Time travel has long fascinated science fiction writers and the science community. ScienceAlert is interested in a physicist who believes he has found what would make this phenomenon possible.
Our current understanding of what time is depends on Einstein’s theory of general relativity. It made possible the understanding of space and time as a single entity: the famous “space-time”. Physicists have verified Einstein’s theory with the utmost precision and are quite certain that it is the most successful description of the structure of the universe.
They also use it to try to find a way to travel through time. So equations have been found that agree with this theory, but can not be applied in practice for two reasons.
The first is that to make a time machine, you need something called exotic matter, which is a matter of negative energy. Thanks to quantum physics, we know that this kind of matter could theoretically be created, but in very small and very unsustainable quantities. And we do not have any concrete evidence of this achievement at the moment.
The second reason is even more important: the paradoxes of time travel defy logic. The most problematic of these is the paradox of cohesion. Here’s a simple example: Imagine making a time machine and going back five minutes. Then you destroy this machine. But after you destroyed it five minutes before you built it, how could you use it five minutes later? And above all, if you can not use it five minutes later, then you can not go back in time and destroy it. In other words, the machine is destroyed if and only if it is not destroyed: this scenario is quite paradoxical. You are still there? It is not over yet.
Eliminate 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 a fact that could possibly happen, it is a concept that shows an inconsistency in the theory itself. Thus, the paradox of cohesion not only implies that time travel is dangerous for the traveler, but that it is simply impossible.
But then some people thought: one can refuse to eliminate the possibility of time travel due to paradoxes, and instead try to eliminate the paradoxes themselves. This is what physicist Igor Dmitrievich Novikov tried to do with the principle of self-consistency. He argues that the past would be unchanged, that we could travel there but without being able to change it: the laws of physics will converge to maintain coherence. So in our example, even if we wanted to destroy the machine five minutes ago, we would not have succeeded.
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: that of multiple stories.
The idea is simple: there would be many parallel timelines and one could go back in time to a first timeline, then change the events to a second, but not to the original. But does the universe allow for multiple parallel schedules? These are just conjectures for the time being, supported by Everett’s interpretation of quantum physics as “multi-world theory”.
Thus, Barak Shoshany and his students have been working for three years on a theory that would make parallel dates compatible with general relativity, which would allow the exclusion of paradoxes of cohesion and the possibility of traveling to the past.