(ORDO NEWS) — Have you ever made a mistake that you would like to correct? Correcting the mistakes of the past is one of the reasons we find the concept of time travel so exciting.
As often depicted in science fiction, with a time machine, nothing lasts forever – you can always go back and change everything. But is time travel really possible in our universe, or is it just science fiction?
Our modern understanding of time and causality comes from the general theory of relativity. The theory of theoretical physicist Albert Einstein combines space and time into a single entity – “spatial time” – and provides a surprisingly complex explanation of how they both work, on a level that cannot be compared with any other established theory.
This theory has been around for over 100 years and has been experimentally tested with extremely high accuracy, so physicists are 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 find out if time travel is possible. It turned out that it is possible to write equations that describe time travel and are completely compatible and consistent with relativity. But physics is not mathematics, and equations are meaningless unless they correspond to something in reality.
Arguments against time travel
There are two main questions that lead us to think that these equations may not be real. The first question is practical: to create a time machine, apparently, exotic matter is required, that is, matter with negative energy.
All matter that we see in everyday life has positive energy – matter with negative energy is not something that can be found just around the corner. We know from quantum mechanics that such matter could theoretically be created, but in too small quantities and for too short a time.
However, there is no evidence that it is impossible to create exotic matter in sufficient quantities. Moreover, other equations may be discovered that allow time travel without the use of exotic matter. So this problem may just be a limitation of our current technology or understanding of quantum mechanics.
The other main problem is less practical but more significant: the observation that time travel seems to defy logic, in the form of time travel paradoxes. There are several types of such paradoxes, but sequence paradoxes are the most problematic.
A popular technique in science fiction, the sequence paradox occurs whenever a certain event occurs that causes the past to change, but the change itself prevents that event from happening in the first place.
For example, consider a scenario where I enter my time machine, use it to go back in time for five minutes, and destroy the machine as soon as I enter the past. Now that I’ve destroyed the time machine, I won’t be able 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 destroyed and not destroyed at the same time, this scenario is inconsistent and paradoxical.
It is a common misconception in science fiction that paradoxes can be “created”. Time travelers are usually warned not to make significant changes to the past and avoid encounters with their past selves for this 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 is a purely theoretical concept that indicates an inconsistency in the theory itself. In other words, sequence paradoxes don’t just imply that time travel is dangerous, they imply that it’s simply impossible.
This was one of the reasons that led the theoretical physicist Stephen Hawking to formulate his Chronology Defense Hypothesis, which states that time travel should be impossible.
However, this hypothesis still remains unproven. Moreover, the universe would be much more interesting if, instead of eliminating time travel due to paradoxes, we could simply eliminate the paradoxes themselves.
But what’s the point of going back to the past if you can’t change the past? My recent work with my students Jacob Houser and Jared Vaughan shows that there are paradoxes of time travel that Novikov’s hypothesis cannot resolve.
This brings us back to the starting point, because even if one paradox cannot be resolved, time travel remains logically impossible.
So, is this the final nail in the coffin of time travel? Not really. We have shown that the possibility of multiple histories (or, in more familiar terms, parallel timelines) can resolve paradoxes that Novikov’s hypothesis cannot resolve. In fact, she can resolve any paradox you present to her.
The idea is very simple. When I exit the time machine, I enter another timeline. In this 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 that I actually used to travel to the past, there is no paradox.
After working on the paradoxes of time travel for the past three years, I’m increasingly convinced that time travel is possible, but only if our universe can allow multiple “parallel realities” to coexist. So, can she?
Quantum mechanics certainly seems to imply this, at least if you stick to Everett’s “many worlds” interpretation, where one story can “split” into multiple stories, one for each possible outcome of a measurement – e.g. a cat is alive or dead. Schrödinger, or whether I arrived in the past.
But these are just guesses. My students and I are currently working to find a concrete multi-history theory of time travel that is fully compatible with general relativity.
Of course, even if we manage to find such a theory, it will not be enough to prove that time travel is possible, but it will at least mean that time travel is not ruled out due to sequence paradoxes.
Time travel and parallel timelines almost always go hand in hand in science fiction, but now we have proof that they must 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.
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