The view from one end of a laser going between two mirrors (timestamp 1:37) is a fairly good demonstration of the camera having to wait for light to get to it.
I thought his method of multiplexing the single channel was very smart. I guess it's more common on 2 channel or high end 4 channel scopes to have a dedicated trigger input, which I've checked this one doesn't have. That said, there're digital inputs that could've been used. Presumably from whatever was controlling the laser.
The video is definitely more interesting than 28 fps but it's also not really 2B fps.
It captures two billion pixels per second. Essentially he captures the same scene several times (presumably 921,600 times to form a full 720 picture), watching a single pixel at a time, and composite all the captures together for form frames.
I suppose that for entirely deterministic and repeatable scenes, where you also don't care too much about noise and if you have infinite time on your hands to capture 1ms of footage, then yes you can effectively visualize 2B frames per second! But not capture.
As I understand it, this is sort of simulating what it would be like to capture this, by recreating the laser pulse and capturing different phases of it each time, then assembling them; so what is represented in the final composite is not a single pulse of the laser beam.
Would an upgraded version of this that was actually capable of capturing the progress of a single laser pulse through the smoke be a way of getting around the one-way speed of light limitation [0]? It seems like if you could measure the pulse's propagation in one direction, and the other (as measured by when it scatters of the smoke at various positions in both directions), this seems like it would get around it?
But it's been a while since I read an explanation for why we have the one-way limitation in the first place, so I could be forgetting something.
No, you cannot escape the conclusion of the limitations on measuring the one-way speed of light.
While the video doesn't touch on this explicitly, the discussion of the different path lengths around 25:00 in is about the trigonometric effect of the different distances of the beam from the camera. Needing to worry about that is the same grappling with the limitation on the one-way speed.
Think of it more like "IRL raytracing", where a ray (the beam) is cast and the result for a single pixel from the point of view is captured, and then it is repeated millions of times.
Even if you had a clock and camera for every pixel, the sync is dependent on the path of the signal taken. Even if you sent a signal along every possible route and had a clock for each route for each pixel (a dizzingly large number) it still isn't clear that this would represent a single inertial frame. As I understand it even if you used quantum entanglement for sync, the path of the measurement would still be an issue. I suggest not thinking about this at all, it seems like an effective way to go mad https://arxiv.org/pdf/gr-qc/0202031
E: Do not trust my math under any circumstances but I believe the number of signal paths would be something like 10^873,555? That's a disgustingly large number. This would reveal whether the system is in a single inertial frame (consistency around loops), but it does not automatically imply a single inertial frame. It's easy to forget that the earth, galaxy, etc are also still rotating while this happens.
The problem (ignoring quantum mechanics) is that the sensors all require an EM field to operate in. So assuming that the speed of light was weighted with a vector in space-time, it would be affected everywhere -- including in the measurement apparatus.
If on the other hand one could detect a photon by sending out a different field, maybe a gravitational wave instead... well it might work, but the gravitational wave might be affected in exactly the same way that the EM field is affected.
No, as he explains in the video, this is not a stroboscopic technique, the camera _does_ capture at 2 billion fps. But it is only a single pixel! He actually scans the scene horizontally then vertically and sends a pulse then captures pixel by pixel.
He did a good job on his setup, but I have to think that adding a spinning mirror would have made everything much faster and easier.
He could then capture an entire line quite quickly, and would only need a 1 dimensional janky mirror setup to handle the other axis. And his resolution in the rotating axis is limited only by how quickly he can pulse the laser.
Of course, his janky mirror setup could have been 2 off-the-shelf galvos, but I guess that isn't as much "content".
Yeah, spinning at a constant rate with an encoder for triggering would probably be a bit more consistent. But potentially more of a mechanical headache. And he does need a pretty big mirror due to being limited by the amount of light he can focus on the sensor, I'm not sure that there are galvos available with such a large area (especially for a reasonable price).
The triggering scheme is completely brilliant. One of those cases where not knowing too much made it possible, because someone who does analog debug would never do that (because they would have a 50k$ scope!.
He scans one line at a time with a mirror into a photomultiplier tube which can detect single photon events. This is captured continually at 2MSample/s (2 billion times per second: 2B FPS) with an oscilloscope and a clever hack.
The laser is actually pulsing at 30KHz, and the oscilloscope capture is synchronized to the laser pulse.
So we consider each 30KHz pulse a single event in a single pixel (even though the mirror is rotating continuously). So he runs the experiment 30,000 times per second, each one recording a single pixel at 2B FPS for a few microseconds. Each pixel-sized video is then tiled into a cohesive image
This video has brough warm and fuzzy memories from my other life. When I was a scientist back in USSR my research subject required measuring ridiculously low amounts of light and I used photomultiplier tube in photon counting mode for that. I needed current preamp that can amplify nanosecond long pulses and have concocted one out of arsenide-gallium logic elements pushed to work in a linear mode. The tube was cooled by Peltier elements and data fed to a remote Soviet relative of Wang computer [0].
2. Increase the precision of the master clock. There's some time smearing along the beam. It's not that hard to make clocks with nanosecond resolution, and picosecond resolution is possible, although it's a bit of a project.
3. As others have said, time-averaging multiple runs would reduce the background noise.
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[ 1.1 ms ] story [ 59.8 ms ] threadIt captures two billion pixels per second. Essentially he captures the same scene several times (presumably 921,600 times to form a full 720 picture), watching a single pixel at a time, and composite all the captures together for form frames.
I suppose that for entirely deterministic and repeatable scenes, where you also don't care too much about noise and if you have infinite time on your hands to capture 1ms of footage, then yes you can effectively visualize 2B frames per second! But not capture.
Would an upgraded version of this that was actually capable of capturing the progress of a single laser pulse through the smoke be a way of getting around the one-way speed of light limitation [0]? It seems like if you could measure the pulse's propagation in one direction, and the other (as measured by when it scatters of the smoke at various positions in both directions), this seems like it would get around it?
But it's been a while since I read an explanation for why we have the one-way limitation in the first place, so I could be forgetting something.
[0] https://en.wikipedia.org/wiki/One-way_speed_of_light
While the video doesn't touch on this explicitly, the discussion of the different path lengths around 25:00 in is about the trigonometric effect of the different distances of the beam from the camera. Needing to worry about that is the same grappling with the limitation on the one-way speed.
Even if you had a clock and camera for every pixel, the sync is dependent on the path of the signal taken. Even if you sent a signal along every possible route and had a clock for each route for each pixel (a dizzingly large number) it still isn't clear that this would represent a single inertial frame. As I understand it even if you used quantum entanglement for sync, the path of the measurement would still be an issue. I suggest not thinking about this at all, it seems like an effective way to go mad https://arxiv.org/pdf/gr-qc/0202031
E: Do not trust my math under any circumstances but I believe the number of signal paths would be something like 10^873,555? That's a disgustingly large number. This would reveal whether the system is in a single inertial frame (consistency around loops), but it does not automatically imply a single inertial frame. It's easy to forget that the earth, galaxy, etc are also still rotating while this happens.
If on the other hand one could detect a photon by sending out a different field, maybe a gravitational wave instead... well it might work, but the gravitational wave might be affected in exactly the same way that the EM field is affected.
He could then capture an entire line quite quickly, and would only need a 1 dimensional janky mirror setup to handle the other axis. And his resolution in the rotating axis is limited only by how quickly he can pulse the laser.
Of course, his janky mirror setup could have been 2 off-the-shelf galvos, but I guess that isn't as much "content".
He mentions this as the inspiration in his previous video (https://youtu.be/IaXdSGkh8Ww).
He scans one line at a time with a mirror into a photomultiplier tube which can detect single photon events. This is captured continually at 2MSample/s (2 billion times per second: 2B FPS) with an oscilloscope and a clever hack.
The laser is actually pulsing at 30KHz, and the oscilloscope capture is synchronized to the laser pulse.
So we consider each 30KHz pulse a single event in a single pixel (even though the mirror is rotating continuously). So he runs the experiment 30,000 times per second, each one recording a single pixel at 2B FPS for a few microseconds. Each pixel-sized video is then tiled into a cohesive image
OMG this was back in 1979-1981.
0. - https://ru.wikipedia.org/wiki/%D0%AD%D0%BB%D0%B5%D0%BA%D1%82...
Some possible improvements.
1. Replace the big heavy mirror with a pair of laser galvos. They're literally designed for this and will be much faster and more precise.
Example:
https://miyalaser.com/products/miya-40k-high-performance-las...
2. Increase the precision of the master clock. There's some time smearing along the beam. It's not that hard to make clocks with nanosecond resolution, and picosecond resolution is possible, although it's a bit of a project.
3. As others have said, time-averaging multiple runs would reduce the background noise.
light moves AT the speed of causality in that frame of time
causality appears to have a MAXIMUM limit in this universe in an "empty" void
but every time you hear a story of how they "slowed down light" what they actually did is make causality more complex in a dense medium, so slower