Relatively simple. An easy guide to relativity

“Albert, do you think you can really explain the Theory of Relativity to me?”

Of course but don’t worry if it seems confusing. It is. It’s not until you start thinking about things and getting confused that you can really start stretching your brain. Thinking is like exercise, if it hurts it must be good for you.

“As long as my brain doesn’t implode, I don’t mind a bit of an ache for a good cause.”

Good. Now, the first thing you need to know is that there are two theories of relativity.

“Two? That means it must be twice as hard to understand as I thought.”

Not at all. The two types of relativity are special relativity and general relativity. The special version was the first one I came up and is a little a simpler than general relativity.

“What’s the difference?”

Special relativity deals with things moving at a constant speed, like light or trains or cars. General relativity was much harder to sort out and took me another ten years to get right. This general theory explains how gravity works.

“How about starting by explaining what the word relativity means.”

It’s called the theory of relativity because it is based around the idea that there is no such thing as moving or staying still. All movement is relative to something else so you can never really know if you are moving or not. There is no such thing as absolute movement.

"But we're moving."

Or are the stars just coming towards us? In a station when the train next to yours starts to move, you might be convinced for a while that you are moving when you are not. That is relative movement or relativity.

“But I know a train really moves because it ends up somewhere else.”

True, but what else is happening during your train journey?

“I don’t understand what you are getting at.”

It is not just the train that is moving. The track and station are moving too.

“No they’re not.”

Of course they are. The earth is rotating and does one complete spin everyday.

“But that isn’t real movement because you can’t feel it.”

You can’t feel the movement but can see the effect, that is why the sun rises and sets. The important thing about relativity is that you need measure your movement against some fixed point. What scientists call a frame of reference.

“What’s the frame of reference on a train?”

Usually the track. The problem is that the track is moving as the world spins. If you were on a train near the equator, say in the middle of Africa, the track itself would be moving at almost a thousand miles an hour around the centre of the earth.

“So the centre of the earth must be a fixed point.”

No the whole Earth is going around the sun, that is where the seasons come from but that’s another conversation. Around the sun your train track would be moving over 60,000 miles per hour.

"Wow…so the Sun is a fixed frame of reference?"

Sorry, the Earth's Sun and the whole solar system are rotating around the centre of the galaxy. Your train track is travelling at a speed of almost a million miles per hour around the centre of the galaxy.

"Is the centre...."

Sorry again, all galaxies are moving away from each other; remember the Big Bang and everything moving apart.

"Doesn't the universe have a centre?"

I'm afraid not. My theories also showed that space is curved. If you travel far enough in a straight line you end up back in the same place. So how can something that has no edge have a centre?

"Hmmm, so what can you rely on?"

Much less than you think. If movement is relative, there can be no such thing as a fixed place because you could never know if you were still or moving. The other important thing I realised in developing the theory of relativity is that there is no such thing as absolute time.

“You are saying time is relative? Relative to what?”

Movement and position can affect the time things seem to happen at. Imagine an explosion seen by two people, one is very close and the other a hundred miles away and both have synchronised watches. Who will see it first?

“The light will reach the closest person first so they will see it a fraction before the person who is further away.”

So they won’t agree on the time of the explosion.

“No but if there were in the same place they would.”

So if two people at the same place they will always agree on timing.

“Yes. I think so”

OK, now imagine two explosives joined by long wires to one detonator placed exactly in the middle. What happens if you push the detonator.

“Both explosives will explode at the same time.”

Good. So what if at the exact moment of the explosion there is another person driving in a fast car past the detonator towards one of the explosives.

“He’ll see the two things explode at the same time if he can manage to look in two directions at the same time.”

As this is a thought experiment will give him the power to see in opposite directions at the same time, but he won’t see the explosions happening at the same time.

“Why not?”

He’ll see the explosion he is heading towards first because in the time it takes the light from both explosions to reach the detonator he will have moved a bit himself towards that explosion. Just like you agreed before as he’s nearer to one explosion he’ll see that explosion first and then other explosion later. So the moving person sees the explosions at slightly different times but they will look simultaneous to someone standing by the detonator. If you can’t get people to agree that things happen simultaneously how can you rely on time?

“Hold on Albert, that’s a trick. If the moving man reaches the detonator at the same time as the light from the two explosions they’ll look simultaneous. Ahah, got you!”

I’m afraid not. The light takes a little bit of time to get to the detonator so if the car is at the detonator just as the light reaches it, then at the time of the explosion the car would have to be a little bit away from the detonator. You already agreed that things will only appear to happen at the same time if people are in the same place. So the time things seem to happen at is different for two people if one is moving relative to the other. There is no such thing as absolute time.

“My brain is definitely about to implode.”

Hold on, you might need your brain for a bit longer. We’ve still a long way to go and lots of things to talk about. The good news is that there is one fixed thing in the universe.

“What’s that?”

The speed of light. That is the heart of my theory of relativity. After convincing myself that time and movement have no fixed meaning, I decided to see what would happen if God had decided the speed of light was the one fixed thing in the universe.

“So what does happen?”

Some very strange but real things. But I think it might be safer to let your brain cool down for a bit before I start telling you about that.

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