Talk:Olbers's paradox
Is this a paradox ? - anon
I know nothing about this, but might not another explanation be that there is so much dust and gas in the universe that light from very different stars is so dim as to be imperceptible, because it is absorbed along the way? Even in a non-expanding universe, you wouldn't expect very very distant stars to be visible anyway... -- Simon J Kissane
I'm not quite clear about the meaning and correctness of the article's last paragraph. Wouldn't thermodynamics forbid us to recycle radiation into matter? --AxelBoldt
Also, I take it that electromagnetic radiation is converted to kinetic energy (heat) all the time. Why should we postulate a hypothetical method for transferring electromagnetic radiation into matter, when there's another observable explanation for how electromagnetic radiation can be converted into another kind of energy? Beyond that if the universe is not bounded, or if it is expanding there's no reason to believe that anything has to happen to the light -- it can just continue to disperse. (But I am certianly no expert in this field...) MRC
With regard to Axel's question: radiation is routinely converted into matter in particle accelerators: most commonly into electron/positron pairs. This is called pair production. The only problem with such recycling is that known methods of matter production result in equal quantities of matter and antimatter, which is not what is observed in nature. Thermodynamics does not forbid recycling. However, it does suggest that a state of equilibrium will eventually be reached, and maintained thereafter. If there is no conversion of energy into matter then the equilibrium state will probably be one in which all except a small remnant of matter has been converted into energy in the form of radiation.
With regard to MRC's points: Yes, EM radiation is converted into kinetic energy. If this were to take place e.g. in a hydrogen gas cloud, some of the kinetic energy would be converted back into low temperature radiation (radio waves). Somebody has already added a paragraph to the main article regarding this possibility, saying that "it would result in strong radiation which is not observed". This also seems to address Simon Kissane's point. However, *some* radiation from gas clouds most certainly is observed.
With regard to light becoming increasingly dispersed in an expanding universe - this is partly covered by the statement about light becoming increasingly redshifted and diminished in brightness in such a universe. However, increasing separation between photons as a possible cause of diminished brightness should perhaps have an explicit mention.
I'll refrain from modifying the main article any further, because I personally favour the idea that energy is recycled into matter, and I find it difficult to evaluate other possibilities objectively.
--Martin Gradwell.
The universe might be infinite, but that doesn't mean the amount of matter/energy in it needs to be infinite. This would easily resolve the paradox.
- True, but then some region of the universe would have more matter/energy than another; the universe would have a "center", which people typically don't like ("why is the center here and not over there?") 207.171.93.45
- Are you sure that's right? How would we define the "extent" (not the right term I'm sure) of the universe except in terms of where there is matter/energy? That is, can there actually be parts of the universe that have no matter and no energy? Mswake 04:36 Jul 24, 2002 (PDT)
Actually, Olbers did not propose the paradox to show that the universe is finite, but that the universe is not transparent, being filled with dust that blocks the light of distant stars. Since the time of Newton, it had been appreciated that if the universe was static (not expanding) as was widely assumed, then it must be infinite, or else the combined gravity of all the objects in the universe would cause it to collapse toward its center of mass.
Olbers (believing the universe to be static and infinite) proposed that the darkness of the night sky showed that the universe was not transparent. However, he did not appreciate the consequences of the first law of thermodynamics (which can be forgiven at his time in history), that if interstellar dust blocked the light of stars, then it would heat up until it shone as brightly as the stars.
I am about to change the main page to reflect this.
- According to Encyclopedia Britannica, Kepler saw it as an argument against an infinitude of stars. Also, the dust wouldn't necessarily shine "as brightly as the stars": it would be invisible microwave radiation, not visible light. AxelBoldt 01:29 Jan 25, 2003 (UTC)
I stand by "as brightly as the stars." It's actually a bit of an understatement. Let me give you a back-of-the-envelope justification.
Consider an infitnite, static universe filled with a uniform scattering of stars. If the distance to any given star is R, then the light recieved from it falls off as 1/R^2. However, if the universe is filled with a uniform scattering of stars, then the number of stars at distance R increases as R^2 (for the same reasons that the surface area of a sphere is 4*pi R^2). So, the light received from stars at distance R is (R^2)*(1/R^2) = a constant, independent of R. This implies that if light from infinitely far away could reach the observer, then all points in this universe would be bathed in infinite luminosity. To radiate this away, they would have to acheive infinite temperature and shine with infinite intensity at all wavelengths.
Or put another way, if a nonexpanding universe is infinitely old, and has contained an infinite number of luminous objects throughout that time, then at the present it must be infinitely luminous at all points.
Or put another way, suppose a dust grain in an infinitely old universe was at one time cold. It absorbed a visible photon, heated up a bit, and radiated away the energy as several microwave photons. However, neighboring dust grains did the same, and in the meantime, it has absorbed several of these microwave photons, in addition to another visible photon, and is a bit hotter. It will radiate this energy away in the infrared, but in the meantime, its neighbors are doing the same... The problem is energy can't be destroyed, and if an infinitely old universe has contained luminous objects during its entire lifetime, then an infinite amount of energy has been released into it, and that energy has to be somewhere.
I think the title of this article is incorrect. His name was "Olbers" so the title should either be "Olbers' paradox" or "Olbers's paradox". A quick consult to my astronomy textbook published in 2001 prefers the latter and notes that the former is acceptable as well. - 66.81.223.216
- Hang on, why has the article been moved to "Olbers' paradox"? Doesn't the above note recommend "Olbers's paradox"? I would certainly prefer it to be there. The omission of the final "s" in the possessive form of a singular noun or proper noun is a rather idiosyncratic rule that a minority of English-speakers follow, and which rather grates on the ears of most of us. -- Oliver P. 14:01 Jan 31, 2003 (UTC)