Question: who invented time and why is it so important?

Jon Copley answered on 18 Jun 2010:

It’s important because without time, I don’t think we’d be able to do science to figure out how the world around us works. Everything could either happen all at once, or events could happen in the wrong order – so we’d get the wrong idea about what causes what.

I don’t think anyone invented time – I think it’s a property of the Universe, like space. But Einstein gave us some new ideas about time (called “Special Relativity”), which I think are fascinating. For example, he deduced that time doesn’t pass at the exactly same rate for everything (or everyone). If something is moving very fast, time passes more slowly for that object (or person) compared with an object or person that is standing still.

That sounds bizarre, but you can show it, using amazingly accurate clocks called atomic clocks. If you synchronise the clocks, and then keep one in London for example, but send another to Tokyo and back on an airliner, then when you bring the two clocks back together again, you can see that one that travelled is running slightly behind the one that stayed still, by the tiniest of fractions of a second, which we can only measure with an atomic clock.

If we’re not moving very fast, that time-warp effect is insignificant, so we never notice it in our everyday lives. But we’re really all in slightly different time zones to each other, by tiny, tiny fractions of a second, because we are all moving relative to each other. I think it would be really nice to demonstrate that for a TV programme by putting one atomic clock in the boot of a fast car, then driving it around a track for many hours (like the Le Mans 24 hour car race), and comparing it with another clock that you kept stationary with you beside the track.

Ok, so why does this matter? Well, as you travel faster and faster, this “time dilation” effect gets bigger and bigger, and time passes slower and slower for you compared with anyone who is not moving. And it turns out that if you could travel at the speed of light, no time would pass for you at all. So if you could travel at the speed of light, *you* could get to the next star “instantly” – or the other side of the galaxy. But time would pass for everyone else – four years would pass here on Earth while you got to the next star, and thousands of years would pass here on Earth if you went to the other side of the galaxy. But for you, travelling at the speed of light, no time would pass in either case.

So next time someone tells you “a light year is the distance that light travels in a year”, that’s not quite correct. From the point of view of the photons in that light, it doesn’t take any time for them to cross that distance. But one year passes for objects not moving at the speed of light, like us here on Earth.