söndag 30 oktober 2011

Helmholtz Reciprocity and DLR/Backradiation

Helmholtz Reciprocity Principle (HRP) states that absorption and emission of light can be viewed as reversed processes arising by reversal of time. Absorption of a light ray by a (black) body is simply emission of a light ray running backwards in time, or the other way around.

Downwelling Longwave Radiation (DLR) and backradiation seek justification by HRP. Kirchhoff's Radiation Law stating that emissivity and absorptivity of a radiating body are equal,
was justified by Planck with reference to HRP.

But is HRP a valid physical principle? What is the relation between HRP and the 2nd Law?

HRP describes reversible physics while the 2nd Law described irreversible physics, and so HRP and the 2nd Law describe different physics.

Is the real physics of absorption and emission the time reversal of each other? Probably not.

The process of absorption is like reading and the process of emission like writing. To state
that reading and writing are the reverse of each other would miss the difference between the active constructive aspect of reading and the passive consumption aspect of reading.

The analysis in Mathematical Physics of Blackbody Radiation shows that absorption and emission are different processes satisfying a 2nd law which does not allow time reversal, and thus indicates that HRP is not valid.

Without HRP the support of DLR/backradiation evaporates.

Note that HRP conforms to a corpuscular theory of light as photon particles for which time reversal is no problem. But such a theory is capable of describing only simple ray tracing physics and not real physics.

21 kommentarer:

1. Downward radiation from the atmosphere is directly observed. All of your attempts to deny its existence are otiose.

2. Maybe not really otiose. Judy Curry for one has given up the idea of DLR/backradiation based on my arguments. Who's next?

3. Downward radiation from the atmosphere is directly observed. You can't "give it up", any more than you can give up the Sun.

4. Curry did, so maybe you can also, if you only give it a try.

5. Curry said she didn't think "back radiation" was appropriate terminology. She didn't say it doesn't exist, and you obviously know that.

6. up to now you've never answered this question unambiguously, but i'll give it a new try: when you say that there is no DLR do you mean that the atmosphere does not emit em-radiation towards the earth or that the em-radiation emitted from the atmosphere towards the earth is not absorbed?

7. What is meant by "emit em-radiation towards the Earth"? Equations?

8. there is no need for equations. the question is well defined.
does the atmosphere emit em-radiation in the direction of the earth, or if you prefer isotropically, or not?

9. Claes, once for all, there is no need for equations. The earth radiates towards the atmosphere with or witout equations and the atmosphere radiates in all directions, also towards the earth. But there is no need for talking about DLR. It is enough with the SB law, in any form you want. Earth is warming the atmosphere, the GHG, and with more GHG, the atm will get warmer, diminishing the radiation from the earth, acc to the SB law(with constant input from sun), which indirectly warms the earth.(I am just talking about radiation effects, no thermodynamics)

10. Yes, equations are needed to make the discussion meaningful. We are talking about physics, not politics.

11. Claes, when others, in other discussion threads, have asked you to give exact mathematical answers to questions, you have instead replied with analogies and similar answers in everyday language. But here, and before, you refuse to answer questions because they are not phrased in terms of equations.

You must see that you are not holding yourself to the same standards you require of others. This is not the form for a scientific discussion.

12. I think there is a problem of method.
The EM energy density given by the SB law is a simple and formal definition, as the other energy density employed in the physics as enthalpy, internal energy, molecular pressure, photonic pressure, … are also formal definition.
Why then we don’t think in the same way when we have to do with all the energy density in whatever physical process and we treat otherwise the sole EM energy density? Is there a valid reason? But it has to be other than the vacuous claim ”Right we are (if we think so)” because it isn’t acceptable.
Michele

13. E$_{down}>0$?

14. I give equations and describe the equations in different ways, sometimes through analogies. To only argue in words or analogies often lead to misconceptions or misunderstanding since the meaning of the words often is unclear. Physics without equations and math is vague physics, and vague physics is difficult to discuss.

15. Yes Claes, you do often use analogies and as you just said analogies are vague and often unclear.

You have on several occasions been asked to give exact mathematical answer to exact mathematical questions about your proposed models, and instead of giving those answers you have given vague answers in terms of analogies with the type of behavior you expect from your model.

The problem is that you have never given the exact mathematical derivations which would either prove or disprove that your model behaves the way that you claim it does.

In an old thread it was convincingly argued that the equilibrium picture that you have proposed for your model is unstable due to the fact that physical waves propagate at a finite, bounded, speed. You have never addressed that problem with a proper mathematical reply.

As a knowledgeable mathematician you know that a single counterexample is all that is needed in order to show that a model or theorem is false. Is the causality problems in your model that counterexample?

16. If your equations proove that there is no IR-radiation from the greenhouse gases towards the earth surface, they are wrong. They exist but are reradiated without directly warming the earth.

17. What counter example?

18. which part(s) of the question:
"does the atmosphere emit em-radiation isotropically?"
you do not understand or you want specified in order to avoid misconceptions?

19. What a tiresome bore you are, Claes. Most of the time you talk in vague, waffly, meaningless ways, and then suddenly you decide that "radiation towards the Earth" is somehow too difficult for you to understand without an equation. You argue like a child. You don't know how to have a sensible discussion. If you can't even answer the question "is there radiation from the atmosphere towards the Earth?" then you have no views worth sharing.

20. You claim that your picture with only one-way transfer of energy is the correct equilibrium picture, and that it is not the case that both a hotter and a colder body will radiate energy at the same time.

The causality arguments given in an older discussion demonstrates that your equilibrium picture will break down as soon a the temperature difference between the two bodies changes faster than information can pass from one body to the other. Without instantaneous transfer of information between two sufficiently distant bodies any model using waves of bounded velocity will break down in the same way, since there is no way for the two bodies to "know" which one is warmer and hence the one that should radiate.

21. I want point out what follows (http://en.wikipedia.org/wiki/Near_and_far_field)
The "far-field", which extends from about …… The power of this radiation decreases as …… and absorption of the radiation has no effect on the transmitter. By contrast, the "near-field", which is inside about …… in the part of the near field closest to the antenna …… absorption of electromagnetic power in the region by a second device has effects that feed-back to the transmitter …… Thus, the transmitter can sense that power has been absorbed from the closest near-field zone ……
Thus, a GHG can interact with the emitting surface of the earth exclusively within a very tiny boundary layer near it.
Michele