The dark night sky

Cecil's Columns/Staff Reports
1

The hypothesis that there aren’t that many stars doesn’t hold up.

Celestial objects brightness is measured by ‘apparent magnitude’. This is a scale that currently runs from -26.73 to a hypothetical 38 (but is inverse - negative means ‘really bright’). The human eye can see anything negative, and up to 6.5.

Thus, the reason the sky is dark at night is that most objects in the universe are so far away that not enough photons from them reach us per second, and our eyes are not that sensitive. Thus, they are just too dim to be seen.

2

To take it a step further, no matter where you look there are many, many stars and galaxies. To infer that that gaps between what we see is because ‘there aren’t enough stars’ steps around the fact that within the gaps we see there are billions of galaxies and stars. They are just so far away that you don’t get hit with photons from them often enough for them to be visible to you.

To put another way, if suddenly every object in the sky became visibly bright, the sky would indeed be filled with light.

3

Yeah, this one didnt make terribly much sense to me. Isn’t the atmosphere a large factor? You can see more stars in space.

4

To go even further, you can calculate the pixelation you would get by using the Hubble Deep Field and some simple math.

Someone should do that :stuck_out_tongue:

Take the number of objects in the deep field and multiply by its coverage ratio and thats how many pixels you would have lighting up the sky… generally…

I believe the deep field is one-two millionth of the sky from an individuals perspective I believe. It contains 10,000 galaxies. So the visible sky would be lit up by about 20 billion pixels. A high def flatscreen has about a million pixels if Im correct. I think if you surrounded yourself with 20,000 flat screens edge to edge displaying white light, I would consider that ‘lit up’.

No time to check my math, go for it.

5

Requisite link to article.

6

I was hoping this thread was about the horrors of light pollution.

It’s not dark at night any more and I hate it.

7

You may want to rethink your answer. :wink:
Unless, of course, you consider yourself much smarter than the entirety of the world’s professional astronomy community. :dubious:

8

Cecil answers this in the column itself. It doesn’t matter how dim the farther stars are; in an unbounded Newtonian universe with an infinite number of stars, the sky would be infinitely bright. The dimming caused by the inverse-square law would be canceled out by the number of stars in any arbitrary cone of observation increasing with the square of the distance.

9

If a star of (let’s say) magnitude 11 cannot be seen by the naked eye, I would assume that a thimbleful of hot glowing ionized hydrogen gas could not be seen at a distance of several million miles. It would be too small, we would be too far away from it, the total light output would not cause enough of it to head in the specific direction of our eyeballs for us to register it.

But if that thimbleful is adjoined top and bottom and left and right with other simlar thimblefuls, and those likewise, and so on, eventually you can see something, yes? After all, our sun is rather spectacularly visible, even much much farther away from it than we are, and if you started out with thimblefuls, somewhere between the first thimbleful and the volume of the sun you’d hit a point were it would be visible. Yes? With me so far?

OK, now suppose, having determined the exact number of thimblefuls sufficient to cause us to be able to see it, we arranged to have 2/3 of them removed, leaving a sort of swiss-cheese latticework of hydrogen-thimblefuls. The loss in light would mean we could no longer see it (it was just sufficiently bright when we started out). Still with me?

Now suppose we have the removed thimblefuls of hydrogen relocated an additional 500 yards farther away from us, and position them so that from our vantage point they are lined up with the holes created when we removed them from the original mass. If we could see them at that distance, they would be in our line of sight, just as the ones we did not remove are in our line of sight. Is the combined arrangement, consisting of the swiss-cheese latticework in front and the other, more spread-out haze 500 yards further away, visible to us? Well, no, because 2/3 of the light source that was just sufficient to be visible has been relocated farther away, making it dimmer.

But if we have someone come in and keep adding additional thimblefuls out there in the midst of the more diffuse haze, will the entire works eventually become visible? It seems a matter of common sense to me that the minimum number of additional thimblefuls required to accomplish that would be the number it takes to cover the same visual field: in other words to fill in the holes that we created when we removed the original thimblefuls. You would need a lot more of them of course because in their original location they took up space like this if you looked at them through binoculars:

O

but 500 yards farther away they look like this:

O
So to get enough light from them at that distance, you need to do something like this:

…O
.OO
OOO
.OO
…O

By extrapolation, it doesn’t really matter how far away they are, as long as you’ve got enough of them to fill in all the holes. A loose latticework at any distance without more stars behind it to shine through the holes is going to mean you can’t see anything in that direction once you’re too far away from it.

But if the univere is infinite, then no matter how loose the latticework is at some randomly-chosen distance (and direction), every single little micropixel of space within the holes in the latticework is going to be filled — some spots at pretty close range, others not until you go a lot farther back, but every single bit of it sooner or later.

So the universe ought to be looking uninterrupted bright white.

(Unless it’s expanding or something)

10

I know I’m getting crankier in my old age, but posts like the OP bother me more than they used to. Olber’s paradox has lasted for 150 years and has been examined by some of the world’s top minds. How could anyone even consider, let alone assert on a message board, that all of them have missed such a trivially simple answer? Wouldn’t the thought occur to you instantly that you must have misunderstood some part of the question if the answer were this obvious? (Not to mention what some conventionally-minded people might deem to be a fairly serious obstacle: that the answer was specifically dismissed in Cecil’s column[!])

This is just one example of a larger malady that occurs in all scientific fields, as when people construct “proofs” of trisecting the angle or announced they have proven that information can be sent faster than the speed of light by wriggling the end of a very long stick or insist that they can divide by zero. Somewhere along the way they’ve fundamentally misunderstood the terms and conditions of the problem.

In Olber’s case, there are two essential underlying assumptions that must be addressed to remove the “paradox.”

  1. The number of stars is infinite.

  2. The universe is of fixed size.

Once stated, it’s easy to see that the OP’s answer neglects assumption 1. It doesn’t address infinity.

As others have already mentioned, even the addition of extremely small numbers can eventually produce an infinite result. (Take the arithmetic sequence 1/2 + 1/3 + 1/4 + 1/5 … Although it adds up slowly, you can exceed any finite number by extending the sequence far enough.) Olber assumed that the universe and the number of stars in it was infinite, having no reason not to do do. In an infinite universe, any sun-sized circle of the sky would have to eventually hold the sum total of a sun’s worth of photons. In fact, infinitely more than that.

As the arithmetic sequence shows, you don’t even need to assume infinity. Since the series will eventually exceed any finite number, and the number of photons from the sun is a finite number, some early point in the sequence will exceed that number.

We now know that the number of stars we can see is a comparative small finite number, much smaller than would be required to add up to the sun… The universe is also not fixed, but expanding. By showing that neither assumption is true, Olber is refuted, although by attributes of the universe he couldn’t possibly know.

But both of these truths are basic high school level astronomy, available in any textbook or popular science book or science magazine or on the web.

That shouldn’t matter. They could be terrifically obscure bits of postgraduate-level knowledge for all the difference it made. The important point is that posts like that of the OP show a lack of basic reasoning skills (and a frightening amount of ego).

If you think you’ve come up with an answer to an age-old problem that would occur to any scientist in the first ten seconds of thought, you’ve misunderstood the problem. Realizing this and what this means is the first step toward scientific understanding.

It will also make me less cranky. That’s a boon to all humanity. :smiley:

11

Surely you can understand this, from the sound of your post you’re a teenager yourself. :stuck_out_tongue:

12

Be proud of your crankiness. Once again, you’ve stepped in and essentially written the post I would have written if I’d taken the time and put the necessary effort into it. You’re like a great labor-saving device for me.

13

Now we know what makes elections! :smiley:

14

Well at least the OP didn’t title his thread Olbers’s paradox solved! At least one POAT thread started like that.

15

But wouldn’t it be wild if someday a guest poster showed up here, posted an original, amazingly correct solution for a famous paradox, or proof for a famous unproven theorem, then vanished, to become one of those mysteries in the history of science?

16

And in the future, our children will study the genuis of Newton, Einstein, Galileo, and BigBootyLover237.

17

Quote of the day.

RR

18

Those are some great attacks on independent thought! Science… dogma… great combo guys. Hey all you researchers out there - freeze! We’re done!

I excluded the infinite universe half of the answer because it is so clearly correct (yes there are two parts to the ‘why is the sky dark’ question). I’m dealing with the more straightforward side of it, the observable - it’s light already got here. 150 years ago no one knew the density side of the equation relating to the observable universe half of the answer. The hubble deep field gives us a great way to calculate it directly.

So far no one has actually attacked my hypothesis. To disprove my hypothesis of ‘density is plenty, it’s dimness that matters’, in my opinion you merely need to answer the following questions:

  1. How dense would the pixelation be in the sky be if every object in the observable universe were visible to the naked eye (lets use current data, not data from 150 years ago, this time)
    2a. Now that we know the pixelation, how dense does the pixelation have to be to consider it ‘lit up’. To me “the entire sky is uniformly white with light from my perspective” covers it.
    2b. Conversely, you could calculate the actual amount of light, based on the pixelation and a minimum brightness for each object, and then ask yourself the same question

I used 2a, and my answer was “we’re way beyond that level of pixelation”, and I put my calculations out there. Not masterful, sure, but enough for someone to attempt to disprove it. Go for it.

If you think discussing this idea is a big deal, whatever, it’s just a simple thought excercise to me. If it contradicts something old, good! If you think that is presumptuous, I think that thought makes you presumptuous. Don’t be afraid to think and don’t blindly bow to authority.

19

Forgot one thing… Olber’s Paradox doesn’t show the sky is dark because the universe is infinite. It shows the universe is infinite because the sky is dark.

Cheers.

20

No one disagrees with the notion that Olber’s Paradox doesn’t apply to a finite universe. That’s the whole point of the original column.