Why can’t we exceed the speed of light? – Science and future

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This article is taken from the monthly Sciences et Avenir – La Recherche n°905-906, July-August 2022. “It is a consequence of special relativity, explains Roland Lehoucq, astrophysicist of the Atomic Energy Commission (Saclay) in his first article from 1905, Albert Einstein, indeed, postulates that the speed of light is invariant whatever the frame of reference, which is equivalent to saying that it cannot be exceeded. Prevalent Galileo at the time, is a simply outrageous postulate, since the velocities were assumed. to add to each other.” To understand this, you need to go back in time. Until the 17th century, philosophers and scientists thought that the speed of light was infinite, which immediately ruled out any possibility of exceeding it. At the beginning of the 17th century, Galileo hypothesized a finite speed of light. It will then take centuries of measurement attempts so that at the end of the 20th century, an international agreement sets it at 299,792,458 m/s in a vacuum. And that speed never varies, as Americans Albert Abraham Michelson and Edward Morley have unwittingly demonstrated. Since Plato, scientists thought that space was occupied by a fluid with various properties, called aether. When in 1801 the English physicist Thomas Young demonstrated the wave nature of light, logically, the ether appeared to be the medium in which light waves propagated, like the swell of the ocean. In 1881, Albert Abraham Michelson, soon joined by Edward Morley, wanted to emphasize the motion of the Earth in relation to this ether. This article is taken from the monthly Sciences et Avenir – La Recherche n°905-906, July-August 2022. “It is a consequence of special relativity, explains Roland Lehoucq, astrophysicist of the Atomic Energy Commission (Saclay) in his first article from 1905, Albert Einstein, indeed, postulates that the speed of light is invariant whatever the frame of reference, which is equivalent to saying that it cannot be exceeded. Prevalent Galileo at the time, is a simply outrageous postulate, since the velocities were assumed. to add to each other.” To understand this, you need to go back in time. Until the 17th century, philosophers and scientists thought that the speed of light was infinite, which immediately ruled out any possibility of exceeding it. At the beginning of the 17th century, Galileo hypothesized a finite speed of light. It will then take centuries of measurement attempts so that at the end of the 20th century, an international agreement sets it at 299,792,458 m/s in a vacuum. And that speed never varies, as Americans Albert Abraham Michelson and Edward Morley have unwittingly demonstrated. Since Plato, scientists thought that space was occupied by a fluid with various properties, called aether. When in 1801 the English physicist Thomas Young demonstrated the wave nature of light, logically, the ether appeared to be the medium in which light waves propagated, like the swell of the ocean. In 1881, Albert Abraham Michelson, soon joined by Edward Morley, wanted to emphasize the motion of the Earth in relation to this ether. According to Galilean thought, if the light of a star reaches us from the direction opposite to that of the displacement of the Earth, we should see this light arrive faster – we will find it – than if the light has to reach it . the Earth in its career in its orbit. The Earth’s orbital speed is about 30 km/s and that of light about 300,000 km/s, the speed of light should vary by about 0.01% depending on its origin. To demonstrate this, Albert Abraham Michelson and Edward Morley are developing an interferometer – which will earn the former the Nobel Prize in Physics in 1907. For six years, the two men will carry out a series of experiments with this instrument, in vain. . Wherever it came from, the light seemed to have an unvarying speed. It is this observation that Albert Einstein inscribed in 1905 in his theory of special relativity. Note c for celerity, the speed of light is independent of the motion of the source and the observer, whatever the frame of reference. The concept of “reference system” can be explained as follows: You are on the freeway and a car overtakes you at a speed higher than yours. Chopped, you accelerate and soon the two vehicles are traveling side by side at the same speed. In this new reference system, the two cars appear motionless relative to each other. Not so with light, explains Albert Einstein, whose speed remains the same regardless of reference frames! “In fact, it was not necessary to make this assumption about light, emphasizes Roland Lehoucq. By reformulating Einstein’s relativity to make it more general, we see that there is an invariant maximum speed whatever the change of reference frame, speed which could be called Einstein’s constant. And it turns out that the experiments of Michelson and Morley have shown that it is that of light. “A curious phenomenon that makes an exception Never reached, the limit c is regularly approached inside the LHC (Large Hadron Collider), the Cern particle accelerator in Geneva, which forms a 27 km ring under the Franco-Swiss border. Protons are accelerated there to 99.9999991% of the speed of light in a vacuum. However, there is a curious phenomenon that allows light to go further… at least when it is slowed down by the medium it travels through. This is especially the case with water, where it travels at 75% of its speed in a vacuum. Then the particles can propagate faster than the photons and a phenomenon analogous to the sound barrier occurs. When an airplane, for example, exceeds the speed of sound in air (1224 km/h), it triggers a shock wave that can be heard in the form of a sonic “bang”. Going beyond the wall of light produces a flash of light. It is this phenomenon that is observed in the pools of nuclear reactors from which a bluish (or ultraviolet) light emanates, called the Cherenkov effect, or Vavilov-Cherenkov after its discoverers. First was the work of Marie Curie who, already in 1910, demonstrated this radiation from water subjected to a radioactive source. But we will have to wait until 1934 for the Russian physicists Sergei Vavilov and Pavel Tcherenkov to give the explanation. Which will be worth a Nobel Prize to the second in 1958. This Cherenkov effect is also at the origin of strange luminous mirages in the visual field (phosphenes) reported by the astronauts of the Apollo missions: this occurred when the particles of the solar winds crossed the fluid of his eyeballs. “In astrophysics, however, some objects are said to be superluminal,” says Roland Lehoucq. In 1981, we observed a beam emitted by the quasar 3C273, the brightest quasar in the sky. Light! We then realized that this was the result of a misinterpretation of the measurements, a kind of optical illusion.” General relativity also establishes a relationship between the mass and energy of a particle and shows that the energy of a particle of non-zero mass increases with its speed. The concept of inertial mass illustrates this well: the more massive the body, the more energy it needs to set it in motion and its acceleration. Therefore , it would take infinite energy for a spacecraft to reach the speed of light in a vacuum. In 1964, the American physicist Gerald Feinberg nevertheless imagined the existence of particles that would permanently evolve above the speed of light, the tachyons (from the Greek tachus meaning fast). There would therefore be, according to him, three classes of particles: those that go slower than light (that is, almost all known particles), those that go as fast as photons and, finally, those that propagate to u exclusively superluminal speed. “It’s a mental experiment, a physics game that has no reality so far, Roland Lehoucq specifies. This allows the consistency of theories to be tested “If tachyons existed, this would give rise to some curiosities. According to Gerald Feinberg, tachyons can go to the past or the future, as defined by Newtonian mechanics. In some works of science fiction they are used like this to send messages to the future or the past to modify the present. “This violates the principle of causality according to which the cause precedes the effect, remarks Roland Lehoucq. But there is no physics without causality!”
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