Taking issue with random walk of photon in star

  • #1
Hello folks, this is my first post. I'm not quite sure that I fully understand the idea of a random walk of a photon that is generated at the core of a star.

I've read this:
http://www3.wooster.edu/physics/jrIS/Files/Walker_Web_article.pdf

My understanding of the theory is this:

A photon gets generated with some initial direction in the star. After traveling some distance (which can be probabilistically determined) is absorbed, then reradiated in some perfectly random direction (any number between 0 and 360 degrees in the theta and phi angles)?

I'll come at this at a different perspective, optics: normal experience playing around with a laser pointer. I point the laser at the wall. Note, the light from the laser pointer doesn't quite do it's random walk in the same way that the photon in the star does (again, unless I'm completely misunderstanding the type of random walk in the star). I point at a position on the wall, and the light from the laser forms a point. If the random walk were completely random in this case, then the light from the laser could've ended up anywhere. What happens with the light from the laser pointer instead is that the light get's absorbed by the molecules in the air then re-emitted opposite the direction of incidence. Shouldn't this happen in a star too?

1) o --> O 2) <O> 3) O -- o -->

Another aspect of common experience: the index of refraction for light in air is strongly dependent upon it's temperature (I don't know if this remains true for plasmas). If variations in the temperature within the star were great enough on a specific scale to produce a difference in the index of refraction, then I could see a variation on the random walk behavior that's described in the theory.

Also, there are some large scale convection structures within stars. I could see "reflection" off of these... but it would only be speculation.

I'm probably way off base with one of my assumptions. Help me understand this?
 
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Answers and Replies

  • #2
On a tangential note: Does lasing occur in stars on any significant scale?
 
  • #3
mathman
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The fundamental difference is that the photons in the star are in a medium of extremely fast moving charged particles, so the reactions are quite different from that in air at room temperature.
 
  • #4
D H
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A photon gets generated with some initial direction in the star. After traveling some distance (which can be probabilistically determined) is absorbed, then reradiated in some perfectly random direction (any number between 0 and 360 degrees in the theta and phi angles)?
Some photons do get absorbed, but many more get scattered. You are applying your knowledge of optics to a region where that knowledge does not apply. Optics simply does not explain things like Compton scattering and thermal emission. A single gamma created in the Sun's core will beget many, many low energy photons by the time the energy leaves the Sun.

I'll come at this at a different perspective, optics: normal experience playing around with a laser pointer. I point the laser at the wall.
That's not a very good analogy. Air is optically clear (in the visible range at least). The mean free path of photons in the Sun's core is less than a millimeter. Here's a better analogy: Point the laser at a fog bank. A very, very thick and very, very large fog bank. By the time the light from the laser exits the fog bank it will have lost all coherency and will not be anything close to monochromatic.
 
  • #5
Drakkith
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Like D H said above, the conditions in the sun are not like they are in the atmosphere. The density of the sun is MUCH MUCH greater than that of the atmosphere and it is composed of different materials which are in a different state.
 
  • #6
Thank you for answering my question. I see where my idea was wrong.

So, is there no real way to model radiative transfer based on plasma properties?

Which states of matter are available in and around the core of the sun? Do we know all of the absorption and transfer properties of those states of matter?

Can you recommend a good book or set of articles on this subject?
 
  • #7
Drakkith
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Thank you for answering my question. I see where my idea was wrong.

So, is there no real way to model radiative transfer based on plasma properties?

Which states of matter are available in and around the core of the sun? Do we know all of the absorption and transfer properties of those states of matter?

Can you recommend a good book or set of articles on this subject?
Most of the sun is composed of Plasma. The core of the sun is simply under much greater pressures than the outer edges of it.
 
  • #8
Perhaps I should explain my question a little more clearly.

There may be multiple states or multiple modes of plasma that are available. Ionized Hydrogen (+1) and Ionized Helium (+1 or +2) along with a soup of electrons; each of these should be described by the same Magneto Hydrodynamic equations. Now, depending on the densities of the plasmas or the conditions of the sun like the high pressures that you mentioned, degenerate forms of matter like metallic hydrogen or something else may be appear.

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

There have been some Earth bound experiments with plasmas. Albeit not simulating exactly the same conditions that exist in the core of the sun, that describe some of the properties of plasmas.

https://e-reports-ext.llnl.gov/pdf/312864.pdf

Is there a general theory (that has been experimentally tested and verified) that can be used to describe the states and properties of matter under different environmental stresses and conditions? Is there such a thing as a general state equation that can be used to describe the properties of any known form of matter?
 
  • #9
Drakkith
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Ah, I see what you are asking now. I wish I could help you.
 
  • #10
I appreciate the fact that you tried. ;)
 
  • #11
Chronos
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If we could do that, we would already have fusion reactors. We cannot recreate anything resembling conditions in stellar cores, so, we don't have a well tested theory of high energy plasma physics.
 
  • #12
Ken G
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A very, very thick and very, very large fog bank. By the time the light from the laser exits the fog bank it will have lost all coherency and will not be anything close to monochromatic.
If you shine a green laser into a fog bank, it stays green-- or is that not what you mean by "close to monochromatic"?
 
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  • #13
Ken G
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If we could do that, we would already have fusion reactors. We cannot recreate anything resembling conditions in stellar cores, so, we don't have a well tested theory of high energy plasma physics.
Yet there is no reason to think we don't understand the physics in the core of the Sun quite well, and every reason to think that we do. For example, it was the physics of the core of the Sun that allowed us to recognize that neutrinos must undergo oscillations as they propagate, because otherwise the observed neutrino flux would not match our understanding of the physics of the core of the Sun. Pretty impressive.
 
  • #14
I think that he was using the example of the fog bank because he was trying to describe how there's a lot of 'stuff' for the beam to go through. A fog bank would scatter the beam. When a gamma ray is produced it can undergo Compton scattering, in Compton scattering the energy from the original gamma photon can be absorbed and the gamma photon's frequency would change. A fog bank won't scatter a beam like that (but there are some nonlinear optical substances that will [[I don't have an example of this right now]]).
 
  • #15
Ken G
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I agree that a laser hitting a wall is not a good analogy for absorption/re-emission in a low density gas. The fog bank won't downgrade the energy of the photons the way absorbing gamma rays does, that's all I was saying, though I suppose one might count conversion to infrared as the analogous process there. We know that Compton scattering cannot be the primary process that turns gamma rays into X-rays in the stellar core, because Compton scattering preserves the number of photons. So most X-ray photons are generated by bremstrahlung in the hot core-- the Compton scattering is just where the heat comes from to do that, so Compton scattering eventually essentially destroys the gamma rays, and bremstrahlung resurrects them as X-ray photons. That is indeed a lot different from shining a laser onto a wall, and so the fog bank is much closer, if we associate the laser light with the gamma rays, and infrared light with the X-rays.
 
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  • #16
Ken G. I appreciate your response. I'll look up Bremstrahlung radiation, and come this time tomorrow, I'll understand it.

Ken G:
"Yet there is no reason to think we don't understand the physics in the core of the Sun quite well, and every reason to think that we do."

I'm well aware that Astronomers are smart folks. :)
 
  • #17
D H
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If we could do that, we would already have fusion reactors. We cannot recreate anything resembling conditions in stellar cores, so, we don't have a well tested theory of high energy plasma physics.
Creating the temperatures reached at the core of the Sun, 13-15 million kelvin or a bit over 1 keV, is child's play. Tokamaks have been doing that since the late 1960s. Controlled fusion that produces more energy than it takes to create and sustain the high temperature plasma requires temperatures an order of magnitude higher than those achieved at the Sun's core. ITER, for example, plans to reach 150 million kelvin.


I agree that a laser hitting a wall is not a good analogy for absorption/re-emission in a low density gas. The fog bank won't downgrade the energy of the photons the way absorbing gamma rays does, that's all I was saying, though I suppose one might count conversion to infrared as the analogous process there.
Those thermal absorptions / reemissions were what I had in mind as a bit analogous to what is happening as energy escapes the Sun. Make the fog bank thick enough and large enough and all vestiges of the original green light (assuming one is using your green laser) will be lost. The laser will heat the fog bank up a bit. The analogy is good in the sense that the mean free path of photons is short in the fog bank but also is rather weak because green photons simply don't have the energy to undergo the high-energy processes that take place deep inside the Sun.
 
  • #18
Ken G
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Those thermal absorptions / reemissions were what I had in mind as a bit analogous to what is happening as energy escapes the Sun. Make the fog bank thick enough and large enough and all vestiges of the original green light (assuming one is using your green laser) will be lost. The laser will heat the fog bank up a bit. The analogy is good in the sense that the mean free path of photons is short in the fog bank but also is rather weak because green photons simply don't have the energy to undergo the high-energy processes that take place deep inside the Sun.
Yes, if the gamma rays are equated to the green laser, and the X-rays are equated to the infrared thermal emission that will eventually result if the fog bank is large enough, then I accept it as a good analogy. I was misunderstanding the way the analogy was getting at the solar core X-rays (and ultimately visible light).
 
  • #19
Are you two graduate students, postdocs, or undergraduates?
If greater than undergraduate; What do you specialize in?
 
  • #20
Ken G
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After-doctoral: stellar astronomy.
 
  • #21
Drakkith
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After-doctoral: stellar astronomy.
Pre-Degree Guy at Work who surfs wikipedia and such. :tongue:
 
  • #22
Chronos
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Agreed, we've been replicating stellar core temperatures [and far beyond] in colliders for decades. Fusion requires both high temperature and high density. A stellar core is conferred both properties courtesy of gravity. Pressure is the missing link in our efforts to replicate fusion - not to mention a few material science issues.
 
  • #23
I'm a grad student who's studying the Atmosphere. Lately I've been doing a lot more computer programming than physics. I find astrophysics interesting... but don't know a whole lot about it.

I also don't know a whole lot about Solid State Physics. (Which I'd assume you'd need in order to fully understand what happens in stars).

Drakkith: We may assimilate you yet. ;)

There doesn't seem to be enough time to study everything that's interesting in life...
 

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