Does White Light Exist? (And two possible explanations for black lines)

[nonequilibrium]

So I'm reading "In Search of Schrödinger's Cat" by John Gribbin, a delightful and concise history of modern physics and I was reading about spectral lines and it said that the dark lines in the solar spectrum can be explained by the fact that there's a cooler cloud of element(s) X around the sun that absorbs those frequencies from the white light by the sun. I understand what it says, but it brings up two (related) questions for me:

1) What if someone brought up the hypothesis "The dark lines are there NOT because there's a cooler cloud of X around the sun, but because X is missing inside the sun (and so are all other element(s) with some of those characteristic frequencies) so the 'white light' the sun emits simply never had those lines/frequencies to begin with," how would one debunk that? The book is not clear on why that is not possible.

2) What is light white actually? If the sun really sends out white light (before reaching the cloud of X), does that imply it has a MASSIVE collection of different types of elements so that all the spectral lines of the individual types add up to make the whole visible spectrum? That seems implausible... Is there another way of creating white light besides adding up spectral lines of specific elements? And what method is used in laboratoria for absorption spectra tests?
(side-Q: the fact blue+red+yellow = white is not talking about the same white, is it? This is more like a psychological white that has little to do with the white previously talked about, correct?)

Thank you!

 

[Jano L.]

Hi,
I think the white light with a broad spectrum is due to the heat radiation of the sun, whose spectrum is continuous and moreless independent of its composition (near to blackbody radiation, whose spectrum is determined only by the temperature of the emitting environment). So even if there is no element X whose emitting frequencies would be f(X)_i, there is nevertheless certain amount of radiation on these frequencies f(X)_i.
But if there are atoms of X in the upper atmosphere, it is possible they absorb this radiation at f(X)_i and reradiate it at other frequencies g(X)_i (emission lines) that could be invisible because the white background overshines them, or they can lose this absorbed energy in some other way.

Jano

 

[nonequilibrium]

Oh very interesting! How is this radiation possible? So it's not generated by electrons falling down, right? (cause that would bring us back to the element spectra) Is it gamma radiation and/or nuclei losing vibrational energy? Maybe I'm forgetting something obvious.

EDIT: in a very chaotic mass free charges probably experience constant acceleration (positive or negative, switching), leading to radiation according to Maxwell, might this be it?

 

[Jano L.]

This is very interesting question. We know the thermal radiation has continuous blackbody spectrum, because it is in a local thermodynamical equilibrium with the matter. This suffices to validate the existence of the continuous spectrum and to calculate its shape using the quantum hypothesis.

But I understand we are more interested in a microscopis description of the process. This is probably more complicated. In sun's matter, atoms (mostly hydrogen) are ionized and the radiation processes should be describable by a classical theory. According to it, the radiation is produced during the accelerated motion of ions and electrons during collisions. Of course, one collision can not produce thermal spectrum - but there is a lot of them, with emissions and reabsorptions of the continuous radiation. Radiation is radiated and reabsorbed by the matter all the time, so in the end, it gets equilibrated with it (to the same temperature). It does not matter what frequencies are preferred by elementary processes - by the microscopic reversibility, if there is low emission at a frequency f, there is also equal low absorption at it, so the total amount of radiation at f rests constant. When part of this radiation escapes to us, we see its thermal spectrum.

Now if there are some atoms with bounded electrons in the way (colder environment), they can absorb some lines of this continuous spectrum. These colder atoms are not in an equilibrium with the radiation (the radiation can be heating them), so their emission is not equal to their absorption, but the absorption is stronger. I guess that is why we see absorption lines in the spectrum, but I am not totally sure. Could somebody help to clarify it?

 

[nonequilibrium]

Thanks again for the swift and clear reply. Indeed, it seems out of statistical mechanics and electrodynamics, the result of a continuous blackbody spectrum follows, that is quite amazing in its own right. It makes me wonder if one is not somewhere implicitly assuming the presence of a mechanism that allows a continuous spectrum? I don't see how S.M. could conjure it up from nothing.

Back to the microscopic description: I agree that sounds like a plausible explanation for the white light from the sun, and also the absorption (although the absorption doesn't seem to be 100% clear, but I think you've got the basis right)

But a blackbody doesn't necessarily have free charges, does it? Does this require a whole new mechanism?

All help appreciated.