In the movie Back to the Future, it was a DeLorean sports car traveling at 88 mph. In Harry Potter and the Prisoner of Azkaban, it was an hourglass necklace. In “Doctor Who,” a phone booth. But in real life, what would time travel, if such a thing were even possible, look like? According to theoretical physicist Ronald Mallett of the University of Connecticut, the answer might be spinning lasers. This, of course, is an oversimplification of Mallett’s theory. Time travel, according to the laws of physics as they are known today, is seemingly impossible. But some theories, such as the theory of general relativity and the wormhole theory, have left the door open a crack. Courtesy of iStock.com/OG photo. Possibilities, and hope, are where Mallett’s interest in time travel began. Following the sudden death of his father at a young age, Mallett read The Time Machine by H.G. Wells. After that, he began reading Einstein. Inspired by the fluidity of space and time, Mallett pursued a doctorate in physics and a career in academia. His interest in the nature of time later drew his attention to black holes. “Rotating black holes can create a gravitational field that could lead to loops of time being created that can allow you to go to the past,” Mallett said in a recent interview with The Guardian. A rotating black hole has two event horizons. Between these two event horizons is a phenomenon called frame-dragging, or the dragging of space-time. Borrowing from this phenomenon, Mallett’s proposition is based on a ring laser’s properties in the context of Einstein’s general theory of relativity. In a 2000 paper published in Physics Letters A, he wrote, “My current research considers both the weak and strong gravitational fields produced by a single continuously circulating unidirectional beam of light. In the weak gravitational field of a unidirectional ring laser, it is predicted that a spinning neutral particle, when placed in the ring, is dragged around by the resulting gravitational field.” A few years later, Mallett proposed that, at sufficient energies, a circulating laser might produce not only frame- dragging but also closed loops in time, allowing time travel to the past and creating the foundation for a time machine based on a circulating cylinder of light. The catch is that, even if such a time machine could be created, it would only allow travel back to the moment when the time loop itself was created. Some of Mallett’s contemporaries have also noted that the time machine would require the ring laser to have a circumference larger than the observable universe. But that sort of problem is for engineers to solve, not theoretical physicists.