At Popular Mechanics, Caroline Delbert has another look at a claim that a specific type of time travel is possible. The biggest barrier to taking time travel concepts seriously (never mind the Terminator films) is this: Time travel would enable a man to go back and kill his grandmother. That means he would not be here today to go back. So how, exactly, is it possible…?

Delbert points to a 2020 open access paper in Classical and Quantum Gravity where University of Queensland researchers Germain Tobar and Fabio Costa

The math itself is complex, but it boils down to something fairly simple. Time travel discussion focuses on closed time-like curves (CTCs), something Albert Einstein first posited. And Tobar and Costa say that as long as just two pieces of an entire scenario within a CTC are still in “causal order” when you leave, the rest is subject to local free will.

“A Scientist Proved Paradox-Free Time Travel Is Possible,” December 26, 2024

In their paper, the researchers say, “This supports the view that complex dynamics is possible in the presence of CTCs, compatible with free choice of local operations and free of inconsistencies.”

At ScienceAlert, David Nield offers an interpretation of their view:

To use a topical example, imagine a time traveler journeying into the past to stop a disease from spreading – if the mission was successful, the time traveler would have no disease to go back in time to defeat.

Tobar’s work suggested that the disease would still escape some other way, through a different route or by a different method, removing the paradox. Whatever the time traveler did, the disease wouldn’t be stopped.

“A Physicist Came Up With Math That Shows ‘Paradox-Free’ Time Travel Is Plausible,” December 19, 2022

Unlike what happened in “A Sound of Thunder” (the butterfly effect) we couldn’t really change past or future events.

You may also wish to read: A Form of Time Travel That Might Be Possible… In world of entropy, time runs in one direction and reversing it would create impossible contradictions, physicists say. The time travel that is likely to be possible would be like having a very good four-dimensional memory — it recreates events but it doesn’t change them.