Interesting and fun, but about 35 seconds in he says that you can’t combine photons together, and that’s just plain wrong.
I know that you can’t mix photons together. So you can’t take a blue photon and a green photon and mix them together to get some other photon. That just doesn’t happen.
Except that it does. You have to do this in a nonlinear medium like certain crystals, but it can be done. It’s called sum frequency generation.
Energy is conserved, so in the example given a 2.5 eV (green) photon added to a 3 eV (blue) photon will result in a 5.5 eV photon, which will be ultraviolet. The addition is not what we see with our eyes, since that’s a different process.
A special case of this is where the two photons being added are the same frequency. This is called frequency doubling, and a common (amongst geeks, at least) example of this is a green laser pointer. The source inside of this is actually an infrared laser emitting at 1064 nm, which then passes through a doubling crystal to produce light at twice the frequency, or half the wavelength: 532 nm. (and cheap laser pointers may not filter the IR from the output, which can be a danger)
I’m guessing the answer is something like “phonon interaction”, but how do you keep angular momentum conserved?
That’s one of the reasons the medium is required, and why this wouldn’t happen in free space.