Optical shock: Scientists have imaged light going faster than itself

A team of researchers at Washington University in St. Louis has taken images of a laser pulse generating an optical Mach cone: the equivalent of a sonic boom, but for light. To make an optical Mach cone, a pulse of light would need to be traveling faster than the waves it’s emitting can propagate forward. But the researchers were able to peel apart the properties of a laser beam, interacting separately with velocity, wavelength, and frequency. They directed the beam through a layered confection of silicone panels, aluminum oxide powder, and dry ice. The source of the light waves was moving faster than the waves themselves as they passed through the layers, leaving behind the optical Mach cone.

Ultrafast cameras have been photographing sweet speed-based optical phenomena like Mach cones for a while, but this is the first time they've been able to catch an optical Mach cone in real time.

Ultrafast cameras have been photographing sweet speed-based optical phenomena like Mach cones for a while, but this is the first time they’ve been able to catch an optical Mach cone in real time. From the (open-access!) report: Liang et al., 2017

To capture the cone itself, the researchers set up CCD cameras next to the cone-generating apparatus. One of the cameras was a streak camera, which exploits the motion of charged particles to create a spatial “pulse profile” that characterizes the light waveform in 3-space over time. Using the streak camera and the CCDs, the researchers captured a 2D sequence of images from three perspectives in a single take. They then spliced the images back together like a CAT scan to make a 3D model of the cone.

Lead author Jinyang Liang hopes that these developments can be pressed into use not just in physics, but in neuroscience. Their imaging setup can capture 100 billion frames a second. With that kind of temporal resolution, researchers could capture neurons firing in real time.

Shock and awesome

Among the other visually rewarding ways we visualize invisible things going at speeds that generate shockwaves are Mach diamonds:


Elon Musk tweeted this image of the first test fire of the Raptor, SpaceX’s shiny new Mars-bound methalox rocket.

The above is a shot of the exhaust plume of SpaceX‘s new methane rocket, the Raptor — the one that could take humans to Mars. Mach diamonds, a visible standing wave in the exhaust plume, are easy to see in the Raptor’s fiery tail. Also called thrust diamonds, these things are a familiar sight for other reasons. They’re often visible behind F-16s, F-22s, and the SR-71 Blackbird, although the Blackbird has to be at quite some altitude for the phenomenon to be crystal-clear.


A jet in supersonic flight over the Mojave. Image: NASA

Shockwaves, it turns out, are pretty easy to photograph if you have the right kind of camera setup. This jet is producing a Mach wave at its nose because the air molecules can’t get out of the way fast enough for the plane to make its way neatly through them — not unlike the physical wave that happens at the bow of a fast-moving ship as it plows through the water. It was photographed with a method called Schlieren photography, which takes high-speed images and then compares the backgrounds, to see where the wave of distortion traveled through the frame.

Kappa Cassiopeiae, whose bow shock stands four light-years away from the star itself

Kappa Cassiopeiae. Image: NASA/JPL

And shockwaves aren’t constrained to places with an atmosphere, either. They also happen at a much, much larger scale. When a body in space gets moving fast enough, it can produce a phenomenon called a bow shock. Kappa Casseiopeiae is an enormous rogue star traveling at 2.5 million miles an hour, which is a quarter of a percent the speed of light. It’s so big, and moving so fast, that its bow shock is twelve light-years across and stands four light-years apart from the star itself. That’s like our sun having a feature that stood as far away as Alpha Centauri. Whoa.

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