[ExI] The subjectivity of entropy, the role of the observer...==> Rational metaethics

Lee Corbin lcorbin at rawbw.com
Fri Feb 29 04:54:54 UTC 2008

Bryan writes, based on what Jef posted about Eliezer's entropy essay

> On Thursday 28 February 2008, Jef Allbright wrote:
>> <http://www.overcomingbias.com/2008/02/second-law.html>
>> "The connection in the other direction is less obvious.  Suppose there
>> was a glass of water, about which, initially, you knew only that its 
>> temperature was 72 degrees.  Then, suddenly, Saint Laplace reveals to 
>> you the exact locations and velocities of all the atoms in the water.  
>> You now know perfectly the state of the water, so, by the 
>> information-theoretic definition of entropy, its entropy is zero.  
>> Does that make its thermodynamic entropy zero?  Is the water colder, 
>> because we know more about it? Ignoring quantumness for the moment,
>> the answer is:  Yes!  Yes it is!" 

The arguments about what entropy is (or how it should be regarded)
are very old, and years ago I spent hours and hours involved in them.

Almost all the time, I stick with this idea:  Temperature of a
gas is the mean kinetic energy of its molecules. 

I doubt if anyone wants to really "subjectivize" what velocity and
mass are---the classical conceptions are simply too useful.  Still, of
course, velocity is relative to an observer, (and so is mass-energy,
unless you speak of a relativistic invariants and so on). But I think
these would be red-herrings to the real issue right here.

> Saint Laplace, supposedly being a mystical being able to reveal this 
> information to you, would have to directly measure all of these pieces 
> of information -- otherwise I would have to argue that it is _not_ 
> colder.

Are you suggesting that the act of measuring those particles affects
them?  That would be true, but I don't see the relevance for the
discussion. A simulation of 100 molecules in a box could be created
in a number of different contexts, and the locations and velocities
known as functions of time.  So without doing more that skimming
Eliezer's paper, and having no time for all the inevitable comments,
can someone briefly explain why I should abandon "Temperature
in gas is the mean kinetic energy of its molecules"?

> In other words, whether it is St. Laplace or yourself that does 
> the measurements: the measurements must be within this universe, else I 
> would have to debate the point that it is colder. But this is just 
> nit-picking, since the basic point that anything that you know more 
> about gets colder.

Suppose I read a magazine article that tells me the rotational energy
of a centrifuge at NASA that is kept going at a constant angular
momentum etc. Should I then in good conscience---having read all
the above---dash off an email telling them to lower their estimate of
its temperature/energy ever so slightly?  After all, I, Lee, now *know*
more about it than I did this morning, hence it must be colder.
(????????)  Sorry. Can't believe that. Sounds like quite a number
of sentences need to be further qualified.


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