THE GIST
- Hong Kong physicists say they have proven a single photon cannot travel faster than the speed of light.
- This demonstrates that time travel is impossible, they say.
Hong Kong physicists say they have proved that a single photon obeys Einstein's theory that nothing can travel faster than the speed of light - demonstrating that outside science fiction, time travel is impossible.
The Hong Kong University of Science and Technology research team led by Du Shengwang said they had proved that a single photon, or unit of light, "obeys the traffic law of the universe."
"Einstein claimed that the speed of light was the traffic law of the universe or in simple language, nothing can travel faster than light," the university said on its website.
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"Professor Du's study demonstrates that a single photon, the fundamental quanta of light, also obeys the traffic law of the universe just like classical EM (electromagnetic) waves."
The possibility of time travel was raised 10 years ago when scientists discovered superluminal - or faster-than-light - propagation of optical pulses in some specific medium, the team said.
It was later found to be a visual effect, but researchers thought it might still be possible for a single photon to exceed light speed.
Du, however, believed Einstein was right and determined to end the debate by measuring the ultimate speed of a single photon, which had not been done before.
"The study, which showed that single photons also obey the speed limit c, confirms Einstein's causality; that is, an effect cannot occur before its cause," the university said.
"By showing that single photons cannot travel faster than the speed of light, our results bring a closure to the debate on the true speed of information carried by a single photon," said Du, assistant professor of physics.
"Our findings will also likely have potential applications by giving scientists a better picture on the transmission of quantum information."
The team's study was published in the U.S. peer-reviewed scientific journal Physical Review Letters.
By proving a single photon can't travel faster than light, scientists say they have proven time travel is impossible.
Top 5 Time Travel Methods from the Movies
Oh, how we often long to travel back in time and change our pasts, to stop some horrible event, to rewrite history. Movies often indulge and inspire us with their time travel adventures, but how many of these have any basis in real science? Let's, for this purpose, ignore how much the movies tug at the heartstrings, entertain us, or tickle our funny bone. We can never forget great ones like "Back to the Future," "Bill and Ted's Excellent Adventure," "Time Cop" and many others. But this is a focus on how the heck we are going to get these movies to come true. Ron Mallett, professor of physics at the University of Connecticut and author of Time Traveler: A Scientist's Personal Mission to Make Time Travel a Reality, has spent his whole career studying the possibilities of time travel and he weighed in on what aspects of the movies are on the edge of possibility and which ones are not. Derived from the expertise of Mallett, here are the top five time travel movies ranked on their basis in science and their true feasibility. Sorry "Hot Tub Time Machine", you didn't make the cut.
Frequency (2000)
Frank Sullivan, played by Dennis Quaid, communicates with his son John (Jim Caviezel) 30 years into the future through a radio. They work together to save Frank's life and to find John's mother's would-be-killer. Frequency's time travel is largely based around an unusual environmental phenomenon in the aurora borealis. This solar disturbance causes Frank's radio to send its signal to the same radio in the future, where John now has it in the same home. The idea that the energy from a solar disturbance could alter spacetime in some way that sends radio waves through time is very much outside the realms of possibility. The aurora borealis could never produce that much energy and if it were to do so, there would likely be some disastrous side effects. But being outside the realms of possibility isn't a problem when it comes to sci-fi time travel. However, Brian Greene, physicist and author of several books including The Elegant Universe, was a consultant for the film, so his input certainly gave the movie some extra weight as far as feasibility goes.
Déjà Vu (2006)
After a ferry bombing, Agent Doug Carlin (Denzel Washington) joins forces with a special team that has technology to see four days into the past. This technology turns out to be a "time window" which Carlin convinces them to use as a time machine and send him into the past. He ends up saving the ferry from the bomber, played by Jim Caviezel. This is a stretch and would involve technology and an understanding of wormholes that we just don't have right now. The "Snow White" project, as it's called in Déjà Vu, somehow controls wormholes or what they call a "time window" to travel through time and space. Wormholes, also called Einstein-Rosen bridges, are a valid theoretical method for time travel, but solely theoretical. Their existence has never been proven and requires something called "exotic matter" to keep them stable. Yet, Mallett says that any movie involving theories of Einstein for time travel is more feasible than those that do not. The movie takes the theory to the extreme in a way that is very entertaining and smart. Brian Greene, who consulted in "Frequency," was also a consultant for "Déjà Vu". You definitely need a theoretical physicist to keep all the timelines straight in this one.
The Time Machine (1960)
H.G. Wells wrote the original book and the movie's lead character is named H. George Wells (Rod Taylor) after him. He uses his machine to travel from the year 1900 through the future seeing two world wars and accidentally ending up in the year 802,701, where he finds a future people who he tries to help. Mallett has this high on his list (of course he says the book is even better) because it's the only movie that accurately states time as a fourth dimension. Although in modern relativistic physics, space and time are combined into one metric called "spacetime." The machine itself; not so feasible. A chair with spinning parts and a lever that controls time travel is something today's scientists have yet to find viable. It's quite miraculous that the fictional H. George Wells was able to invent it in the early 1900s. Another positive note for this version: it does not have him traveling to the past beyond the invention of his machine. The movie doesn't explain this rule, but it does follow the true physics of time travel. Mallett says, "Since it is the device that creates the effect, then is not possible to go back before the device was created."
Star Trek (2009)
Time travel is featured in the "Star Trek" series many times, but the way the most recent movie dealt with time travel makes it very easy to understand without having to be a physics expert. "They actually brought in a number of current ideas," says Mallett. The movie's use of parallel universes is done very well and explained very well. There is a valid scientific theory in quantum mechanics that says there are many parallel universes. Plus, when you go back into the past you actually arrive in the past of a parallel universe, where you can change the future of that universe, but in your time those things have already happened. It's also important to know that once you are in this alternate reality you cannot, as another time travel movie suggests, go back to the future. Black holes are a very popular method for sci-fi time travel and one that could actually be possible. Einstein's general theory of relativity basically says that gravity effects time and since black holes have gravity so strong that light can't even escape, they create the possibility for time travel. If you were able to get close to a black hole, time would slow down. When you escaped, time outside would have passed a lot more quickly. Making things more complicated, when the black hole is rotating, it can cause time to be twisted into a loop that allows you to go into the past. That still leaves some scientific flaws in the movie, but nobody's perfect. It was still one of the most (if not the most) entertaining "Star Trek" movies.
Planet of the Apes (1968)
Put briefly, astronaut Taylor (Charlton Heston) unknowingly goes on a journey to a future Earth where apes have guns, don't believe in the possibility of flight, and rule the planet. Humans are still around in this future, but are mute and quite unintelligent. The best thing about this movie is that they talk about real scientific theories in explaining how they traveled through time. "According to Dr. Hasslein's theory of a vehicle traveling near the speed of light, the earth has aged nearly 700 years since we left it, while we've aged hardly at all," says Taylor in the opening scene of the movie. Actually, it's based more on theories of a man named Albert Einstein. His special theory of relativity says that time for a moving clock slows down. In 1971, the Hafele-Keating experiment proved this with very accurate atomic clocks. One was on Earth and another was flown around the world on a passenger jet. When the jet landed, the clock on the jet was about 50 nanoseconds behind the one on Earth, just like Einstein predicted in 1905. "This means, for a space traveler traveling close to the speed of light, that this effect will happen dramatically. A few years will pass for those on board, but when the rocket lands, decades will have passed on Earth," explains Mallett. So as Heston and his crew travel near the speed of light, they would in fact be traveling through time relative to those of us on Earth. Getting a ship close to the speed of light is the practical challenge in this case, but very plausible in theory. The portion of the movie in which apes control the planet is a little more questionable.
