Visualizing Photons in Motion at a Trillion FPS | Hacker News Reader

Visualizing Photons in Motion at a Trillion FPS

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ColinWright, HN's own biological duplicate content detection bot ;).

(disclaimer: I find what you do very valuable, please keep doing that)

One of the things that are unrealistic about "bullet time" as portrayed in games and movies, is that the world doesn't dim as time slows down. Slow the rest of the universe down, and the amount of light energy entering your pupil per unit time slows down as well.

One of the things that came up in the criticisms of Khan Academy's teaching of slope/rate is that lots of people don't internalize a conception of sensory intensities as rates. To some people out there, things like speed are feelings of intensity divorced from the notion of X things happening in a second.

This is probably especially true for things like brightness.

The eye normalizes for orders-of-magnitude difference in luminance. Presumably, it makes for a good cinematographic choice to avoid the effect, for the same reason it would be grating to be hit with a lens flare every time a shot tracks from indoors to outdoors.

On the other hand, bullet-time would, and should, be red-shifted, which is a whole 'nother thing.

Yes, but for certain extreme instances of bullet time indoors, I suspect the difference in intensity would be significant to the action. It would be like navigating a room at night with just one dim red LED providing light. Or else, people with that superpower would have to set off a flash-bang grenade when using their power.

Another interesting thought: bullet-time would seem colder. Depending on how things were supposed to be working (let's say your whole body was put into a different relativistic reference frame from everything around you), this would mean that things like air would become liquid/solid and impossible to move through.

Interestingly, this would mean that you could have a civilization of Speedsters living comfortably inside a star.

Another interesting thought: bullet-time would seem colder.

Yes, but then you also have friction and the required speedup of energy consumption and heat generation. Drag and friction increases with the square of velocity, so going ludicrously fast is going to generate ludicrous heat.

Also recall that there's an absolute zero. Basically, there's limited downside and much greater potential upside.

Interestingly, this would mean that you could have a civilization of Speedsters living comfortably inside a star.

Or on a Neutron Star, as in Robert Forward's Starquake. (With the speedup coming from the much greater speed of strong force mediated nuclear reactions.)

It depends on your "bullet-time" model whether it's red-shifted or not. Much "bullet-time" is explicitly just a special effect, i.e., the idea is that the action is still occurring in real time but we're having it slowed down so we can see it, understand it, and enjoy it. In that case we are simply shooting the scene with a high-FPS camera. "Technically" we should indeed get a dimmer scene, just as real high-speed cameras do, but it's not that unrealistic that it's compensated for, again, just as in real life when the Mythbusters go to a high-speed camera shot they've already compensated for that.

If you have a "real" time slow-down, exactly what happens depends on your model, especially where the photons cross whereever the effect is occurring. It is true that one model could be that a photon crossing into the effect keeps the same "real" Hertz, in which case you'd see it red-shifted. However, since in the end the entire idea of such sharply-localized time variance without a corresponding gravity field is complete nonsense, the truth is that no matter how you slice it the result is gibberish, so it's just as sensible to let the field "keep" the old photon's frequency too.

However, I'd point out that "real" time slowing in the movies is fairly unusual. Mostly it's the first kind. There's a number of sci-fi books with a variety of "real" time slowing effects, though.

Even cameras which vary speed during an unbroken shot automatically compensate so the exposure stays the same, but doing so has other interesting effects, depending how it's done:

a) Compensating with shutter so that the integration time is constant means the quality of motion blur changes during the ramp. For example a shot that starts at 24fps and ramps to 120fps keeping a shutter time of 1/240th would start out with really crisp-looking "skinny shutter" motion, and you might see flickering and strobing if there's much camera motion. The end of the ramp would have normal looking amounts of motion blur. If there are CRTs or fluorescent lights in shot they'd flicker at different rates throughout the ramp. It can be a dead giveaway that something's about to happen if a shot starts with really tight motion blur like that.

b) Compensating with lens aperture is much less common because the depth of field changes, but starting with everything in focus, the focus becomes much tighter with the background way out of focus at high speed. Reportedly, mechanical problems tend to manifest as the aperture overshooting its mark or bouncing back leading to brightness variations at the end of the ramp.

c) Compensating with ISO/gain is possible these days, which would avoid the above problems but would end up with a noisier image for the high-speed end.

Recent bullet time shots are either fully CG, shot with an array of cameras, or cheated by hanging everything on wires, but it's interesting to check if any of these things from the days of 16mm skate films with the classic hang-time dips are re-created :)

I believe for the action scenes in 300 they recorded the whole lot at 120fps and selectively dropped or merged frames to get back to 24fps

>bullet-time would, and should, be red-shifted //

How so? As I understood it bullet-time, eg in The Matrix, is an internal perception of time slowing; they're not changing the flow of time in the Matrix just choosing to perceive the computer generated scenes at a modified rate. The external "physics" is identical: Just as in a video player the images are fixed by the physics whether their presentation time corresponds to normal time (standard playback) or their presentation time is slowed (slow motion).

Perhaps we better not get in to a deep discussion of made-up physics?!?

Perhaps we better not get in to a deep discussion of made-up physics?!?

Perhaps we'd better. Media physics misconceptions can produce hilariously large errors of scale.

I generally interpret these scenes as the individual speeding themselves up to an insane level and everything else 'appears' to slow down in comparison.

Maybe this is why everyone in the Matrix is wearing black sunglasses 24/7

This photography isn't really about "photons" or any amount of energy measurements of radiation emission at the subatomic scale. This is more about capturing macro-scale events at very precise time scales and exceedingly high shutter speeds, and detecting how quickly an arbitrary amount of light will traverse the components of a still life.

There's also a lot of distortion introduced into the final composited images, enough that they cease to look like photographs of normal physical objects, and start to look more like computer generated images of ray traced 3D models.

The camera filming itself in perpendicular mirror? Would we see an appreciable delay for each diminishing image, or what?

Does this have any practical applications, or is it just a (very cool!) art project?

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