Time travel is a frequently discussed topic in the philosophy of time. One supposed problem with time travel is that it would allow for ‘causal loops’ — conundrums that make little sense in our third dimensional existence. If you’re prepared for some mind-warping hypotheticals, read on!
What are causal loops?
When an event occurs (eg. the construction of a time machine), that event has a cause — you invested millions of dollars in time travel research. And that cause has its own unique cause (you first became interested in time travel after reading about it online). You can keep going further and further back, with each event having its own unique cause, all the way back to the start of the universe. If time travel were possible, though, we might run in to a problem with causal explanations going in circles.
Let’s say that while you’re first reading about time machines, a future version of yourself suddenly shows up in a time machine, hands you the keys, then walks off. The existence is this time machine is “illegitimate,” as there’s no reason why it FIRST came in to existence. Your future self had a time machine because you eventually would have made one. BUT now that you have been given a time machine before doing so, you no longer go through that process. So in now altered future, your future self wouldn’t have a time machine to come back and give you, which means he wouldn’t have come back and given you the keys, so you would have built it… etc, etc. Hopefully you see the Catch-22 here.
If you’re still confused, watch this video for a clear example of a causal loop from Bill & Ted’s Excellent Adventure.
What does this mean for those without time machines?
Recall how I mentioned that these causes go all the way back to the beginning of the universe. Well, why does causation suddenly get to stop there? If every event needs to have an independent cause, then it seems we could never find a “first cause,” and therefore we could never even get started. The only way to resolve this issue to drop the requirement that every event must have an independent cause. But, when you drop that demand, suddenly causal loops aren’t so problematic. There’s no ‘independent’ cause to start off the loop, but it doesn’t need one, in the same way regular chains of causation don’t need a first cause. As Meyer puts it, “causal loops are no more mysterious than infinitely descending causal chains.”
This conundrum speaks to our very limited understanding of the nature of time. After all, we live in the third dimension so we see time as linear, when in fact it is anything but. Imagine explaining depth to someone who lived in a 2-D world — it wouldn’t make any sense to them!
With that thought, I’ll leave you to contemplate the nature of reality ;)