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Grothendieck
1st March 2016
Alexandre Grothendieck died in 2014. Tired of living, handicapped by the fact of gradually becoming blind, he let himself die. The world thus lost the brightest living mathematician.
Alexandre, when I met him, in Mormoiron
We had met in 1988, at the time when he had refused the Crafoord Prize. What immediately brought us together was our perception of the role of the military in scientific research. It was to me that he said "I would rather be shot than wear a uniform". Over the years, I admit I have felt the same allergy, after having seen people like the polytechnician Gilbert Payan, who has since died, working to develop "cancerous weapons" (I remember the document he gave me, coming from military research and titled "evocation of cancers").
I remember an issue of the CNRS mail, giving voice to the military and titled: "Researchers, we need to talk to each other". At that time, the General Director, or perhaps the head of the "Physics for Engineering" department, had written: "we lack contracts with the army to satisfy the demands of researchers".
Throughout my entire career, the military have always been in my way, until I finally abandoned my MHD work. Simply because their applications could only be military at that time. Yes, one can be surprised to see that work carried out in a garage by Jean-Christophe Doré, thanks to donations from readers, could lead us to appear in major international conferences in the field. All of this with experiments carried out in a simple glass bell jar, where we operated under low pressure. But this pressure is simply the one that exists at high altitudes, where the Americans are already operating their hypersonic Aurora vehicle.
My wife often reassures visitors when I go off on a tangent like this:
*- When my husband wants to talk about making an omelette, he starts by telling you about the unhappy childhood of the chicken. But don't worry, he will eventually get back to the original topic. *
Yes, it's true, when approaching the subject of Grothendieck, many memories emerge. And with the benefit of hindsight, I completely share his attitude of rejection, his flight, which some may have perceived as the manifestation of a disturbed mind. But no, it was a deliberate, thoughtful choice, which one could call a "strong act", something few would do, let alone dare. Because even the most abstract mathematics can lead to deadly applications. The applications to robotics, to the search for autonomy for combat robots, drones, equipped with artificial intelligence is an example. Alexandre, who saw further than many, knew that all of this was in the making. This refusal of military funding for the IHES has symbolic value.
Returning to what I said earlier, how could experiments carried out by Jean-Christophe Doré in his garage in Rochefort, with permanent magnets and the most rudimentary devices, possibly interest the military to the highest degree? It all seems absurd. But in rarefied air, plasmas behave in a very particular way. Why study plasma physics? Because if you want to make a flying machine operate at very high altitude, well beyond the 30 kilometers reached by the fastest plane, the SR-71, flying at 3500 km/h and below 150 km of altitude where spy satellites can no longer venture, due to not being slowed down by the atmosphere, you have to fly at speeds of the order of 10,000 km/h.
The SR-71
Yes, the higher you fly, the faster you have to go. At 10,000 meters, the standard altitude for civil flights, a speed of 900 km/h is required, essential. At such an altitude, at 600 km/h, a liner would fall like a stone. At 15,000 meters, it was the Concorde, operating at Mach 2. And beyond that, the domain of the fastest spy plane in the world, which no Soviet missile ever managed to intercept, because it flew faster than the missiles directed at it!
Military forces from several countries are now trying to establish themselves in this "intermediate space", a major strategic issue. Even the French have joined in. But there is a big difference between the idea and the reality. If you try to use a simple scramjet, you face the very high temperature resulting from the recompression of air through a shock wave in the engine's air intakes. To avoid this, you need to recompress the air in a "soft" way, using MHD.
When this air arrives at a speed V, if it is subjected to a transverse magnetic field B, it immediately generates an electromotive field E = V B. The physicist would write it more precisely as V X B, because this electric field induced by the velocity completes the two vectors V and B through the classic "three-finger rule". This electric field causes current to flow in the gas.
It doesn't matter how it happens. What matters is that we can then extract electrical energy from this rarefied air flow, more easily the lower the pressure, as it ionizes easily, like the rarefied gas filling our fluorescent tubes. Under these conditions, a current I will appear in the gas, which, combined again with the field B, gives a force I x B (Laplace force) that tends to slow down the gas. Naturally: we convert the kinetic energy of the incoming air into electrical energy. This is the price to pay for this direct conversion.
Thus, we can consider slowing down and recompressing this air without heating it too much. Whereas in a shock wave, the kinetic energy is abruptly converted into thermal energy, into heat.
What to do with this electrical energy? We send it to the back of the device, where it contributes to accelerating the air, thus contributing to propulsion. This trick is called "MHD bypass".
Note in passing that a turbojet operates a "mechanical bypass", since at the back of the engine the gas drives a turbine which, coupled to the shaft, drives the compressor located at the other end.
All of this seems quite plausible. But in the conditions under which we operate, a plasma instability develops in a few millionths of a second, electrothermal instability, discovered by my friend Evgueni Velikhov in 1964. Instabilities in plasmas are a plague. It is these instabilities that condemn the ITER project.
It turns out that I am one of the best specialists in plasma instabilities, internationally. And in particular, the only European specialist in the Velikhov instability, which I was the first to master, in 1965. That's how it is. Without mastering this subject, it is impossible to consider a hypersonic project operating in rarefied air. Up there, this instability is the key.
Satisfying a time demands such as that of Jean-Christophe Doré, I accepted to conduct some experiments in rarefied air, which immediately opened wide the doors of international conferences (Vilnius, Lithuania, Bremen, Germany, Jeju, Korea, Prague, Czechoslovakia), and of peer-reviewed journals (Acta Ph...