How is Time Relative?

Isn't time the same for everyone?

It's all relative. A phrase often coined by physicists, but what does it actually mean. Well, basically it means that observations made are entirely subject to the observer. What one person may measure, won't necessarily be the same as what someone else measures. Depending on where you're standing, what you're doing, and what you can see, all has an effect on what you can determine about the world.



A good example of this is the times when you're looking out of a window while riding a train. The bushes that line the rail way are zooming past at incredible speed, a green and brown blur. Meanwhile, near the horizon, the hills and fields are gently rolling by providing a pleasant viewing experience. That is relativity, if you assume instead that you are the one stationary, then as far as you're concerned the bushes are moving a lot faster than the fields are.

What has all this got to do with time, you ask? Surely time is the same for everyone, we all age at the same rate, and we all obey the same clocks. Well, technically that isn't the case. Sure on a day to day basis, it is a good way to live, but if you were to take it to the extreme it can make a big difference.

Everyone's favourite theoretical physicist,
Albert Einstein.

So, everything is relative and will be different for me than someone else. Well, sort off, everything except for the speed of light. The velocity of a photon is the same in any frame, even if you're traveling at a million miles an hour, light will still travel away from you at 300,000,000 meters per second. It is a fact of the universe and the foundation upon which Einstein built his theories. And this fact allows us to prove that time varies for people at different speeds.

If for a moment you consider a device which uses a beam of light to measure time, call it a "light clock". Compromised of two mirrors and a single photon (particle of light), one "tick" of the clock is counted as the light bouncing from one mirror to the other and back again. All well and good, while stationary the light bounces back and forth. But what happens when you start to move? Well, the mirrors are no longer stationary and are moving as the light tries to bounce between them.  So you end up with the light having to move on a diagonal line to bounce off the mirror and return.

On the left is the stationary light clock. On the right is the clock which is moving, showing the increased travel of
the light beam.

With a basic understanding of geometry we know that the diagonal line is longer than the vertical one - thank you Pythagoras - and so the light has traveled further. Combine this with the aforementioned constant that is the speed of light (doesn't change for anyone), it means that the clock has taken longer to complete a "tick". Which means that time is slowed down!

Yes, time slows down the faster you go. Compared to someone standing still. So if you spent your whole life traveling faster than everyone else you would live longer, because less time would have passed for you than them. It seems bonkers but it's true, it has been tested with high accuracy atomic clocks and an airplane. As well as being the answer to some initially puzzling particle physics conundrums. That was a brief look at Einstein's Theory of Special Relativity, and how basic geometry proves time slows down as you speed up.

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