Wibbly Wobbly Timey Wimey: Is Time Travel Possible?

As a scientist and long-time Doctor Who fan, this weekend is one of my favourite times of year at the National Space Centre. It’s our annual Science of the Time Lords extravaganza, a loving homage to the fantastical science and science-fiction behind the Doctor’s adventures.

The crux behind all of these adventures is the Doctor’s ability to travel backwards and forwards in time – an ability that is always a tool, sometimes a blessing, often a curse, and occasionally even a weapon (remember the Moment from The Day of the Doctor?).

But is time travel actually possible? Is it science fact or science fiction?

The answer is that it depends on which direction you want to go. If you want to travel into the future, then it’s absolutely possible. In fact, it’s even commonplace, every time you travel at speed. But to travel into the past, well, that’s hard and presently still very much in the realm of science fiction.

Speed of light

So, how can I travel forward in time, you ask?

Before 1905, we would have said that any kind of time travel was most definitely in the realm of science fiction. But then a young patent clerk by the name of Albert Einstein astonished the world by publishing a theory of space and time with rather bizarre consequences.

Einstein began by considering the speed of light. In every day experiences, light seems to travel instantaneously across a room. Switch a lamp on and immediately we see light, with no noticeable delay. But make careful measurements of light across large distances (as Ole Rømer did in the 1670s when he studied light travelling between Jupiter and Earth) and it becomes clear that light does have a speed limit, it just travels really, really, really fast. 300,000 kilometres per second to be precise.

As well as a speed limit, Einstein also suspected that the speed of light was the same for all observers.

This is definitely not true for other moving objects. Imagine you’re driving down a dual carriageway at 100 kilometres an hour and you measure the speed of the car approaching from the opposite direction, also driving at 100 kilometres an hour. Because of your combined speeds, the other car will appear to be speeding towards you at 200 kilometres an hour. But that’s just apparent speed. Someone standing on the side of road would measure each car as travelling 100 kilometres an hour.

In 1887, two American scientists tried to measure the speed of light relative to the motion of the Earth, hoping to find different apparent speeds. But to their surprise, the speed of light was always the same, regardless of the speed of the observer [Read: Michelson-Morley Experiment].

Let's build a clock

For those of you who are anxiously awaiting the possibilities of time travel, don’t worry we’re nearly there! Because if we can speed up or slow down a clock, then we can move into the future faster or slower than other people, effectively travelling in time.

So how can a clock be slowed? Well to show you that, I first need to build a clock. Now, a second is arbitrary. It’s a human invention. We don’t have to count in seconds or minutes. But we do need something that can tick out time at exactly a constant rate. What do we know that always travels at exactly a constant speed no matter what? Light! The speed of light is constant. So let’s build a clock from a beam of light bouncing back and forth between two mirrors, ticking out constant intervals of time forever (see top image on the left). Pretty simple.

Now imagine we put this light clock on moving vehicle. Could be a skateboard, could be a rocket. Imagine watching this clock as an observer on the ground as it speeds past. The path the light would travel would look something like the second image on the left.

Here’s where things get interactive. Grab a ruler or a bit of string to measure the length of the path that the light travels from the bottom mirror to the top mirror, for both the stationary light clock image and the moving light clock image. Which path is longer?

The light in the moving clock appears to travel a longer distance between mirrors, meaning the ticks of the clock are slower. Interesting.

For more details on relativity, watch this video narrated by our very own National Space Academy director Anu Ojha.

So how does this help us time travel?

Well, let’s imagine we can board a rocket and fly out amongst the stars for ten years, travelling at 99% the speed of light (never mind that we don’t actually have the technology to do this yet). If we return to Earth after ten years at this speed, we’d be shocked to find that time had travelled at a different rate for us compared to people left on Earth. We would have aged by ten years, but everyone else on Earth would have aged by 71 years in the mean time!

Moving clocks run slow. Effectively, we would have sped through to the future quicker than everyone else, simply by moving very fast. Pretty cool eh? This scenario is known as the Twin Paradox. It’s complicated by the fact that the rocket has to change directions to come back to Earth, but the key idea is that time passes at different rates for different people depending on their speed.

The Doctor does this at an even quicker pace – a few seconds elapse for him and his companions in the Tardis and suddenly they emerge hundreds or even thousands of years into the future. This is also possible in real life. You just have to find out a way to travel very, very, very close to the speed of light.

Going back in time

But the Doctor can also travel just as easily back in time. In the very first episode, the Doctor travels all the way back to 100,000 BC (An Unearthly Child) and gets involved in a Stone Age power struggle. This feat is something still firmly in the realms of science fiction. Different speeds (or indeed different masses as Einstein showed in his follow-up theory of General Relativity in 1915) can warp time slower or faster or even stop it altogether. But they can’t rewind time.

Some scientists have speculated on the possibility of natural worm holes woven into the space-time fabric of the universe. These could in theory allow travel between different locations and different times (including into the past), but so far there’s no evidence to suggest worm holes exist beyond the equations of theorists.

We’ll have to leave those historic adventures to the Doctor. And probably for the best. Even in the hands of a Time Lord, meddling with the past can lead to some pretty messy consequences.