Re: "quantization"

[Igal Galili <igal@vms.huji.ac.il>]

> > Let me ask, without making no powerful reference (which is almost never a
> > problem), what is wrong in saying that energy is a physical entity which
> > conservation follows the time invariance of physics laws?
>
> I think Noether's theorem's consequence is usually stated a bit
> differently, but I understand that energy conservation is a
> necessary counterpart (not a consequence) of the time invariance
> of physical law. Let me also point out that energy conservation,
> like energy itself, is a mathematical abstraction. The time
> invariance of physical law is a quantitative statement as well.
>
> Leigh

Well, ANY physical concept is not more than a mental construct (see Dewey's
aftermail quote of Einstein).   It does not prevent us from using them.  As
far as I know, the energy conservation follows the explicit demand of time
invariance expressed in math terms.  But in any case (that is whether you
are going to derive it or restrict yourself to a claim, which is sufficient
to the great majority of students), I am convinced (because I have enough
supporting experience) that to provide some theoretical ecology,  almost
regardless the level of physics instruction, is essential and contributes a
lot to people's understanding.  A mere joking on the subject is a play
which is good only for the great performers (e.g., Feynman) who can
compensate listeners later on with a plenty of other brilliant pieces.  In
a regular case (e.g. myself) it might not help: " What is good for Jupiter
is not for an ox."  I feel much better (and it guarantees the similar to my
students), when I do relate energy conservation with the time invariance (a
causal connection) and generalize this approach to other conservation laws
(within the known limits).  Let me make a reference to Landau & Lifshits
Vol. I (Mechanics)  and express a pity that this point is not widely
adopted in INTRODUCTORY physics courses.
Igal.