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Asked by to Laurence, Aimee, Chris, Dave, Greig on 17 Jun 2014. This question was also asked by .
Question: Time travel is theoretically possible, but do you think it would work in real life? And if so, would we be able to come back?
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Laurence Perreault Levasseur answered on 17 Jun 2014:
Actually, there is only one sense in which time travel is possible. The further away we look in space, the further back in time we see. That is because light travels at a finite speed. For example, when I look at the sun, I see it as it was 8 minutes ago, since it took 8 minutes to the light in order to travel form the sun to us. The closest star is at 4 light-years away from us. This means it takes light 4 years to travel from this star to us, and so we always see it as it was 4 years ago. Therefore, looking far far away is like traveling back in time in the history of the Universe.
However, the time travel that you are probably thinking of is the type of travel with a space shuttle, right? That time travel isn’t really possible. Here is why. Relativity tells us that nothing can travel faster than the speed of light. What happens as a spaceship get closer and closer to the speed of light is that, form our point of view on earth, time on the spaceship starts to slow down. If the spaceship reaches the speed of light, it’s time completely stops. So there is no way it can accelerate even more to go over the speed of light, its time has stopped!!! So it can’t go back in time!
If we were actually going to do this, then, the people in the shuttle would age much more slowly than the people on Earth, because their time would go very slowly. So by the time they’d get back to earth, they’d be much younger than, say, their own children. I guess you could say this is a way of traveling in the future, but as I said, there is no way to go physically in the past, apart from looking at the stars 🙂
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Dave Jones answered on 20 Jun 2014:
Nailed it, Laurence! Time travel is only possible with astronomy and even then we can only look backwards, not forwards, and even then we can’t actually go back just see what was happening far away!
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Aimee Hopper answered on 21 Jun 2014:
We are already time travellers! We constantly move forwards in time – you can see that on your watch! Backwards time travel is a lot harder, and required a lot more knowledge about the universe before we can really say if it is possible.
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Greig Cowan answered on 25 Jun 2014:
Time travel in the sense that we can jump about forward or back probably isn’t possible, but we are always travelling forward in time, one second at a time. What is interesting is that we believe all physical processes (such as the movement of a car or the emission of a photon of light) are all symmetric when it comes to time. By that I mean the equations that describe the process make sense for both positive and negative times. However, we only experience time in “one direction” as we move forward, second by second. This is called “time’s arrow”.
Comments
Aimee commented on :
… unless we discover that stable wormholes actually exist and infact connect space-time in ways we never thought possible… 😛
Laurence commented on :
hum… that could allow people to seemingly travel faster than light, but it would still not allow to travel back in time. The problem is, if you can travel back in time, you can make something called a ‘closed time-like curse’. Here’s what it means:
say I shine a photon and I travel back in time with it, and then I arrange to make it evolve forward in time again (just by letting time flow) and I bring it back to the point where I brought it back in time at first. Then I add another photon, and I do the same again, bring the 2 photons back it time, then make them 3, then 4, then 5, and eventually there will be infinitely many photons, without time having gone forward at all!! This close time-loop of adding more and more photons is what is called a ‘closed time-like curve’.
The problem is that infinite number of photons has infinite energy, and so it will make the whole fabric of spacetime collapse around it. (presumable the ‘wormhole’ will have collapsed before – but i’m not sure before is the right word since we are talking about time travel here… – this happens, rendering time-travel impossible).
Sorry Aimee, but good trial 😛
Aimee commented on :
good response, but couldn’t that be avoided by having the wormhole one-way only, so you wouldn’t be able to cause that feedback loop?
If we’re jumping around our own universe, then we shouldn’t have issues regarding entropy/energy either, as it’s a self contained unit – we’d just be moving it about.
Laurence commented on :
Well… there really are three possibilities here. I think this is going to be a bit technical but I’ll try to keep it as simple as possible. I’ll need to talk about the causal light cone, though.
The future light cone of a person, call him Bob, is basically all the space-time points that Bob can influence in the future. That means if Bob shines a light in all directions, the future light cone is made of all the points in space-time that will receive that light. For example, a point 2 light-minutes away from Bob won’t be inside the future light cone of Bob until 2 minutes after he shined the light.
The past light cone of Bob is the exact opposite. It’s all the space-time points that can have influenced Bob in the past. So all the space-time points that could have ’caused’ Bob, or from where a light ray hitting Bob could have been emitted.
Ok, back to time travel. The wormhole can bring Bob to 3 places:
– in his future light cone,
– in his past light cone,
– outside his light cone completely.
If the wormhole brings Bob to his future light cone and he can’t come back, that’s no problem. It’s the same thing as traveling in a very fast space shuttle so that Bob ages more slowly: he’ll end up living in the future.
If the wormhole brings Bob to his past light cone, the problem we run into is the one I described in the previous comment with the ‘closed time-like curve’: Bob will always be able to cause a ‘feedback loop’. So that’s not possible. Space-time would collapse and at best destroy the wormhole or form a black hole.
The last possibility is the one you are referring to, Aimee (I think 😉 ): the wormhole brings Bob very far (in space-time), so far that he ends up outside of both his past and future light cones. In that case, the place where he ends up is so far that there is no way for him to tell if he is in the future or in the past, because he lost all his reference points of what ‘future’ means and what ‘past’ means. This is because he has no causal connection with anything of where he came from. The technical way of saying this is that the distance separating him form his starting point is called ‘space-like’, and this means its always possible to find a frame where those two points are simultaneous. That is, there is no (frame-independent) way of saying which of the starting or the finishing point of the wormhole is in the past and which is in the future. So that means: no way to travel back in time 🙂
Aimee commented on :
that’s a very interesting way of looking at it 🙂 Thank you 🙂