Re: On the uncertainty principle for photons. An experimental counter
- From: CarlB <carl@xxxxxxxxxxxxxxxx>
- Date: Sat, 15 Dec 2007 18:12:40 -0500 (EST)
On Dec 10, 10:29 pm, DRLunsford <antimatte...@xxxxxxxxx> wrote:
On Dec 4, 3:08 pm, kvblake <kvblake2...@xxxxxxxxx> wrote:
Do you mean that when a photon is created there emerges a EM field at
once in the whole universe or the existing EM field changes at a
constant value immidiately everywhere?
I read about localized photons in Mandel's quantum optics.
Regards: Kevin
No, I mean exactly what relativity implies - things that go at C
cannot be localized in any way. The electromagnetic field already
exists everywhere in the universe, and can absorb and release energy-
momentum anywhere and any-when. The photon as a thing, a localized
object, does not exist, and can't possibly exist.
In fact the entire idea of propagation is what is at issue here. In
relativity, the world is 4-d, and propagation is a primitive fact,
like distance in Euclidean geometry, and can't be reduced. So the EM
field absorbs energy here, and releases it there, with the difference
of here and there being an interval lying on the light cone. Photons
are the units of absorption and release. The 4-d-ness of the world
really has to be taken at a face value.
The conflict between relativity and QM is stark enough without making
it worse. Neither ceases to be true because of the other.
-drl
QED works with "point particles" and the photon is a point
particle like any other. Feynman is clear on this subject and
he is correct. There are a number of ways of seeing this.
(a) Experimentally, we are unable to distinguish between a
photon with a small (uh, very very small) mass and a massless
photon. Theoretically, you can assume that a photon has a
small mass, make a calculation, and then let the mass
go to zero. The limit will be well defined and you will get the
right answer as if you had left the photon massless from the
beginning.
Therefore, our theory and experiment cannot tell you what
the mass of the photon really is. If you are a gambling man,
you can place your bet anywhere along the real line from
- 1.0e-56 kg to +1.0e-56 kg and your number for the photon
mass will be compatible with experiment, and therefore,
ipso-facto, equally likely as the value "0" that it is assumed
to be. An oft quoted fact about nature is that anything that
is not forbidden is mandatory. A massive photon is not
forbidden. And the set of photon masses where the photon
is truly a massless particle, as compared against the
experimentally allowed photon masses, is a set of
measure zero, a mighty thin assumption to stand upon.
(2) The definition of "point particle" in a QFT is quite clear
and the photon is one of these. Some might confuse that
discussion with the question of whether or not a photon
can be "localized". You might as well ask if an electron
can be localized. Both are on the same basis with respect
to quantum mechanics as each other.
Yes, it is true that it is easy to find good QM references
that say the photon does not have a position-space wave
function, but these references are out of date. Margaret
Hawton solved the problem about a decade ago. For a
recent article of hers, see
http://arxiv.org/abs/0705.3196 and references.
The above is not well known, despite being published
in Phys Rev A. For example, see:
http://arxiv.org/abs/quant-ph/0508202
.
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