Good explanation. One detail though: it is one pixel at a time, not one line at a time. Basically does the whole sequence for one pixel, adjusts mirror to next one, and does it again. The explanation is around the 8 minutes mark.

Just want to make it clear that in any one instant, only one pixel is being recorded. The mirror moves continuously across a horizontal sweep and a certain arc of the mirror's sweep is localized to a pixel in the video encoding sequence. A new laser pulse is triggered when one pixel of arc has been swept, recording a whole new complete mirror bounce sequence for each pixel sequentially. He has an additional video explaining the timing / triggering / synchronization circuit in more depth: https://youtu.be/WLJuC0q84IQ

The author explained that he originally attempted to pulse the laser at 30 KHz, but for the actual experiment used a slower rate of 3 KHz. The rate at which the digital data can be read out from the oscilloscope to the computer seems to be the main bottleneck limiting the throughput of the system.

Overall, recording one frame took approximately an hour.

Can we not find why the strange behavior of a double split experiment occurs using this setup?

Thanks for the explanation. Honestly, your explanation is better than the entire video. - I watched it in full and got really confused. I completely missed the part where he said the light is pulsing at 30kHZ and was really puzzled at how he is able to move the mirror so fast to cover the entire scene.

Yup, this technique also allows oscilloscope capture signal with frequency higher than their Nyquyst bandwidth.

The downside is it only works with repeative signal.

I believe this technique is known as "equivalent-time sampling".

At the point where the light is getting reflected on the mirror, it is unfocused - those galvos look too small. But a pair of larger mirrors in the same arrangement could work.

The slowmo guys did a video about a similar setup at CalTech https://youtu.be/7Ys_yKGNFRQ

It’s super cool that AlphaPhoenix is able to get comparable results on his garage. These academic versions use huge lab bench optic setups. They wind up with technically higher quality results, but AlphaPhonix’s video is more compelling.

Does anyone have a $50,000 scope they could just give to this dude? He seems like he would make great use of it.

Hmm, it's a clever hack, but they would use an oscilloscope with an "External trigger" input, like most of the older Rigols. That would let you use the full sample rate without needing to trigger from CH2

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.

Frankly it's uncommon to not have a trigger input. I'm not sure I've ever seen a DSO in person without a trigger in.

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.

>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.

It is not different phases, but it is a composite! On his second channel he describes the process[0]. Basically, it's a photomultiplier tube (PMT) attached to a precise motion control rig and a 2B sample/second oscilloscope. So he ends up capturing the actual signal from the PMT over that timespan at a resolution of 2B samples/s, and then repeating the experiment for the next pixel over. Then after some DSP and mosaicing, you get the video.

>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?

The point here isn't to measure the speed of light, and my general response when someone asks "can I get around physics with this trick" by answer is no. But I'd be lying if I said I totally understood your question.

[0] https://www.youtube.com/watch?v=-KOFbvW2A-o

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.

It would look something like this[1] except with slower visual propagation.

Note that this camera (like any camera) cannot observe photons as they pass through the slits -- it can only record photons once they've bounced off the main path. Thus you will never record the interference-causing photons mid-flight, and you'll get a standard interference pattern.

[1]: https://www.researchgate.net/figure/The-apparatus-used-in-th...

AlphaPhoenix mentions in the description that he wants to try and image an interference pattern, and it seems possible.

Though it wouldn't really be showing you the quantum effect; that's only proven with individual photons at a time. This technique sends a "big" pulse of light relying on some of it being diffusely reflected to the camera at each oscilloscope timestep.

Truly sending individual photons and measuring them is likely impractical as you'd have to wait for a huge time collecting data for each pixel, just hoping the photons happens to bounce directly into the photomultiplier tube.

No. It's strictly because of travel time from the "observed spot" to the camera. He explains it in the back half of the video, but for his setup (and every camera everywhere, including your eyes!), it doesn't matter just how long light took to get to the point where it scatters, but also how long it takes for it to get from that point to the detector. If the camera is placed far enough to the side, that time delay is close enough to identical for all samples, but if the camera is in line with the laser, there is a substantial time difference for light scattering close to the camera to light scattering near the far wall.

He actually explains it. It's due to the round trip time increasing from further away water droplets when filmed at an oblique angle. Light hitting a droplet 3m away has a 3x times longer round trip than light hitting a droplet 1m away.

Yes, the laser was fired at 3 KHz, while the mirrors were slowly scanning across the room.

For each laser pulse, one microsecond of the received signal was digitized with the sample rate of 2 billion samples per second, producing a vector of light intensity indexed by time.

A large number of vectors were stored, each tagged by the pixel XY coordinates which were read out from the mirror position encoders. In post-processing, this accumulated 3D block of numbers was sliced time-wise into 2D frames, making the sequence of frames for the clip.

It's pretty clear he had a computer repeat the experiment that many times in reasonably rapid succession rather than doing it "himself", but yes...

Shere are so many levels this could be answered at.

All light in a narrow cone extending from the camera gets recorded to one pixel, entirely independently from other pixels. There's no reason this would be blurry. Blur is an artifact created by lenses when multiple pixels interact.

There is a lens in the apparatus, which is used to project the image from the mirror onto the pinhole, but it is configured so the plane of the laser is in focus at the pinhole.

What I don't understand is how the projection remains in focus as the mirror sweeps to the side, but perhaps the change in focus is so small.

It’s in focus because only a small angular area from the scene contributes to the light that reaches the sensor. You get blur when sensor elements (individual pixels) receive light from too wide of an area.

Who detonated 2073600 bombs?

Scanning a single pixel over an image? How does that work with an explosion? The laser pointer is reproducible

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).

I think a spinning mirror would make it a lot harder. He's only moving the mirror after the "animation" finishes. So it's capture video, step by 1 pixel, capture video, step by 1 pixel, capture video, etc... He's replaying the scene ~1 million times, for 1 million unique single pixel 2 billion fps videos.

Are you suggesting it would be easier if the mirror spun at 2 billion revolutions per second?

But I think he does capture an entire line quite quickly. As I understood it, he “scans” a line of pixels in seconds.