# Filming light in motion?

Regarding this video http://www.youtube.com/watch?v=Y_9vd4HWlVA, which claims to "film" light in its motion.

Is it not an absolute nonsense?

Even if photons could even be "seen" (meaning, returning light them self), isn't this video look unrealistic?

You can see that the area around the "light" is getting brighter, but that absurd.. light is not a flash light... light does not illuminate the area around it, because it does not "emit" more light to all the directions..

Am I missing something?

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You misunderstood his claims. Because the time frame is so short, they shoot beams of light over and over again, repeating the experiment millions of times in the blink of an eye (or run the experiment for a long time to improve the quality). They statistically average all of the exposures to get the video you see. Each individual exposure might only detect a few photons, and it's possible this research group uses other tools to discard erroneous hits as noise.

They don't "see" the photons in transit. That is also a misunderstanding. It only sees photons that hit the camera. The principle of his femtosecond photography is that the effective exposure is on the same time frame as what it takes for the photons to travel in the environment. That means the motion you see is due to the travel time differences between different photons emitted from the source. All photons you see are basically scattered, the ones you see early in the video were scattered sooner than the ones you see later on, that's all.

You can see that the area around the "light" is getting brighter, but that absurd.. light is not a flash light... light does not illuminate the area around it, because it does not "emit" more light to all the directions..

In order to have a coherent discussion, you also need to understand the specifications of the light source. I revise this if I see new information, but my current understanding is that his light source for the coke-can example is basically a pulsed laser. In other videos, it seems they send out light in all directions, so in that case it's more like an LED pulsed very fast, or they could have used a pulsed laser and spread the beam with a lens.

In order for a light source to give a "pulse" visible in this video, it has to pulse very very fast. For some math, consider that a packet of light is emitted which has a length of $5 cm$. That would mean the source remains "on" for a time of:

$$\Delta t = \frac{5 cm }{ 3.0 \times 10^8 m/s^2} = 0.17 ns$$

This is very fast. A common reference I think about is the fact that computer processors have speeds of $3 GHz$, which gives a clock cycle of $0.3 ns$. There is a limit to how fast technology can do certain tasks, and it is no coincidence that these limits lie somewhere close to the speed of processors.

In short, it's not nonsense, only new.

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It´s not absolute nonsense, but you have to be careful about how it is made. They shoot many short duration laser beams, and photograph them at the same position at time (let us call it $t_i$). If you do that many times you get a picture of a light packet frozen in position (say, the bottom of the bottle). Them by doing the same for each $t_i$ and sequencing all such shots you get a movie. That's doable, all you need is very good time resolution for the camera, which they claim to have.