Isn’t part of the reason the night sky is dark that there is nothing for light to reflect off of? Where there are things to reflect starlight, the sky is light; our moon, other planets, satellites, astronauts, UFO’s, Star Destroyers, etc.
Am I getting this all wrong?
Pretty much. What do you think the sun is reflecting off of to make it visible when it is up?
The column being discussed is, of course,
Why is the night sky dark? (17-Apr-1992)
Quand les talons claquent, l’esprit se vide.
Maréchal Lyautey
I assumed that since the sun is relatively close, and we can look at it without much interference, it doesnt need to reflect off of anything to see. We can see the stars, too, by looking at them. I thought the question was, “Why is the night sky dark?” not “Why can’t we see the stars?”
Seems like a better answer would be that the night sky is dark because that’s the way it naturally looks in space. Then you have to turn the question around and ask why the day sky is not (AKA “why is the sky blue?”). Of course this has been answered about a bazillion times as Tyndall Effect (AKA Rayleigh Scattering).
The column was discussing Olbers Paradox- which was “Why is the Night Sky dark - IF there is an infinite number of stars?” That question was important back before the Big Bang became the accepted model of the universe, because scientists thought there might be an infinite universe with an infinite number of stars. Since now it is generally accepted that there is only a finite number of stars the apparent paradox is resolved.
An article in the November 1999 issue of Scientific American offers another potential reason.
The Fate of Life in the Universe by Lawrence M. Krauss and Glenn D. Starkman
Given that the universe is expanding, there are galaxies and stars so far away that their rate of recession exceeds c. (They claim this is allowed since it is space-time expanding, not simple movement.) At this rate, the light from these objects is red-shifted to zero energy, so we can’t see them. (Unfortunately, this article wasn’t included in their on-line page)
What about the fact that we humans can’t see all wavelengths of the electromagnetic spectrum? If other objects in the universe are “shining” in the IR or radio wavelength, we can’t see them. Wouldn’t they appear dark to us?
nebuli,
You wrote:
Correct, but that was Cecil’s response to the question. If you read the original question by Bill and the first post in this thread by Bobby O, they are not asking for the explanation to Olbers’s Paradox. They were not expecting to see a sky so dense with stars that you can’t see gaps… Olbers’s Paradox. They want to know, given that the density of stars is basically as it appears, why don’t the individual pinpoint stars don’t flood the sky with light like billions of massive floodlights.
In essence, Bobby O’s assessment is correct, for this question.
Of course, not having access to the article that was referred to (written by Robert Cowen), it’s entirely possible that Cecil was responding to elements of this article that we are not privy to.
Oops! Need a proof reader… Five yard penalty, excessive use of “don’ts”… That should have read something like:
“why don’t the individual pinpoint stars flood the sky with light like billions of massive floodlights.”
Why don’t stars act like floodlights?
Because they are point sources. If you fly in a private plane at night over a long straight interstate, the headlights all look like individual points. They do not mush out into one long ribbon of light.
If there is no dust to scatter the light, you will only see the rays from the star that are aimed straight for you.
Headlights also have reflectors placed behind them so that ALL the rays )for all practical purposes) exit the headlamp parallel to each other and in the same direction. Stars are spherical and give off approximately equal luminosity in all directions.
JoeyBlades wrote:
If you read the original question by Bill and the first post in this thread by Bobby O, they are not asking for the explanation to Olbers’s Paradox. They were not expecting to see a sky so dense with stars that you can’t see gaps… Olbers’s Paradox. They want to know, given that the density of stars is basically as it appears, why don’t the individual pinpoint stars don’t flood the sky with light like billions of massive floodlights.
The original question was:
I’m enclosing an article that poses a question that had never occurred to me before: Why is the night sky dark? According to the author of the article, Robert Cowen, “the traditional answer holds that the universe is expanding so fast that light from the distant stars is degraded and thinly spread.” Another theory suggests, “the darkness is better explained by the simple fact that the universe is of finite age. Galaxies have not had time to flood the sky with starlight.” Excuse me, but aren’t we overlooking the obvious here? --Bill, Nanaimo, British Columbia
The explanations given in this question are regarding Olber’s Paradox. I didn’t follow Bill’s question about the obvious, but apparently you did. I guess that was in regards to seeing the stars with the distribution they have, etc.
JoeyBlades: In essence, Bobby O’s assessment is correct, for this question.
After your interpretation, yes. From my understanding of the original, Olber’s paradox was the question.
Bobby O: I assumed that since the sun is relatively close, and we can look at it without much interference, it doesnt need to reflect off of anything to see. We can see the stars, too, by looking at them. I thought the question was, “Why is the night sky dark?” not “Why can’t we see the stars?”
Bobby, your interpretation is a bit backwards. Stars are light emission sources, and thus much easier to see than planets, asteroids, moons, and dust that are light reflectors. Easiest demonstration - look up at night and count the visible stars. Then count the visible extra-solar planets. Hint: 0.
pldennison: Headlights also have reflectors placed behind them so that ALL the rays )for all practical purposes) exit the headlamp parallel to each other and in the same direction. Stars are spherical and give off approximately equal luminosity in all directions.
Not really parallel, but all pointing in the same general direction. However, the light from stars travels far enough before it gets here that the light rays are essentially coherent - all aligned in parallel rays. This is process that is emulated to make laser coherent. They use tubes with mirrors to simulate a long travel distance to let only the coherent light out.
This was a nice column. Astronomers would basically agree with Cecil’s answer - the finite age of the universe is the key reason why the night sky is dark. However, the two other effects he indicated with bullets in his column can’t be ruled out so glibly, and do play a role to some extent. The light emitted by the most distant galaxies is “redshifted” so they’re brightest in infrared light, which our eyes don’t see (a previous posting mentioned this). Also, when dust obscures the heart of a galaxy, it does start glowing itself, but again in infrared light, which we can’t see.
pachyderm
I’m not so sure that the infrared effects cannot just be dismissed–even though we can’t see infrared, we have a certain amount of sensitivity to it. If the night sky was ablaze with infrared radiation, we’d know about it pretty quickly.
Hmm, maybe I better go back and reread cecil’s column now.
Although we cannot see infrared, I can assure you that if the sky radiated an infinite amount of infrared, we’d notice.
Or, rather, we wouldn’t notice, since it’s hard to notice anything after being vaporized.
John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams
Also remember also that stars in space are not evenly spread out. In our own galaxy, the band we call the Milky Way shines a bit more brightly with stars because we’re looking from the inside of a big flat pancake-shaped group of stars. But really, all the individual stars you see are part of the galaxy – there’s just more of them between us and the outer edge.
But even the bright part of the galaxy isn’t blindingly bright… and the next decently sized galaxy visible from the northern hemisphere is the Andromeda Galaxy, and even that just looks like a faint smudge in the darkest of skies. All the other galaxies further away just look like tiny points even in big telescopes. Objects get dimmer with the square of their distance, and those galaxies are very far away, and also very far apart from each other.
There’s another reason the sky is black at night: A star that formed nine billion years ago at a distance of ten billion light-years would not be visible to us for another one billion years, right?
Maybe as the universe ages, it’ll get brighter as more stars form?
(Yes, I do know that stars age and die and become either a neutron star or a black hole or something I’ve not heard of. But surely the number of stars in the universe won’t remain consistent. Zero Population Growth for stars? If the amount of matter in the universe is finite, then the number of stars will remain essentially the same? Is the birth rate equal to the death rate? Or will the number decrease according to the Laws of Thermodynamics, the “Heat Death” of the universe? Or will the universe collapse upon itself because neutrinos have mass and therefore the universe is too massive to expand forever?)
I like thinking about this stuff…
>< DARWIN >
__L___L
Just nitpicking here, but this is not exactly true. While it is true that the laser does use a tube with mirrors to only let coherent light out, the important part of a laser is embedded in the etymology of the word-Light Amplification by Stimulated Emission of Radiation. That is, the laser tube doesn’t just act as a orientation filter, but it also acts to amplify only the light that is oriented along the axis of the tube.
If one were simply to place a light source in a tube with mirrors on the ends, it would still act as a point source of light and not as a source of coherent light.
TheDude
Yes, but in a flat, uniform universe, the number of objects at a given distance increases with the square of the distance, too, so that cancels out the inverse-square law, and it remains the case that if the universe were infinite, the night sky should be infinitely bright.
John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams