Re: Photon Energy...Huh?

From: PD (pdraper_at_yahoo.com)
Date: 03/28/05 

Date: 28 Mar 2005 12:49:19 -0800

mme...@cars3.uchicago.edu wrote:
> In article <1112022451.154874.42250@z14g2000cwz.googlegroups.com>,
"PD" <pdraper@yahoo.com> writes:
> >
> >bruce_pearson@hotmail.com wrote:
> >> Physics Newbie here...
> >> The words "energy" and "photon" are often put together, as in
"Photo
> >> energy".
> >>
> >> Since, e=mc^2. and a photon is a massless particle, how does it
> >aquire
> >> energy without aquiring mass??
> >>
> >> I'm sure a dumb question, but, unless I understand this I won't
get
> >> farther.
> >>
> >> Thanks.
> >>
> >> BP
> >
> >A much better definition of energy is that quantity which, when
added
> >to other contributions to the energy, is conserved in a closed
system.
> >
> That's a tad circular. Try "the end point variation of the action
> under time translation" or, alternatively, "the generator of
> infinitsimal time translations".

I'm not sure mine's so circular, but it's also not so useful. Yours is
more useful, but it offers little physics insight to the untrained.

I think my point to the neophyte would be this:
In a closed system (where this can easily be defined), we note that
there are certain quantities which, when added up for the system, turn
out to remain constant in the system, no matter what happens inside the
system. This is quite a remarkable statement, especially the last
clause. Why this is so is not completely understood, but it is true
nonetheless. The best we can do is say that these "conservation laws"
are related to symmetries exhibited by the laws that govern the set of
forces acting within the system, but this really only changes the
question from why those quantities are conserved to why nature exhibits
those symmetries, though the latter seems to have more intuitive
appeal. The trick, of course, is figuring out how to calculate the
quantities which, when added up, are conserved. Historically, this has
amounted to a combination of stumbling on the right thing (or something
very close to the right thing) or knowing some details about the system
(e.g. the Lagrangian of the system, which in turn tells you something
about the symmetries of the system).

The fundamental issue is "Does how to calculate it amount to a
definition?" I maintain not, otherwise all these folks that say
"momentum is m*v" would argue they have a definition. Likewise, coming
up with a guaranteed-to-work recipe for how to calculate it under any
conditions also presumes knowing more about the system (e.g. the
Lagrangian). I favor a *definition* that describes why the quantity is
important, but that's just me.

PD

>
>
> >A similar but independent definition applies to momentum.
> >
> Just the same, in fact, with time translation replaced by space
> translation.
>
> Mati Meron | "When you argue with a fool,
> meron@cars.uchicago.edu | chances are he is doing just the
same"

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