Faster-than-light, a curious concept, is all but reality. Due to the recent expanses in the field of quantum mechanics, faster-than-light travel has come up once again. Permanently locked in the bounds of science fiction, FTL travel would break one of the universe’s most fundamental laws: causality.

First off, it is important to define faster-than-light, or FTL. FTL is any particle traveling at a velocity faster than light, hence why it was given the name faster-than-light. Theoretically, this would enable people to travel faster than the expanding universe, and cross vast distances of empty space in relatively small amounts of time.

Practically, this concept isn’t possible. Due to the theory of special relativity proposed by Albert Einstein, nothing can move faster than light. Not only would  faster-than-light travel be impossible, but it would imply time travel as well as space travel.

“Yes, it [FTL travel] would enable us to cross space very quickly and would be great for humanity to achieve,” said Mr. Everett, when asked whether or not he thought FTL travel should be a priority for science.

Causality, or cause and effect, would be broken due to an observer witnessing the events at different times than the FTL user. For example, say that someone wanted to use FTL travel to make their way to Andromeda galaxy. Simply enough, they travel there faster than light could. This all sounds fine so far. However, let’s say someone in between Earth and Andromeda sees this take place. They would witness someone else arriving at Andromeda far before they would ever see  leave Earth. The observer sees the effect before the cause happens, because they come in contact with the result before the light from the cause ever arrives at the observer’s location.

Thus, the observer could then use FTL to inform someone at Earth to prevent the trip from ever taking place which would be communicating backwards in time relative to the observer and the ship at Andromeda. The observer already knows what is going to happen in the future relative to Earth. Since the ship arrives without ever even leaving Earth, cause and effect is therefore broken, as is the very fabric of space-time. This is why faster-than-light travel would imply time travel.

In addition to causality, it would require infinite energy in order to travel faster than light, as the increasing velocity would appear to increase the mass relative to a stationary observer.  However, that’s not very fun, and it doesn’t have much to do with time travel.