In science fiction works, as long as there is a time machine, we can travel back to the past at any time. But is time travel actually feasible in our universe? Or is it just a concept confined to science fiction?
Modern understanding of time and causality stems from the theory of general relativity. Einstein combined space and time into one, creating the concept of “spacetime.” This concept explains the mechanisms of both in an extraordinarily complex and subtle manner. It has been over a hundred years since the theory of general relativity was introduced, and it has been experimentally validated with extreme precision, leading physicists to believe that general relativity is the key to accurately describing the causal structure of the universe.
For decades, physicists have tried to use general relativity to analyze whether time travel is possible. Some equations they derived indicate that time travel is entirely compatible with relativity. However, physics must still rely on the foundation of mathematics, and these equations would be meaningless if they do not correspond to real-world phenomena.
Time Travel Paradoxes
For decades, physicists have analyzed whether time travel is possible.
There are two major issues that make these equations seem impossible. The first is a practical problem: Creating a time machine may require negative energy matter. All the substances we observe in daily life are positive energy substances (positive energy), but negative energy is something we have not yet observed in reality.
However, quantum mechanics tells us that theoretically, negative energy matter can be created, albeit in extremely small quantities and for an extremely short duration. At the same time, there is no evidence that we are completely incapable of generating negative energy. Furthermore, we may find equations that allow time travel without requiring such materials. Thus, this problem lies only in our current technical state or our limited understanding of quantum mechanics.
The second issue is the paradoxes. A common scenario in science fiction works: an event occurs that leads us to want to change the past, but after the past is changed, it prevents that event from happening, thus creating a paradox.
For example: Suppose you have created a time machine, use it to go back five minutes, and immediately destroy that machine. Since the machine has been destroyed, you will not be able to use it after five minutes. But because you cannot use the time machine, you cannot go back in time to destroy it, so the machine is not destroyed. Can you then go back in time and destroy it? In other words, the time machine can only be destroyed if it has not already been destroyed. But clearly, it is impossible for a machine to be simultaneously in two states of “destroyed” and “not destroyed.” This situation is clearly contradictory.
Eliminating Paradoxes
A common misconception in science fiction is that paradoxes can be “created.” Time travelers are often warned not to change too much about the previous past and not to encounter their past selves. Examples of this can be found in many time travel-themed movies (such as the Back to the Future trilogy).
However, in physics, paradoxes are not actual events that can occur; rather, they are purely theoretical concepts that indicate a contradiction within the theory itself. In other words, a self-consistent paradox not only means that time travel is a dangerous action, but also implies that it may be completely impossible.
This is one of the motivations behind Hawking’s “chronology protection conjecture.” The theoretical physicist Hawking believes that time travel is impossible. However, the conjecture has yet to be proven. Instead of dismissing time travel due to paradoxes, we might eliminate the paradoxes themselves, making the universe much more interesting.
To resolve time travel paradoxes, theoretical physicist Igor Dmitrievich Novikov proposed the theory of “self-consistent conjecture.” The basic idea of this theory is that you can travel to the past, but you cannot change the past. Novikov pointed out that if you go back five minutes and try to destroy the time machine, you will find that you cannot do so at all, because the laws of physics will attempt to intervene and protect the consistency from destruction.
Time travel is not completely ruled out by self-consistent paradoxes.
New Theories
But what is the use of going back to the past if you cannot change it? According to a recent study by Barack Shoshenny, an assistant professor of physics at Brock University in Canada, some time travel paradoxes as per Novikov’s self-consistent conjectures may not be resolvable. This would mean that even if just one paradox cannot be eliminated, time travel is logically impossible.
Shoshenny’s research indicates that as long as the existence of multiple histories (or parallel timelines) is allowed, we can resolve paradoxes that Novikov’s conjecture cannot solve. In fact, any paradox can be resolved in this way.
The idea is quite simple: When you step out of the time machine, you enter a different timeline; you can do anything you want on that timeline (including destroying the time machine), and the timeline you are currently in will not change. Since you cannot destroy the time machine in the original timeline, there are no paradoxes at all.
For the past three years, Shoshenny has been researching time travel paradoxes. He is increasingly convinced that time travel is entirely possible, but only if our universe allows for the existence of multiple timelines. So the question arises: Could there be multiple timelines in the universe?
From the perspective of quantum mechanics, the answer seems to be “yes.”
But these are just conjectures. Shoshenny and his students are currently seeking a time travel theory that incorporates multiple histories and is fully compatible with general relativity. Of course, even if they discover such a theory, it will not be enough to prove that time travel is possible. But at least it means that time travel is not completely ruled out by self-consistent paradoxes.
Time travel and parallel timelines always appear together in science fiction, but researchers have now demonstrated that they are inseparable in the real world. General relativity and quantum mechanics tell us that time travel is possible; but if time travel is possible, then there must also be multiple histories.
