Patrick Powers wrote:
Bjoern Feuerbacher wrote in message ...

Patrick Powers wrote:


[snip]


The position of a photon is uncertain with a probability distribution.
So is this uncertainty part of the collapse of the wave function or is
the photon actually moving in an uncertain manner?

The question is rather vague. "collapes of the wave function" usually
refers to the fact that after an observation, the wave function is
suddenly an eigenfunction of the relevant observable. This has little to
do with the position of the photon being uncertain.


Aha, I was thinking the position becomes certain when measured, but
this is not correct.

The position is only certain within the limits of the measurement.

I.e. the wave function collapes to something like a narrow Gaussian
distribution centered on the position where one thinks to have observed
the photon, with a width corresponding to the resolution of the measurement.



How about this: before the
measurement/observation is made, we have only a vague idea of where a
photon might be. We can show probability distributions but no more.

After the measurement, this is still valid. The only difference is
that the distribution has become narrower.


After an observation/measurement the uncertainty has been reduced to
(at best) the Heisenberg minimum.

Huh? Heisenberg's principle applies to measurements of *both* position
*and* momentum.


It is also true that the means used
to measure/observe are uncertain in several ways.

Yes, obviously.


The same sort of siTuation obtains concerning the number of photons.

Well, let's say, a similar sort.


So the former is
true. The mean of the distribution moves steadily at a constant
speed, and there is some true certainty in the situation.

I don't understand totally what you want to say here, but it sounds
right. ;-)


I meant that with certain kinds of uncertainty images would always
become blurred with travel through space.

Well, they do. A wavepacket becomes broader with passing time.


They would be blurred with
respect to time: we would see photons from the same point source with
varying ages. I've never heard of such a thing, so there is some true
certainty with photons.

I don't see how one should be able to measure the "age" of a photon...


Bye,
Bjoern