If something is allowed by the laws of physics, then scientists can assume that it probably exists. Under that reasoning, certain exotic structures of spacetime called closed timelike curves may be real—and they may allow a message to travel from the future to the past.
A new study has calculated how much information can be sent backward through time via closed timelike curves. Albert Einstein’s general theory of relativity predicts that these spacetime pathways can form under intensely bending, rotating space—such as around a spinning black hole. “Spacetime can curve around so much that you can be innocently going forward in time and then you meet yourself in the past,” says study co-author Seth Lloyd, a quantum information scientist at the Massachusetts Institute of Technology.
According to general relativity, in a rotating black hole, the singularity—the theoretical point of infinite density at the center—is really a one-dimensional ring, with closed timelike curves arcing around it. No one knows if these spacetime structures actually exist in our universe, but they are plausible. We do know, however, that black holes are plentiful in space and that most of them spin. “So they might very well exist,” Lloyd says.
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Inspiration for the study came in part from a movie. “In early 2025 I watched the film Interstellar,” says Kaiyuan Ji, a graduate student at Cornell University who, with his advisor Mark Wilde, collaborated with Lloyd on the new research. The findings were published recently in Physical Review Letters.
In the movie, an astronaut played by Matthew McConaughey travels up close to a black hole and sends a message to his daughter in the past. Ji realized the plot was mathematically equivalent to a question he and his colleagues had posed in previous research.
The group decided to investigate how best to use closed timelike curves to transmit information between the future and the past. “The strategy has a different structure than communicating forward in time,” Ji says. “The key difference is that the sender in the future has memory of what happened in the past, and that causes a causal loop. You now have the ability to bend the probability of success.”
The researchers assumed that the channel might have some noise—interference preventing the maximum amount of information from passing through. But the sender’s memory of the past can help counter that noise, they found. “Let’s say you drop a message into a black hole in the future and it emerges from the same black hole in the past, but the message gets corrupted or parts of it get lost,” Lloyd says. “The receiver in the past can say, ‘Hey, if you’re going to send me a message last Tuesday, I know the closed timelike curve was super noisy then. Can you send multiple copies or try on Wednesday?’”
The findings could have interesting implications for quantum computing, says Giulio Chiribella, a quantum information scientist at the University of Hong Kong, who was not involved in the study. Chiribella has studied the probability of simulating closed timelike curves in a laboratory on Earth.
“We don’t know if [these curves] exist in our universe, but we do know that if they exist, they have powerful consequences,” he says. “For example, they induce radically new scenarios where the order of events becomes indefinite, boosting quantum computation and quantum communication beyond the limits of conventional setups.”
In the past scientists have found that closed timelike curves can’t be used for paradoxical time travel. An experiment in a 2011 paper co-authored by Lloyd simulated these pathways in a laboratory and effectively sent a photon (a particle of light) back in time by less than a second. The researchers were curious whether the photon might be able to destroy the past version of itself—akin to a person traveling back in time to kill their grandfather, thus preventing themselves from ever being born. The so-called grandfather paradox is a prickly aspect of time travel, but in the case of closed timelike curves, it seems that quantum physics permits only self-consistent versions of time travel. In other words, you can visit the past, but you can’t change the future—no grandfather murder allowed.
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