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Olbers' Paradox

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Re: Olbers' Paradox

#51

Post by Kranar »

ravi-delia wrote: post #44
Kranar wrote: post #32

Earlier quoted context omitted.

Saying "stars that are further away are more redshifted" is Hubble's law and leads almost directly to the big bang. It alone, however, does not explain the paradox. For example even with red shifting, you could have a bright night sky if there are infinitely many stars and the speed of light is instantaneous.

It doesn't really, no? Physicists were well convinced of the big bang for ages before it popped out that the expansion was speeding up, not slowing down. Also, it's pretty hard to imagine a universe where light travels infinitely fast, but also redshifting still exists.

Both the big bang and the observation that galaxies are moving away from the Earth at speeds proportional to their distance originated from the same physicist, George Lemaitre in 1927. The observation came first and then subsequently Hubble formalized it further into Hubble's law and Hubble's constant in 1929.

It's hard to imagine a lot of things, including that the speed of light is the same for all observers, but that's not particularly relevant. Any discussion of the nature of the universe including the big bang itself is going to involve things that are hard to imagine.

Re: Olbers' Paradox

#52

Post by lurquer »

Kranar wrote: post #50
lurquer wrote: post #38

Earlier quoted context omitted.

'Infinitely large' isn't a prerequisite to the paradox. Rather, it's an infinite number of stars. Or, more precisely, an infinite number of stars spread out rather uniformly.

It's not possible to have an infinite number of stars in a finite region. However, you could have a finite number of stars that form a cover over some point.

I agree.

I was just pointing out that the thread keeps referring to ‘infinitely large’ universe. Perhaps it’s just shorthand for ‘infinite number of stars’. No big deal. Obviously, if you had 10 stars in an infinitely large universe, there is no ‘paradox.’

Re: Olbers' Paradox

#53

Post by ravi-delia »

Kranar wrote: post #51
ravi-delia wrote: post #44

Earlier quoted context omitted.

It doesn't really, no? Physicists were well convinced of the big bang for ages before it popped out that the expansion was speeding up, not slowing down. Also, it's pretty hard to imagine a universe where light travels infinitely fast, but also redshifting still exists.

Both the big bang and the observation that galaxies are moving away from the Earth at speeds proportional to their distance originated from the same physicist, George Lemaitre in 1927. The observation came first and then subsequently Hubble formalized it further into Hubble's law and Hubble's constant in 1929. It's hard to imagine a lot of things, including that the speed of light is the same for all observers, but t…

> Both the big bang and the observation that galaxies are moving away from the Earth at speeds proportional to their distance originated from the same physicist, George Lemaitre in 1927. The observation came first and then subsequently Hubble formalized it further into Hubble's law and Hubble's constant in 1929.

Ope, right you are. Got my history a little mixed up there.

> It's hard to imagine a lot of things, including that the speed of light is the same for all observers, but that's not particularly relevant.

I mean, sure, but if light was infinitely fast it would lack every single quality that might make it redshift. Is it possible? Sure, anything is possible, but you'd need something that isn't a wave and doesn't act like a wave to independently behave exactly like a wave in a single specific circumstance. I'm sure someone could try and construct such a theory, but it would need a lot of epicycles to make it work. If light was infinitely fast, it really wouldn't be light anymore, at least not as we understand it.

ed: I guess relativity does kind of account for redshift in a way that looks the largely the same as it would with the standard Doppler effect combined with a classical electromagnetic wave, but for different reasons. Still, infinite speed is a bigger leap

That being said, rereading the original comment I see that yeah, even if redshifting wasn't tied to relative velocity the fact that distant galaxies are more redshifted absolutely solves Olber's paradox. I short circuited from "observe redshift" to "universe is expanding" due to velocities, but as long as the energy of distant light fades out fast enough obviously it doesn't matter why.

Re: Olbers' Paradox

#54

Post by Kranar »

ravi-delia wrote: post #53
Kranar wrote: post #51

Earlier quoted context omitted.

Both the big bang and the observation that galaxies are moving away from the Earth at speeds proportional to their distance originated from the same physicist, George Lemaitre in 1927. The observation came first and then subsequently Hubble formalized it further into Hubble's law and Hubble's constant in 1929. It's hard to imagine a lot of things, including that the speed of light is the same for all observers, but t…

> Both the big bang and the observation that galaxies are moving away from the Earth at speeds proportional to their distance originated from the same physicist, George Lemaitre in 1927. The observation came first and then subsequently Hubble formalized it further into Hubble's law and Hubble's constant in 1929. Ope, right you are. Got my history a little mixed up there. > It's hard to imagine a lot of things, includ…

Something being a wave does not require a finite velocity and in fact there is nothing in physics inconsistent with an instantaneous velocity of light. We know that the two-way speed of light is finite, but all currently known laws of physics are consistent with an instantaneous one-way speed of light [1]:

https://en.wikipedia.org/wiki/One-way_speed_of_light

I am not suggesting that light is instantaneous, only that there is nothing inconsistent with it being so, and that consequently trying to reason about these matters on the basis of what appeals to our intuitions is not sufficient to come to any solid understanding of this phenomenon.

Re: Olbers' Paradox

#55

Post by vba616 »

ravi-delia wrote: post #45
vba616 wrote: post #35

Earlier quoted context omitted.

>The paradox itself is that if the universe is both infinite and eternal then we shouldn't have a dark sky I've never really felt like I understood the paradox, since the way people explain it, it sounds to me like they're just denying the idea that an infinite sum can have a finite value. Like, why couldn't the brightness of the sky in an infinite universe be any value at all, depending on the density? The argument…

I mean, it's not like some abstract denial of convergent sums. You can work out the sum yourself, it doesn't converge. The density doesn't matter if it's consistent through the universe (which is one of the possible outs, but our universe is indeed consistent on the very large scale). The article explains it pretty well, and it's not hard to formalize: > To show this, we divide the universe into a series of concentri…

I read that, I just reflexively feel like it must be the wrong math problem to be solving.

Doesn't it imply the observer has existed forever? Which doesn't seem to me like an obvious assumption.

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