The material of the vacuum


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The laser intensity will gush matter vacuum Michel Alberganti

Keywords: energy, empty, material creation, particles, antimatter

The biography of the equation E = mc 2 is far from complete. Laremarquable illustration given in the documentary fiction broadcast by Arte on Sunday, October 16 (A biography of the equation E = mc2, Gary Johnstone) could soon experience an exciting new chapter. In Applied Optics Laboratory (LOA), common to the National School of Advanced Technologies (Ensta) at the Ecole Polytechnique and CNRS, Palaiseau (Essonne), Gérard Mourou is approaching when he will bring forth of matter from the void ...

"Void is the mother of all matter," he says with a certain jubilation. In the perfect state, "it contains a gigantic quantity of particles per cm3 ... and just as many antiparticles". Hence a zero sum that leads to this apparent absence of matter that we name ... emptiness. What challenge the dictionary definition for which, since the fourteenth century, the latter is a "space that is not occupied by matter." It was to count without the antimatter and without the famous formula E = mc², that Albert Einstein deduced from the special relativity a hundred years ago, in 1905.

Why invert this formula by producing matter from emptiness? For Gérard Mourou, the applications will go from the creation of a new relativistic microelectronics to the study of the Big Bang and the possibility of simulating black holes. What he calls "extreme light" makes it possible to develop proton therapy, able to attack tumors without damaging surrounding cells, a "nuclear pharmacology" and the ability to control the radioactivity of a material with a single button. Not to mention the manufacture of extremely compact accelerators that can compete with the gigantic facilities of CERN Geneva. The control of light is far from having reached its limits. The LOA works with the laser, one of the most spectacular achievements of the discoveries that earned Albert d'Einstein the Nobel Prize in 1921.

Gérard Mourou played a major role in increasing the power of this coherent ray of light obtained for the first time in 1960. In 1985, he developed a method called chirped pulse amplification (CPA) (The World of 8 June 1990). "Overnight, we made a source that stood on a table and whose intensity matched that of facilities the size of a football field," says Gerard Mourou.

beachcomber

Physicists have been struggling for twenty years on the appearance of nonlinear phenomena at intensities of about 1014 W / cm2 (W / cm2) which degraded the wave and caused the destruction of the solids in which the lasers were born. Gérard Mourou used sources producing very short pulses (picosecond, 10- 12 second), one of the characteristics of which was to contain a wide range of frequencies. "To solve the problem, before amplifying the impulse, we stretched it by ordering the photons", indicates the researcher who, to explain the CPA, uses the analogy of a bunch of cyclists facing a tunnel. To avoid a blockage during a crossing of the front, it is necessary to slow down some riders before the obstacle.

Gérard Mourou proceeds in the same way with the frequencies. After separating them, it imposes different paths to each color using a diffraction grating. After the amplification of each frequency, it is "enough" to perform the reverse operation to find a pulse profile identical but much more intense. With the CPA, the intensity has been climbing again to reach ... 1022 W / cm2 today, 1024 W / cm2 in 2006.



"Until a certain value of the intensity, the magnetic component of the incident wave remains negligible compared to its electrical component, explains Gérard Mourou. But from 1018 W / cm2, it exerts a pressure on the electron. The latter, until then subject to a simple "swell", is suddenly swept away by a surging wave that drives him to reach his own speed, that is to say that of the light. We then enter the relativistic nonlinear optics. The torn electrons turn their atoms into ions that "try to retain the electrons, which creates a continuous electric field, that is, electrostatic, of considerable intensity." This transforms the alternating electric field of the incident light wave into a continuous electric field.

This "extraordinary" phenomenon generates a titanic field of 2 teravolts per meter (1012 V / m). "CERN on a meter ...", summarizes Gérard Mourou. At 1023 W / cm2, the electrostatic field will reach 0,6 petavolt per meter (1015 V / m) ...
For comparison, the Stanford Linear Accelerator Center (SLAC) accelerates particles up to 50 gigaelectronvolts (GeV) on 3 km. "In theory, we can do the same on a distance of the order of the diameter of a hair," says the researcher. In his time, Enrico Fermi (1901-1954) believed that to reach the petavolt, the accelerator should go around the Earth.

"The electrons pushed by the light end up pulling the ions behind them," continues Mourou. From now on, the boat carries anchor. The initial light generated a beam of electrons and ions. The LOA has managed to accelerate electrons up to 150 mega-electronvolts (MeV) energies over distances of a few tens of microns. He intends to push first to GeV, and much later.

Mini Big Bang

In parallel with this development that could eventually compete with large particle accelerators, Gérard Mourou said he was very close, still thanks to the huge luminous intensities obtained, to "slam the void", that is to say, to reveal " something "where there was nothing in appearance.

In reality, it is not a magical operation but, "simply", to reveal what was invisible. Theoretical goal is an intensity of 1030 W / cm2. To obtain this value, physicists consider the vacuum as a dielectric, that is to say an insulator. In the same way that a too strong intensity "snaps" a capacitor, it is possible to "slam the vacuum".

But what will happen then? What strange particles will flow they vacuum? Again, the mystery is stale. There will be a couple electron-positron. A particle and its antiparticle, which are lighter and therefore those who, in the words of Einstein, will claim the least energy to be produced. And that minimum is also well known: 1,022 MeV.

Thus, everything seems ready for that matter makes its first appearance from the vacuum in a laboratory. This mini-Big Bang could even happen before the 1030 W / cm2. Mr. Mourou think that using X-rays or gamma rays, it would be possible to reduce the threshold to around 1023 1024 W / cm2. But it is precisely the objective of the LOA for the coming years

Article in 19.10.05 edition of the World


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