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Olbers' Paradox
- lproven 5y agoReminds me of the classic Barrington J Bayley short story "the Knights of the Limits"... > ‘No, she’s quite a way off,’ Corngold said, taking a look at a meter. ‘Roughly a googol olbers.’ > ‘Your gadget can see that far? But good God – how do you find a single object at that distance?’
- cdirkx 5y agoFrom a mathmatical perspective: does the assumption that "any line of sight from Earth must end at the surface of a star" hold? There are countable infinite number of stars, and uncountable number of places in the night sky (I think?). Can a sphere be covered by a countable infinite number of points?
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- pingpongrandom 5y agoCool concept, but doesn't consider planets, or nebulae, or other space objects that can block light. I have no expertise to speak with authority on this, so please, weigh in on this consideration. But wouldn't this break the "homogeneity" assumption? EDIT: Thinking about this further, could you make approximation rules about the things that block light, too? e.g. By the nature of a star's mass, you can assume that some opaque object is likely between Earth and any given star with some likelihood? EDIT2: Thanks to everybody for the replies - this really helps clarify! Cheers!
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- thechao 5y agoThere's no "blocking light": anything that "blocks light" would get hot from absorbing that light and, then, reradiate. By this time (some point along the curve of "infinitely old universe") everything should be radiating a lot.
- trhway 5y agoOr the energy may get captured and stored as chemical for example.
- mananaysiempre 5y agoBlocking light means absorbing energy. In an eternal universe, you have to reach thermal equilibrium eventually, so you have to shed that energy somehow, and for a constant amount of matter just hanging around in empty space radiation is the only logical possibility. At the end of the day, your absorber will reach the same temperature as the emitter and emit the same amounts of light that it absorbs on frequencies where it does absorb, though perhaps in different directions. This is the insight behind Kirchhoff’s law and blackbody radiation.
- theptip 5y agoIf you read the section on "The Paradox" in the article, it's basically using a simplifying geometric assumption to divide the universe into concentric shells, each of which contributes the same intensity of light to the observer. If space is sparse enough that any light reaches us from shell N+1, then you'll get the same amount of light from every shell, and so you'd have a fully-saturated bright sky. So there are possible infinite/steady-state universes universes (very dense, lots of dust) where you don't see shell N+1. But since we can see stars from shell N+1 and other shells, we know that we don't live in that universe. Therefore our universe isn't a steady-state / infinite time one.
- bmn__ 5y ago> but doesn't consider planets, or nebulae, or other space objects that can block light This is addressed in the article. ctrl+f cloud
- mhh__ 5y agoIf you imagine a curve of the light from a star, the dip in the curve from a planet passing in between us and the star is actually relatively small. They don't block that much light (and what they do block heats the planet, which in turn will radiate lower energy light)
- sebow 5y agoWhen I first encountered this idea it quickly jumped to me that the universe does not appear to be "static", or at least our observation of it is constantly changing. This also plays into homogeneity, and the last assumption of being "infinite" is somewhat irrelevant, because if the first 2 aspects don't hold, being infinite still won't make the sky appear 'filled', at least past the point of our observation. Of course there's still the theory of an imploding universe which could shrink back together all matter and thus maybe achieve this effect, but even then we still don't have a static characteristic, it only makes an argument for an 'infinitely occurring universe', maybe something akin of the theory proposed by Penrose.(which however still has a lot of unanswered questions, imo)
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- AdamH12113 5y agoA more concise explanation can be found in the Usenet Physics FAQ: https://math.ucr.edu/home/baez/physics/Relativity/GR/olbers.html https://math.ucr.edu/home/baez/physics/Relativity/GR/olbers....
- WinterMount223 5y ago> too. It does act like a radiation shield, exponentially damping the distant starlight. This is wrong, damping is not exponential but to the fourth root.
- vikingerik 5y agoA root is a fractional exponent; I can see that as a valid description.
- WinterMount223 5y agoExponentially means that it grows like exp(x).
- adhesive_wombat 5y agoAlso it's an important distinction, because an exponential will always eventually grow (or shrink, depending on sign) faster than a polynomial, no matter how high order.
- BoardsOfCanada 5y agoAssuming an infinite amount of anything seems to be a good way to end up with a paradox.
- ravi-delia 5y agoTrue sometimes perhaps, but not in this case. There's really no "paradox", just an interesting observation. An infinitely old, infinitely large, non-expanding universe is incompatible with the stellar density we see. As it turns out, not only is our universe not infinitely old but it is expanding. So-called paradox resolved, a conclusion easily reached the minute it was formulated, which was of course the point.
- pazimzadeh 5y agoWhat about time?
- IntrepidWorm 5y agoOne cannot even assume time is infinite- there are theories postulating infinite time and finite time, as well as theories postulating that time may not really exist at all.
- ravi-delia 5y agoI mean yeah, but not in a "will cause contradiction" kind of way. You can totally think about infinite time without paradox, as well as infinite space, infinite most things really. You can also think about finite things obviously, if you're into that kind of thing.
- nusaru 5y agoSo basically, we don't exactly know why the night sky is mostly dark? (At least, that's what I got out of this)
- mabbo 5y agoWhat we know is that an infinitly old and large universe is not possible given our understanding of physics and the sky being dark at night. That's all.
- Kranar 5y agoAn infinitely large observable universe is not possible, however an infinitely large and infinitely old universe is still possible with dark sky at night.
- mabbo 5y agoHowso? If the universe is infinitely large and old, and light works as well believe it does, then light should come from all directions. That's the entire point of this paradox. Light as we know it moves without limit. If anything is blocked by it, the blocking object would heat up until it glowed just as brightly.
- lurquer 5y ago'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.
- Kranar 5y agoIt'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.
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- SamLeBarbare 5y agoConstant speed of light is the main problem… More infos: https://en.m.wikipedia.org/wiki/Variable_speed_of_light https://en.m.wikipedia.org/wiki/Variable_speed_of_light Paper: http://www.januscosmologicalmodel.com/pdf/1988-ModPhysLettA-1.pdf http://www.januscosmologicalmodel.com/pdf/1988-ModPhysLettA-...
- 015a 5y agoIs there a good explanation as to why this paradox isn't explained by universal expansion? From wikipedia: "To any observer in the universe, it appears that all of space is expanding, and that all but the nearest galaxies (which are bound by gravity) recede at speeds that are proportional to their distance from the observer."; Wouldn't this imply that, there was probably a time in the distant past when the night sky was saturated brilliantly with light, but as things expand apart, we simply see less-and-less because those infinitely distant galaxies are so far away that their light cannot ever reach us? Additionally, and maybe related; we can only see 13 odd billion or so lightyears away. The universe could be infinite; there could be a point of light at every "pixel" of the night sky, twenty billion light years away; we simply cannot see that far, as their light cannot travel faster than the rate they're expanding away from us. Is that accurate? Additionally; space is pretty empty, but not empty. Even ignoring all this, its not unreasonable to think that, over truly universal distances, this matters. We look at the night sky and see nothing; we point a powerful telescope at that same place and see millions of objects, but still darkness between them. Is there an argument against the notion that, maybe, the light from objects even further away was just absorbed naturally before it could get here? Or maybe our telescopes are not sensitive enough? Additionally; the paradox makes an assumption that space is uniform, but even our naïve observations of the universe prove this to be inaccurate. Its tremendously non uniform; mostly thanks to gravity. Stellar matter is not distributed uniformly; it coalesces into galaxies. Galaxies are not distributed uniformly; they coalesce into galactic neighborhoods, fibers which stretch across the cosmos. There are many regions like the Bootes Void, which contain magnitudes fewer galaxies as we'd expect. There's the Great Attractor, an abnormally massive area which affects the placement of galaxies around it. Maybe on an absolutely, truly, insanely large scale, trillions of lightyears, infinite lightyears, that non-uniformity averages out, but again it comes down to; the universe may be infinitely large, but it doesn't seem to be infinitely old.
- ravi-delia 5y agoExpansion does fix this problem, as does a "young" universe. We actually have both, which would be nice if it wasn't terrible. There was a time that the universe was saturated with light, which we now see as the cosmic background radiation, but it wasn't from stars. The universe was basically very hot, and thus glowed brightly (it's more complicated than that, physics was weird before things cooled down, but still).