You might think that this should be an easy question to answer — just look at the laws of physics and find out if they allow you to travel backwards in time or not—but unfortunately it is not that simple. Jacques Pienaar, PhD student at the University of Queensland explains.
We don’t have a single set of physical laws that applies in every situation. We have one theory that describes the movements of the smallest particles in nature, which is called quantum mechanics, and another that describes how things attract each other using gravity, called general relativity.
Having two separate theories is fine as long as we can ignore one of them whenever the other becomes important. For example, if we want to describe how an electron moves in an atom, we need quantum mechanics, but the mass of the atom is so tiny that we can basically ignore its gravitational field. On the other hand, if we want to describe the motions of the planets, we need a theory of gravity, but since we don’t care about the exact positions of every atom in the planet, we can ignore quantum mechanics. But as soon as we are faced with a situation where both the movements of tiny particles and the effects of gravity are important, we are out of luck, because we do not have a theory of quantum gravity that can describe both of these effects in a single system.
And quantum gravity is exactly what we need to answer questions such as “is time travel into the past possible?”
Fortunately, there is a kind of time travel that we can talk about without a theory of quantum gravity: time travel into the future. It turns out that Einstein’s two theories of relativity tell us almost everything we need to know about visiting the future.
Einstein’s theories of relativity come in two parts: the special theory of relativity, which deals with objects that are travelling very fast, but does not deal with gravity; and the general theory of relativity, which deals with gravity. Each theory gives us a way to travel into the future, without having to do it the boring way of just waiting for the future to arrive.
What is the theory of special relativity?
Imagine that you are running after the school bus. If you run slower than the bus, the bus will get further and further away from you. The rate at which the bus gets away from you depends on how much faster it moves compared to your running speed. If you could run at the same speed as the bus, then it would remain the same distance away from you at all times, just as if you and the bus were standing still. In that case, we say that the bus is stationary relative to you, even though you and the bus are in fact moving very fast, but relative to the people standing at the bus stop.
Now, instead of a bus, imagine trying to catch up to a beam of light, which travels at the very high speed of exactly 299,792.458 metres per second, or about 1,080 million kilometres per hour. Unlike a bus, it turns out that no matter how fast you run, a beam of light always moves away from you at exactly the same speed of 299,792.458 m/s. How can this be possible?
According to Einstein, as you accelerate to faster speeds, time slows down for you compared to people who have not accelerated. Although it looks to the people standing at the bus stop as though you are catching up to the beam of light, from your point of view the beam of light seems to be travelling away from you just as fast as before you accelerated. Of course, this means that once you give up your mad chase and return to the bus stop, you will find that in the last ten minutes of your own time, the people at the bus stop have aged by many years!
This effect is called time dilation and it’s normally so small that we don’t notice it. Usain Bolt, the fastest man alive, is able to run 100 metres in about ten seconds. After each sprint, Bolt is about ten millionths of a billionth of a second (or as a scientist might say, ten femtoseconds) younger than he would have been, had he just been standing still. It is a consequence of Einstein’s theory that we can never accelerate all the way up to the speed of light. However, at least in theory, we can get as close as we want to the speed of light, for example using a very powerful rocket. The closer we get to the speed of light during our journey, the more time will have passed for the people on the Earth when we return. Makes sense? How fast would you need to go in order to see Earth in a hundred years time?
Suppose you were to travel in a rocket ship at half of the speed of light, which is about 540 million kilometers per hour. When you return, for every day that has passed for you, a hundred years will have passed for people on Earth. However, it would take about a trillion megajoules (that is one exajoule) to accelerate to that speed. Roughly speaking, if we took the Tsar Bomba, the biggest atomic bomb ever detonated, and multiplied it by one thousand, we might just have enough energy to get you up to half the speed of light. If you could stay in one piece, that is.
A better way to travel into the future would be to go sit near a very heavy object, such as Jupiter or, better yet, a black hole. According to Einstein’s theory of general relativity, heavy objects cause space and time to curve, leading to what we normally refer to as gravitational force. What does it mean to say that time is curved near a heavy object? Well, it means that time passes more slowly for people close to the heavy object than for people far away from it.
A black hole doesn’t really have a surface because all of its mass is concentrated at a single point called, the singularity. However, there is a special distance from this point called the event horizon, which marks the point beyond which even light cannot escape the gravitational pull of the black hole. So, to avoid getting trapped inside a black hole, you have to stay above the event horizon. The good news is that you can slow down time as much as you like (in theory) by getting closer and closer to the event horizon without actually crossing it. For a black hole with the same mass as the Sun, which weighs just under two thousand billion billion billion kilograms, or a million yottagrams, the event horizon is about 2.8 kilometres away from the singularity. If you hovered about a thousandth of a millimeter away from the event horizon, every day that passes for you would be a hundred years for someone far away from the black hole. But don’t let any part of your body cross that horizon, or it might get sucked in, causing you to be stretched until you break apart, a process that Stephen Hawking has called spaghettification, or the noodle effect.
While time travel into the future is possible, therefore, it would be extremely difficult. It might just be easier to freeze yourself in a block of ice and hope that people in the distant future will be able to revive you with new technology. As for time travel into that past, that too might be possible in theory – but that is a different story.