David Lewis on the logical possibility of time travel

L asked:

I am currently writing an essay on the topic of the logicality of time travel and I have been researching various philosopher’s ideas on time travel. However, I am really confused over David Lewis’ well-respected article on the ‘Paradoxes of Time Travel.’ Lewis clearly shows he believes time travel could be possible and he goes on to say that time travellers would not be able to change the past. However, he admitted that one would alter the past just by being in the past.

I am not sure if I have misread his work or if Lewis has made a contradiction. If by merely travelling back to the past an individual will cause an alteration to the past then how can time travel be possible if Lewis claims one cannot change the past? If I have understood his argument correctly (though I have probably just read it wrong), his argument appears to be logically flawed.

I was wondering, if you are familiar with his work, you would be able to explain?

Thank you!

Answer by Craig Skinner

Wouldn’t it be great to hand in an essay uncovering a logical howler by a famous philosopher!

Maybe next time.

You have misread Lewis. He doesn’t say one could ALTER or CHANGE the past, only that one could AFFECT it.

But you can be forgiven, for Lewis falls a little short, here and there, of his usual exemplary clarity. First, he doesn’t spell things out, contrasting altering/changing with affecting. Secondly, and having regard to your remark about ‘he admitted that one could alter the past just by being in the past’, his text is confusing, as follows (talking of the time traveller):

‘he changes the past from the unactualized way it would have been without him to the one and only way it actually is. To ‘change’ the past in this way,… it is enough just to be there…’

What he means here is that the TT, by being there, affects the past, has an effect on events, not that he changes it. There is no change to the past. Lewis contrasts here what ACTUALLY happens with what COULD HAVE happened had circumstances been different (eg had the TT not been there). In short he is alluding to counterfactuals and possible worlds. Of course he is a maestro in both of these fields, but I think they are a different kettle of fish from time travel.

It is a mistake to think that there could be different versions of the past, one that happened originally, and a new one when a time traveller goes back and takes a hand in events.There is only one past, it’s over, fixed, done and dusted. Any actions by time travellers have already been built in to the past. Simple example. Next month I go back to 1215 and am a signatory to the Magna Carta. This means that anybody who has studied the document in the last 800 years will have seen my signature on it. I affected the past (by being there, as a time traveller, taking part in the events, all those years ago), but I didn’t CHANGE the past.

You talk of the logicality of time travel (to the past). So here are my views on the two main alleged logical paradoxes.

The Grandfather Paradox

The fact that I am here means my grandfather wasn’t killed as a lad. But I could go back in time and shoot him, in which case I couldn’t exist. Contradiction, hence time travel impossible.

Not so. My grandfather wasn’t killed as a lad. So, if I were there as a time traveller, I didn’t succeed in killing him. Certainly, if I tried repeatedly, a series of amazing freak accidents would have occurred: the gun jams on my first try; bullets turn out to be blanks on second try; I fire at the wrong boy on my third try, etc. All very strange, but then time travel is strange so we can expect some of its implications to be strange.

The story of the history student is another amusing example. Appalled by the carnage of the First World War, learning that it was triggered by the assassination of Archduke Ferdinand, she resolves to go back and prevent it. She studies all the archives, knows exactly where the assassin will be etc, goes back, approaches him, then trips and bumps into the assassin, who was a poor shot and would have missed, nudging his arm so that the line of fire now finds its mark. Far from preventing, she has triggered the war. Horrified, she later goes back to stop her other self from tripping, but her attempt just causes the trip, and so on.

The only way in which I could go back and kill my grandfather is if time is branching or there are parallel universes. Here I go back, kill the old boy, and the universe divides into two, one where I dont exist, the other where I came from. Alternatively my travel takes me to a parallel universe just like ours up to the time of my arrival but with a different future in that I kill the old boy and never get born in that universe (but am there as a visitor to do the killing, so no paradox). However I think we can deal with the oddities of time travel without invoking branching/parallel universes.

The Free Knowledge Paradox

Rummaging in a cupboard, I find a notebook with details of how to build a time machine. After twenty years of toil I build it, then travel back twenty years to leave the instructions in the cupboard. I am a physicist tired of failed attempts to find a theory of quantum gravity. I travel two hundred years into the future, look up the accepted theory (aha!), write it all down, return and submit it to a journal whereupon it becomes the Nobel-winning accepted theory.

Where does the knowledge come from in these cases. I dont know, but knowledge is knowledge whatever its source, one could say.

For a really strange loop how about in two hundred years physics has advanced enough for time travel and for us to make universes having laws of nature of our choice, one of us travels back 13.7 billion years and sets off the Big Bang that started our universe

Incidentally, we cant change the future either, we can only affect it. Just like the past, there is only one version.


Answer by Shaun Williamson

I am not familiar with this article by David Lewis but I am interested in the logic of time travel. If you time travel back to the 16th century then, since you didn’t exist in the 16th century you are not really travelling back to the 16th century. You are travelling back to a possible alternative 16th century. When you return to the present you are not really returning to the present you left. You are travelling back to a possible alternative present which now contains the fact that you also existed in the 16th century. The present that you left didn’t contain this fact.

I don’t know if time travel will ever be possible but the logic of time travel is perfectly consistent as long as you accept the notion of possible alternative worlds.


Answer by Geoffrey Klempner

I glanced at the article by David Lewis when I wrote my Afterword to the reprint of David Gerrold’s sci-fi time travel classic The Man Who Folded Himself but didn’t read it properly. As a result I completely missed the point that Craig has picked up on, that time travel that avoids the grandfather paradox is logically possible without positing alternative worlds.

That’s what comes from being lazy. Mea culpa. My bad.

Alternative worlds are still the only way to go if you want to have fun changing the known facts of history — such as prevending Kennedy’s assassination or the attack on the Twin Towers. But all you would be doing, as I noted in my essay, is saving Kennedy or the people in the Twin Towers in an alternative world, not the original one.

Let’s say your brand new Apple tablet which you put on the dinner table mysteriously disappears. 50 years ago a strange object (which we would today recognise as an Apple tablet) materialised in a diner in Nebraska, and was subsequently taken to Area 51 where it was studied as a possible alien artefact.

That’s how time travel works. The time traveller disappears and reappears (from their perspective) in the past. In real-time, however, the event of the time traveller materialising in the past preceded, as it must, the event of the time traveller pressing the button and disappearing.


One thought on “David Lewis on the logical possibility of time travel

  1. I only read Craig Skinner’s response (he answered the question so I didn’t need to read the others). Brilliant answer to the same question I had. Perfectly explained. Moreover, it stopped me from giving a strawman argument against Lewis in my exam this week!

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