That’s what a recent paper uploaded to ArXiv says. Ivan Farkas explains at ScienceAlert:

If our universe is one of a typical class,

in such universes only forward movement is possible. As if driving down an endless one-way street with no service roads, turnarounds, or even an extended shoulder where one could pull over to enact a sneaky (illegal) U-turn.

And, really, this is the basis of time travel, which has much different definitions depending on whether one is talking science or science fiction:

“People expect a result about time travel to need heavy machinery, energy conditions, quantum effects, maybe a singularity. It needs none of that. The whole content is bookkeeping about directions,” [researcher HongSheng] Zhang adds. (November 3, 2026)

Shortly before his retirement, Stephen Hawking (1942–2018) held a party for time travelers, with the usual trappings, except for one thing. He did not send out the invitations until a day after the party. Needless to say, no one showed up holding one. Hawking felt that that ruled out time travel in this universe.

Farkas adds that the new research shows us what we are up against if we want to reverse time:

Finally, this offers a thought-and-math experiment of the type that has been influential throughout scientific history, allowing us to explore the abstruse principles that define our existence and possibilities.

But the Universe, as always, has the final say, Zhang concludes:

“What it does not do is tell us whether nature permits any of this. That remains open, and it is the question I would most like to see answered.”

But we are a step closer: “If you want a time machine, you now know exactly what you must break.”(November 3, 2026)

Maybe the time traveler must first find a different universe.