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Article Length : 534 Words

The principle of relativity was first expressed by Galileo, not Einstein. Galileo stated his relativity principle in the following way. He said that if you see an object which appears to move in a straight line with a constant speed, then you cannot tell if you are moving or if the other object has the motion. There is no experiment which can tell who is moving and who is at rest where no forces are acting. One person’s idea of motion is another person’s idea of rest.

Einstein said, imagine that you and I are in two spaceships. The faster you move with respect to me, the slower you seem to age to my eyes. I notice that your clock runs slower than mine, your pulse runs slower... Absolutely everything in your spaceship runs slower.

We can apply some symmetry here and conclude from Einstein’s relativity principle that when you look at me, you should see me ageing slower than you as well! That is to say we both see each other as thinking slower, talking slower, eating slower... everything is slowed down and the effect is symmetrical and proportional to our mutual degree of motion.

This is not really any paradox, even though it seems like it has to be. Common sense tells you that if I see you ageing slower, you should see me ageing faster. Einstein says that has to be wrong if relativity is right. Why? Because if you saw me ageing faster and I saw you ageing slower, we really would be able to tell from an experiment who is moving and who is at rest, just by paying attention to who saw time go faster and who saw time go slower. The only way to preserve the relativity principle is to say that you and I should see the same effects for a single motion, always. So we both see the other as ageing slower.

And of course for all good laws in physics you can do an experiment to see how mother nature thinks about these things. Well mother nature says our common sense is wrong and relativity really is right. She very much cares about this bizarre idea of symmetry.

It just so happens that there are particles which will reliably decay and break apart into smaller pieces in a definite, given time. They are like perfectly reliable clocks in

the sense that their decay rates are very regular indeed.

But according to Einstein, time is completely relative so these decay times should not be a true constant but must also depend on the motion of the particle with respect to the laboratory and so also the scientists watching the decay take place. The faster the particle seems to move through the lab, says Einstein, the longer the particle should appear to live before breaking apart.

And it turns out to be exactly right. Decaying particles really do have a lifetime which depends entirely on how fast we see them moving! Because the law is so reliable we can even work out how fast the particle was moving by knowing the time it took to decay as compared an identical particle at rest with the lab.