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Save those little specks of light up there, space is awfully dark. It’s such a fundamental assumption, I never thought to ask why. But as someone on the sci.astro Google group recently wondered, given that there are billions upon billions of stars in the night sky, most brighter than our own sun, shouldn’t they light up the night like a Fourth of July fireworks display? Actually, there’s already a name for this question — Olbers’ Paradox, after German astronomer Heinrich Wilhelm Olbers who described it in 1823 — but it was also asked, perhaps most elegantly, by Edgar Allen Poe (of all people) in his 1848 essay Eureka:
“Were the succession of stars endless, then the background of the sky would present us a uniform luminosity, like that displayed by the Galaxy –since there could be absolutely no point, in all that background, at which would not exist a star. The only mode, therefore, in which, under such a state of affairs, we could comprehend the voids which our telescopes find in innumerable directions, would be by supposing the distance of the invisible background so immense that no ray from it has yet been able to reach us at all.”
Well, Edgar, it seems that the complete answer involves quite a bit of math (”Nevermore!” quoth I after completing pre-calculus in the 11th grade), but the short answer is mathless, fascinating, and a two-parter:
1) The universe is not infinite, and neither is the speed at which light travels. If they were, then the night sky would be as bright as the surface of the sun. As it happens (according to prevailing theories, at least), the universe Banged itself into existence about 13.7 billion years ago (give or take a few hundred million years), so that’s all the time light has had to travel from distant stars to our neck of the woods. It would hold, then, that while finite, the universe is considerably more than 13.7 billion light years across, which would help explain why the night sky isn’t blinding, and why even our most powerful telescopes can still find patches of nothingness in the Heavens.
2) The universe is expanding, and with that expansion comes a phenomenon known as redshift — kind of like a visual Doppler effect. Starlight is moving away from us, which shifts its frequency higher, sometimes right out of the visual spectrum and into the microwave spectrum. So huge portions of what would otherwise be potentially blinding starlight has become a steady, cosmic microwave background radiation.
Pretty cool, huh?
Comments (22)
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“13.7 million years ago”
I think thats supposed to be BILLION :)
posted by ashmantle on 9-10-2007 at 6:36 am
On Part 2:
As a transmitter is approaching, the Doppler effect is noted by the apparent increase in the frequency of the transmission. The frequency appears to drop as the transmitter moves away.
If the stars are moving away from us, then the frequency should appear to be dropping. That would drop us through the (invisible) infrared range into the microwave range.
posted by DaveR on 9-10-2007 at 7:33 am
Well, since I just learned at the dilbert blog that cold is the absence of heat, then dark is the absence of light.
neener, neener, neener.
posted by Going Like Sixty on 9-10-2007 at 11:12 am
Little thing called the “inverse cube law”. Doesn’t take calculus to figure this one out, although I could give you the equations if you’re anal enough to ask for them. Basically, the further you are from a light (or gravitational) source, the weaker the effect that souce has for you. Flashlight shining in the face at one meter (like the cop catching you porking your favorite squeeze, or the most available one) at the end of Lover’s Row is bright, the same flashlight at 100 meters provides little to no usable illumination.
Like we used to say in the US Army, I can see you from 10 meters with my light, you can see (”spot” in artillery and sniper lingo) me at 1000 meters. But it’s just a teeny tiny little speck at that range…
Remember what the Hitchikers’ Guide to the Galaxy (via Douglas Adams) said, “Space is really REALLY big.”
posted by Doc on 9-10-2007 at 11:38 am
To “Going Like 60″ – sorry, you ought to get your information from someplace other than a comic strip blog:
Cold and dark are not the absence of anything, they are reduced levels of heat and light.
The mean temperature of the Universe is about 3 degrees Kelvin. There’s still heat out there, just not much of it if you average it out over intergalactic spaces. The complete absence of heat would be zero Kelvin, and that will not happen – see 3rd Law of Thermodynamics. Heat’s gotta go someplace, even if it IS into the Great Void.
If/when all the stars burn out, the Universe will still have a mean temp of 3 Kelvin.
posted by Doc on 9-10-2007 at 11:47 am
A Brief History of time also explains Doppler effect with sound:
‘A nascar or ambulance is high pitched sounding as it approaches, lower pitched as it descends. This is because of the sound waves catching up with, or running away from the source as it moves. All depending on your position from the vehicles direction.
When astronomers looked at the stars to see this effect, they expected about half to be blue, and half red. But to their surprise everything in the universe was moving away from us at an incredible rate. ‘
Thats a paraphrase from memory so excuse me, but you should check out the illustrated version for more.
But what about dark Matter. Its almost a guarantee that it exists (heck technically humans are dark matter)
We can see it w/ gravitational pulls, Microwaves and it helps explain the absence of light. Theres lots of things sucking up the light(black holes) or blocking it.(dark matter)
posted by Eric E on 9-10-2007 at 2:21 pm
Are we getting a bit complicated? Lets go back to Meteorology 101 –
We see light when it is reflected or refracted, i.e. there must be something for the light to illuminate for us to sense it. My cube-graveyard of an office is only lit because the surfaces in here reflect certain wavelengths of light.
It is the same with sensing other energies: to know something is hot we must feel something warmed, to hear a sound something must vibrate.
For “space” to be illuminated, it must have some endless surface or reflect or bend light to us. The gasses that stars are made of refract certain wavelengths of light to us (which are seen through the lens of our own atmosphere) and are red/blue/yellow stars/planets. Unless, of course, the backdrop of “space” is a color absorbing black, or there is what resembles a solid mass of planets and stars visible form Earth…
posted by manderson on 9-10-2007 at 2:40 pm
In this article, you say “Starlight is moving away from us, which shifts its frequency higher…”. If it is moving away, that would shift the frequency lower.
posted by Elias on 9-10-2007 at 4:17 pm
Ok, physicist here. I’ll do my best.
Redshift means the frequency gets lower. It’s the wavelength that gets larger.
Cold is a relative term, unless you’re talking absolute zero. Anything that has a lower temperature than you (or whatever you’re comparing) is “cold” and anything that has a higher temperature is “hot”.
Dark, on the other hand, is tricky. Any spot of vacuum, in the absence of photons, is dark. Space, on average, has a very weak “background radiation” on the microwave scale. These are photons left over from the big bang that got stretched out as space expanded. Overall, we can call space “dark” because there is no light coming from it that is visible to us.
On dark matter: Humans are not made of dark matter. In fact, nothing we can see is made of dark matter. We don’t know much about dark matter, but we do know that it consists of “exotic” particles that are not protons, neutrons, electrons, or any other known particle. Dark matter does not absorb light- it doesn’t even interact with light. That’s why it’s called “dark”. It is affected by gravity, however, and that’s how we know it’s there.
Stars do not refract light- at least not in a significant way. We only see certain wavelengths of light because those wavelengths represent photons released when electrons drop to lower energy levels in an atom. Each element has its own signature emission spectrum, which is how we know what stars are made of. Sometimes bits of the spectrum will be missing, and then we know that some sort of “dust cloud” that absorbs that wavelength is in between us and the star.
Stars also do not reflect light. In fact, a star is about as close as you’ll get to a perfect “blackbody”. That’s an interesting topic too, if you’re interested in the history of quantum mechanics.
As you can see, I can go on like this forever. But I’ll be merciful and shut up now.
posted by Iris on 9-10-2007 at 5:57 pm
Does anyone remember C.S. Lewis’ “Out of the Silent Planet”? He describes the space travel under the theory that space is extremely bright because of the sun. I always thought it interesting even though not possible- apparently he thought about this question too. :)
posted by Julia Rae on 9-10-2007 at 9:38 pm
If/when all the stars burn out, the Universe will still have a mean temp of 3 Kelvin.
Well, as the universe is expanding it’s average temperature will continue to drop, getting ever closer to 0 Kelvin. C.S.Lewis is right, space in the vicinity of our sun is well lit, it continues to look dark because there’s not a lot to illuminate. Some colleagues of friend took some persuading that brightly lit broadcast images of the ISS and shuttle are lit up by the sun rather than powerful spot lights.
posted by kafircake on 9-11-2007 at 2:43 am
That’s one theory.
A less popular, but still possible, theory is that light slowly loses energy over time and therefore continually drops to lower and lower frequency. If that were not true, then light would be equivalent to a perpetual motion machine, which physicists all agree can not exist, since it would violate laws of physics.
To say the universe is not infinite is quite presumptuous. How does anyone know that? haha No one *knows* that. It is simply something most scientists would like to believe.
It is quite valid to consider that an infinite sized universe originated at the time of a big bang. There is no reason to suppose that a universe originating from the big bang would have to be finite.
Additionally, it is possible that the big bang originated in an already preexisting universe of infinite size.
The only caveat is that the *visible* universe is of a finite dimension … but that is just a consequence of the limit of the speed of light.
One last point … the big bang itself is questionable. If light is not a perpetual motion machine, then there is no need to have an expanding universe to explain its red shift over long distances and times.
posted by William in Boston on 9-11-2007 at 11:16 am
I’ve often wondered if the “Big Bang” was only one of a succession of Big Bangs…
posted by Sheralyn on 9-11-2007 at 5:19 pm
Dumb questions with smart answers #23: Why is space dark?
Dumb answer: Lack of light.
posted by PM Toone on 9-12-2007 at 7:41 am
if there are so many objects in space, it would stand to reason that millions of objects are behind other objects. so a star we see in the sky could have hundreds, thousands or millions of objects behind it “blocking” that light.
when the planet moves, stars become visible while others go away from sight. the ones that become visible then have other stars behind it that become non-visible. and this is only in a 2d plane since that is our way of “seeing” objects. we cant look around an object 50 million light years away until the planet moves to a different angle, which then blocks more. etc etc etc.
posted by t on 10-8-2007 at 11:03 am
The amazining universe.I just can’t get enough of it!
posted by WUGGY6X9 on 10-8-2007 at 1:49 pm
hey the universe isn’t dark….
there are things which is hidden from our eye sight as our retina can’t be focus all the things to our brain.
Besides is all under gravity control.
No gravity, no life exist!
posted by maumau on 10-8-2007 at 7:22 pm
I just have trouble imagining space being infinite. It’s hard to wrap my head around the idea that space is not some actual finite space. I think of space like a huge room, and what’s outside surrounding the room? I’ve thought about that for years, and it still blows my little pea-brain away…
posted by Vickey on 2-6-2009 at 6:21 am
is the universe is expanding, what is it expanding into? what is on the other side…
posted by Dummy on 2-6-2009 at 8:26 am
Thank you William in Boston– your remarks regarding the finality of the universe are quite on point. To expand on it:
The assumption by physicists that the universe is finite is more a relic of conservation of mass than anything else– if the “big bang” theory is correct, then scientists make the assumption that the original element of the big bang must have had a finite mass. However, things we “knew” to be true have not proven to be “universal” (pun INTENDED), especially regarding Newton’s laws, which have lost a lot of meaning in terms of “space” and the atomic level since Einstein’s theory of relativity.
Besides that, the possibility that space is not finite (i.e. infinite) has an infinite amount of possible (not necessarily plausible) explanations, but that’s another conversation.
This page reminds me of those binge-drunk conversations my fellow engineers and I used to have regarding space’s finality, like, “if the universe is infinite, then is there someone somewhere doing the exact same thing and talking about the exact same topic as us right now?”
posted by Joel on 2-6-2009 at 9:48 am
Astrophysicist here:
your answer is basically correct except for one thing: the Universe is not necessarily finite in extent. There’s no way to know. We DO know that it’s bigger than the horizon (the total distance light could travel since the Big Bang). There’s just no way to prove that it’s infinite or finite if there’s no way to see outside this horizon. Thankfully, the solution to Olber’s paradox doesn’t depend on this point. The age of the Universe and redshift are good enough by themselves.
posted by Brad on 2-6-2009 at 10:39 am
very interesting!
but this is what makes my brain almost short circuit-
so the universe is still expanding, so the space that the universe inhabits is even bigger…but what’s containing *that* space?
like- where is all of this stuff being held, and how could just infinite space exist?
makes me think of all the ancient myths about nothing but Chaos existing, and from Chaos came the gods.
posted by em on 2-7-2009 at 4:33 am