> Any idea what exactly is going on in that photo?

My take on it, based on the explanation in the video, is that
it's similar to the technique used in high-speed oscilloscopes,
where the display is not of a single event, but an assemblage
of samples of a repetive and predictable waveform

So the light 'moving' across the screen is not made up of just
one pulse of photons, but is a movie of a series of pulses captured
by the camera, their positions being reflected to it by tilting the
mirror. But in effect, as they are all identical, it might as well be
the same photons

Similar to how lower-speed strobes can capture water droplets
in a stream or freeze cartwheels in old cowboy films

This page is more helpful

http://www.tested.com/news/mit-photographs-light-at-one-trillion-frames-per=
-second-video/3278/

"Finally, our algorithm uses this captured data to compose a single 2D
movie of roughly 480 frames, each with an effective exposure time of
1.71 picoseconds"

Photons travel at 300,000,000,000,000,000 pm/s, implying that an
individual photon will have moved about 0.5mm in the exposure time,
which is seriously blurred. But an impressive technical feat nonethless

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