# Back to the Future

Mathematically, time travel is possible. A paper which was recently published in Classical and Quantum Gravity, ‘Traversable acausal retrograde domains in space-time’, revealed that the potential to become a time traveler has just become realistic than ever.

In real life, time travel may not be as fancy as in the Back to the Future, but it is or will be, somehow achievable.

Travel using wormholes

Travelling forwards is relatively easy.

The 102-year-old Einstein’s famous theory of general relativity showed that time passes at different speeds depending on how fast you are, relatively. The higher the speed (compared to other people), the slower the time passes on you. If one person leaves Earth and travels at a speed very close to the light-speed for several years, many years could have gone by on Earth. A famous example is known as the ‘twin paradox’, where the traveler will become much younger than his twin after he finishes his travel.

The only problem of traveling fast is that it is hard to reverse, as travel faster than light-speed is impossible.

Thanks to the relativity theory, the problem is solved by creating short-cuts between time-spaces—wormholes. Einstein’s theory argues that space and time are inseparable and they curve in the presence of mass.  If we can create wormholes with certain levels of gravity and lead to the formation of time-space curves, they can act as tunnels to connect very distant parts of the universe.

Travel back in time is much difficult than travel to the future.

First of all, Wormholes can only take you to the point when they were created. If there were natural wormholes that were formed in the Big Bang, or any time before the point where the wormhole machine is invented, future visitors may be able to travel back to see us. But it seems like the chance is little, and this may also explain why we haven’t encountered those visitors so far. There is no wormhole for them to travel to here.

More sadly, Kip Thorne, a famous American theoretical physicist who was the consultant of Interstellar, combined general relativity with quantum physics suggests that wormholes collapse as soon as its formation.

Furthermore, even if we managed to preserve these wormholes long enough to actually travel using wormholes, a new issue arises – the ‘Grandfather paradox’.  If we changed the past, we will change the future, and even the smallest cause can have a large effect.

The good news is, The Novikov self-consistency principle showed one possible solution to prevent the generation of paradoxes on backward time travel. In this model, the word only has one timeline. Since the time-machines (any instrument that commences the time travel) can only present in a world with the universal laws of physics, only self-consistent events can occur. Any attempt that a time traveler does to try to change the future, will eventually become part of the history. For instance, if a person traveled back in time to kill his grandfather. However, this person knows (observed) that his grandfather is not dead at the point he starts his time travel, hence, his chance to kill his grandfather is 0. This means any endeavour that has a possibility greater than 0 can prevent him from killing his old man. Even if he managed the killing, he will find out that the man he killed is not his biological grandfather (or any logical reason), that his ‘real grandfather’ is still alive.

Additionally, Kip Thorne also found that for wormhole travels, no initial conditions create grandfather paradox.  A simple mass, like a spacecraft, could never generate such paradoxes.

However….

Parallel universes and timelines

News reported that scientists explained the dark area in the cosmic microwave background could be the consequence of a collision between universes. The cosmic microwave background covers the entire sky, and it is mostly equally distributed and the same temperature – but one cold spot stands out.

This raise makes the time travel more complicated: the Novikov self-consistency principle cannot be applied to more than 1 timelines, will the history be able to change then?

Scientists still have a long way to go.