Relatively Weird. How to get younger, thinner and fatter all at the same time.

“OK, Albert. Go on hit me with the weird stuff from your relativity theory”

You’ve got the idea that movement is relative from our last conversation?

“That bit’s fine.”

Good. The rest of the theory is based on the simple idea that the speed of light in space is constant. It doesn’t matter how fast the source of light is moving, or the speed of someone looking at that beam of light. Also nothing can go faster than the speed of light.

"If I came out of the headlights of a car that was flying through space at half the speed of light, I'd end up going one and a half times faster than you, wouldn't I?"

Unfortunately not, but a good question. Light from the headlights of a car isn't travelling at the speed of light plus the car's speed, but simply at the speed of light.

“How can that happen?”

It’s simple enough really you just have to change a few other rules to make sure that the speed of light is always the same. If the speed of light is always going to be the same, it just means that time slows down as you get faster. The good news is you stay younger longer. The bad news is you get squashed and get heavier and heavier. I’m afraid this next part will sound more like something out of Alice in Wonderland than science so I hope you are ready for this. And you people reading out there, if you are a bit sensitive it might be better to read on with your eyes closed.

“So basically you invented the idea that clocks slow down just to make your theory about the speed of light work?”

Well not exactly, I just started imagining how the universe would work if the speed of light was constant. The rest, slowing down clocks and even E=Mc² followed from that.

“OK Albert, start by explaining to me why fixing the speed of light makes you convinced that clocks can change speed.”

Imagine someone leaning out of a train travelling at 100 miles per hour and another person standing beside the track. Both of them throw an apple in the direction the train is moving. Which apple will be moving faster?

“The one from the train. It’s starting at 100 miles per hour so it must be moving faster than the one thrown from a standing start.”

Exactly. So change the apple for a torch. Does the light leave the torch faster if the torch is moving?

“I would think so, but you just told me I’m wrong.”

Yes, you are wrong. Even stranger is that to the person on the train and the person beside the track the same beam of light will appear to be going the same speed.

“How can that be true? If I am following a beam of light at half its speed it will move away from me at only half the speed of light.”

Unless time slows down, of course. There is a fairly simple formula that tells you how much time must slow down to make sure that the speed of light will always be the same no matter how fast you are moving.

“Of course, why didn’t I think of that? Albert are you crazy? Why would time slow down?”

To make the universe work properly, time has to slow down as you speed up.

“So why had no-one noticed this slowing down of speeding clocks before you?”

Because it doesn’t happen until you are going very fast.

“How fast is very fast?”

A reasonable fraction of the speed of light, which is something no human has ever done except in their imagination.

“What’s the fastest speed a human has ever travelled at?”

The fastest humans are astronauts. They have to reach a speed of more than 25,000 miles per hour to escape the earth’s gravity. But that’s a tiny fraction of the speed of light, only 0.004%.

“So go on explain what happens at almost the speed of light.”

If you could buy a ticket for a space ship going at 99% of the speed of light, time would go seven times slower than on Earth. If you tried just to spend your sleeping hours whizzing around the moon at 99% of the speed of light, you would face a slight problem. Everything in your body would, from an Earth point of view, slow down. From your point of view, if you could see what was happening on Earth, it would be like watching a video on fast forward. So if you go to sleep on your spaceship for eight hours sleep on Monday evening and came back to Earth the next morning it wouldn't be Tuesday but Thursday morning.

“Is that like time travel?”

It is a sort of time travel but it only lets you go into the future, you can't go back in time with this trick. If you slow down even a bit the effects gets much less. At 10% of the speed of light, a mere 67 million miles per hour, you’d gain less than two days if you spent a whole year travelling. At the end of the day it is easier, cheaper and more effective just to lie about your age like everyone else does than use relativity to stay young.

“How fast is time travelling for us then?”

We are travelling at the speed of light, so for us it is passing normally but compared to earth our clocks are ticking infinitely slowly.

“Infinitely slowly?”

Well when we started out, earth was back in 1223 BC and it’s now 1905 the year I published my theory of special relativity. Does it feel like we’ve been travelling for three thousand years?

“No, but…”

Well there you have it. We could travel the entire universe in no time. We can travel as fast as imagination. You can imagine yourself anywhere in the blink of an eye.

“If we went faster than the speed of light would time go backwards?”

We'll never know because nothing can go faster than the speed of light.

“Why not?”

Because as you get near to the speed of light your mass increases very rapidly. At half the speed of light, a space ship would have a mass 15% more than parked in a space dock. At 99% of the speed of light the mass would have increased sevenfold. At 99.99999% of the speed of light the mass would have increased by over two thousand times. As the mass of a space ship increased it would need more and more energy to accelerate. If it ever reached the speed of light, its mass would be infinite and it would require an infinite amount of energy to accelerate it to a speed greater than the speed of light. Since that would take more energy than is in the whole universe, it can't happen.

“Talking about energy, where does E=Mc² fit in?”

There are some other laws of the universe about conserving energy or momentum that I needed to tweak to work with relativity. These formula were worked out for moving objects but it turned that at rest, there was one part of the formula still needed. That leftover bit was E=Mc² which showed that mass and energy are related to each other.

“Tweak? The atomic bomb exists because you worked out how to tweak a mathematical formula. Is there anything else you are going to tell me changes?”

The other thing that changes as you approach the speed of light is your length. In fact everything you try to measure in the direction you are travelling, including distances, shrinks. Everything travelling with you shrinks by the same amount, so you could never measure this shrinkage because anything you tried to measure yourself with would also have shrunk. But someone else travelling slower than you, in relative terms of course, would see you as squashed. Since squashing sounds a little unscientific, scientists call this squashing the Fitzgerald Contraction, named after an Irishman George Francis Fitzgerald who invented it in 1889 more than fifteen years before I worked out special relativity in 1905.

“So Fitzgerald discovered relativity before you?”

No he was trying to explain how ether could exist even if we couldn’t detect it in experiments. He was changing the rules of physics to make old theories work, I was changing them to make my new theory work.

“So what’s to say you are right and he is wrong?”

I kept the strangest fact about relativity until now. When scientists have tested some of these crazy relativity ideas, like time slowing down, they have turned out to be true. Perhaps we’ll talk about that next time, just to prove to you that even though I may be dead I’m not dead crazy.

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[Note. The limericks used here are attributed to that famous author Anonymous, if anyone knows their origin I will give their creator(s) the belated fame they deserve.]

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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