On 11-12-2017 15:12, Bruno Marchal wrote:
On 10 Dec 2017, at 23:38, Bruce Kellett wrote:
On 11/12/2017 2:19 am, Bruno Marchal wrote:
On 09 Dec 2017, at 00:03, Bruce Kellett wrote:
On 9/12/2017 4:21 am, Bruno Marchal wrote:
Similarly, a shroedinger car, once alive + dead, will never become
a pure alive, or dead cat. It will only seems so for anyone
looking at the cat, in the {alive, dead} base/apparatus.
Superposition never disappear, and a coin moree or less with a
precise position, is always a superposition of a coin with more or
less precise momenta. The relation is given by the Fourier
transforms, which gives the relative accessible states/worlds.
I pointed out that for a macroscopic object such as a coin, the
uncertainty relations give uncertainties in positions and/or
momentum far below any level of possible detection.
Of possible practical detection. That is good FAPP, but irrelevant
for theoretical consideration.
This is a purely rhetorical objection, Bruno. And when you trot this
out, as you do regularly, I know that your purpose is to obfuscate,
and hide the fact that you have no rational argument to offer.
You confuse physics and metaphysics. The difference is not rhetorical,
but fundamental in this thread.
We actually do detect quantum uncertainties for macroscopic objects
routinely when doing typical quantum experiments. Interference
experiments involving photons is a good example. Suppose we have an
interferometer that has mirrors in it, the photons bounce off the
mirrors and at some spot the different possible paths come together and
you can then detect or not detect photons there.
One can then ask why the momentum absorbed by the mirror when a photon
bounces off it, does not destroy the interference pattern. One may
consider here a thought experiment where the mirrors are freely floating
in a magnetic field. But that's not actually necessary, if you could in
principle detect the momentum from the recoil of the photons, then you
won't get interference and in general the interference becomes weaker if
you can in principle get partial information.
The answer to this question is that macroscopic objects such as the
mirror in interferometers do not have sharply defined momenta. In fact,
you could argue that unless the mirror surface is not located to well
within the wavelength of light, you obviously wouldn't get interference,
and applying the uncertainty relations then also gives you an
uncertainty in the momentum. But this doesn't tell you what the
uncertainty in the momentum typically is.
The uncertainty in the center of mass position can be estimated crudely
as the thermal De-Broglie wavelength. A displacement well within this
length scale will not lead to the environment interacting appreciably
differently with it. So, the uncertainty in the position will be of the
order of h/sqrt(m k T). The interpretation is then that a wavefunction
spreading beyond this length will effectively collapse back to within
this length scale due to the environment effectively having located the
center of mass within this scale.
The uncertainty in the momentum is then of the order of sqrt(m k T), and
this can actually be quite large for large objects. This large
uncertainty in the momentum in absolute terms explains why you can
actually do quantum experiments using macroscopic measurement devices.
Saibal
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