This article is from Sciences et Avenir – La Recherche n°905-906, July-August 2022.

“This is a consequence of special relativityRoland Lehook, an astronomer at the Atomic Energy Commission (SACLE), explains. In his seminal essay in 1905, Albert Einstein suggested that the speed of light is invariant whatever the frame of reference, which is tantamount to saying that it is insuperable. According to the dominant Galilean physics of the time, this was simply an extreme assumption, since the velocities were supposed to add to each other.”

To understand, you have to go back in time. Until the 17th century, philosophers and scientists assumed that the speed of light was infinite, immediately rejecting the possibility of exceeding it. Early in the 17th century, Galileo proposed the theory of a finite speed of light. It would take centuries of measurement effort, so at the end of the 20th century, an international agreement put it at 299,792,458 m/s in a vacuum. This velocity never varies, as the Americans Albert Abraham Michelson and Edward Morley inadvertently demonstrated. Since Plato, scientists have thought that space is comprised of a fluid with various properties called the ether. When the English physicist Thomas Young demonstrated the wave nature of light in 1801, logically, the ether appeared as a medium in which light waves propagated like the swells of the ocean. In 1881, Albert Abraham Michelson, together with Edward Morley, wanted to highlight the motion of the earth relative to this ether.

This article is from Sciences et Avenir – La Recherche n°905-906, July-August 2022.

“This is a consequence of special relativityRoland Lehook, an astronomer at the Atomic Energy Commission (SACLE), explains. In his seminal essay in 1905, Albert Einstein suggested that the speed of light is invariant whatever the frame of reference, which is tantamount to saying that it is insuperable. According to the dominant Galilean physics of the time, this was simply an extreme assumption, since the velocities were supposed to add to each other.”

To understand, you have to go back in time. Until the 17th century, philosophers and scientists assumed that the speed of light was infinite, immediately rejecting the possibility of exceeding it. Early in the 17th century, Galileo proposed the theory of a finite speed of light. It would take centuries of measurement effort, so at the end of the 20th century, an international agreement put it at 299,792,458 m/s in a vacuum. This velocity never varies, as the Americans Albert Abraham Michelson and Edward Morley inadvertently demonstrated. Since Plato, scientists have thought that space is comprised of a fluid with various properties called the ether. When the English physicist Thomas Young demonstrated the wave nature of light in 1801, logically, the ether appeared as a medium in which light waves propagated like the swells of the ocean. In 1881, Albert Abraham Michelson, together with Edward Morley, wanted to highlight the motion of the earth relative to this ether.

According to Galilean thinking, if the light from a star comes from the opposite direction of the Earth’s displacement, we should see this light coming faster – we are going to encounter it – and catch the light. Earth is on its run in its orbit. Given that Earth’s orbital speed is about 30 km/s and the speed of light is about 300,000 km/s, the speed of light should vary by about 0.01% depending on its origin. Albert Abraham Michelson and Edward Morley develop an interferometer to prove it – and it would earn them the first Nobel Prize in Physics in 1907. For six years, these two men conducted many experiments with this device, in vain. . Light seemed to have a constant speed no matter where it came from.

This observation was recorded by Albert Einstein in his special theory of relativity in 1905. Graded C As for celerity, the speed of light is independent of the motion of the source and observer, regardless of the reference system. The concept of “reference system” can be explained as follows: You are on the highway and a car overtakes you at a higher speed than you. Stalled, 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 to be motionless relative to each other. Not so for light, explains Albert Einstein, whose speed remains the same regardless of reference frames! “In fact, there is no need to make this assumption about lightRoland Lehoucq points out. By reformulating Einstein’s relativity to further generalize its foundations, we see that there is a maximum speed that does not change regardless of the change of frame, and this speed can be called Einstein’s constant. Michelson and Morley’s experiments proved that it belonged to light. “

A curious phenomenon that is an exception

Never reached, limit C It regularly approaches the LHC (Large Hadron Collider), CERN’s 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 pass through. This is especially true of water, which travels at 75% of its speed in a vacuum. Particles can propagate faster than photons, and a phenomenon similar to sound barrier occurs. For example, when an aircraft exceeds the speed of sound in air (1224 km/h), it generates a shock wave that can be heard in the form of a “bang”. Going beyond the wall of light creates a flash of light. This phenomenon is observed in pools of nuclear reactors, from which bluish (or ultraviolet) light is emitted, called the Cherenkov effect or Vavilov-Cherenkov after its inventors.

This radiation from water exposed to a radioactive source was demonstrated as early as 1910 by the work of Marie Curie. But Russian physicists Sergey Vavilov and Pavel Chernov would have to wait until 1934 to provide an explanation. The latter was awarded the Nobel Prize in 1958. This Cherenkov effect is also the origin of the strange luminous mirages in the visual field (phosphenes) reported by astronauts on the Apollo missions: these occurred when particles of the solar wind passed through. Fluid from their eyelids. “However, in astronomy, some objects are said to be superluminal, Roland Lehoucq says. In 1981, we observed a jet emitted by quasar 3C273, the brightest quasar in the sky. Amazingly, this jet of matter was faster than light! We realized that this was the result of a misinterpretation of measurements, a kind of optical illusion. “

General relativity establishes a relationship between the mass and energy of a particle and proves that the energy of a particle of non-zero mass increases with its velocity. This illustrates well the concept of inertial mass: the more mass a body has, the more energy it needs to set it in motion and accelerate it. So a spacecraft would need infinite energy to reach the speed of light in a vacuum.

In 1964, the American physicist Gerald Feinberg hypothesized the existence of particles, tachyons (from the Greek. Tachus meaning speed). Thus, according to him, there will be three types of particles: those that travel slower than light (that is, almost all known particles), those that travel as fast as photons, and finally, those that travel at superluminal speeds.

“It’s a thought experiment, a physicist’s game that has not been realized to date, Roland Lehoucq says. This allows us to test the consistency of the hypotheses. “If tachyons exist, it would cause some strangeness. According to Gerald Feinberg, tachyons can travel to the past or the future, as defined by Newtonian mechanics. In some science fiction they are used to send messages to the future. Or the past to modify the present. “It violates the principle of causation that causes a prior effect, Remark Roland Lehoucq. But without causality there is no physics! “