On 6/30/2021 8:07 AM, John Clark wrote:
On Mon, Jun 28, 2021 at 7:46 PM Bruce Kellett <[email protected]
<mailto:[email protected]>> wrote:
/> I think John's trouble here is that he still adheres to David
Deutsch's concept of worlds. Deutch talks as though every
component of a superposition is a separate world. This leaves
Deutsch no language to talk about decohered worlds, pointer
states, and all the other usual apparatus of quantum interpretations./
Not so, "superposition" is just a word that means a collection of
particles that exist in very different physical states at exactly the
same time,
It's more that a particle or a system of particles exist in a single
physical state, which is represented by different components in our
basis for the Hilbert space. A particle that is spin UP is in a
coherent/superposition /of spin LEFT and spin RIGHT, relative to a
Stern-Gerlach measurement. Aside from measurement, any pure system is
just some vector in Hilbert space. I has components relative to
whatever basis we choose for its representation...and it's a
"superposition" of those components. The only reason this is at all
different from classical mechanics is that in the quantum case we can't
just measure the all the components of the vector. In general we can at
most measure/prepare half of them (c.f. Holevo's theorem). So
measurement results are determined by what instruments we can devise and
we usually write Hilbert space vectors in terms of components we can
measure/prepare.
in other words it's a word that people like to use when they just
don't want to say that the universe has split.
MWI doesn't split worlds until some measurement-like interaction
decoheres the coherent superposition. So making a LEFT/RIGHT
measurement on a spin UP atom in an SG decoheres the state into a
mixture of LEFT or RIGHT which Everett takes to define different worlds.
Brent
In Many Worlds if the mathematics says that 2 things could happen then
2 things do happen. Usually when a universe splits the two never
recombine again, that's why we usually don't see weird quantum effects
in our everyday lives, and that's why making a Quantum Computer is
hard. But If the difference between universes is very very small then
a skilled experimenter can make them become identical again and
recombine, and that produces interference. However the difference
between the universes rapidly grows larger and the task of making them
identical again rapidlybecomes more difficult, so when the difference
becomes larger than the microscopic level the possibility of them
becoming identical again becomes ridiculously small, like in classical
physics and the possibility that by pure random chance all the air
molecules in the room you're in right now will go to the other side of
the room and you'll suffocate to death. That's why you never see
somebody as large as a human being use quantum tunneling to walk
through a brick wall even though such a thing is theoretically possible.
We don't always see a superposition of states, in fact usually we
don't. If you flip a coin and it comes out heads then you are NOT
living in the world where it came out tails. In a roughly similar way
if you do the two slit experiment and see that the photon goes through
slit A then you are not living in the world where the photon went
through slot B. But the 2 slit experiment can be a littledifferentfrom
the simple coin toss example.
If after the universe splits and the photon goes through both slits
they then hits a photographic plate (or a brick wall) then both
photons in both universes are destroyed and thus there is no longer
any difference between the two, so the universes will merge back
together. Then and only then you will see evidence that the photon
went through both slits (aka. Interference) on the photographic plate
even if you send the photos through one at a time.
If you got rid of the film (or the brick wall) and let the photon head
out into infinite space after it passes the slits then the two
universes will never recombine, and so of course you will never see a
interference effect. The beautiful part of the theory is that it
doesn't have to explain what an observer is and that's why a brick
wall will work just as well as a photographic plate.
A measurement, if for some reason you'd like to use that word, is a
change made in the universe, and it doesn't matter if that change is
made in a conscious being or not. In one universe the photon hits the
screen at point X, and in another universe the photon hits the screen
at point Y, and in yet another universe the photon doesn't hit the
screen at all because it doesn't pass through either slit. If there
happens to be an observer watching all this he splits too, and they
all have different memories about what happened. And it doesn't
matter if nobody is watching, the universe splits anyway. In Many
Worlds if you like you could replace the word "measurement" with the
word "change" and you don't need to use the word "observer" at all, so
you don't need to ponder the question of if a cockroach can observe
things and make the universe split.
John K Clark See what's on my new list at Extropolis
<https://groups.google.com/g/extropolis>
v42
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