Traveling at the speed of light: when science fiction challenges scientists
Will traveling at a speed greater than the speed of light ever be possible? This idea, explored for decades by science fiction writers, is studied by physicists - in theory only...
Will traveling at a speed greater than the speed of light ever be possible? This idea, explored for decades by science fiction writers, is studied by physicists - in theory only...
Nothing can go faster than light in a vacuum. It is a physical constant that has determined our understanding of the universe since the end of the 20th century . The study of its speed goes back to Antiquity and to the first thought experiments, a method making it possible to find an explanation for a phenomenon, here physical, by the sole means of the imagination.
After several centuries of debate, Ole Rømer, a 17th century Danish astronomer , was the first to demonstrate that the speed of light is finite, which means that it can be calculated. Its exact value is established at 299,792,458 meters per second (the value of 300,000 kilometers per second is an international standard). It applies to any object emitting light: the speed of light emitted by a star and the speed of that emitted by a bulb is the same.
“The speed of light is the ultimate speed that we can reach in the space-time in which we live,” explains Marie-Christine Angonin, professor at Pierre and Marie Curie University and vice-president of the Paris Observatory. “The speed of light is actually the speed of propagation of energy, whatever form of energy it is. »
Our current knowledge does not allow any object to move faster than 300,000 kilometers per second. “To be able to reach this ultimate speed, a massive particle must, if we follow general relativity, receive an infinite quantity of energy. It must disintegrate to become purely energetic. It is then that it will be able to go at the speed of light” supports Marie-Christine Angonin.
But let's admit for a moment, as science fiction writers have been doing for decades now, that it is possible. Let's ignore the incalculable number of material challenges this would present and dwell on the two solutions imagined by science fiction to travel at the speed of light and beyond: hyperspace and warp, imaged in many productions, from Star Trek to Star Wars to the most recent example, Buzz Lightyear, the latest Disney Pixar* movie. At the start of the film, Buzz Lightyear, his superior, and a crew of over a thousand scientists and technicians are returning home from a space mission. About 4.2 million light-years from Earth, their ship's navigation system alerts them that they are near an unexplored planet potentially rich in resources. Due to an error in judgment, they severely damage their ship and find themselves stuck on this planet, called T'Kani Prime. The team of scientists is therefore working on the development of a super-fast fuel. And with each attempt, Buzz's character fights an insane battle against time: the four minutes of his first test flight equal four years on T'Kani Prime,
HYPERSPACE, A NARRATIVE JUSTIFICATION
To move from one world to another easily, the characters of works of science fiction as diverse as Dune and Star Wars , travel at superluminal speeds. It is a narrative process rarely explained in detail, making it possible to justify interstellar or intergalactic travel on human time scales. An essential imperative to build a story with which the reader or the spectator can identify. It is still necessary that the elements of explanation seem credible.
The authors quickly had to find theories that were both “scientifically acceptable” and understandable by the greatest number – hence the absence of details. Although the means of interstellar travel vary from one work to another, most involve concepts from general and special relativity theories, such as Wheeler-Misner's wormholes or Einstein's bridge. Rosen. A wormhole as imagined by Charles W. Misner and John A. Wheeler designates the hypothetical connections between two distinct regions of spacetime. On one side a black hole , on the other a white hole . The wormhole would thus form a shortcut in space-time.
For Einstein and his collaborator the physicist Nathan Rosen, the atomic nature of matter could, if one pushed the Schwarzschild solution, be interpreted as the existence of a bridge between two sheets of space-time. This theory constitutes in fact the first serious speculations on possible shortcuts in space-time or passages between parallel universes.
"Schwarzschild's solution considers a mass that is spherically symmetric, and we are outside of this mass, which can be the Sun, the Earth, a black hole... And there is a distortion of the space-time that makes a hole. From the moment we have a hole, we can imagine that it is made in two places in the universe and that it causes a curvature” illustrates Marie-Christine Angonin. The problem is that if the Schwarzschild solution imagines the existence of wormholes, or even already curved space-times, when entering the black hole it would be very unlikely to succeed in coming out through the white hole. We would fall to one side or the other, as if we were stuck in a vat.
Diagram of a negative mass wormhole and hyperspace.
“Now, if we add a charge, which would make electrical energy in addition to gravitational energy, it would circumvent this problem and get pushed back [to the other side of the wormhole]. Like a gigantic magnet, we would be attracted to the North Pole and then repelled by the South Pole. Only once you've done that, you can't go back the other way. In the context of narratives providing for a return trip, this initial shortcut would certainly make it possible to move quickly, but would force us to go around the universe to return to the starting point. "It's as if you took a shortcut to go to church but to come back home, you had to go around the village" image Marie-Christine Angonin.
To be able to travel back and forth through the wormhole, as in Interstellar , would require an energy with a lower density than a vacuum, a mass that could push us back towards a white hole when approaching it. . "For that, we would have to have a negative mass somewhere," notes Marie-Christine Angonin. “It would be anti-gravity, that is to say a place from which we would be rejected instead of being attracted. Anti-gravity is very present in science fiction, but in everyday life, it has never been highlighted, even in vacuum energies. »
Notions difficult to interpret by neophytes, any science fiction fans they may be. This is why the hyperspace narrative justification was developed. The vessels piloted by the fictional characters pass between two ends of a wormhole at a speed greater than the speed of light, thus justifying a rapid movement that does not delay the plot. Note that the hyperspace staged in science fiction only borrows certain terms from science, but that it does not correspond to any scientific theory.
DISTORTION AND THE ALCUBIERRE METRIC
In 1994, Mexican physicist Miguel Alcubierre developed a theoretical means of superluminal motion that would not violate the physical principle that nothing can travel faster than the speed of light in a vacuum. The Alcubierre metric, in theory compatible with the special theory of relativity, imagines the possibility of bending space-time. The spacecraft does not itself move faster than light, but is caught in a "bubble" allowing it to move from point A to point B faster than light would. Said bubble is formed by the expansion of the space behind the vessel on the one hand, and the contraction of the space in front of the vessel on the other hand.
Only here, if this theory respects a priori the laws of physics, thus bending space-time to travel faster than light also requires energy whose density is lower than vacuum. However, the principle of negative mass, imagined by Einstein, is still today only an unverified hypothesis and difficult to verify.
"The problem is that a particle that goes faster than light will always remain faster than light, because to slow it down would require infinite energy," says Marie-Christine Angonin. “Conversely, bringing a massive object to the speed of light would require an infinity of energy. And this is not a theoretical infinity, it has been demonstrated by experience. It is for this reason that in particle accelerators, it is not cubes of matter that are sent but a single particle, because a colossal amount of energy is already needed for it to be simply relativistic . »
That there are space-time bubbles that travel at the speed of light, or even faster than light is theoretically possible. But the major problem would be to put a ship in one of these bubbles. “We would have to stop the bubble, make it go slower than the speed of light, and that is not possible. Technically, there is no physical way to cross this barrier of the speed of light” supports Marie-Christine Angonin.
But then, if we consider that the “distortion engines” – long theorized by the screenwriters of Star Trek – cannot be developed because of the very need for negative energy, will they ever be nothing but fiction? Not necessarily. Research by an international group of scientists on other classes of "distortion engines" was published in 2021 in the journal Classical and Quantum Gravity .
Scientists in this group, called Applied Physics, have looked into a theoretical "distortion engine" that does not require negative mass. For this to be physically possible, they say, the ship would have to look like a shield, flat in front and behind. Their demonstration, although incomplete, has the advantage of showing that from a mathematical point of view, a warp engine could be built.
“Alcubierre's metric was an unrealistic idea and completely impossible to implement in everyday physics for multiple reasons. [...] What these scientists have demonstrated is that the Alcubierre metric is one solution among many others, which are easier to implement, so to speak. It's interesting, but that doesn't mean that it can be done,” emphasizes Marie-Christine Angonin. “If we take into account the fluctuations of the vacuum, this kind of experiments will only exist in a theory of quantum space-time with very small energies and therefore with very small particles. A spaceship at the speed of light is completely unrealistic at present. »
Despite the interest of scientists in these questions for many years, superluminal travel should therefore remain, for a long time, the prerogative of science fiction authors.
https://www.nationalgeographic.fr/espace/voyager-a-la-vitesse-de-la-lumiere-quand-la-science-fiction-met-les-scientifiques-au-defi?fbclid=IwAR17KfJW70jkH9lp4kjO7DtigZVmx9YPKrwxqRXwGJIVxcE6GzJrYTkACf4cientifiques%20au%20d%C3%A9fi