Sunday, October 02, 2016

A video of light itself



Photography is all about the light. Without light an image cannot be captured. And it is often the quality of the light that makes a photo, in the aesthetic as well as the functional sense.

Now, researchers in Spain and at MIT have managed to image light itself. Which means that we can see light in motion. No, not the movement of other things in light. But the process of light moving.

For example, we can watch a video of a mirror forming a reflection. That's exactly what this video shows. Science-fiction has once again caught up with the world we live in. This is more than a bit mind boggling, so I hope to explain.

The process is documented on the dedicated website "Femto-Photography: Capturing and Visualizing the Propagation of Light". Let me start with the obvious.

Light travels awfully quickly. In fact, its speed represents the upper bound on how fast anything in our universe can travel. You will be familiar with this concept from countless SF films, where this idea is mentioned only so it can immediately be circumvented.

In order to measure the passage of something through space, a fast frame rate is required. Conventional film runs at 24 frames per second, meaning that an image is captured in one-twenty-fourth of a second. At this slow rate, the image flickers, but our eye "smooths out" the sequence to give us a characteristic aesthetic impression that signifies "film".

Digital cameras routinely run up to 60 frames a second. High-speed cameras capture video at 250 frames per second or higher. Velten et al. are using an exposure time of 1.85 picoseconds per frame. "Pico" is a metric (SI) prefix that you are likely not familiar with. So, a small diversion.

You will know that "milli" means one-thousandth or 10-3. And "micro" means one-millionth or 10-6. Next smallest is "nano", one-billionth or 10-9. After this, words start to lose their meaning. The prefix "pico" means one-trillionth or 10-12. And "femto" is one-quadrillionth or 10-15. Each unit is a thousand times smaller than the one before.

This team is using an effective exposure time of 1.85 picoseconds. Which means that their capture rate is half a trillion frames per second. In this regime, light travels approximately half a millimetre between each frame.

Since there are no shutters capable of this extreme frame rate, a one-dimensional imager is swept quickly (!) through the second dimension, to form a 2D cross-section. Because this does not give enough data for a reasonable pixel resolution, the same process is repeated exactly in synchrony many times, in order to build up an image. The mind boggles.

Since this "camera" is moving quite close to the speed of light itself, time-space distortion results as a relativistic effect. A process of "time-unwarping" is required to translate from the camera's space-time coordinate systems to our own as viewers. Einstein comes in handy yet again.

That much I can understand, but the next fact is odder still. The faster we take pictures, the less time light is given to reach our sensor. Every photographer knows this relationship. At the insane frame rate under consideration here, we run up against the limitations of quantum physics.

If a scene is lit with a 100W bulb, any single exposure captures only a single photon of light. That is, an elementary quantum component. One of them. This is not enough to build an image, so some sort of statistical process beyond my understanding is used to integrate many passes across the same scene.

This is all an insane technical achievement that results in some beautiful imagery. I have watched that video over and over. We all live in the future.

A few words about reporting on science
I don't normally publish anything on science on this blog, because I don't feel qualified. Even though I have a degree in Theoretical Physics I have never practised science professionally. I am painfully aware of how poorly science, especially physics, is normally reported in the media.

This is because the language of this work is mathematics. Any attempt to convey meaning in a natural language like English inevitably falls afoul of translation issues. And often, the report conveys little more than the author's own wishes.

The myriad reports on teleportation, faster-than-light travel, and various psychic phenomena -- all apparently corroborated by "scientists" -- attest to this problem. The internet (and my Facebook stream) is full of such misleading articles, reported by those who wouldn't understand a metaphor if it hit them in the face.

We don't need such fantasies to realise that the world is already predicated on science-fiction. We live in a time of wonders.

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