Why is the Sky Dark at Night?
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Tony Darnell explains Olbers' Paradox.
Sep 3, 2010 11:35 PM
Re: Why is the Sky Dark at Night?
This was great. This paradox would never have occurred to me without the explanation given here, but indeed it is a paradox if you don't understand the red shifting of light from the universe's expansion.
Still, surely they have neglected to mention interstellar dust, no? Even the center of our own galaxy is mostly obscured by this opaque matter ( I won't call it "dark matter"). Or would microwave radiation pass through that anyway, perhaps, because of its wavelength...?
Still, surely they have neglected to mention interstellar dust, no? Even the center of our own galaxy is mostly obscured by this opaque matter ( I won't call it "dark matter"). Or would microwave radiation pass through that anyway, perhaps, because of its wavelength...?
By: ice-9
Re: Why is the Sky Dark at Night?
Wouldn't this opaque matter become too hot (relatively to the ~0° Kelvin in outer space) and begin radiating itself in the long time?
Re: Why is the Sky Dark at Night?
Interstellar dust is usually too rarefied to be opaque. It has a reddening effect instead, because it scatters blue light more effectively. And even when it is dense enough to be opaque to visible light, it is still mostly transparent at longer wavelengths.
By: quisph
Re: Why is the Sky Dark at Night?
This makes sense, but doesn't negate my point: In the visible spectrum, interstellar dust keeps the sky from being "bright" in the sense that it is in the daytime. If you live far enough away from a city to look at the Milky Way at night, you can see that the galaxy's center is obscured by a "blackness" that is interstellar dust. It's definitely there and definitely opaque to the majority of visible wavelengths.
By: ice-9
Re: Why is the Sky Dark at Night?
But only in the visible spectrum, and even then, opaque dust is the exception, not the rule. That's the point. The center of the Milky Way is obscured *because* it's the center of a galaxy, and galaxies are where dust particles are formed. And these particles are of such a size that longer wavelengths, like microwaves, generally pass right through. Isn't that what you were asking with your last question?
By: quisph
Re: Why is the Sky Dark at Night?
why would god do that to us? not give us microwave eyes? sheesh.
By: ritterkreuz
