This video by AlphaPhoenix is absolutely incredible! I do research in (nonimaging) optics and I am used to thinking about the propagation of light. Still, there is something amazing about seeing a real recording of the propagation of a real beam of light. I also love the fact that you see artefacts due to how long light from different parts of the scene takes to reach the camera
Thermophotovoltaics is really cool. It is an old idea but recently several groups (including the group behind fourth power) have shown much better experimental performance than before, towards the level where this is starting to look like a real solid-state heat engine.
The idea is to use a photovoltaic cell (“solar cell”) to convert thermal radiation to electricity. A regular solar cell has limited efficiency because the sun has a wide spectrum and a single material is not efficient across the whole spectrum. With thermophotovoltaics, the hot surface is so close to the cell that you just reflect the “bad” photons back to the hot surface to recycle them instead of losing their energy.
In theory, a more efficient alternative to a traditional solar cell is to use the sunlight to heat a surface to ultra-high temperatures and then run a thermophotovoltaic cell on that hot surface, but this is easier said than done.
As an outsider I do think it looks like the competitor Antora Energy has a simpler approach: instead of pumping the heat using high-temperature liquid (with lots of moving parts), they just use thermal radiation to transfer the heat inside their battery.
The reason we get a lot of light from the sun is not that the sun is particularily "bright" (high radiance) compared to other stars, it is because the the sun has an absolutely huge apparent size in the sky compared to all the other bright objects we can see.
Let's say you go to one of the illuminated areas that paid for reflectorbital-light and look up. What would you see? You would see a tiny bright spot flying past, with an angular size of about 10^-10 steradians [1].
This tiny spot has the same "brightness" (radiance) as the sun, because a mirror preserves radiance. However the mirror looks about 10 000 times smaller than the sun from your perspective (the sun has an angular size of about 10^-5 steradians). This means that the satellite would only give you 0.01% of the light compared to the real sun.
If you could somehow take 10k satellites and use them to illuminate the same spot, you could technically get it to resemble real sunlight. But imagine what this would look like: These satellite would need to be many enough / huge enough to cover a very significant portion of our sky, on the order of the apparent size our actual sun. They would be spread out in a sun-synchronous orbit, so they would be visible at dusk with this size, from all points on the earth. Would we really want that?
The founder has been thinking about using mirrors to collimate the sunlight to get around this problem, but it won't work. The collimator design he showed in a 2022 article [2] would decrease the focal spot from a 5km diameter to some smaller diameter as intended, but it would do so by throwing away light, not by increasing the brightness in this smaller spot. This is given by conservation of ètendue, one of the fundamental laws of nonimaging optics (where I do research).
[1] They are planning a 100sqm mirror at 600km altitude, which gives a solid angle of (100 m^2)/(600e3 m)^2
> You make a big ring (roughly of the same area of the spot you are trying to make on the ground)
Unfortunately, this is not how the size of the spot on the ground is decided. Sunlight, even if reflected by a perfectly shaped mirror, spreads by approx 1 meter every hundred meters. At the "edge of space" at 100km, your spot already has a 1km diameter, in reality with a higher orbit and imperfect mirror & tracking it will be much larger. The size of your (ideal) mirror decides the brightness of the spot, not the size of the spot.
Liquid mirrors in space seem like a cool concept though!