scott wrote:
>>> If a cat's status is not decided until someone looks inside
>>
>> Since that isn't true, it's a meaningless question to ask.
> 
> I thought the whole point of that experiment was that according to QM 
> the cat was both alive and dead until you actually tried to look?  Maybe 
> something changed since I read about it?


To be clear, strictly speaking this is not "according to QM" but rather 
according to the Copenhagen Interpretation of QM (which is currently the 
most popular interpretation of the mathematics).

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Kevin Wampler wrote:
> scott wrote:
>>>> If a cat's status is not decided until someone looks inside
>>>
>>> Since that isn't true, it's a meaningless question to ask.
>>
>> I thought the whole point of that experiment was that according to QM 

>> the cat was both alive and dead until you actually tried to look?  
>> Maybe something changed since I read about it?
> 
> 
> To be clear, strictly speaking this is not "according to QM" but rather
 
> according to the Copenhagen Interpretation of QM (which is currently th
e 
> most popular interpretation of the mathematics).

I thought it was "according to the Schrödinger's wave equations"?

-- 
Darren New, San Diego CA, USA (PST)
   Human nature dictates that toothpaste tubes spend
   much longer being almost empty than almost full.

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Darren New wrote:
>>> I thought the whole point of that experiment was that according to QM 
>>> the cat was both alive and dead until you actually tried to look?  
>>> Maybe something changed since I read about it?
>>
>>
>> To be clear, strictly speaking this is not "according to QM" but 
>> rather according to the Copenhagen Interpretation of QM (which is 
>> currently the most popular interpretation of the mathematics).
> 
> I thought it was "according to the Schrödinger's wave equations"?


I should have checked to see that Warp had already written the same 
thing I had, oh well.

At any rate, the relevant part of the wave equations here is really the 
fact that they describe the probabilities of the outcomes of 
measurements.  In terms of "what's actually going on" any explanation 
which can reproduce these probabilities is on equal footing WRT the 
equations, and stating that the cat in superposition before measurement 
and "collapses" when we open the box is only one such possible explanation.

Granted, it's worth noting (as you're aware) that the nature of QM puts 
some pretty severe restrictions on what sorts of explanations can match 
the observed measurements, but it's not as if the Copenhagen 
Interpretation is the only possibility: 
http://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics

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Kevin Wampler wrote:
> At any rate, the relevant part of the wave equations here is really the 
> fact that they describe the probabilities of the outcomes of 
> measurements.  

Right. And the question is "what's a measurement" in this case. That's the 
outstanding question.  Since measuring a superimposed state can give you an 
answer different from any possible "outcome" per se, figuring out when a 
"measurement" has occurred is the problem.

> and stating that the cat in superposition before measurement 
> and "collapses" when we open the box is only one such possible explanation.

For us, maybe. Not for the cat.

-- 
Darren New, San Diego CA, USA (PST)
   Human nature dictates that toothpaste tubes spend
   much longer being almost empty than almost full.

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Warp wrote:
>   If it looks like a wave, feels like a wave, smells like a wave, what is it?
> Not a wave, it seems. It just fakes being one quite well.

In general I agree with Neeum that it's not meaningful to discuss 
weather it's a "wave" without defining what you even mean by a wave, but 
I still thought I'd point out one way in which the "waves" in QM behave 
differently from classical waves.

This was mentioned by Feynman in the short video linked by Darren, and 
it's that in multiple particle systems the wave function doesn't just 
describe the probability of finding a single particle at a given 
position, but rather the probability of finding the entire system of 
particles in a given configuration.  This is distinct from the way that 
classical waves behave, and is the source of effects of "non locality" 
in quantum systems which would not arise from a system with classical waves.

If you want to still call this a wave (perhaps a wave in the 
configuration space of the system?) then that's clearly fine, but it's 
worth keeping in mind that it's a different sort of wave than one 
generally means in classical systems.

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Darren New wrote:
> Kevin Wampler wrote:
>> At any rate, the relevant part of the wave equations here is really 
>> the fact that they describe the probabilities of the outcomes of 
>> measurements.  
> 
> Right. And the question is "what's a measurement" in this case. That's 
> the outstanding question.  Since measuring a superimposed state can give 
> you an answer different from any possible "outcome" per se, figuring out 
> when a "measurement" has occurred is the problem.

Exactly why I phrased my sentence the way I did.  The Schrödinger 
equations don't directly say anything about what state the cat's in, or 
even if it's in a deterministic state or not -- it's how you treat the 
process of measurement that does, and that's a philosophical questions 
rather than a scientific one at this point (and thus not part of QM 
proper but of its interpretation).

More specifically, AFAIK there are some interpretations of QM which give 
the cat a deterministic state, and others which don't (and I assume 
others which regard the question as meaningless).  All are in agreement 
with he Schrödinger wave equation, so it's not the equation that's the 
issue here.


>> and stating that the cat in superposition before measurement and 
>> "collapses" when we open the box is only one such possible explanation.
> 
> For us, maybe. Not for the cat.

Indeed, but I don't understand what this has to do with the Schrödinger 
equations, since it seems (as I mentioned) to be only a problem with the 
Copenhagen interpretation.  Perhaps I'm missing what point you're making?

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Kevin Wampler wrote:
> If you want to still call this a wave (perhaps a wave in the 
> configuration space of the system?) then that's clearly fine, but it's 
> worth keeping in mind that it's a different sort of wave than one 
> generally means in classical systems.

I think that's really the problem. When I say "it's not a wave", I'm saying 
you don't get any of the same measurements from one particle that you'd get 
from a wave. There's some wave-like stuff going on, but not in a way that 
you'd normally say the photon *is* a wave.


For more fun confusion, what does multi-world say about stuff?

http://en.wikipedia.org/wiki/Quantum_suicide_and_immortality

-- 
Darren New, San Diego CA, USA (PST)
   Human nature dictates that toothpaste tubes spend
   much longer being almost empty than almost full.

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Kevin Wampler wrote:
> All are in agreement 
> with he Schrödinger wave equation, so it's not the equation that's
 the 
> issue here.

OK. I'm now more educated, I think.

> Indeed, but I don't understand what this has to do with the Schröd
inger 
> equations, since it seems (as I mentioned) to be only a problem with th
e 
> Copenhagen interpretation.  Perhaps I'm missing what point you're makin
g?

Only that the quantum equations don't *seem* to apply to the macroscopic 

world, for reasons unknown?

-- 
Darren New, San Diego CA, USA (PST)
   Human nature dictates that toothpaste tubes spend
   much longer being almost empty than almost full.

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Darren New wrote:
> Kevin Wampler wrote:
>> All are in agreement with he Schrödinger wave equation, so it's not 
>> the equation that's the issue here.
> 
> OK. I'm now more educated, I think.

It's a point which is often neglected.  Just to be clear though, the 
issue is more subtle than I'm making it sound and AFAIK most 
interpretations which give a deterministic state to the cat have run 
into problems (for instance when incorporating them into special 
relativity).  Also such interpretations are necessarily non-local.


>> Perhaps I'm missing what  point you're making?
> 
> Only that the quantum equations don't *seem* to apply to the macroscopic 
> world, for reasons unknown?

Yeah, IANAP (I am not a physicist) but as far as I'm aware this is 
correct.  Although I think there's some interpretations which describe 
why this is the case perfectly well, they just might come with other 
philosophical implications that are undesirable (for instance I'd put 
the many-worlds interpretation into this class).

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Kevin Wampler wrote:
> Also such interpretations are necessarily non-local.

I read recently that someone did a Bell's Inequality type of experiment with 
  more than 20 angles measured in 3D and from that somehow deduced locality. 
Interesting.

> Yeah, IANAP (I am not a physicist) but as far as I'm aware this is 
> correct.  Although I think there's some interpretations which describe 
> why this is the case perfectly well, they just might come with other 
> philosophical implications that are undesirable (for instance I'd put 
> the many-worlds interpretation into this class).

Yah. I suspect it's much more mundane, like the "decoherence" section of the 
paper Neeum posted.

-- 
Darren New, San Diego CA, USA (PST)
   Human nature dictates that toothpaste tubes spend
   much longer being almost empty than almost full.

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