
The Quantum Theory and Reality (1979) [pdf] - xtacy
https://www.scientificamerican.com/media/pdf/197911_0158.pdf
======
nabla9
Wave function collapse is not observed physical phenomenon. Some
interpretations of QM require it to exist, but there is no empirical evidence
of that happening.

Apparent wave function collapse happens when a wave function in a
superposition of several eigenstates appears to reduce to a single eigenstate.
The apparent wave function collapse collapse is mathematically equivalent of
quantum decoherence where the wave function never really collapses but the
states gets entangled with the observer.

If somebody were able to formulate and experience that would show the
difference between decoherence and collapse, that would be new physics and we
would be able to rule out some interpretations of quantum mechanics. Until
that happens, 'shut up and caluclate' seems to be valid course of action.

As far I understand, the philosophical difference between apparent and actual
wave function collapse is that in the apparent collapse probabilities of other
states get so close to zero that they don't matter, in actual collapse they
are exactly zero.

The assumption that human consciousness has something to do with setting all
other states to zero is weird one and can't completely understand the
assumptions behind it. I guess the idea is that we would not experience the
world as we experience it now if there is just continuing decoherenće.

~~~
cygx
The philosophical difference is that without collapse, all of the eigenstates
contributing to the intial state will get entangled with their own 'copy' of
the environment, which is where the moniker _many worlds interpretation_ comes
from.

~~~
nabla9
What is unclear to me in the many-worlds hypothesis is the reason whey these
'copies' don't constantly interact with each other and lead to continuous
decoherence.

How this is justified?

~~~
justinpombrio
Each of the worlds ends up distinct, e.g. all the air molecules end up in
slightly different positions. Worlds that different don't interfere with each
other. (This is predicted by the math, the math is the justification.)

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david927
Ok, I'm going to hijack this thread to get an answer to something I've always
struggled with: how does 'observation' cause the wave function to collapse.

It won't happen in a closed box, but what about an open box in a closed room?
What about a closed box with a live video camera? What about a live video
camera whose display no one is watching?

I'm sure this is a basic question but for the life of me, I've never really
understood it. Thanks.

~~~
jackson1372
The standard story about measurement/observation and quantum collapse is not
the only interpretation of the available data.

There are, broadly, three kinds of interpretations of what's 'really' going on
behind the scenes:

1\. Collapse - The world exists in an indeterminate state until observation
occurs, when the world collapses into one a single determinate state. The
probabilities of quantum mechanics map onto the the different parts of the
unobserved indeterminate state.

2\. Many Worlds - Quantum phenomena cause the world the branch into multiple
worlds. The probabilities of quantum mechanics represent the 'share' of
reality that branches in each direction.

3\. Hidden variables - The probabilities of quantum mechanics are artifacts of
our inability to know all the relevant variables. Measurement necessarily
involves causal contact, and causal contact will always disturb some of the
relevant variables in unpredictable ways. It would be cool if we could observe
what goes on when measurement occurs, but to do that would require
measurement! So we're stuck with hidden variables.

The public tends to hear the collapse interpretation most often. Physicists
tend to like the many worlds interpretations. Philosophers of science tend to
like the hidden variables interpretation, because the other options require an
incoherent metaphysics. (I'm a Philosophy PhD student.)

People say that the hidden variables interpretation is ruled out on
experimental grounds, but this is demonstrably false. Experimental data shows
that hidden variables, if they exist, violate locality:

Locality - Causal interaction is a local phenomena. No action at a distance.

So long as you're willing to abandon locality, hidden variables can work.
Given that the other two interpretations posit equally weird things,
abandoning locality won't seem so weird.

For more on this, see Tim Maudlin's excellent paper, "Three Measurement
Problems"

[https://www.academia.edu/32885328/Three_measurement_problems](https://www.academia.edu/32885328/Three_measurement_problems)

~~~
platz
> Physicists tend to like the many worlds interpretations.

According to Sean Carroll, this is because if you 'buy' the mathematics of the
wavefunction, even before collapse you _already_ have to accept "many worlds"
at the quantum level. "many worlds" for them simply means superposition/linear
combinations. After all, if you _already_ accept "superposition" at the level
of quantum states, you don't have to invoke anything _new_ to deal with so-
called "collapse", if everything simply stays as superpositions i.e. linear
combinations.

Or, in other words, if you assume classical behavior _first_ , and you need to
get that out of superpositions at the quantum level, you need wavefunction
collapse.

but if you _start_ with superpositions at the quantum level, you already have
superpositions, and then classical behavior can simply be derived from
locating yourself in one of those superpositions.

Explained this way, "many worlds" doesn't seem so shocking, if you have
already resigned yourself to quantum level superpositions.

I think of many worlds as kind of a literal interpretation that quantum
superpositions are fundamentally real, as opposed to quantum superpositions
merely being a very accurate mathematical model. (Most scientists think QM
represents something real instead of some lucky equations.)

~~~
westoncb
This sounds a lot like someone has fallen for a subtle confusion of map and
territory...

~~~
DavidSJ
At some level, if we take scientific theories seriously as descriptions of the
world out there, and not merely tools for prediction, then we have to assume
some sort of isomorphism between the map (our theory) and the territory (the
world), even if they're not the same thing. This isn't only for quantum
theory.

~~~
jackson1372
> we have to assume some sort of isomorphism between the map (our theory) and
> the territory (the world)

But a mathematical model != a theoretical model. The very same mathematical
model will be compatible with uncountably many theoretical models. (By
'theoretical model' I mean something like 'an interpretation of what the math
is representing'.)So you can't read off theoretical structure from
mathematical structure. And so you can't read off the structure of the world
from mathematical structure.

~~~
lmm
> (By 'theoretical model' I mean something like 'an interpretation of what the
> math is representing'.)So you can't read off theoretical structure from
> mathematical structure. And so you can't read off the structure of the world
> from mathematical structure.

Occam's razor favours a theoretical model that corresponds more closely to the
mathematics, rather than one that adds a bunch of epicycles to arrive at a
different interpretation. If you follow your logic then you can never reject
geocentrism, because it's possible to create a geocentric model that generates
the same predictions as a heliocentric one; nevertheless we would generally
say that heliocentrism is "more true" and "more physically real" than
geocentrism.

~~~
alien_at_work
What Occam's razor favours is irrelvant. It has no predictive power [1]. The
more complex answer is just as likely to be the correct one.

[1]
[http://scienceblogs.com/developingintelligence/2007/05/14/wh...](http://scienceblogs.com/developingintelligence/2007/05/14/why-
the-simplest-theory-is-alm/)

~~~
lmm
Physics tends to be simpler than we originally thought, and Occam's razor
correctly predicts this: electricity, magnetism, and light turn out to be
facets of the same phenomenon, electricity and the nuclear forces turn out to
conform to the same theory, electromagnetism, gravity, and spacetime turn out
to behave in the same way. Your link's lazy "11 dimensions! OMG!!!1one" non-
argument is in no way an adequate refutation of that history.

~~~
alien_at_work
You're cherry picking. You pick the places where Occam's razor picks the
correct answer and ignoring all the places it doesn't. For any _new_ situation
we can't predict which of those possibilities the answer will fall. Hence my
statement that Occam's razor has no predictive powers.

I just linked to the first article I found that talked about the subject. And
it's interesting that you look for something lazy instead of addressing more
serious issues like Newton's model vs Einstien's. Very clearly the (much!)
more complex answer is more correct than the simple one. Here's [1] another
link discussing the issue and gives 2 examples.

[1]
[http://neuralnetworksanddeeplearning.com/chap3.html](http://neuralnetworksanddeeplearning.com/chap3.html)
Search for "Occam"

~~~
lmm
> I just linked to the first article I found that talked about the subject.
> And it's interesting that you look for something lazy instead of addressing
> more serious issues like Newton's model vs Einstien's.

You should give links you're willing to stand by. I didn't go looking for
something lazy, I went looking for talk about fundamental physics (where
Occam's Razor is appropriate, and what we're talking about) and found only the
barely even wrong throwaway line about M-theory.

> Very clearly the (much!) more complex answer is more correct than the simple
> one.

Newton and Einstein don't make the same predictions. When you include the
amount that you'd have to add on to Newton's theory to generate the
predictions Einstein gives you about the behaviour of light, Einstein ends up
simpler.

> Here's [1] another link discussing the issue and gives 2 examples.

The "new particle" example shows the opposite of what's claimed. Bethe's only
reason to dismiss a new particle as an explanation was Occam's razor. The part
about Newton/Einstein is just wrong about the history; relativity wasn't
developed as an effort to explain the orbit of Mercury, it was developed out
of Maxwell and Lorentz's work on electromagnetism. A universe in which the
only reason to believe relativity was those deviations in the orbit of Mercury
probably would be a universe in which the true theory was Newtonian gravity
with small correction terms, not a universe in which relativity was true.

~~~
alien_at_work
>Bethe's only reason to dismiss a new particle as an explanation was Occam's
razor.

Thank you for pointing out the other fault of Occam's razor: both sides of
most arguments tend to assume the razor is on their side. This aspect was
addressed in the second article I linked.

~~~
lmm
It's possible to make a mistake in the application of any principle,
particularly when the mistake suggests you've made an important discovery. I
think there would have been a clear consensus among third-party physicists
that the razor favoured Bethe in that case.

------
whatshisface
It's worth mentioning that "quantum theory" has no implications for
consciousness, especially given the headline proclaiming the opposite. There
are plenty of interpretations that don't involve consciousness at all - and
hopefully my use of the word interpretation clues you in to the fact that this
is isn't really a physics question.

Weird philosophy aside, this does look like a good explanation of the Bell
inequality.

------
platz
No, you don't need consciousness. Since 1979, the concept of decoherence has
replaced consciousness in the understanding of QM.

------
virgil_disgr4ce
I'm deeply sick of the absurdly drawn-out living death of the Copenhagen
interpretation, and its even more absurd "consciousness is made out of magic"
descendants. This is bordering on straight-up irresponsible misinformation.

~~~
fiatjaf
What do you mean? What is the death of the Conpenhagen interpretation?

~~~
pikchurn
The Conpenhagen interpretation is flat out wrong by any reasonable philosophy
of science. You can choose many-worlds interpretation, or pilot-wave theory
and either one would give you a consistent, simpler explanation of what is
going on then some mumbo-jumbo hocus pocus about consciousness and observation
of cats in boxes, with fewer assumptions to boot, and no paradoxical
conclusions that require religious like mysteries to explain.

Yet for inane reasons the Conpenhagen interpretation is still ALL that is
taught to the next generation of physicists, who in turn teach it to their
students. Only the weird physics students (like me) who go "wtf?" in class and
refuse to believe the teacher go out and learn pilot-wave theory (my
preference) or many-worlds interpretation to re-inject some sanity into the
world.

~~~
thethirdone
> The Conpenhagen interpretation is flat out wrong by any reasonable
> philosophy of science.

AFAIK, this is an exaggeration to the point of incorrectness.

The Copenhagen interpretation may be a poor interpretation which introduces
absurd complicated ideas rather than the simpler ideas of other
interpretations, but that doesn't make it "flat out wrong".

~~~
pikchurn
You ignored the "philosophy of science" part. Philosophy of science is how we
decide which of two theories is "correct" when they offer the same
experimental predictions. See also my reply to a sibling comment.

