Olbers's Paradox - Why is the sky dark at night?

A couple of thought on this...

Donald Hamilton wrote; "Olbers thought that if the universe was infinitely large and infinitely old..."

I think the issue here is the age of the universe. The current estimate is, I believe, about 13.7 billion years.

Now, it would seen obvious that we could not see anthing more than 13.7 billion light years (LY) away, since the light from more distant objects has not had sufficient time to reach the Earth.

As with so many obvious things, this is untrue - thanks to the expansion of the universe, we actually see light from objects that are now much further away. The expansion means that at very great distances, these galaxies are receeding from us faster than the speed of light. This sounds odd, but does not violate special relativity as the velocity is due to the expansion of space itself, rather than motion *through* space. The limit of the observable universe (IIRC - I stand to be corrected) lies at roughly 40 billion LY.

The night sky is dark because the light from more distant galaxies has not yet arrived (and since, as has been recently discovered, the expansion rate is increasing, it never will).

If the universe was static (not expanding) and was infinitely old and large, then Olbers would have been right, and the whole sky would have the same brightness as the sun.

Something similar to this might actually happen on any planets unfortunate enough to exist deep within the densely - packed groups of stars called Globular Clusters.

One other thing - I'm not sure about this, but I don't think redshift affects the visual appearance of a star. A rapidly receeding star does not look 'dimmer' or 'redder' to the naked eye. That's because stars emit radiation over a broad swathe of the electromagnetic spectrum, not just in visible light.

So as the visible light from the star is redshifted into the infra-red, light from the UV part of the spectrum replaces it.

Redshifting (or blueshifting for approaching objects) is detected by analysing the position of absorbtion and emission lines, specific frequencies of light fixed by the quantum properties of atoms.

Dan Ibekwe

Manchester, UK

If humans were a nocturnal animal it would have evolved eyes similar to these. The following is a description of the contrast between the human eye and the nocturnal eye. See the link at the end of the article for more info..

"What appears as pitch black to a human may be dim light to a nocturnal animal. The reason lies in the structure of the eye itself.

Pupils - Nocturnal animals tend to have proportionally bigger eyes than humans do. They also tend to have pupils that open more widely in low light. So, at the outset, nocturnal eyes gather more light than human eyes do.

Rods and cones - After the light passes through the pupil, it is focused by the lens onto the retina, which is connected to the brain by the optic nerve. The retina is an extremely complex structure. It's made up of at least 10 distinguishable layers, and is packed with more sensory nerve cells than anywhere else in the body.
The retina is home to two different kinds of light receptor cells -- rods and cones. (Both are named after their relative shapes.) Cones work in bright light and register detail, while rods work in low light, detecting motion and basic visual information. It is the rods that become highly specialized in nocturnal animals. In fact, many bats, nocturnal snakes and lizards have no cones at all, while other nocturnal animals have just a few.

Tapetum - Many nocturnal eyes are equipped with a feature designed to amplify the amount of light that reaches the retina. Called a tapetum, this mirror-like membrane reflects light that has already passed through the retina back through the retina a second time, giving the light another chance to strike the light-sensitive rods. Whatever light is not absorbed on this return trip passes out of the eye the same way it came in -- through the pupil. The presence of the tapetum can be observed at night when a pair of glowing eyes reflects back a flashlight or some other light source. (Interestingly, different animals have different color tapeta, a fact that can aid in nighttime animal identification.)

Circular vs. slit pupils - One consequence of having extremely light sensitive eyes, is that they must be adequately protected during the day. Some animals accomplish this with a retractable eye flap. Others rely on their pupils. The circular pupil, because of the way the muscle bunches as it contracts, is the least efficient at closing rapidly and completely. A slit pupil, with two sides that can close like a sliding door, is far better at this task, which is why so many nocturnal eyes have slit pupils. These apertures can be vertical, horizontal, or diagonal."

http://www.pbs.org/wgbh/nova/kalahari/nightvision.html

So you see - this is the reason why preceive the night to be dark. It is simply the way our eyes evolved.

Don Hamilton http://novan.com

Mr. Hamilton,

I think you strayed from the point of Olber's Paradox, going off on a tangent about our eyes, etc. If there had been a star in every direction that was equally as intrisically bright as our sun, then the sky should be blaizing with the same light as the surface of the sun. Instead of having any "night" at all... we should be baked to a crisp! (Actually, we would be vaporized and ionized plasma.)

This doesn't happen because of Hubble expansion, the galaxies are receeding at an higher rate the further they are from each other. This causes a dopplar red-shift to ever less energetic light reaching us from the more distant stars. So instead of a series of stars at the same brightness with distance, we have a series of decreasing brightness with distance, which allows the stars, perhaps infinite stars, to "fade away" with distance allow us to exist, and wonder at the distant points of light in the night sky.

Finally, the owl is in no way crippled in the day time. It uses the same basic combination of rods and cones to allow it to see in both day and night conditions as we do. It may see better in the night than do day time hunters, but it isn't that much better. It's real advantage for hunting at night is superb hearing and sound location. It can hear a mouse russling in the leaves and swoop down on it even when it can't see it in near total darkness. It's eyesight is primarily used to avoid flying into objects. In the daylight, the owl, like all birds, has better color vision than we do! We are only trichromats, while the birds are tetrachromats. Compared to the birds (owls included) we are colorblind!!

In terms of dark adaption, we can see as well as the owl. Our rods are just as sensitive and can detect a single photon. I recall with great fondness one moonless night I spent camping in a mountain meadow. I could walk about freely, able to see every rock, every tuft of grass, as clearly as though it were merely a cloudy day. The trick is that I became fully dark adapted, never once resorting to using a flashlight. Admittedly, I couldn't see as much fine detail, the rods having neural circuitry that pools groups of rods to give better signal to (quantum) noise, at the expense of resolution. But the owl rods work in the same way, trading off signal to noise ratio for resolution. The owl has higher number of rods than we do, so it has better resolution, but those rods individually are no more sensitive than ours. The owl has paid the price for increased night time resolution in terms of reduced daytime resolution... BUT NOT light adaption. It can and does see in the daytime.

Many night hunting animals do have one advantage over us. They have a special reflective layer behind the light sensing nerves to allow the rods a 'second chance' at capturing light that may have slipped past them. We don't have that. Instead, we have dark pigment, so that light that slipped past us *won't* be reflected! This allows us to see sharper images in higher contrast, undistracted by internally scattered light. This is the trade-off, sharper images or more light capture in the dark.

--Candice H. Brown Elliott