You Could Send Messages to Your Past Self With This Mind-Melting Technology, Scientists Say

5 min readAlbert Einstein introduced a thought experiment in 1907 that, in true Einstein fashion, is still making waves more than a century later. In it, he proposed a technology, called the tachyonic anti-telephone, that could send faster-than-light messages into the past.If this tech actually worked, it would create some wild paradoxes, even making it possible to reshape your past actions. But there’s a hitch: We are still looking for tachyons—the hypothetical, faster-than-light particles that would underlie the tachyonic anti-telephone. Although, that doesn’t mean that we should give up on our time travel dreams; the thought experiment is still helping us understand the limits of physics—and it could rewrite what we know about the way the universe works.Waiting for a supernovaIn the theory of special relativity, Einstein showed that electrons and other particles that start out slower than the speed of light can never be raised up to beyond the speed of light, explains Robert Ehrlich, PhD, a professor emeritus of physics at George Mason University who researches tachyons.“The reason is that [acceleration] would take an infinite amount of energy,” Ehrlich says. “But Einstein specifically did not rule out particles that always go faster than light. That’s the catch.”There is one fundamental particle whose mass has been hard to constrain and maybe could go faster than light depending on that mass, Ehrlich adds. It’s called a neutrino, and it’s a neutrally charged particle produced by stars. In 2011, a widely discredited experiment result briefly made some members of the physics community think they had observed particles going faster than light, though the findings have since been widely discredited. But there could be another pathway to finding such particles.Ehrlich authored a paper published on the preprint server Arxiv in which he uses a nearby supernova that exploded in 1987, called SN 1987, to better constrain neutrino masses observed during the event. He is currently submitting the paper for peer review. The research is part of a larger set of Ehrlich’s work about “superluminal,” or faster-than-light neutrinos, which remain theoretical but are scientifically interesting to him.“If a supernova emits its neutrinos in a very, very brief burst, then the change in arrival time of [different] neutrinos tells us something about the neutrino mass,” Ehrlich says. Forty years ago, three detectors observed neutrinos coming from the supernova in the seconds following the explosion, but a fourth, smaller detector picked up a brief burst of five neutrinos about five hours beforehand.Notably, this fourth detector was located under Mont Blanc in the Alps, which Ehrlich says made it better shielded from background radiation (or interference) than the other three detectors used. However, Ehrlich cautions that most physicists don’t believe the neutrinos are emitted in a short time, even when looking at SN 1987. He also acknowledges most other physicists feel the Mont Blanc detection was unrelated. If the neutrinos were found to be related to the supernova, those could help constrain the mass—but there are other explanations for the neutrinos, such as having the supernova “bang” (emit) twice, he acknowledged.SN 1987 exploded only about 168,000 light-years from Earth. That is far away to us, but in cosmic terms it’s close: it took place in one of the nearest galaxies to our own, called the Large Magellanic Cloud. Ehrlich says the ideal would be for another supernova to happen close by to validate the results. But there’s a rub: these sort of close-to-home explosions only happen twice a century on average. Alternatively, he says it’s possible for physicists to measure the mass of a neutrino on Earth, which is what the KATRIN experiment is trying to do in Germany.Killing your grandfatherTachyons are a tough enough topic on their own, but imagining a device that can communicate with the past is also difficult. That said, it’s a topic closely related to the research of Barak Shoshany, PhD, an associate professor of physics at Canada’s Brock University. Since 2019, he has focused much of his work on time travel and faster-than-life travel. Shoshany is particularly interested in the notion of causality—such as what would happen if we could theoretically send a message back in time.Shoshany explains that most of his peer-reviewed work in this field acts as a series of thought experiments to test the limits of physics. “None of them discuss anything that can exist in our universe based on currently known physics,” he says. “However, the idea is that by exploring these models we can learn about the limitations of current theories, and understand time and causality better, even if the models themselves are unrealistic.”In the everyday world, causality works like this: an event produces an effect, such as knocking a glass off a table and seeing it fall and then shatter on the ground. But—as all the best time travel movies show—defying our linear timeline gets causality completely twisted.An oft-cited example of causality issues is, theoretically, using time travel for a person to go into the past to kill their own parents or grandparents, thus making it impossible for the murderer to be born in the first place. That situation is known as a paradox—a problem when two things that cannot be true simultaneously (the killer existing, or not) actually do exist at the same time.So where does the “tachyonic antitelephone” become paradoxical? It’s a device that appears to violate the laws of physics, as communication cannot go faster than light when following Einstein’s rules of special relativity, which explain how space and time are governed and how they relate to each other. In one series of lectures published in SciPost Physics Lectures Notes in 2019, Shoshany explored what would happen if you tried to send a message into the past using a similar device.“In simple terms, let’s say that you are able to send a message from 2027 to 2026,” Shoshany says. “You decide to do the following experiment: if you receive a message in 2026, you will not send one in 2027, and if you do not receive a message in 2026, you will send one in 2027. Since the message you send in 2027 is the same one you receive in 2026, this is obviously a paradox; the message is sent if and only if it is not sent.”While being able to call into the past sounds like it should have practical implications, because the particles they are based upon—tachyons—have not been proven, there is currently no effect on our lives, Shoshany says. For example, there is no foreseeable way to build improved communications devices using the so-far non-existent tachyons.It would take a high bar of proof to accept tachyons as real particles, Shoshany adds. “These particles are forbidden by quantum field theory, which is one of the fundamental theories of modern physics, and [that] has been verified experimentally to extremely high precision,” he says. “Tachyons have never been observed in any experiment. If they do exist, then we would need to rewrite—or at least substantially modify—our current theories of physics, but there is no reason to assume they exist.”For now, it appears tachyons and tachyonic anti-telephones are simply thought experiments, yet scientists continue to study them to toe the limits of physics. While you won’t foreseeably be using such a device to communicate with the past, it’s still fun to imagine how you could defy the limits of time. As long as you don’t accidentally wipe yourself from the timeline, that is.Elizabeth Howell (Ph.D., she/her) is one of a few space journalists in Canada. She has written five books, and was Space.com's former staff reporter in spaceflight. As a freelancer, she has written or edited articles about astronomy and space exploration for outlets such as Payload Space, Air&Space Magazine, Sky & Telescope and Salon. Elizabeth holds university degrees in journalism, science and history and also teaches an astronomy course, with Indigenous content, at Canada's Algonquin College. Aside from watching several astronaut missions launching from Florida and Kazakhstan, Elizabeth once lived like an astronaut at the Mars Society's Mars Desert Research Station in Utah.