The matter of emptiness

The intensity of the laser will cause matter to spring from the void by Michel Alberganti

Keywords: energy, vacuum, matter, creation, particles, antimatter

The biography of the equation E = mc 2 is far from complete. The remarkable illustration given by the fictional documentary broadcast by Arte on Sunday October 16 (A biography of the E = mc2 equation, by Gary Johnstone) could soon experience a new exciting chapter. At the Applied Optics Laboratory (LOA), common to the National School of Advanced Techniques (Ensta), the Polytechnic School and the CNRS, Palaiseau (Essonne), Gérard Mourou is getting closer to the moment when he will be able to elicit matter from a vacuum ...

“Emptiness 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”. From where a zero sum which leads to this apparent absence of matter which we call… the vacuum. What challenge the definition of the dictionary for which, since the fourteenth century, the latter is a "space which is not occupied by matter". This was counting without antimatter and without the famous formula E = mc², which Albert Einstein deduced from special relativity a hundred years ago, in 1905.

Why reverse this formula by producing matter from a vacuum? For Gérard Mourou, the applications will range from the creation of new relativistic microelectronics to the study of the Big Bang and the possibility of simulating black holes. What he calls “extreme light” allows the development of proton therapy, capable of attacking tumors without damaging surrounding cells, “nuclear pharmacology” and the possibility of controlling the radioactivity of a material with a simple button. Not to mention the manufacture of extremely compact accelerators that can compete with the gigantic facilities at CERN in Geneva. The control of light is therefore far from having reached its limits. The LOA works with the laser, one of the most spectacular results of the discoveries which won Albert Einstein the Nobel Prize in 1921.

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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) (Le Monde du 8 June 1990). “Overnight, we made a source that stood on a table and whose intensity equaled that of installations the size of a football field,” explains Gérard Mourou.

Surf wave

For twenty years, physicists had stumbled over the appearance of non-linear phenomena at intensities of around 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, ie 10-12 seconds), 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,” says the researcher who, to explain the CPA, uses the analogy of a peloton of cyclists facing a tunnel. To avoid a blockage during a frontal passage, it is necessary to slow down some runners before the obstacle.

Gérard Mourou does the same with frequencies. After having separated them, he imposes different paths on each color using a diffraction grating. After the amplification of each frequency, it "suffices" to perform the reverse operation in order to find a pulse with the same profile but much more intense. With the CPA, the intensity started to climb again to reach… 1022 W / cm2 today, 1024 W / cm2 in 2006.

“Up to 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 pressure on the electron. The latter, until then subjected to a simple "swell", is suddenly carried away by a breaking wave which carries it along until it reaches its own speed, that is to say that of light. We then enter into the relativistic nonlinear view. The torn electrons transform their atoms into ions which "try to hold the electrons, which creates a continuous electric field, that is to say electrostatic, of considerable intensity". The alternating electric field of the incident light wave is thus transformed into a direct electric field.

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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 giga-electronvolts (GeV) over 3 km. "In theory, we could do the same over a distance of the order of the diameter of a hair", assures the researcher. In his time, Enrico Fermi (1901-1954) estimated that, in order to reach the petavolt, the accelerator would have to go around the Earth.

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

Mini Big Bang

At the same time as this development which could, in the long term, compete with large particle accelerators, Gérard Mourou says he is very close, again thanks to the enormous light intensities obtained, to "crack the void", that is to say to reveal " something ”where there was apparently nothing.

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In reality, it is not a question of a magical operation but, "simply", to reveal what was invisible. The theoretical objective is an intensity of 1030 W / cm2. To obtain this value, physicists consider vacuum as a dielectric, that is, an insulator. In the same way that an excessively strong current makes a capacitor "snap", it is possible to "crack the vacuum".

But what will happen then? What strange particles will spring from the void? Here again, the mystery is cleared. It will be an electron-positron couple. A particle and its antiparticle, which are the lightest and therefore those which, according to Einstein's formula, will require the least energy to appear. And this minimum is also well known: 1,022 MeV.

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

Article published in the 19.10.05 edition of Le Monde

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