Is time travel possible?


  • [Required] Ch.7 of Paul Horwich (1987) Asymmetries in Time MIT Press.
  • [Required] David Lewis (1976) "The Paradoxes of Time Travel" Reprinted in Lewis (1986) Philosophical Papers Vol. II. Oxford University Press.
  • A.P. French (1968) Special Relativity Van Nostrand. [the nuts and bolts of special relativity]
  • Albert Einstein (1920) Relativity : The Special and the General Theory Methuen. [the best informal introduction to the ideas of relativity theory by the man himself.]
  • Butterfield, Hogarth and Belot (1996) Spacetime Dartmouth.
  • Paul Davies (1995) ''About Time' Viking. (popular science)
  • John Earman (1995) Bangs, Crunches, Whimpers, and Shrieks Oxford. [Chapters on time travel, cyclical time and singularities. Presupposes a lot of physics background.]
  • Adolf Grünbaum (1964) Philosophical Problems of Space and Time Routledge. [Chapter 10 is on the question of whether time flows.]
  • Halliwell, Perez-Mercader and Zurek (1994) Physical Origins of Time Asymmetry Cambridge University Press.
  • Le Poidevin and MacBeath The Philosophy of Time Oxford University Press. [A collection of philosophy articles.]
  • Steven Savitt (ed.) (1995) Time’s Arrows Today : Recent Physical and Philosophical Work on the Direction of Time Cambridge University Press. [Two chapters on time travel. An easier one by Horwich and a difficult one by Earman.]


  • Traveling forward in time is actually possible according to special relativity (time dilation).
  • The interesting issue is whether backward time travel (BTT) is possible.
  • Senses of "possible": logical, physical, technical, ...

We can't go back in time yet. Why should we care?

  • Many physicists argue that it is physically and technically possible.
  • Raises interesting philosophical issues about time, identity, causation, etc.

An aside: The Gödel metric spacetime

A rotating universe with no expansion


Argument against BTT

  • If BTT is possible, a person can go back in time to when he was a baby and kill his earlier self.
  • This would lead to a contradiction. It would be true that he did not grow up and it is also true that he grew up and went into the time machine.
  • Since BTT leads to a contradiction, BTT is not possible.

Some responses we shall set aside

  1. When a person goes back in time, he enters a different possible world in another dimension. He grew up in world W1 and went back to kill himself in W2. But the baby in W1 did not die.
  2. Just because something leads to a contradiction does not mean it is impossible.


  • A simpler response: All that the argument shows is that if BTT is possible, then auto-infanticide is not possible.
  • It implies that if BTT is possible, auto-infanticide attempts will necessarily fail. (killing the wrong person, guns malfunctioning, slipping on banana peel, etc)
    • Some might think this is rather strange, but it might be replied that it is merely a matter of logical consistency.

"Changing the past" paradox

  1. If BTT is possible, a person can go back and change the past.
  2. But it is not possible to change the past.
  3. So BTT is not possible.

Horwich's inference:

  • If BTT will be freely available, we would expect many attempts to change the past.
  • But these attempts are bound to fail.
  • These attempts will correlate with repeated coincidences of accidents happening.
  • Such correlations are rather improbable.
  • So it is improbable that BTT will be freely available.
  • Explanations?

Duplication paradox

  1. If BTT is possible, a person (e.g. Jimmy) can go back and visit his earlier self, e.g. when he was a baby at time t.
  2. But there would be a contradiction - Jimmy was a baby at time t, and Jimmy was not a baby at time t.
  3. So BTT is not possible.

Horwich's reply

  • Need to relativize properties to a frame of reference.
  • "Being F at time t" becomes "being F at time t as measured by a clock associated with a reference frame R"
  • "Jimmy was a baby in 1980 as measured by the Earth clock" and "Jimmy was not a baby in 1980 as measured by Jimmy's Rolex" can both be true.

Does this reply really work?

"Something from nothing" paradox

Causal loops with no explanation

  1. If BTT is possible, a person X can go back in time and give his earlier self an item.
  2. His earlier self passes on the item to his later self who eventually takes it into the time machine.
  3. But then the item comes from nowhere, which is not possible.
  4. So BTT is not possible.


  • Does it make a difference whether the item is a physical object or whether it is a piece of information?
  • David Lewis's example:

@Recall the time traveller who talked to himself. He talked to himself about time travel, and in the course of the conversation his older self told his younger self how to build a time machine. That information was available no other way. ... But where did the information come from in the first place? Why did the whole affair happen? There is simply no answer. The parts of the loop are explicable, the whole of it is not.@

  • Lewis: "Strange! But not impossible, and not too different from inexplicabilities we are already inured to." eg God, big bang, random decay.

"Why no visitor" objection

  1. If BTT is possible, we should already come across visitors from the future.
  2. We have not come across visitors from the future.
  3. Therefore BTT is not possible.


  • BTT can be possible even if no BTT will ever take place.
  • Even if BTT will take place, time travelers might not choose to visit some other period.