An image of a green fedora hat, which serves as the logo for this site.Gordy's Discourse

Time travel's paradoxes resolved External link icon.

Time travel has always been a problem because it creates paradoxes. Perhaps the most famous is the Grandfather Paradox. If you travel back in time and kill your grandfather when he was a child, your parents wouldn't have been born and therefore neither would you. So if you don't exist, how could you travel back in time in the first place to kill your grandfather?

Yet Einstein's General Theory of Relativity predicts the possibility of time travel. In physicists' parlance they're called Closed Timelike Curves. Time travel is theoretically possible, although finding or creating closed timelike curves is another matter altogether. You can't just buy one from eBay.

Let's for one highly speculative moment assume we did have a closed timelike curve; how do we use it without running into any of the paradoxes?

What's really at odds here are General Relativity and the Laws of Dynamics, because the latter suggests determinism such that if I know all there is to know about some particular object, I can predict exactly where that object will be in the future.

When we launched Apollo 11 at the moon we could be fairly sure our aim was correct and it would get to the moon. This is the laws of dynamics in action.

Perhaps you can see how time travel could interfere with that. If I nip back in time and nudge Apollo 11 so it no longer points at the moon, the laws of dynamics will fail and the Apollo 11 crew would think magic was occurring because this just isn't how the laws of physics work.

So something's got to give.

Germaine Tobar, a clever young physicist, and his supervisor, Dr Fabio Costa, have done some very complex mathematics and believe they've figured out a way around the problems.

What they've demonstrated is that events would somehow recalibrate to avoid paradoxes. Tobar describes it like this:

Say you travelled in time, in an attempt to stop COVID-19’s patient zero from being exposed to the virus.

However if you stopped that individual from becoming infected, that would eliminate the motivation for you to go back and stop the pandemic in the first place.

He then goes on to say:

In the coronavirus patient zero example, you might try and stop patient zero from becoming infected, but in doing so you would catch the virus and become patient zero, or someone else would.

No matter what you did, the salient events would just recalibrate around you.

And he concludes:

The range of mathematical processes we discovered show that time travel with free will is logically possible in our universe without any paradox.

Going back to my Apollo 11 example, if I nudged the rocket, something, somehow would nudge it back. If it didn't we'd have the paradox of me preventing a moon landing that didn't happen in the first place, and thus I'd have no reason to go back in time to prevent it.

I tried the read the paper, but I just don't understand enough of the maths. Apparently, though, the maths checks out and this is no mean feat when you think about it. Sure, the chances of it ever happening are fantastically remote, but even resolving the paradoxes of time travel in theory is an amazing achievement.

It's great fodder for science fiction writers.