity or soundness. In cases where the reasoning requires a certain acceptance of a complexity of cause, those making the claim typically choose what they believe to be the handful of supporting arguments that are likely to give their conclusion the greatest amount of inductive strength… they hedge their bets, in other words, by choosing primary and secondary and etc. propositions in proportion to those propositions expected contribution as premises for their conclusion.

In cases where the question of causation is particularly complex, it’s common to discover that the handicappers and oddsmakers miss some important detail which, had it been known before hand, would have changed the relative merits and valuations used in selecting the supporting propositions believed to deliver the greatest inductive strength. Ooops. The temptation is to apply 20/20 hindsight or historical revisionism to ones argument in order to avoid admitting that it turns out that what seemed at the time to be an inductively strong argument was, in fact, NOT a strong argument at all; and that one’s conclusion turned out to be correct mostly by accident.

Say a conclusion, X, is basically supported by premises p1-p9. Now say at least a third of the premises have to be valid for ones conclusion to be passable. If one selects p1-p5 and neglects p6-p9 due to ignorance or etc, and suddenly discovers that three of the five premises ones chosen aren’t any good after the fact… but lucks out in that p4-p9 are ALL good. The temptation is to try to snake out of admitting the original argument with p1-p5 was unsupported… likewise there’s a reverse temptation to refuse to acknowledge that an opponents PARTICULAR argument didn’t support his conclusion, and refusing to conceed the conclusion on these grounds alone, knowing full well in hindsight that there WAS an argument(s) supporting the conclusion in question.

In the Iraqi war case: Bush (and Clinton) can’t deliver WMD’s, which they made their primary justification for the invasion. They do a 20/20 hindsight trick by claiming that “if we hadn’t invaded they WOULD have had WMDs”“, which is unknowable and logically cynical (a fallacious ‘reverse causation’).

If instead the Bushies pointed out that the UN inspectors took bribes from Hussein, and that the Oil for Food payoffs make it unlikely that they’ll be able to find WMDs, then they’d have a better case. As it should be obvious to just about anyone that bribing weapons inspectors is done for a lot of reasons, but that paying them off to NOT find something that’s NOT in Iraq isn’t one of ‘em. This would be an example of including additional extra supporting premises after the fact, rather than subtracting or substantively changing the original primary premises.

Am I being vague?