Have you ever made a mistake that you wish you could change? The concept of time travel is fascinating because of how to correct past mistakes. In science fiction, with a time machine, nothing is permanent, you can always change it. Is time travel possible in our universe, or is it just science fiction?
Our understanding of time and causality is a result of general relativity. Albert Einstein's theory combines space and time into a single entity and provides a remarkably intricate explanation of how they both work.
Physicists are fairly certain that this theory provides an accurate description of the Universe because it has existed for more than 100 years and has been verified to extremely high precision.
Physicists have been trying to figure out if time travel is possible using general relativity. You can write down equations that describe time travel and they are compatible with the theory of relativity. If equations do not correspond to reality, they are meaningless.
There are two main issues that make us think these equations are not realistic. Building a time machine seems to require exotic matter which is negative energy.
Positive energy is what we see in our daily lives, and it's not something you can just find lying around. From quantum mechanics, we know that matter can be created in too small a quantities and for too short a time.
There is no proof that it is possible to create exotic matter in sufficient quantities. Other equations can allow time travel without exotic matter. This may be a limitation of our current technology or understanding of quantum mechanics.
The observation that time travel seems to contradict logic is more significant than the practical issue.
Consistency paradoxes are the most problematic type of paradoxes.
Consistency paradoxes happen when there is a certain event that leads to changing the past, but the change itself prevents this event from happening in the first place.
Imagine a scenario where I enter my time machine, use it to go back in time five minutes, and destroy it as soon as I get to the past. I would not be able to use the time machine five minutes later.
I can't destroy the time machine if I can't use it. I can destroy it back in time because it is not destroyed. If the time machine is not destroyed, it will be destroyed.
This scenario is not consistent because it cannot be destroyed and not destroyed at the same time.
Time travellers are usually warned not to make significant changes to the past and to avoid meeting their past selves for this exact reason. This can be found in many time travel movies, such as the Back to the Future trilogy.
In physics, a paradoxes is not an event that can actually happen, it is a purely theoretical concept that points towards an inconsistency in the theory. Consistency paradoxes imply that time travel simply cannot be done.
This was one of the reasons for Stephen Hawking to come up with his theory that time travel should be impossible. However, this is not proven yet.
If we could eliminate paradoxes, the universe would be a lot more interesting.
The self-consistency conjecture is a theoretical physicist who states that you can travel to the past, but you cannot change it.
If I tried to destroy my time machine in the past, I would find it impossible. Consistency would be preserved by the laws of physics.
If you cannot change the past, what is the point of going back in time? My recent work with my students shows that there are paradoxes in time travel. Time travel is impossible if only one of the paradoxes can be eliminated.
Is this the final nail in the coffin of time travel? Not quite. Allowing for multiple histories or parallel timelines can resolve paradoxes that are not possible with the current conjecture. It can resolve any paradoxes you throw at it.
The idea is very simple. When I leave the time machine, I go in a different direction. I can destroy the time machine without changing anything in the original timelines I came from.
I can't destroy the time machine in the original timeline, which is the one I used to travel back in time.
I've been working on time travel paradoxes for the last three years, and I've become more convinced that time travel is possible, but only if our Universe can allow multiple histories to coexist. Can it?
If you subscribe to the "many-worlds" interpretation, one history can be split into multiple histories, one for each possible measurement outcome.
These are speculations. A concrete theory of time travel with multiple histories that is compatible with general relativity is currently being worked on by my students and I. Even if we can find a theory that1-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-6556
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. If time travel is possible, then multiple histories must also be possible.
Barak Shoshany is an assistant professor at Brock University.
This article is free to use under a Creative Commons license. The original article is worth a read.
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