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Re: Is photon a wave packet ?



I am trying to understand Roger Haar's posting (below).

I am trying to imagine a photon on its way from the Sun to Earth. I am
assuming this photon came from an atomic transition. I understand the
energy of the upper state had a finite uncertainty so the energy of this
specific photon can't be known until we measure it. That in itself would
not necessarily mean the photon is not monochromatic. It just means we
don't know the exact energy of this photon even if we somehow know it came
from an n=3 to n=2 transition in hydrogen.

I am trying to picture what happens in space between the Earth and the Sun.
At the moment of detection the photon doesn't span this whole distance does
it? Isn't there a beginning (a wave front) and ending (a wave behind) to
this thing as it travels in space. It takes over 8 minutes to get here, and
the transition time in the atom is way shorter than that. Four minutes into
its travel isn't there space ahead of the photon where the photon hasn't
been yet, and space behind it where it has been but isn't any more? But, if
there is some localization, doesn't that require a wave packet, and doesn't
that require polychromatic?

Are we saying that the energy difference in the atom is not exact, and over
the time of the electronic transition (and hence during the creation of the
photon) the energy difference between states is not constant, thus the
photon ends up not being monochromatic, and this means it is a wave packet,
and that explains the localization?


Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817


...the light associated with a transition from single isolated
excited atom is not truly monochromatic. It has a
natural line width or energy spread. The energy of
the upper state of a transition has a certain
amount of uncertainty in it otherwise the state
would have an infinite lifetime and there would be
no transition. This is NOT a statistical thing,
individual discrete photons just do not have an
exact energy until their energy is measured.

Roger Haar