How to use a time machine

Another problem with time travel would be altering events of the past. Take the Grandfather Paradox: if someone travelled back in time to kill their ancestor, then the possibility of their existence in the future would be zero (3). Thus, they would have been unable to time travel to begin with to have killed their ancestor. This issue of causality is expanded upon through the Novikov Self-Consistency Principle (4). This states that if an event causes a paradox or changes the past, the possibility of this event occurring would be impossible. However, this principle is not widely accepted by time travel enthusiasts.

Now, whilst your TARDIS machine may be nothing but a prop at this point in time, it could still help provide evidence on the possibility of time travel. Take this example: you set up two duplicates of the same clock that read the same time and placed one into a rocket that blasts off into space. The rocket orbits around the Earth and then returns and is compared to the clock that remained on Earth. You would find that less time has passed on the clock that was in the rocket. Why? Because moving clocks run slower than stationary clocks. That is, as you move faster through space, you move slower through time. This is known as Time Dilation (5).

An example of time dilation is the comparison of time on the International Space Station (ISS) to the time on Earth. Astronauts who have spent 6 months in the ISS have aged 0.005 seconds less than people on Earth (6). This does not seem like much because the astronauts are not traveling close to the speed of light. To see the effects of time dilation multiply, one would need to be very close to the speed of light. If you were to travel in space at 90 per cent the speed of light, whilst everyone on earth would age by 22 years you would only have aged by 9!

Speed is not the only thing that affects how fast we age, gravity also affects our experience of time. A stronger gravitational field means that time travels slower in that field. For instance, your feet age slower than your head considering the slightly smaller gravitational pull on your feet compared to your head. Now take a black hole; we know that black holes have immensely strong gravitational fields where one hour near a black hole would equal approximately 100,000,000 years for a person on earth (7).

So what would happen if you travelled through a black hole?

No one really knows what occurs inside a black hole but we know trying to enter will likely turn you into spaghetti (8). That being said, we can only observe things that go as far as the event horizon of the black hole, so once something has entered it, we do not know what has happened. Black holes have however, been especially useful in theoretically explaining the possibility of time travel. Placing someone in a strong gravitational field or having them experience motions close to the speed of light would have them experience time slower compared to someone on Earth. This brings us to wormholes. 

Einstein’s theory of general relativity predicts the existence of wormholes which would theoretically permit time travel. To travel to a galaxy that is 2.5 million light years away with the fastest rocket on earth would be impossible as it would take longer than a human lifetime. This is where wormholes come to the rescue. A wormhole would provide us with a shortcut to our location of interest. Imagine folding a paper in half and poking a pen through it to represent your route of travel. You are essentially skipping the length of the paper and traveling from one end to the other.