"Understanding energy via the laws of physics"

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Erik Orsenna

And more good news... is that I get to welcome Etienne Klein. I have great admiration for Mr Klein because he is, as we know, Research Director at the AEC. He is a physicist and, above all, he has this incredible generosity, this giving energy, which allows us to grasp some of the most complex issues in physics.

So the first question is, basically: What is energy? And what is the difference between energy and power?

Etienne Klein

Hello everyone.

“Energy” is a very old word – a Greek word used by Aristotle. In essence, it is something that changes the state of a system. Over time, the word has acquired multiple meanings, which are somewhat problematic. Energy is both “energy” – what we currently call energy – but it can also mean power, strength, momentum, dynamism, will. In short, the answer is not very clear. 

Erik Orsenna

Basically, it’s everything that causes change. 

Etienne Klein

It’s the thing that makes change possible.

Erik Orsenna

The force that makes change possible. 

Etienne Klein

The change of matter, of its properties... It allows for the transfer of energy between one system and another. Contrary to popular belief, however, the word “energy” came late to the scientific field. Its first mention is in 1717, in a letter from scholar and mathematician Jean Bernoulli to Pierre Varignon, a Jesuit. In this letter, Bernoulli defines energy for the first time as the production of force through displacement.

This is what we now call mechanical energy. But the term would not go on to dominate physics. For this, we have to wait until the nineteenth century, with the birth of thermodynamics. 

In 1847, a great German physicist called Helmholtz writes a book entitled “Über der Erhaltung der Kraft” on the conservation of power or force. “Kraft” means “power” in German. And in fact, the distinction between energy and power at the time was not very clear. 

Erik Orsenna

So in the beginning, it's a kind of mysterious force. 

Etienne Klein

It’s the notion of “force” as associated with “energy”. And Helmholtz uses this meaning. In the book, he defines what is now called the first law of thermodynamics – the principle of the conservation of energy. But he confuses energy and force.

And it’s only in 1884 that a well-known physicist, Max Planck, the future founder of quantum mechanics, who wrote his PhD on the concept of entropy in physics publishes a book on the conservation of energy that repeats Helmholtz’s title almost word-for-word: "Über der Erhaltung der Energie”.

Here, Planck puts forward a very abstract definition of energy. Energy is a concept that is defined by the fact that, over time, the energy associated with a body is conserved. So energy is defined by the principle of the conservation of energy. By the fact of its self-preservation. In other words, you can't create energy from nothing. You can't produce it.

The expression “energy production” makes no sense. When you say that you produce energy, you imply that you can create it from nothing, from nothingness. The only thing you can do is take energy’s present form and change it to another form. For example, if you have mechanical energy, you can transform it into electrical energy. The other thing you can do is take energy from one system and transfer it to another system. These are the only possible options.

And if you take this seriously, it means that the expression “energy production” makes no sense. Neither does the expression “energy consumption”. You cannot consume energy. For example, you fill your car with a litre of petrol and you drive. After a while, your fuel has disappeared. Your petrol tank is empty. You say, “I have consumed energy”. But that’s wrong. You have consumed petrol. However, the energy that was in the petrol was used to propel your car as mechanical energy. While you were driving, the tyres heated the road, producing heat energy. You may have turned on your lights, etc., producing light energy. And then you heated the air. In the end, there is exactly as much energy as there was in the litre of petrol you consumed. The thing that you consumed was never energy. You have not consumed any energy. You have rather created entropy. By this, I mean you have taken energy in a highly ordered form – i.e. the energy in a litre of petrol – and you have dispersed it in the form of heat. You have created chaos, without consuming energy.

Erik Orsenna

So can an analogy be made with Lavoisier in chemistry? Nothing is lost, nothing is created, everything is transformed. It's kind of the same thing in terms of chemistry.

Etienne Klein

Yes. Except that Lavoisier was thinking about mass. Mass is not conserved. It's energy that is conserved. That's something really fundamental.

And energy should not be confused with power. Often, in our way of thinking, we confuse these two. For example, we take TNT to symbolise energy. It's an explosive. But in fact, a kilo of TNT contains 10 times less energy than a litre of petrol. TNT should rather be the symbol of power, i.e. the speed with which energy is released. TNT explodes; while petrol burns slowly. So it has a lot of energy, but it delivers little power.

Erik Orsenna

So power is the speed at which energy is dispersed. 

Etienne Klein

It’s the flow of energy. Power is measured in Watts and energy in Joules. The unit for energy that we use and that appears on our electricity bills is the kilowatt-hour. A kilowatt, 1,000 watts, is power multiplied by the number of hours, by time. Power multiplied by time gives us energy.

In fact, the human body needs 2.5 kilowatt-hours every day to fuel its metabolism, to function. And we acquire that energy through meals. A good meal equates to a kilowatt-hour. Lunch, dinner and breakfast make up 2.5 kilowatt-hours.

To give you an order of magnitude and an analogy, a kilowatt-hour equates to the kinetic energy of a 10-tonne truck driving at 100 kmph. So when you see a 10-tonne truck driving at 100 kmph you can say: that’s the energy equivalent of a meal. 

Erik Orsenna

So that’s why it can feel so heavy on the stomach. 

Etienne Klein

“Heavy goods”, as they say!

The energy needed to climb Mont Blanc, for example, is also a kilowatt-hour. You need a good meal to climb Mont Blanc. Or rather, when you climb the Mont Blanc, you consume the energy of a good meal.

Our body needs 100 watts of power. That is, when you take 100 watts, and you multiply it by the number of seconds in a day, it gives you 2.5 kilowatt-hours. So it's power that our metabolism actually consumes. The body uses 70 watts; the brain, which takes priority, uses 30 watts. That's why, for example, if you’re in the mountains and you get cold hands, the first thing you should do is put on a hat, not gloves. Because when it’s cold, as preference is given to your brain, all the heat in your hands will go up to your brain to restore its missing energy. So putting a hat on makes your hands feel less cold. And then you can put on some gloves. They’re not useless.

A good way to see and measure how we live is to calculate the total energy that we consume each evening – not only as fuel for our bodies but also as movement, light, heat.

Erik Orsenna

I suppose that calculation will depend on the day? 

Etienne Klein

Yes. So you have a total amount of energy. You write it in out kilowatt-hours and you divide by 2.5. This gives you the number of “energy slaves”, to use a popular expression, that have worked for you. These are machines. For example, your microwave and your car worked for you. And when you do the maths, you see that the average Frenchman has about 200 “energy slaves”. It's as if there were 200 fictional people who have worked for us by donating their bodies’ energy to our various uses.

Erik Orsenna

So what do you think about the current discussions, projects and holdups relating to what we call the “energy transition”? How do you see all this?

Etienne Klein

To me, there's some semantic ambiguity here. In the French language, the subject of the verb “to change” (changer) is what stays the same when we say that it changes. The Greeks, for example, saw a paradox between the relative concepts of identity and change. They said, 2If something has changed, this means that it is no longer has the same identity. And if it has the same identity, this means that it has not changed.” 

This paradox, which divided Greek philosophy, was solved by a linguistic trick. When you say that X has changed – X is a man, or an object – you can still do something to X at the end of the change process. So X, as a subject, has not changed. What has changed is a property of X.

If you say that Mr. So-and-so has changed, you actually mean it’s his haircut or his waistline, perhaps, that has changed. But it’s still the same Mr. So-and-so. In other words, when we say that X has changed, we mean that we are still dealing with X at the end of the change process. So a change is not a replacement. X didn't become Y.

When we talk about the energy transition, we imply that a change will come about. And until we have identified the constant factor within the transition, the issue remains blurred. This is a transition that requires us to change what, exactly? And which will lead us to retain what?

Do we want to maintain our way of life, for example? Do we want to retain the portion of fossil fuels we use, for instance to produce our electricity? Until we specify these things, it’s just a vague slogan saying that we need to change things, without detailing the elements we want to change.

Erik Orsenna

We can see this quite clearly, for example, with the so-called “electric car”. These tangible issues, we are told, are right in front of our eyes.

Etienne Klein

Yes, the electric car. I have nothing against it. But it still causes problems. The issue is how we produce the electricity. And then there are the rare earth elements in the batteries. Do we have the capacity to manufacture batteries in very large numbers for a very long time? Can we recycle them?

For me, these are fundamental questions. To just say that we're going to replace combustion vehicles with electric cars is somewhat hasty. The announcement has multiple secondary effects that will need to be examined.

Erik Orsenna

Especially if you haven't changed your lifestyle quite as you imagined.

Etienne Klein

Say that you have a litre of petrol. You have two options: either, you put it into a power plant, which will generate an electric current that you will use to charge the battery of your electric car; or, you take that litre of petrol and put it directly into a car's combustion engine. Which gives the best yield? 

This is a small, interesting calculation to make when you want to understand the value of switching to electric over sticking to thermal energy. 

And then there are the laws of physics. I am of the same mind as Richard Feynman, the Nobel-prizewinning physicist, who said, “Nature cannot be fooled.” By this he meant you can say anything you want, but if your words violate the laws of physics then your solutions won't be applicable. You won't be able to use them. And there are some well-known physical constraints. Aside from the fact that total energy remains constant, what if I ask you how much matter you need to produce a kilowatt hour? In other words, how much matter do you need to obtain the energy equivalent of a good meal? Physics’ answer: it depends. It depends on the fundamental force that you use to produce the energy.

You could use gravity – a force that is not very intense. For example, you can drop some water and collect the energy it acquires when it falls, its kinetic energy. You need 10,000 cubic meters of water falling from a height of 40 meters. So it takes 10 tonnes of water to produce a kilowatt-hour. I like to see this when I look at a dam. I think to myself: Hey, that’s one good meal that just dropped.

Now, if you use electromagnetic force, the amount of matter you need to produce a kilowatt-hour is about a kilo. And you absorb about a kilo of matter during every meal. If, on the other hand, you use nuclear forces – a strong nuclear interaction, for example, as in the fission of uranium – the answer is a few milligrams. If you use nuclear fusion with deuterium and tritium, then it’s a few micrograms.

That's the basic data. There are energies that are “concentrated”, as they say; and others that are less concentrated. And we cannot change these parameters. So when we talk about the energy transition, we obviously need to think about uses. We need to move towards saving more energy. But we must also decide on our preferred choice of primary energy sources.

Erik Orsenna

You are a research director at the AEC, the Atomic Energy Commission. What is the AEC looking for? What are its hopes? And what obstacles does it still face?

Etienne Klein

Let’s say that the AEC is committed to a lot of very long-term projects. Nuclear fusion is a long-term project.

Erik Orsenna

Can you tell us about nuclear fusion?

Etienne Klein

We are in the habit of saying that nuclear fusion is energy from the sun. Obviously, that’s not true. In the sun, there is hydrogen in the beginning. Protons, all alone. All protons have a positive electrical charge. So if you bring them closer together, electric repulsion will prevent their fusion. The electric repulsive force will make them move away from one another. But nature has come up with an interesting ploy: from time to time, one of the two protons that could fuse transforms into a neutron via radioactivity. This is what’s known as beta radioactivity.

And so if one of the two protons becomes a neutron – i.e. “neutral” – there is no more electric repulsion. Fusion can occur and form what is called a deuterium nucleus consisting of a proton and a neutron. This process of converting a proton into a neutron is very rare. That's why the sun takes billions of years to burn its hydrogen.

The power it delivers, even in the hottest places, is therefore very low: 800 watts per cubic meter. The sun is a rotten nuclear reactor. Eight hundred watts per cubic meter – that’s less than your body. Your body is 100 watts; and your volume is much less than one cubic metre. This means that per unit of volume, your body radiates more energy than the sun.

So our plans for the earth do not involve copying the sun, which is a very bad nuclear reactor. The energy it sends us is only due to its enormous size. There are a lot of cubic meters, so its total radiated power is very great. But per unit of volume, it’s very low.

Our efforts on Earth are therefore rather different. We don’t want to use the beta radioactivity by which a proton transforms into a neutron. We're going to use another type of nuclear interaction – a so-called “strong nuclear interaction”. As the name suggests, it’s strong. Here, we take nuclei that are isotopes of hydrogen: deuterium, with a proton and a neutron; and tritium, with a proton and two neutrons. Next, we put them into a gas. We create a gas that we will heat to extremely high temperatures, 100 million degrees. Under these conditions, the energy given to these nuclei will be sufficient to bypass the electrical repulsion that prevents their fusion. They will be able to fuse to make a nucleus of helium 5, with two protons (1+1) and three neutrons, which is radioactive and which will emit a neutron of fourteen MeV. MeV is a unit of energy.

And that's the energy we will recover to heat the water and make electricity.

The big challenge is to control this kind of reaction. We have a large, global-scale project in Cadarache called ITER, which is scheduled to start soon. We hope it will tell us how to make industrial reactors producing electricity based on this principle.

Erik Orsenna

How “soon” is soon?

Etienne Klein

This is not a solution for the energy transition we were just talking about. It's a much more distant deadline. And it's up to our generation, let's say, to make sure that it's possible. We are not the ones who are going to decide whether or not to install fusion reactors around the world.

We have to study this, because our generation has burned more oil than any generation before it. In a sense, we emptied the fridge. When a friend lends you an apartment with a full fridge, and if you empty it during your stay, the least you can do is go shopping so that when he comes home there is something to eat.

Our generation has a responsibility to study the processes that will enable future generations to have an electricity “consumption” that is similar to ours.

Erik Orsenna

Imagine you are the President of France. What programme would you be launching for what is – falsely – called the “energy transition”?

Etienne Klein

First of all, I would put lessons about energy on TV, at all hours of the day. Our citizens’ opinions on energy depend on the symbolic halo associated with the various energies. A philosopher of techniques called Gilbert Simondon expressed the idea that it is the narratives into which we insert technologies – or energies, in this case – that determine the opinions we draw about them.

There is a symbolic halo around nuclear power, wind power, hydroelectric power, renewables, etc. And once you are convinced that this is the “good” energy, you no longer need any know-how. In other words, when we talk about energy, there is a kind of decorrelation between activism and know-how. As soon as you become an activist, you're “for” something. And there's no need to understand this thing that you are “for”. For example, I ask my friends who are anti-nuclear:  Guys, why are we using uranium in nuclear power plants? What’s so special about uranium? Why don't we use aluminium or tungsten? And they don't know the answer.

Likewise, if I ask my pro-nuclear friends Why do we use uranium in nuclear power plants? They don't know either. 

In other words, it’s as if the fact of having a strongly held opinion absolves us of the obligation or the desire to learn about the object of this same opinion. And that's not OK. If we want to be able to discuss the kind of shared belief that we as a society are going to have as we continue to address the question of energies, we need to do so in possession of the facts.

Our knowledge has to be a factor in our decision. As I mentioned earlier, nature can’t be fooled. So to have the debate, we have to start from a common knowledge base. It is not a question of championing an academic conception of citizenship. A citizen who does not know anything about physics is not a worse citizen than another. But when it comes to debating issues that have a major scientific component, I think that knowledge sharing needs to come before debate.

Erik Orsenna

Thank you, Etienne Klein. One last question: do you have a programme for transforming curiosity (like mine, in this case) into intelligence (like yours)?

Etienne Klein

It is a question that contains a flattering, excessive component that throws me off balance. The surge of emotion it produces causes me to lose my intellectual capacities.

Erik Orsenna

Is emotion an energy? 

Etienne Klein

Of course. Curiosity, too. Psychological studies have shown that knowledge acquired under conditions where no emotion is involved is very poorly assimilated. A teacher who shows emotion disseminates their knowledge much better than one who seems indifferent to their words. So, yes. Emotion is a part of learning.

Erik Orsenna

Etienne Klein, thank you. Something tells me that you should have been a Rolling Stone, and that's really the biggest compliment I can give. For the first time in these discussions, I will allow myself to clap.

Etienne Klein

I have one objection: I'm too young.

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Etienne Klein is a physicist and research director at the AEC and doctor of science philosophy. He directs the AEC Research Laboratory on Material Sciences (LARSIM). He teaches philosophy of physics at CentraleSupélec. The author of numerous books and winner of several awards, he is a member of the French Académie des Technologies. He recently published “Genius Ideas” (Champs-Flammarion, 2021) and “Short circuits” (Gallimard, April 2023).