AMA | September 2024

10518.652 - 10541.228 Sean Carroll

So photons directly interact with electrons and protons and so forth, but they don't directly interact with each other. That doesn't sound exactly what your friend is getting at, but that's a true statement that I would trust. Okay, Jeff Davis says, there are a lot of unsolved problems in cosmology, the Hubble tension, the nature of dark matter, formation of supermassive black holes, etc.