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 cosmic 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 the hyperfine-transition 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 somebody 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, in line with Pythagoras's theorem. 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. Please Register or Log in to view the hidden image! Public domain image by Mdd4696, see Wikipedia. It’s the same kind of thing with gravitational time dilation. It's not quite the same, but it's still simple. It 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 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 what space is. Please Register or Log in to view the hidden image! 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. Not 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.