If the universe extends forever and if it’s full of stars, why is the night sky dark? This is a question that has been asked by philosophers and scientists since Antiquity. Johannes Kepler sought an answer, as did Edmond Haley, many years after him. Just as an observer sees trees in all directions when standing in a forest, every line of sight in an infinite universe should end with the twinkling of a star. The net result should be a sky ablaze with heavenly light. Not only should the night sky be as bright, if not brighter, than during the day but the heat from all those suns should be sufficient to boil the Earth's oceans away! Therefore, the starry scene depicted in the striking picture that accompanies this article, should appear to be missing stars when compared to gazing into the Cosmos above.

Read the full blog entry

Maybe, I´m too naive. But I never understood the dark-night-sky-paradox. We don´t even see all the stars in our own galaxy (I´m not talking about dust hiding them!). So why should we see more distant stars. I could agree to the wellknown arguments if all the stars were at the same distance (chose any distance you like). Place the trees of the forest one mile apart from eachother. Where is the forest gone now?

Hi dhd40,

You seem to be asking about Olber's Paradox. The idea was that if the universe were infinite, infintely old, and uniformly populated with stars (and not expanding, and light doesn't get tired with age), that you can demonstrate that even if the apparent surface area off all stars combined within 100 lightyears of here is very tiny, that in every possible direction we should see the surface of a star somewhere.

We now know that a lot of the presumptions are wrong (light probably doesn't get tired, but the rest are wrong). We also know that a lot of the surface area we'd be seeing would be dim red dwarfs, which aren't quite as bright as looking at the Sun, but there you have it.

__________________
Forming opinions as we speak

yes, it´s Olber´s paradox I´m talking about. quote: ´that in every possible direction we should see the surface of a star somewhere´. And exactly that´s my problem. Why? If we can´t see (surfaces of) stars in our own galaxy, because they are too faint (too few photons to hit my eye) , why should we be able to see even fainter stars more distant than e.g. 100 000 light years? I wonder whether someone could calculate the number of photons arriving at Earth from a sunlike star at a distance of some 100 million lightyears. As far as I rember, our retina needs approximately 6 to 8 photons per second (?) to send a signal to our brain. Maybe, the boundary condition ´per second´ can explain the miracle??

Hi Guys:

You each bring up good points- ones that should have been better explained in the article, in retrospect.

For example, the reason stars appear faint, to the point that we can no longer see them with our eyes, is due their distance which, in turn, causes their light to become redder. As their light becomes reddened, fewer photons reach us at wavelengths that our eyes can sense, thus, the stars become visually dimmed and very distant stars become invisible.

I also agree that there are a lot of dwarf stars. But these release light which is tilted to the red end of the spectrum from the outset. Their red light is then further shifted to the red, due to distance, and thus cannot be seen visually.

However, if starlight was not reddened due to distance and if the Universe were infinitely old, then it would be highly likely that the night sky would glow brightly in the light of the infinite number of stars throughout the Universe.

Here's a link that also offers some insight:

http://math.ucr.edu/home/baez/physic...GR/olbers.html


Jay

thanks for the link, a good one.
BUT there it says: After all, if you move the Sun twice as far away from us, we will intercept one quarter as many photons, but the Sun will subtend one quarter of the angular area. So the areal intensity remains constant.

Well, maybe. But at some distance the angular area of the sun (star) will fall below our eyes´ angular resolution. And therefore the sun (star) will "disappear".

When distant light is shifted to the red, we can't see it anymore. But shouldn't we feel it then as infrared warmth?
Shouldn't we be barbequed under a dark sky?

__________________
Es gibt keine Tatsachen, es gibt nur Interpretationen. (There are no facts, only interpretations.) Friedrich Wilhelm Nietzsche

As I recall, the fully dark adapted human eye (equals 20 minutes + of total darkness) will respond to a single photon. This has been verified experimentally. The mechanism is a cascade response in the retina, I think from the night vision pigment rhodopsin. While a single photon does not have enough energy, it triggers a landside response which is perceptible in the brain.

My apologies for not being able to give references, but our idiot firewall (Blue Coat) has decided that Wikipedia is an adult entertainment site! Oh Powers That Be, protect us from those who would protect us from ourselves.

Ah, sanity has returned to Blue Coat. (See previous post). After checking the Wiki article on Olber's paradox, they vote for red shifting, but leave the beyond 13.7 billion year space-time limit open, and dismiss the fractal solution.

Yes, it's rhodopsin for human night vision, but Wiki didn't cover the lower limit in the night-vision article.