More on the Second Law

Since writing my previous post on the second law of thermodynamics I’ve been wondering just how seriously to take the premise that human intention has the power to reverse it. Feedback so far has been limited, so I’ve mostly been focusing on other things, but I’ve also done some quick research.

One interesting thing I found from wikipedia was that there seems to be a fair amount of controversy and confusion regarding the definition of entropy. This is in some ways remarkable, given (i) the fact that it is THE key concept around which the second law is generally formulated, and (ii) the second law is taken to be so fundamental in physics. How can we be so sure of a law when we can’t even agree about the definition of the key concept around which it’s formulated?

One aspect of the controversy concerns the idea that entropy is a measure of how disordered a system is. This idea seems to be going out of vogue, at least in the teaching of chemistry in the US, essentially on the grounds that it confuses students and distracts from more precise definitions in the context of real-world, earth-bound chemical processes. But if we really want to make a fundamental law out of the idea that entropy tends to increase, then it seems to me that we need to do better than just “heat tends to flow from a hotter to a colder body”. It doesn’t, not at the scale of stars and galaxies. That’s why, in the words of Roger Penrose, the sun is a “hot spot in a cold sky”, which is basically why life on earth is possible.

In any case I would welcome views on this. Don’t feel the need to be an “expert” in order to comment. Sometimes experts get wrapped up in their own jargon, while “non-expert” views are necessary to reconnect with day-to-day reality. (Expert views are also welcome of course.)

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About Peter Wicks

International consultant. I bring clarity to complex and confusing situations and identify the most promising solutions.
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2 Responses to More on the Second Law

  1. Matt says:

    I don’t understand your claim that entropy doesn’t hold for stars and galaxies. Heat from the Sun flows from it (which is hot) to space around it (which is cooler). That’s an increase in entropy.

  2. peterwicks says:

    Thanks for the comment (and sorry for being slow to moderate…)

    I don’t say that the idea of increasing entropy doesn’t hold for stars and galaxies. What I do say is that *the idea that heat tends to flow from a hotter to a colder body* doesn’t hold where gravitational effects are important. The process you mention (i.e. radiation) doesn’t significantly involve gravitation, so heat indeed flows from the hotter to the colder “bod(ies)”. But what about when the star is being formed? There you have essentially cold matter clumping together – due to gravity – and getting hotter as it does so. Meanwhile more matter falls into (and becomes part of) the star, which seems to me to be a case of heat flowing from the colder to the hotter body.

    This is not what kills the second law, of course. This type of gravitational clumping is itself an entropic process, in the sense that there are far more microstates corresponding to the “star” macrostate than there are to the “cloud of cold matter not yet congealed into a star” macrostate. But continuing with the argument I started to develop in my earlier (31/08/2011) post, all this really means is that we have defined a more precise macrostate when we talk about a cloud of cold matter than when we talk about a star.

    There is plenty that can be said about why we do this, but my hunch is that part of the reason at least is that we are sensitised, presumably by natural selection and the nature of (this part of) the universe that we inhabit, to distinguish more precisely between states the closer they are to Big Bang conditions. So for example, we can tell the difference between all the air being squashed into the corner of the room and other states, but we can’t tell the difference between the different states that correspond to the air being spread throughout the room at roughly equal temperature and pressure. This is what gives us the impression that entropy is always increasing (except to the extent it can be exported to the environment) and we are headed towards thermal death. Whereas in reality, when we design low-entropy states and then take action to bring them about, we are actually reducing entropy.

    That last sentence needs to be nuanced, of course. When we build a cathedral, for example, we certainly do so by exporting a lot of entropy. But this is still “entropy” in the sense of the usual categorisations we make between macrostates. We eat food, we burn fuel, etc…all this involves evolution from smaller, well-defined macrostates (e.g. “oil fields”) towards large, amorphous (to our perception) macrostates (e.g. increased atmospheric CO2). And most of the work is done relatively “mindlessly”, i.e. without much care.

    One objection to my argument might be that even the initial creative process involves exporting entropy, since it’s a mental process that takes place in the brain, which is a dispersive system. I’m not completely sure how to counter that (or indeed whether it can be countered) at this stage, but still the fundamental subjectivity (pointed out inter alia by Penrose in his book Road to Reality) of the concept of entropy leads me to doubt whether the second law is as fundamental as it is generally taken to be.

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