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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.



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 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.


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 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.


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 601 to 625 out of 1090:

  1. each time a collission occurs, there is a net loss of energy to gravity What?? gravity gains energy? I dont understand this comment at all.
  2. Camburn @598, the atmosphere does not cause the Earth to accelerate in space, so there is no loss of energy to gravity. In fact, the paths of all molecules are accelerated to towards the surface, but because the atmosphere is evenly distributed over the Earth (to a reasonable approximation), the Earth is accelerated in all directions equally by this interaction, and hence not accelerated at all. The energy gained by gravitational acceleration of particles in the atmosphere is returned by elastic collisions at the surface. Further, collisions do not expend heat energy. You are assuming the statistical properties of a group of molecules must also be the properties of individual particles. The energy involved in the collision of particles is the kinetic energy of the particles, plus their individual rotational and vibrational energies. Collisions can result in the redistribution of energy amongst the particles, and the rotational/vibrational states, and can result in the emission of IR radiation. But there is no "heat energy" as a distinct property from the kinetic energy, and rotational/vibrational energy of the particles; and sum of the energies of the particles (including photons) involved is always conserved.
  3. A loss of energy to gravity???!!??? What in the world is that?
  4. Tom C@602: 1. Conservation of energy. Gravity is a force. Everything in our atmosphere/earth sphere is affected by gravity. It consumes energy. Without it, we would all float etc. Yes, kinetic energy is kinetic energy. But even kinetic is the result of heat and can be converted to heat. Friction is an example. When two molecules, even tho of minisucle weight, collide, energy is not only transferred, it is expended because they do not bond, rather they collide and go a different direction and they will go in a different direction at a slightly slower speed. That energy is absorbed by gravity. Even our atmosphere, while of atomic weights, requires an expenditure of energy to stay aloft. This energy comes from the sun as I know of no other source. A physics prof was trying to get this into my mind in college. Not sure I ever really totally grasped what he was trying to show, but rattling my old cobwebs I am trying to understand it still. It gets back to including all energy in any equation talking about energy. Maybe he was all wet, but the older I get the more I think he understood something quit clearly that I didn't.
  5. Tom Curtis (RE:597), "1) The energy absorbed at the surface is the Incoming Solar Radiation absorbed at the Surface (approx 161 w/m^2) plus the Back Radiation absorbed at the surface (approx 333 w/m^2)." OK, here is my question then to you: If you agree that the atmosphere cannot create any energy of its own, and 78 W/m^2 of the 239 W/m^2 (239 - 161 = 78) entering the system never reaches the surface, then where is the 333 W/m^2 of back radiation coming from if 239 W/m^2 is also leaving the system? 40 + 78 + 121 = 239 W/m^2 leaving. 396 W/m^2 - 239 W/m^2 = 157 W/m^2 emitted down to the surface. 161 + 157 = 318 W/m^2 at the surface (396 W/m^2 required). Also, 157 W/m^2 + 97 W/m^2 = 254 W/m^2 (333 W/m^2 required).
  6. RickG @ 591 "Specifically, what is it that you guys don't understand about this diagram? You are trying to make it into something that it is not." I'm not trying to make this diagram into anything. I question the basis by which the energy is stored in the atmosphere...if not temperature. So I ask again how do you know is there...can it be measured? And by what means is the atmospheric energy stored?
  7. Sorry Camburn, it still makes no sense. You say "Gravity is a force." Then you say "That energy is absorbed by gravity." So energy is absorbed by a force. Please elaborate on the physical process there, I'm at a loss.
  8. Tom Curtis (RE: 597), "Saying that 239 w/m^2 becomes 396 w/m^2 is to directly assert the non-conservation of energy." How do you figure? I said the 239 W/m^2 entering the system becomes 396 W/m^2 at the surface. The amount leaving is still 239 W/m^2. Power in = power out = Conservation of Energy.
    Response: [DB] Fixed unclosed html tag.
  9. Tom Curtis (RE: 597), "5) The surface radiation is a function of temperature and emissivity, which is not 1 at any location, though very close to 1 at most." OK yes, temperature and emissivity, which for all practical purposes is 1 because the surface is a near perfect black body radiator.
  10. Philippe@607: Now you are understanding why I didn't understand. I was trying to explain his reasoning, which I did a poor job at doing. His lectures etc on that still tickle my brain tho. I know there is something there that I didn't understand.
  11. Tom Curtis (RE: 599), "RW1 @595, all energy "emitted" from the surface is radiative only because we do not talk about "emitting" convection, or evapo/transpiration." OK, this is the crux. Is 396 W/m^2 radiated from the surface or not? "Not all energy flux from the surface, however, is radiative. In fact, only 80% of it is." Agreed. "And some of the energy flux carried by convection and evapo/transpiration makes its way to space. Do you deny that?" No. As I said, trade offs do occur but an equal and opposite amount less is then returned to the surface. Less energy returned to the surface than what initially left in kinetic form will cool the surface, which reduces the amount of emitted radiation by an equal and opposite amount.
  12. Camburn @604: 1) First and most importantly, the action of a force on an object never consumes energy. It only ever changes its form, and its location. This concept is actually very hard to grasp for most people because it is counter intuitive based on our everyday experience. In our everyday experience, a body in motion will come to rest unless an external force acts on it. This is the experience fairly well captured by Aristotle's laws of motion, but it is an illusion based on not taking into account the effects of friction and air resistance. In space (or by careful experiment) we can see that Newton's laws reign supreme, and that: a) A body at rest or in a state of steady motion will remain at rest or in a state of steady motion unless an external force acts on it; b) Force equals mass by acceleration; and c) For every action, there is an equal and opposite reaction. So, if we consider a gas molecule heading towards space. Gravity indeed acts on it, and it decelerates, losing energy in the process. But gravity equally, and oppositely acts on the Earth at the same time, accelerating it so that it has more energy. In fact, it gains exactly as much energy as the molecule loses. If we follow the path of the molecule under gravity, and ignore all the other collisions (which cancel out in effect), eventually the molecule will collide with Earth, resulting in another exchange of energy that cancels out the exchanges that took place under gravity. (To tell this story completely accurately, I would need to include gravitational potential energy, which shows up in General Relativity as very small changes in mass; but this is just a comment on a blog so you'll have to settle with the short and dirty version). Anyway, gravity does play a crucial role in the atmosphere. It is because of gravity that the atmosphere is dense near the surface, and thin away from the surface. A secondary consequence of this is that molecules move rapidly (the gas is warm) near the surface and slowly (the gas is cool) away from it; and it is possible to predict those temperature relationships using Newtons theory of gravity. But it does not result in a loss of energy, because all forces only result in the exchange of energy, never its loss. 2) Heat that is generated by friction is just kinetic the energy of the molecules that make up the substance being warmed as they vibrate in position; or in the case of a gas, move around in their container. But because that heat is just the motion of those small particles, the small particles themselves do not lose energy to friction. 3) The molecules in a gas do need to have significant kinetic energy to stay aloft. That is the energy of motion that they have because of the temperature of the gas. If the gas cools, they slow down, and will eventually not be able to escape the surface (which will mean the gas has condensed as a liquid or solid). However, they have this energy, the do not need to expend energy because any forces just rearrange the energy, not dissipate it. Further, they do not have frictional energy losses. The do lose energy by the radiation of IR gases, which needs to be replaced directly or indirectly from the sun - but for practical purposes they do not lose energy over and above that.
  13. Tom Curtis (RE: 599), "And some of the energy flux carried by convection and evapo/transpiration makes its way to space. Do you deny that?" Let me try to explain this a little better. Yes, some of the kinetic energy flux into the atmosphere from the surface by thermals and latent heat can end up being radiated out to space. However, if this occurs, it also must result in an equal amount of energy less being returned to the surface. If less energy is returned to the surface than the amount of energy that left the surface, the surface will cool. As a result, the cooler surface will radiate an equal amount less than the amount radiated out to space from latent heat and thermals. Thus, the net effect of latent heat and thermals on the radiative budget is zero, as Conservation of Energy dictates, because all the energy leaving at the top of the atmosphere is radiative.
  14. Tom@612. I'm saving that one. Your physics explanations are always good, but this one has bells, bows, balloons and buzzers. Outstanding.
  15. RW1, would you consider the following simple model: The model is a simple box with mirrored back and sides. We will assume the mirrors are 100% efficient, and reflect all light. The box is covered with a material that is completely transparent to all light coming from outside the box. However, it is half mirrored on the inside, reflecting exactly 50% of all light from the inside of the box, and transmitting without loss the remainder. The box is not a model of the greenhouse effect; but it does have the virtue that any thermodynamic issues raised by the greenhouse effect are also raised by this box, but in a simplified form. In this box, we have the following equalities: 1) Incoming light (A) = Outgoing light (C) (by virtue of conservation of energy). 2) Light reflected from the lid (D) = light transmitted by the lid (C) = Outgoing light (by virtue of the defined half mirrored property of the lid). 3) Light reflected from the back = light striking the underside of the lid (B) = light transmitted by the lid (C) plus light reflected by the lid (D) (by virtue of conservation of energy). Therefore 4) Light reflected from the back of the box (B) = light reflected from the back of the lid (D) plus Incoming light (A) = 2 x A (again, by conservation of energy. By simplifying the situation, ie, by getting rid of any concerns about convection and light absorbed by the atmosphere etc, we should be able to raise any issues you have with the consistency of the GHE with the laws of thermodynamics without getting hung up on trivia. Do you agree? Do you also agree with me that this simple model does not violate any laws of thermodynamics?
  16. 615 Tom Curtis I like, understand and agree with your model, but..
    The box is not a model of the greenhouse effect; but it does have the virtue that any thermodynamic issues raised by the greenhouse effect are also raised by this box, but in a simplified form.
    it doesn't do the thermodynamics justice. (For fun) To do that the 'mirrored' walls should be perfectly black, perfectly insulated and with infinite heat capacity; the radiation hitting them would be perfectly converted into heat and the walls would black-body radiate. Then if the half-mirrored front was, instead, transparent at wavelength λ (give or take), and opaque else where: the box would heat up until radiance from the walls reaching the window at λ equaled the radiance coming in... With a little more messing around (2nd window, transparent at a different wavelength, etc.) you'd have a green house 'box'.
  17. les @616, with some minor amendments, and one major one, your variant box would much better model the greenhouse effect. However, it is not clear that it raises any issues of thermodynamics not raised with my simpler box. So, unless it is clear that we cannot get agreement that no additional thermodynamic arise in the simple model, than I would rather stick with that. If it becomes clear that we cannot get that agreement, we should introduce one variation at a time until we can isolate the actual issue in dispute. The one major disagreement, by the way is the infinite heat capacity. An infinite heat capacity would imply the walls of the box would neither heat nor cool, no matter who much radiation they absorbed or emitted, which would itself violate the laws of thermodynamics (I think) and certainly not accurately model any real physical process.
  18. Tom@617 ... absolutely, hence the "for fun" remark - it was only that, sometimes, one good model deserves another. as for the heat capacity - I hesitated long and... then well, didn't have time to think that one out. Indeed the heat capacity would determine how long it would take to "heat up until" - and an infinite heat capacity would result in it taking an infinite time to heat up. Boundary conditions, hu? always a good way of doing a sanity check on a model - which is the point of your model; so back to that.
  19. Tom Curtis@615 1. A = C ok 2. C can not equal D without violating the 2nd law. Otherwise you have doubled your light/energy with a mirror and a filter. Light can not brighten due to it's own reflection. If your box was fully enclosed such all surfaces are reflective save two small aperture. One aperture to receive light the second to radiate light, do you really think the light/energy will increase beyond it's input? Now change your perfectly reflective interior to one with an emissivity of 1 (black body). Do you believe C will be grater then A. Again no. Black body emission represent the maximum conferred energy for light input. Therefore, a surface with emissivity less then one will NEVER radiate more then it's black body equivalent...regardless of it's own reflection or it' own re-radiation (back radiation).
  20. 619 Ryan - hint: the walls are reflective. All the photons which do D (i.e. don't escape when they hit the front window) will bounce around (lets say B') till they hit the window again and are either C' or D', then B''/C''/D'', B'''/C'''/D''' etc. till they do.
  21. "till they do." - oops, sorry should be "till the sum of Ds = A"... Or do you think it's possible that a significant number of photons will bounce around the box for ever without leaving?
  22. les@621 Like I said C can equal A, but C cannot equal D.
  23. 622 Ryan.. Yes, and I'm pointing out that, really - they are (all) integrals. That may be a problem of notation rather then violation of a fundamental physical property.
  24. L.J.Ryan C will equal D. This scenario is very similar to the spreadsheet I posted here. You can model Tom's box using a 5 column spreadsheet thus; In row 1 type A, blank, B, C, D (to represent the quantities on Tom's diagram) In row 3 type 100, 0, =A3+B3, =C3*0.5, =C3*0.5 In row 4 type 100, =D3, =A4+B4, =C4*0.5, =C4*0.5 Copy row 4 into the next 30 lines of the table. You will find equilibrium reached after about 17 iterations and that Tom's calculations match in every detail. To check the conservation of energy you must let the accumulated energy in the box dissipate. To do this copy row 30 into 31 and set cell A31 to 0. Copy row 31 to the next 15 or so cells. If you sum column A and D (don't forget, column D represents Tom's arrow C) you will find they are equal.
  25. Tom, I really like this. Same idea as Science of Doom example but much simpler.

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