Already seen in cinemas, could a "return to the future" one day become reality? Even though the question seems like science fiction, responding positively is perhaps not so unrealistic... As proof: in his posthumous work entitled "Brief Answers to Big Questions", the most famous contemporary physicists - Stephen Hawking - considers the possibility of traveling back in time as a "very serious question".
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This was enough for scientists to decide to take another look at the question that's kept Humanity spellbound for centuries. In an article published in The Conversation, Peter Millington - a researcher and member of the Particle Cosmology Group of the Faculty of Physics and Astronomy at the University of Nottingham in the United Kingdom - draws up an extensive inventory of scientific knowledge in the field of time travel.
A daily experience
"Let's start with our daily lives, and the way we take for granted the opportunity to call our friends and family wherever they are in the world to find out what they're doing. This is something we can also never truly understand: the signals that carry their voices and images travel incredibly fast...but these signals take a finite time to reach us," says Peter Millington.
As the researcher explains, this example perfectly illustrates "our inability to access the present of a person far away from us, [which] is at the heart of Einstein's space-time theories". According to the German scientist, space and time can't exist as such. The speed of light being absolute - 299,792,458 meters per second in a vacuum - moving at this pace would then be fixed in time; one would be able to see the rest of the world move by in "fast forward" mode.
From there, to say one can exceed the speed of the light, this would make it possible to go back in time. There's only one step that science unfortunately doesn't have the authorization to cross...
The principle of cause and effect without going topsy-turvy
"To accelerate a human being at the speed of light requires infinite energy, not to mention going beyond this speed, but even if we could, time wouldn't simply go back. Rather, talking about this would be meaningless, as the law of causality would be violated and the principle of cause and effect would lose its meaning," says Peter Millington.
The challenge of time travel could seem lost... But hope is perhaps not quite snuffed out yet! And for good reason: besides his theories on space-time, Einstein had also developed the idea that gravity would be a consequence of the way in which the mass of a body curves within space-time. A revelation that could be the beginnings of a solution to travelling back in time...
"If we concentrate a sufficient mass, space-time becomes so curved that even light can no longer escape its gravitational attraction, and a black hole is formed. And if you were about to approach the edge of the black hole - its "event horizon" - the ticking of your watch would be infinitely slow compared to those who are away from it, so we could bend space-time in the right way to close it in on itself; so the answer is "maybe," says Peter Millington.
The hope of wormholes?
As the researcher explains, such a folding of space-time would constitute a "traversable wormhole", a structure that would connect two regions of space-time, and that one could cross. Not impossible on paper, but hardly conceivable according to the laws of classical physics. "However, modern theories of quantum mechanics [don't prevent this]," reveals Peter Millington.
"Finding a coherent theory combining quantum mechanics with Einstein's theory of general relativity, however, remains one of the greatest challenges in theoretical physics," concedes the researcher. Finally, the ability to travel a day in time seems well compromised. "Is time travel really possible?" The answer is probably no," says Peter Millington. "Despite his mischievous optimism, Hawking acknowledged that the still largely unknown laws of physics that will someday supplant Einstein's help prevent large objects like you and me from skipping casually back and forth through time," warns the scientist. To make a "return to the future" risks remaining a mere cinematographic fantasy forever...
