There is a chance we will be able to travel through dimensions and time

It’s everyone’s favourite science question: Is time travel really possible?

There have been hundreds of movies and TV series that have explored this – some more intelligently than others – and there’s always plenty of science-sounding jargon bandied about, like wormholes and higher dimensions and singularities and parallel realities.

But what does proper physics have to say on the matter? Well, luckily for you this is something I have studied over the past three decades. In fact, my first book, Black Holes, Wormholes and Time Machines, based on my Institute of Physics Schools lectures, was published exactly 20 years ago.

One can begin by asking the simple question: Do the laws of physics allow for the possibility of time travel? You may be surprised to hear that not only do we know time travel to be possible, but that it is routinely demonstrated by experiments, albeit on a tiny scale.

But travelling through time depends on whether we want to get to the future or the past, because one is easier than the other. Isaac Newton believed that time was something that we have no control over – that it goes by at a constant rate, relentlessly ticking by the seconds, minutes, hours and years everywhere in the Universe at the same rate.

Then, in 1905, Einstein published his theory of relativity and brought about a scientific revolution. He showed that time is not absolute or independent of us but can be stretched and squeezed depending on how fast we are moving. In fact, by travelling close to the speed of light you can slow time down so that when you stop, less time has gone by for you than in the outside world and you will find yourself, quite literally, in the future.

Another way of getting to the future is predicted by Einstein’s theory of gravity (we physicists call it General Relativity), which he completed in 1915, which showed that gravity also slows down the passage of time. So, time runs slower at sea-level than it does on top of a mountain, where it would in turn run more slowly than out in space – basically the stronger the pull of gravity (in this case the closer you are to the centre of the Earth, the slower clocks will tick).

But the effect of Earth’s gravity on time is very tiny and so not that interesting. But if you could find yourself a black hole and orbit around it a few times (while being careful you don’t get sucked in), then its incredibly strong gravitational field would dramatically slow your time down. When you return to Earth, everyone will comment on how young you look considering the many Earth years you have been away.

Time travel into the past, on the other hand, turns out to be much trickier. But General Relativity – which is still our best theory on the nature of time – doesn’t completely rule it out. It states that spacetime can be curved around to create what’s called a closed time-like curve, which is a bit like a ‘loop-the-loop’ on a rollercoaster that takes you through both space and time back to the point you started, but before you left.

So, if time travel to the future is possible and time travel to the past is, while difficult, not yet ruled out, what are we waiting for? Why haven’t we built a time machine yet? Is there something we’re not understanding about the nature of time? Well, very possibly.

One problem was famously highlighted by Stephen Hawking who asked why, if time travel to the past were possible, we haven’t been visited by time travellers from the future yet. After all, our time is, to them, in the past. If future generations ever succeed in building a time machine, then surely there will be some who would wish to visit the early 21st century. Of course, it may be that time travellers from the future are indeed among us but simply choose to keep a low profile, or simply don’t fancy a package tour to 2019.

It turns out that this issue is easy to resolve. If we ever succeed in building a time machine, then it turns out that it would only take us as far back as the moment it was switched on. So, the reason we don’t see time travellers from the future is because time machines have not been invented yet.

Parallel dimensions

However, there are many other really mind-boggling paradoxes that time travel throws up. The most famous one was explored in the film, Back to the Future. What if you were to go back in time and prevent your parents from ever meeting? Do you suddenly pop out of existence because you were never born? And if you were never born, you would never have grown up to become a meddling time traveller, so your parents will have met after all, and you were born, in which case you do travel back in time and prevent your birth… and so on.

The only sure way of resolving this paradox is if there exist parallel universes. This is an idea that may not be so crazy as it sounds, and physicists are currently research the idea. If such parallel dimensions are real, then time travel unavoidably slides the traveller into a parallel world in which he or she is able to mess with the past without it affecting them.

So, stopping your parents from meeting in a parallel universe just means you will never be born in that universe.

If I were a betting man I would say that time travel to the past will one day be shown to be impossible, even in theory. Getting to the future, on the other hand, just requires building a fast-enough spaceship to get you to the nearest blackhole.

Beware though that if you reach the future, there may be no coming back if you decide you don’t like it there.

Professor Jim Al-Khalili is a quantum physicist, author and broadcaster. His new blockbuster sci-fi thriller, Sunfall, is out now.

The Future Of Everything

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