Correcting past mistakes is one of the reasons we find the concept of time travel so exciting. As is often depicted in science fiction, nothing lasts forever with a time machine - 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 general relativity. The theory of theoretical physicist Albert Einstein combines space and time into a single entity - "space time" - and provides a surprisingly complex explanation of how they work at a level unmatched by any other established theory.
This theory has been around for over 100 years and has been experimentally tested to extremely high accuracy, so physicists believe it provides an accurate description of the causal structure of our universe. For decades, physicists have tried to use general relativity to find out if time travel is possible. It turned out that you can write equations describing time travel that are completely consistent with the theory of relativity. But physics is not math, and equations are meaningless if they don't correspond to something in reality.
There are two main questions that lead us to conclude that these equations may not be true. The first question is practical - apparently, exotic matter is needed to create a time machine, that is, matter with negative energy. All matter we see on a daily basis has positive energy - matter with negative energy is not something you find just around the corner. We know from quantum mechanics that theoretically such matter could 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 can be discovered that allow time travel without the use of exotic matter. So this problem may simply be a limitation of our current technology or understanding of quantum mechanics.
The second main problem is less practical but more significant - time travel seems to defy logic, in the form of paradoxes. There are several types of such paradoxes, but the most problematic are sequence paradoxes. A popular science fiction technique, the sequence paradox, occurs whenever an event occurs that causes the past to change, but the change itself prevents that event from happening.
For example, consider the scenario where I enter my time machine, travel back in time five minutes, and destroy it as soon as I enter the past. Now that I've destroyed the time machine, can I 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 will be destroyed if and only if it is not destroyed. Since it cannot be destroyed and not destroyed at the same time, this scenario is incoherent and paradoxical.
A common misconception in science fiction is 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 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 points to an inconsistency in the theory itself. In other words, sequence paradoxes not only imply that time travel is dangerous, but also that it is simply impossible.
This was one of the reasons that led theoretical physicist Stephen Hawking to formulate the defense of chronology hypothesis 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 caused by paradoxes, we could simply eliminate the paradoxes themselves.
Perhaps if someone had tried to destroy the time machine five minutes in the past, it would have been impossible. The laws of physics would somehow conspire to maintain consistency. Even if one paradox cannot be resolved, time travel remains logically impossible. Is this the final nail in the time travel coffin?
The possibility of multiple stories, or parallel timelines, can resolve paradoxes. In fact, it can solve any paradox you present. The idea is very simple. When I exit the time machine, I go to another timeline. In this timeline, I can do whatever I want, including destroying the time machine, without changing anything in the original timeline where I come from.
Since I cannot destroy the time machine in the original timeline that I actually used to travel back in time, there is no paradox. Quantum mechanics certainly seems to imply that one story can "split" into many stories, one for each possible outcome of the measurement - e.g. Schrödinger's cat is alive or dead.
Physicists are currently working to find a specific theory of time travel that is fully compatible with general relativity. Of course, even if such a theory is found, it will not be enough to prove that time travel is possible, but at least it will mean that time travel is not ruled out due to sequence paradoxes.
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