Science Sundays with John Duffield: Time Travel

You can beat an egg, but you can’t beat a good movie. And there are some real crackers out there. The Terminator was a classic, with that remorseless skeletal T-800 that just won’t stop. Another cracker was 12 Monkeys, which featured a scary way to save the planet. Then there’s Men in Black3, Looper, and Lost in Space. Or Frequency or Déjà Vu or Source Code. And if chick-flick is your thing there’s Kate and Leopold, The Lake House, and The Time Traveller’s wife. They all feature time travel, and I loved them all, and more besides. One of my special favourites was Timescape starring Jeff Daniels, where he and his other self hammered the church bell to sound the warning. Dang Ding Dang. Brilliant. I loved it even though I know that time travel is science fiction.

Yes, science fiction. Time travel to the past is impossible. Not because of the grandfather paradox or the chronology protection conjecture. But because we don’t even “travel forward through time”. That’s just a figure of speech. So is “the flow of time”. If you open up a clock you don’t see time flowing through it like it’s some gas meter. You see little cogs and things, moving. In a grandfather clock there’s a pendulum swinging. In a quartz wristwatch there’s a crystal vibrating. In an atomic clock it’s microwaves moving. Clocks always feature some kind of regular cyclic motion, and they clock this up to show you some kind of cumulative display that you call “the time”.

You must have seen some movie or TV program where some guy has a device that can stop time. Did you notice that the gizmo doesn’t so much stop time as stop motion? It’s the same kind of deal with bullet time. It’s not time going slower. It’s bullets going slower. Or just hold your hands up. See that gap between them? That’s a space, and you can see it’s there, even though you can’t see the space itself. Now waggle those hands. That’s motion, and you can see that too. But can you see time? No. Can you see time flowing? No. Can you see any actual travelling through time? No.

Have you ever heard Hawking saying you can “travel” to the future on a superfast train? Imagine you’re on it. The train moves fast. Through space. And because it does, the local rate of motion in your body brain and clocks has to reduce. It’s called time dilation, but it’s all down to motion really. It occurs because the maximum rate of motion is the speed of light. If that train could move at the speed of light, your local motion would be zero. If it wasn’t, adding the train motion to your local motion would mean your total motion exceeds the speed of light. That’s a no-no. Special relativity says no, and the wave nature of matter says no. Time dilation doesn’t really mean you travel to the future faster. All it means is your local motion is reduced compared to mine. You never were actually travelling through time. You were travelling through space. And whilst you were doing that I could have watched you every step of the way. You could have looked out the window and watched me every step of the way too. You didn’t disappear up some time tunnel to start living your life in the middle of next week. You just fast-forwarded through the week. And when you step off the train it’s me who meets you. I don’t miss you by a week.

It’s the same kind of thing with gravitational time dilation. That can be idealised via the stasis box, which is kind of like the ultimate refrigerator. Yes, it’s something out of science fiction too, just like time travel. But it’s kind of fun to fight fire with fire. No motion of any kind occurs inside this stasis box. So when I put you inside, electromagnetic phenomena don’t propagate. So you can’t see, you can’t hear, and you can’t even think. Hence when I open the door a week later, to you it’s like I opened the door just as soon as I closed it. And get this: you “travelled” to the future by not moving at all. Instead everything else moved. And all this motion wasn’t through time, or spacetime, it was through space. Yes, the stasis box is science fiction, but don’t forget, we can freeze embryos now. In the future maybe we’ll be able to freeze an adult. Then you could “travel” to the future by stepping into a glorified freezer. But you aren’t really travelling to the future. You aren’t moving. Instead everything else is.

So if there isn’t really any travelling forward through time, how are you going to travel back in time? Arrange for the big guy in the sky to press his cosmic rewind button? Whilst leaving your memories intact? Don’t think so. Besides, that’s not time travel. That’s just one up from dragging the bar on a YouTube video. And despite what you may have heard about closed timelike curves, that’s not time travel either. That’s just Groundhog Day, only you wouldn’t know it. The thing is, spacetime is a just mathematical model. It presents the dimensions of space along with a time dimension, depicting all times at once. It’s like you film something with a movie camera, then develop the film, then cut it up into individual frames and form them into a stack. And because of that, it is static. It isn’t something you can move through. It isn’t space, despite what Minkowski said.


Spacetime depiction by John D Norton, see Einstein for everyone course HPS0410

You cannot look up to the clear night sky and point to a world line or a light cone, because these things are abstractions, like spacetime is an abstraction. Yes, general relativity refers to curved spacetime, but only to give the equations of motion through inhomogeneous space. And the time dimension is in itself derived from motion, such as the motion of clocks, and the motion of light. So that time dimension just isn’t the same as the dimensions of space. You can hop forward a metre, but you can’t hop forward a second. And you can’t hop backward a second either.

You can read more about this stuff in A World Without Time: The Forgotten Legacy of Godel and Einstein. It’s a book featuring philosophy and relativity and what Einstein and Godel used to talk about in Princeton. It doesn’t say time does not exist, it’s more like time exists like heat exists. And just as you can’t literally climb to a higher temperature, you can’t literally travel forward in time. Or backwards. No way, no how.

Bear all this in mind next time you see some celebrity physicists talking about time travel on the Discovery Channel. Like those time-travel movies, it’s science fiction. So it isn’t great physics. But a time travel movie can still be a great movie. There is no time travel, but there is no Santa Claus either, and you can still have a great Christmas.

13 comments on “Science Sundays with John Duffield: Time Travel

  1. Mark
    September 29, 2013 at 6:07 pm #

    I think you’re muddled in your understanding. If you get in your starship and nip off to Alpha Centauri and back and find that more time has passed on earth than on your watch then that’s time travel to the future. To take an extreme case with starship speeds close to the speed of light, you could come back and find that a million years has passed on earth. By any definition, that is time travel to the future. And everything I have described is mainstream physics.

    • duffieldjohn
      September 30, 2013 at 4:30 pm #

      I’m not at all muddled Mark. You could watch me go to Alpha Centauri and back through your advanced telescope, and at no point do I disappear from the present. I don’t really “time travel to the future”. I travel through space. And time does not literally pass. Cogs and gears move within my clock. Electrical impulses move within by brain. The Earth moves. And so on. With respect, what you’ve described isn’t mainstream physics, it’s popscience physics. Have a look at the recent physicsworld article about time:

      Note the comments. Amrit, Vadim, and jcnsmith share this the empirical view. I don’t know if Lee Smolin does. But other physicists/cosmologists who have written whole books about time usually seem to be peddling mystery rather than addressing the simple physics.

  2. Graham N Booth
    September 29, 2013 at 7:07 pm #

    ‘some celebrity physicist’: that would be the only man in history ever to have motor neuron disease for several decades and still be alive…

  3. duffieldjohn
    September 29, 2013 at 11:27 pm #

    I said physicists, Graham. Follow the link to the YouTube video and there’s several guys talking about “The Truth About Time Travel”.

  4. Gareth
    September 30, 2013 at 9:28 pm #

    John, that is a nice explanation of time dilation. You had me worried for a minute – I was not sure if you believed in it!

    Like you, I enjoy time travel stories while not believing it is really possible.

    So I assume you have read Smolin’s “Time reborn”. I think he is saying there may in fact be a “present moment” throughout the universe, even though relativity seems to deny that.

    I am guessing you would say that that is redundant. If there is only motion we do not need a “present moment” whether it is absolute (throughout the universe) or relative (to a particular observer)?

    What about the feeling we have of being in the present moment, with the past behind us, and fixed, and the future ahead, and yet to be decided? Does that require an explanation?

    • duffieldjohn
      October 1, 2013 at 7:31 am #

      Thanks Gareth. Sorry, no, I haven’t read Lee Smolin’s book. But I suppose I’d better. Meanwhile I’m thought of as being a relativity guy*, but I don’t have an issue with a “present moment”. Simplify the universe by forgetting about gravity and expansion, then imagine all we have is light beams. Then “the present moment” is essentially defined by their light-path lengths. At the present moment they’re 13.8 billion light years long and counting.

      As for the feeling of being in the present moment with the past behind and the future ahead, put yourself in Einstein’s shoes and imagine you’re riding a light beam. Look backwards, and there’s 13.8 billion light years behind you. Alternatively you could look forwards!

      * An essay that’s well worth reading is “The Other Meaning of Special Relativity” by Robert Close. IMHO when you introduce the wave nature of matter, you realise WHY special relativity works.

  5. Dr Evil
    October 1, 2013 at 12:28 pm #

    Dr Kaku does his best!

    The Langoliers was a nice twist on time travel too.

    • duffieldjohn
      October 3, 2013 at 3:30 pm #

      Yes, the Langoliers was a nice twist. Shame it wasn’t a big-budget movie. There’s just loads of productions I could have mentioned. I didn’t even mention Dr Who. I was never that keen on Dr Who as it happens. Even when I was a kid I noticed that towards the end it’s all crisis and he’s got like ten seconds to save the Earth. But wait a minute, he’s got a time machine! Laws of Time? More like Laws of TV.

      As regards Michio Kaku, best if you google to find out what physicists think of him.

      • John Barrett
        October 7, 2013 at 11:58 am #

        Ah but in the classic Pyramids of Mars ( and also City of Death now I come to think of it ) the Doctor uses the TARDIS to arrive BEFORE the villain does, to thwart his devilish plan.

        I’ll get me coat, it’s the one with the long scarf.


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