I Think Faster Than Light Travel is Possible. Here's Why.

happen at equal times are on these straight lines, not horizontal lines. Then you can see that for Bob the order of events is that the egg first falls to the ground and then falls off. It seems that for Bob the temporal order of the

faster

than

light

ship is backwards, crazy! The first reaction you may have to this is: Who cares what Bob sees? I mean you can watch this video backwards and that doesn't mean I actually spoke backwards. That seems fine to me. The second reaction is to point out that this is not what Alice or Bob see anyway.

You can't see a ship coming

faster

than a

light

ship for the same reason you can't hear a supersonic plane coming. What do you want to see it with? Instead, both Alice and Bob will only see the spaceship after it has passed and then watch it move away in both directions. And again, you may say, so what? I mean, gravitational lensing distorts galaxies into rings, okay, but that doesn't mean the galaxy is a ring. It's just a strange trick of our perception. And that's totally right... But, you know, physicists have noticed it too. The thing is, this wasn't the whole argument.

T

here

is a piece missing that says like this. Imagine that you are Bob and that t

here

is actually a spaceship that can go faster than light and, according to you, goes back in time. Let's not ask what this means but what you can do with it. If time on the spaceship really does progress this way, then you'll be able to give the guys a message as they pass. They take your message to Andromeda, deliver it to another faster-than-light spaceship, and the second ship brings the message back to you. Then it would arrive before you shipped it.

This means that you could send messages to yourself back in time, and that causes a lot of problems. Imagine that this video disturbs you a lot and you send a message to your younger self not to watch it, then you would never have sent the message in the first place, did you or didn't you watch it? This type of construction is also called temporal closed loop, it is a loop in time. The argument then concludes that if faster-than-light

travel

were

possible

, that would lead to causality paradoxes, so it must be im

possible

. But this argument is also wrong.

The reason is that just because according to Bob there is a spaceship going in that direction with time moving the spaceship forward in a direction that Bob calls backward in time, that doesn't mean that if a spaceship goes in that direction then its internal direction of forward movement in time would be that way. If the time direction on the ship is going in that direction, they won't be able to deliver a message to your younger self. Instead, your younger self can send a message there, and there's nothing strange about that. Physicists have reason to suppose that time in the spacecraft could pass this way, but it is not a good reason.

It is because in special relativity all observers must be treated the same way. In Special Relativity, if you

think

this is possible, then it must also be possible. But Special Relativity is special because it does not contain gravity and this means that it does not actually describe reality. For this, we need general relativity. And while the time

travel

argument is correct in special relativity, it is not correct in general relativity. I know this video is difficult, so let's take a moment to appreciate where we are. I summarized the usual argument for why faster-than-light travel leads to time travel paradoxes.

I'm about to explain why this argument doesn't apply in the real universe. The usual argument uses special relativity, according to which only relative velocities are physically relevant. In special relativity, you can't be at a speed of absolute zero, that just doesn't make sense. But the real universe contains things, as you've probably noticed. You can take all of this and calculate the average speed at which it moves. And then you can define absolute rest as motion that has no velocity relative to the average of all of that. Since you like technical terms so much, it's called a "co-moving framework".

It is the frame of reference that moves along with matter in the universe. We are currently not at rest relative to the average of things in the universe because the Earth rotates around the Sun and the Sun rotates around the center of the Milky Way and the Milky Way is hurtling toward something called the great attractor that no one really knows about. . what it is. If you wanted to be at rest with the universe you would have to run at 300 kilometers per second in this direction. No wait. This. Or this? Okay, so there is matter in the universe that moves in one direction and not another.

But what does this have to do with the history of time travel? Suppose you are Alice again, but now you are Alice in a universe with general relativity and you move with things, you are in the co-movement framework. And now suppose that faster-than-light travel is only allowed forward in time in this particular frame. In this case, you can't loop in time, regardless of what Bob

think

s he sees. The co-moving frame defines a direction as progress in time. The only thing Bob can do is send two signals to Andromeda, and there is nothing paradoxical about that. You may now be wondering what the movement of matter should have to do with the possibility of faster-than-light travel.

This is a very good question whose answer is: quite possibly nothing. I just used this as an example. It is an example to show that traveling faster than light does not necessarily imply time travel paradoxes. The latter simply does not follow from the former. To add one last reason why we should not trust the argument that faster-than-light travel is impossible is that we know that our current theory of space-time, General Relativity, cannot be correct because it does not work together with Theory quantum. That's why we need a theory of quantum gravity and we don't have one yet.

However, we know that causality and locality are very much confused in quantum mechanics, and the same is probably true in quantum gravity. That's why I think it's extremely unlikely that any argument for faster-than-light travel will survive in the yet-to-be-discovered theory of quantum gravity. Of course, you already know that no one has figured out how to travel faster than the speed of light. But I hope I have managed to convince at least some of you that the formal reasons you may have heard against it are on shaky grounds. That's why I think physicists should think a little more about faster-than-light travel.

At least then maybe humans wouldn't be so boring. ​When I was in high school, my physics teacher told me that very few people understand Einstein's theories. Maybe that was once correct, but I can tell you with confidence that it is no longer the case today. I think everyone can understand Einstein's theories today, but passively watching YouTube videos won't get you there. You have to actively engage with the material. Our sponsor Brilliant can help you with this. Brilliant dot org offers courses on a wide variety of science and math topics, and adds new content every month. The best thing about their courses is that they are all interactive with visualizations and follow-up questions, so you can check your understanding right away.

For some background on the physics in this video, check out, for example, his course on special relativity. It will shed light on how it actually works with reference frames and Lorentz transformations. When I need to update my knowledge or want to learn something new, the first thing I do is search for it on Brilliant. Now I even have my own course on Brilliant, which is an introduction to quantum mechanics. Covers topics such as interference, superpositions and entanglement, the uncertainty principle, and Bell's theorem. It is a course for beginners that you can take without prior knowledge. And then you can expand on this perhaps with their courses on quantum objects or quantum computing.

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