Have you ever made a mistake that you wish you could undo? Correcting past mistakes is one of the reasons we find the concept of time travel so fascinating. As science fiction often portrays, with a time machine, nothing is more permanent – you can always go back and change things. But is time travel really possible in our universe, or is it just science fiction?
Our modern understanding of time and causality comes to us from general relativity. The theory of theoretical physicist Albert Einstein combines space and time into a single entity – “spacetime” – and provides a remarkably complex explanation of how they work, on a level unmatched by any other established theory.
This theory has existed for more than 100 years and has been verified experimentally with very high precision. Physicists are therefore almost certain that it provides an accurate description of the causal structure of our universe.
For decades, physicists have been trying to use general relativity to determine if time travel is possible. It turns out that one can write equations that describe time travel that are fully compatible and consistent with relativity. But physics is not mathematics, and equations are meaningless if they don’t correspond to anything in reality.
Arguments against time travel
Two main questions make us think that these equations may not be realistic. The first is practical: building a time machine seems to require exotic matter, i.e. negative energy matter.
All matter that we see in our daily lives has positive energy – negative energy matter is not something that can be found just anywhere. Quantum mechanics teaches us that it is theoretically possible to create such matter, but in too small quantities and for too short durations.
However, there is no evidence that it is impossible to create exotic matter in sufficient quantities. Also, other equations can be discovered that allow time travel without requiring exotic matter. Therefore, this problem might just be a limitation of our current technology or our understanding of quantum mechanics.
The other main problem is less practical, but more significant: it is the 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 the coherence paradoxes.
A popular science fiction trope, coherence paradoxes occur whenever a certain event leads to altering the past, but the change itself prevents that event from happening in the first place.
For example, consider a scenario in which I enter my time machine, use it to go back five minutes, and destroy the machine as soon as I enter the past. Now that I’ve 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, then 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 if it is not destroyed. Since it cannot be both destroyed and undestroyed simultaneously, this scenario is inconsistent and paradoxical.
Eliminate the paradoxes
A common misconception in science fiction is that paradoxes can be “created”. Time travelers are generally warned not to make significant changes to the past and to avoid encountering their past selves for that very reason. Examples of this can be found in many time travel films, such as the Back to the Future trilogy.
But in physics, a paradox is not an event that can actually happen – it’s a purely theoretical concept that highlights an inconsistency in the theory itself. In other words, the coherence paradoxes not only mean that time travel is a dangerous business, but that it is simply impossible.
This is one of the reasons that prompted theoretical physicist Stephen Hawking to formulate his Timeline Protection Conjecture that time travel should be impossible.
However, this conjecture has not yet been proven. Also, the universe would be much more interesting if, instead of eliminating time travel due to the paradoxes, we could just eliminate the paradoxes themselves.
An attempt to resolve the paradoxes of time travel is theoretical physicist Igor Dmitriyevich Novikov’s self-consistency conjecture, which essentially asserts 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 in the past, I would find it impossible to do so. The laws of physics would somehow conspire to preserve consistency.
Introduce multiple stories
But what’s the point of going back in time if you can’t change the past? My recent work, in collaboration with my students Jacob Hauser and Jared Wogan, shows that there are time travel paradoxes that the Novikov conjecture cannot resolve. This brings us back to square one, because if a single paradox cannot be eliminated, time travel remains logically impossible.
So, is this the final nail in the coffin of time travel? Not quite. We have shown that allowing multiple histories (or, in more colloquial terms, parallel timelines) can resolve paradoxes that the Novikov conjecture cannot eliminate. In fact, it can resolve any paradox.
The idea is very simple. When I step out of the time machine, I step out into a different timeline. In this timeline, I can do whatever I want, including destroying the time machine, without changing anything about the timeline I came from.
Since I cannot destroy the time machine in the original timeline, which is the one I actually used to time travel, there is no paradox.
After working on the paradoxes of time travel for the past three years, I am increasingly convinced that time travel is possible, but only if our universe can allow multiple stories to coexist. So is it possible?
Quantum mechanics seems to suggest so, at least if you subscribe to Everett’s “many worlds” interpretation that a story can “split” into multiple stories, one for each possible measurement outcome – for example, if Schrödinger’s cat is alive or dead, or whether or not I’ve arrived in the past.
But these are only speculations. My students and I are currently struggling to find a concrete theory of time travel with multiple histories that is fully compatible with general relativity. Of course, even if we manage to find such a theory, it wouldn’t be enough to prove that time travel is possible, but it would at least mean that time travel is not ruled out by the coherence paradoxes.
Time travel and parallel timelines almost always go together in science fiction, but now we have proof that they must go together in real science as well. General relativity and quantum mechanics tell us that time travel might be possible, but if so, multiple histories must also be possible.
By Barak Shoshany, assistant professor of physics at Brock University.