There have been many times when we wished we hadn't. Correcting the mistakes of the past is one of the reasons we find the concept of time travel so fascinating. With a time machine, as is often portrayed in science fiction, nothing is permanent anymore.
You can always go back and make or change our mistakes or failures. But is time travel really possible in our universe, or is it just science fiction?
Our modern understanding of time and causation comes from general relativity. Theoretical physicist Albert Einstein's theory combines space and time into a single entity - "space-time" and offers a highly complex explanation of how both work, incomparable to any other established theory.
This theory has been around for over 100 years and has been experimentally verified with extremely high precision, so physicists are fairly confident that it provides an accurate description of the causal structure of our Universe.
For decades, physicists have been trying to use general relativity to figure out whether time travel is possible. It turns out that we 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 actually correspond to anything.
Arguments Developed Against Time Travel
There are two main issues that make us think these equations may not be realistic.
The first issue is practical: building a time machine seems to require exotic matter, matter with negative energy.
Every substance we see in our daily lives has positive energy – matter with negative energy is not something you can find lying around.
We know from quantum mechanics that such matter could theoretically be created, but in very small quantities and for very short periods of time.
However, there is no evidence that it is impossible to create sufficient quantities of exotic matter. Also, other equations can be discovered that allow time travel without requiring exotic matter. Therefore, this problem may simply be a limitation of our current technology or understanding of quantum mechanics.
The other important issue is less practical but more important.
It is the observation in the form of time travel paradoxes that time travel seems counterintuitive.
There are several types of such paradoxes, but the most problematic are consistency paradoxes.
Consistency paradoxes, a popular trope in science fiction, occur when a certain event causes the past to change, but the change itself prevents that event from happening in the first place.
For example, consider a scenario where we enter my time machine, use it to go back in five minutes, and destroy the machine as soon as it reaches the past. Since I destroyed the time machine, five minutes later it would be impossible for me to use it.
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 only if not destroyed.
This scenario is inconsistent and paradoxical, as it cannot be destroyed and destroyed at the same time.
There is a common misconception that paradoxes can be “created” in science fiction.” Time travelers are often warned not to make significant changes in the past and therefore not encounter their past selves. 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 something that can actually happen - it's a purely theoretical concept that points to an inconsistency in the theory itself. In other words, consistency paradoxes don't just mean that time travel is a dangerous endeavour, it means that it's not possible.
This formulated the chronology preservation assumption of theoretical physicist Stephen Hawking, who stated that time travel should be impossible. However, this assumption has so far been unproven.
Moreover, the universe would be a much more interesting place if we could eliminate the paradoxes themselves, instead of eliminating time travel because of the paradoxes.
One of the attempts to resolve time travel paradoxes is theoretical physicist Igor Dmitriyevich Novikov's self-consistency assumption. This essentially states that you can travel to the past but cannot change it.
According to Novikov, if I had tried to destroy my time machine five minutes ago, I would have found it impossible to do. The laws of physics would somehow conspire to maintain consistency.
But what's the point of going back in time if you can't change the past?
Physicist Shoshany's latest work shows that there are time travel paradoxes that Novikov's conjecture cannot resolve.
This brings us back to the beginning, because if even a mere paradox cannot be dispelled, time travel remains logically impossible.
Is this the last nail in the coffin of time travel? Not exactly. We have shown that allowing multiple dates (or parallel timelines in more familiar terms) can resolve paradoxes that Novikov's conjecture cannot.
The idea is very simple. When I exit the time machine, I enter a different timeline. In this timeline, I can do anything I want, including destroying the time machine, without changing anything in the original timeline I came from.
Since I couldn't destroy the time machine in the original timeline, which is actually the timeline I used to travel back in time, there is no paradox.
After working on time travel paradoxes for the past three years, I am increasingly convinced that time travel is possible, only if our Universe can allow multiple dates to coexist. Well, can he?
Quantum mechanics certainly seems to suggest that a date can be "divided" into multiple dates, at least if you subscribe to Everett's interpretation of "many worlds".
We can provide one example for each possible measurement result.
Like history of whether Schrodinger's cat is alive or dead or whether I've been there.
But these are just speculations. My students and I are currently trying to come up with a concrete multiple-history theory of time travel that is fully compatible with general relativity. Of course, even if we managed to come up with such a theory, that wouldn't be enough to prove that time travel is possible, but at least it means that time travel isn't ruled out by coherence paradoxes.
Time travel and parallel timelines almost always go hand in hand in science fiction, but now we have proof that they should go hand in hand in real science as well.
General relativity and quantum mechanics tell us that time travel is possible, but if it is, then multiple histories should also be possible.
Source: Barak Shoshany, Assistant Professor of Physics, Brock University. – Sciencealert