The quantum world operates by different rules than the classical one we buzz around in, allowing the fantastical to the bizarrely normal. Physicists have described using quantum entanglement to simulate a closed timelike curve—in layman’s terms, time travel.

Before we proceed, I’ll stress that no quantum particles went back in time. The recent research was a Gedankenexperiment, a term popularized by Einstein to describe conceptual studies conducted in lieu of real tests—a useful thing when one is testing physics at its limits, like particles moving at the speed of light. But a proposed simulation involves “effective time travel,” according to the team’s recent paper in Physical Review Letters, thanks to a famously strange way that quantum particles can interact.

That interaction is called quantum entanglement, and it describes when the characteristics of two or more quantum particles are defined by each other. This means that knowing the properties of one entangled particle gives you information about the other, regardless of the distance between the two particles; their entanglement is on a quantum level, so a little thing like their physical distance has no bearing on the relationship. Space is big and time is relative, so a change to a quantum particle on Earth that’s entangled with a particle near a black hole 10 billion light-years away would mean changing the behavior of something in the distant past.

The recent research explores the possibility of closed-timelike curves, or CTCs—a hypothetical pathway back in time. The curve is a worldline—the arc of a particle in spacetime over the course of its existence—that runs backwards. Steven Hawking posited in his 1992 “Chronology protection conjecture” paper that the laws of physics don’t allow for closed timelike curves to exist—thus, that time travel is impossible. “Nevertheless,” the recent study authors wrote, “they can be simulated probabilistically by quantum-teleportation circuits.”

The team’s Gedankenexperiment goes like this: Physicists put photonic probes through a quantum interaction, yielding a certain measurable result. Based on that result, they can determine what input would have yielded an optimal result—hindsight is 20/20, just like when you can look over a graded exam. But because the result was yielded from a quantum operation, instead of being stuck with a less-than-optimal result, the researchers can tweak the values of the quantum probe via entanglement, producing a better result even though the operation already happened. Capiche?

The team demonstrated that one could “probabilistically improve one’s past choice,” explained study co-author Nicole Yunger Halpern, a physicist at the National Institute of Standards and Technology and the University of Maryland at College Park, in an email to Gizmodo, though she noted that the proposed time travel simulation has not yet taken place. 

In their study, the apparent time travel effect would occur one time in four—a failure rate of 75%. To address the high failure rate, the team suggests sending a large number of entangled photons, using a filter to ensure the photons with the corrected information got through while sifting out the outdated particles.

“The experiment that we describe seems impossible to solve with standard (not quantum) physics, which obeys the normal arrow of time,” said David Arvidsson-Shukur, a quantum physicist at the University of Cambridge and the study’s lead author, in an email to Gizmodo. “Thus, it appears as if quantum entanglement can generate instances which effectively look like time travel.”

The behavior of quantum particles—specifically, the ways in which those behaviors differ from macroscopic phenomena—are a useful means for physicists to probe the nature of our reality. Entanglement is one aspect of how quantum things operate by different laws.

Last year, another group of physicists claimed that they managed to create a quantum wormhole—basically, a portal through which quantum information could instantaneously travel. The year before, a team synchronized drums as wide as human hairs using entanglement. And the 2022 Nobel Prize in Physics went to three physicists for their interrogation of quantum entanglement, which is clearly an important subject to study if we are to understand how things work.

A simulation offers a means of probing time travel without worrying about whether it’s actually permitted by the rules of the universe.

“Whether closed timelike curves exist in reality, we don’t know. The laws of physics that we know of allow for the existence of CTCs, but those laws are incomplete; most glaringly, we don’t have a theory of quantum gravity,” said Yunger Halpern. “Regardless of whether true CTCs exist, though, one can use entanglement to simulate CTCs, as others showed before we wrote our paper.”

In 1992, just a couple weeks before Hawking’s paper was published, the physicist Kip Thorne presented a paper at the 13th International Conference on General Relativity and Gravitation. Thorne concluded that, “It may turn out that on macroscopic lengthscales chronology is not always protected, and even if chronology is protected macroscopically, quantum gravity may well give finite probability amplitudes for microscopic spacetime histories with CTCs.” In other words, whether time travel is possible or not is a quandary beyond the remit of classical physics. And since quantum gravity remains an elusive thing, the jury’s out on time travel.

But in a way, whether closed-timelike curves exist in reality or not isn’t that important, at least in the context of the new research. What’s important is that the researchers think their Gedankenexperiment provides a new way of interrogating quantum mechanics. It allows them to take advantage of the quantum realm’s apparent disregard for time’s continuity in order to achieve some fascinating results.

The headline and text of this article have been updated to clarify that the team describe a way that time travel can be simulated; they did not simulate time travel in this experiment.

More: Scientists Tried to Quantum Entangle a Tardigrade