Why is the sky dark at night?

[Darnok]

Very simple question: why is the sky dark at night?

[Frida Space]

In two words: Olbers' paradox.

[Silver_Swift]

If you don't just mean because the rest of the earth is between you and the most prominent source of light for lightyears, I'll let minutephysics explain:

. Edited July 20, 2015 by Silver_Swift

[K^2]

I'll let minutephysics explain:

You really shouldn't. That video is full of errors. First of all, there is no point in space from which light has not reached us, because while it's true that some parts of the universe are more than 14bly away from us, it's only due to expansion of the universe being faster than light. Light from eeach point still reaches us eventually, and since all known universe started in the same point, there isn't a part of it which we cannot see.

Second, it is absolutely not true that infinite, unchanging universe would result in bright sky. In fact, if this was true, our local group alone would have been sufficiently large. We should se stars in all directions. We do not, because distribution of stars is not uniform. Stars are bound in galaxies, galaxies in groups, groups in clusters, clusters in filaments. It's this fractal structure that guarantees that number of stars a given distance away from us does not rise as rapidly as brightness of stars drops due to the same distance.

Red shift is a factor. But if the stars were uniformly distributed at the same density they are found in Milky Way galaxy, it wouldn't have made a difference. There would be enough stars in our immediate neighborhood, to close to us for red shift to make an impact, to make the entire sky glow as a star's surface.

I'm glad the video did not bring up another common misconception that interstellar dust is the cause of it. While it's true that we'd see a lot more light if there was no dust, if universe was, indeed, filled with stars uniformly, the interstelar dust would heat up due to starlight to the point where it itself glows same as the stars around it. So while interstelar dust makes the sky even darker, it's not the cause of it being dark to begin with.

[Hcube]

Very simple question: why is the sky dark at night?

because there are about 12,000km of rock between you and the sun, wich is the only important light source around. The light from the stars is way too weak to enlight anything (inverse square law with the light propagating in all directins

[WinkAllKerb'']

because a black hole sink and eat all the light xDr

[Bill Phil]

A better question would be: "Why is the sky bright during the day?" The answer is that there's a light source. If there were no sun there would be no day. And also no life... But, you get the idea.

The sky isn't dark during th night, it's the whole side of the Earth entering its own shadow.

[Scotius]

Anyone read "Nightfall" by Isaac Asimov? If not, feel ashamed and go read it ASAP.

[Albert VDS]

You really shouldn't. That video is full of errors. First of all, there is no point in space from which light has not reached us, because while it's true that some parts of the universe are more than 14bly away from us, it's only due to expansion of the universe being faster than light. Light from eeach point still reaches us eventually, and since all known universe started in the same point, there isn't a part of it which we cannot see.

MP meant visible light and he also explains about CBR.

Second, it is absolutely not true that infinite, unchanging universe would result in bright sky. In fact, if this was true, our local group alone would have been sufficiently large. We should se stars in all directions. We do not, because distribution of stars is not uniform. Stars are bound in galaxies, galaxies in groups, groups in clusters, clusters in filaments. It's this fractal structure that guarantees that number of stars a given distance away from us does not rise as rapidly as brightness of stars drops due to the same distance.

An infinite, unchanging universe as in no expansion of space and do demise of stars.

[ZetaX]

An infinite, unchanging universe as in no expansion of space and do demise of stars.

And as K^2 already said: that does not imply a bright sky. You need homogenity and a few other assumptions for that.

[K^2]

Homogeneity is sufficient, actually.

[ZetaX]

Homogeneity is sufficient, actually.

Not really:

A too young universe won't do due to the limited speed of light (not sure about ~4 billion years, the original one even assumed infinite age; but 100 years definitely wouldn't do); as would probably the very hypothetical scenario of having large black holes everywhere (which might become somewhat more plausible in a homogenous universe). Sure, those assumptions are minor, but they are to serve as an example that homogenity itself does not formaly imply the result (only after inserting some assumptions that probably are true in real life; but if one goes into hypothetical scenarios, then all conditions should be stated as good as possible).

Also, didn't we have posts about this in the past¿ I think I listed a couple of things that give the bright sky, and a couple that would not by themselves (including homogenity).

[YNM]

Some important (wrong) assumption from Olber while he contemplated for it long ago :

1. The sky is all homogenous (so, no galaxies, just stars, stars everywhere) -> today known to be wrong, but would still be the same out of the galaxies itself in turn, if only this is considered.

2. The Universe is infinitely old -> today known to be wrong as well. Can still be pretty bright if only this is considered.

3. The Universe is infinitely huge -> unknown, but luckily we don't need to solve this one to prove the paradox is wrong.

4. The Universe is static, not expanding / contracting -> known to be wrong today too. This one is tied to two previous point though - an expanding universe allows for an age (and a contracting one as well), unlike a static universe, and allows for a finite size.

Alignment and other things are just bogus - Olber knew that stars may visually overlap or dust may be around, but all would be heated until it shines like the surface of a standard (average ?) star.

[WinkAllKerb'']

what if a photon "fade"/"vanish" at some point for some reason ? ... just a though

(beside the fact it could prevent to see the next closest "universe" as we know it ? primitive soup / atom and molecule more or less related metaphorically)

Edited July 21, 2015 by WinkAllKerb''

[Skyler4856]

That's a bit like asking why a vacuum has lower pressure than a bottle of air?

There's very little light causing the electrons in your photoreceptors to excite, therefore you're in the dark about the surrounding environment.

[NFUN]

what if a photon "fade"/"vanish" at some point for some reason ? ... just a though

(beside the fact it could prevent to see the next closest "universe" as we know it ? primitive soup / atom and molecule more or less related metaphorically)

As one gets further away from the emitter of light, less photons will reach the observer as the angular diameter of the body gets smaller and smaller. Assuming there is nothing in the way of the light's path, and it doesn't get moved due to gravity, light will reach the target. However, if no light was actually emitted directly at the object, obviously no light would reach it, and sufficiently distant bodies would receive few, if any, photons from each other. Energy would probably not just disappear en route without an object or force forcing it to. This is in response to your first question, your second statement is gibberish. It is, in fact, your responsibility to transcribe your thoughts in a medium that can be understood, or else you may not get much conversation.

[WinkAllKerb'']

yep i really like to see the universe like a living molecule slowly floating in space and photon like small living thing too. it does the tricks pretty well and sound logic and not against any known science law right ; maths specialist ?

(yup nfun but if photon has smaller thing inside them we can't measure yet ? including some sort of life span related process thing ? )

just asking now i use to be rudimentary at "math" i prefer "feeling" approach much more

wich is strange because in shcool i was pretty good in science and always given back white sheet in philosophy even at final exam before university ... just to give you an example without assiting to lesson and "playing magic the gathering all day" and not working the evening at home i got 18/20 on the first exam "matteer science" in university then i stopped and done something else ¯\_(ツ)_/¯ ... regarding that i could also say that i consider that having facilities easyness to understand "some" thing without the need of any effort isn't especially helpful in daily life 30 years later but hey you know it's not really like it's the kind of thing you can choose at the beggining wich is also equal that when not much thing can challenge you you tend to be bored easily seeking challenging thing also

Edited July 21, 2015 by WinkAllKerb''
mode "how the irony on/off" xDr

[K^2]

Not really:

A too young universe won't do due to the limited speed of light (not sure about ~4 billion years, the original one even assumed infinite age; but 100 years definitely wouldn't do); as would probably the very hypothetical scenario of having large black holes everywhere (which might become somewhat more plausible in a homogenous universe). Sure, those assumptions are minor, but they are to serve as an example that homogenity itself does not formaly imply the result (only after inserting some assumptions that probably are true in real life; but if one goes into hypothetical scenarios, then all conditions should be stated as good as possible).

An infinite static universe cannot be young. It can either be eternal or have closed time. (Via relativity - no preferred frame.) Either case prevents light cone limits. All other considerations, including black holes, are trumped by thermodynamics.

Infinite, static, homogeneous is sufficient condition for bright sky. Or uniformly lit, I should say. After all, you can have a universe with no stars at all.

[ZetaX]

An infinite static universe cannot be young.

Why ¿

[MilesPalacios]

Go on sky site you will get your answer there

[K^2]

In which coordinate system?

[ZetaX]

In which coordinate system?

Ours. I don't see how other frames of reference are relevant.

Edit: actually, why is it relevant¿ The infiniteness of space and the finiteness of time does not depend on an observer moving slower than c.

Edited July 21, 2015 by ZetaX

[K^2]

Ours.

I don't know why we bothered with relativity in the first place. We should have just used "ours" coordinate system and be done with that whole nonsense.

Sorry. I'm being a bit rude, but the lack of preferred coordinate system is kind of the whole point. And any fixed duration will be different in a different coordinate system. And if you happen to have an infinite amount of space in any one coordinate system, I can give you a coordinate system in which duration of time is arbitrarily long. Moreover, if you have a coordinate system in which time is finite but space is not, any coordinate system moving with respect to it at any velocity will have finite space.

Once you throw in field theory and uncertainty, the idea that you can have infinite space without infinite or cyclic time becomes silly.

[ZetaX]

I don't know why we bothered with relativity in the first place. We should have just used "ours" coordinate system and be done with that whole nonsense.

Instead of indeed being rude, you could at least answer the question properly...:

but the lack of preferred coordinate system is kind of the whole point.

No it is not. It is the point of relativity, yes, but obviously not of bright skies. It's our sky we are talking about, not any other.

And any fixed duration will be different in a different coordinate system. And if you happen to have an infinite amount of space in any one coordinate system, I can give you a coordinate system in which duration of time is arbitrarily long.

Arbitrary long is still finite. The whole point of "infinite time" is that it is actually infinite (which then automatically is independent from the frame of reference).

Moreover, if you have a coordinate system in which time is finite but space is not, any coordinate system moving with respect to it at any velocity will have finite space.

Why¿ Only the past is necessarily finite, we have no assumption on the future.

Edited July 21, 2015 by ZetaX

[RainDreamer]

Maybe because the universe is expanding at the same speed, or even faster than the speed of light? Just my candy wrapper in the pocket (not even 2 cent).