### Einstein’s cool way to make time go slower

#### How fast is time? If you’re trapped in the office or enduring a maths class or something dull you’d no doubt tell me it drips past. Just as you’d also tell me it moves damn fast if you’re enjoying the latest Star Wars movie.

But that’s personal perception of time. To physicists, no matter how fast or slow you perceive time to be, it’s actually all moving at the speed of light. Relatively speaking.

It’s one of the consequences of Einstein’s theory of Special Relativity, which he published more than a century ago. Let me explain.

According to our friend Albert, if an observer on Earth were to watch a spaceship accelerating away from us, up to (nearly) the speed of light, they’d also notice that as the spaceship moved faster and faster, time on it (relative to Earth) moved more and more slowly.

When you get to the speed of light, time on the spaceship – to an Earth observer – would seem to stop. Actually, you can’t get there for other reasons. But you can think of the relative velocity as, in effect, stealing some of the rate of time – and time, therefore, moves at lightspeed.

Cool, isn’t it. But wait, there’s more. Einstein’s other theory – General Relativity – drew in the nature of space-time. According to General Relativity, the universe we perceive exists in space-time, a four-dimensional entity that comprises the three dimensions we know and love – plus time.

If you bend space-time, you also alter the rate at which time passes. And that bending can be done by energy (mass) – which, according to Special Relativity, are the same thing. The more bent space-time is, the slower time goes. If you bend space-time to infinity, which is what happens in a singularity (‘black hole’), time stops – which is why we refer to black holes as being surrounded by an ‘event horizon’. It’s because that’s the point where escape velocity reaches lightspeed, and where events – to an outside observer – stop.

We actually experience this effect, indirectly, every time we use GPS. It’s like this. A satellite in orbit around the Earth experiences time dilation caused by its velocity relative to Earth’s surface (‘Special Relativity’). At that velocity the dilation is miniscule. It also experiences time acceleration caused by it being further away from Earth’s centre than the surface, thus in a less heavily distorted part of space-time (‘General Relativity’). The two don’t cancel each other out.

Ordinarily that wouldn’t matter – we’re talking about miniscule differences that humans can’t ordinarily perceive. But GPS relies on very, very accurate time measurements. Sensitive enough that they are affected by the phenomenon – and so the satellite clocks have to be calibrated to take into account the net effect of relativity.

Sometimes that effect is called ‘frame dragging’ (relative to a reference point), though Einstein personally hated the term.

So there you have it. Time travels at the speed of light, and you can change the rate at which time passes by bending space-time or moving at near light-speed, relative to the reference object. It is, of course, all relative – hence the name of the theories.

Is that super cool, or what?