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Philosophy week seven – time travel


Alasdair Richmond

Time Travel and Philosophy

These lectures were great and there is so much interesting reading to do I doubt if I will finish it before the end of the course. What I loved most about this particular part of the course was that I understood all the language.

So what did I learn?

Our first lecture by the wonderful Dr Alasdair Richmond was entitled

 “What might time travel be anyway?”

It sounds obvious doesn’t it? Time travel is travelling to the past or the future  But we looked at it in more detail than that. A lot more detail.

We began by considering a 1976 paper by David Lewis called ‘The Paradoxes of Time Travel’. But before going into the paradoxes we need to consider what it actually is. Lewis talked about forward and backward time travel differently mainly because backward time travel is more tricky.

With forward time travel both the person travelling and time itself are going in the same direction but at different speeds or durations however you want to look at it. In this case an example might be that five minutes of personal time equates to fifty years of external time. For backward time external time is moving in the opposite direction so five minutes of personal time might equate to a negative fifty years of external time.

Source; Guardian

We then looked at Einstein‘s Special Theory of Relativity which predicts that “the rate at which time passes is not an absolute, not an invariant but varies according to relative speed [sic. – velocity]. In other words, the greater the relative velocity between two systems, the closer that relative velocity comes to the speed of light, the more that the rate of temporal passage diverges in those two frames of reference.

So OK when I said I understood all the words I lied a little about this bit. It’s as clear as it’s ever going to be. The important thing that I understood is that :-” the Special Theory of Relativity says that if I travel fast enough relative to the Solar System, I can make the rest of my life span tens or hundreds or millions or even billions of years.  So forward time travel is very deeply embedded in Einstein’s Special Theory of Relativity and we have decades and decades of very well-supported physical results which suggest that these divergences between frames of references really, really occur.”

Source ;theonano.wordpress.com

Backwards time travel is a bit more dodgy which is maybe why no one has gone forward (that I know of). How would they get back again to tell us about it or even to prove it. The General Theory of Relativity says that if we have an enormous amount of mass or an enormous density of mass or enormously rapid movement of mass, then time travel in both directions might be possible.

Kurt Gödel designed some models that suggested it was possible.  Here is the beginning of it from Wikipedia.

Like any other Lorentzian spacetime, the Gödel solution is defined by giving the metric tensor in terms of some local coordinate chart. It may be easiest to understand the Gödel universe using the cylindrical coordinate system presented lower down, but here we will give the chart that Gödel originally used. In this chart the metric is defined by:

 ds^2= \frac{1}{2\omega^2} \, \left( -\left( dt + \exp(x) \, dz \right)^2 + dx^2 + dy^2 + \frac{1}{2} \exp(2x)\, dz^2 \right)
 -\infty < t,x,y,z < \infty

where \omega is a nonzero real constant.

So there you have it. I understood at least half the words in that paragraph.

Grandfather Paradoxes

Source : tamilandvedas.wordpress.com

So David Lewis said that it wasn’t possible to create a contradiction or a paradox. For example you can’t go back in time and kill your grandfather (supposing you would want to) because then you wouldn’t have been born.

Lewis thinks that logical contradictions can’t occur. Where he thinks the Grandfather Paradox argument goes wrong is in assuming that time travel necessarily generates the ability to enact logical contradictions. In his analysis each moment, each time, happens only once. He says that the very premise is faulty.

‘If it was possible to travel backward in time, it would be possible to generate paradoxes’.

To look at this properly, we need the notion of something being compossible. This means possible relative to a set of facts or certain states of affairs. So it is possible for me to understand the theory of relativity. I have learned other things so I am capable of learning that too. But I don’t because I was never taught it to an extent that I understood it.

So, Lewis says, backward time travel can be logically consistent provided you bear in mind that what’s possible relative to one set of facts may not be possible relative to another set of facts. So you could go back and maybe kill someone but that person couldn’t possibly be your grandfather because otherwise you wouldn’t exist. Clear? Good, let’s continue.

Two senses of change

So we can see that Lewis believes a time traveller could effect the past only to the extent that it doesn’t change a truth in the time from which he came.  If you went back to kill your Grandfather you could as long as the person you killed wasn’t actually your Grandfather , or anyone else’s for that matter unless he had already had his children.

Lewis said that everything only happens once. If all your actions already exist in the past how can you effect the past?  Lewis thought it could happen in two senses.

1. Replacement change

If you drop a glass of water then you have replaced it with glass fragments and a puddle. That is a replacement change of a concrete object. Replacement changes happen to concrete objects but not to times or events.

Because everything only happens once then Lewis said that this was not possible– you can’t ‘replacement-change’ any time past, present or future. All consequences of a persons actions have already happened in the ‘now’  including”every blade of glass trodden , every squashed butterfly”. Nothing can be changed.

2. Counterfactual change

This is a change contrary to the facts. You know all those times you say “if only such and such had happened” but it didn’t and so you are stuck with what ever the outcome. If only I had managed to catch the early train I wouldn’t have been late. But I didn’t so I was so I didn’t hear the director saying that really important point at the beginning…..etc There is not two versions of this even only the one where I didn’t catch the early train. If I had then history would have been different.

Lewis maintains that time travellers can have an impact in the counterfactual sense.  That is something different might have happened if the time traveller had not been there.  For example you could go back in time and save your grandfather from a mortal accident.

Causal Loops

Source : tvtropes.org

From a backwards time travel point of view  a causal loop is a chain of events that loops back in time, so that an event turns out to be among its own causes.

For example if I photocopied the entire works of Shakespeare and then travelled backwards in time and gave them to Shakespeare in 1588. He then goes ahead and puts on all these plays and writes them down and they in turn become incorporated in the book I copied to take to him.

But who wrote all those plays?

 Imagine that you are home one evening, and the ‘phone rings and an oddly familiar voice says “Don’t say a word, write these instructions down and follow them to the letter.” And the voice proceeds to recite some instructions for how to construct and operate a time machine, and you follow the instructions and you discover the time machine has deposited you in the recent past. And once in the recent past, you call your own ‘phone number. An oddly familiar voice says “Hello?” and you say into the phone, “Don’t say a word, write these instructions down, and follow them to the letter”. And you proceed to give your younger self the instructions that you remember, for how to build and operate a time machine. (David Lewis)

There are no paradoxes with these examples. All the information is consistent with reality. Although counter intuitive Lewis believes that it is not impossible. After all, he added, “where does any information come from? ”  For each event, you can appeal to an earlier event, and an earlier event still, and an earlier event still, back and back and back, but there’s no end to the chain. And so there’s no answer to the question, “Where does the whole chain come from?” The chain itself has no origin, has no end point. So that’s an infinite linear chain. There comes a point when you can’t go back to an earlier event. What came before the Big Bang? What is North of the North Pole?  Causal loops are no more problematic than any other chain of event.

So there we have it, time travel. Apart from a few more theories like those of David Deutsch and Michael Lockwood, I think I have covered it all.


  1. Julie Tomlinson says:

    Thank you for providing the link to your blog in the Modern/Postmodern class discussion! This time travel summary is excellent, and I can add something I recently read….

    Some physicists and philosophers believe there are infinite dimensions, each containing every possibility (and every branch of every possibility) of everything that has ever occurred and anything that can ever occur. And when you time travel backward, you can only go to a branch that doesn’t form a contradiction. For instance, you can go back to a branch that includes your maternal grandfather’s death before your mother was born – going back all the way through your lineage. The only branches you can’t go back to are ones that cause contractions, loops, etc.

    Also, many (most?) theories/philosophies I’ve read say there is no such thing as “now,” and past and future are not distinct, but our 3-D perception only gives us the ability to see what we perceive as the past, and only allows us the perspective of moving into the future. So, theoretically, we can go either direction. Beyond that, the theories I’ve read get into physics lingo that’s about 3/4 over my head, but it’s still fascinating.

    The newest book I’ve read about this in is Jim Holt’s “Why Does the Earth Exist?” I discovered it when I read its review written by our Mod/Postmod philo instructor, Michael Roth: http://www.huffingtonpost.com/michael-roth/review-of-jim-holts-why-d_b_2661060.html. It’s a great book, and Holt is very witty – parts of it had me laughing out loud.

    Thanks again! I’ll be back to your blog soon….

    – Julie

    • Louise Taylor says:

      Hello Julie and thanks for coming by and commenting.
      That is very interesting information. I have just submitted my essay for this course and I chose the subject of time travel as it fascinates me. I concluded in that essay that I didn’t think we would be able to move in the same ‘dimension’ not based on what you said here because I hadn’t heard about it but based on the Higg’s boson and string theory.
      Doing this philosophy course really made me think about a lot of things and whether they are real. If now doesn’t exist either then where and when are we. I don’t think will spend too much brain power on it as I have other things to worry about like what’s for tea.

      Thank you for the link to the book review. It sounds like a great book to get hold of. I have so many books to get through at the moment so It will have to wait in line.

      See you in class :))


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