Re: Quantum question?

[Jack Uretsky <jlu@HEP.ANL.GOV>]

I add to Leigh's exceptions to Ben Crowell's posting:
        A "photon" is an object with definite energy and momentum.  It
is detected through it's interaction with matter (e.g. the Compton
effect).  The momentum and location of the point of interaction are
conjugate variables that are subject to the uncertainty principle.  If
the point of interaction is left undetermined, then the momentum of
an interacting photon may be determined with arbitrary precision.
        Using the known fact that a photon has zero mass then leads you
to an energy determination that has the same precison as the momentum
determination.  Since E=hf, f the frequency, the frequency is known to
the same precision as the energy.
        This is why it is meaningless to speak of the "linewidth" of
a single photon (although it is meaningful to speak of its energy shift
from some expected value, or the uncertainty in the determination of the
energy of a particular photon).
                                Regards,
                                        Jack

Adam was by constitution and proclivity a scientist; I was the same, and
we loved to call ourselves by that great name...Our first memorable
scientific discovery was the law that water and like fluids run downhill,
not up.
                          Mark Twain, <Extract from Eve's Autobiography>

On Fri, 28 Jan 2000, Ben Crowell wrote:

> Leigh Palmer wrote:
> > A single photon doesn't have a linewidth. Linewidth pertains to the
> > distribution of energies in an ensemble of photons.
>
> Not true. A photon could only have zero line width if it was a
> sine wave of infinite length. How this effect compares in importance
> with thermal broadening, for instance, depends on the situation.
>
>         Ben Crowell
>         Fullerton College