Stanford scientists invent a dark matter camera
This story originally appeared on popular science.
On the outskirts of Chicago, about 34 miles west of Lake Michigan, there is a hole in the ground that goes down about 330 feet. Long ago, scientists had the well drilled for a particle physics experiment that has long since disappeared from this world. Now, in a few years, they will reuse the well for a new project with the mystical name MAGIS-100.
When MAGIS-100 is complete, physicists plan to use it to detect hidden treasures: dark matter, the mysterious invisible Something this is believed to constitute a large part of the universe; and gravitational waves, ripples in spacetime caused by cosmic shocks like black hole collisions. They hope to find traces of these elusive phenomena by observing the quantum signatures they leave behind on raindrop-sized clouds of strontium atoms.
But actually observing these atoms is trickier than you might think. To perform similar experiments, physicists have so far relied on cameras comparable to those of a smartphone. And while the technology might work well for a sunset or a photo of tasty food, it limits what physicists can see at the atomic level.
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Dark matter camera developed by Stanford makes the most of mirrors
Fortunately, some physicists may have an upgrade. A research team from different groups in Stanford, California, has created a unique camera that sits on top of a dome of mirrors. The extra reflections help them see what light is entering the lens and determine the angle a certain area of light is coming from. This, they hope, will allow them to peer into a cloud of atoms like never before.
Your cell phone or DSLR camera doesn’t care where the light is coming from: it captures the intensity of the photons and the colors reflected from the wavelengths, little more. For taking pictures of your family, city skyline, or the Grand Canyon, that’s fine. But for studying atoms, that leaves a bit to be desired. “You shed a lot of light,” says Murtaza Safdari, a graduate student in physics at Stanford University and one of the creators.
Physicists want to preserve this information because it allows them to paint a more complex 3D picture of the object (or objects) they are studying. And when it comes to the finicky analyzes that physicists love to do, the more information they can get at once, faster and better.
One way to get this information is to set up multiple cameras, allowing them to take pictures from multiple angles and stitch them together for a more detailed view. It can work very well with, say, five cameras. But some physics experiments require measurements so precise that even a thousand cameras might not be enough.
Thus, in a Stanford basement, researchers decided to create their own system to circumvent this problem. “Our thinking…was basically, can we try to completely capture as much information as possible, and can we retain directional information?” Safdari said.
Their resulting prototype, made from off-the-shelf, 3D-printed components, resembles a shallow dome, dotted with a series of small mirror-like dots inside. The pattern appears to form a fun optical illusion of concentric circles, but it’s carefully calculated to maximize the light hitting the camera.
How the Dark Matter Camera Works
For the MAGIS-100 project, the subject of the shot – the cloud of strontium atoms – would be in the dome. A brief flash of light from an external laser beam would then scatter over the mirror points and through the cloud at a myriad of angles. The lens would pick up the resulting reflections, how they interacted with the molecules, and where they bounced off.
Then, from this information, machine learning algorithms can reconstruct the three-dimensional structure of the cloud. Currently, this rebuild takes several seconds; in an ideal world, it would take a few milliseconds or even less. But, like the algorithms used to train self-driving cars to adapt to the surrounding world, the researchers believe the performance of their computer codes will improve.
Although the creators haven’t had time to test the camera on atoms yet, they have tried it out by scanning sample parts of the appropriate size: 3D-printed letter-shaped parts the size of strontium droplets they intend to use. The photo they took was so clear that they were able to find flaws where the small letters D, O, and E differed from their intended design.
For atomic experiments like MAGIS-100, this equipment is distinct from any other on the market. “The state of the art is just cameras, commercial cameras, and lenses,” says Ariel Schwartzman, a physicist at SLAC National Accelerator Laboratory in California and co-creator of the Stanford setup. They scoured photo equipment catalogs to find something that could see into a cloud of atoms from multiple angles at once. “Nothing was available,” says Schwartzman.
What’s next for the dark matter camera
To complicate matters, many experiments require atoms to rest at extremely cold temperatures, barely above absolute zero. This means they need low light conditions – shining a bright light source too long could heat them up too quickly. Setting a longer exposure time on a camera can help, but it also means sacrificing some necessary detail and information in the final image. “You allow the cloud of atoms to diffuse,” says Sanha Cheong, a graduate student in physics at Stanford University and a member of the camera construction team. The mirror dome, on the other hand, aims to use only a brief laser flash with an exposure of a few microseconds.
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The next challenge for the creators is to place the camera in MAGIS-100, which will require a lot of tinkering to fit the camera into a much taller tree and into the void. But physicists are hopeful: a camera like this could do much more than detect dark effects around atoms. Its designers plan to use it for everything from tracking particles in plasma to measuring quality control of small parts in the factory.
“Being able to capture so much light and information in a single shot with the shortest possible exposure opens up new doors,” Cheong says.