Scientists Capture a “Sonic Boom” of Light

A new, ultra-fast camera recorded the phenomenon for the first time

Light Cone
Jinyang Liang and Lihong V. Wang

Most people are familiar with sonic booms, even if they don’t know exactly how they work. NASA explains that air reacts like a fluid to objects that are moving faster than the speed of sound. This speedy object rapidly forces surrounding air molecules together, causing a wave-like change in air pressure that spreads out in a cone called a Mach cone, like the wake of a boat. As the shock wave passes over an observer on the ground, the change in air pressure produces the sonic boom.

Previous research suggested that light could also produce a similar cone-shaped wakes, called a "photonic Mach cone," reports Charles Q. Choi at LiveScience. But they had no way to test the idea. Now, researchers at Washington University in St. Louis have developed an ultrafast camera that can actually catch the light boom in action.

Choi reports that optical engineer Jinyang Liang and his colleagues fired a green laser through a tunnel filled with smoke from dry ice. The interior of the tunnel was surrounded by plates made of silicone rubber and aluminum oxide powder. The idea was that, since light travels at different rates through different materials, the plates would slow down the laser light, which leave a cone-shaped wake of light.

Though clever, this setup wasn't the star of the study—it was the “streak” camera that the researchers developed to capture the event. Choi reports that the photography technique, called lossless-encoding compressed ultrafast photography (LLE-CUP), can capture 100 billion frames per second in a single exposure, allowing the researchers to capture ultrafast events. The camera worked, capturing images of the light cone created by the laser for the first time. The results appear in the journal Science Advances.

“Our camera is different from a common camera where you just take a snapshot and record one image: our camera works by first capturing all the images of a dynamic event into one snapshot. And then we reconstruct them, one by one,” Liang tells Leah Crane at New Scientist.

This new technology could open the door to some revolutionary new science. “Our camera is fast enough to watch neurons fire and image live traffic in the brain,” Liang tells Choi. “We hope we can use our system to study neural networks to understand how the brain works.”

In fact, LLE-CUP may be too powerful to watch neurons. “I think our camera is probably too fast,” Liang tells Kastalia Medrano at Inverse. “So if we want to do that, we can modify it to slow it down. But now we have the image modality that’s miles ahead, so if we want to reduce speed we can do that.”

The technology, Liang tells Crane, can be used with existing cameras, microscopes and telescopes. Not only can it look at the functioning of things like neurons and cancer cells, Crane reports, it could also be used to examine changes in light in objects like supernova.

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