# Light: The speed limit of the universe

04 June 2019

## Interview with

Matt Bothwell, University of Cambridge

## LIGHT-RAYS

We’ve covered humans, animals and cars but when it comes to extreme speed, there’s one thing that takes the crown; light. Light is the fastest thing in our universe but how do we know that? And what happens when we get to such a high speed? With Izzie Clarke and Katie Haylor is Matt Bothwell from Cambridge University’s Institute of Astronomy. Just how fast is the speed of light?

Matt -  Very fast is the answer. The number is 299,792,458 metres per second.

Katie - You're going to have to put that into context for me.

Matt - Yes, of course. That's a number that's so big it doesn't make any sense, right. It's so fast you can get to the moon in about one second and you can get to the sun in about eight minutes even though that's 150 million miles away.

Katie - Right. Well whilst I wrap my head around that, how do we know that the speed of light is the number you've just quoted?

Matt - Right. So there's been lots of attempts to measure the speed of light over history. Galileo tried to do it by shining lanterns from hillsides and timing it but as we know now, obviously, the speed of light is far too fast for that to be effective. The first person to have a decent stab at measuring the speed of light was a Danish astronomer called Roemer. He was observing the moon Io, that's a moon of Jupiter, and we can see Io orbiting Jupiter and when it goes behind Jupiter that's an eclipse. What he noticed was that when the solar system is configured that the earth is moving towards Jupiter, the eclipses seem to happen more frequently than when we were moving away from Jupiter and that can only happen if light has a finite speed and we were catching up with the signals, if you like, when we were travelling towards Jupiter.

Katie - So crucially, the distance that the light was having to travel was changing depending on where the earth was?

Matt - Exactly. And it was taking either more time or less time depending on which way round.

Katie - How well did he do in terms of getting the answer?

Matt - For a historical experiment he got pretty close to the actual answer, yeah.

Katie - And crucially, the speed of light is constant, right?

Matt - That's right. It's actually quite a bizarre thing to get your head around. It doesn't behave the way we expect speeds to. If you imagine, if you could throw a tennis ball at 50 mph for example, if you were to stand on a moving train going 50 miles an hour and throw the same tennis ball, the speed of the tennis ball would be 50+50, a 100 miles an hour because the speeds add together. The speed of light doesn't behave that way. The speed of light is about 300,000 kilometres per second. If you were to stand on a train and shine a torch out in front of you, that light is still going at 300,000 kilometres a second. It’s not the speed of light plus the speed of the train. The speed of light is always the same no matter how you measure it.

Katie - So are we getting into the realms of relative motion versus what light does which is something different?

Matt - Right, exactly, yes. In order for the speed of light to be constant in every reference frame, space and time themselves need to distort to keep this so it's so. So basically this is the time dilation and length contraction that people might have heard of.

Katie - Oh, my gosh. By reference frame, do you just mean from any point in time, space, the universe?

Matt - Exactly, yeah. If I'm sitting at my bedroom and I measure the speed of light, I'll get the value it says in the textbook. If I'm shooting off in a rocket to the stars at some phenomenal speed and I measure the speed of light, I’ll get the same results.

Katie - So you talk about time dilation, is this the idea that because the speed of light is constant, to get a certain distance time has to change to compensate? Is that kind of what we're talking about here?

Matt - Exactly. The way it works is that if an object is moving very very fast then an external observer will see time for that object as moving more slowly. So if you take a clock and then you fling it off at some phenomenal speed and then you watch the clock it will appear to be running slowly, which sounds very theoretical but that's an experiment we've actually done. We've taken atomic clocks and put them on jumbo jets flying around the world, and the clock that flies around the world ends up slower than the clock that was sitting on the ground.

Katie - Wow! So this isn't just my perception of time?

Matt - Yeah, it's not a perceived thing. Time really is going slower.

Katie - Apart from light obviously, which travels at the speed of light, can anything else travel that fast?

Matt - Yeah. There are a few things that travel at this speed and so we call it the speed of light - it's actually a lot more general than that, it’s just the maximum speed of the universe. And anything that is massless, any particle that is massless, will travel at this speed. The most famous ones are photons right. They are particles of light, they have zero mass and so they travel at the speed of light. But there are other particles like gluons, they are little particles that glue protons together, they are also what we call massless particles so they travel at the speed of light. The force of gravity travels at the speed of light. Yes, so it's not just limited to light itself.

Izzie - Things that are massless travel at the speed of light so why is that mass relationship so important?

Matt - Anything that has mass in the universe like a person, or a star, or a ball, or anything takes energy to move it. If you want to move a person it going to take energy to give them a shove. So anything with mass takes more and more energy the faster you want to go. And then to take anything with mass and make them travel at the speed of light would take an infinite amount of energy, but anything that has zero mass travels at the speed of light automatically.

Izzie - Ah, I see.

Katie - Back here on earth, are there any applications for understanding the speed of light that we might use every day?

Matt - There are definitely applications. We used the speed of light to define the length of a metre. In the past, metres were defined in various different ways including the size of the earth and they had a natural physical object which was a metre. Nowadays, we use the speed of light to define a metre so that number that I gave before the speed of light is exact, there is no uncertainty because that's how we define our metre.

We have to deal with the speed of light in various ways on earth. Often it's an obstacle to workaround; for example, when you're designing computer chips the speed of light is the limit to how fast signals can move around and they have to design them accordingly. And even in fields as far off as finance for example, the speed of light is important. So with high-frequency trading selling stocks and shares at fantastic numbers of times per second, price information can only get sent out from places like London and New York at the speed of light, it can't go any faster. And so companies that do this high-frequency trading will want to get as close as possible to the centre so that the speed of light can reach them as soon as possible and shave off some crucial milliseconds.

Katie - Ah, so geographically as close as possible. Does milliseconds really make a difference, milliseconds?

Matt - They do, yeah. You wouldn't think it but yeah, with supercomputers running algorithms on the stock market milliseconds is where it counts.