On 11-12-2017 23:11, Bruce Kellett wrote:
On 12/12/2017 1:51 am, smitra wrote:
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.
There is a fairly serious error in your analysis. You use an
expression for the momentum, p = mv = sqrt(3mkT), which applies to
molecules in an ideal gas. Mirrors in quantum experiments are not
molecules in an ideal gas! What is more, molecules in an ideal gas are
not located within their de Broglie wavelengths. You forget that the
uncertainty principle applies to the uncertainty in measurement
results, and the molecules of the gas are not constrained such that
their position uncertainty is that small.
In other words, you are talking nonsense.
No, your arguments are totally wrong here.
The thermal de Broglie wavelength is a measure for the coherence length
of the molecules in a gas and this then gives the coherence length in
momentum space via the uncertainty relation (if you want to invoke
measurement here, you can say that the environment consisting of all
other molecules effectively "measure" the position of the center of
mass). To a good approximation this also applies to atoms in a solid,
the fact that a solid is not an ideal gas doesn't actually matter all
that much for the coherence length.
So, the mistake you made here is to assume that the coherence length is
given by the volume a molecule is known to be in, which is obviously
wrong.
Saibal
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