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# Why is the sky dark at night

HS Thomas
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Posts: 3404
• Heat energy flows from hot areas to cold areas, heating the cold areas until the temperatures are equal.
• In an enclosed space with no avenue of escape for energy, any source of heat, no matter how small, will eventually heat the entire space to its own temperature.
• If you change the volume of a gas, you also change its temperature. If you decrease the volume, the temperature rises. If you increase the volume, the temperature drops

• Is this proof that the universe is expanding ?
[ April 18, 2004: Message edited by: HS Thomas ]

Jeroen Wenting
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Posts: 5093
no it's not
The universe is on the whole a constant temperature which would mean there is no net energy being produced in it.
As we know there's energy being produced (fusion in stars for example) there must be energy being dissipated somehow.
Expansion is one way to do that.
Another avenue of proof is that all celestial objects portray a redshift, meaning they're moving away from us.
Unless the universe is expanding there should be some objects that do not show this redshift because they have nowhere to move to except towards us.

HS Thomas
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Posts: 3404
But given the number of stars there are in an infinite space shouldn't there be bright light at night ? If the universe is expanding , that may explain why the sky is dark at night.
Quoting from somehwere but wouldn't like to post the link yet. You guys can up with something better.

in a stable, infinite universe, the night sky should blaze with the light of the stars that lie in all directions, even those far away.
First, realize that each large region of space is very much like any other -- it receives some heat energy, and it radiates some also, and a typical region is in balance with its neighbors -- it radiates just as much energy as it receives.
Therefore, instead of trying to imagine infinity, just imagine a large cube of space with perfect mirrors for walls. The mirrors reflect perfectly, so if you are inside and look in any direction, you see what appears to be an infinite distance.
Energy that reaches an edge of the square "wraps" around to the opposite edge, just as though the square was lined with mirrors. Careful thought will show that this simple method effectively mimics an infinite space.

[ April 19, 2004: Message edited by: HS Thomas ]

Jim Yingst
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Posts: 18671
But given the number of stars there are in an infinite space
Depends what we mean by "infinite". Current usderstanding is that the volume of space is finite, but increasing. However thanks to the curvature of space we have wraparound. You can keep going infinitely in any direction, eventually passing through the same region you started in. In that sense, the mirrored cube discussed in your quote is a fairly apt model.
shouldn't there be bright light at night ? If the universe is expanding , that may explain why the sky is dark at night.
It's not clear to me how expansion would affect the argument you quoted either way. That is, it seems as though there's still "infinite space" (that is, a finite amount of space infinitely repeated) even if it is expanding.
It's hard to discuss this effectively without a lot of diagrams and some math which I've now mostly forgotten. But I think the main reason why we don't see a big bright sky is that while there may be an "infinite" number of stars (or a finite number infinitely repeated, roughly speaking) the same argument also applies to the empty space in between the stars. There's a lot of stars out there, but a whole lot more empty space. There's no real reason to think that just because you could travel infinitely far in a particular direction, you must eventually hit a star. In fact in the case of the mirrored cube, there are quite a few paths for which you could prove that you will never hit a light. Of course the universe isn't so orderly, so it's harder to prove. But consider - even if the path you follow does eventually hit a star, you could then adjust the path by a tiny angle in any direction, and it will miss that star. There are still going to be plenty of paths nearby that miss all the stars in the area.
Alternately if you don't accept that argument - even if you hit a star, if it's far enough away, it doesn't matter. EM radiation from a point-source dissipates in intensity as it radiates outward; it's inversely proportional to the square of the distance. Meaning that if the distance is large enough, you can be looking straight at a star and not see it. The intensity of the light from the star can be too small to be detected by the humand eye, or (at another level) the most advanced imaging systems we have available.
I think all these counterarguments essentially reduce to: even if there were an infinite number of stars, there'd still be a whole lot more empty space. than stars, and the empty space "wins". This ends up being true whether the universe is expanding or not, IMO.

John Smith
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Posts: 2937
I think all these counterarguments essentially reduce to: even if there were an infinite number of stars, there'd still be a whole lot more empty space. than stars, and the empty space "wins".
There is another variable, which is the 10 billion years (the age of the Universe) that the heat had to dissipate. The way that I read the original explanation for why the sky is dark at night is that if the volume was constant, then 10 billion years would be more than enough to even out the temperatures in all points of the volume. Therefore, the volume is expanding.
So, the the "war" becomes of that between the empty space, the stars, and the time. Which one wins is not immediately clear to me, but I think it would not be that hard to estimate it. After all, we know the average density of starts in the known universe, we know its age, and we know the star temperatures. All of these are not precise, of course, and they may be different beyond the observable Universe, but I think the bulk part can be estimated.

Chicken Farmer ()
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Posts: 1932
Ok, this doesn't have to do directly with why the sky is dark, but is somewhat related.
Currently in the night sky we can see all sorts of planets. Mercury, Venus, etc. If I understand it correctly, the planets all are in the same plane. When it's night time, we're facing away from the Sun.
Therefore, to see the planets at night, doesn't that mean that they would have to be behind us?

Joel McNary
Bartender
Posts: 1840
That is correct -- which is why you would never see Venus or Mercury at midnight.
However, in the evening and pre-dawn morning, we are not facing directly away from the sun. Therefore we can see Mercury and Venus.
And, to be technical, all planets except for Pluto are in the the same plane. Of course, that raises the "Is Pluto truly a Planet?" question
[ April 19, 2004: Message edited by: Joel McNary ]

Ellen Zhao
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Posts: 581
why is the sky dark at night?
one of the possibilities could be: the human being's eyes are not sensible for the various low-frequency or high-frequency rays at night. So that people feel it's dark at night.
maybe some certain kinds of plants or animals are wondering: why is the sky so bright at night. :roll: They just sense different parts of spectrum.

Gregg Bolinger
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Posts: 15304
6
This is an interesting discussion. But it seems overlly technical to me to explain our side of the planet being shadowed when we are facing away from the sun.
Shine a flashlight on one side of a basketball in a dark room. Light doesn't bend around objects. The night sky is dark because there is not enough light for it to not be dark. Maybe my explination is too simple, but does it have any merit?

HS Thomas
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Posts: 3404
Stars don't move but just hang in space. Perhaps if they oscillated a bit the sky would brighten up at night ? For each slice of time, each star receives heat from its neighbors according to the rules of heat transfer. The "stars," of course, are sources of energy, so they do not cool off -- they maintain their temperature even as they deliver energy to their environment.
One of the arguments against this statement is that there might be dust clouds that hide the light and heat of distant stars. The response to this (as you may have guessed by now) is that, over time and in a stable universe, the stars would heat the dust clouds to the stars' temperature.
Another objection is that the distant stars might be too small and far away to be seen locally. The response to this is that, at increasing distances, there are more and more stars, so their smaller apparent size is made up for by their greater numbers.
As stated by Jim here there would seem to be a third objection:
There's a lot of stars out there, but a whole lot more empty space. There's no real reason to think that just because you could travel infinitely far in a particular direction, you must eventually hit a star. In fact in the case of the mirrored cube, there are quite a few paths for which you could prove that you will never hit a light. Of course the universe isn't so orderly, so it's harder to prove. But consider - even if the path you follow does eventually hit a star, you could then adjust the path by a tiny angle in any direction, and it will miss that star. There are still going to be plenty of paths nearby that miss all the stars in the area.

This discussion leads to several interesting points:
* In a stable universe filled with stars, a dark night sky is not possible .
* As scientists learned more about the physics of matter and energy (thermodynamics), they realized the dark night sky contradicted their new knowledge.
* New scientific theories are judged in several ways. For example, a theory might create new problems to be solved, be neutral, or it might solve unrelated problems. Here are some of the problems "solved" by the Big Bang Theory:
o It provides a reason for the recession of the galaxies, the "Red Shift."
o It provides an explanation for Olbers' Paradox.
o It explains a quirk in Einstein's Relativity Theory.

This last point deserves elaboration. When Albert Einstein developed his Relativity Theory, he saw a problem right away -- the universe described by his equations was unstable. It relied critically on the density of matter -- if the density was above a certain critical value, the universe would begin to fall in on itself, which would increase the density further, leading to a runaway collapse. If the density was below this critical value, the opposite would happen -- the universe would expand forever. Einstein's equations didn't permit a stable universe.
Einstein therefore added a term to his equations -- a "cosmological constant" -- that would counter the effect of gravity at great distances and balance the universe. Einstein was not happy with this constant -- it had no obvious physical explanation, it seemed ad hoc, but he saw it was necessary so that his equations could describe the universe that was known at the time. This was before the work that led to the Big Bang Theory, which made the cosmological constant unnecessary. Einstein later called the cosmological constant the "biggest mistake of my life." (Actually, the second biggest. The first was triggering The Cold War.)
Einstein could simply have said, "Oh well, your knowledge of the universe is not good enough for my equations -- you need to look more carefully through your telescopes," but this was too much even for Einstein, known for his occasional bold pronouncements. He thereby missed his chance to predict the expanding universe.
Now you know why the sky is dark at night.
[ April 19, 2004: Message edited by: HS Thomas ]

Nathaniel Stoddard
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Posts: 1258
I don't see how the universe having an infinite number of stars implies not having dark night skies. Just because there is an infinite number of stars does not necessarily mean that the night sky would not be dark at all. I consider is analgous to the fact that infinite number sets like our good friends, integers, are indeed countable though infinite.
As for
In an enclosed space with no avenue of escape for energy, any source of heat, no matter how small, will eventually heat the entire space to its own temperature
, doesn't this contradict the 2nd(?) law of thermodynamics stating that the amount of energy remains constant?
I should really read this entire thread before I hit this button, but here goes. Personally, I'm happy with the basketball method.

HS Thomas
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Posts: 3404
Originally posted by Jeroen Wenting:
no it's not
Another avenue of proof is that all celestial objects portray a redshift, meaning they're moving away from us.
Unless the universe is expanding there should be some objects that do not show this redshift because they have nowhere to move to except towards us.

The Doppler effect - some galaxies show this red shift if they are moving away and some a blue if advancing.
Doppler shift is what makes a car sound lower-pitched as it moves away from you. It turns out that a special version of this everyday effect applies to light as well -- if an astronomical object is moving away from the Earth, its light will be shifted to longer (red) wavelengths.

Jim Yingst
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Posts: 18671
[HST]: In an enclosed space with no avenue of escape for energy, any source of heat, no matter how small, will eventually heat the entire space to its own temperature
[NS]: doesn't this contradict the 2nd(?) law of thermodynamics stating that the amount of energy remains constant?

Right. It would be more correct to say that the source of heat and the rest of the space will eventually reach the same temperature - but this temperature will be lower than the original temperature of the heat source. The heat source will decrease in temperature, and the remaining space will increase in temperature, until they arrive at the same point.

Nathaniel Stoddard
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Posts: 1258
So, we're going to all eventually freeze to death?

HS Thomas
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Posts: 3404
Not if enough new stars spring to life nearby or the Earth can be shunted to a new Sun. ( Hope God knows how to use a point and click mouse. Copy and paste would seem to fit in better with the natural order of the Universe
double )
Evidence of a dark night sky contradicts the thermodynamics laws in space, apparently. At night you get to look at space and stars without hindrance from sunlight and space (night sky) is dark. Just to put a damper(or flagrant foul) on the Basketball theory.
[ April 19, 2004: Message edited by: HS Thomas ]

John Smith
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Posts: 2937
There are some wild theories that may relate to the topic discussed here in The Fabric of the Cosmos : Space, Time, and the Texture of Reality. This is a sequel to The Elegant Universe which some of the ranchers mentioned before.
I have not read the "The Fabric of the Cosmos" yet, but a collegue of mine mentioned one of the stipulations in there. Apparently, the entropy of the Universe now is actually lower than it was billions of years ago. As contradictory as it may sound to the second law of thermodynamics, it may explain why the sky didn't heat up yet. The book is on my way to me, I'll update you guys on the developments.

Jim Yingst
Wanderer
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Posts: 18671
Evidence of a dark night sky contradicts the thermodynamics laws in space, apparently.
Ummm... how is that? What thermodynamic laws are you talking about? For the ones mentioned so far, a key point is that it still takes a really, really long time. Yes, the stars will eventually burn out (unless the universe collapeses first instead) but that's unlikely to effect most of us in the near future; I'm not terribly concerned. But the fact that the stars haven't burned out yet doesn't mean they won't ever.
At night you get to look at space and stars without hindrance from sunlight and space (night sky) is dark.
Yesss... that's kind of the definition of "night", isn't it?

Ashok Mash
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Posts: 1936
because the universe is in power-save mode??

HS Thomas
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Posts: 3404
Originally posted by Ashok Mash:
because the universe is in power-save mode??

An intriguing thought.. Stars share their power in the star ecosystem and it bodes well that stars are immobile.This discussion is an excellent cure for insomnia Highly recommended !
[ April 20, 2004: Message edited by: HS Thomas ]

HS Thomas
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Posts: 3404
Originally posted by Jim Yingst:
Evidence of a dark night sky contradicts the thermodynamics laws in space, apparently.
Ummm... how is that? What thermodynamic laws are you talking about? For the ones mentioned so far, a key point is that it still takes a really, really long time. Yes, the stars will eventually burn out (unless the universe collapeses first instead) but that's unlikely to effect most of us in the near future; I'm not terribly concerned. But the fact that the stars haven't burned out yet doesn't mean they won't ever.

No particular law- just the science of heat which I believe includes entropy. In a nutshell, if stars have been shining for a long time, billions of years they should have heated up the universe to their temperature. Even at night the whole sky would be as bright as the sun and every direction one looks would be on a star or on a cloud of dust that would have been heated up to be as hot as the stars. Light from distant stars hasn't had the time to reach us yet The distance is so great that it would take billions of years to reach us ,and that is one reason the sky is dark at night. Another is that no star has been shining longer than 10 to 15 billion years the time of the Big Bang.
And also, analysis of Doppler effects shows that more galaxies are moving away rather than towards us.
Enjoy the night sky. Question is : Is it getting even darker ? If so, our universe is saying goodbye to us.
[ April 20, 2004: Message edited by: HS Thomas ]