Time travel

Colin - As far as we know, traveling through time does also require traveling through space. We know of two ways you can travel into Earth's future and potentially one way you can go backwards, but all of them require traveling through space. It's not a kind of Tardis situation, where you, get in a machine, you close the door and then you open up and you're in exactly the same place. You physically have to move through space in order to move through time.

Chris - You are saying then that we could travel in time, but we would have to move in the course of doing it. But if we move and something gets in the way in the process, so say you flew across our solar system and you cross the orbit of various planets, you are gonna coincide in space and time, literally as well as metaphorically with those objects. Do you end up just spaghettified?

Colin - Not spaghettified, but any object can be given four coordinates, right? Three space coordinates and one time coordinate. The chair I'm sitting in has three dimensions in space and one in time. If I move the chair tomorrow then it's no longer there. If any two objects share those four coordinates, they're gonna be, they're gonna hit each other. If you happen to go on your big loop around space to achieve your time travel and you crash into mars, then you crash into mars.

Chris - Of course, there is one other way that we can deform or distort the passage of time. That's by either going very fast or going near things which are very massive. Both effects actually are happening very visibly on earth right now, because if we didn't take into account the fact that Earth's gravity distorts time, our GPS system wouldn't work.

Colin - They're the two ways of traveling into the future that are kind of alluded to. You've got two options like you say, go fast and return to the earth and you'll realise that more time has passed on the Earth than you think, so you just skipped ahead into the future. But again, that requires you to move through space. Or you go and hang out near something really massive, like a black hole, and then return to the Earth and you have the same effect, but that still requires you to leave the earth and come back. The GPS satellites, that's the best piece of evidence I can give that time travel is real. Every time you push that button on your maps app on your phone, that brings up that blue dot, you are using the physics of time travel. As you say, the system works because your phone receives a signal from the satellites in the GPS, but to do that, you need three or four satellites. It works out how long it's taken to arrive at your phone. The quicker you arrive, the nearer you are to that particular satellite. But to do that, the satellites have to have to timestamp the signal so you know how long it's taken. They have atomic clocks onboard that keep track of time, but time is running at a different rate for them. As you said, for two reasons, one they're traveling at speed. Then two they're slightly further outside Earth's gravitational field than we are down here. So, we have to artificially change the time onboard those clocks to hold them back into our time. If we didn't do that, the blue dot on your phone would be out by 10 kilometers in one day. So, people don't always buy into time dilation, when they hear it for the first time, they say this is just a gimmick, it can't be real, but you rely on it every time you push that button on your phone.