If parallel timelines exist, time travel may be limited to those who have walked through the same tunnel

If parallel timelines exist, time travel may be limited to those who have walked through the same tu ...

Have you ever made a mistake that you wish you could undo? One of the reasons why we see the concept of time travel so fascinating. As often described in science fiction, with a time machine, nothing is permanent anymore you may always go back and modify it. Is it true in our universe, or is it just science fiction?

Our current understanding of time and causality comes from general relativity. The Theoretical physicist Albert Einsteins theory combines space and time into a single entity spacetime, providing a remarkable detail of how they both work, at an all-time high. This theory has existed for over 100 years and has been experimentally tested to great accuracy, therefore physicists are fairly certain it provides an accurate description of our causal structure.

Physicists have been attempting to use general relativity to figure out if time travel is possible. It turns out that you may write down equations that describe time travel as well as being fully compatible and compatible with relativity. However, physics isn''t mathematics, and equations are useless if they do not correspond to anything in reality.

Arguments against time travel

Two main problems make us think these equations might be unrealistic. The second issue is a practical one: building a time machine appears to require exotic matter, which is the matter with negative energy. Everything we see in our daily lives has positive energy-matter with negative energy isn''t something you can just find lying around. From quantum mechanics, we know that such matter can theoretically be created in too small quantities and for too short periods.

There is no evidence that it is impossible to create exotic matter in sufficient quantities. Further, other equations may be discovered that allow time travel without requiring exotic matter. This issue may just be a limitation of our current invention or understanding of quantum mechanics.

The other major issue is less practical, but more significant: it is the observation that time travel appears to contradict logic in the form of time travel paradoxes. There are several kinds of similar paradoxes, but the most common are consistency paradoxes.

When there is a certain event that causes the past to change, a popular trope in science fiction and consistency paradoxes happen, but this change impedes the event from happening in the first place.

Consider a scenario where I enter my time machine, use it to return in time five minutes, and destroy it as soon as I get to the present. Now that I destroyed the time machine, it would be impossible for me to use it five minutes later.

If I cannot use the time machine, then I cannot go back in time and destroy it. 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 simultaneously. This scenario is unquestionably and paradoxical.

Eliminating the paradoxes

Paradoxes are often created in science fiction. Time travelers are often warned not to make significant changes to the past and to avoid meeting their past peers for this exact reason. Examples of this may be found in many time travel movies, such as the Back to the Future trilogy.

However, a paradox isn''t an event that can happen, it is a purely theoretical idea that indicates an inconsistency in the theory itself. In other words, consistency paradoxes do not imply time travel is a dangerous task, they simply fail to be possible.

This was one of the motivations for theoretical physicist Stephen Hawking to write his chronology protection conceiv, which states that time travel should be impossible. However, this predicament so far remains unproven. Moreover, the universe would be a much more interesting place if we could instead eliminate time travel due to paradoxes.

Igor Dmitriyevich Novikovs, a theoretical physicist, is trying to resolve time travel paradoxes, claiming that you can travel to the past, but that you cannot modify it.

According to Novikov, if I attempted to destroy my time machine five minutes in the past, I would find that it is impossible to do so. According to the laws of physics, they may be used to maintain consistency.

Introducing multiple histories

If you cannot change the past, what''s the value of returning in time? My recent research, combined with my students Jacob Hauser and Jared Wogan, shows that there are time travel paradoxes that Novikovs conjecture cannot resolve. This takes us back to normal one, because if even one paradox cannot be eliminated, time travel remains logically impossible.

Is this the final nail in the puzzle of time travel? It''s not quite. We demonstrated that allowing for multiple histories (or in more familiar terms, parallel timelines) can alleviate the paradoxes that Novikovs conjecture cannot. In fact, it can alleviate any paradox you throw at it.

When I exit the time machine, I see a different timeline. It''s possible to, for example, to destroy the time machine without altering anything in the original timeline I was coming from. Since I cannot destroy the time machine in the original timeline, which I actually used to travel back in time, there''s no denying.

After spending the last three years on time travel paradoxes, I have become increasingly convinced that time travel may be possible, but only if our universe permits multiple histories to exist. So, can it?

Quantum mechanics seems to imply that, at least if you subscribe to Everetts'' many-worlds interpretation, where one history can split into several histories, one for each possible measurement outcome, for example, whether Schrodingers cat is alive or dead, or whether or not I arrived in the past.

These are just speculations. My students and I are currently attempting to develop a simple approach of time travel that is completely compatible with general relativity. Nevertheless, this might not be sufficient to prove that time travel is possible, but it would at least mean that time travel is not ruled out by consistency paradoxes.

Time travel and parallel timelines are almost always going hand-in-hand in science fiction, but now we have proof that they must go hand-in-hand in real science. General relativity and quantum mechanics make us think time travel may be possible, but if it is, then multiple histories must be permitted.

This article is republished from The Conversation under a Creative Commons license.

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