“Can we slow down the light?”, Asked Benoit Nennig on our Facebook page. Science and the future We answer our internet user’s question during our weekly question. Thank you all for your participation, loyalty and curiosity.

The speed of light in a vacuum is always the same. This equates to 299,792,458 meters per second, often circular at 300,000,000 m / s or 300,000 km / s; We usually denote this constant by the letter “c”. This is one of the most important posthumous statements on the subject General relativity Created a revolution in our view of the world a hundred years ago. This theory later proved itself; Thanks to Ai and it was proven again Very large scale experience, Conducted over 16 years, includes monitoring Pulsars.

Of course, photons and light-producing particles have no mass (so the previous answer to a weekly question: light Do not rush to achieve speed). The moment the photons are emitted, they evolve at a constant speed, c. But this is only valid in a “vacuum”, or at least an atmosphere that is filled enough to be considered empty, like the air we breathe.

The speed of light depends on the medium through which it propagates

This is because light does not propagate in the same way depending on the environment in which it operates. It is time to introduce a new parameter: Refractive Index. The value without this unit is the characteristic of an environment: it describes how light works there. In fact, depending on the atoms that make up matter, it will partially deviate as light propagates there, thus changing direction. To give an idea, the refractive index of air is 1: light has a straight line.

In most environments, light remains the champion of speed. One exception can be observed in water: “If the speed of light in a vacuum is 299,792 km / s, it is only 225,563 km / s in water and 200,000 km / sec in glass, so light can be ‘overtaken’ in a physical environment.”, Explains The On its website. As explained in another question of the week, it is so possible Exceeding the speed of light In water, when it remains below its velocity in a vacuum: it is calledeffet Tcherenkov.

⁇ # Do you know : Do you know where the distinctive blue color of the swimming pool in the reactor building comes from? # JudyPhoto ⁇ pic.twitter.com/2N3wRDhshZ

– EDF Fessenheim (DFEDFfessenheim) January 30, 2020

Charged particles, as they travel through water, disrupt the electronic layers of the atoms: the electrons change their energy level, and then emit when they return to a more stable level. Electromagnetic waves In the visual spectrum. If the particle travels at a lower speed than these electromagnetic waves, they will compensate for each other and the light will not be visible. But if it exceeds the speed of light, they are superimposed differently: they are said to be “in phase” and then a bluish light appears. Thanks to the electrons emitted by the fuel spent in the cooling pools of nuclear power plants, this effect won its inventor Powell Crenkov the 1958 Nobel Prize in Physics.

Using Bose-Einstein condensate, light is slowed down in a vacuum

So only some very light particles and some media can overcome the speed of light. But the light slows down, is that possible? Thanks for the strange state of matter: the answer is yes Content of Bose-Einstein. This corresponds to the cooling of a gas of bosons near absolute zero (below nanokel): they are then placed in the same quantum state and act as a macroscopic quantum wave.

Before explaining how to slow down light, another concept needs to be introduced: Induced electromagnetic transparency. As mentioned above, each material has a refractive index, which describes how light propagates through it. The higher it rises, the more it will deform as it passes, so it will take longer to cross. Most materials have indices between 1 and 2, with some exceptions Diamond Whose index reaches 2.4. At a very high index, for example, in the order of several billion, so light will have a chaotic motion and will not reach its point of arrival until it is too late: so it will slow down.

This is where electromagnetic induced transparency comes into play. With a laser, Bose-Einstein condensates can be made transparent to a narrow wavelength, where they are usually completely opaque. The refractive index of the condensate varies greatly depending on the wavelength of the incident light. So when light passes through a condensate, it scatters: each wavelength changes differently in the material. When the condensate passes through the targeted wavelength, it interacts very strongly and almost stops. Light then becomes what we call it Slow light : The speed of the group usually decreases as a wave, but each photon progresses at the same speed individually. Using this method, the researchers were able to slow down the light 17 meters per second, And others were able to stop it before restarting it, creating so-called so-called Light off !