Purple Magazine
— The Cosmos Issue #32 F/W 2019

christophe galfard

astrophysics CHRISTOPHE GALFARD

from galileo to newton, from einstein to hawking, from the big bang to the big bounce and a crossfire of competing mathematical theories, astrophysics never stops changing our understanding of the universe. the more we know, the more questions we have

interview by OLIVIER ZAHM
all pictures by THE HUBBLE SPACE TELESCOPE

OLIVIER ZAHM — Let’s begin with this idea of a contemporary renaissance. Do you agree that in the past 20 years or so, we’ve been living through a period of great discoveries in our decipherment of the starry sky, with its dizzying temporal depths and what appears to be its expansion?
CHRISTOPHE GALFARD — Science is going down two paths simultaneously. There is theoretical research, and there’s technological, experimental innovation, which entails the manufacture of devices and tools that have never existed — so that we can observe or verify something that we’ve never seen.

OLIVIER ZAHM — Instrumentation?
CHRISTOPHE GALFARD — Right. Theoretical instrumentation. For centuries, we’d see things, but we couldn’t really explain them. If we look back on all that from today’s perspective… Let’s say we were trying to find a theory that could explain why or how an arrow moves if I shoot it from a crossbow — and predict where it will end up. Galileo and Newton were the ones who, in a way, suddenly invented modern science by opening the way to a theoretical vision. Thanks to them, we now have models, in our heads, of what occurs around us.

OLIVIER ZAHM — First Galileo, then Newton.
CHRISTOPHE GALFARD — Galileo was the first to think: “Ah! Perhaps there are natural laws, and perhaps we can discover what they are.” He laid down the idea of science: that is, the quest for regularities that we can verify, predict, confirm, or deny. Before him, our notions were more mythological, if you will.

OLIVIER ZAHM — Aristotelian?
CHRISTOPHE GALFARD — Even earlier. The most important thing was to have a coherent frame of reference. It didn’t matter too much whether a theory of the world matched reality. I’m simplifying, of course, but the important thing was to convince others that a given system of the world was best. There was a sort of philosophical version of Speakers’ Corner, where everyone would set forth his story or vision.

OLIVIER ZAHM — For example, the Greeks with their four elements.
CHRISTOPHE GALFARD — Right: earth, water, air, and fire, each with its properties. That “explained” why heat would rise and cold descend, why water was always to be found over earth, and so forth. It was beautiful and worked very well. Indeed, it still works for some, almost 2,500 years later. But it was just one story.

OLIVIER ZAHM — So, Galileo destroyed the myth.
CHRISTOPHE GALFARD — To judge by the texts that have come down to us, he was certainly one of the first, if not the first. He inaugurated the era of science, an era that’s much more bound to reality and heeds experiment. Then Newton took a greater leap forward, which was to affirm that the laws that hold here on Earth hold all over the universe. It seems trivial, but no one had ever thought that physical laws were the same up above, that they were universal. People thought, after Aristotle, that there was a sort of solid, crystalline sphere that the stars were attached to and that turned around us. Galileo aimed a telescope at the sky, saw Jupiter, saw that there were moons revolving around Jupiter, and in one go demonstrated that not everything revolved around the Earth. From that point, there was a sort of transformation. We began to stop thinking of ourselves as the center of things. Newton and his universal law of gravity endowed us with the colossal power to understand a universe that, in the years to come, proved to be amazingly bigger than anything anyone had ever imagined.

OLIVIER ZAHM — And far beyond what we can see with the eye.
CHRISTOPHE GALFARD — If you go round the Earth and look at all the visible stars, and you have good eyes and do your looking on dark nights, you can count 5,000 to, let’s say, 10,000 of them. In our galaxy, though, there are 200 billion stars, and we know of a thousand billion galaxies, each containing several hundred billion stars. Those are figures that boggle the mind.

OLIVIER ZAHM — It’s interesting to have you cite them.
CHRISTOPHE GALFARD — So, we advanced confidently, thinking that the law of gravity applied everywhere.
Yet there were problems. Mercury, for example, didn’t revolve around the sun exactly as Newton predicted. That was a clue that a new theory of gravity became necessary as we approached the sun. That new theory escaped us for more than 150 years — until the early 20th century. When it was discovered, our concept of gravity changed.

OLIVIER ZAHM — You’re speaking of Albert Einstein?
CHRISTOPHE GALFARD — Yes. He was the one who understood why Mercury, the nearest planet to the sun, did not revolve as we might have expected from Newton’s equations. At first, scientists thought, “There must be a hidden planet shifting it off its trajectory,” but we couldn’t find the mysterious planet. And, in fact, it doesn’t exist.

OLIVIER ZAHM — Was it just a mistake in the calculations?
CHRISTOPHE GALFARD — It’s that Newton’s theory no longer applies perfectly because the gravity gets too strong. Because Mercury is so near the sun, we have to change frames. We need another vision. Einstein gave it to us. In passing, he transformed everything we thought we knew about our universe. Before him, we thought the universe had always been there, changeless. We thought things moved within the universe, but the universe itself didn’t move. Suddenly, though, we received a completely different vision of the whole universe. Einstein’s universe is not fixed. It can change.

OLIVIER ZAHM — What did Einstein discover to turn things on their head like that?
CHRISTOPHE GALFARD — In his vision of gravity, space and time are not external to reality. They’re part of the universe itself. They are its fabric, and this fabric can be deformed, twisted, and stretched.

OLIVIER ZAHM — He said that space and time are physical realities?
CHRISTOPHE GALFARD — Yes, absolutely. The two can even be different from one place to another. It’s an extremely odd concept and impossible to visualize. The time shown by the watch on your wrist won’t elapse in the same way all over the universe. It won’t elapse in the same way if you’re going faster or slower with respect to someone else.

OLIVIER ZAHM — I learned that it also doesn’t elapse the same way if you’re at the foot or the summit of a mountain.
CHRISTOPHE GALFARD — That’s true. The difference is ridiculously small, but it’s been measured.

OLIVIER ZAHM — A small difference, yes, but extremely precise nuclear clocks have detected it.
CHRISTOPHE GALFARD — To give you an example: for one of the astronauts who’s spent the most time in space — about a year and a half, I think — the difference in time between him and us who have remained on the Earth is 0.02 seconds in all. But it could be years if you go much faster, or if you end up near a planet with much greater gravity.

OLIVIER ZAHM — What’s the next scientific revolution?
CHRISTOPHE GALFARD — Not the next — the one that’s taking place at the same time. We speak of the infinitely small. Quantum physics. Officially, that kind of physics saw the light of day in 1900, with Max Planck. For almost a century, our knowledge of reality has exceeded the reach of our senses. While we were with Newton, what we knew matched our intuitive perception of space and time. But there is a beyond — both in the very big and in the very small.

OLIVIER ZAHM — A reality bound by other laws.
CHRISTOPHE GALFARD — Exactly. Let’s stick with the very small for now. There exists a reality outside our senses that obeys other laws — extraordinarily effective laws that were deciphered in the 20th century. A plethora of discovered particles in the 1970s, for example, had been predicted by those theories. We built particle accelerators — like the one at CERN [European Organization for Nuclear Research] in Geneva — for that purpose. Smashing known particles into one another releases energy, and that energy can transform into matter. It might seem like alchemy, but it follows from quantum physics. And this matter that was predicted by the theory has been detected.

OLIVIER ZAHM — Could you speak in greater detail about how theory has preceded observation? For example, in the detection — in 2015, I think — of gravitational waves.
CHRISTOPHE GALFARD — It’s very simple mathematically. Today it’s an exercise you do in your first years at university. Whereas in 1915, when Einstein developed his theory, there weren’t many who could do it, but now it’s standard practice.

OLIVIER ZAHM — [Laughs] That’s mean to Einstein…
CHRISTOPHE GALFARD — No, not to Einstein — because for him it was completely revolutionary. If I had told you 2,000 years ago that the Earth was round, it would have given you a headache, I assure you. Now it’s all right. You’ve seen photos and such. Jokes aside, it’s a simple calculation that takes a half-page. It’s really easy.

OLIVIER ZAHM — Is it a beautiful equation?
CHRISTOPHE GALFARD — It’s easy, and it’s elegant, yes.

OLIVIER ZAHM — And what does it say?
CHRISTOPHE GALFARD — It says that if you move a bit of energy in the universe, the motion creates waves in the universe’s very structure, in spacetime. A wave in space and time that propagates at the speed of light. Like a wave on the surface of the ocean. It’s a beautiful idea, and the result is simple. What’s hard is to detect it.

OLIVIER ZAHM — And that’s called a gravitational wave.
CHRISTOPHE GALFARD — Exactly.

OLIVIER ZAHM — And why does it go at the speed of light?
CHRISTOPHE GALFARD — That’s one of its properties. And the fact that it goes at the speed of light is a prediction made by Einstein. There’s an equation that corresponds to a wave, like a wave on the surface of water. When there’s a wave on the surface of water, there’s a “v” that stands for speed and tells you how fast the thing propagates. And this “v” in Einstein’s calculation is the speed of light.

OLIVIER ZAHM — Why is the speed of light a constant? Is there an answer to that?
CHRISTOPHE GALFARD — It is only a constant in a vacuum. We might say that it’s a postulate that for now is confirmed by experiment.

OLIVIER ZAHM — Is light an electromagnetic field?
CHRISTOPHE GALFARD — When you hold a magnet up to a fridge, before the magnet touches the fridge you feel an attraction or a repulsion. But you see nothing between the two things. It’s light that’s being exchanged — virtual light that’s exchanged and carries the electromagnetic force. Light is a way of seeing…

OLIVIER ZAHM — The electromagnetic field.
CHRISTOPHE GALFARD — Right. It’s part of that. And there’s no reason in principle for the speed to be fixed.

OLIVIER ZAHM — A speed of 300,000 kilometers [186,000 miles] per second.
CHRISTOPHE GALFARD — That’s right. Bravo.

OLIVIER ZAHM — Well, it’s not exactly that. It’s a little bit less, which is also strange.
CHRISTOPHE GALFARD — No. The kilometer is something we’ve defined. If we’d defined the kilometer as the distance that light travels in a second, it would be one kilometer per second. But the standard unit we’ve chosen doesn’t give us a round number. It would have been strange for it to work out exactly — really, really strange. Einstein posited the constant speed of light in a vacuum as an axiom, as a principle, but that’s not an explanation.

OLIVIER ZAHM — That’s a bit Greek, isn’t it — laying down axioms in the middle of science?
CHRISTOPHE GALFARD — No, not really. That’s what axioms are. Science is made up of them. You start with a principle. You suppose, for example, that it sounds wrong for the laws of nature to apply only on the Earth, and you suppose that they apply throughout the universe. You posit that they’re everywhere the same, and then you look to see that they’re the same. Or not. And if you’re wrong, as is usually the case, you have to start over.

OLIVIER ZAHM — So, in this case it was posited that the speed of light was constant, and so far no one has contradicted that.
CHRISTOPHE GALFARD — That’s right. And it’s thanks to the speed of light that we know that the same goes for gravitational waves. An enormous collision in space — like, say, that of two black holes — generates gravitational waves, which will fade in intensity little by little as they get farther from the shock. Such a wave was first detected in 2015. It was a gravitational wave generated more than a billion years ago by the collision of two black holes. The detection was also visual, confirming that gravitational waves travel at the same speed as light.

OLIVIER ZAHM — What’s the state of theoretical research these days? Are there various theories in development?
CHRISTOPHE GALFARD — Yes. There have been many over the past few decades, and they’ve given us rather farfetched predictions about parallel universes, extra dimensions, a double to matter that hasn’t yet been detected and that’s supposedly all around us.

OLIVIER ZAHM — You mean antimatter?
CHRISTOPHE GALFARD — In this case, I’m talking about something called supersymmetry, which is part of string theory — which changes pretty much everything. These theories have developed a colossal mathematical richness. Just as the Earth is minuscule by comparison with the rest of the universe, Einstein’s theory is minuscule by comparison with the set of abstract theories that have been created over the past 30 years — especially string theory, which is anomalously rich. We can use that theory to create just about anything mathematically. For the moment, though, it makes no precise prediction amenable to experiment.

OLIVIER ZAHM — That’s dizzying, what you’re saying. These are mathematically complex theories?
CHRISTOPHE GALFARD — Extremely complex.

OLIVIER ZAHM — And you’ve got young scientists wandering through it, darting about like rabbits?
CHRISTOPHE GALFARD — Exactly. [Laughs]

OLIVIER ZAHM — They’re going off in every direction. [Laughs] And having a ball?
CHRISTOPHE GALFARD — It’s a joyous thing to wander around in there, I must say.

OLIVIER ZAHM — And the problem is that we lack the means to perform experiments, to verify things?
CHRISTOPHE GALFARD — We haven’t the slightest idea for an experiment that could tell us: “It’s this rather than that.” It might be too far from our experimental basis, or maybe the theory’s no good, or else maybe we haven’t yet produced the little genius to help us prove it all.

OLIVIER ZAHM — So, we have a surplus of theories and a deficit of experimental means?
CHRISTOPHE GALFARD — It’s hard to say. Some of the machines we use today to make all kinds of discoveries take 20, 25, 30 years to design and build. You need to get the green light, the funds, and so on.

OLIVIER ZAHM — What about the new telescopes in space?
CHRISTOPHE GALFARD — Several extraordinary telescopes are continually scanning the sky, and there’s one in particular that I’m impatiently awaiting: the James Webb telescope, the Hubble’s successor. That is necessarily going to open our eyes to a thousand things. You never know, but it might shed a little light on the mysterious stuff we call dark matter.

OLIVIER ZAHM — Experiments, and not calculations, have shown us that such matter exists, right?
CHRISTOPHE GALFARD — A bit of both — because to analyze and make sense of the results of an experiment, we need a theory. Uranus, for example, has a bizarre orbit around the sun. The orbit itself isn’t bizarre; it’s bizarre with respect to Newton’s theory. If you believe Newton’s theory, you predict that something is disturbing Uranus. If you don’t believe it, you say that Uranus revolves that way because the laws we use are wrong. In the same way, when you look at how, say, the stars move in distant galaxies, you can check to see whether it matches Newton’s or Einstein’s theory. As it turns out, the orbit of Uranus doesn’t match either of them. So, either we say that Newton and Einstein are wrong, or we say that they’re right, but there’s something missing.

OLIVIER ZAHM — Dark matter?
CHRISTOPHE GALFARD — Yes, and there’s apparently five times more of it than of the regular matter we know. For every kilo of Olivier Zahm, there are five kilos of dark matter.

OLIVIER ZAHM — And why is it not visible?
CHRISTOPHE GALFARD — Because it doesn’t react to light.

OLIVIER ZAHM — This is some freaky stuff. [Laughs]
CHRISTOPHE GALFARD — And since you’re asking about recent discoveries — we’ve discovered yet another thing about the expansion of the universe: the expansion is accelerating.

OLIVIER ZAHM — Explain that to us.
CHRISTOPHE GALFARD — If you imagine a universe that’s expanding because this strange thing happened at the start…

OLIVIER ZAHM — Like the Big Bang?
CHRISTOPHE GALFARD — Yes, but it’s not an explosion. I don’t like that picture. But let’s say it’s that. The Big Bang, an explosion, and the universe gets bigger. In other words, the distant galaxies all get farther away from one another and from us. Necessarily, you’d say, gravity would slow down the expansion because it slows down and attracts everything. Gravity only attracts things to one another. It’s not repulsive. So, the stars and the galaxies and such should retreat more and more slowly.

OLIVIER ZAHM — Gravity’s attractive…
CHRISTOPHE GALFARD — Well, what we observe is the reverse. We observe that instead of slowing down, the galaxies are getting farther apart, faster and faster. There’s an acceleration in the expansion. And they should be doing it slower and slower. What we see, though, is the reverse.

OLIVIER ZAHM — They’re zipping along.
CHRISTOPHE GALFARD — And that’s not possible unless somewhere in the void that’s more or less everywhere, there’s a force, an anti-gravitational energy, that makes things repel. And that is what we call dark energy.

OLIVIER ZAHM — It’s not dark matter.
CHRISTOPHE GALFARD — No. And there’s a lot of it. Dark energy and dark matter make up 95% of the energy in our universe. Everything you learned about in school, everything I’ve told you about so far, concerns just the 4% or 5% that’s left over. And we’ve discovered that in the past 20 years.

OLIVIER ZAHM — Dark energy and dark matter.
CHRISTOPHE GALFARD — Yes, 24% dark matter, unknown and invisible; 70% percent dark energy, unknown and invisible; and 4% or 5% normal matter, known — the stuff we see everywhere.

OLIVIER ZAHM — The stuff we detect with the Hubble telescope or with the next… What’s it called?
CHRISTOPHE GALFARD — James Webb… Our universe is so stuffed with mystery that it’s super-exciting. We don’t have the answers, and, at the same time, we have technologies that might allow us to gather more and more information. Consequently, every now and then we can enjoy the sense of finding a little piece of the puzzle and adding to our understanding of reality. But we don’t know how big the puzzle is. We don’t know if there are a billion pieces or just a few. [Laughs]

OLIVIER ZAHM — So, it’s piece by little piece.
CHRISTOPHE GALFARD — Until we find the big missing piece, and it all suddenly makes sense. For now, though, we don’t have it. So, yes, there is indeed a sort of experimental emulation for the scientists of our era. But there’s also the mild frustration of not being able to manage a decisive experiment, to show something that isn’t predicted by the two other theories: Einstein’s gravity and quantum physics.

OLIVIER ZAHM — Because, in your opinion, there’d have to be something unexpected or a mistake somewhere to elicit a complementary theory?
CHRISTOPHE GALFARD — There’d have to be something stumping us and forcing us to reconsider everything.

OLIVIER ZAHM — A new theory…
CHRISTOPHE GALFARD — We know there exists a theory over and above gravity and quantum physics, and that might encompass the two. We haven’t found it, but we’ve known it’s there since Stephen Hawking.

OLIVIER ZAHM — Your mentor.
CHRISTOPHE GALFARD — Yes. He showed, among other things, that we can’t understand black holes without reconciling the two theories. But how to reconcile them is precisely what we don’t know.

OLIVIER ZAHM — He managed to show that they had to be connected?
CHRISTOPHE GALFARD — He managed to bring them into contact. He managed to provide our only result from what we call quantum gravity, the only result we know of from that unknown theory: the evaporation of black holes. He assigned a quantum temperature to a gravitational object.

OLIVIER ZAHM — So, what you’re saying is at that point, at that temperature, the two theories have come into contact.
CHRISTOPHE GALFARD — Yes. That discovery made Hawking into a physics superstar in the 1970s, but he did other things that are just as crazy. He demonstrated quite a number of things. He was the one who showed, mathematically, that the Big Bang must have taken place.

OLIVIER ZAHM — Oh, yeah? Even if it was predicted by Einstein…
CHRISTOPHE GALFARD — Actually, no. It wasn’t predicted by Einstein. It wasn’t him. Einstein denied the idea.

OLIVIER ZAHM — Really?
CHRISTOPHE GALFARD — He changed his mind later. Others suggested it, but Hawking demonstrated it mathematically. He was a hell of a genius.

OLIVIER ZAHM — So, Hawking is sort of the last great figure?
CHRISTOPHE GALFARD — No, no. There are others. He was the last great media star. But there are other geniuses.

OLIVIER ZAHM — But he’s the one who made possible a third kind of physics.
CHRISTOPHE GALFARD — You mean quantum gravity? He didn’t create it, no. But it’s true that he’s the first and, for now, the only one to have gotten a result that derives from the unknown theory. It’s a bit as if he’d found a…

OLIVIER ZAHM — A little key?
CHRISTOPHE GALFARD — Or the lock, rather: a keyhole. But we don’t yet have the key. We can only look through the hole and see a little of what happens on the other side, Hawking radiation, and we think there are certainly lots of things around it, but we don’t see any of it because we don’t yet have the key.

OLIVIER ZAHM — And what’s around it might be string theory?
CHRISTOPHE GALFARD — Well, maybe. It’s still a candidate.

OLIVIER ZAHM — There are many theories competing for the title of supreme grand theory?
CHRISTOPHE GALFARD — Yes. There’s also loop quantum gravity. The name’s a little less sexy than string theory, but…

OLIVIER ZAHM — Is it more complicated?
CHRISTOPHE GALFARD — No. Just the name. String theory is sort of the stuff of dreams. A violin, or some other stringed thing you like.

OLIVIER ZAHM — S&M.
CHRISTOPHE GALFARD — [Laughs] To each his own. At any rate, string theory is a little dreamier than a title like “the loop theory of quantum gravity.” So, there are several competing theories, and maybe none of them will turn out to be the one. We don’t know.

OLIVIER ZAHM — So, everyone in the world is working toward the same thing.
CHRISTOPHE GALFARD — It’s the truly great, great researchers who are dealing with that. Or the young ones who want to shine, although they often end up quickly drowning — that is, until one of them hits on it and explains everything to everyone else. It’s the holy grail.

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