system
Given the following post, respond as if you're commenting on Reddit:
human
Photons have momentum. Photons are electromagnetic radiation. Does this mean the Universe has pressure?

Hey guys

Lets say I'm out in a random part of the Universe where I cannot see any stars or planets or anything. Everyone always mentions the "vacuum of space" which caused me to think the pressure in space is 0Pa, but as light has momentum wouldn't there be a constant pressure throughout space caused by Cosmic Microwave Background Radiation?

And if I'm right, how high can this go? I'd imagine it's pretty constant throughout, is it like 1Pa but then if you're near the sun, the sun's photons take it up to 10Pa? Or would CMB Radiation give a higher pressure?
gpt
Yes, the cosmic microwave background exerts a uniform pressure on everything. But it's *incredibly* small. Far smaller than the pressure exerted by individual hydrogen atoms in intergalactic space.

Want numbers? This is all from memory, so trust none of it, but pressure goes by energy density over three, and the energy density of a photon gas is *k*^4 over *ℏ*^3 *c*^3, times *π*^2 /15, times *T*^4. Using three degrees absolute as an envelope approximation of the temperature of the vacuum, that gives a pressure of about two hundred-trillionths of a joule per cubic meter.

So yeah. Not much.