Olbers' Paradox is a simple, if nonintuitive, question. It goes like this : why is the night sky dark?

Just because the sun went down doesn't mean that the night sky should be dark. There are, after all, all those stars up in the sky shining down on us. And, if the universe is infinite, as posited by Issac Newton, then the number of stars in it is infinite, meaning we get an infinite amount of light hitting the earth at all times! The weather forecasts would read, infinitely bright, clear skies with no humidity and an infinitely high temperature that will kill you dead.

Clearly, there is not an infinite number of something involved. We ain't dead, right? Johann Kepler, who came before Newton, gave this brief thought. His solution? The universe is finite. Of course, that leads you to ask, where does the universe end? No experiment can find the edge of space. We are forced to discount that thought. Still, why is the night sky dark? Why aren't we dead from heat?

Let's say that even though there is infinite space, there are a finite amount of stars. Even then, there should be enough energy to cook our collective goose. Stick a star in a very, very, very large room (say, big as the Milky Way). As long as the star shone continuously, the room would eventually heat up. Make it like space and remove the air, and there's less matter to heat up, so it happens quicker! Dump in all the rest of the known stars, and there's more than enough heat energy to go around. We'd fry. Even if you take into account the fact that a star will not burn forever, it's still enough energy.

Maybe there's matter out there that's blocking the energy from reaching us? Possibly, but in the end, that matter would heat up, and then it would radiate out energy, as per the law of black body radiation; the heat would reach us anyway. It would just have to get through traffic. And we are still bright, hot, and dead.

Maybe the universe is young and we get to enjoy it before we get baked? Maybe.

The accepted answer to this paradox lies in the current cosmological explanation of the end times - the Big Bang. According to Big Bang theory, we've got two main factors cutting down on energy. One is that energy from stars from a long way away will not reach us for billions of years. We're saved for a time by the very hugeness of the universe. The second part is that the redshift phenomenon, where quickly receding galaxies actually look more red than others, means that the energy is actually getting knocked down. We're not getting the full brunt of the assault. I have never worked out the equations myself, but I'm told that it only sometimes comes out in our favor.

Yet we live on, totally unconcerned about Olbers' Paradox or dying from the energy of a near-infinite gaggle of stars bombarding us, searing the sky and ourselves. If we've got everything right, we should be ash by now, but we aren't, so what's going on?

Yet another node borne of my weak-willed attempts to teach myself physics. I was born yesterday - you have been warned.

weStLY : great questions! As for the time scale, it's pretty much an unknown. For all we know, it will happen, but due to whatever effect we don't know about, the heating of the universe won't happen for trillions of years. Or maybe it won't happen at all. Or maybe it will become noticeable in the next 1000 years. I am not truly familiar with the exact calculations (just the base laws), so I can't hazard a guess. From what I've read, people disagree strongly on this.

As for evolution, well, it's hard to evolve past the point where the heat would cause the proteins in your body to break down. There is a temperature where life as we know it becomes impossible.

As for the 'why's of creation : I ain't touching that with a light-year long pole.

Stephen Hawking and Jim Hartle 1983 suggested that the Universe is finite but does not have any boundaries. ("Wave function of the universe", Physical Review vol D28, pages 2960-2975) So far, all observations and calculations agree with this. There is no boundary and there is absolutely nothing outside this nonexisting boundary. A finite number of stars in a finite universe. (I have no idea if this contributed in a constructive way the way I had hoped.)

On request, on "no boundaries": "No boundaries" is a mathematical boundary condition, which seems to be correct compared to what has been observed in fluctuations in background microwave radiation. I cannot draw any physical or philosophical conclusions of this, myself, unfortunately.

Most important though, and the commonly accepted solution to Olber's paradox is that the universe is so young that only a tiny fraction of all light has reached us yet.

You can get to the heart of the paradox quite easily, actually. But it never talks about infinite brightness of the night sky -- "just" that every portion of the sky (day or night, looking at the sun or elsewhere) should be as bright as the sun. (This is also enough to kill you outright)

First, suppose you're in an infinite universe. Stars, each roughly as bright as the sun, are scattered throughout it.

But you don't see a distant star as brightly as you do the sun! The star radiates energy uniformly in all directions, so its brightness (the amount of energy it radiates onto us) decreases with the square of its distance. So a star twice as distant is only a quarter as bright as the sun. So are we saved?

Not really. The (expected) number of stars increases with the square of their distance, since we're assuming a uniform distribution. So we expect four times as many stars when we double the distance. At X times the distance from the sun, we expect to find X2 stars, each radiating 1/X2 times as bright as the sun. This exactly cancels out, so we end up expecting the brightness of the sun impinging on us from every area of the sky. (All this can be made precise; trust me).

What "night sky"? (Almost) any point of it should be as bright as the sun!

You may have noticed that this is not quite true. Here are some reasons why not, along with some "reasons" which don't manage to explain why.

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