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From: dlzc on 9 Mar 2010 12:48
Dear marcofuics:

On Mar 9, 7:19 am, marcofuics <marcofu...(a)netscape.net> wrote:
> On Mar 9, 3:11 pm, dlzc <dl...(a)cox.net> wrote:
> > On Mar 9, 6:14 am, marcofuics <marcofu...(a)netscape.net> wrote:
>
> > > In my opinion only massive particles could
> > > be positioned, not massless.
>
> > No quantum object has a "position", only a
> > measurement of position with an uncertainty.
> > The fact that massless particles move at c
> > simply makes the problem "harder".
>
> So, you mean Quantum(FT) scenario.
> For example: does a photon have its position?

If you discount QM, yes, it has a position. But this is just a model,
not some Reality.

> No... the position-measured for the photon
> simply means that <<photon>> has collapsed
> in X eigenstate, of the sensor, i.e. we have
> measured the position of the sensor.....

If you are discounting QM, as you say you want to, then measuring a
photon's position is just a measurement problem.

> But speaking in a RR (or classical)
> scenario? Can we conclude something about
> the problem of position? [I dont want inspect
> the QM scenario... for the time being]

The difficulty is in detecting the passage of a photon through an
envelope of known size. Without absorbing it. So we could replace
one photon with a very short light pulse, so some members could be
lost. Or use photons trapped in quantum wells...

It is just not a helpful concept, since it pretends / assumes photons
are billiard balls...

David A. Smith
From: "Juan R." González-Álvarez on 9 Mar 2010 16:01
marcofuics wrote on Tue, 09 Mar 2010 05:14:45 -0800:

> In my opinion only massive particles could be positioned, not massless.
> Massless particle does move at speed of light c; so it is unreachable by
> whatever observational-frame.. then for this reason each observer sees
> it moving at the same c speed. This means that for massless particles
> talk about position has no sense. Any idea?

Photons have no position operator associated to them. See for instance

Simple proof of no position operator for quanta with zero mass and nonzero
helicity. 1980: J. Math. Phys. 19, 1382-1385. Jordan, T.F.

This is also true for some massive particles, e.g. Dirac electrons, which
always move at c.

In relativistic quantum electrodynamics, position is not an observable, unlike
in quantum mechanics.


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From: "Juan R." González-Álvarez on 9 Mar 2010 16:02
dlzc wrote on Tue, 09 Mar 2010 06:11:02 -0800:

> Dear marcofuics:
>
> On Mar 9, 6:14 am, marcofuics <marcofu...(a)netscape.net> wrote:
>> In my opinion only massive particles could be positioned, not massless.
>
> No quantum object has a "position", only a measurement of position with
> an uncertainty. The fact that massless particles move at c simply makes
> the problem "harder".

Nope.

>> Massless particle does move at speed of light c; so it is unreachable
>> by whatever
>> observational-frame.. then for this reason each observer sees it moving
>> at the same c speed.
>
> Because it is difficult to measure a position, only means it is not at
> rest.

Nope.

>> This means that for massless particles talk about position has no
>> sense.
>> Any idea?
>
> Quantum mechanics does not care about position, speed, path, or
> duration. I think it has a good idea.

Nope.

> David A. Smith





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From: "Juan R." González-Álvarez on 9 Mar 2010 16:04
Tom Roberts wrote on Tue, 09 Mar 2010 09:02:28 -0600:

> marcofuics wrote:
>> In my opinion only massive particles could be positioned, not massless.
>
> Hmmmm. This depends in detail on what you mean.
>
> For instance, with a suitable detector, photons of sufficient energy can
> be localized to better than a millimeter in all three directions, along
> with timing to a few picoseconds. "Sufficient energy" basically means
> high enough to liberate an electron from a photocathode; blue light has
> reasonable efficiency for this. Resolution degrades for gammas, because
> larger detectors are needed to get reasonable efficiency.

Nope.

> But in QED, photons do not "have" a position, in the sense that position
> is not a property of photons, and in the sense that they do not follow
> any trajectory. All one can do is measure them. But one can measure
> them.

Right.


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http://www.canonicalscience.org/publications/canonicalsciencetoday/canonicalsciencetoday.html
From: dlzc on 9 Mar 2010 17:38
Dear "Juan R." González-Álvarez:

On Mar 9, 2:01 pm, "Juan R." González-Álvarez
<nowh...(a)canonicalscience.com> wrote:
> marcofuics wrote on Tue, 09 Mar 2010 05:14:45 -0800:
>
> > In my opinion only massive particles could
> > be positioned, not massless. Massless
> > particle does move at speed of light c; so
> > it is unreachable by whatever
> > observational-frame.. then for this reason
> > each observer sees it moving at the same c
> > speed. This means that for massless particles
> > talk about position has no sense. Any idea?
>
> Photons have no position operator associated
> to them.

That's OK, Marco was not interested in a QM treatment at this time, if
you'd have bothered to read before sniping.

David A. Smith
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