Brian Cox

So we could come back a million years in the future and look at the Earth and find out what had happened. You can't go back, as far as we can tell. So you can't build a time machine to go backwards. So these are time machines. The world is built such that a time machine...

So we could come back a million years in the future and look at the Earth and find out what had happened. You can't go back, as far as we can tell. So you can't build a time machine to go backwards. So these are time machines. The world is built such that a time machine...

So we could come back a million years in the future and look at the Earth and find out what had happened. You can't go back, as far as we can tell. So you can't build a time machine to go backwards. So these are time machines. The world is built such that a time machine...

A way to think about it, the way that we teach it in undergraduate physics, is that, so in Einstein's theory, there are events, which are things that happen in space-time. So that would be an event. It's something that happens. Our conversation now is a thing that happens, space-time. And what Einstein's theory tells you is it's about the relationship between events.

A way to think about it, the way that we teach it in undergraduate physics, is that, so in Einstein's theory, there are events, which are things that happen in space-time. So that would be an event. It's something that happens. Our conversation now is a thing that happens, space-time. And what Einstein's theory tells you is it's about the relationship between events.

A way to think about it, the way that we teach it in undergraduate physics, is that, so in Einstein's theory, there are events, which are things that happen in space-time. So that would be an event. It's something that happens. Our conversation now is a thing that happens, space-time. And what Einstein's theory tells you is it's about the relationship between events.

So let's say that we wanted to come back here tomorrow. That would be another event. We meet again tomorrow. And you can see how much time has passed between those events. In Einstein's theory, the amount of time that has passed... is the length of the path you take over spacetime between the events. So it's just like saying, in a sense, what's the distance between Austin and Dallas, right?

So let's say that we wanted to come back here tomorrow. That would be another event. We meet again tomorrow. And you can see how much time has passed between those events. In Einstein's theory, the amount of time that has passed... is the length of the path you take over spacetime between the events. So it's just like saying, in a sense, what's the distance between Austin and Dallas, right?

So let's say that we wanted to come back here tomorrow. That would be another event. We meet again tomorrow. And you can see how much time has passed between those events. In Einstein's theory, the amount of time that has passed... is the length of the path you take over spacetime between the events. So it's just like saying, in a sense, what's the distance between Austin and Dallas, right?

And you'd say, okay, well, it depends what route you go. Well, what's interesting in Einstein's theory, the only complication is the length of the path you take between events is the time measured by a clock that's carried along that path. So that's how much.

And you'd say, okay, well, it depends what route you go. Well, what's interesting in Einstein's theory, the only complication is the length of the path you take between events is the time measured by a clock that's carried along that path. So that's how much.

And you'd say, okay, well, it depends what route you go. Well, what's interesting in Einstein's theory, the only complication is the length of the path you take between events is the time measured by a clock that's carried along that path. So that's how much.

If you're carrying your watch with you and you go between here and tomorrow, you go this way, you go off and maybe you fly to Dallas and back or something and then come back again. There's a particular length. Someone else can take a different path, obviously. And so a different amount of time will pass for them between those two things that happen. Just because of that one fact.

If you're carrying your watch with you and you go between here and tomorrow, you go this way, you go off and maybe you fly to Dallas and back or something and then come back again. There's a particular length. Someone else can take a different path, obviously. And so a different amount of time will pass for them between those two things that happen. Just because of that one fact.

If you're carrying your watch with you and you go between here and tomorrow, you go this way, you go off and maybe you fly to Dallas and back or something and then come back again. There's a particular length. Someone else can take a different path, obviously. And so a different amount of time will pass for them between those two things that happen. Just because of that one fact.

A very infinitely small but measurable amount of time. It's a tiny amount unless you travel, someone goes close to the speed of light or someone goes near a black hole or something where the space time is all distorted. Then you can get big effects. But it's still completely measurable.

A very infinitely small but measurable amount of time. It's a tiny amount unless you travel, someone goes close to the speed of light or someone goes near a black hole or something where the space time is all distorted. Then you can get big effects. But it's still completely measurable.

A very infinitely small but measurable amount of time. It's a tiny amount unless you travel, someone goes close to the speed of light or someone goes near a black hole or something where the space time is all distorted. Then you can get big effects. But it's still completely measurable.

I mean, they are quite big effects, these, in the sense that for the satellite navigation system, for example, GPS. The clocks on the satellites tick at a different rate to the clocks on the ground. And it's quite a big effect.

I mean, they are quite big effects, these, in the sense that for the satellite navigation system, for example, GPS. The clocks on the satellites tick at a different rate to the clocks on the ground. And it's quite a big effect.