Getting to grips with relativity: Why Luke and Leia are different ages

Travelling through hyperspace to explore distant planets sounds all very good, but all of that lightspeed travel could have a slightly unpredictable and unusual effect on how we would experience time. Heather Wark has been researching relativity with the help of Harry Cliff from Cambridge University and David Tricker from The Perse School, Cambridge...

Heather - Space is a very big place, and to get anywhere at a reasonable rate we’re either going to have to cheat and bend space to create shortcuts across it, or, more realistically, travel extremely quickly at, or close to, the speed of light - which is the fastest that theory says we can go. But flitting around the universe at these sorts of speeds could have some unfortunate side effects, not least for the aging process.

For a start, Star Wars twins Luke and Leia Skywalker would actually finish up being years different in age by the end of the story. Why? Because of Einstein's theory of special relativity...

Harry - One of the basic assumptions of special relativity is that speed of light is the same not matter how fast you’re moving.

Heather - Cambridge University Physicist, Harry Cliff…

Harry - And when you go at very very high speeds close to the speed of light, strange things start to happen. The laws of physics are very different to the ones we’re used to in our everyday lives. For example, time runs at different speeds depending on how quickly you’re moving, distances can stretch or contract and everything is just rather unusual and peculiar.

Heather - Indeed it is. Because if I were travelling in a car at the speed of light and turned on my headlights, I would measure the light travelling away from my car illuminating the road ahead as travelling at the speed of light. But a policeman with a speed camera beside the road would also measure the lights from my car as approaching him only at the speed of light, not twice the speed of light.

This seems totally wrong because if I get my friend Lewis to stand a few yards away and shoot me with his dart gun… it hurts a lot less than if he rides towards me on his bike and shoots me again...

Because this time the dart is travelling faster having been launched from a moving bicycle. And we can prove this is happening if we do a slightly more accurate experiment with the help of physics teacher, David Tricker, from the Perse School in Cambridge...

David - We’ve got a trolley here with a projectile launcher on top of it so we can launch a ball vertically outwards from the trolley. When the trolley’s stationery, not surprisingly, the ball being launched vertically upwards… comes straight down again and lands in the trolley.

Heather - Now, if we do the experiment again and make the trolley move along and fire the ball upwards, the ball still lands in the trolley because the ball has the trolley’s motion and its own motion relative to the trolley.

David - When the trolley is moving, the ball is launched vertically upwards from the perspective of the trolley. But, of course, we watching the trolley move see that the ball is also moving horizontally. So that means when the ball goes up, it then comes down again and meets up again with the trolley, so it lands on the trolley.

Heather - But with light this doesn’t happen. It always travels at the same speed of light for any observer, and the mind-boggling implications of this is that when you move at the speed of light time has to change to keep the speed of light constant for everybody.

Einstein realised this would happen himself, and he came up with a thought experiment called “the twin paradox” to explore it. Harry cliff again…

Harry - The twin paradox is basically looking at this idea that time runs at different speeds depending on how fast you’re moving. So let’s say you have two identical twins, they’re exactly the same age, and one of them stays on Earth and the other one goes on some really long journey into space at close to the speed of light.

Then, because time runs at different speeds depending on how fast you’re moving, the twin that’s travelled out round the universe and comes back, well actually, for him or her, less time would have passed than for the one left on Earth.

So let’s say it’s Luke Skywalker leaving Hoth in the Empire Strikes Back and travelling to see Yoda on Dagobah. If he’s got a wristwatch, he leaves Hoth just after the battle and he goes very close to the speed of light and arrives at Dagobah, he’ll measure a certain time. It could be like a couple of days or something. But, if Princess Leia who’s still on Hoth is measuring how long that took, she might measure a much longer period of time, so she might measure several weeks, or months, depending on how close to the speed of light Luke’s been travelling.

Heather - And in fact, in one of the journeys documented in the Empire Strikes Back, Leia would end up more than two years older than Luke when they’re finally reunited. But this isn’t fanciful theory; we know it happens and we’ve got good experimental evidence that Einstein was right. Time does change when we travel very fast and we use this fact every time we turn on a GPS device…

Harry - These GPS satellites orbit the Earth; they’re moving quite quickly, so time runs at different speeds for them than it does on the Earth. And the way that GPS works is by having really accurate clocks that communicate with each other to exchange time information. So you have to be able to take into account these effects. If you didn’t, then GPS would basically wander away from rather than having an accuracy of a couple of metres, it would go to having the accuracy of a mile or several hundred miles over the course of quite a short period of time, and it would be totally useless. So you have to take relativity into account if you want GPS to work for example.

Heather - So it’s all down to relativity, and perhaps that’s why the actors in Star Wars all look so leathery these days.