Tell me if you've heard this one before. You build a time machine and go back in time and accidentally kill your grandfather. What happens then? Do you disappear from the universe in a puff of pixie dust because if your grandfather dies in your time-travel timeline, you would never be born?
The "grandfather paradox" is one of the most famous of all time travel paradoxes. It's one of many paradoxes that make time travel not only impractical, but theoretically impossible.
Is it really?
The problem for the naysayers is that you should never tell a good scientist something is impossible until they've tested and exhausted all other possibilities. And given the utter strangeness of quantum mechanics and other quirky mathematical postulates, there are some tantalizing hints that maybe — just maybe — the "theoretical impossibility" of time travel might not be so impossible after all.
The "grandfather paradox" is a fun thought experiment. But surprisingly, it can be tested both mathematically and experimentally. "The results are compelling because they hint at potential breakthroughs in our knowledge of how time works, even though they don’t necessarily mean time travel itself is possible," writes Jonathan O'Callaghan in Popular Mechanics.
“We should not be thinking time travel is impossible because it would lead to paradoxes,” says Lorenzo Gavassino, PhD, a mathematical physicist at the University of Cambridge in the U.K. “The problem is that the universe tends to break down before.” O'Callaghan adds that the universe might find ways to stop it—such as forming a singularity before it could happen.
The universe is a pretty darn strange place, with stars made of diamonds and planets that may be totally covered in an ocean miles deep. Why not time travel?
Quantum mechanics already has a theory that suggests alternative timelines. The "many worlds theory" suggests that a quantum system splits whenever it interacts with its environment — a process called quantum decoherence. The new system is not aware of the other system, so it wouldn't experience a grandfather paradox.
There are other theories like a closed timelike curve, which arises from Einstein’s theory of general relativity. "Under that theory, it’s possible for a closed timelike curve to bring you back to your starting point in space—and time—and return to an earlier point in your life," suggests O'Callaghan.
“A closed timelike curve is a trajectory through spacetime that loops back on itself in the time dimension,” says Nicole Yunger Halpern, PhD, a theoretical physicist at the National Institute of Standards and Technology (NIST). “A particle on a closed timelike curve may meet its former self.”
Popular Mechanics:
The suggestion of such paths raises paradoxes, though. What would happen if you prevented yourself from being born (the grandfather paradox)? Would you ever set off on the journey at all? Or what if you received a notebook from yourself that told you how to travel back in time along the curve, and then gave it to your younger self. Where did the notebook come from? It would be information with no origin, known as the bootstrap paradox.
Or what if you went back in time to stop yourself from being in an accident only to cause it? This is known as the predestination paradox. If your attempt to change the past was the cause of it, does this mean the past itself cannot be altered? When it comes to time travel, the possibilities and questions never seem to end. Yet the ideas, while largely conceptual, can be tested to a degree, particularly in the quantum realm.
Kater Murch, PhD, a physicist at the University of California, Berkeley, has been conducting such tests to investigate closed timelike curves, using the unusual properties of quantum entanglement—that two quantum particles, even when separated, share a state.
"Quantum entanglement" is perhaps the most bizarre aspect of quantum mechanics. Entanglement is a real, observable behavior of nature in which two or more particles become linked so that measuring the state of one instantly dictates the state of the other, regardless of the distance between them.
It is an empirical fact confirmed by decades of laboratory experiments (using photons, electrons, and atoms). The 2022 Nobel Prize in Physics was awarded to Alain Aspect, John Clauser, and Anton Zeilinger specifically for experimentally proving that quantum entanglement is a real property of the universe.
How weird is entanglement? It's possible to have a camera take an image of a shooting star after the event had happened. “Quantum mechanics creates certain circumstances where you can do that, where we can orient a sensor in hindsight,” says Murch.
Vlatko Vedral, PhD, a quantum scientist at the University of Oxford, says this can be solved if entanglement is considered not just across space but across time too. This is known as temporal entanglement, which he has shown to be mathematically possible. The particle outside the curve would be entangled to both versions of the particle inside the curve.
That could help solve another problem, known as the black hole information paradox, postulated by the late Stephen Hawking, which suggests black holes can emit radiation but do not record information about what fell into them.
“Maybe black hole evaporation could be seen as a process that converts spatial into temporal entanglement,” Vedral says, explaining how entangled particles that remain outside the black hole can retain information, and avoid the paradox, so that information can escape.
Emily Adlam, an assistant professor of the philosophy of quantum mechanics at Chapman University, takes us down a very deep, very strange rabbit hole.
"In a 2022 paper, Adlam suggested an 'all at once' model for the universe where the whole universe might exist as one complete structure across time, fitting together 'like a game of Sudoku,'" writes O'Callaghan, quoting Adlam.
In this universe, time as we know it doesn't exist. That would be a relief for a younger me who always managed to be rushing not to be late. No time means no reason to rush.
I'm down with that.
