On 8/21/2018 6:51 AM, Bruno Marchal wrote:
On 21 Aug 2018, at 14:53, Bruce Kellett <bhkell...@optusnet.com.au
<mailto:bhkell...@optusnet.com.au>> wrote:
From: *Bruno Marchal* <marc...@ulb.ac.be <mailto:marc...@ulb.ac.be>>
On 21 Aug 2018, at 02:20, Bruce Kellett <bhkell...@optusnet.com.au
<mailto:bhkell...@optusnet.com.au>> wrote:
From: *Bruno Marchal* <marc...@ulb.ac.be <mailto:marc...@ulb.ac.be>>
On 20 Aug 2018, at 13:18, Bruce Kellett
<bhkell...@optusnet.com.au <mailto:bhkell...@optusnet.com.au>> wrote:
You didn't respond to my earlier post in which I discussed the
symmetry breaking occasioned by Alice's measurement interaction
with the singlet state. I copy the relevant parts of my earlier
post here:
"The fact that Alice's interaction with the state is unitary and
can be reversed does not mean that the original symmetry still
exists in some sense. If I place a large weight at some point on
the circumference of a bicycle wheel, the rotational symmetry of
that wheel is lost. The fact that I can reverse the process by
removing the imposed weight does not mean that the altered wheel
is still rotationally symmetric in some wider view.”
OK, but when the heavy object is removed, at that moment, the
symmetry is back. Then, when Alice makes the measurement, the
symmetry is lost from her point of view, but the general symmetry
of the state has not changed. It is only not retrievable by Alice
(unless quantum erasure, amnesia, etc.).
Bruno, you have not made the least effort to understand the point I
made above,
Stop speculating on people.
I am merely responding to what you wrote. No speculation involved.
How do you know I did not make some effort. Maybe you imagine that I
am clever or something. You might need to develop some sense of pedagogy.
or to respond to it intelligently.
Sop making judgement.
There has been no intelligent response. No judgement involved.
That is a contradiction.
It is difficult to believe that you are actually discussing this in
good faith. You just keep repeating your own misunderstandings of
the situation.
This is discussed since the beginning of QM. Stop talking like if
only you understand Everett.
Well, it does not appear as though you do either. You keep adding in
infinities of observers that are not part of Everett's formulation of QM.
There are two sort of infinity here. One which I hope you agree with,
like when Alice measure the position of an electron prepared in the
state of lowest energy level of an electron around a proton. The
electron state is a superposition of all position possible in the
corresponding orbital. After measurement she is entangled with that
electron, and we have an infinity of Alice. OK? (I assume of course
some classical QM; that might need some correction when GR is used).
This assumes that Alice has used a measuring instrument whose
interaction is spherically symmetric. It is because her instrument has
an infinite (or at least very big) number of possible results that there
are an infinity (or many) Alice's.
The other sort of infinity, the one which I think you disagree with,
is typical for the superposition of tensor products, like the singlet
state ud - du. Before measurement Alice has the same probability of
finding u, or d for any measurement she can do in any direction.
But direction is chosen via her thought processes which are effectively
classical. Her wf is not rotationaly symmetric. It could be arranged
that some quantum random number generator is used to set the detector
angle to X. In that case the multiverse would split into many different
branches when the qrng result decohered and output X. But this event
would still leave Alice and Bob spacelike separate in the world where
the qrng output X. There will be many branches corresponding to the
many possible values of X. But in each branch the change of the wf when
Alice measures the spin along X will be a non-local splitting into
"up-X" or "down-X". At least that's conventional QM.
Brent
Both Alice and Bob are maximally ignorant of their possible
measurement results. The MW on this, or a MW way to interpret this, to
keep the rotational symmetry, is that we have an infinity of couples
Alice+Bob, with each couple being correlated. If not, some implicit
assumption is made on u and d, like it is a preferred base.
And yes, I do assume locality, if only to illustrate that the MW does
not force the presence of FTL influence (without transfert of
information, which actually would require a third person indeterminacy
in Nature, which I doubt).
It is just a consequence of ud-du = u’d’-d’u’, and the fact that this
implies maximal ignorance of Alice (and Bob) whatever spin-direction
is chosen. After the choice of Alice, and her measurement, neither
Alice and Bob will be able to access a different world. All Alice and
Bob will have to interpret the state like if it was s simple (two
terms) superposition. It is like suppressing the global phase of the
state.
The measurement that Alice makes destroys the symmetry. That is all
there is to it. There is not some wider symmetry that is preserved.
That is Bohr theory. Not Everett. A measurement does not change
anything in the big picture. It collapses wave and destroys
symmetries only in the relative first person mind associated to
bodies doing the experience.
It is not Bohr's theory, it is quantum mechanics. You appear to
believe that symmetry cannot be destroyed,
The symmetry is destroyed from the perspective of the one doing the
experiment. But it is extended to the couple Alice + the singlet
state, although “rational symmetry” might be have its usual definition
slightly enlarged.
even though I have given clear examples where this happens.
It was using some collapse. It seems to me.
The symmetry is destroyed totally, not just in the mind of the
experimenter. If the symmetry is still preserved in some bigger
picture, it is up to you to prove this. But you have not been able to
do so. It is just an assertion on your part. And that assertion
happens to be false.
You seem to believe that a measurement has to change something in the
physical reality (besides the brain of the observer). But that does
not happen in the MW. Measurement is only self-entanglement. It broke
the symmetry of the singlet state, but enlarge it on the system
Aice+singlet state.
Bruno
What you have to do is to work through the application of the
Schrödinger equation for this situation, without invoking any
collapse, and demonstrate that the symmetry is still present in the
total wave function. I contend that you will not be able to do this,
because the interaction with the singlet state destroys the
rotational symmetry. This is really a trivial observation since the
Stern-Gerlach magnet itself is not rotationally symmetric.
Bruce
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