Doctor Who’s 50th anniversary: Is time travel actually possible?

With Doctor Who celebrating its 50th anniversary today with Day of the Doctor, it's a good time to talk about time travel.

We are, of course, travelling through time every moment of our lives. We're just doing so in one direction — forwards — and going second by second. However, this is more about whether science fiction is on to something, and we can actually skip past large blocks of time to see the far future, or skip backwards to see the past.

Travelling faster into the future is fairly easy, as it turns out. Every time you drive in your car, ride a bus or take a train ride, the increase in your speed slows down your personal time ever so slightly. The effect is even greater if you increase your speed, flying in a plane or heading up into orbit to whip around the Earth at thousands of kilometres per second. This is due to the effect of time dilation, since time slows down as you approach the speed of light. Now, we're only talking incredibly tiny fractions of a second here, so that's not going to make much of a difference to anyone.

Skipping into the far future requires extremely fast speeds, approaching the speed of light. A ship flying at 99 per cent of the speed of light to Alpha Centauri and back would return a little over 8.6 years later. However, for anyone on the ship, the journey would have only taken 1.2 years. Increase your speed to 99.99 per cent of lightspeed and the same 8.6 years would still pass on Earth, but those on board the ship would only experience about 4.5 days on the trip.

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This same kind of effect happens with gravity. The stronger the gravity, the slower time ticks away. We have practical examples of this, since satellites in orbit have to be specially programmed so their clocks run slow, so they'll match up with clocks here on Earth. Relative to clocks here on Earth, a satellite's speed slows its clock down ever so slightly — by about seven microseconds for each day — but the lower gravity it experiences speeds the clock up by about 45 microseconds per day. If the engineers didn't program in a fix for that, things would get dicey and GPS satellites would be useless.

So, to travel far into the future, you'll need something really massive, like a neutron star or a black hole. Spend some time near it, and when you leave you'll find that many more years would have passed for everyone else.

Going into the past is a lot trickier, and it is likely impossible. There are always the questions of paradoxes and alternate timelines/universes, but it may not even be physically possible. Wormholes that link together two points in spacetime could do it, if we can actually find some. There are some ideas to twist the fabric of spacetime back on itself, by using infinitely-long cylinders or donuts, which might work, however, you could only use that to travel as far back as the twist was created. Put one of these twists into orbit today, and no matter when we used it, we could only travel back to today. So, it's very unlikely that anyone will be traveling back in time to change anything that's already happened.

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One thing I always find interesting about the concept of time travel in science fiction is that it usually conveniently forgets about space. In just the span of one hour, the Earth's surface rotates by 1,670 kilometres, the planet moves 107,300 kilometres in its orbit around the sun, and our entire solar system moves 72,000 kilometres through the galaxy (not to mention how much our galaxy moves through the universe and how much space expands in that time). So, if you have your standard time machine sitting on the surface of Earth and you step through — if time is the only thing you're moving through — go forward or back by even a second or so, and chances are you're going to end up floating in the empty vacuum of space. That wouldn't make for a very good story, though.

In any case, enjoy the Doctor Who 50th anniversary special! I know I will!

(Image courtesy: Wikimedia Commons)

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