The strange physics of time

25 February 2020

Interview with 

David Tong, University of Cambridge

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We’ve looked at how time ticks along, and how it can change over a day, or across the world. But time can get very strange, very fast, as Adam Murphy found out when he went to speak to University of Cambridge physicist David Tong…

Adam - Measuring longitude lets you travel far. Well, what happens when you travel fast, really fast, speed-of-light fast. Then physics gets a little bit strange and something called time dilation kicks in, put together by Albert Einstein. This theory says that the faster you move, the slower time will run for you compared to someone else. But that opens up a lot of questions like what does the person who is going fast feel? Well, thankfully theoretical physicist, David Tong from the University of Cambridge is on hand to help.

David - I feel exactly the same thing as I did before. I feel exactly the same thing as you do, but you think that I am ageing more slowly than you are. The important speed, and it's a speed limit in the universe, is the speed of light. Light travels at 300 million meters per second. That's fairly fast. And the surprising thing is that it travels at that speed. No matter how fast you're travelling. So you stand still, somebody shines light at you, you think it's travelling towards you at 300 million meters per second, roughly. Now you run towards it as fast as you can. Let's say you run towards it almost at the speed of light. You still see it coming towards you at 300 million meters per second. So, uh, we had things happen with time and in fact with space, when you're travelling at these speeds close to the speed of light.

Adam - And that's all very, very strange indeed. But how commonplace could something like this possibly be?

David - Yeah, it sounds weird, but now we just do experiments on a daily basis where these effects occur. One of the first times we saw this is in something called cosmic rays. So cosmic rays are particles which travel across the universe before they finally hit the earth. When they hit the upper part of the Earth's atmosphere, they typically split into many other particles. It's kind of like having an LHC particle collider, but put at the top of the atmosphere. And one of the things they create as a particle called a muon.

A muon doesn't live for very long. It lives for 1 millionth of a second, roughly speaking. Um, and 1 millionth of a second isn't long enough for the muon to make it down from the top of the atmosphere to the earth where we can detect it. And yet we see many of these muons coming from the top of the atmosphere. And the reason is that although the millionth of a second wasn't long enough, the muon thinks it doesn't take that long. The muon's travelling so fast that it thinks time has slowed down, and from its perspective it's lasted its usual millionth of a second before it disappears, but that was then long enough for it to come down. Now these days we do these experiments routinely. We do them in particle colliders, managing to speed up particles to very close to the speed of light and when they get to those speeds, their lifetime is extended often by up to about 3000 or 4,000 times.

Adam - And it's not just speed that can mess with time. Big, heavy things can as well.

David - Einstein had two theories of relativity. There was the special theory of relativity from 1905: this is the theory that tells us what physics looks like as we approached the speeds of light, that's where the time dilation due to traveling at high speeds comes from. And 10 years later he had what's called the general theory of relativity, which is our best theory of gravity. He replaced Newton's theory of gravity that had been around for a couple of hundred years. General relativity says that what we think of as gravity is actually due to the bending and warping of space and time, and so if you have ever heavy objects in the universe like stars or planets, then a space warps around them but also time warps around them. The net effect is that again, there's a time dilation.

When you're close to a heavy object, time goes slower for you, so you age less quickly if you're close to a heavy object, then if you're far away. Yeah, GPS systems only work because of general relativity; or maybe that's a slight exaggeration. If we didn't include general relativity, GPS would be off by a few meters. There was an experiment done in the 1970s by Hafele and Keating. It was a very simple experiment. They got some atomic clocks. I guess it wasn't simple to build atomic clocks, but they got some very precise atomic clocks, and then they just bought a ticket for them on a plane and they flew around the world commercial. I think business class! There's lovely photos of them actually have them sitting in one seat and his big atomic clock strapped into the seat next to them. The plane did one route around the world. It landed and they tested the atomic clock that had been on the plane versus atomic clock that was just left behind, and it's exactly as was predicted by the theory of relativity.

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