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A common climate change denier argument is that back-radiation proposed by climate-change theory violates the second law of thermodynamics because it implies that a cold body warms a hot body.

My understanding of this argument is that it's incorrect because the cold body and warm body are both emitting thermal radiation, the thermal radiation from the cold body to the hot body slows the rate at which the hot body cools down and the thermal radiation from the hot body to the cold body warms the cold body up. When considered together, the overall heat flow is from hot to cold, no violation of the second law involved.

This explanation has made me wonder about the following scenario. Consider two black bodies of different temperature near each other in free space, connected by a unidirectional thermal radiation filter, such that thermal radiation from the cold body towards the hot body passes through but thermal radiation from the hot body to the cold body is blocked or redirected in some other direction. I'll assume that the filter is 100% efficient for this hypothetical example.

Would this system allow heat to flow from cold to hot? Would that flow be considered a violation of the second law of thermodynamics?

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    $\begingroup$ What you're describing sounds kind of like Maxwell's demon. $\endgroup$
    – Sandejo
    Aug 29, 2021 at 3:47
  • $\begingroup$ What are the cold and hot bodies in the denier's argument? $\endgroup$
    – Chiral Anomaly
    Aug 29, 2021 at 4:14
  • $\begingroup$ @ChiralAnomaly In the denier's argument, the cold body is the atmosphere, the hot body is the earth's surface. The denier's argument is that climate change theory invokes backradiation, radiated energy from the atmosphere to the surface of the earth. Denier's claim the existence of backradiation violates the second law. Climate change theory proponents argue that the backradiation doesn't violate the second law because you have to consider the entire net heat flow which still goes from hot to cold. Denier's don't like this defense. $\endgroup$
    – Rising Maverick
    Aug 29, 2021 at 13:25

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Would this system allow heat to flow from cold to hot? Would that flow be considered a violation of the second law of thermodynamics?

Yes, such a system would violate the 2nd law of thermodynamics. Therefore, we can conclude that unidirectional thermal radiation filters do not exist.

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  • $\begingroup$ Indeed, they would be instances of Maxwell's demon. $\endgroup$
    – Andrea Alciato
    Aug 29, 2021 at 9:03
  • $\begingroup$ @AndreaAlciato Very similar, although the original "Maxwell's demon" thought experiment worked with atoms/molecules rather than photons. $\endgroup$
    – gandalf61
    Aug 29, 2021 at 9:27
  • $\begingroup$ there is something en.wikipedia.org/wiki/Thermal_diode $\endgroup$
    – hyportnex
    Aug 29, 2021 at 10:51
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    $\begingroup$ you are right, my excuse is that I have no idea what the OP is describing $\endgroup$
    – hyportnex
    Aug 29, 2021 at 11:49
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    $\begingroup$ Please don’t do that. It is against the site policies to edit a question after it has received answers in a way that invalidates the answers received. I have reverted the question to the original. Please ask a new question about the circulators $\endgroup$
    – Dale
    Aug 29, 2021 at 16:16

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