branch of physics concerned with heat, work, temperature, and thermal or internal energy

Thermodynamics is a branch of physics that studies the movement of energy and how energy instills movement. It studies the effects of changes in temperature, pressure, and volume on physical systems at the macroscopic scale. Using statistics, its findings are explained as the collective motion of their particles. 19th century physicists defined three Laws of thermodynamics to sum up the basic principles of the subject; in the 20th century, an unofficial "zeroth law" was added.


  • Every mathematician knows it is impossible to understand an elementary course in thermodynamics.
    • V.I. Arnold, "Contact geometry: The geometrical method of Gibbs' thermodynamics," in Proceedings of the Gibbs Symposium, D. Caldi and G. Mostow, eds. (American Mathematical Society, 1990), p. 163.
  • Nicolas Léonard Sadi Carnot, Reflections on the Motive Power of Heat and on Machines Fitted to Develop Power (1824)
    • In order to consider in the most general way the principle of the production of motion by heat, it must be considered independently of any mechanism or any particular agent. It is necessary to establish principles applicable not only to steam engines but to all imaginable heat-engines, whatever the working substance and whatever the method by which it is operated.
    • Machines which do not receive their motion from heat... can be studied even to their smallest details by the mechanical theory. ...A similar theory is evidently needed for heat-engines. We shall have it only when the laws of Physics shall be extended enough, generalized enough, to make known beforehand all of the effects of heat acting in a determined manner on any body.
    • The production of heat alone is not sufficient to give birth to the impelling powerː it is necessary that there should also be cold; without it the heat would be useless. And in fact, if we should find about us only bodies as hot as our furnaces... What should we do with it if once produced? We should not presume that we might discharge it into the atmosphere... the atmosphere would not receive it. It does receive it under the actual condition of things only because.. it is at a lower temperature, otherwise it... would be already saturated.
    • Heat can evidently be a cause of motion only by virtue of the changes of volume or of form which it produces in bodies. These changes are not caused by uniform temperature but rather by alternations of heat and cold.
    • The driving power of heat is independent of the agents used to realize it; its value is uniquely fixed by the temperatures of the bodies between which the transfer of caloric is made.
  • A theory is the more impressive the greater the simplicity of its premises, the more different kinds of things it relates, and the more extended its area of applicability. Therefore the deep impression that classical thermodynamics made upon me. It is the only physical theory of universal content which I am convinced will never be overthrown, within the framework of applicability of its basic concepts.
    • Albert Einstein (author), Paul Arthur, Schilpp (editor). Autobiographical Notes. A Centennial Edition. Open Court Publishing Company. 1979. p. 31 [As quoted by Don Howard, John Stachel. Einstein: The Formative Years, 1879-1909 (Einstein Studies, vol. 8). Birkhäuser Boston. 2000. p. 1]
  • Newton and his theories were a step ahead of the technologies that would define his age. Thermodynamics, the grand theoretical vision of the nineteenth century, operated in the other direction with practice leading theory. The sweeping concepts of energy, heat, work and entropy, which thermodynamics (and its later form, statistical mechanics) would embrace, began first on the shop floor. Originally the domain of engineers, thermodynamics emerged from their engagement with machines. Only later did this study of heat and its transformation rise to the heights of abstract physics and, finally, to a new cosmological vision.
    • Adam Frank, About Time: Cosmology and Culture at the Twilight of the Big Bang (2011)
  • [I]n the nineteenth century, even the theory of heat could be reduced to mechanics by the assumption that heat really consists of a complicated statistical motion of the smallest parts of matter. By combining the concepts of the mathematical theory of probability with the concepts of Newtonian mechanics Clausius, Gibbs and Boltzmann were able to show that the fundamental laws in the theory of heat could be interpreted as statistical laws following from Newton's mechanics when applied to very complicated mechanical systems.
  • Nick Lane, Transformer: The Deep Chemistry of Life and Death (2022)
    • Even the laws of thermodynamics... can be recast in terms of informationShannon entropy, the laws of bits of information. But this view generates its own paradox at the origin of life. ...Place information at the heart of life, and there is a problem with the emergence of function ...the origin of biological information. There are problems... in understanding why we age and die... diseases... and how experiences can give rise to conscious mind. ...A far better question ...what processes animate cells and set them apart from inanimate matter?
    • The driving force of metabolism is thermodynamics. ...[I]n this context ...the chemical need to react (to dissipate energy) in the same way that water needs to flow downhill.
    • If the Krebs cycle is ordained by thermodynamics, then it should take place spontaneously in some suitably propitious envirnonment, even in the absence of genes.
  • James Clerk Maxwell, Theory of Heat (1871)
    • The whole science of heat is founded Thermometry and Calorimetry, and when these operations are understood we may proceed to the third step, which is the investigation of those relations between the thermal and the mechanical properties of substances which form the subject of Thermodynamics. The whole of this part of the subject depends on the consideration of the Intrinsic Energy of a system of bodies, as depending on the temperature and physical state, as well as the form, motion, and relative position of these bodies. Of this energy, however, only a part is available for the purpose of producing mechanical work, and though the energy itself is indestructible, the available part is liable to diminution by the action of certain natural processes, such as conduction and radiation of heat, friction, and viscosity. These processes, by which energy is rendered unavailable as a source of work, are classed together under the name of the Dissipation of Energy.
      ** "Preface"
    • Heat may be generated and destroyed by certain processes, and this shows that heat is not a substance.
      ** Ch.3 "Calorimetry"
  • Isn’t thermodynamics considered a fine intellectual structure, bequeathed by past decades, whose every subtlety only experts in the art of handling Hamiltonians would be able to appreciate?
    • Pierre Perrot, "A to Z Dictionary of Thermodynamics"
  • Thermodynamics is a funny subject. The first time you go through it, you don't understand it at all. The second time you go through it, you think you understand it, except for one or two small points. The third time you go through it, you know you don't understand it, but by that time you are so used to it, it doesn't bother you any more.
  • If the water flow down by a gradual natural channel, its potential energy is gradually converted into heat by fluid friction, according to an admirable discovery made by Mr Joule of Manchester above twelve years ago, which has led to the greatest reform that physical science has experienced since the days of Newton. From that discovery, it may be concluded with certainty that heat is not matter, but some kind of motion among the particles of matter; a conclusion established, it is true, by Sir Humphrey Davy and Count Rumford at the end of last century, but ignored by even the highest scientific men during a period of more than forty years.
  • Thermodynamics is more like a mode of reasoning than a body of physical law. ...we can think of thermodynamics as a certain pattern of arrows that occurs again and again in very different physical contexts, but, wherever this pattern of explanation occurs, the arrows can be traced back by the methods of statistical mechanics to deeper laws and ultimately to the principles of elementary particle physics. ...the fact that a scientific theory finds applications to a wide variety of different phenomena does not imply anything about the autonomy of this theory from deeper physical laws.
  • The Second Law recognizes that there is a fundamental dissymmetry in Nature... All around us are aspects of the dissymmetry: hot objects become cool, but cool objects do not spontaneously become hot; a bouncing ball comes to rest, but a stationary ball does not spontaneously begin to bounce. Here is the feature of Nature that both Kelvin and Clausius disentangled from the conservation of energy: although the total quantity of energy must be conserved in any process (which is their revised version of what Carnot had taken to be the conservation of the quantity of caloric), the distribution of that energy changes in an irreversible manner. The Second Law is concerned with the natural direction of change of the distribution of energy, something that is quite independent of its total quantity.
  • The choice (or accident) of initial conditions creates a sense of time directionality in a physical environment. The 'arrow' of entropy increase is a reflection of the improbability of those initial conditions which are entropy-decreasing in a closed physical system. ...
    Everywhere... in the Universe, we discern that closed physical systems evolve in the same sense from ordered states towards a state of complete disorder called thermal equilibrium. This cannot be a consequence of known laws of change, since... these laws are time symmetric—they permit... time-reverse... The initial conditions play a decisive role in endowing the world with its sense of temporal direction. ...some prescription for initial conditions is crucial if we are to understand... A Theory of Everything needs to be complemented by some such independent prescription which appeals to simplicity, economy, or some other equally metaphysical notion to underpin its credibility. The only radically different alternative... a belief that the type of mathematical description of Nature... —that of causal equations with starting conditions—is just an artefact of our own preferred categories of thought and merely an approximation... At a deeper level, a sharp divide between those aspects of reality that we habitually call 'laws' and... 'initial conditions' may simply not exist.
    • John D. Barrow, Theories of Everything: The Quest for Ultimate Explanation (1991) pp. 38-39.
  • The second law of thermodynamics is, without a doubt, one of the most perfect laws in physics. Any reproducible violation of it, however small, would bring the discoverer great riches as well as a trip to Stockholm. The world’s energy problems would be solved at one stroke. It is not possible to find any other law (except, perhaps, for super selection rules such as charge conservation) for which a proposed violation would bring more skepticism than this one. Not even Maxwell’s laws of electricity or Newton’s law of gravitation are so sacrosanct, for each has measurable corrections coming from quantum effects or general relativity. The law has caught the attention of poets and philosophers and has been called the greatest scientific achievement of the nineteenth century. Engels disliked it, for it supported opposition to Dialectical Materialism, while Pope Pius XII regarded it as proving the existence of a higher being.
    • Ivan P. Bazarov, "Thermodynamics" (1964)
  • If one applies this to the universe in total, one reaches a remarkable conclusion. ...Namely, if, in the universe, heat always shows the endeavour to change its distribution in such a way that existing temperature differences are thereby smoothened, then the universe must continually get closer and closer to the state, where the forces cannot produce any new motions, and no further differences exist.
    • Rudolf Clausius (1864) as quoted by Helge S. Kragh, Entropic Creation: Religious Contexts of Thermodynamics and Cosmology (2013)
  • The more the universe approaches this limiting condition in which the entropy is maximum, the more do the occasions of further change diminish; and supposing this condition to be at last completely attained, no further change could evermore take place, and the universe would be in a state of unchanging death.
  • In the year 1900 Max Planck wrote...  , where   is the energy of a light wave,   is its frequency, and   is... Plank's constant. It said that energy and frequency are the same thing measured in different units. Plank's constant gives you a rate of exchange for for converting frequency into energy... But in the year 1900 this made no physical sense. Even Plank himself did not understand it. ...Now Hawking has written down an equation which looks rather like Plank's equation...  , where   is the entropy of a black hole,   is the area of its surface, and   is... Hawking's constant. Entropy means roughly the same thing as the heat capacity of an object. ...Hawking's equation says that entropy is really the same thing as area. The exchange rate... is given by Hawking's constant... But what does it really mean to say that entropy and area are the same thing? We are as far away from understanding that now as Planck was of understanding quantum mechanics in 1900. ...[T]his equation will emerge as a central feature of the still unborn theory which will tie together gravitation and quantum mechanics and thermodynamics.
  • The law that entropy always increases holds, I think, the supreme position among the laws of Nature. If someone points out to you that your pet theory of the universe is in disagreement with Maxwell's equations — then so much the worse for Maxwell's equations. If it is found to be contradicted by observation — well, these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation.
  • The paradox that immediately bothers everyone who learns about the second law is this: If systems tend to become more disordered, why, then, do we see so much order around us? ...It seems to conflict with our "creation myth": In the beginning, there was a big bang. one is saying that the second law of thermodynamics is wrong, just that there is a contrapuntal process organizing things at a higher level.
    • J. Doyne Farmer, The Third Culture: Beyond the Scientific Revolution ed. John Brockman (1995)
  • Nothing in life is certain except death, taxes and the second law of thermodynamics. All three are processes in which useful or accessible forms of some quantity, such as energy or money, are transformed into useless, inaccessible forms of the same quantity. That is not to say that these three processes don't have fringe benefits: taxes pay for roads and schools; the second law of thermodynamics drives cars, computers and metabolism; and death, at the very least, opens up tenured faculty positions.
  • Life is nature's solution to the problem of preserving information despite the second law of thermodynamics.
    • Howard L. Resnikoff, The Illusion of Reality (1989), ISBN 0387963987, p. 74
  • The reactions that break down large molecules into small ones do not require an input of energy, but the reactions that build up large molecules require an input of energy. This is consistent with the laws of thermodynamics, which say that large, orderly molecules tend to break down into small, disorderly molecules.
    • Stanley A. Rice, Life of Earth: Portrait of a Beautiful, Middle-aged Stressed-out World (2011)
  • No one has yet succeeded in deriving the second law from any other law of nature. It stands on its own feet. It is the only law in our everyday world that gives a direction to time, which tells us that the universe is moving toward equilibrium and which gives us a criteria for that state, namely, the point of maximum entropy, of maximum probability. The second law involves no new forces. On the contrary, it says nothing about forces whatsoever.
    • Brian L Silver, The Ascent of Science (1998)
  • A good many times I have been present at gatherings of people who, by the standards of the traditional culture, are thought highly educated and who have with considerable gusto been expressing their incredulity at the illiteracy of scientists. Once or twice I have been provoked and have asked the company how many of them could describe the Second Law of Thermodynamics. The response was cold: it was also negative. Yet I was asking something which is the scientific equivalent of: Have you read a work of Shakespeare's?
    • C. P. Snow, 1959 Rede Lecture entitled "The Two Cultures and the Scientific Revolution".
  • My own introduction to entropy was as an undergraduate mechanical engineering student. Neither I nor any of the other students knew anything about the molecular theory of heat, and I bet that the professor didn't either. The course... was so confusing that I...couldn't make any sense of it. Worst of all was the concept of entropy. We were told that if you heat something a small amount, the change in thermal energy, divided by the temperature, is the change of its entropy. Everyone copied it down but no one understood what it meant. It was as incomprehensible to me as "The change in the number of sausages divided by the onionization is called floogelweiss."
    • Leonard Susskind, The Black Hole War: My Battle with Stephen Hawking to make the World Safe for Quantum Mechanics (2008)
  • Organic evolution has its physical analogue in the universal law that the world tends, in all its parts and particles, to pass from certain less probable to certain more probable configurations or states. This is the second law of thermodynamics. It has been called the law of evolution of the world; and we call it, after Clausius, the Principle of Entropy, which is a literal translation of Evolution in Greek.
  • It is impossible by means of inanimate material agency, to derive mechanical effect from any portion of matter by cooling it below the temperature of the coldest of the surrounding objects. [Footnote: ] If this axiom be denied for all temperatures, it would have to be admitted that a self-acting machine might be set to work and produce mechanical effect by cooling the sea or earth, with no limit but the total loss of heat from the earth and sea, or in reality, from the whole material world.
  • 1. There is at present in the material world a universal tendency to the dissipation of mechanical energy.
    2. Any restoration of mechanical energy, without more than an equivalent of dissipation, is impossible in inanimate material processes, and is probably never effected by means of organized matter, either endowed with vegetable life or subjected to the will of an animated creature.
    3. Within a finite period of time past, the earth must have been, and within a finite period of time to come the earth must again be, unfit for the habitation of man as at present constituted, unless operations have been, or are to be performed, which are impossible under the laws to which the known operations going on at present in the material world are subject.
    • William Thomson, Mathematical and Physical Papers, Vol.1 p. 512 (1882) "On a Universal Tendency in Nature to the Dissipation of Mechanical Energy" originally from the Proceedings of the Royal Society of Edinburgh for April 19, 1852, also Philosophical Magazine, Oct. 1852
  • In classical physics, most of the fundamental laws of nature were concerned either with the stability of certain configurations of bodies, e.g. the solar system, or else with the conservation of certain properties of matter, e.g. mass, energy, angular momentum or spin. The outstanding exception was the famous Second Law of Thermodynamics, discovered by Clausius in 1850. This law, as usually stated, refers to an abstract concept called entropy, which for any enclosed or thermally isolated system tends to increase continually with lapse of time. In practice, the most familiar example of this law occurs when two bodies are in contact: in general, heat tends to flow from the hotter body to the cooler. Thus, while the First Law of Thermodynamics, viz. the conservation of energy, is concerned only with time as mere duration, the Second Law involves the idea of trend. Milne developed his cosmology by taking this idea of trend to be fundamental, regarding the expansion of the universe as its supreme manifestation.


  • Zeroth: You must play the game.
    First: You can't win.
    Second: You can't break even.
    Third: You can't quit the game.
    • A common scientific joke expressing the four laws. Often attributed to C. P. Snow without a source, such as in Mahon, Tom (2011). Reconnecting.calm.  The first to third laws were expressed like this in Astounding Science Fiction, November 1956[1] by Dr. Wayne Batteau of Harvard University.
    • Variants:
      • You can't win; you can only break even.
        You can only break even at absolute zero.
        You can't reach absolute zero.
      • Zeroth: You must play the game.
        First: You can't win.
        Second: You can't break even, except on a very cold day.
        Third: It doesn't get that cold.
      • Zeroth: You must play the game.
        First: You can't win.
        Second: You can't break even, except on a very cold day.
        Third: It doesn't get that cold, even in Wisconsin.
      • Zeroth: There is a game.
        First: You can't win.
        Second: You must lose.
        Third: You can't quit.


  • Murphy's Law about Thermodynamics: Things get worse under pressure.
  • If you think things are in a mess now, JUST WAIT!
  • "Old Chemists never die: they reach thermodynamic equilibrium"
  • S happens.
    • Bumper sticker on the second law of thermodynamics.
  • You can't unscramble eggs.

Also see

Wikipedia has an article about:
  1. Archive of Astounding Science Fiction November 1956[1]