On Mon, Feb 06, 2012 at 08:36:44PM +0100, Evgenii Rudnyi wrote:
> On 05.02.2012 23:05 Russell Standish said the following:
> >
> >The context is there - you will just have to look for it. I rather
> >suspect that use of these tables refers to homogenous bulk samples
> >of the material, in thermal equilibrium with a heat bath at some
> >given temperature.
>
> I do not get your point. Do you mean that sometimes the surface
> effects could be important? Every thermodynamicist know this.
> However I do not understand your problem. The thermodynamics of
> surface phenomena is well established and to work with it you need
> to extend the JANAF Tables with other tables. What is the problem?
The entropy will depend on what surface effect you consider
significant. Is it significant that the surface's boundary bumps an
dimples are so arranged to spell out a message in English? What if you
happen to not speak English, but only Chinese? Or might they not be
significant at all? All of these are different contexts.
Ignoring surface effects altogether is a perfectly viable model of the
physical system. Whether this is useful or not is going to depend,
well, on the context.
>
> It would be good if you define better what do you mean by context
> dependent. As far as I remember, you have used this term in respect
> to informational capacity of some modern information carrier and its
> number of physical states. I would suggest to stay with this example
> as the definition of context dependent. Otherwise, it does not make
> much sense.
It makes just as much sense with Boltzmann-Gibbs entropy. Unless
you're saying that is not connected with thermodynamics entropy...
>
> >If we were to take you at face value, we would have to conclude that
> >entropy is ill-defined in nonequlibrium systems.
>
> The entropy is well-defined for a nonequilibrium system as soon as
> one can use local temperature. There are some rare occasions where
> local temperature is ambiguous, for example in plasma where one
> defines different temperatures for electrons and molecules. Yet, the
> two temperatures being defined, the entropy becomes again
> well-defined.
This is circular - temperature is usually defined in terms of entropy:
T^{-1} = dS/dE
>
> >More to the point - consider milling whatever material you have
> >chosen into small particles. Then consider what happens to a
> >container of the stuff in the Earth's gravity well, compared with the
> >microgravity situation on the ISS. In the former, the stuff forms a
> >pile on the bottom of the container - in the latter, the stuff will
> >be more or less uniformly distributed throughout the containers
> >volume. In the former case, shaking the container will flatten the
> >pile - but at all stages the material is in thermal equilibrium.
> >
> >In your "thermodynamic context", the entropy is the same throughout.
>
> No it is not. As I have mentioned in this case one just must
> consider surface effects.
Hence the context.
>
> >It only depends on bulk material properties, and temperature. But
> >most physicists would say that the milled material is in a higher
> >entropy state in microgravity, and that shaking the pile in Earth's
> >gravity raises the entropy.
>
> >Furthermore, lets assume that the particles are milled in the form
> >of tiny "Penrose replicators" (named after Lionel Penrose, Roger's
> >dad). When shaken, these particles stick together, forming quite
> >specific structures that replicate, entraining all the replicators
> >in the material. (http://docs.huihoo.com/reprap/Revolutionary.pdf).
> >
> >Most physicists would say that shaking a container of Penrose
> >replicators actually reduces the system's entropy. Yet, the
> >thermodynamic entropy of the JNAF context does not change, as that
> >only depends on bulk material properties.
>
> We are again at the definition of context dependent. What are saying
> now is that when you have new physical effects, it is necessary to
> take them into account. What it has to do with your example when
> information on an information carrier was context dependent?
>
Who decides what physical effects to take into account? This is not a
question of pure relativism - I'm well aware that some models are much
better than others at describing the situtaion, but even in the case
of Penrose replicators described above, their ability to adhere and
fragment may or may not be relevant to the situation you are trying to
model.
--
----------------------------------------------------------------------------
Prof Russell Standish Phone 0425 253119 (mobile)
Principal, High Performance Coders
Visiting Professor of Mathematics hpco...@hpcoders.com.au
University of New South Wales http://www.hpcoders.com.au
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