Energy regularly makes the front page of our media, but do our ways of saying it do it justice? Do they take into account what we know about her? This question is important for two reasons. The first is that if we talk about energy by betraying its essence or its properties, we are likely to think it wrong. The second is that nature will not let itself be fooled by our language games: if the solutions we formulate are impracticable in practice, they will not be solutions.
Thus, once it is firmly established that the energy of any isolated system has the property of remaining constant, it becomes misleading to speak of “energy production”, because this expression implies that energy could arise nothingness alone. In reality, what we can do is either change the form that energy takes (transform electrical energy into thermal energy, for example), or transfer energy from one system to another system. It is never a creation ex nihilo. In short, to have energy, you must already have it or receive it…
For these reasons, we should not speak of “energy consumption” either. Because consuming a kilojoule is in no way making it disappear: it is taking it in a very ordered form (for example electricity) and converting it into an exactly equal quantity of energy in another less ordered form ( hot air for example). In short, “to consume energy” is to create “entropy”. Entropy is a quantity that characterizes the tendency of a physical system to undergo spontaneous transformations: the greater its value, the lower the capacity of the system to transform itself. By evolving, any system increases its entropy, that is, weakens its tendency to evolve further. In fact, entropy measures the “quality” of the energy available in a system: during its successive transformations, its energy becomes less and less usable, then ends its life in the form of heat.
Of the “liberation” of energy
In December 2022, an inertial confinement nuclear fusion experiment was performed in California at the National Ignition Facility (NIF). It consisted in bombarding with 200 laser beams a small target containing deuterium and tritium, two isotopes of hydrogen. Deuterium nuclei contain one proton and one neutron, tritium one proton and two neutrons. In the target compressed by the light power, these nuclei can fuse, giving rise to nuclei containing two protons and three neutrons, i.e. helium 5. The latter quickly transform into helium 4 nuclei by expelling one of their neutrons, which then possesses great energy.
During the Californian experiment, the lasers delivered 2.05 megajoules (roughly the energy delivered by a croissant being swallowed) and the nuclear fusion reactions they induced provided 3, 15 megajoules. The physicists would therefore have succeeded, “to produce for the first time more energy than that contained in the beams of light”. As if they had created energy out of nothing!
In reality, it did not happen. But these experiments are remarkable because they created the conditions for transforming part of the mass of the merging nuclei into energy. How ? Through the formula m = E/c2. Ultimately, they are machines for transforming mass energy into another form of energy, without violating the law of conservation of energy in the slightest. In this case, we should therefore speak of “liberation” rather than “production” of energy: part of the energy hitherto confined in the mass of the nuclei of atoms has come out of its walls.
And above all, we hardly insisted on the fact that to deliver their 2.05 megajoules, the lasers first had to swallow several hundred of them! Which is enough to show how far we are from an industrial-type achievement: the laser shots, instead of occurring once a day as is the case at the NIF, should be carried out 10 times per second 24 hours out of 24! Nuclear fusion by inertial confinement is therefore not one of the solutions that can be envisaged in the short term to achieve what is called “the energy transition”.