Re: COLLISION 2

[Brian McInnes <bmcinnes@PNC.COM.AU>]

+Since writing the e-mail below  and before sending it off, I caught
up on the mail for the last few hours.  Jim Green has already dealt
with the mattes well,  but I'll send this contribution off anyway
because it does cover a couple of points that Jim doesn't mention.
BTW, Jim, I agree with his point about the reality of the photon.


   Mike Monce wrote:
>        Webster's gives: transfer   a) to convey from one place to
> another: TRANSPORT, b) to cause to pass from one to another: TRANSMIT , c)
> TRANSFORM, CHANGE.
>
>        A photon leaves the sun with h-nu of energy, travels through
> space, strikes the surface of the earth and is absorbed by an atom.
>
> Questions:
>        1) Didn't the internal energy of the sun decrease (change) by
>           h-nu?
>
>        2) Didn't the internal energy of the earth increase (change) by
>           h-nu?
>
>        3) How did this happen if the photon did not transfer the energy?
> (see a) and b) and c) above).


It seems that the crux of this discussion is whether energy is a THING
that can moved from one place (system) to another.

Feynman certainly used blocks as an analogy for energy and had young
Dennis looking for and finding them all over his bathroom.  But it was
an analogy used to show conservation and I argue that we read it
wrongly if we believe its purpose was to show that energy is a thing
that can be carried in and out of the bathtub.

In Mike's example the photon travels from the sun to the earth where
it is absorbed by an atom.  As a result changes have occurred to some
of the properties of the sun system and the earth system.  Consider
two of these properties: energy and entropy.  The energy of the sun is
reduced, the energy of the earth is increased.  These energy changes
are equal in magnitude and opposite in sign. As a result there is a
temptation to claim that the energy has been transferred as though in
the words of of the dictionary definition quoted above it is some sort
of stuff that has been conveyed from one place (the sun) to another
(the earth) or has passes from the sun to the earth.

But before we succumb to that temptation, let's look at what has
happened to that other property, entropy.  The entropy of the sun is
reduced, the entropy of the earth has increased.  These entropy
changes are NOT equal in magnitude, although they are opposite in
sign.  (As a consequence of the lower temperature of the earth, its
increase in entropy is less in magnitude than the entropy loss of the
sun.)  Here surely there is no temptation to claim that entropy has
been transferred.

Both energy and entropy are properties of systems; they are not things
in their own right that can be conveyed.  It is an easy step from the
idea of energy or entropy or colour as attributes that can be
conveyed/transported/transmitted to the idea of energy or entropy or
colour being a fluid that can flow from one system to another.  All
three of these and other properties of systems can be changed but they
are not things that can be conveyed or that can flow.

Brian McInnes