# Olbers' paradox: why is the night sky dark?

Started 6 months ago | Discussion thread
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 Re: Olbers' paradox: why is the night sky dark? In reply to trisd, 5 months ago

trisd wrote:

Jonny Boyd wrote:

The brightness perceive is a result of the flux through our eyes. If you can measure the flux then you know what we will see. If you measure a uniform flux, then you know that we will perceive a uniform brightness. Do you dispute that?

There is no any uniform flux. The math used has a single value result. That is not uniform, that is a sum, a total, or net value. According to that you would conclude the two shells on these images below are also uniform and equally bright.

You're avoiding the question. I am claiming two things:

1. The brightness that we perceive is directly connected to the flux experienced by each photoreceptor i.e. photoreceptors experiencing a high flux will be bright, those with a low flux will be dim.
2. If each photorecptor experiences the same flux, then we will consequently perceive the sky to be of uniform brightness.

Do you dispute either of those and if so, on what basis?

Luminance is a form of brightness. And since the angular area of each shell is the same, then the flux, which corresponds to perceived brightness, will be the same.

What are you trying to say? That these two images above are equally bright, or that I should have made stars on the right image have the same brightness as the stars in the first shell?

If you take the field of view covered by the stars in the first image and work out the angular flux, then do the same for the stars in the second image, you will get the same result Because each star in the second image covers a quarter of the field of view, but there are four times as many. Basically the amount of light for each field of view that ends on a star is the same.

Perhaps you can explain where I've gone wrong with these terms? There's nothing in the links you provided that says I'm wrong. On the contrary, I've quoted them myself before.

I told you already. Luminance and flux is not the same thing as brightness.

What causes us to perceive brightness? The arrival of photons at our photodetectors. How do we quantify that? Luminance and flux.

You quoted from http://www.astro.cornell.edu/academics/courses/astro201/olbers_paradox.htm They use flux and luminance to quantify brightness. Are they wrong?

The paradox is about brightness. And they are not talking about luminance or flux, they are talking about net amount of light or total intensity, which is again not the same thing as flux or luminance, and it is not the same thing as brightness.

They don't talk about photons or photoreceptors by name but it's still perfectly valid to mention them.

I don't know how to make it simpler. My point is that every point in the sky has uniform brightness, therefore you are wrong and the inverse square law supports Olbers' paradox instead of explaining it away.

NASA: -"...if you look at a shell twice as far, each star is only a quarter as bright, but there are four times as many stars, so each shell is equally bright."

What part of "each star is only a quarter as bright" do you not understand?

Eqch point in the sky will have a number of stars proportional the the square of the distance. Why can't you accept that? That's why they say 'but there are four times as many stars'

Cornell: -"...distant stars will APPEAR to be fainter than nearby ones, but at the same time there will be many more stars at larger distances, so that the two effects should cancel."

What part of "distant stars will appear to fainter" do you not understand?

What part of 'the two effects should cancel' do you not understand? Nobody disputes the bit about fainter stars. The problem is everyone else then agrees with the second bit while you ignore it.

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

- "Note that brightness varies with distance; an extremely bright object may appear quite dim, if it is far away. Brightness varies inversely with the square of the distance."

What part of "brightness varies inversely with square of the distance" do you not understand?

Again, there will be more stars in a given area so the effects cancel out.

As I've repeatedly told you, you see the results of all the shells, not the shells individually. If you could see each shell separately then of course they would look different. But you look at the night sky then you see them together. When you do this, then every point in the sky looks the same in Olbers' paradox. That what my maths shows you.

NASA: -"...if you look at a shell twice as far, each star is only a quarter as bright, but there are four times as many stars, so each shell is equally bright."

Where do you see they are talking about any other shells or some combination of shells?

That would be the very next sentence which says ' If you have an infinite number of shells, you end up with infinite brightness!'

Cornell: -"...distant stars will APPEAR to be fainter than nearby ones, but at the same time there will be many more stars at larger distances, so that the two effects should cancel."

Where do you see they are talking about any other shells or some combination of shells?

'Therefore the flux arising from each shell is constant, that is, stars at any given distance from us contribute the same amount of flux as stars at any other distance. In every direction our line of sight should intercept the surface of a star and thus the sky should be bright'

The bit where they say 'any other distance' means they are referring to more than just two shells.

Oh and look, they use flux and luminnce.

That's the brightness of a single object, not the brightness of a shell.

And? Do you mean to say brightness of each star in a shell does not, for some strange reason, vary inversely with the square of the distance?

No-one has ever said that. However you will see more stars in the same area of sky, so the effect will cancel.

As is patently obvious from my quote, what they work out for two shells they then apply to infinite shells. Hence then saying that with infinitely many shells there would be a bright sky, and that with more shells there will be more light. The calculations done for two shells are valid for all shells and the final conclusion is reached by considering all shells. If you don't understand that, then you don't understand the paradox.

Stop hallucinating. They don't talk about any combination of shells when they wrongly conclude brightness of each shell is independent of distance and that each shell is equally bright.

The conclusion they reach is valid for any shell. They then apply it to every shell.

What would you do differently? Do you honestly think only two shells of arbitrary thickness should be considered?

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