system
Given the following post, respond as if you're commenting on Reddit:
human
If there are billions of stars in the sky and their light is hitting earth, why does it ever get dark?
gpt
This is a very good question. Such a good question, in fact, it has a special name. It's called "Olbers's paradox." After a guy named Olbers.

The answer to the question wasn't fully understood until just a few decades ago. It was once thought that (a) there's dust and junk in the way that scatters the starlight, or (b) light is capable of "canceling out" through destructive interference, but both of those are quite wrong.

The right answer comes in two parts: the finite speed of light, and a phenomenon called "redshift."

The universe is infinite in extent; this was suspected basically forever, and confirmed through observation in recent years. Since the universe is also *homogenous* — that is, pretty much the same pretty much everywhere — that means if you draw any imaginary line out from the surface of the Earth, that line *will* sooner or later intersect a star.

But the key phrase there is "sooner or later." In point of fact, a random imaginary line drawn outward from the surface of the Earth almost certainly *won't* intersect a star in less than about fifteen billion light-years. If you extend the line farther than that, yes, you will certainly hit a star *eventually,* but within about fifteen billion light-years, you are not likely to.

Why is that fifteen-billion-light-years figure important? Because points that are farther away from us than fifteen billion light-years *cannot be seen,* because the universe is less than fifteen billion years old. Since light crosses one light-year in a year, light from farther away from us than fifteen billion light-years hasn't yet had time to get here.

Does that mean that if we imagine an impossibly distant future, many hundreds of billions of years hence, the night sky *will* be blindingly bright because the light from those unimaginably distant stars will have had time to reach us?

No, because of the other thing: "redshift." For reasons that are too complex to explain here, light from distant things looks "redder" to us than it would if those things were closer. For *very* distant things, things ten billion light-years away or more, those things are "red-shifted" to the point where we can no longer see their light at all; their light is now in the infrared part of the spectrum, or even farther down into the microwave band.

Even more distant things — stars a hundred billion light-years from here — would be even more greatly dimmed by redshift, even if the universe were old enough for that light to have reached us.

So that's why the night sky is dark. First, because there simply aren't enough stars in the universe that are *close enough to us* to be seen in the night sky, and second, because anything that's really distant is too dimmed by redshift for us to see it with our eyes.