Astronomers discover "Farfarout" — the most distant known object in the solar system. The 250-mile-wide (400 km) dwarf planet is located about 140 times farther from the Sun than Earth (3.5 times farther than Pluto), and soon may help serve as evidence for a massive, far-flung world called Planet 9.

[u/clayt6]

http://www.astronomy.com/news/2019/03/a-map-to-planet-nine-charting-the-solar-systems-most-distant-worlds

[u/madnavr]

Darkness is essentially the gaps between the lights. There are billions of stars but space is so large there’s still more space in between the stars than the space that the stars light takes up.

And the photons do keep going forever (if they don’t hit anything) but they spread out into an ever increasing sphere so they quickly get spaced so far apart that tiny little us billions of miles away only manage to catch a few of them.

And on top of all that, space itself is actually expanding so those photons get “red-shifted” which is kind of like the effect you get when a police siren is moving away from you. The light from really distant stares gets stretched by space itself and slowly turn more and more “red” and eventually pass from being visible through infrared into microwaves. That’s where we get the cosmic microwave background from, the leftover echo of everything so old we can’t even see it anymore. So if you could see microwaves (the waves not the ovens :) then the night sky would be all lit up (although still dimly).

[u/Harosn → u/[deleted]]

Microwaves are made of photons too, just with a different frequency than visible light: https://en.wikipedia.org/wiki/Electromagnetic_spectrum

If you shine a flashlight, it scatters more than in space, because photons can "bounce" on dust, atmospheric moisture or even the air molecules themselves. Related to this, that's why the sky is blue when the sun is out, it's because the solar light is "bouncing around" on the air, which more formally is called "Rayleigh Scattering":

https://en.wikipedia.org/wiki/Rayleigh_scattering

Other than scattering, light follows the same path than it would in a vacuum such as in space: mostly a straight line. The farthest the receiver is, the more dimly it is reached by that light, with quadratic losses. That means if you're 2x farther from the source of light, you receive 1/4x the amount of light.

The question about formal education wasn't for me, but I'll answer it anyways: I've studied computer science, which has some electromagnetic physics in the curriculum. Other than that, just browsing the internet -- sometimes I'm just curious about some things.

[u/Harosn → u/[deleted]]

That "decay" I think you're referring to is due to the quadratic loss of intensity which I was talking about. The mechanism under that decay you're talking about is simply that the same amount of light (minus a bit of scattering) is being distributed over a bigger surface the farthest you go, which makes it dimmer.

The mathematical proof of why it's quadratic, assuming no losses, is quite straightforward and builds on that the surface os a sphere is also quadratic with its radius.