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What Are Causal Loops and How Can We Fix Time Travel?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

Is Time Travel Possible?

Time travel is a paradoxical topic that drives many people crazy trying to wrap their heads around the seemingly insurmountable conclusions it leads us to. These causal loops are numerous and certainly worth exploring for their fun contradictions.

But what I want to know is how can we conquer these issues? It would seem to not be a problem because we haven’t had any known time travelers…but that is side-skirting the issue. What can we reason our way through to help us resolve time travel paradoxes?

Action Causal Loops

To be clear, not all time traveling creates equally perplexing causal loops, or when an action eventually leads back to creating itself (if a then b, b then c, c then a—as an example). If we are closed, then each action leads to only one effect, and so we can follow the loop as a singular path back into itself like a circle. But an open loop offers at least one event that can be caused by more than one thing, like how an off ramp leads to a road already there (Monton 54-6).

Time travel stories involve a character entering a loop typically, and so their story is an open causal loop. The common theme is backward causality, or when effect precedes cause. This flies in the face of our everyday experiences and so is naturally something we find impossible to occur. So, with this established, let’s play (Ibid).

Are the past and future just as real as the present? Eternalism says so, which is unlike presentism or that the moment is all that’s real with the past being what has happened and the future as possibilities but both truly inaccessible to us at any time. Bradley Monton presents his ideas in an eternalist stage in a method of causal loop resolution for certain cases. To be perfectly clear, he isn’t denying that causal loops are a feature of time travel, he is just presenting cases where causal loops don’t form from our time traveling adventures (56-7).

To do so, he first clarifies a worldline as the path an object takes in 4-D space and the personal time of an object as the age a person would report themselves as. Why is this needed? Well, as you move about the Universe one could trace out the path you took based on where you were at and at some time. That is represented by the worldline and each moment you are on it you would have a personal time of an object to report as, about the time since your origin (Ibid).

He also clarifies the difference between discontinuous time travel and continuous time travel. The former is a worldline with a jump between two times on a worldline but a personal time of an object reporting no age difference between the two. The latter is where each moment on the worldline matches per unit time with a personal time of an object’s age increase and each moment happens after another (57).

As far as how time travel can occur, Monton points out that simple movement in space-time constitutes forward movement in time. Objects in spacetime also offer geometrical means for time travel, like frame-dragging of black holes or the tunneling of wormholes. So, to summarize Monton lays out continuous time travel, discontinuous time travel, and space-time geometry time travel. No matter which one we are working with, he shows how causal loops can be eliminated (Ibid).

In Monton’s example of causal loop reconciliation, he postulates a world populated with one particle which doesn’t undergo spooky action at a distance as quantum mechanics allows for normally. If it moves forward, then backward, and then forward in time, it never interacted with itself and so we have no causal loop (57-8).

Okay, simple enough…but as Monton confesses, “one might point out that the argument implicitly relies on a contentious substantivalism view about time.” That is, time goes on at a constant rate like a river and we go up or down it. This is different from the relationist viewpoint, where time changes as an object does. So Monton attempts to remove the causal loops regardless of a substantivistic or relationalistic viewpoint (Ibid).

Instead, consider a world with many particles with a majority moving in the same direction of time. Note that this method satisfies either a substantivistic or relationalistic viewpoint. The former is met because time sets its own direction no matter what we do. That’s easy. The latter is met because the particles determine the direction of time and we had a majority moving in the same direction. There are three types of point-particles, which Monton simply labels as A, B, and C types for simplicity. Also, for simplicity, spooky action at a distance doesn’t apply. Space-time is Newtonian and not Einsteinian, so worldlines correspond to time travel and no spacetime geometry time travel is occurring (58-61).

Our world has two regions separated by a force field, with A particles on one side, B particles on another, and C particles able to traverse between the two regions. Also, A particles can cross over but they become B particles upon entering the other region. With this set-up, an example of discontinuous time travel would be particle A at time t2 going back in time and appearing in the other region at time t1, becoming a B particle. No loop is possible because B particles are trapped and so we have avoided a causal loop (Ibid.

Of course, a few objections to this can be raised. How can you tell the particle apart from the others? Is it actually the same particle if it changed? Monton does a little bit of hand-waving, claiming he isn’t proving the existence of discontinuous time travel but rather a mechanism for it allowing for no causal loop (Ibid).

As for continuous time travel in this scenario, that would involve an A particle crossing over, becoming a B particle and then going back in time to…well, it can only go as far back as its formation, for the force field prevents travel to the other region. Hence, we have a case of causal isolation and a prevention of a causal loop once again. Two down, one to go. How about space time geometry time travel? Simply make the barrier a wormhole, with the passage through it stretching time out. Boom, that’s it (60-1).

As I was reading Morton’s work, I was constantly thinking to myself, “This isn’t realistic at all.” But then I paused a moment and thought about the theoretical implications. It would be up to a scientist to develop the model, and it should be possible through some trickery I would imagine (I just cannot think of what that could be at this time) (62).

Object Causal Loops

While causal loops cover many different paradoxes, it is sometimes prudent to look at subsets of them with their own curious features. Hanley does this based on the causal loop, and the first one we will look at is the object loop. It is the predestination paradox essentially, with an object being found, used but wears out, is replicated via facsimile or transcription, sent back and time, and becoming the very object that was found in the first place. No biggie, right?

Well, as Hanley points out, how would you describe the manufacturing of the object? When was it exactly first made? Even worse, how do you describe the age of it? We often tell time by how long an object has been around or how used up it is, but with this causal object loop such attempts fail. In fact, it leads to what he calls the restoration problem, where if we replace or alter our object, then each molecule that is involved with that is sent back and somehow must be separated so it can once again become a part of the loop. Something must undo what happened so it can happen again during the time traveling process! (Hanley 131-5)

You could attribute a potential solution as a coincidence factor, or that each particle that is involved in the replacement was always there because it was freed up to be via destruction of the old piece—a highly unlikely event. This is known as the replacement solution (Ibid).

Information Causal Loop

As we shall see, coincidence will not be singular to object loops. Information loops are when a traveler knows something, goes back in time, and causes the event that will eventually lead to the knowledge the traveler gains in the first place. Where did the info originate from in the first place? Some of these loops can be explained by coincidence…again. Maybe the information was conveyed without intent or was just a happenstance. But information that is uncovered by someone could conceal it and send it back in an altered form, but this too presents problems. The only real possible solution Is once again a replacement one (Hanley 136-142).

More loops are out there, and I intend to talk about them. I just need more time…

Works Cited

Hanley, Richard. “No End in Sight: Causal Loops in Philosophy, Physics, and Fiction.” Synthese. Vol. 141, No. 1. Print. 131-42.

Monton, Bradley. “Time Travel Without Causal Loops.” The Philosophical Quarterly. Vol. 59, No. 234. Print. 54-62.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2021 Leonard Kelley