• Question: what do you think about time travel

    Asked by anon-217218 to Savannah, Philippe, Lucy, Joanna, Harrison, Edoardo on 13 Jun 2019.
    • Photo: Joanna Huang

      Joanna Huang answered on 13 Jun 2019:

      A very cool concept! Actually, this was one of the concepts that originally got me interested in physics. If you continue doing physics for your GCSE’s, you’ll learn about relativity and how time travel is theoretically possible.
      Here’s a little taster:
      The faster you move, the slow each tick of the clock. This effect is very small at low speeds, but if you approach the speed of light, then the effect is quite significant! If you were to get into a ship that travels close to the speed of light, fly it for 1 hour, then land back on earth, you’ll find that you’ve “travelled” to the future. In reality, you haven’t jumped a wormhole or anything, it’s just that the in the spaceship, one minute can be the equivalent of one year outside the spaceship!

    • Photo: Savannah Clawson

      Savannah Clawson answered on 13 Jun 2019: last edited 13 Jun 2019 3:26 pm

      I think time travel as an idea is super cool and it isn’t just stuck in the realm of science fiction either! In physics, we treat time as a dimension, just like the other 3 dimension that we have in space (forward-backward, side-to-side, and up-and-down). In fact, we don’t treat time as separate to space at all and we merge the dimensions together into what we call spacetime (inventive, I know). We can describe all points in space and time by values in spacetime so every event in history is completely mapped out. Your entire lifetime can be represented by a line through spacetime. This line may be wibbly and wobbly but it will always be continuous because it must connect each event that you experience to the next.
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      As we all know, it is very easy to travel forward in time – we all do this every day! But is it possible to jump really far ahead in time or to even go backwards?
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      Einstein’s theory of special relativity shows us that time passes at different rates for objects travelling at different speeds: the faster you travel, the more time slows down for you. If you were to zoom off into the cosmos at really really fast speeds and then returned some time later to meet your friend back on Earth, they would have actually aged more than you because more time has passed on Earth than it has for you on your spaceship (or your preferred mode of space travel). You are now a time traveller into the future! This is one way that we know time travel can happen. But can we go back in time to change history etc?
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      Now mathematically, it should be possible for this spacetime to be bent in such a way that you could create closed loops. This means that you would always appear to be moving forward along your line in spacetime but eventually you would come back to exactly where you started and even go to times before this! This might sound completely crazy but there is nothing in Einstein’s theory of general relativity (different to special relativity mentioned earlier) that says it isn’t allowed – or at least we haven’t found a reason why it can’t happen yet. There are lots of arguments for why this shouldn’t be allowed to happen – e.g. if you go back in time and kill your grandparent then you can never be born to go back in time to kill your grandparent so you grandparent is alive but then you go back in time to kill them…. do you see where I’m going with this? We get stuck in a paradox called the “grandparent paradox”. We could then say that maybe there are rules that prevent us from doing silly things like killing our granddad (what did he do wrong anyway?) but this then causes problems with our idea of free will – i.e. our ability to make our own choices.
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      So to summarise, this is a really cool topic in physics and time travel is definitely possible in some ways but maybe not in the classical way we think about it in sci-fi movies where we can just jump to any time in the past (although we’re not sure about this yet). I think I have definitely already given you enough to think about here so I won’t go on any more – your question is just so interesting that I got carried away!

    • Photo: Harrison Prosper

      Harrison Prosper answered on 14 Jun 2019:

      In principle, according to relativity, you can get to my future quicker than I can! One question I like to ask my students is: what is the elapsed time between say 2019 and 3019? Most look at me puzzled, especially when I answer: anywhere between 0 and 1000 years! If you were to zoom away at close to the speed of light and travel 500 light years away from Earth, then come back to Earth, the Earth would now be in 3019. But, you may have aged only a few years! So, you would have got to my descendants future quicker than they did! So this is one form of time travel. However, time travel as most people think of it means being able to zip back and forth in time. There is one way one could travel back in time and that is to travel faster than the speed of light! But, according to our current understanding that is impossible. But, if you could travel faster than the speed of light, you would be able to return to the past. But, here is the classic problem. Suppose you went back in time and accidentally killed your mother before you were born. Then you would not have been born, you would not exist, and therefore you could not have gone back in time and accidentally kill your mother thereby preventing your birth! Such contradictions are called paradoxes and time travel causes all sorts of strange paradoxes. However, remarkably, in the 1980s, the theoretical physicist Kip Thorne of Caltech, USA, published a paper in a serious scientific journal in which he showed (theoretically) how to build a time machine! Since then, the study of time machines has entered theoretical physics. As an experimental physicist, however, I see no way in which the technology required to amass the gargantuan amount of negative energy that these time machines typically require is remotely possible. So, I remain extremely skeptical that any such machine exists in the Universe.

    • Photo: Philippe Gambron

      Philippe Gambron answered on 14 Jun 2019:

      It is impossible, because of causality. If you could go back in time, this would cause time paradoxes. You could meddle with events that caused your current situation, causing a paradox. However, it is possible to alter time. For example, if you travel very fast or get close to a black hole, time will slow down for you. But in no way can this be used to circumvent causality.

    • Photo: Edoardo Vescovi

      Edoardo Vescovi answered on 14 Jun 2019: last edited 14 Jun 2019 6:45 pm

      Let me distinguish traveling into the past and future.

      1) Time travel to the future is happening right now to everybody. We’re getting into the future at different rates though. You’ll learn the math, grab the idea for now.
      — If you could attach a clock to two people, one still and one running, the person running would experience less time. This is a “kinematic” (= speed) effect and predicted by Einstein’s “special” relativity in 1905.
      — If you could attach a clock to two people, one at sea level and one on mountains, the person at sea level (which feels heavier in stronger gravity) would experience less time. This is a “gravitational” effect and predicted by Einstein’s “general” relativity in 1916.

      2) Time travel to the past brings paradoxes (= contradictions, see other answers) so impossible. Wait! Are paradoxes real or apparent? Nobody knows. One workaround may be the “many-worlds interpretation”. A time traveler would end up in a different universe than his. Messing up with that “past” doesn’t have effect on the “present” he came from, so no contradiction! Time traveling becomes no more than hitching a ride from a “nearby” universe. This theory goes on with saying that actually all possible events and histories can occur in a bigger collection of universes, the multiverse. It has deep consequences on what our mind perceives and reality is. This is called “many-minds interpretation”.

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