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From: marcofuics on 9 Mar 2010 08:14
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?
From: dlzc on 9 Mar 2010 09:11
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".

> 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.

> 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.

David A. Smith
From: marcofuics on 9 Mar 2010 09:19
On Mar 9, 3:11 pm, dlzc <dl...(a)cox.net> wrote:
> 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".

So, you mean Quantum(FT) scenario.
For example: does a photon have its position?
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.....

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]
From: Tom Roberts on 9 Mar 2010 10:02
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.

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.


> 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.

You say "This means...", without any logical connection that actually implies
that. Just placing words in sequence is not an argument.

Counterexample: a light pulse travels with speed c to all
observers, and can be easily localized by any of them.
The difficulties are related to quantum phenomena, not
speed.


Tom Roberts

From: marcofuics on 9 Mar 2010 10:12
On Mar 9, 4:02 pm, Tom Roberts <tjroberts...(a)sbcglobal.net> wrote:

> You say "This means...", without any logical connection that actually implies
> that. Just placing words in sequence is not an argument.

I am wondering in a general (abstract) scenario; i dont want to anchor
reasoning to a particular framework.... so it is just a gedanken
experiment.
First point: Do you agree with the assumption that "if a body (....a
particle, an entity) has no mass; then this means that it moves at c"?
And no one can go at the same c speed (or at least no massive
observer)?

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