There is a fiction I've read years ago that mentioned satellites becoming makeshift weapons by overheating exposed objects (think reactors, gas trucks, oil refineries) by acting as a solar furnace [1] via mirrors.
Not sure/don't recall how it deals with practical issues such as clouds and distance/intensity, but good enough for a story I guess.
I thought this was interesting because it doesn’t really seem like an applicable top level claim, surely this is referring to a specific furnace, not all solar furnaces?
Then this got me thinking if there is some universal upper bound constraint to these temperatures. E.g. if I recall a telescope can’t make a source object brighter than it actually is, and this just seems like a thermal telescope, so I wonder if that principle applies here or not.
It applies, but also in practice the maximum temperature is lower than the theoretical upper bound.
> "In general, there's no way to "overlay" light beams on each other, because the whole system has to be reversible".
Behold, my two torches.
Edit: and also don't forget that the Sun is not a point source and has an appreciable angular size, further making it impossible to focus it with a reasonably-sized lens or mirror.
There are some non-linear effects that _are_ dependent on the wavelength, but not important to matter here.
Masers are not useful as powerful transmitters, but only as amplifiers with a very low noise or as generators with a very stable frequency.