• Question: Time travel... could it exist, and would it involve the speed of light? Could you actually travel backwards or forwards in time while staying still?

    Asked by anon-251974 on 28 Apr 2020.
    • Photo: anon

      anon answered on 28 Apr 2020: last edited 28 Apr 2020 10:28 am

      Hey! So backwards time travel, unfortunately, isn’t allowed by our current understanding of physics. I’d love for that to change, but at the same time I really don’t think it’s likely.

      Forwards on the other hand, that’s very different. I mean, we’re already travelling forwards in time, just all at pretty much the same rate, which is a bit boring. If you want to get a head start on everyone else though, you just need to travel really fast, which is where the speed of light comes in. The closer you get to the speed of light, the faster everything behind you seems to progress. If you want to see how whacky this can get, have a look at the Twin Paradox.

      Now unfortunately we’re not very good at travelling that fast. But there is one other way you can cheat time and travel differently to everyone else, and that’s to stand next to something really heavy. REALLY REALLY heavy, like a black hole, or a star. The closer you are to something heavy, the slower time runs for you. What that will look like though is everyone else zipping around really fast. You won’t notice anything unusual yourself, until you look at someone that isn’t stood next to the heavy thing.

      We know this because it’s happening to us right now. The Earth is pretty heavy, and we have people spending time up in orbit, further away from Earth than us. We’ve been able to measure, using really precise clocks, that time runs slightly slower for us on Earth compared to the people in orbit. In fact, the core of the planet is ever so slightly younger than the surface of the planet, by about 2 years.

      So to summarize, backwards time travel currently doesn’t look possible. Forwards time travel certainly is, but it’s really hard to do, and wouldn’t really look like it does in sci-fi. You wouldn’t even notice it happening until you carefully measured time somewhere else.

      Disappointing, I know…

    • Photo: Ry Cutter

      Ry Cutter answered on 28 Apr 2020:

      Like Joel said, no 🙁

      The reason for this, we need to go faster than the speed of light. The big problem, anything with mass is limited to less than speed of light travel. Here’s the weird thing, as we push mass closer and closer to the speed of light, it gets heavier! This pretty much stops us being able to force matter going too fast to accidentally slip backwards in time 🙂

      Great Question,

      Ry

    • Photo: John Bridges

      John Bridges answered on 28 Apr 2020:

      Another way of thinking about this is looking at thermodynamics. Some scientists like to point out that it is the Second Law of Thermodynamics which gives us the direction of time as we experience it. The total entropy (state of disorder) of an isolated system (like say our Solar System) can never decrease over time, so there is no going back to the beginning of the Solar System!

    • Photo: Stewart Martin-Haugh

      Stewart Martin-Haugh answered on 28 Apr 2020:

      Forwards time travel is happening right now – you’re moving forward in time at a speed of 1 second per second.

      What’s interesting about time travel is that most of the laws of physics seem to work the same forwards as backwards. But you can tell if you watch a video of someone frying an egg whether it’s going forwards or backwards. Does that explain why we can only seem to go forwards in time? Could you perceive time differently than how we do?

      If you like thinking about this, I really recommend watching Arrival, an/or reading Story of Your Life by Ted Chiang, the short story it’s based on.

    • Photo: Susan Cartwright

      Susan Cartwright answered on 28 Apr 2020:

      You are currently travelling forwards in time, at the rate of one second per second, but I’m assuming you want something a bit more exotic, so…
      In the context of our current theories, i.e. General Relativity (which is very well tested experimentally, and will not be easy to supersede), yes, time-travel is closely linked to the speed of light. In principle, we know how to travel to the future: get on a very fast spaceship (travelling at, say, 99% of the speed of light, which would be extremely hard to achieve but does not break any physical laws), travel away from the Earth for one year, turn round, and come back at the same speed. Two years have passed for you, but you will find that 100 years have passed on Earth. So you have effectively travelled into the future. The faster you go, the larger the time difference: if you could do 99.9% of the speed of light, your 2-year trip would result in 1000 years having passed on Earth.
      Backward time travel, on the other hand, does not appear to be possible. There are various weird geometries in General Relativity which can lead to the formation of “Closed Time-like Curves”, whereby you can travel in a loop and return to your starting point not only in space but also in time, but there doesn’t seem to be any way of ending up earlier in time than your starting point.
      So, sorry, there doesn’t seem to be any way to go back ad find out what really happened to the dinosaurs! (Which is a great pity: I’d really like to see some of the weird four-winged feathered dinosaurs from China…)

    • Photo: anon

      anon answered on 28 Apr 2020:

      So, seeing as the others already covered this, just to “troll” you a bit:
      The speed of light is a postulated limit. As in, Einstein just put it out there and the rest nodded (eventually) and said “makes sense”. But we still have no direct evidence that it is indeed a limit – but our experiments and observations sure suggest that it is. There was very recently a huge hype about experiment that supposedly detected a particle moving faster than the speed of light – this turned out to be a measurement error, but inspiring!
      A lot of things have been taken for granted before because “it must be so” or because anything else was inconceivable. Take Newton’s law of motion for instance – completely revolutionary in its time, but two centuries later, turns out it’s an approximation! Very useful yes, but an approximation nevertheless.

      But what IS “time travel”? Get in a blue box and whoosh? One could say we go back each time we remember something. But memory is a fickle thing, we actually modify it slightly each recollection.
      We could set up a VR experience that would be realistic to the extent of our knowledge. There is a bus in Washington DC that can take you for a ride on Mars for instance – so a travel to the “not-so distant future”, if you will. Then there is geology and archaeology – looking at tangible objects that have “memories” of the past. Extending this to space, each meteorite is a very physical “record” of the formation of the solar system.

      My point is, since we effectively hallucinate our reality (yes, really! there’s a great TED talk on this – have a look) – being somewhere physically and mentally can overlap enough to count as actual experience.

    • Photo: Greg Wallace

      Greg Wallace answered on 28 Apr 2020: last edited 28 Apr 2020 2:42 pm

      Contiuing along Oleg’s line of reasoning, I think the question of time travel is at least partly within the realm of philospophy.

      The laws of physics, as we understand them now, do say something about whether time travel is possible but these predictions are practically impossible to test. Also, as I understand it*, The two halves of modern physics, general relativity and quantum mechanics, have very different ideas of what time acutally is (*please check this with a professor). So why should we believe what they have to say on the subject of time? They can’t both be right.

      So, current understanding aside, what can we learn about time travel just from first principles?

      The famous Grandfather paradox comes in to play here. If you can time travel, you could travel back in time and prevent your grandparents meeting, which in turn would prevent you from being born, which would prevent you from being around to prevent them meeting, hence the paradox.

      Some might say that this contradiction means time travel is logically impossible, let alone physically. There is a way of resolving this though; You are allowed to travel back in time but you are not allowed to change anything. In other words you can have time travel, but you cannot have free will. Is that a price you’re willing to pay?

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