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Why is the second law of thermodynamics undisputable?

On his website Professor Hawking says the following:

The cosmologist, Sir Arthur Eddington, once said, 'Don't worry if your theory doesn't agree with the observations, because they are probably wrong.' But if your theory disagrees with the Second Law of Thermodynamics, it is in bad trouble.

Why is that? Why does the second law of thermodynamics seem to prescribe how theories are done? Why does it seem to have this power?

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    $\begingroup$ Great question! $\endgroup$
    – innisfree
    Oct 26, 2015 at 10:58
  • $\begingroup$ What's the original source for Eddington's quote? It's similar to this unsourced quote by Crick: A theory should not attempt to explain all the facts, because some of the facts are wrong. $\endgroup$
    – innisfree
    Oct 26, 2015 at 11:07
  • $\begingroup$ @innisfree: According to Wikiquote it comes from Eddington's "The Nature of the Physical World" (1928) - Published versions of his Gifford Lectures delivered in the University of Edinburgh (January - March 1927) $\endgroup$
    – RedGrittyBrick
    Oct 26, 2015 at 11:12
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    $\begingroup$ 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. - Eddington $\endgroup$
    – innisfree
    Oct 26, 2015 at 11:31
  • $\begingroup$ You may be interested in reading the last chapter of Herbert Callen's Thermodynamics and an introduction to thermostatistics (i.e. the postlude 'Symmetry and the conceptual foundations of thermodynamics'). This conceptual basis is quite different to other physical theories and this difference is the reason Eddington, Einstein and Hawking place it on a different footing than other laws. $\endgroup$
    – Emilio Pisanty
    Oct 26, 2015 at 12:35

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This type of blanket pronouncements by quite venerable people in physics are the bread and butter of crackpottery.

Theorists should worry if their theories disagree with the standard physics theory that has been built up to their research, not just the second law of thermodynamics. Lorenz invariance is very important, energy conservation and the other conservation rules etc. Entropy and thermodynamics are a small part of the great volume of physics disciplines.

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.

These blanket pronouncements should be buried , not pointed out or quoted out of context . Maybe he is just talking of cosmological theories:

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.

Physics as we know it is based on two frameworks, the quantum mechanical one and the classical which emerges from the quantum mechanical. Within these frameworks there are ranges of the variables where theories developed are accurate and validated by observations and experiments, and ranges where new formats have to be proposed.

The second law of thermodynamics plays no role in particle physics for example, except like the famous song "the ankle bone is connected to the leg bone, the leg bone is connected to the knee bone..." . The Standard Model is an adequate summary of observations and data. It is very important in cosmological models but the inflaton model was proposed just to get away from thermodynamic limitations, and it is considered very successful in modeling the observations, though it has inflationary entropy problems.

Observations and measurements can be wrong, a recent one was the superluminal neutrino business,for example, where the measurement was a hardware error. Generally though technology and the number of physicists checking data is so large, and the drive to reproduce results very great, that the probability of a wrong measurement lasting for long in the data banks is very small.

All in all I would consider it a not to be quoted as wisdom quote.

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    $\begingroup$ I disagree. I don't think that these sorts of statements are meant as a put-down of known physical theory, but rather to emphasize the fact that the conceptual basis of thermodynamics is quite different to e.g. quantum mechanics, and it is more concerned with the symmetries of the underlying physical laws than with their details. I also don't think the statements that correspondence with observation are really meant seriously. $\endgroup$
    – Emilio Pisanty
    Oct 26, 2015 at 12:38
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    $\begingroup$ I agree with @EmilioPisanty. I expect that, as time goes by, what we consider the "fundamental" description of Nature will still change many times, as its scope encompasses larger and larger domains. However, most people have no doubt that (a proper version of) the second law of thermodynamics will continue to hold, irrespective of what our current pet fundamental theory is. $\endgroup$
    – Yvan Velenik
    Oct 26, 2015 at 13:27
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    $\begingroup$ @EmilioPisanty They are not taken seriously by serious researchers for sure, but they are ammunition for all the funny "theories" coming down the pipe. $\endgroup$
    – anna v
    Oct 26, 2015 at 13:31
  • $\begingroup$ @YvanVelenik Our view of nature and our models change even within the working life of a physicist, BUT the change is smooth, the old is not destroyed, just its region of validity is restrained. Thermodynamics emerging from statistical mechanics will always hold where the region of validity remains where the models are validated. It is new boundary conditions and new dimensions ( small or large) that have forced quantum dynamics on us, and cosmological theories , as the inflation before 10^-32 seconds with effective quantization and the inflaton field. $\endgroup$
    – anna v
    Oct 26, 2015 at 13:35
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    $\begingroup$ If your theory disagrees with Maxwellian EM - say it predicts a 1%-level in some exotic new parameter range - then it has an outside chance of matching experiment, and it supplements rather than supplants classical EM. If your theory disagrees with thermodynamics - say it predicts a 1%-level violation of the second law in some exotic new parameter range - then as Hawking puts it, it is in bad trouble, and it doesn't really just 'supplement' standard thermodynamics, it breaks pretty violently with it. $\endgroup$
    – Emilio Pisanty
    Oct 26, 2015 at 13:42

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