Time travel has long fascinated science fiction writers and the scientific community. ScienceAlert is interested in a physicist who thinks he has found what would make this phenomenon possible.
Our current understanding of what time is depends on Einstein’s general theory of relativity. It has made it possible to perceive space and time as a single entity: the famous “space-time”. Physicists have verified Einstein’s theory with the greatest accuracy and are fairly certain that it is the most successful description of the structure of the universe.
They also use it to try to figure out a way to travel in time. Equations consistent with this theory have therefore been found, but they cannot be put into practice for two reasons.
The first is that in order to build a time machine, you need something called exotic substance, which is negative energy substance. Thanks to quantum physics, we know that this type of substance could theoretically be created, but in quantities that are too small and too unsustainable. And we currently have no concrete evidence of this achievement.
The second reason is even more significant: the paradoxes of time travel defy logic. The most problematic of these is the context paradox. Here’s a simple example: Imagine building a time machine and walking five minutes back. Then you destroy this machine. But since you destroyed it five minutes before you built it, how could you use it five minutes later? And most of all, if you can not use it five minutes later, then you can not go back in time and ruin it. In other words, the machine is destroyed if and only if it is not destroyed: This scenario is quite paradoxical. Are you still there? It is not finished yet.
Remove the paradoxes
In science fiction works, these paradoxes are eliminated. For example, the hero avoids changing anything that may affect the future. But in physics, a paradox is not just an event that can possibly happen, it is a concept that points to an inconsistency in the theory itself. So the context paradox not only implies that time travel is dangerous for the traveler, but that it is simply impossible.
But then, some people thought to themselves: one can refuse to eliminate the possibility of time travel because of the paradoxes, and instead try to eliminate the paradoxes oneself. This is what the physicist Igor Dmitrievich Novikov tried to do with the principle of self-consistency. He argues that the past would be immutable, that we could travel there, but without being able to change it: the laws of physics would converge to maintain coherence. Thus, in our example, it would not succeed even if we would destroy the machine five minutes before.
The Novikov conjecture alone does not solve all the paradoxes of time travel. However, Barak Shoshany, an assistant professor at Brock University, and two of his students suggested that another theory could solve all the paradoxes: the one about multiple stories.
The idea is simple: there would be several parallel timelines, and one could go back in time in a first timeline and then change events on another, but not in the original. But does the universe allow multiple parallel timelines to exist? These are merely speculations for now, which are supported by Everett’s interpretation of quantum physics called “many worlds theory”.
Thus, for three years, Barak Shoshany and his students have been working on a theory that would make parallel chronologies compatible with general relativity, which would make it possible to exclude contextual paradoxes and consider traveling in the past.