Watching laser beams fly through the air makes for dramatic battles in sci-fi films, but they're not so easy to see in real life. In order to observe a laser, or any other light source, photons from it must directly hit your eyes. But since laser photons travel in a tightly-focused beam, all heading in the same direction, you can only see them when the laser hits something that reflects a portion of the light and produces a visible dot.
A tiny proportion of photons scatter off air molecules, but normally these are too faint to see. You can get around this by firing a laser through smoke, giving the photons more molecules to scatter off – but that's not the effect we see in the movies.
"The challenge was to have a movie of light moving directly in air," says Genevieve Gariepy of Heriot-Watt University in Edinburgh, UK. "We wanted to look at light without interacting with it, just looking at it passing by."
To make this work, she and her colleagues constructed a camera sensitive enough to pick up those few scattering photons. It is built from a 32 by 32 grid of detectors that log the time a photon arrives at them with incredible precision, equivalent to snapping around 20 billion frames a second.
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