How a zeptojoule sensor helps hunt for dark matter

HostMost of the stuff in our universe is missing. We can see stars and planets, but there's this huge amount of invisible weight out there that we call dark matter. We know it's there because it pulls on everything else, but we have never actually touched it or seen it directly. It's like trying to find a ghost in a dark room just by feeling the floor shake.

HostPeople have built massive tanks deep underground to catch this stuff, but so far, we have come up empty. Now, there's a new idea that involves a tool so sensitive it can feel a single zeptojoule. What's a zeptojoule, and why is that the key to finding this hidden part of the world?

GuestTo get a handle on a zeptojoule, you have to think really, really small. Imagine you have a single grain of sand. Now, imagine lifting that grain of sand just a tiny bit, maybe the width of a human hair. The energy you used to do that's huge compared to a zeptojoule. In fact, a zeptojoule is about the amount of energy in one single light wave, or a single photon. It's a billionth of a billionth of a thousandth of a joule. It's such a tiny amount of energy that for a long time, we didn't even have tools that could feel it. But we think some types of dark matter might be very light. If they're light, they won't hit our sensors like a bowling ball. They'll hit them more like a single speck of dust. If your sensor isn't sensitive enough to feel that tiny zeptojoule tap, the dark matter just sails right through without you ever knowing it was there.

HostSo we have been looking for bowling balls when we should've been looking for dust? That seems like a big jump. Is there a reason we think dark matter is that light, or are we just looking there because we ran out of other places to hide?

GuestWell, it's a bit of both. For a long time, the best guess was that dark matter was made of heavy particles. We built these huge, heavy detectors to find them. But after decades of looking, we found nothing. That has pushed a lot of people to look at the other end of the scale. There are these theoretical particles that are extremely light, and if they exist, they would be everywhere. They would be flowing through you and me right now like a ghost wind. The problem is that when one of these light particles hits a normal atom, it barely moves it. It's like a mosquito flying into a freight train. The train doesn't even notice. To find these light particles, we need a sensor that's more like a feather hanging on a silk thread. Something that reacts to the tiniest possible nudge.

HostBut if the sensor is that sensitive, how do you keep it from going off all the time? I mean, a stray bit of heat or even a tiny vibration from a truck driving by miles away must feel like an earthquake to something that can sense a zeptojoule.

GuestThat's the biggest hurdle. To make this work, you have to get things incredibly cold. We're talking almost as cold as it's possible for anything to be. We use these tiny wires made of special metals that let electricity flow through them with zero resistance. We call this superconductivity. When the wire is that cold, the electricity just glides along without any friction. But the wire is balanced on a knife edge. If even a tiny bit of energy hits it, like one zeptojoule from a dark matter particle, it warms the wire up just enough to break that perfect flow. Suddenly, there's friction. The electricity hits a wall, and that creates a little spike in the signal. That spike is our bell ringing. It tells us something just hit the wire.

HostI'm struggling with the idea that something so light it only carries a zeptojoule of energy could actually be the missing weight of the whole universe. If it's that weak, how does it hold entire galaxies together?

GuestIt's all about the numbers. A single snowflake doesn't weigh much, but an avalanche can bury a house. If these particles are as light as we think, there must be a staggering amount of them. There would be billions of them in every cubic inch of space. Each one only gives a tiny, tiny pull of gravity, but when you add them all up across the whole sky, they outweigh all the stars and gas we can see by five to one. The zeptojoule sensor is our way of finally hearing the sound of a single snowflake in that avalanche.

HostBut wait, if these particles are everywhere and hitting us all the time, why has it taken us this long to build a wire that can feel them? It feels like we should've noticed this ghost wind a long time ago if it's that thick.

GuestIt's because dark matter is very shy. It doesn't like to talk to normal matter. Most of the time, it passes right through the earth without hitting a single thing. It's like a person walking through a forest who never touches a leaf or a branch. The only way we can catch it's by making the forest incredibly dense and the leaves incredibly sensitive. These tiny superconducting wires are the most sensitive leaves we have ever made. We lay them out in a grid, almost like a spider web, and wait. We're hoping that out of the trillions of particles passing through, one of them will finally snag a thread and give it a tug.

HostSo it's a waiting game. But even if we see a tiny spike on a screen, how do we know it's dark matter and not just some random glitch in the fridge or a bit of background radiation?

GuestThat's where the hard work happens. You have to shield the whole thing behind feet of lead and copper. You put it deep underground to block out cosmic rays. Once you have blocked out everything you can possibly think of, you look for a specific pattern. For example, as the earth moves through the galaxy, we should be headed into the wind of dark matter at certain times of the year and away from it at others. If our zeptojoule spikes go up and down with the seasons, then we know we're not just looking at noise. We're looking at the map of the universe itself.

HostThe idea that the weight of the sky might be hidden in a tap so small we can barely name it's wild.

GuestWe're finally building tools that can feel the heartbeat of the vacuum.

HostThese tiny wires might be the only way to touch the invisible wind that holds the stars in place.