Just so you know, it’s 2022, and we still don’t know what 95% of the universe is made of.

In 1933, Fritz Zwicky, a Swiss astronomer, observing a cluster of galaxies 321 million light years away, noticed something odd. The gravitational pull of the galaxies was much too weak to keep the stars traveling on their edges from escaping into outer space. Something invisible is keeping these galaxies stable. Eighty-nine years later, a team of physicists using a recycled particle accelerator buried 20m below the German city of Hamburg might be on the verge of finally figuring out what that something is.

The tunnel.

Gravels crunch under the tires as we pull up on a damp Friday afternoon in September. We park between empty trailers and buses, taking advantage of this free parking sanctuary, wedged between what only city folk could call a forest and deserted soccer fields.

Sandy has been waiting for us in the drizzle. “You found the spot ok?” she asks in French, her thick expressive eyebrows and piercing eyes betraying a sense of excitement, as if this were only the start of an expedition. An impressive mane of chestnut hair frames a thin face, marked in the right places by gentle smile and laughter lines. Not yet forty, she’s been a research engineer at DESY – a German physics research centre – for just a few years, working on the ALPS II experiment which she introduces with an appropriate “well… this is it” and a gesture towards the silvery building behind her.

A gate to the worn-out base of a 1980s Bond villain, whose threat to destroy the earth was never taken seriously, never got the money, and now struggles with the upkeep. A rusting 3-meter-high reservoir - buttressed by rotting wooden pallets - and what can only be described as “pieces of equipment” clutter the access ramp to the 4-storey, faded-blue garage door.

We’re alone. She ushers us through the unmanned gate, badging us in one by one and informing the security officer through a rusty interphone that “we’re going down to the tunnel”. A smell of oil and damp concrete hits as we enter through a human-sized side door. As we step into the elevator, Sandy utters a few words about hoping we won’t get stuck and how there’s no phone signal in the elevator shaft. The floor numbers are messed up. Having stepped in from outside, we find ourselves on floor 7 already. Pressing “0”, we’re shakily zipped below ground, the doors opening into a dusty hallway. Withered security signs and the odd rotary telephones – “they still work” says Sandy with a sense of wonder – adorn the hastily painted concrete walls. At a steady pace, we walk through a dim cavern – maybe 10 meters high, the resting place of a colossal out of service particle detector – and through heaps of concrete blocks - each one big as a baby elephant -, into a tunnel.

Here we are. This is where a merry team of technicians, engineers and physicists is getting ready to prod the fabric of physical reality.

Matter.

Your mother-in-law, that chocolate cake you’ll bake tonight, MACS J1149 Lensed Star 1 - a.k.a. Icarus, the most distant star we’ve ever observed, 14-billion light-years from Earth - and the screen you’re reading this on are made of the same building blocks of matter: atoms.

Physics superstars such as Marie Curie, Niels Bohr, Erwin Schrödinger, and Wolfgang Pauli started experimenting, measuring, and building mathematical models - which we today call quantum physics - to describe and use matter at the subatomic scale in the 1920s. Since then we’ve continued exploring and fine-tuning our understanding of fundamental physics to where we, as in scientists and engineers, use it every day to understand how stars work, or to build the things that make life in a material world comfortable.

But, since the end of the 19th century, astrophysicists have been amassing heaps of evidence telling us that “ordinary matter” only accounts for a small portion of our universe. Today, 5000 years after the first traces of writing, one year after the 13th iPhone iteration, humans cannot say what the vast majority of the universe contains. That brings us back a peg or two. We have ideas, we even have names - dark energy and dark matter - but we have no certainty.

Dark Matter.

From the 1920s, astronomers studying rotating galaxies recognized things weren’t stacking up. One of them, Fritz Zwicky, observed stars at the outer edge of distant galaxies forming the Coma cluster. He measured the speed at which they were circling their galactic centers and found they were going much faster than could be explained by the gravitational effects of the visible galaxies. Something invisible to our eyes and instruments, dark matter and dark energy, generating a massive gravitational pull on these fast-moving stars, were missing from our map of the universe. While Zwicky’s calculations turned out to be way off due to it being 1933, his conclusions have been upheld by 90 years of research in cosmology - more observation of rational speeds, measures of the speed at which the universe expands, gravitational lenses...

To detect “something” - dark matter or not - we use our senses or build machines to augment them. Telescopes and microscopes enhance our sense of sight by focusing light in a way that works for our eyes; a Geiger counter augments our ears by emitting a click every time a radioactive particle disintegrates. The key here is that everything we build to bridge the gap between the far away or the invisible and our natural senses is made of atoms. Dark matter interacts so little - if at all - with atoms that we might never build a machine that can give us a hint of its presence.

Of course, we have ideas, or rather hypotheses. And by “we”, I mean theoretical physicists. Their job is to build mathematical equations and models that describe reality. By stretching these models to their limits, by asking them not to predict the results of experiments we’ve performed in the lab, but of experiments we haven’t done yet, they reveal different options to explain physical reality. Experimental physicists then come in and try to debunk these different theoretical versions. Probing different models has led theoretical physicists to postulate the existence of new particles that might explain dark matter: WIMPs - Weakly Interacting Massive Particles; FIMPs - Feebly Interacting Massive Particles; or Gravitinos are some of them, alongside other types of explanations such as primordial black holes.

The darling candidate of the physics community for dark matter today is the Axion. It appeared in physics theories at the end of the 1970s and takes its name from a brand of detergent. Because if its existence is confirmed, it would not only explain dark matter but also clean up some other discrepancies in our understanding of fundamental physics: strong interaction forces, imbalance in the amount of matter and antimatter, or even dark energy. The fine details of physics are lost to my mind, so is the humour.

Teams in the US, Europe, Australia or South Korea are now racing to detect these Axions, and we should either find them or rule out their existence within the next 10 years.

Light through a wall.

“In about two weeks we’ll start turning on the lasers, and no one will be allowed in here anymore.” Sandy reminds us of the privilege it is to be down here.

We’re in the old particle accelerator tunnel, a thin donut 6,3km in circumference - a walk around the entire thing takes an hour and a half - where the team is getting ready to fire up the ALPS II experiment and look closely at any traces of Axions.

The ALPS II machine is a testament to the team’s ingenuity - and their will to extract as much science as possible out of every euro of funding. They’re not just re-using an old tunnel but are refurbishing 200m of supra-conducting magnet segments from the HERA particle accelerator – the Hadron-Elektron-Ringanlage which ran from 1992 to 2007 - to fit their needs.

“The laser comes from that cabin,” Sandy points at a small porta-potty-like white box 100m to our left, “and on the other side, 100m to our right, is where the light detectors are. This larger clean room over here is where we’ll put the wall.”

She points at the third cabin, just to our right. In between these hubs of equipment, the particles of light - and even maybe Axions - travel in recycled accelerator segments, a concentric assembly of tubes and magnets about half a meter in diameter.

As we go through the process of packing our entire bodies in white over-shoes, suits, and headgear, designed to keep even the smallest traces of dust out of the clean room, Sandy explains the concept behind the experiment. “We’ll shoot a light at a wall and see if we can detect any of it coming out on the other side.” This doesn’t sound like the cutting-edge physics experiment I was hoping to hear about. Then comes the - slightly - more technical version: a powerful laser is shot from the first hut towards the middle clean room, the one containing the wall. As it travels down the first 100 meters of accelerating tubes, the laser is subject to an intense magnetic field (cue the refurbished supra conducting magnets). The yet unproven theory tells us that a few photons - particles of light - from the laser beam will transform into Axions under these conditions. And since Axions don’t interact with atoms, they’ll just zip through the wall - somewhat disappointingly, not a real wall - and onto the second 100m arm of the machine. There, another magnetic field transforms the Axions into photons -a.k.a. light - again, which can be detected in the last hut.

Contemplating the vacuum vessel - a thick, sealable, silvery bathtub which is apparently a challenge to seal correctly - that will soon host the “wall” and a maze of optical lenses, Sandy runs through the numbers and explains the challenges of getting the machine to work. The magnets need to be cooled down to minus 269 °C and all the instruments kept in an ultra-high vacuum. That’s the complicated but doable part. For the laser to work just right, it needs to bounce back and forth between two mirrors 100m apart.

“Their position needs to be precise to less than the width of a single atom,” she says. Her eyes – the only part of our bodies poking out of our protective white suits – beam with wonderment. My mind frantically tries to grasp the enormity of that short sentence, desperately looking for a relatable metaphor, which never comes. There is no relatable comparison to “being precise to less than the size of an atom”.

“One detector on the receiving side of the experiment uses a supra-conducting film kept at exactly the right conditions so that the energy brought by a single photon will tip it into a conducting state. We can detect that change of state using other complicated equipment that we’re installing on the floor below.” For your information, a single photon doesn’t hold a lot of energy. A billion billion of them still hold a million times less energy than a lazy cat, to give a precise reference point. Figuring out how to keep the film just below the detection threshold is enough to keep a small team physicists and engineers busy over a few years.

What are the chances?

“It will take about 10 months to tweak everything before the experiment can start,” Sandy says.

Theory tells us that one out of every 10^13 - shorthand for 10,000,000,000,000 - photons might transform into an Axion in the first magnetic field, and one of 10^13 - the same overwhelming number - of these Axions will turn back into a detectable photon in the second magnetic field. Each day, the team will send 100,000,000,000,000,000,000,000,000 photons at that wall, hoping to detect one on the other side.

And when the experiment starts mid 2022, they’ll need less than two weeks to draw their first conclusions. Ten years of work, compressed into a few days of measurements. If there is light on the other side of the wall, pats on the back will be warranted. If not, the team will still add their contribution to the collective knowledge of humanity: we’ll know there is no Axion in the specific range of energies they are probing.

Whatever the outcome, work is already well underway for the next experiment. As we head down the corridor again, Sandy tells me:

“It’s an underground telescope.”

“A telescope, placed under the ground surface?” I enquire, happy to point at the obvious flaw in the setup.

“Yes! Plus, it has a cute name.”

I might have to come back explore another underground lab then.

©️ Xavier Auclert

Photo Copyright: © DESY / Heiner Müller-Elsner. Assembling the superconducting magnets.