From: *Bruno Marchal* <marc...@ulb.ac.be <mailto:marc...@ulb.ac.be>>
On 21 Aug 2018, at 14:53, Bruce Kellett <bhkell...@optusnet.com.au
<mailto:bhkell...@optusnet.com.au>> wrote:
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).
I am OK with this, as I have said before. This is just the infinity that
comes with measuring position or momentum of a particle in a wave packet.
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. 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.
But that is not part of quantum mechanics in Everett's or any other
interpretation. It is an infinite superposition that you have added on
for your own reasons. I have previously offered some suggestions as to
how you could create such a superposition in a conventional way. One
obvious possibility is to have Alice choose her measurement angle
according to some random quantum process, such as radioactive decay.
But the problems with any such suggestion are obvious. Firstly, Alice
does not choose her measurement angle in that way, so there is no
super-superposition created. Secondly, this construction does not
restore the rotational symmetry in any case. You might have an infinite
number of Alices, measuring the singlet at all possible angles, but that
multi-multiverse is not rotationally symmetric either! All it needs is
for Alice number 7,234,826 to poke her tongue out and the rotational
symmetry is lost! Of course, you could add yet more multiverses to cover
every possible deviation of Alice from the stationary state. But the
process rapidly becomes ridiculous.
So this Rube Goldberg construction of additional multiverses of
superpositions does not actually restore stable rotational symmetry. So
why propose such a construction? William of Ockham will rise out of his
grave to haunt you for such pointless extravagance of entities!
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.
And what is the problem with regarding it like this? Even if you add in
these arbitrary multi-superpositions, you end up with an Alice making a
single measurement in some particular direction and communicating with
her partner Bob, who was always in the same world. All your additional
worlds add only smoke and confusion -- they do not actually change
anything of substance. Alice's measurement on the non-separable state
destroys the original rotational symmetry of that state -- and nothing
that you can do will ever restore that symmetry.
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.
I don't think there is any way in which you can "enlarge" the definition
of rotational symmetry and still talk sense.
even though I have given clear examples where this happens.
It was using some collapse. It seems to me.
What collapse is involved if I add an asymmetrical weight to a bicycle
wheel to destroy its rotational symmetry?
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.
As I have pointed out, there is no way in which you could ever make such
an idea work. No matter how much you enlarge the Alice+singlet system,
the rotational symmetry can always be trivially broken.
Face it Bruno, broken symmetries are universal in physics.
Bruce
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