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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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The greenhouse effect and the 2nd law of thermodynamics

What the science says...

Select a level... Basic Intermediate

The 2nd law of thermodynamics is consistent with the greenhouse effect which is directly observed.

Climate Myth...

2nd law of thermodynamics contradicts greenhouse theory

 

"The atmospheric greenhouse effect, an idea that many authors trace back to the traditional works of Fourier 1824, Tyndall 1861, and Arrhenius 1896, and which is still supported in global climatology, essentially describes a fictitious mechanism, in which a planetary atmosphere acts as a heat pump driven by an environment that is radiatively interacting with but radiatively equilibrated to the atmospheric system. According to the second law of thermodynamics such a planetary machine can never exist." (Gerhard Gerlich)

 

At a glance

Although this topic may have a highly technical feel to it, thermodynamics is a big part of all our everyday lives. So while you are reading, do remember that there are glossary entries available for all thinly underlined terms - just hover your mouse cursor over them for the entry to appear.

Thermodynamics is the branch of physics that describes how energy interacts within systems. That interaction determines, for example, how we stay cosy or freeze to death. You wear less clothing in very hot weather and layer-up or add extra blankets to your bed when it's cold because such things control how energy interacts with your own body and therefore your degree of comfort and, in extreme cases, safety.

The human body and its surroundings and energy transfer between them make up one such system with which we are all familiar. But let's go a lot bigger here and think about heat energy and its transfer between the Sun, Earth's land/ocean surfaces, the atmosphere and the cosmos.

Sunshine hits the top of our atmosphere and some of it makes it down to the surface, where it heats up the ground and the oceans alike. These in turn give off heat in the form of invisible but warming infra-red radiation. But you can see the effects of that radiation - think of the heat-shimmer you see over a tarmac road-surface on a hot sunny day.

A proportion of that radiation goes back up through the atmosphere and escapes to space. But another proportion of it is absorbed by greenhouse gas molecules, such as water vapour, carbon dioxide and methane.  Heating up themselves, those molecules then re-emit that heat energy in all directions including downwards. Due to the greenhouse effect, the total loss of that outgoing radiation is avoided and the cooling of Earth's surface is thereby inhibited. Without that extra blanket, Earth's average temperature would be more than thirty degrees Celsius cooler than is currently the case.

That's all in accordance with the laws of Thermodynamics. The First Law of Thermodynamics states that the total energy of an isolated system is constant - while energy can be transformed from one form to another it can be neither created nor destroyed. The Second Law does not state that the only flow of energy is from hot to cold - but instead that the net sum of the energy flows will be from hot to cold. That qualifier term, 'net', is the important one here. The Earth alone is not a "closed system", but is part of a constant, net energy flow from the Sun, to Earth and back out to space. Greenhouse gases simply inhibit part of that net flow, by returning some of the outgoing energy back towards Earth's surface.

The myth that the greenhouse effect is contrary to the second law of thermodynamics is mostly based on a very long 2009 paper by two German scientists (not climate scientists), Gerlich and Tscheuschner (G&T). In its title, the paper claimed to take down the theory that heat being trapped by our atmosphere keeps us warm. That's a huge claim to make – akin to stating there is no gravity.

The G&T paper has been the subject of many detailed rebuttals over the years since its publication. That's because one thing that makes the scientific community sit up and take notice is when something making big claims is published but which is so blatantly incorrect. To fully deal with every mistake contained in the paper, this rebuttal would have to be thousands of words long. A shorter riposte, posted in a discussion on the topic at the Quora website, was as follows: “...I might add that if G&T were correct they used dozens of rambling pages to prove that blankets can’t keep you warm at night."

If the Second Law of Thermodynamics is true - something we can safely assume – then, “blankets can’t keep you warm at night”, must be false. And - as you'll know from your own experiences - that is of course the case!

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!


Further details

Among the junk-science themes promoted by climate science deniers is the claim that the explanation for global warming contradicts the second law of thermodynamics. Does it? Of course not (Halpern et al. 2010), but let's explore. Firstly, we need to know how thermal energy transfer works with particular regard to Earth's atmosphere. Then, we need to know what the second law of thermodynamics is, and how it applies to global warming.

Thermal energy is transferred through systems in five main ways: conduction, convection, advection, latent heat and, last but not least, radiation. We'll take them one by one.

Conduction is important in some solids – think of how a cold metal spoon placed in a pot of boiling water can become too hot to touch. In many fluids and gases, conduction is much less important. There are a few exceptions, such as mercury, a metal whose melting point is so low it exists as a liquid above -38 degrees Celsius, making it a handy temperature-marker in thermometers. But air's thermal conductivity is so low we can more or less count it out from this discussion.

Convection

Convection

Figure 1: Severe thunderstorm developing over the Welsh countryside one evening in August 2020. This excellent example of convection had strong enough updraughts to produce hail up to 2.5 cm in diameter. (Source: John Mason)

Hot air rises – that's why hot air balloons work, because warm air is less dense than its colder surroundings, making the artificially heated air in the balloon more buoyant and thereby creating a convective current. The same principle applies in nature: convection is the upward transfer of heat in a fluid or a gas. 

Convection is highly important in Earth's atmosphere and especially in its lower part, where most of our weather goes on. On a nice day, convection may be noticed as birds soar and spiral upwards on thermals, gaining height with the help of that rising warm air-current. On other days, mass-ascent of warm, moist air can result in any type of convective weather from showers to severe thunderstorms with their attendant hazards. In the most extreme examples like supercells, that convective ascent or updraught can reach speeds getting on for a hundred miles per hour. Such powerful convective currents can keep hailstones held high in the storm-cloud for long enough to grow to golfball size or larger.

Advection

Advection is the quasi-horizontal transport of a fluid or gas with its attendant properties. Here are a couple of examples. In the Northern Hemisphere, southerly winds bring mild to warm air from the tropics northwards. During the rapid transition from a cold spell to a warm southerly over Europe in early December 2022, the temperatures over parts of the UK leapt from around -10C to +14C in one weekend, due to warm air advection. Advection can also lead to certain specific phenomena such as sea-fogs – when warm air inland is transported over the surrounding cold seas, causing rapid condensation of water vapour near the air-sea interface.

Advection

Figure 2: Advection fog completely obscures Cardigan Bay, off the west coast of Wales, on an April afternoon in 2015, Air warmed over the land was advected seawards, where its moisture promptly condensed over the much colder sea surface.

Latent heat

Latent heat is the thermal energy released or absorbed during a substance's transition from solid to liquid, liquid to vapour or vice-versa. To fuse, or melt, a solid or to boil a liquid, it is necessary to add thermal energy to a system, whereas when a vapour condenses or a liquid freezes, energy is released. The amount of energy involved varies from one substance to another: to melt iron you need a furnace but with an ice cube you only need to leave it at room-temperature for a while. Such variations from one substance to another are expressed as specific latent heats of fusion or vapourisation, measured in amount of energy (KiloJoules) per kilogram. In the case of Earth's atmosphere, the only substance of major importance with regard to latent heat is water, because at the range of temperatures present, it's the only component that is both abundant and constantly transitioning between solid, liquid and vapour phases.

Radiation

Radiation is the transfer of energy as electromagnetic rays, emitted by any heated surface. Electromagnetic radiation runs from long-wave - radio waves, microwaves, infra-red (IR), through the visible-light spectrum, down to short-wave – ultra-violet (UV), x-rays and gamma-rays. Although you cannot see IR radiation, you can feel it warming you when you sit by a fire. Indeed, the visible part of the spectrum used to be called “luminous heat” and the invisible IR radiation “non-luminous heat”, back in the 1800s when such things were slowly being figured-out.

Sunshine is an example of radiation. Unlike conduction and convection, radiation has the distinction of being able to travel from its source straight through the vacuum of space. Thus, Solar radiation travels through that vacuum for some 150 million kilometres, to reach our planet at a near-constant rate. Some Solar radiation, especially short-wave UV light, is absorbed by our atmosphere. Some is reflected straight back to space by cloud-tops. The rest makes it all the way down to the ground, where it is reflected from lighter surfaces or absorbed by darker ones. That's why black tarmac road surfaces can heat up until they melt on a bright summer's day.

Radiation

Figure 3: Heat haze above a warmed road-surface, Lincoln Way in San Francisco, California. May 2007. Image: Wikimedia Commons.

Energy balance

What has all of the above got to do with global warming? Well, through its radiation-flux, the Sun heats the atmosphere, the surfaces of land and oceans. The surfaces heated by solar radiation in turn emit infrared radiation, some of which can escape directly into space, but some of which is absorbed by the greenhouse gases in the atmosphere, mostly carbon dioxide, water vapour, and methane. Greenhouse gases not only slow down the loss of energy from the surface, but also re-radiate that energy, some of which is directed back down towards the surface, increasing the surface temperature and increasing how much energy is radiated from the surface. Overall, this process leads to a state where the surface is warmer than it would be in the absence of an atmosphere with greenhouse gases. On average, the amount of energy radiated back into space matches the amount of energy being received from the Sun, but there's a slight imbalance that we'll come to.

If this system was severely out of balance either way, the planet would have either frozen or overheated millions of years ago. Instead the planet's climate is (or at least was) stable, broadly speaking. Its temperatures generally stay within bounds that allow life to thrive. It's all about energy balance. Figure 4 shows the numbers.

Energy Budget AR6 WGI Figure 7_2

Figure 4: Schematic representation of the global mean energy budget of the Earth (upper panel), and its equivalent without considerations of cloud effects (lower panel). Numbers indicate best estimates for the magnitudes of the globally averaged energy balance components in W m–2 together with their uncertainty ranges in parentheses (5–95% confidence range), representing climate conditions at the beginning of the 21st century. Figure adapted for IPCC AR6 WG1 Chapter 7, from Wild et al. (2015).

While the flow in and out of our atmosphere from or to space is essentially the same, the atmosphere is inhibiting the cooling of the Earth, storing that energy mostly near its surface. If it were simply a case of sunshine straight in, infra-red straight back out, which would occur if the atmosphere was transparent to infra-red (it isn't) – or indeed if there was no atmosphere, Earth would have a similar temperature-range to the essentially airless Moon. On the Lunar equator, daytime heating can raise the temperature to a searing 120OC, but unimpeded radiative cooling means that at night, it gets down to around -130OC. No atmosphere as such, no greenhouse effect.

Clearly, the concentrations of greenhouse gases determine their energy storage capacity and therefore the greenhouse effect's strength. This is particularly the case for those gases that are non-condensing at atmospheric temperatures. Of those non-condensing gases, carbon dioxide is the most important. Because it only exists as vapour, the main way it is removed is as a weak solution of carbonic acid in rainwater – indeed the old name for carbon dioxide was 'carbonic acid gas'. That means once it's up there, it has a long 'atmospheric residency', meaning it takes a long time to be removed. 

Earth’s temperature can be stable over long periods of time, but to make that possible, incoming energy and outgoing energy have to be exactly the same, in a state of balance known as ‘radiative equilibrium’. That equilibrium can be disturbed by changing the forcing caused by any components of the system. Thus, for example, as the concentration of carbon dioxide has fluctuated over geological time, mostly on gradual time-scales but in some cases abruptly, so has the planet's energy storage capacity. Such fluctuations have in turn determined Earth's climate state, Hothouse or Icehouse – the latter defined as having Polar ice-caps present, of whatever size. Currently, Earth’s energy budget imbalance averages out at just under +1 watt per square metre - that’s global warming. 

That's all in accordance with the laws of Thermodynamics. The First Law of Thermodynamics states that the total energy of an isolated system is constant - while energy can be transformed from one component to another it can be neither created nor destroyed. Self-evidently, the "isolated" part of the law must require that the sun and the cosmos be included. They are both components of the system: without the Sun as the prime energy generator, Earth would be frozen and lifeless; with the Sun but without Earth's emitted energy dispersing out into space, the planet would cook, Just thinking about Earth's surface and atmosphere in isolation is to ignore two of this system's most important components.

The Second Law of Thermodynamics does not state that the only flow of energy is from hot to cold - but instead that the net sum of the energy flows will be from hot to cold. To reiterate, the qualifier term, 'net', is the important one here. In the case of the Earth-Sun system, it is again necessary to consider all of the components and their interactions: the sunshine, the warmed surface giving off IR radiation into the cooler atmosphere, the greenhouse gases re-emitting that radiation in all directions and finally the radiation emitted from the top of our atmosphere, to disperse out into the cold depths of space. That energy is not destroyed – it just disperses in all directions into the cold vastness out there. Some of it even heads towards the Sun too - since infra-red radiation has no way of determining that it is heading towards a much hotter body than the Earth,

Earth’s energy budget makes sure that all portions of the system are accounted for and this is routinely done in climate models. No violations exist. Greenhouse gases return some of the energy back towards Earth's surface but the net flow is still out into space. John Tyndall, in a lecture to the Royal Institution in 1859, recognised this. He said:

Tyndall 1859

As long as carbon emissions continue to rise, so will that planetary energy imbalance. Therefore, the only way to take the situation back towards stability is to reduce those emissions.


Update June 2023:

For additional links to relevant blog posts, please look at the "Further Reading" box, below.

Last updated on 29 June 2023 by John Mason. View Archives

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Comments 826 to 850 out of 1481:

  1. Re #824 "damorbel #820: You attribute a quotation from message #804 to me. It was actually written by KR." Very sorry!
  2. damorbel #821: "The Sun is at about 5780K ... The Sun occupies only a small part of the sky so we do not get a the full 5780K here, only 279K." someone; mods, NATO, the UN ... anyone please make it stop! The abuse of physics here is right up there with any human rights violations! (just so as this isn't pure rant: you do see the full temperature give or take attenuation - just not integrated over the whole of our 'aperture' (the sky), nor do we see it's full irradiance; but that does not affect the light spectrum and, therefor the temperature we see, as such)
    Response:

    [DB] You have my sympathies, FWIW. I would reiterate an earlier suggestion I made: DNFTT. And we all know by now the players in this drama. If you see recycling of earlier arguments, please point them out for deletion and possible stronger action. Thanks!

  3. LJ, it's the same old question: where does the energy being emitted by the atmosphere go? It cannot choose its path. If a molecule of CO2 3 inches above a fallow field in Idaho emits a photon downward at 3:00 in the afternoon on July 29th, and that photon is not impeded before striking the sun-warmed molecules that make up the soil, what happens to the photon? Does it slam on the brakes and say to itself "Damn, I almost violated the alleged 2nd Law of Thermodynamics!"? Or does it hit the soil and "bounce off"? Can it be absorbed? More home experiments! Take two pots of boiling water, both with a constant heat source of 90C. Place a 50C heat source ten feet (so there can be no question of convective interference) above pot no. 2. Will the temperature of pot no. 2 increase at all? Will the 50C heat source add its energy to the 90C source and make the water hotter than for pot no. 1? Yah, ok, DB. I'm done--and I barely got started.
  4. Re #814 your comment (as mod) was:- "Response: [muoncounter] Perhaps you should have checked the link in my response to #784. Unless you are a different damorbel, you gave us the parable of your disdain for textbooks some months ago. Do try to keep track of your own words; they are there for all to see." If you look at #784 carefully. I wrote :- "But I don't know which textbook I am suppose to read or whether it is a requirement for scientists to read text books. Personally I recommend original works, textbook contents are at least 2nd hand if not much more; at university my tutors always advised original texts, they had a low opinion of published textbooks." - I was responding to DB's comment in my #783 where he links to scaddenp #753. I responded to scaddenp's remark in 753 where he wrote:- "I asked if the experiment didn't go your way, whether you would be prepared to abandon your view and read the textbook. (ie, behave like a scientist)." This is of course a personal attack on me and I usually avoid responding to them. But, since you are in a special position as a moderator, I thought it would be a good idea to let you know the origin of these remarks that I, for one, see as highly irrelevant.
    Response:

    [DB] The comment is not an attack on you personally; that would be ad hominem and would be disallowed. The remark in question was directed to your very own words. Please be consistent and do not feign ignorance. The definition of "is" has already been debated.

    [muoncounter] You repeated the substance of a prior comment, which adds nothing to the current discussion. You've done the same thing a number of times. If you find the instruction to stop that particular behavior a personal affront, so be it.

  5. damorbel - the gambit is a way to end the argument. If you arent philosphically prepared to accept experimental evidence as the arbiter, then yes it is a well-deserved attack on you and meant to expose you to other reader of this. On the other hand, if you do accept that reality is the arbiter, then then the game is played like this: An experiment is proposed: (you can propose it). You calculate by any means you like, the outcome of the experiment. I am sure you mean to do within your understanding of physics. Someone else (not you), calculates the experiment via the relevant textbook physics. If you are right, then time for us to help you polish a paper. If textbook is right, then time for you to go back to school and stop complaining the climate scientists dont understand physics.
  6. What's this talk of photons having temperatures in Kelvins? A photon's energy depends on its frequency or wavelength, not on the temperature of its source, which is what Damorbel seems to imply. The better informed here correct me please, as this is the way I see it: the temperature of an EM radiation source affects the spectrum of the radiation and that's about it. An individual photon at a given frequency couldn't care less whether it came from a 5 gazillion degrees source or a light bulb, does it? If it does, how exactly does that manifest? A different spin angular momentum? Or what?
    Response: There is a common misconception that all photon sources output photons of only a single frequency that is determined by the temperature of the source. In fact, the blackbody radiation curve is a distribution of photons of multiple frequencies, with an increase in temperature causing a shift in that distribution of emitted photons so that more of the higher frequency/energy photons are emitted relative to the lower frequency/energy photons, but there still is emission of photons of multiple frequencies.

    Consequently, when somebody receives a photon of a given frequency, that person can state only the relative probabilities of the temperature of that photon's source. That photon could have come from a source of any temperature. As a commenter said a bit ago, photons do not carry ID cards.
  7. Ryan, your assertion that the earth, if receiving energy faster than it looses it, will not warm beyond 255K here violates the first law of thermodynamics - what you propose does not conserve energy. Either you are wrong or the Laws of Thermodynamics are wrong - take your pick.
  8. Re #831 You wrote:- "What's this talk of photons having temperatures in Kelvins? A photon's energy depends on its frequency or wavelength, not on the temperature of its source" The temperature of a particle (in an ideal gas) is measured by the amount of energy (Joules) in the particle. The Boltzmann constant relates the energy to the temperature in Kelvins, the formula is E = 3/2 kT where k is the Boltzmann constant 1.3806504×10^−23J/K Photons are considered to be energetic particles, the term photon gas is used frequently. As energetic particles photon energy can be given as temperature or e/v (electron volts) Photon energy is also a function of the oscillation frequency of the electron that originates the photon, so photon energy is given by the formula E = hv where h is the Planck constant = 6.62606896×10^−34 J/s and v the frequency You wrote:- "...the temperature of an EM radiation source affects the spectrum of the radiation and that's about it." Not just the spectrum but the energy also. You wrote:- "An individual photon at a given frequency couldn't care less whether it came from a 5 gazillion degrees source or a light bulb, does it? If it does, how exactly does that manifest? A different spin angular momentum? Or what? " Each photon is created by an individual electron that gets its energy from the particle that where the electron is found. A photons energy is directly related to the temperature of the particle emitting it. When a photon is emitted it carries momentum, there is a recoil reaction on the particle emitting the photon which means the emitting particle loses the amount of momentum taken away by the photon. This is just the same but on a smaller scale, as a bullet leaving a gun. Thus photon energy is directly related to the source temperature and the photon 'knows' this because the frequency v is a direct function of the temperature.
  9. Re #831 (in the grey area) someone wrote:- "That photon could have come from a source of any temperature. As a commenter said a bit ago, photons do not carry ID cards." If they wrote that, then it is not correct. Photons are emitted (and absorbed) by individual (accelerating) charged particles. The source of a photon characterises it by the energy the photon has. The photon keeps this energy (unless it changes energy in a gravitational field) until it is absorbed by another charged particle, even if it has to cross the universe before this happens.
    Response:

    [DB] I am simply gobstoppered. Please think about what you just wrote some more. As written, does-not-parse.

  10. #831: "As a commenter said a bit ago, photons do not carry ID cards." #834: "If they wrote that, then it is not correct. Photons do carry ID cards? Will the madness never cease? 'Get the new EZ-photon identification card! Never get held up by those pesky laws of physics again! With EZ-photon you too can make your own decisions about what forms of matter you choose to interact with. EZ-photon! Because reality is just so passe!'
  11. Damorbel, you are digging yourself into a bottomless pit of nonsense. This sentence makes no sense, and wouldn't even if the syntax was correct: "Each photon is created by an individual electron that gets its energy from the particle that where the electron is found." You say this: "photon energy is given by the formula E = hv" That is, in fact, correct. Where in that formula is the temperature of the source hidden? In other words, what distinguishes the energy of a photon at a given frequency emitted by a source at a certain temperature from the energy of a photon at the same frequency coming from a source at a different temperature? There are only 2 terms to the energy of a photon, one is a constant. You are saying that, if the other is also kept constant, the product of the 2 can nonetheless be different according to a factor that is not part of the equation. Do you realize how idiotic that is?
  12. Damorbel @831 Each photon is created by an individual electron that gets its energy from the particle that where the electron is found. A photons energy is directly related to the temperature of the particle emitting it. This is incorrect. Molecules emit photons when they transition from one quantum state to a less energetic one. The frequency of the photon is determined by the difference in energy between the two states, as related by E = hv. The energies of the quantum states are fixed, determined by the atomic makeup of the molecule and the strength of the bonds between those atoms. Thus the frequency (and hence energy) of the photon is not determined by its temperature. Temperature will control the intensity of the radiation at a given frequency, since that will determine the proportion of molecular in excited quantum states that can decay. IR radiation is emitted by vibration of atomic nuclei within a molecule, Microwave by rotation of the molecule as a whole, and visible/UV radiation is emitted by electrons. Two photons of identical frequency are not "tagged" by their emitting source. However the spectrum (plot of frequency versus intensity) of a given molecule (especially a gas) is a sufficient finger-print to identify the substance uniquely, and the relative intensities of the various frequency bands can often be used to infer temperature of the emitting substance.
  13. Re #835 & #386 If you find what I wrote in #833 unclear check this link and find - when a cosmologist talks - . when a cosmologist talks about the 'temperature' of a photon Then tell me what the problem is with 'the temperature of a photon'.
    Response: [Dikran Marsupial] I suspect there is a good reason the article talks of an "equivalent temperature" rather than simply a "temperature".
  14. damorbel, That article is talking about curve matching an observed wavelength spectrum to a known emission profile at a given, not the temperature of an individual photon. Individual photons of equal wavelength are identical regardless of source temperature.
  15. Re #839 Bibliovermis you wrote :- "Individual photons of equal wavelength are identical regardless of source temperature." If photons of 'equal wavelength' are 'identical' then they have the same energy also, which according to the link means they have the same 'temperature'. The only difference betwen a photon and a particle moving at less than c is that a photon must be absorbed to give up its energy. When you talk about 'curve matching' and 'spectrum' you are no longer talking about individual particles (including 'photons' as particles). These terms form part of statistical mechanics, the science of large collections of particles. But the concept of temperature is not confined to 'large collections of particles', temperature is an intensive property, meaning individual particles have a temperature also.
    Response: [Dikran Marsupial] No, it means they have the same "equivalent temperature", it does not imply they were emitted by bodies of the same temperature.
  16. Re #838 in Response: [Dikran Marsupial] you wrote :- "I suspect there is a good reason the article talks of an "equivalent temperature" rather than simply a "temperature"." There is. Photonic energy is regarded as electromagnetic, although when they are created they take mechanical momentum from the emitting particle and give it (the momentum ) up when absorbed. But photons are not mechanical 'objects'; they have no mass so can't collide. Collision is how mechanical particles exchange momentum (thus energy), according to kinetic theory. Temperature is essentially a mechanical concept, that is why the energy of a photon gives it an 'equivalent' temperature.
  17. Discussing an individual photon's 'temperature' is a bit of semantic play, which is why quotes are used. Individual photons with equal 'temperature' (energy / wavelength / frequency) are identical regardless of source temperature.
  18. Re #840 in Response: [Dikran Marsupial] you wrote :- "No, it means they have the same "equivalent temperature", it does not imply they were emitted by bodies of the same temperature." Photons are generated in different ways, but when they are generated by molecular motions they have energy directly related to the temperature of the particles. Einstein wrote a paper about this in 1916 "Zur Quantentheorie der Strahlung" and I've never seen it contradicted. Photonic energy is regarded as electromagnetic, although when they are created they take mechanical momentum from the emitting particle and give it (the momentum ) up when absorbed. But photons are not mechanical 'objects'; they have no mass, so they can't collide. Collision is how mechanical particles exchange momentum (thus energy), according to kinetic theory. Temperature is essentially a mechanical concept, that is why the energy of a photon gives it an 'equivalent' temperature.
    Response: [Dikran Marsupial] You are still not making the distinction between the "effective temperature" of a photon/emitting particle and the temperature of the emitting body. Oh well, you can lead a horse to water...
  19. This has gone on long enough. I would like to encourage all to abide by this principle we keep on talking about yet keep on ignoring: DNFTT. Of course, every time we try, they spew another humongous piece of absurdity and we can't help but point it out. We have to stop doing that. They have all done has done an excellent job of demonstrating the extent of their confusion and no amount of redirecting can reconcile them with reality. At some, point, one's mind must be acknowledged as having declared itself. We're long past that.
  20. Re #842 Bibliovermis you wrote :- "Individual photons with equal 'temperature' (energy / wavelength / frequency) are identical regardless of source temperature." This is precisely what Einstein's 1916 paper is about, he shows how the electromagnetic 'Planck black body spectrum' is equivalent to the Maxwell-Boltzmann energy distribution in an ideal gas.
  21. Re #843 Response: '[Dikran Marsupial] You are still not making the distinction between the "effective temperature" of a photon/emitting particle and the temperature of the emitting body.' Sorry, I must have missed something; "effective temperature"? I had not realised this matter had been commented on. Can you help me?
    Response: [Dikran Marsupial] I think you need to read the article that you introduced to the discussion here a little more carefully. Note the author talks about the 'temperature' of a photon, the quotes imply that the meaning of temperature was not the usual meaning of the word.

    [muoncounter] The article uses 'equivalent temperature,' rather than 'effective temperature'.

    [Dikran Marsupial] You are quite right, mea culpa - oh the irony! ;o)

  22. Re #846 Response: [Dikran Marsupial] I think you need to read the article that you introduced to the discussion here a little more carefully. Note the author talks about the 'temperature' of a photon, the quotes imply that the meaning of temperature was not the usual meaning of the word. I think the last sentence of Paul Walorski's article sums up the matter quite well:- "So it's not so much that the photons are all at a temperature of 2.7K but rather that they appear as if they were emitted by a single blackbody which was itself at a temperature of 2.7K." That would only be true if the Planck spectrum and the Maxwell-Boltzmann distribution were equivalent. My use of the word 'equivalent' is deliberate. Re #844 Philippe Chantreau. I'm sorry you have this reaction but the 2nd Law is what is in question. The 2nd Law of Thermodynamics it is very well established, it is not easy to grasp all its implications and failure to take them into account has brought many a beautiful hypothesis crashing down. I'm afraid the concept of temperature is just about as close to the heart of the 2nd Law as you can get.
    Response: [Dikran Marsupial] The "they" in the sentence is the key there, you can't tell from a single photon the temperature of the emitting body, you need to look at the distribution of energies of a large number of photons and do some curve-fitting (and make an assumption or two).
  23. Re #847 Response: [Dikran Marsupial] The "they" in.... you can't tell from a single photon the temperature of the emitting body," Indeed you can't. But what you do know is the amount of energy the emitting particle has (or more accurately 'had') and that is (was) its temperature. Further: "you need to look at the distribution of energies of a large number of photons and do some curve-fitting (and make an assumption or two" Only true if you have a large number of particles. If you have a large number of particles ('real' particles - not photons) they are continually colliding and thus exchanging energy. Because of this they all have different energies and thus different temperatures but the critical point is they have an averge energy that corresponds to the measured (average) temperature.
  24. An object, at 20C, has an 80% absorptivity for 6 micron photons. Absorptivity is unchanged by temperature - the temperature is for later reference. (q) The object is struck by a 6 micron photon from a hotter object (40C) which includes 6 microns in it's emission spectra. What is the probability of absorbing the photon? (a) 80%. (q) The object is struck by a 6 micron photon from a cooler object (0C) which includes 6 microns in it's emission spectra. What is the probability of absorbing the photon? (a) 80%. Photons do not carry ID cards (the earlier quote was originally from me, I believe) indicating the temperature of the emitting object. The temperature of the emitting object is not encoded in the energy of an individual photon. Absorption depends only upon the individual photon energy and the (separate) object absorptivity spectra. You cannot refuse that 6 micron photon because you somehow "know" that it came from something colder. A spectra of photons can be statistically analyzed to determine the temperature required to emit that spectra (given some idea of the emission spectra of the object), but individual photons have energies, not temperatures. And each individual photon adds to the energy of the absorbing object. --- All of these 2nd law objections are based upon one or more such fundamental misunderstandings of physics, and are hence incorrect. The radiative greenhouse theory is entirely supported by thermodynamics.
  25. Damorbel, you've got nothing to say. Cut the BS and answer the substantive questions: Energy of a photon E=h.v Where is the temperature of the source? You have not the slightest clue of what you are babbling about and neither does LJR. "Trolling" is the only accurate way to describe what both of you did on this thread.

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