There is an observation that time travel seems to contradict logic, in the form of time travel paradoxes. There are several types of such paradoxes, but the most problematic are consistency paradoxes.

A popular trope in science fiction, consistency paradoxes happen whenever there is a certain event that leads to changing the past, but the change itself prevents this event from happening in the first place.

Consider a scenario where I enter my time machine, use it to go back in time five minutes, and destroy the machine as soon as I get to the past. Now that I destroyed the time machine, it would be impossible for me to use it five minutes later.

But if I cannot use the time machine, then I cannot go back in time and destroy it. Therefore, it is not destroyed, so I can go back in time and destroy it.

The time machine is destroyed if and only if it is not destroyed. Since it cannot be both destroyed and not destroyed simultaneously, this scenario is inconsistent and paradoxical.

Time travellers are usually warned not to make significant changes to the past and to avoid meeting their past selves to avoid creating a paradox. Examples of this may be found in many time travel movies, such as the Back to the Future trilogy.

But in physics, a paradox is not an event that can actually happen — it is a purely theoretical concept that points towards an inconsistency in the theory itself. In other words, consistency paradoxes don’t merely imply time travel is a dangerous endeavour, they imply it simply cannot be possible.

• This physics theory essentially states that you can travel to the past, but you cannot change it.
• If I tried to destroy my time machine five minutes in the past, I would find that it is impossible to do so. The laws of physics would somehow conspire to preserve consistency.
• This theory has many loopholes and is mired with paradoxes and inconsistencies.

Allowing for multiple histories (or in more familiar terms, parallel timelines) can resolve the paradoxes that are eclipsing all the other theories. In fact, it can resolve any paradox you throw at it.

The idea is very simple. When I exit the time machine, I exit into a different timeline. In that timeline, I can do whatever I want, including destroying the time machine, without changing anything in the original timeline I came from. Since I cannot destroy the time machine in the original timeline, which is the one I actually used to travel back in time, there is no paradox.

• Time travel could be possible, but only if our universe can allow multiple histories to coexist. So, can it?
• Quantum mechanics certainly seems to imply so, at least if you subscribe to Everett’s “many-worlds” interpretation, where one history can “split” into multiple histories, one for each possible measurement outcome.
• General relativity and quantum mechanics tell us that time travel might be possible, but if it is, then multiple histories must also be possible.