Time travel may not break reality, experiments suggest. It may reveal that all of reality exists all at once

You’ve probably heard this one before. Scientists asked themselves what would occur if one builds a time machine, decides to go back in time, and kill his grandfather. What occurs then? Would one cease to exist, meaning he never builds the time machine and goes back in time – creating an impossible loop?

This is the grandfather paradox, one of the most famous of all time travel paradoxes. It is one of many that posit what occurs in some illogical time travel scenarios. For the most part, these are just thought experiments – but some can actually be tested too, either mathematically or even experimentally. The results are compelling because they hint at potential breakthroughs in our knowledge of how time works.

“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.

Time travel would work in multiple hypothetical ways, scientists think. One way is the many-worlds theory, in which our universe splits into parallel timelines. Going back in time would create a new timeline, so killing one’s grandfather would begin a new branch of reality, and no paradox is invoked.

A more popular idea of time travel involves following some sort of causal loop in spacetime, often called a closed time-like curve, which arises from Einstein’s theory of general relativity. Under that theory, it’s possible for a closed time-like curve to bring you back to your starting point in space – and time – and return to an earlier point in your life.

“A closed time-like 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 time-like curve may meet its former self.”

The suggestion of such paths raises paradoxes, though. What would occur 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 modify 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 don’t 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 time-like curves, using the unusual properties of quantum entanglement – that two quantum particles, even when separated, share a state.

“You can make certain entanglement circuits that simulate these closed time-like curves,” Murch says. “If you can simulate time travel, are there ways you can leverage that in some useful way?”

One of Murch’s experiments, published in 2024, was inspired by a moment when he saw a shooting star overhead. He wondered, would it have been possible to have a camera take an image of the shooting star after the event had occurred? “Quantum mechanics creates certain circumstances where you can do that, where we can orient a sensor in hindsight,” he says.

In his team’s experiment, they entangled two qubits, tiny superconducting electrical circuits that display quantum behaviour. One acted as a sensor. When exposed to a magnetic field, the sensor’s spin would modify, recording the strength of the field. But the size of this modification depended on how the sensor’s spin and magnetic field were oriented. The team showed that if they measured the second qubit after, they could modify the sensor’s orientation in the past. It was as though the information about which way the sensor should point had travelled back in time to when the two qubits were entangled.

“It’s kind of the amazement of entanglement,” Murch says. “If I do something to one of them, it affects the other one in the past [and] in the future. So, entanglement allows you to break chronology in some mathematically equivalent way.”

No time travel was actually involved in the experiment, but it did demonstrate the principles of the bootstrap paradox. “Does it prove you can send information backwards in time? No,” says Murch. “But in some circumstances, you can modify the orientation of the sensor in the past.”

It’s also possible that time as we know it doesn’t really exist

There are many theoretical quirks of closed time-like curves. One in particular is the idea of one entangled quantum particle traveling along the curve, but another staying outside it. What would occur when the particle on the curve met the younger version of itself?

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, has been looking into a different idea, called retrocausality, where an event in the future can affect one in the past. “Most of the time we expect causation goes from the past to the future,” Adlam says. But it’s possible, particularly in quantum mechanics, that some properties are best explained by causation “going in the other direction of time instead,” she says.

Or perhaps time is stranger still. 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,” Adlam says. Combining this with retrocausality could be a solution to the bootstrap paradox, in which a person went back in time to illogically modify the past. There would be no paradox, because the past and future occurred simultaneously.

Adlam says the presence of clear causation in the universe today, though, means retrocausality is unlikely to be something we can utilize. “Because we exist in this regime where there’s this strong causal asymmetry in one direction, we can infer we’re unlikely to see strong signs of controllable causal influences in the backward direction,” she says.

A different way to consider the passage of time is that, on a closed time-like curve, entropy – the modification in the universe from order to disorder as time passes – might begin to flow in the opposite direction until it reaches its original point at the start of the curve. In 2024, Gavassino studied this as a way to resolve some paradoxes.

Gavassino compares it to the popular movie Groundhog Day. “By the time you hit 24 hours, you need to reset somehow because you go back to the beginning,” he says. But instead of thinking of it like a straightforward 24 hours, there would be a reversal halfway through. “There is a person that goes from zero to 12, and a time-reversed image of that person that goes from 24 back to 12,” he says. “They go in opposite directions.”

 

yogaesoteric
October 17, 2026