The first illustration in Jaco de Swart (33)’s thesis on the history of dark matter is the photo of a distortion pedal for an electric guitar, with the type name Dark Matter. In addition to being a physicist and philosopher (and chicken keeper and plastic picker), De Swart is also primarily a bassist. After the interview he has to go straight to Utrecht to practice with his band X Raiders, once seen at The world goes on, at Paaspop and at the Zwarte Cross Festival. Almost every week the five childhood friends perform somewhere. Hard rock music that plays with styles and genres, and where everything is always slightly different than you expect.
De Swart (tall tall, big curls, red-painted nails on the left hand, sweatshirt with the texts Mortal Coil and Reaping Death on the sleeves) is a part-time scientist; He puts at least 20 percent of his time and energy into music and it should remain that way in the future. He doesn’t really see such a big difference either: in both cases it’s about creativity, walking uncharted paths, never taking anything for granted and letting yourself go.
And no, De Swart, who will be leaving for the renowned Massachusetts Institute of Technology next summer, has no ambition to solve the mystery of dark matter with his own hands – others can do that.
It’s a riddle that has puzzled astronomers for decades. The bottom line: There must be a lot more matter in the universe than can be seen with telescopes. Almost six times as much, to be exact. This is apparent from, among other things, speed measurements in the universe. The outer regions of galaxies such as our own Milky Way, for example, spin much faster than you might expect. That can only be explained if those galaxies are much heavier than they look: without the gravity of enormous amounts of dark matter, the stars would be flung into space at high speeds.
Something similar applies to the rates of motion of galaxies in clusters — colossal swarms of hundreds of such galaxies. Those speeds are also much too high and indicate the presence of invisible stuff. And there are many other clues to the existence of dark matter.
No one knows what that dark matter consists of. Everything indicates that it must be about unknown elementary particles, but they have never been found, and the question is whether that will ever work. De Swart doesn’t really like that; he is especially interested in the history of the mystery. ‘For me, it’s only more fun and interesting if there isn’t a solution yet.’
How does one come to study physics and philosophy at the same time?
‘I started with physics – ‘real’ science, I thought. But there I soon missed a broader perspective. I also did a few nice courses next to it and was captivated by the philosophy. Physics is very factual and deals with the external world; philosophy is more reflective and much more about yourself. How does knowledge work; what does the term ‘evidence’ actually mean? In both worlds I was a bit of an odd one out, always asking different questions than the rest. Dark matter is a nice link: how is it possible that something about which so little is still known could play such an important, central role in science? The title of my dissertation is not for nothing How Dark Matter Came to Matter.†
How old is the riddle?
‘The term ‘dark matter’ was first introduced a hundred years ago, in 1922, by the Groningen astronomer Jacobus Kapteyn, in the May issue of The Astrophysical Journal† But Kapteyn didn’t use that description in the contemporary context at all – just as someone must have mentioned a brown bar long before the term took on its current meaning. Eleven years later, so did the Swiss-American astronomer Fritz Zwicky, sometimes referred to as the “father of dark matter.”
Kapteyn had no idea that the universe was bigger than our own Milky Way galaxy. He did show that you can in principle track down invisible matter – and calculate its mass – by measuring the speeds of stars. Zwicky made such measurements for individual galaxies in a cluster several hundred million light-years away. But in his day there was still a lot of uncertainty about the expanding universe and the observations were scarce. Zwicky did signal that something was going on, but he didn’t really put the mystery of the dark matter on the map either.’
When was dark matter really considered a serious problem?
‘It started with the work of the American astronomer Vera Rubin. Rubin and her colleague Kent Ford showed in the late 1960s that the outer regions of galaxies rotate much faster than you might expect. Radio astronomers – also in the Netherlands – made comparable measurements. The term ‘dark matter’ does not even appear in the first publications by Rubin and Ford. It wasn’t until the mid-1970s that everyone became convinced of the existence of vast amounts of mysterious dark matter.
‘That was due to the rise of cosmology – the study of the universe as a whole. Thanks to all kinds of new technologies, often indirectly derived from the Cold War, it was possible to describe the structure and evolution of the entire universe in a physical way. A new generation of young cosmologists wanted to answer the question of how much mass the universe as a whole contains, because it fixes the future: does space always expand after the Big Bang, or does gravity ever stop expanding?
‘The latter option was popular, but then the cosmos had to contain a lot of invisible matter. And then suddenly there were three ‘problems’ that could be solved with one overarching idea: the movements of galaxies in clusters; the rapid rotation of galaxies, and the “missing matter” of the cosmologists. That one idea was the current concept of dark matter, which we owe in large part to the work of Jim Peebles, who received the Nobel Prize in Physics for it in 2019.’
And then particle physicists suddenly turned to the riddle.
‘Yes, around the same time there was a symbiosis between cosmology and particle physics – anyone who studies the Big Bang needs both fields. I am convinced that the problem of dark matter played an important role in that synthesis. Peebles showed that the mysterious stuff must consist of unknown elementary particles and that this also explains the origin of the large-scale structure of the universe, with its clusters and superclusters of galaxies.
‘In the 1970s, particle physics was also one big success story. Physicists had developed the Standard Model, which describes all known particles and forces. Powerful particle accelerators were built at Cern in America and Europe. New discoveries followed each other in rapid succession. There was a strong feeling of ‘we understand how that works, we will find that dark matter particle’. But yes, we are now more than forty years later and the riddle is still not solved.
‘It is therefore not surprising that there has recently been a renewed interest in alternative theories of gravity, such as that of my co-supervisor Erik Verlinde. According to him, dark matter does not exist at all, but we must learn to understand gravity and the fundamental laws of nature in a whole new way. Much is at stake; fascinating.
‘My own research into the history of dark matter shows in any case that science is by no means a linear process and that you have to be careful with the term ‘conclusive evidence’. Moreover, dark matter is a different problem for everyone. For particle physicists it’s about new physics beyond the standard model, for astronomers it’s about explaining the properties of the universe and for people like Verlinde it’s about our understanding of gravity.’
The term ‘dark matter’ is now a hundred years old. Will we ever see the solution to the mystery?
‘For many scientists, the quest is starting to get frustrating. I’m working on an anthropological study of the underground Xenon experiment in Italy, which has been a long-standing search for dark matter – without success so far. On the one hand, there is the romanticism of the search for something fundamental; on the other, the banality of daily work, in which an army of students is busy cleaning literally every screw to perfection, in order to make the experiment as sensitive as possible. There’s something tragic about that combination.
“On the other hand, gravitational waves – minute ripples in space-time – were also first measured a century after they were predicted by Albert Einstein. And biologists also hunted for some sort of genetic building block of life long before DNA was discovered.
‘I do think that we need to reflect on the role of science in organizing our society. We are in the midst of an ecological crisis; the earth is on fire. But physicists are very good at ignoring the rest of the world. They are already dreaming about an even bigger and more expensive particle accelerator as a new weapon in their crusade to understand nature. Is it all really worth it? When is it enough? I think that is a field of tension that physics has yet to find a way into.’