[Phys-L] Re: entropy and electricity

[John Denker <jsd@AV8N.COM>]

Fayngold, Moses wrote:

>  Now, suppose we have a single photon whose polarization is
> equally likely to be found in one of two distinct states,
> namely, either along the x- or along the y-direction,
> after we will have passed it through, say, a polarizing
> beam-splitter.

That description is ambiguous.

>  Does this mean that before that the photon's entropy is
> 1 bit?

That depends on how we disambiguate the question.  There are
two extreme cases:

a) Unpolarized light has density matrix
           [ .5    0 ]
           [         ]
           [  0   .5 ]
and has an entropy density of one bit per photon.

b) Light 100% linearly polarized at 45 degrees to the XY axes has
density matrix
           [ .5   .5 ]
           [         ]
           [ .5   .5 ]
and has an entropy density of zero.

c) And you can have anything in between.

......

>  Third, whose "not knowing" the entropy is? Knowing or not
> knowing is the state of the observer,

True.

> while the entropy is
> the objective state of a system.

False, for reasons explained in detail at
   http://www.av8n.com/physics/thermo-laws.htm#sec-s-context

> This has been already
> discussed pretty extensively on this Forum.

So why bring it up again?
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