Re: Understanding MinEP and MaxEP
- From: Tim Tyler <tim@xxxxxxxxxxx>
- Date: Sat, 1 Oct 2005 14:38:58 -0400 (EDT)
William Morse <wdmorse@xxxxxxxxxxxx> wrote or quoted:
> "Perplexed in Peoria" <jimmenegay@xxxxxxxxxxxxx> wrote in
> > I agree with what you are saying both with regard to what Lotka's
> > logic was, and also with regard to the validity of that logic.
> >
> > However you seem to be missing one key point which leads you to
> > mistakenly believe that there is a significant distinction between
> > maximum work and maximum entropy production. The thing you are
> > missing is this:
> >
> > In a 'climax' ecosystem at steady state, biomass does not accumulate!
> >
> > Therefore, you can't use accumulation of biomass as a measure of
> > 'work'. Biomass may represent 'work' to a growing organism, but to an
> > ecosystem it is secondary production. Ultimately, all of the
> > potential energy available to the primary producers is going to be
> > converted to heat. (It may be converted to heat by either biological
> > or non-biological mechanisms, but it is going to become heat in any
> > case). Now heat is not the same as entropy - you have to divide heat
> > by temperature to get entropy. You get the most entropy if the heat
> > production is done at nice low biological temperatures (by leaves or
> > termites) rather than at high temperatures (black-body minerals or
> > forest fires) from the original potential energy source (sunlight or
> > cellulose). So maximum work really does come pretty close to maximum
> > entropy production if you count the 'work' as simply a secondary
> > energy source.
>
> But what if the energy is transferred without producing heat (i.e.
> reversibly). The termites are merrily turning wood into more termites,
> but every step of the reaction is very close to equilibrium. This is
> maximum work (in terms of amount of termites produced) but it is MinEP.
You can convert energy from one form to another with relatively little
entropy gain, but in practice if you try to use the energy to *actually*
do something with, then it is hard to avoid dissipating that potential
energy as entropy.
Living organisms are analogous to a fridge in this context. They are
trying to maintain an ordered state (their genomes) in an environment
where such an ordered state would not naturally occur, one that has
its own entropic forces that actively resist such accumulations of order.
Why can't such order be maintained reversibly? Try building a fridge
without a power supply and you will soon see the problem.
In *theory*, you could have a near-reversible fridge - that maintained a
low temperature inside it by using lots of very-effective insulation.
However, real fridges are not like that - in practice they use *lots*
of energy to preserve the ordered state inside them.
So it is with living systems: you can have reversible living systems in
/theory/ (e.g. under simulation) but in reality, organisms are usually
far from being reversible - and leave a trail of garbage in their wake.
Even under simulation, if you have a reversible living system, the
chances are high that a predator or parasite that will arise that will
be irreversible. There are usually many more ways to be irreversible
than there are to be reversible - and energy is more easily wasted than
it is conserved.
--
__________
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- From: William Morse
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