But is it really possible to time travel in our Universe or is it just science fiction? Our modern understanding of time and causality derives from general relativity. Albert Einstein’s theory combines space and time into a single entity, spacetime, and provides an extraordinarily intricate explanation of how they both work, on a level unmatched by any other established theory.
This theory exists for more than 100 years and it has been experimentally verified with extremely high accuracy, so physicists are confident enough that it provides an accurate description of the causal structure of our Universe. For decades, physicists have tried to use general relativity to figure out whether time travel is possible.
It turns out that you can write equations that describe time travel and are completely compatible and consistent with relativity. But physics isn’t mathematics, and equations don’t make sense if they don’t match anything in reality.
Arguments against time travel
There are two main issues that make us think that these equations may be unrealistic. The first problem is practical: building a time machine seems to require exotic matter, which is matter with negative energy.
All matter we see in our daily life has positive energy – negative energy matter is not something you can just find lying around. From quantum mechanics we know that such matter can theoretically be created, but in quantity too small and for too short a time. However, there is no evidence that it is impossible to create exotic matter in sufficient quantity.
In addition, other equations can be discovered that allow time travel without requiring exotic matter. Therefore, this problem may just be a limitation of our current technology or understanding of quantum mechanics. The other main issue is less practical, but more significant: it is the observation that time travel appears to contradict logic, in the form of time travel paradoxes.
There are several types of such paradoxes, but the most problematic are the coherence paradoxes. An extremely popular subject in science fiction, the paradoxes of coherence they occur whenever there is a certain event that leads to changing the past, but the change itself prevents this event from occurring in the first place.
For example, consider a scenario where I walk into my time machine, use it to go back five minutes, and destroy the machine as soon as I get into the past. Now that I have destroyed the time machine, it would be impossible for me to use it five minutes later.
But if I can’t use the time machine, I can’t go back in time and destroy it. Therefore, it is not destroyed, so I can go back in time and destroy it. In other words, the time machine is destroyed if and only it is not destroyed. Since it cannot be at the same time destroyed and not destroyedthis scenario is inconsistent and paradoxical.
Time: eliminate paradoxes
There is a common misconception in science fiction that paradoxes can be “Created”. Time travelers are generally cautioned not to make any significant changes to the past and to avoid meeting their past self for this very reason. Examples of this can 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 indicates an inconsistency in the theory itself. In other words, the paradoxes of coherence do not simply imply that time travel is a dangerous endeavor, they simply imply that it cannot be possible.
This was one of the reasons why theoretical physicist Stephen Hawking formulated his conjecture on history protection, which states that time travel should be impossible. However, this conjecture remains unproven so far.
Furthermore, the Universe would be a much more interesting place if instead of eliminating time travel due to paradoxes, we could simply eliminate the paradoxes themselves. An attempt to solve the time travel paradoxes is the self-consistent conjecture of theoretical physicist Igor Novikov, which essentially states that you can travel to the past, but you cannot change it.
According to Novikov, if I tried to destroy my time machine five minutes ago, I would find that it is impossible to do so. The laws of physics would somehow conspire to preserve consistency.
Presentation of multiple stories
But what’s the point of going back in time if you can’t change the past?
The recent work of Dr. Shoshany together with his students Jacob Hauser and Jared Wogan, shows that there are time travel paradoxes that the Novikov conjecture cannot solve. This brings us back to square one, for if even one paradox cannot be eliminated, time travel remains logically impossible.
They have indeed shown that allowing multiple stories (or more familiarly terms, parallel timelines) can solve the paradoxes that Novikov’s conjecture cannot. Indeed, it can resolve any paradox that is thrown at it.
The idea is very simple. When I get out of the time machine, I go out in 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.
After working on the paradoxes of time travel for the past three years, they have become increasingly convinced that time travel may be possible, but only if our Universe can allow for multiple stories to coexist. So, can it be?
Quantum mechanics certainly seems to imply that, at least if you embrace interpretation of Everett’s many worldsin which a story can be divided into several stories, one for each possible measurement result, for example, whether Schrödinger’s cat is alive or dead, or whether or not I arrived in the past.
But these are just speculations. The students and Dr. Shoshany are currently working to find a concrete theory of time travel with multiple stories that is fully compatible with general relativity.
“Of course, even if we could find such a theory, it would not be enough to prove that time travel is possible, but it would at least mean that time travel is not excluded by coherence paradoxes.” Shoshany reports
Time travel and parallel timelines almost always go hand in hand in science fiction, but now we have proof that they have to go hand in hand even in real science. General relativity and quantum mechanics tell us that time travel may be possible, but if it is, then multiple stories must also be possible.
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