Comments on: Why is the sky dark at night?

By: 9. The Universe, in Brief « Tikkunista!

9. The Universe, in Brief « Tikkunista! — Fri, 26 Oct 2012 13:41:33 +0000

[...] Why Is The Sky Dark At Night? Bad Astronomy Discover [...]

By: Chalkboard

Chalkboard — Fri, 05 Oct 2012 22:17:43 +0000

Ah yes, the intellectual equivalent of “yah huh…nut uhh…yah huh if this way…ok, but nut uhh if that way” Thanks Phil, keep up the good work you do for those of us with brain pans of a lessor volume.

By: Phillip Helbig

Phillip Helbig — Fri, 05 Oct 2012 16:04:31 +0000

@#58: OK, this is a bit better; one can argue that the brightness isn’t infinite but equal to the average surface temperature of a star. OK, but still that is not what we observe. (This is of course an idealized thought experiment; if the photon density in space were the same as that at the surface of a star, then stars could not exist, but that is not relevant to the paradox.) If the stars have always been shining and the universe is infinite, then every point would indeed be as bright as the surface of a star.

@#55: Yes, this is possible, but would require that we are at a special place in the universe.

By: Nick L

Nick L — Fri, 05 Oct 2012 04:09:03 +0000

That should be Planck length. (We don’t live in a lumber universe regardless of the number of blockheads you might know)

By: Nick L

Nick L — Fri, 05 Oct 2012 03:39:36 +0000

“you can figure out that the Universe is neither infinite in space nor in time.”

Unless an infinite universe has virtually all of its diameter at around a plank length.

By: Speedy Light

Speedy Light — Thu, 04 Oct 2012 22:52:57 +0000

[...] phase velocity are the same in a vacuum, but in any other medium this is not, in general, the case."It's a common misconception that light always travels at a constant speed. Light slows down when it …in a vacuum — where there's no matter, only emptiness. When light travels through matter such as [...]

By: TB

TB — Thu, 04 Oct 2012 17:04:30 +0000

@53. Please pardon my sloppy writing earlier. I understand what you are saying, but I still disagree. Even if you have a detector which integrates all photons from all directions, you still get a finite light density. The reasoning is simple. If you have some star density which is uniform over space and time (no die-outs or blockages) you will get a photon density which is, in effect, the average temperature of the volume. If you double the star density, you will double the photon density because you have effectively doubled the energy density inside your “black body”. So if we could put a detector in the middle of intergalactic space we could get an approximate value for the star density in the universe. Obviously this is not zero, as my previous post may have implied, but neither would it be infinite or the same as being inside of a star. (I will read Harrison as you suggested, but I have taken many physics courses and have been involved in various camera light detection systems for several years, so I think I already know a bit about this).

By: ceramicfundamentalist

ceramicfundamentalist — Thu, 04 Oct 2012 15:18:25 +0000

Matt B., #57
“there are directions you can draw a ray so that it doesn’t touch any points with integer coordinates.”

interesting thought, but the human eye can’t see a point, it has a limit of practical resolution. would any starless area of the sky be large enough to be resolved by a human eye (or for that matter even the most advanced telescope)? if not, then the sky would still look bright in every direction.

By: Cosmonut

Cosmonut — Thu, 04 Oct 2012 10:44:59 +0000

Couple of things glossed over in the video, but really important.

As stars get farther away, their light gets fainter. So maybe that will take care of it ? Turns out not, due to a neat mathematical fact.

Consider a sphere 1 AU in radius centred on Earth. It will just about touch the Sun and thus only 1 star on the surface of the sphere.

Now consider a sphere say 1 million times larger. Every star lying on the surface of that sphere is 1 million times further away, so the light they contribute is 1/(1 million)^2 times that of the Sun.

But on average, if a sphere of size 1 AU touches just the Sun, a sphere of 1 million AU touches (1 million)^2 stars ! So, the overall light coming from that sphere = Same as from the Sun.

Now if space goes on till infinity, we get an infinite amount of light hitting us ! That’s the paradox.

By: Matt B.

Matt B. — Wed, 03 Oct 2012 22:16:21 +0000

Infinity of volume and consistent density of stars doesn’t necessarily lead to covering the entire sky. In a Euclidean plane, there are directions you can draw a ray so that it doesn’t touch any points with integer coordinates. In a real-life infinite universe, the stars wouldn’t be spaced so exactly, so I would expect a fractal of light and dark.

It might be possible to use calculus to prove that an infinite number of stars doesn’t necessarily add up to 100% of the sky, but I’m not up for that task right now.

Oh, and I should add: It’s /TEM-por-al/, not /tem-POR-al/. (I often hear a similar mistake with “electoral”.)