Our common everyday view of time is actually an out of date one.

It goes back to Isaac Newton, who believed that time was something independent that we have no control over. That time goes by at a constant rate (someone once said: at a rate of one second every second – stupid thing to say of course as you cannot measure something against itself).

Newtonian time is that of a hypothetical cosmic clock that counts the seconds, minutes, hours and years everywhere in the Universe at the same rate, because it is independent of the Universe. It is outside of space.

So, along comes Albert Einstein in 1905 and changes all this. He said that time and space are not independent, that time and space are mixed up in a way that means no two people really measure the same rate of time if they are moving relative to each other. And this effect gets bigger the faster they are moving. So if you strap a clock to a high-speed rocket (and I mean high speed – faster than anything we have at the moment, because it has to be travelling close to the speed of light itself – the fastest speed in the universe at 300 million m/s or 187 thousand miles per second).

Their time, their biological clock everything is running slower

So then you look at the arms of the clock as it flashes past. You will see it ticking by the seconds at a slower rate than an identical clock by your side. Now this has nothing to do with how long it takes the light to reach your eyes or the speed of the rocket affecting the mechanics of the clock. It is literally that time itself has slowed down for the rocket clock. If there was a person on board the rocket you would see them doing everything is slow motion. Their time, their biological clock everything is running slower.

So Is Time Travel Possible?

Now this conclusion, which by the way is not science fiction of just a theory – it is fact, we see it all the time in our experiments with subatomic particles that DO travel at such speeds – this conclusion comes from Einstein’s special theory of relativity. Not the word ‘relativity’ there. It means all motion is relative. So if you were on board that rocket, you could just as well say that the rocket wasn’t moving and that the earth was moving at close to light speed in the opposite direction. In fact, you’ll see earth clocks slow down and everything on earth running in slow motion.

So is this just an optical illusion. Well no. It means no one has a monopoly on how fast time goes by or how long it takes for something to happen. And things get really interesting when you not only travel very fast, but speed up and slow down. Einstein later showed that when you accelerate and decelerate it is not just your speed that changes, but time itself runs a little slower. And even gravity, that stuff that you thought Newton had sown up, turns out to affect how fast time goes by. So time on earth goes by slightly slower than time out in space because we are immersed in earth’s gravitational field.

All this slowing down of time actually gives us a way of travelling into the future.

All this slowing down of time actually gives us a way of travelling into the future.

Basically, you need one of two things: either a superfast rocket than can take you near light speed (all that speeding up and slowing down will slow you time down) or find yourself a strong gravitational field like a that of a neutron star or black hole to orbit around for a while – careful you don’t fall in though. (For which I suppose you’d need a superfast rocket anyway just to get there in a reasonable time).

So here it is: because your time slows down, for you the trip takes a shorter duration (let’s say one year). But back on earth clocks are running at their normal rate so count more years going by. When you get back you will find that maybe 10 years have gone by on earth (the numbers are not important, the closer you can get to the speed of light, or the stronger the gravitation field you find, the more the difference between your journey time and the earth’s.

So if you return to earth one year older and everyone on earth is 10 years old, what you have done is literally time travelled nine years into the future. So time travel into the future is indeed possible. All we need to build is a fast enough rocket. We wouldn’t be breaking any laws of physics, and in fact we know it would work because it has already been done on a smaller scale. In 1971 scientists fly atomic clocks twice around the world (east and west) then compared them with clocks on the ground after the journeys. They were out by a few billionths of a second, just as the theory predicted. In fact, the travelling clocks should have run slower because they were moving but fast because they were further away from the earths gravity, but the two effects don’t cancel exactly.

So time travel into the future not only can be done but it has been done, even if it is just a few nanoseconds. But, I hear you complain, this isn’t real time travel surely. It no different to suspended animation. Well, I grant you it doesn’t need the future to sort of be already out there waiting for us. But this is still real time travel you get into your rocket (time machine) travel around in it for a while and when you get out again you are in the future. You have basically fast-forwarded to the future and got there before everyone else.

Can we Visit the Past?

The issue to address now is whether, having got to the future, you don’t like it and want to return to your own time. In fact, are we able, according to what we know of the laws of physics and the nature of time, to travel back into our past?

Einstein’s theory of gravity is ‘better’ than Newton’s law of gravity in that it is more precise. Unfortunately the maths is horribly difficult.

Well, ten years after Einstein published his work on what is called Special Relativity (time being the forth dimension, E=mc2 and all that business) he completed his greatest work, which is called his General Theory of Relativity. This is a theory about gravity and how it is really all down to how space-time is warped. It sort of replaces Newton’s law of gravity that we learn at school because that is wrong (or, at least not very precise). The maths of general relativity is horribly difficult (compared, that is, with Special Relativity, which is so simple that any fool can understand it).

Luckily for us, lots of clever people have solved the equations of general relativity and figured out what it all means. As far as time travel is concerned, general relativity does allow for what are called a closed timelike curves. These are circular routes through curved spacetime in which time itself is bent round in a circle. If you were to follow such a path you would feel like you were just travelling through space normally but would in fact arrive back at your starting point before you set off – time travelling into the past. Of course this is all ‘in theory’.

Time Loops

It has been known for a long time that general relativity allows for the existence of such time loops. One of the first people to study them was a guy called van Stockum in 1937. He suggested that if you happened to have an infinitely long cylinder of very densely-packed material spinning rapidly in empty space, then general relativity predicts that the region of spacetime surrounding the cylinder would be twisted around it and could contain a time loop.

Most scientists don’t believe that travelling back to the past is possible because of the many paradoxes it throws up.

Then in 1974 a young American by the name of Frank Tipler took van Stockum’s idea a step further. He suggested that we might be able to get away with a cylinder just 100 kilometres long and 10 kilometres wide. How we might build this and out of what kind of material we have no idea. However, Tipler pointed out that these were practical problems and, anyway, who knows what might be technologically possible in the distant future. To use a Tipler cylinder time machine, you would have to orbit around it a few times then return to Earth, arriving back in the past. How far back depends on the number of orbits you made.

So we have our current best theory about the nature of time (general relativity) that says time travel to the past is at least theoretically possible. What is stopping us from building a time machine? More to the point, why is it that most physicists you might care to ask, would say it’s impossible. They don’t base this on proper physics arguments but simply because it goes against common sense.

What you have to remember about time travel to the past is that you are allowed to meddle with history as long as things turn out the way they do

The reason why most scientists don’t believe that travelling back to the past is possible in practice is because of the many mind-boggling paradoxes it throws up. For example, what if you were to go back in time, to last year say, and kill your younger self. What happens then? Do you suddenly pop out of existence as the younger you slumps to the ground? After all, the older you can’t now have existed. But… if you died last year.. who KILLED you? It seems you are unable to kill yourself because you must survive the assassination attempt to become the assassin. What you have to remember about time travel to the past is that you are allowed to meddle with history as long as things turn out the way they do. You cannot change the past. Of course this paradox does not imply that time travel into the past is forbidden, just that there are certain rules associated with it.

Where Are All the Time Travellers?

Stephen Hawking once famously asked why, if time travel to the past were possible, we have not been visited by any time travellers from the future. After all, our time, to them, is 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. But here are two, more sensible, reasons.

First of all, if we ever do 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 (because of the way it would hook up space and time). So, the reason we don’t see time travellers from the future is because time machines have just not been invented yet.

Another explanation is even weirder than time travel. It turns out that our universe may not be the only one, but is just one of an infinite number of parallel universes. In that case, time travel to the past unavoidably slides the traveller into a parallel world. This idea kills two birds with one stone. Firstly it addresses why we haven’t met any time travellers from the future: There are so many of these parallel realities that our universe is just not one of the lucky few that have been visited. Secondly, parallel universe removes any paradoxes. If you were to travel to the past, you slide into a parallel universe and can kill the other you without any problem, apart from, one hopes, a guilty conscience of course.

Prof Jim Al-Khalili

Surrey Uni

You can download the word version of this article here: Al-Khalili-What-is-time (1)