“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.
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?
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.
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.]
This is the story of a great journey that started with a great thought. One day in 1895 a boy looked into a mirror and wondered what the universe would look like if he could travel on a beam of light. That sixteen year old boy was Albert Einstein and that one thought started him on the road to discover his Theory of Relativity. The great man has been reinvented as Albert 2.0 to come back and blog about a journey through space on a beam of light and explain the science behind everything from atoms, blackholes to global warming. If you've just joined and want to start at the beginning use the index on the left. If you're bored try these links below just for fun.
UNSCRAMBLE EINSTEIN'S BRAIN
PRACTISE SAVING THE WORLD FROM ASTEROIDS
ALIEN CONTACT CALCULATOR
HEAR THE REAL EINSTEIN TALK ABOUT E=Mc2.
Saturday, July 7, 2007
“OK, Albert. Go on hit me with the weird stuff from your relativity theory”