Time Travel May Not Break Reality, Experiments Suggest. It May Reveal That All of Reality Exists All at Once.

7 min readHere’s what you’ll learn when you read this story:Time travel paradoxes like the grandfather and the bootstrap raise questions about whether the past can truly be changed.Scientists can’t prove time travel is possible, but they are testing its core principles using quantum and mathematical experiments. These involve entanglement, closed timelike curves, and retrocausality.Research suggest paradoxes may be avoided under certain conditions—for example, if timelines branch, or if entropy reverses. It’s also possible the universe exists as a unified structure across time.You’ve probably heard this one before. You build a time machine, decide to go back in time, and kill your grandfather. What happens then? Do you cease to exist, meaning you never built your time machine and went 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 happens in some illogical scenarios if time travel is possible. For the most part, these are fun 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, even though they don’t necessarily mean time travel itself is possible.“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,” meaning the universe might find ways to stop it—such as forming a singularity before it could happen.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 your 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 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.“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.”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.“You can make certain entanglement circuits that simulate these closed timelike 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 happened? “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 behavior. One acted as a sensor. When exposed to a magnetic field, the sensor’s spin would change, recording the strength of the field. But the size of this change 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 change the sensor’s orientation in the past. It was as though the information about which way the sensor should point had traveled back in time to when the two qubits were entangled.“It’s kind of the magic 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 mean you can send information backwards in time? No,” says Murch. “But in some circumstances, you can change 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 timelike curves. One in particular is the idea of one entangled quantum particle traveling along the curve, but another staying outside it. What would happen 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 change the past. There would be no paradox, because the past and future happened 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 timelike curve, entropy—the change 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 must 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.”The reversed image would have experienced hours 24 back to 12, so once the original person reaches 12 hours, “you will see a movie going backward” of the subsequent 12 hours, Gavassino says. “After 24 hours, you’re back to the beginning.”In practice, Gavassino thinks the universe itself would not allow closed timelike curves to exist, not only making time travel extremely unlikely but preventing any paradoxes from occurring. “The laws of physics probably break down before forming a closed timelike curve,” he says, or maybe the universe “creates some sort of singularity” to stop one from occurring.It’s also possible that time as we know it doesn’t really exist, Vedral says; it’s simply a concept we use to explain the universe, but one that has no fundamental basis, meaning no paradoxes ever arise. Or perhaps the answer to many paradoxes is simpler still. Maybe there are pathways through time that only work because the past is not altered.“Imagine going back in time and not murdering your grandfather,” Yunger Halpern says. There would be no paradoxes at all—because none could ever truly exist.Jonathan O'Callaghan is an award-winning freelance space journalist from the UK, currently based in Bangkok. His work has appeared in The New York Times, the BBC, Scientific American, New Scientist, and many other publications. He writes about astronomy, astrophysics, and space exploration, and sometimes appears on TV or radio as a space expert. He enjoys football, running, padel, spending time with his children, and exploring the world.