Two topics which we will see are closely related to each other had grabbed the attention of physicists as the twentieth century dawned. The first was the theory of the ‘ether’; the second was the discovery that the speed of light never varied.

I use the word theory to describe the ether because the idea was born out of little more than an instinct that space must be filled with something, that there couldn’t possibly be such a thing as ‘empty space’. This mysterious something, for want of a better name, was commonly called the ether, and it was assumed to be around us everywhere. How profoundly strange, though, that scientists should choose to follow an instinct when there was absolutely no evidence – despite extensive testing – to support it.

On the other hand, I use the word discovery to describe the speed of light being as being unchanging or constant. This is because as far back as the mid 19th century the Scottish physicist James Clerk Maxwell had measured with great precision the speed of light to be 299,792 kilometres per second. It is a great pity of nature that the figure falls just short of a nice round 300,000, that would have been lovely. But 299,792 it is. Incidentally, click here to whizz off at a tangent and find out more about why James Clerk Maxwell was a pretty cool kind of guy.

But let’s get back to the speed of light. Science in the mid 1800s was good enough not only to measure the speed of light, but to measure it many times in all manner of experiments in any which way you please. The speed of light was measured in the direction that the Earth was spinning and also against the direction of spin. That was a clever idea because you’d expect the ether to interfere with the speed of the light in one direction or the other at least by slowing it down. Thing was, the speed of light was always found to be the same in every situation, i.e. 299,792 kilometres per second. This rather put the cat among the pigeons as far as the champions of the ether theory were concerned: whatever the ether was, it had no effect at all on the speed of light. It was assumed that there must have been mistakes in those experiments that seemed to show the speed of light was unchanging. How could it possibly be that the speed of light was always the same! A lot of time was spent trying to detect those mistakes.

Enter Einstein

Einstein came right out and said the unsayable: the fact that the existence of the ether could not be proved meant that it did not exist. But that still left the speed of light problem. It maybe doesn’t sound like a big deal to say that the speed of light is the same in every situation. But it truly is utterly amazing, and to understand just how big a deal it is, we need to go back to our marble rolling. Let’s relocate the marble from your kitchen to a train carriage. It can be one of those continental ones with a corridor down one side. I loved those when I went inter-railing in the early 90s.

Let’s add some figures into it, and let’s say that from start to finish you watch entranced as the marble covers a distance of three metres in five seconds down the corridor. Let’s assume the marble rolls at the same speed until it stops at the conductor’s feet, i.e. not accelerating or slowing down, or the maths really will trash my head. Remember that speed for the moment: 3 metres every 5 seconds.

Remember as well your Grandad who is standing on the Moon. Actually, the Moon might make things tricky (more about why a bit later when we come to the Theory of General Relativity and gravity). For now we’ll relocate your Grandad to the top of the nearest  snow-covered mountain overlooking the train track. He watches the train as it glides effortlessly through the Alps. He looks through the glass ceiling of your carriage and admires your marble as it rolls along the corridor at a speed of 3 metres every 5 seconds. But just a second. The marble is in the corridor and the corridor is in the train and the train is whizzing along at a fair old pace. Let’s say at 100 kilometres per hour.

So now comes the big question for you and for your Grandad: what is the speed of that marble?

You would each give a different answer, n’est-ce pas? You would say, it is travelling at a speed of 3 metres every five seconds. And your Grandad’s answer? Using the Theorem of the Addition of Velocities all he has to do is add the two speeds together. This simple calculation would give the total speed of the marble.

So: (3 metres per five seconds) + (100 km per hour, or 10,000 metres per hour, or 166.66’ metres per minute, or 13.88’ metres per five seconds) = 16.88’ metres every five seconds. That is how fast your Grandad says the marble is moving.

All very interesting (I hope you are thinking), but aren’t we straying from the point which is to do with the speed of light? Ah ha! Here comes the crunch. Supposing that there on the train you had a truly amazing marble. Supposing you also had equally amazing measuring equipment which clocked your marble not at a speed of 3 metres every five seconds, but instead at a speed of 299,792 kilometres per hour. Yes, that’s right: your humble marble has rolled down your train carriage at the speed of light. You are beside yourself with excitement! You immediately grab your mobile and ring your Grandad and ask him what speed he measured for the marble. Because he has just the same highly accurate and truly amazing measuring equipment at his disposal, of course.

Here we go! Are you ready for it?

“Ee, lad,” says your Grandad, taking off his flat cap and rubbing his forehead. “Funny thing is, I measured 299,792 as well.”

And there you have it. The speed of light is always 299,792 kilometres per hour. No matter what ‘momentum’ you give it, no matter whether the source of the light is moving in the direction of the ray of light or against it. Et voilà: Einstein’s Theory of Special Relativity.

Imagine having a race with a beam of light. If your Grandad fired a beam of light across the desert (I’m just thinking of a vast, open space when I say ‘desert’) and at exactly the same time you set off running on your amazing bionic legs also at the speed of light, do you think that you would be able to keep level with the start of the beam of the light? After all, you and the beam of light are travelling at the same speed, right?

In fact, you would be amazed to see the beam of light zip past you at the speed of light.

Mind boggling!