Inverse square law explains Olbers' paradox?

[humbleteleskop]

Hello,

This is the thread I originally wanted to respond to, but it's closed:
https://www.physicsforums.com/showthread.php?t=650126


I also found this on Wiki-talk page, which seems to be the same argument:
http://en.wikipedia.org/wiki/Talk:O...uare_law_actually_does_explain_dark_night_sky

a.) left image representing first shell contains 10 bright stars

b.) right image representing second shell has 40 stars each 4x less bright

c.) total light received is the same, but does that make them equally bright?


Now, my question is, are those two images indeed correct representation of the paradox? And if so, are those two images really supposed to be equally bright? I mean, are they? They certainly don't look equally bright. So what's the trick, is this some kind of optical illusion, or something?

 

[Drakkith]

The trick is that the light is spread out more in the 2nd picture, which leads to it looking dimmer overall. If you concentrated all the light into one spot the image would "appear" to us to be brighter because it is more noticeable. In other words, the difference in brightness between different areas of the image is greater in the first picture than in the second picture, and our eyes and brains have an easier time seeing this greater difference.

 

[humbleteleskop]

The trick is that the light is spread out more in the 2nd picture, which leads to it looking dimmer overall. If you concentrated all the light into one spot the image would "appear" to us to be brighter because it is more noticeable. In other words, the difference in brightness between different areas of the image is greater in the first picture than in the second picture, and our eyes and brains have an easier time seeing this greater difference.

Ok. But what I'm trying to point out is that those two images are not equally bright in the same way Olbers' paradox would have us believe, from Wikipedia:

So if we receive the same amount of light from each shell, but individual stars from further away appear dimmer than closer stars, then I see no paradox since that's what we actually observe in reality and is explained by the inverse square law.

 

[Jonathan Scott]

Ok. But what I'm trying to point out is that those two images are not equally bright in the same way Olbers' paradox would have us believe, from Wikipedia:

So if we receive the same amount of light from each shell, but individual stars from further away appear dimmer than closer stars, then I see no paradox since that's what we actually observe in reality and is explained by the inverse square law.

You're confusing the issue with the use of the word "dimmer" here. The point is that the brightness of any part of the surface of an individual star is approximately independent of distance. If it's further away, the angular size is smaller, but the brightness over the smaller angular area is essentially the same, so it's only dimmer in the sense that it covers a smaller area of the sky.

 

[humbleteleskop]

You're confusing the issue with the use of the word "dimmer" here. The point is that the brightness of any part of the surface of an individual star is approximately independent of distance. If it's further away, the angular size is smaller, but the brightness over the smaller angular area is essentially the same, so it's only dimmer in the sense that it covers a smaller area of the sky.

I suppose that Wikipedia animation is misleading in more than one way. We are talking about human eyes here, all those stars (galaxies) appear as point light sources and their "apparent magnitude" does indeed get dimmer with the distance, according to inverse-square law.

http://en.wikipedia.org/wiki/Inverse_square_law
http://en.wikipedia.org/wiki/Apparent_magnitude


Besides, Olbers' paradox is taking it into account, so even if it was not really true the two pictures in my first post would still correctly represent what the paradox actually states, and my primary objective is to establish just that. I believe Drakkith confirmed those two pictures do represent the paradox correctly. I take it you disagree, so I'm asking you to reconsider.

 

[Jonathan Scott]

I suppose that Wikipedia animation is misleading in more than one way. We are talking about human eyes here, all those stars (galaxies) appear as point light sources and their "apparent magnitude" does indeed get dimmer with the distance, according to inverse-square law.

http://en.wikipedia.org/wiki/Inverse_square_law
http://en.wikipedia.org/wiki/Apparent_magnitude


Besides, Olbers' paradox is taking it into account, so even if it was not really true the two pictures in my first post would still correctly represent what the paradox actually states, and my primary objective is to establish just that. I believe Drakkith confirmed those two pictures do represent the paradox correctly. I take it you disagree, so I'm asking you to reconsider.

Calculate what the sun looks like from twice as far away. It appears half the linear size and a quarter of the area, but the amount of the surface area of the sun you can see in a given angular area has multiplied by 4 at the same time. This factor cancels out with the inverse square effect of the distance so that the surface is emitting the same amount of power per angular area (normally known as solid angle) as seen from any distance. So overall it is a quarter of the total brightness, but each part of the surface has the same brightness (in terms of power per angular area - usually known as solid angle - at the observer) regardless of the distance.

For a distant star, we cannot necessarily resolve it as anything other than a point, but Olbers' paradox does not require us to be able to do so.

The limitations of human eyesight are not relevant here (except that at least our lack of resolution prevents distant stars from damaging our retina!) If you replace a single star by four stars which are twice as far away (making the same total light) then human eyes may not be able to resolve each one but their contribution to the overall level of light is unaffected by the method of observation, and the angular area they cover is the same.

Olber's paradox doesn't even require any particular radial distribution of stars to work, except that there should be sufficient stars that every line outwards from the observer should eventually encounter a star. If that happened, the sky would be as bright as the sun in all directions.

As far as I can see, the Wikipedia animation is accurate and helpful.

 

[humbleteleskop]

Calculate what the sun looks like from twice as far away. It appears half the linear size and a quarter of the area, but the amount of the surface area of the sun you can see in a given angular area has multiplied by 4 at the same time. This factor cancels out with the inverse square effect of the distance so that the surface is emitting the same amount of power per angular area (normally known as solid angle) as seen from any distance. So overall it is a quarter of the total brightness, but each part of the surface has the same brightness (in terms of power per angular area - usually known as solid angle - at the observer) regardless of the distance.

I don't disagree with that, but we are talking about point light sources, thus inverse-square law applies. Do you agree?


Therefore, if we represent the first shell with 10 bright stars, we must represent the second shell with 40 stars where they are 4 times less bright than stars in the first shell, like this:

...so the total amount of light received from both shells is the same. Right?

 

[Drakkith]

Don't get caught up in "point sources". Whether we can resolve an object or not doesn't matter, the amount of light received is still the same. If it helps, just imagine that we have a perfect optical system capable of resolving any object, no matter how small/distant.

 

[humbleteleskop]

Don't get caught up in "point sources". Whether we can resolve an object or not doesn't matter, the amount of light received is still the same. If it helps, just imagine that we have a perfect optical system capable of resolving any object, no matter how small/distant.

I agree we need not to worry about much detail for our hypothetical scenario, it is supposed to be generalization. And I agree the amount of received light is the same from each shell regardless of any such detail. The point where we disagree is when you say 40 grey dots are as bright as 10 white dots.

Imagine we arrange those dots into one little white square on the left and one big grey square on the right. Yes, they both "received" the same amount of light, but they are not equally bright. The white box/dots will always stand in contrast against the grey box/dots regardless of their size or quantity.


More importantly however, I'd like if we could first establish this: the stars in Olbers' paradox are considered as point light source and thus inverse-square law applies, so that stars in every shell have the same apparent size as stars in a previous shell and are four times less bright.

 

[Jonathan Scott]

I agree we need not to worry about much detail for our hypothetical scenario, it is supposed to be generalization. And I agree the amount of received light is the same from each shell regardless of any such detail. The point where we disagree is when you say 40 grey dots are as bright as 10 white dots.

Imagine we arrange those dots into one little white square on the left and one big grey square on the right. Yes, they both "received" the same amount of light, but they are not equally bright. The white box/dots will always stand in contrast against the grey box/dots regardless of their size or quantity.


More importantly however, I'd like if we could first establish this: the stars in Olbers' paradox are considered as point light source and thus inverse-square law applies, so that stars in every shell have the same apparent size as stars in a previous shell and are four times less bright.

40 dots of 1/4 the brightness do add up to the same total luminosity as the 10 dots. If you move and adjust the shape of the dots (without changing the area) until they formed a contiguous area of the same shape, there should be no visible difference between the two. Obviously the human eye can detect contrast more readily when the power is concentrated into fewer larger dots, but contrast is not relevant here, only the total power and the total solid angle from which it is being emitted.

Olbers' paradox would not work if stars were indeed points, as it is necessary for each star to subtend a finite solid angle, however small. The "apparent size" is not relevant.

 

[someGorilla]

There's a different issue at work here. Normally the light emitted by a computer screen is not a linear function of the RGB values.
Try setting your screen's gamma correction to 1. You will see how your two pictures have more or less the same brightness.

 

[Drakkith]

I agree we need not to worry about much detail for our hypothetical scenario, it is supposed to be generalization. And I agree the amount of received light is the same from each shell regardless of any such detail. The point where we disagree is when you say 40 grey dots are as bright as 10 white dots.

Imagine we arrange those dots into one little white square on the left and one big grey square on the right. Yes, they both "received" the same amount of light, but they are not equally bright. The white box/dots will always stand in contrast against the grey box/dots regardless of their size or quantity.

We need to be specific here. The total amount of light emitted by the dots is the same. However, they are not equally bright because brightness is the visual perception of the luminance of a surface, which itself is a measure of how much light is emitted per unit area. In other words, with 40 dots the same light is spread out over a larger surface area and the image is dimmer. Note that in reality you aren't looking at the picture as a whole and deciding how bright it is. You are looking at the picture and seeing that the dots are brighter in one picture than the other. Our eyes and visual system simply aren't designed to accurately measure overall light intensity in this manner. It's much easier to see and compare distinct light sources than an overall scene.

One thing to understand here is that if we look at both images from far enough away so that we can't resolve each dot individually, the two images will appear equally bright. Think of each dot as a flashlight. If the flashlights are so far away that we can't resolve each one as an individual light source, then the combined light from the 10 brighter flashlights looks equally as bright as the 40 dimmer flashlights.

More importantly however, I'd like if we could first establish this: the stars in Olbers' paradox are considered as point light source and thus inverse-square law applies, so that stars in every shell have the same apparent size as stars in a previous shell and are four times less bright.

It doesn't matter if they are considered point sources or not, the inverse square law still works just fine. And I don't understand what you're saying about the apparent size of the stars. Point sources are not resolved and don't really have an apparent size.

 

[humbleteleskop]

40 dots of 1/4 the brightness do add up to the same total luminosity as the 10 dots. If you move and adjust the shape of the dots (without changing the area) until they formed a contiguous area of the same shape, there should be no visible difference between the two. Obviously the human eye can detect contrast more readily when the power is concentrated into fewer larger dots, but contrast is not relevant here, only the total power and the total solid angle from which it is being emitted.

Luminosity is a measure of emitted light, brightness is a measure of received, or better to say perceived, light. Luminosity refers to an actual object and is objective property, brightness refers to an image of an object and is subjective property. As Drakkith points out we have to be careful about various types of light measurement which often have subtle but important differences.

Luckily, in this case, we can avoid having any kind of semantic argument or disagreement due to differences in our definitions and interpretations. Instead of talking about words, we shall talk about pictures, and instead of talking about image brightness, we shall talk about whether our images are simply true or false. The goal is of course to establish correct visual representation of Olbers' paradox and thus conclude if it is indeed different from what we actually observe, or not.


Therefore I state, if we represent the first shell with 10 bright stars, we must represent the second shell with 40 stars 4 times less bright than stars in the first shell, like this:

...so the total amount of light received from both shells is the same.

True or false?

 

[Drakkith]

That's right. But note that while each star in the 2nd shell is 4 times dimmer, it is also 1/4 the apparent size that the stars in the 1st shell are (assuming we can resolve them). Understand that Olber's paradox isn't about point sources. Point sources are the result of our inability to resolve far away objects. That just means that the the size of the airy disk of the focused light is larger than the image of the object at the focal plane.

 

[humbleteleskop]

We need to be specific here. The total amount of light emitted by the dots is the same. However, they are not equally bright because brightness is the visual perception of the luminance of a surface, which itself is a measure of how much light is emitted per unit area. In other words, with 40 dots the same light is spread out over a larger surface area and the image is dimmer. Note that in reality you aren't looking at the picture as a whole and deciding how bright it is. You are looking at the picture and seeing that the dots are brighter in one picture than the other. Our eyes and visual system simply aren't designed to accurately measure overall light intensity in this manner. It's much easier to see and compare distinct light sources than an overall scene.

I agree it was a mistake to talk about brightness of an image as a whole, we are really only concerned about brightness of the stars. The stars will eventually completely fill our image and then "image brightness" will make more sense in relation to whether it is uniformly bright or not, but until then it's misleading.

One thing to understand here is that if we look at both images from far enough away so that we can't resolve each dot individually, the two images will appear equally bright. Think of each dot as a flashlight. If the flashlights are so far away that we can't resolve each one as an individual light source, then the combined light from the 10 brighter flashlights looks equally as bright as the 40 dimmer flashlights.

Yeah, but that's like photographing a photograph. I consider the image we are talking about to be the final image formed in our brain when we look at Olbers' paradox night sky. I'm not sure we are talking about the same thing, or at least not from the same perspective, so I'd rather concentrate on things we absolutely agree on and I think we should proceed from there.

It doesn't matter if they are considered point sources or not, the inverse square law still works just fine. And I don't understand what you're saying about the apparent size of the stars. Point sources are not resolved and don't really have an apparent size.

If we consider them all as point light sources it will simplify our visualization. Surely with naked eye we can not resolve the size of any star or galaxy. Maybe a few, I don't know, but for our generalized hypothetical scenario I believe considering them all as point sources is a very sensible thing to do. With "apparent size" I was trying to take into account visual artifacts where a point light source may appear smudged over some area as opposed to being illumination of a single pixel.

 

[Jonathan Scott]

Ever looked at the Milky Way on a dark night? You will see that there is an impression of light, forming a milky band, but you will not be able to resolve many of the stars. Being able to resolve them does not affect the total light received.

Note that if the light from an image is blurred, making it larger, this does not affect the overall balance, because it is the amount of light received compared with the amount of background hidden behind the star itself which determines the average brightness per angular area. That is why the assumption of point sources does not work.

 

[humbleteleskop]

That's right. But note that while each star in the 2nd shell is 4 times dimmer, it is also 1/4 the apparent size that the stars in the 1st shell are (assuming we can resolve them).

How am I supposed to draw that? They are already point light sources, they can not get any smaller, only dimmer.

Understand that Olber's paradox isn't about point sources. Point sources are the result of our inability to resolve far away objects. That just means that the the size of the airy disk of the focused light is larger than the image of the object at the focal plane.

...in astronomy, stars are routinely treated as point sources
http://en.wikipedia.org/wiki/Point_source

...so for stars and other point sources of light
http://en.wikipedia.org/wiki/Luminosity

...images of point sources (such as stars)
http://en.wikipedia.org/wiki/Astronomical_seeing

 

[humbleteleskop]

Ever looked at the Milky Way on a dark night? You will see that there is an impression of light, forming a milky band, but you will not be able to resolve many of the stars. Being able to resolve them does not affect the total light received.

I didn't say total light received is affected by anything. I said total light received is the same for both of those two images.

Note that if the light from an image is blurred, making it larger, this does not affect the overall balance, because it is the amount of light received compared with the amount of background hidden behind the star itself which determines the average brightness per angular area. That is why the assumption of point sources does not work.

...in astronomy, stars are routinely treated as point sources
http://en.wikipedia.org/wiki/Point_source


Where do you get your information from? What are you suggesting how those two pictures should look like? Are you saying I should make the stars in the second shell brighter?

 

[Jonathan Scott]

I didn't say total light received is affected by anything. I said total light received is the same for both of those two images.

...in astronomy, stars are routinely treated as point sources
http://en.wikipedia.org/wiki/Point_source


Where do you get your information from? What are you suggesting how those two pictures should look like? Are you saying I should make the stars in the second shell brighter?

The two shells have the same total light. If the images of the shells were represented correctly, and were then reduced in size until you couldn't resolve the points, they should look similar (as for the Milky Way analogy).

For purposes of Olber's paradox, the effective surface brightness of the sky is determined by the light being received from each visible star divided by the angular area (solid angle) that the star occupies against the background. This is the same for similar stars at all distances, regardless of what the individual star looks like to the human eye.

This is like spray-painting something. It doesn't matter whether the drops are big or small; by the time the surface is completely covered, they have all joined up and overlapped, and you only see the final surface.

When we are dealing with light from individual stars, they are of course approximately point sources. However, for Olber's paradox, one also needs to take into account the angular area (solid angle) of the source. The suggested assumption that a star is actually a point source but emitting a finite amount of energy would be equivalent to infinite brightness per angular area, which isn't going to give a sensible result for Olber's paradox.

And it really doesn't matter if the observed image is out of focus, so light from different stars gets mixed up, as the total amount of light is unchanged and the fraction of the background which each star "covers" is unchanged, even if it cannot be resolved.

 

[Drakkith]

If we consider them all as point light sources it will simplify our visualization. Surely with naked eye we can not resolve the size of any star or galaxy. Maybe a few, I don't know, but for our generalized hypothetical scenario I believe considering them all as point sources is a very sensible thing to do. With "apparent size" I was trying to take into account visual artifacts where a point light source may appear smudged over some area as opposed to being illumination of a single pixel.

Considering the stars as point sources may simplify things in many cases, but it doesn't get at the heart of Olber's paradox. The key lies in the fact that stars have a finite apparent size and in an eternal infinite universe they would fill the sky completely, so that no matter where you look it would be like looking at the surface of a star.

Note that nothing of what I just said involves point sources or resolving power. I've only talked about how in an eternal infinite universe your line of sight would fall on the surface of a star somewhere. The inverse-square law still applies here, but it's kind of a red herring that takes the focus away from the real explanation, which is that the luminosity per solid angle doesn't decrease with distance. For example, the Sun is about 0.5 degrees (30 arcminutes) across as seen from Earth. If we move out to 2 au the Sun is now 15 arcminutes across, which means the apparent area has dropped to 1/4 of what it was at 1 au. However, a section of the Sun 1x1 arcminute in area has the exact same luminosity at both 1 au and 2 au.

So, knowing that the luminosity per solid angle doesn't decrease with distance, and that in an eternal infinite universe our line of sight would always fall on the surface of a star, we can say that the sky would be extremely bright if our universe were infinite and eternal.

The explanation involving concentric shells and point-sources just muddles up everything, as it takes the focus away from the above.

 

[humbleteleskop]

If the images of the shells were represented correctly...

That's all I'm trying to do.

So tell us, should I make the stars in the second shell brighter, darker, bigger, smaller, more stars, less stars?



Here are instructions, by the way:
To show this, we divide the universe into a series of concentric shells, 1 light year thick. Thus, a certain number of stars will be in the shell 1,000,000,000 to 1,000,000,001 light years away. If the universe is homogeneous at a large scale, then there would be four times as many stars in a second shell between 2,000,000,000 to 2,000,000,001 light years away. However, the second shell is twice as far away, so each star in it would appear four times dimmer than the first shell. Thus the total light received from the second shell is the same as the total light received from the first shell.

http://en.wikipedia.org/wiki/Olbers'_paradox

 

[Jonathan Scott]

So tell us, should I make the stars in the second shell brighter, darker, bigger, smaller, more stars, less stars?

No. If the representation of the conceptual diagram is accurate, with 1/4 the light to represent stars twice as far away, then the total light is the same in both without requiring any change.

In practice, a computer display is very unlikely to give the correct result. The worst source of error will be that the black background will not be completely black, and may well provide more light than the star representations. Also, at scales where the resolution of the star becomes sub-pixel, requiring the pixel to be only partially illuminated in proportion, the limited number of light levels on a digital display would make it difficult to represent the situation accurately.

On top of that, the human eye is not going to be able to get an impression of the average light level unless the pixels are close enough together for the eye to be unable to resolve them individually, and given the very low light level the environment would have to be otherwise dark for the eye to have enough sensitivity to tell the difference from all black.

 

[humbleteleskop]

...in an eternal infinite universe they would fill the sky completely, so that no matter where you look it would be like looking at the surface of a star.

The paradox explicitly states the stars in each shell are four times dimmer than in the previous shell. We know that would be a fact thanks to very well proven inverse-square law. It means great majority of all those stars would be at least four times less bright than the stars in the first shell, which is quite a difference and pretty stark contrast. It's much worse actually as after just the third shell the stars would be so dim they would practically be black to the naked eye, just like in the real world. Where are all those dim, dimmer, and very dim stars in your picture, how do you account for them?

The explanation involving concentric shells and point-sources just muddles up everything, as it So, knowing that the luminosity per solid angle doesn't decrease with distance, and that in an eternal infinite universe our line of sight would always fall on the surface of a star, we can say that the sky would be extremely bright if our universe were infinite and eternal.

Does that mean the reason when I look in the direction of the galaxies in the Hubble Deep Field with my naked bulging eyes and see nothing but black is not due to inverse-square law but because there is an edge to the universe?

 

[Drakkith]

Does that mean the reason when I look in the direction of the galaxies in the Hubble Deep Field with my naked bulging eyes and see nothing but black is not due to inverse-square law but because there is an edge to the universe?

Yes. Because the universe is not eternal and infinite (and static), only a very small number of directions will actually fall on the surface of a star. Note that the "edge" isn't an edge in space, but an edge in time. In other words, the universe has a finite age and light has only had 13.7 billion years or so to travel. So light that has to travel longer to reach us hasn't had time to do so because it hasn't existed for that long. And this doesn't even take into account the effect of expansion, which redshifts light from great distances out of the visible range anyways.

 

[Jonathan Scott]

The paradox explicitly states the stars in each shell are four times dimmer than in the previous shell. We know that would be a fact thanks to very well proven inverse-square law. It means great majority of all those stars would be at least four times less bright than the stars in the first shell, which is quite a difference and pretty stark contrast. It's much worse actually as after just the third shell the stars would be so dim they would practically be black to the naked eye, just like in the real world. Where are all those dim, dimmer, and very dim stars in your picture, how do you account for them?

The "four times" case is just a simple illustrative example. A more general example (assuming a hypothetically uniform universe) is that each spherical shell of some standard thickness has a number of stars which is proportional to the square of the radius, but the apparent brightness of those stars is inversely proportional to the square of the radius. Each shell then contributes the same total luminosity, up to the point where some stars in more distant shells are hidden behind stars in closer ones, so the outer shells are then only filling in the gaps in the inner ones, until the entire sky is covered by stars of some size. This is very similar to the "spray paint" example.

All that is actually needed for the paradox to work is that every line of sight from the observer eventually hits a star. And the fact that the sky is dark proves that that this does not occur, demonstrating that a simple model of an infinite uniform universe is wrong.

As we have said before, the fact that the human eye can't see lots of tiny specks compared with fewer large ones is not relevant.

 

[abitslow]

Time to draw some diagrams. Define a sphere of diameter D, at a distance d from a point U. In this scenario, D is a star's Photosphere, and U is your eye. Assume that all light given off by the disc is radial (meaning on a straight line between the center of the sphere and each point on the surface of the sphere). With this model, how many lines intersect at U?
One.
If that point is giving off 1,000,000 photons steadily every second, point U will receive 1,000,000 photons steadily every second. Right? With me so far? Ok, given this simplified model, how does distance, d, reduce the number of photons reaching point U? (hint: think of the words:"it don't"). Increase d by a factor of a million, reduce it by a factor of 1000, it doesn't matter, there are still the exact same number of photons going through point U every second. Olber's Paradox is (in this simplified form) the question of why, if the number of stars is infinite, why doesn't every line of sight from point U end up on a surface of one of those infinite number of spheres? Working with this simple model, the ONLY solution in a static Universe (no dust, no inflation, etc.) is that another sphere between U and the first sphere partially eclipses that first sphere so that the photon from the first sphere hits the closer sphere. You can see, perhaps, how each sphere only contributes one single line which will intersect at U, but will obscure many lines heading towards U from farther away. Is this the solution to the paradox? No, unfortunately it isn't. We need to go back to the assumption that only radial lines emanate from each sphere. In reality, each point of a star's Photosphere gives out a lot of photons in all directions (not just radially). So, we need a new diagram. This one with spheres of radius D and various distances from point U (we still assume your eye is a "point-sized" detector, we could treat it otherwise, but this is good enough if we are careful.) So, instead of just lines, this diagram needs areas and lines. Draw as many spheres as you want, at various distances and with varying "overlap"(not physically on top of each other but in each other's line of sight). Now, you need to draw two lines for each sphere, the lines are to be tangent to the surface (tangent to the circle) AND intersecting point U. For each star, you can only draw (on paper) two such tangent lines, one on either edge. Draw them one at a time. Start with the one nearest U.
With your straightedge, line up one edge of the circle (tangent) and point U. Starting at point U draw a line segement which goes to the circle and continues on to the edge of the paper (thorugh any stars). Repeat with the other edge of that circle. These two lines represent two things: first all of the light from the circle to U and second, the shadow of that Star (for areas behind it will be in that star's shadow and not visible to U). Shade in the area from that circle to U outlined by the two (intersecting at U) lines and the circle of the star. This represents the photons going from the surface to point U. Using a different color, shade in the area in back of the circle (the 'shadow'). From this area no light can get past the star and hit U, so no lines need be drawn in this area. Repeat the same procedure with the next closest circle. There are now four possibilities: 1. The star is clear of any closer star and so you do the same thing as before or 2. The star is partially in the shadow of a nearer star, with one edge exposed. In this case the area to be shaded is defined one tangent line and the edge of the shadow you've already shaded. As before shade in the light area, and the shadow area in back of it, using the same two colors (or cross-hatches, or patterns, etc.) 3. The third possibility is that the circle is partially
obscured by two tangent lines, and if so you color in the light area and also the shadow in back using the shadow lines as a guide. 4. The star is completely in one circle's shadow. Do nothing in this case.
Continue until done with all of them. Perhaps you can see that if you drew enough circles and placed them randomly (here we assume they're all the same diameter, D) that eventually the point U would be COMPLETELY surrounded by light. IF you also assume (correctly) that each point from every star (every circle) is giving off the same number of photons, then just like in the first drawing, the number of photons from each "line of sight" is equal REGARDLESS of distance. If one star isn't sending light toward U, then a star in farther away (or closer) is, for ANY line you care to draw between U and infinity. Distance is not the problem. If we include dust and other things which create shadows between us and the stars, then of course this analysis is wrong. But the dust isn't (we think) the major issue with Olbers Paradox. Again, to be clear, the inverse square law has no (significant) part in the solution: distance isn't an important part (in a static non-expanding Universe). Boy, I've set up the problem, but have run out of patience and won't write the answer. Others have already, but there seems to be much that is misleading, also.
Hints to solve "paradox".
1. The first stars, we think, are only ~13½ billion years old (the Universe is NOT static), stars farther away (and we do think there are stars farther) than 13½ billion light years can NOT contribute (yet) to the light we see.
2. The Universe is expanding, meaning that the distances between galaxies is increasing, meaning there ARE some lines of sight which will never intersect a star's surface (in a finite amount of time). Another way to put that is that many lines of sight between us and the 'edge' of our Observable Universe do NOT intersect a star.
3. Most of the Milky Way Galaxy can NOT be seen from Earth - there IS too much dust in the way (at visible wavelengths).
4. Because the Universe is expanding, light does what we call "redshift" (as it travels through billions of light-years, near by (within thousands or hundreds of thousands of light years) light is affected so little, that you wouldn't notice. Redshift means blue light turns red, red turns into radio waves... far enough (long enough) and almost ALL the light will be converted into microwaves... There IS no Olber's Paradox. If we could see microwaves, then we would see the sky lit up like daylight all the time. (This is called the Cosmic Microwave Background).

 

[humbleteleskop]

Yes. Because the universe is not eternal and infinite (and static), only a very small number of directions will actually fall on the surface of a star.

I said I am looking in the direction of Hubble Deep Field. So that's not the answer, it doesn't even address the question.

Note that the "edge" isn't an edge in space, but an edge in time. In other words, the universe has a finite age and light has only had 13.7 billion years or so to travel. So light that has to travel longer to reach us hasn't had time to do so because it hasn't existed for that long.

This is what Hubble sees there:

http://en.wikipedia.org/wiki/Hubble_Extreme_Deep_Field

These galaxies are 13.2 billion years away and obviously their light has been reaching us for at least last 10 years. So that's not the answer.

And this doesn't even take into account the effect of expansion, which redshifts light from great distances out of the visible range anyways.

Whatever light from those galaxies red-shifted there is apparently plenty left in the visible spectrum. So that's not the answer.


The answer is well known and proven fact called Inverse-square law.

http://en.wikipedia.org/wiki/Inverse-square_law
http://en.wikipedia.org/wiki/Apparent_magnitude


Which is why they had to make exposure time last almost a month and why this portion of the night sky looks completely black to the human eye.


- "The exposure time was two million seconds, or approximately 23 days. The faintest galaxies are one ten-billionth the brightness of what the human eye can see."
http://en.wikipedia.org/wiki/Hubble_Extreme_Deep_Field

 

[Drakkith]

I said I am looking in the direction of Hubble Deep Field. So that's not the answer, it doesn't even address the question.

Let's get one thing straight here. the inverse-square law is the reason that individual objects get dimmer as distance increases. No one's arguing against that.

The reason you can't see those galaxies when you look towards the Hubble Deep Field is because they are too dim for your eyes to detect them. The reason the sky is mostly black is because there is a very large distance between most visible objects in space and light from more distant objects that would "fill in the gaps" has not yet had time to reach us.

These galaxies are 13.2 billion years away and obviously their light has been reaching us for at least last 10 years. So that's not the answer.

Yes, it is the answer. The light from all of those galaxies has had time to travel across space to us. Light from objects much further away has not.

Whatever light from those galaxies red-shifted there is apparently plenty left in the visible spectrum. So that's not the answer.

Redshift is a definitive factor for galaxies at the extreme edge of the visible universe. This is one of the main reasons that the James Webb Space Telescope has been designed to observe primarily in the infrared range. Note that almost all of the galaxies visible in the Hubble Deep Field are NOT at the extreme edge of the visible universe, but are much closer. So expansion hasn't redshifted their light enough for it to be out of the visible range. There are a few galaxies near the edge of the visible universe that you can see in the picture, but they are very small and very red.

The answer is well known and proven fact called Inverse-square law.

The inverse-square law explains why objects get dimmer as the distance increases. That's all. The law itself does not explain Olber's paradox.

I don't understand the issue here. Even the wikipedia article on Olber's Paradox gives the answer right in the opening paragraph.

If the universe is static and populated by an infinite number of stars, any sight line from Earth must end at the (very bright) surface of a star, so the night sky should be completely bright. This contradicts the observed darkness of the night.

The fact is that the universe is neither eternal nor static, so most sight lines from Earth do NOT end at the surface of a star. Most end at the surface of last scattering which is where/when the Cosmic Microwave Background Radiation was emitted. (Which was mostly in the infrared and visible range, but has since been redshifted to the microwave range) Note that even galaxies are mostly empty space. The average distance between stars is very large and even though a single galaxy looks like a single, large object, it is actually made up of a large number of small objects whose emitting surface area is MUCH smaller than the apparent surface area of the galaxy. We see all these emitting sources blurred together because we don't have the resolving power to see each one individually.

 

[humbleteleskop]

The "four times" case is just a simple illustrative example.

No, it is direct consequence of inverse-square law, which is the key premise Olbers' paradox was formulated around. The whole point of the paradox is to show how inverse-square law is not the answer because it is the most obvious possible answer, hence "paradox".

A more general example (assuming a hypothetically uniform universe) is that each spherical shell of some standard thickness has a number of stars which is proportional to the square of the radius, but the apparent brightness of those stars is inversely proportional to the square of the radius. Each shell then contributes the same total luminosity, up to the point where some stars in more distant shells are hidden behind stars in closer ones, so the outer shells are then only filling in the gaps in the inner ones, until the entire sky is covered by stars of some size. This is very similar to the "spray paint" example.

You did not address the question. I can only assume that you mean to assert how the same total luminosity means equal brightness. And that is not true.

Luminosity is not brightness. Each shell does not contribute the same luminosity, they contribute the same intensity. Intensity is not brightness either. Brightness is a function of intensity per unit area. Same intensity spread over larger area makes it less bright than if the same intensity was focused over smaller area. Brightness is also a function of resolution, sensitivity and exposure time.

 

[russ_watters]

That's all I'm trying to do.

So tell us, should I make the stars in the second shell brighter, darker, bigger, smaller, more stars, less stars?

The most correct way to do it I can think of would be to make the first set of stars 4 pixels and the second set 1 pixel, but 4x as many. All would be the exact same brightness.

 

[humbleteleskop]

The most correct way to do it I can think of would be to make the first set of stars 4 pixels and the second set 1 pixel, but 4x as many. All would be the exact same brightness.

I agree. But then we have to draw the third shell, so we are back looking at the same pickle and we can't just keep increasing the size to compensate. After all the first shell is according to the paradox thousand million light years away, those stars really shouldn't appear much bigger.

My main objection against scaling the size however would be because the paradox explicitly states the stars in the second shell are actually dimmer.

 

[humbleteleskop]

Time to draw some diagrams. Define a sphere of diameter D, at a distance d from a point U. In this scenario, D is a star's Photosphere, and U is your eye. Assume that all light given off by the disc is radial (meaning on a straight line between the center of the sphere and each point on the surface of the sphere). With this model, how many lines intersect at U?
One.
If that point is giving off 1,000,000 photons steadily every second, point U will receive 1,000,000 photons steadily every second. Right? With me so far? Ok, given this simplified model, how does distance, d, reduce the number of photons reaching point U? (hint: think of the words:"it don't").

I'm afraid that's incorrect. We can talk about it in terms of numbers of photons. Light intensity is indeed a measure of amount of photons per unit time per unit area, but that's exactly where and how inverse square law applies.

http://www.astronomynotes.com/starprop/s3.htm

 

[russ_watters]

I agree. But then we have to draw the third shell, so we are back looking at the same pickle and we can't just keep increasing the size to compensate. After all the first shell is according to the paradox thousand million light years away, those stars really shouldn't appear much bigger.

Yes, it is difficult to make an accurate diagram.

My main objection against scaling the size however would be because the paradox explicitly states the stars in the second shell are actually dimmer.

I don't think that's true; where are you seeing it?

Is it based on the assumption that stars are point sources? It just isn't true, no matter how many times you say it. It is an approximation that sometimes works, but doesn't here.

Perhaps it would be useful if you considered what it would look like if you captured two or four or an infinite number of stars in one pixel.

 

[russ_watters]

I'm afraid that's incorrect. We can talk about it in terms of numbers of photons. Light intensity is indeed a measure of amount of photons per unit time per unit area, but that's exactly where and how inverse square law applies.

http://www.astronomynotes.com/starprop/s3.htm

Your diagram doesn't show the areas. Again, you are misusing the inverse square law and need to start paying closer attention to how it actually works.

 

[humbleteleskop]

I don't think that's true; where are you seeing it?

http://en.wikipedia.org/wiki/Olbers'_paradox
To show this, we divide the universe into a series of concentric shells, 1 light year thick. Thus, a certain number of stars will be in the shell 1,000,000,000 to 1,000,000,001 light years away. If the universe is homogeneous at a large scale, then there would be four times as many stars in a second shell between 2,000,000,000 to 2,000,000,001 light years away. However, the second shell is twice as far away, so each star in it would appear four times dimmer than the first shell. Thus the total light received from the second shell is the same as the total light received from the first shell.

Is it based on the assumption that stars are point sources? It just isn't true, no matter how many times you say it. It is an approximation that sometimes works, but doesn't here.

I don't like assumptions. I simply see no other way to visually represent that sentence I quoted above.

Perhaps it would be useful if you considered what it would look like if you captured two or four or an infinite number of stars in one pixel.

I want to draw what the paradox postulates and I don't see any such bunching effect has relevance, but if you have some idea how it might actually come in play just tell me about it and I'll incorporate it in the picture so we can see how it fits.