Katherine Freese and I are seated at a plush hotel bar in Boston’s Back Bay neighbourhood when she tells me that billions of unseen, undiscovered particles are likely to be zooming through my body at that very second. For someone like me, that idea is unsettling. For her, it’s another thrilling hypothesis about the universe.

Freese is a theoretical astrophysicist, who splits her time between the University of Michigan and Stockholm University. She has dedicated her career to untangling some of physics’ greatest mysteries — cooking up thrilling ideas about the cosmos, disproving others — while knocking down barriers for women in a famously male-dominated field. Much of her work has centered around an enormous question: what is dark matter, that elusive cosmic component whose gravity appears to bind the galaxies? She thinks those hypothetical invisible particles whooshing at us could hold the answer. 

Freese’s interest in science was first piqued as a child. Both of her parents were biologists, making the dream of a career in the sciences seem possible. “I had a woman role model every day,” she says. At 16, she went to Princeton and became one of the university’s first women to major in physics. After college, she put off offers from a dozen graduate programmes to travel the world. She ended up falling in love with Tokyo and spent nearly two years there, working as an English teacher and a hostess in a bar.

Then, while on a trip to South Korea, she fell ill and found herself in the hospital for an emergency appendectomy. Laid up in her bed after the procedure, Freese started reading one of the only books she had brought with her: Spacetime Physics, an introduction to special relativity by two American physicists. It exhilarated her. Years later, in her book The Cosmic Cocktail, she would describe that time in the hospital as a celestial wake-up call. “I felt that if one elementary text could substantially alter my perceptions of the universe, then I had to go back to school for a deeper understanding,” she writes. “If physics from the early 1900s could be so fascinating, what would the next millennium bring?” She headed back to the United States to enrol at Columbia University.

Astrophysicist Katherine Freese

There was, in fact, a more recent puzzle grabbing the attention of the physics world arising from research by astronomer Fritz Zwicky in the 1930s. While calculating the mass of the Coma Cluster, a dense clot of more than a thousand galaxies, and the total light it gave off, Zwicky found that the total was one-hundred-fold heavier than the light would suggest. He concluded that there must be some hidden “dark matter” making up the difference.

Subsequent experiments over the years have determined that dark energy and dark matter make up the overwhelming majority of the universe. Everything we see and know — from this magazine and the chair you’re sitting in to mountains and rivers, the sun and its planets —makes up less than five per cent of the total. The rest is invisible, unknown, but it’s out there.

In The Cosmic Cocktail, Freese imagines the universe as a tall, icy concoction: three ounces of dark matter, seven ounces of dark energy, a half-ounce of hydrogen and helium gas. The rest — all the other chemical elements, stars, supermassive black holes, and so on — would be just a splash added on top.

Freese regards dark matter as the secret ingredient to the universe: it’s the glue that helped bring visible matter together into the structures that we see today. “We wouldn’t exist if it hadn’t been there,” she says. But what is it? We still aren’t so sure. Dark matter doesn’t emit light or any detectable radiation. Over the decades, physicists have proposed, and then ruled out, a number of possible explanations. It’s not diffuse hot gas. It’s not snowballs of hydrogen. It’s not rocks or dust.

Freese recalls her student days in the 1980s — first at Columbia, then the University of Chicago — as an invigorating period: “Particle physics was brand new at the time and it was pretty damn exciting.” The physics world had divided into two opposing factions in the war over dark matter’s identity. They called themselves the MACHOs (Massive Compact Halo Object) and the WIMPs (Weakly Interacting Massive Particles).

The MACHOs thought dark matter might be massive astronomical bodies — like white dwarfs, the burnt-out shells of dead stars — skulking out in the “halo” of our galaxy. We wouldn’t have seen them yet, because these shadowy bodies remained undetected by not giving off any light. The WIMPs suggested that dark matter was made up of a type of particle, like the protons and neutrons found in the atoms in our bodies. Weighing up to 10,000 times more than protons, they rarely interacted with the particles around them, explaining why we hadn’t noticed them yet.

Freese has researched both sides of the debate and now finds herself on the side of the WIMPs. These “massive” particles would be those same billions of particles that she suggests are going through our bodies every second of every day. As the Earth rotates around the sun, it plows through a cosmic sea of them.

Existing physics theories already posit that there should be particles out there that we haven’t yet discovered. Scientists can also model in the lab how many WIMPs might have survived from the early universe to the present day. Their calculations show that the number of WIMPs would match up nicely with the amount of dark matter believed to be present now. And there’s another reason to join team WIMP: in 2000, Freese and her colleagues looked at the different MACHO candidates, such as white dwarfs and brown dwarfs (or faint stars), and figured out there weren’t enough of them out there to explain the whopping size of dark matter. 

Now, around the world, physicists are on the hunt for WIMPs. At the Large Hadron Collider outside Geneva, they’re trying to create WIMPs by smashing particles together in its 27-kilometre-long underground facility. Other experiments are ongoing at the South Pole and in Canada and Japan. Scientists at the DAMA/LIBRA particle detector, located under 1.5km of rock at Gran Sasso in Italy, claim to have found an aspect of dark matter that Freese and her colleagues first proposed in 1986. They suggested that the “WIMP wind” of particles should fluctuate as the Earth whizzes by. However, the Italian experiment’s results have yet to be confirmed.

Astrophysicist Katherine Freese

Meanwhile, Freese is looking for other innovative ways to catch a glimpse of dark matter. In one bizarre recent experiment, she teamed up with biologists to try to catch WIMPs as they ran into an unusual lab-made detector: a sheet of gold lined with strings of DNA. She is also pursuing the idea that dark matter might have made its way into some of the first stars after the Big Bang. If that’s the case, these “dark stars”, which would have been one billion times brighter than our sun, might be glimpsed when NASA’s powerful James Webb Space Telescope launches in October 2018.  

With all these people looking for dark matter, is there a chance we’ll know soon what it actually is? “We hope so,” says Freese, who has seen the research on dark matter progress throughout a career in which, she says, she has felt constantly aware of her status as a woman in a field populated mostly by men.

Aside from being one of the first women to receive an undergraduate degree in physics from Princeton, Freese later became the first woman appointed to the physics department at Michigan. When she became pregnant in the late 1980s, she had to handle all her responsibilities as a new professor at the Massachusetts Institute of Technology (MIT) without the luxury of any maternity leave.

In some ways, things have improved for female physicist. According to the American Physical Society, 20 per cent of all bachelor’s degrees in physics are earned by women, up from 10 per cent when Freese graduated from college. Advanced degrees have progressed at a similar rate. And the struggles like those she once experienced at MIT are far less common for female professors today. Yet sexist attitudes still pervade physics, she says, as evidenced by recent sexual harassment scandals and the scientifically proven biases of hiring committees. “It’s still going on,” she says. And women tend to doubt their own abilities more than men, she asserts: “I doubted myself until I was 40.” She urges young women getting into the sciences “not to talk yourself out of it”.

Her experiences might explain why some of her proudest recent accomplishments have centered around bringing scientists together. For the past three years, Freese has served as Director of Nordita, the Nordic Institute for Theoretical Physics in Stockholm. Last year, she persuaded Stephen Hawking to spend a week discussing his work on black holes.

The institute also hosted a ball entitled “The Spacetime Odyssey Continues”. “It’s not every day you see 80 physicists in tuxes and ball gowns in a mirror hall,” she says. “It was really amazing.”

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