High School What Is Surprising About Wave Function Collapse?

[abrogard]

Well thanks for all that. I'm probably interjecting at this point - you're all busy talking esoteric details amongst yourselves. I am out of my depth in the maths and in the general consideration, being untutored in both. The maths isn't so important to me because I can take it on faith.

But the lack of general understanding is more of a stumbling block because that means I can't even follow the thread.

But I'm getting something out of it all. I set off in one direction following one link and then get led to others, and others, and others...

If I track them all down and eventually get to understand them all I'll become an expert of quantum physics of some sort. Never going to happen. Couldn't, wouldn't and not what I aspire to either.

It is now probably time for me to go away and devote myself to all that reading and trying to understand. But before I do I'll record what I understand so far:

The surprising thing about the 'collapse' of the wave function is that the prevailing view is that the particle simply doesn't exist. It is not seen as a probability of it being here or there. It is seen simply as 'a probability', a 'non-thing', a 'potential thing' that springs into being when we go to measure the location of the 'thing' that has recently disappeared.

That explains that.

Tangential or flowing on or allied with that or whatever I've discovered that a particle is a wave and a wave is a particle, apparently. And coincidentally saw in a recent New Scientist I think it was, the first ever photograph (!) of this 'wavicle' http://www.abc.net.au/radionational/feed/6344892/podcast.xml

That's about all I 'know' right now. I've come across a whole range of claims/theories and tried to store them all on my pc and unfortunately mislaid nearly all of them right now - tucked away on some hard drive in some dir somewhere, in some guise or other, pdf, youtube vid, html, I don't know, I'm fairly chaotic obviously.

Some claiming the whole quantum thing is proof that consciousness is the be all and end all. Another man claiming to prove that the whole thing is absurd and based on false premises. Another claiming Einstein and others made basic mistakes in their maths with the Lorentz contraction formula in the very early days. It has led me over to the Big Bang debate where there's a man claiming the whole Hubble doppler shift thing is a mistake and the missing dark matter is simply H2 and the red shift is due to ( I think ) magnetic fields in the vastness of space, something like that...

Fascinating. Bewildering and fascinating.

I note much of what I read is dated and I wonder what today's consensus is regarding all these questions.

If it is known where there is perhaps a sort of 'news sheet' or something that gives details of the current prevailing wisdom on these matters I'd like to know about it for I've been unable to find it.

 

[atyy]

The surprising thing about the 'collapse' of the wave function is that the prevailing view is that the particle simply doesn't exist. It is not seen as a probability of it being here or there. It is seen simply as 'a probability', a 'non-thing', a 'potential thing' that springs into being when we go to measure the location of the 'thing' that has recently disappeared.

Yes, exactly. So since probability refers to things that do exist but about which we have incomplete knowledge, we use different words to describe the 'non-thing' like "quantum state" (very correct) or "wave function" (informal, but in context most people understand that you mean the quantum state). The closes term that is used to distinguish the quantum state from normal probability is "probability amplitude" (slighht incorrect use of a formal term, but again, people usually understand from context).

 

[bhobba]

The surprising thing about the 'collapse' of the wave function is that the prevailing view is that the particle simply doesn't exist. It is not seen as a probability of it being here or there. It is seen simply as 'a probability', a 'non-thing', a 'potential thing' that springs into being when we go to measure the location of the 'thing' that has recently disappeared.

Actually its silent on such things - we have interpretations where its a very real particle.

Some claiming the whole quantum thing is proof that consciousness is the be all and end all. Another man claiming to prove that the whole thing is absurd and based on false premises. Another claiming Einstein and others made basic mistakes in their maths with the Lorentz contraction formula in the very early days

Most of the above is crank rot. For example I have engaged that Lorentz contraction guy - he is an idiot. We now understand relativity a lot better than when Einstein wrote his famous paper eg:
http://www2.physics.umd.edu/~yakovenk/teaching/Lorentz.pdf

Even if Einstein made a mistake, he didn't, but assuming he did, proofs like the above are entirely independent of it. All of them needed to have an error - that's about as likely as 2+2 is not 4 because everyone made a mistake and didn't spot it.

Thanks
Bill

 

[stevendaryl]

A measurement has a definite outcome because you constructed your measurement apparatus to give you one.

How does one construct a system so as to have a definite outcome?

 

[stevendaryl]

Does removing collapse solve this problem? You still need an observer to say when an observation occurs (ie. when do you apply the Born rule), or you need something extra beyond standard quantum theory.

I agree. To me, the "minimalist interpretation", which makes the minimal ontological commitment ends up defacto giving special status to certain variables, namely the macroscopic results of measurements. Since presumably measurements are interactions like any other, describable by QM, that seems ad hoc to me, if not inconsistent.

Conceptually, it almost works to divide the world into macroscopic versus microscopic, and to view QM (and the microscopic world) as just a peculiar way of computing probabilities for the evolution of the macroscopic world. But it doesn't really make sense to divide it like that, because there is no principled cutoff for something being macroscopic. Dividing it by measurement versus everything else seems even more ad hoc than macro versus micro.

 

[Jimster41]

QM founders highlighted that there is no QM without classical Mechanics. Classical Mechanics is about a word where we neglect microscopic details. You have heat pressure, mean values and probabilities. The is no Schrodinger cat in this word. But you have to use CM to describe the apparatus in a laboratory, its environment and so on.
So we need a frontier. The problem is not to find where this frontier is: You put it where you want! It may include an observer who looks at the apparatus.
This frontier has to be seen as a boudary in space time. You can choose it to wrap only the particle between two moments or the whole laboratory between 2014 and 2015.
Once you have this frontier QM tells you that this boudary is a black box. Not a black hole but not so far. Inside the box you have amplitudes of probabilities that you have to sum Outside you have probabilities. Inside you have Schrodinger equation outside you have
https://en.wikipedia.org/wiki/Diffusion_equation
The is no collapse in QM. Seen from the outside QM has given a probability to the boundary that YOU hav chosen. I think that the question of when did the collapse occurred has no sense.
Collapse is an interpretation of QM for observers who live in a classical word where there is no amplitudes to collapse.

Interesting, and surprising. So did the founding fathers of QM think that there could be a classical world without QM, once they had discovered QM?

 

[naima]

Heisenberg's point of view was interesting. He did not say: we have an electron with an energy E1 and it jumps to a random energy E2 < E1 . He was only interested by what he could measure: the spectrum of the emitted photons. His matrix gives a probability to each couple of (E1, E2) .
I think that the good point of view would be to consider only couples (preparation, output) as the objects of our studies. And to calculate their probabilities. This is another way to say what i said in post 24. Where dou you need collapse?

 

[naima]

Interesting, and surprising. So did the founding fathers of QM think that there could be a classical world without QM, once they had discovered QM?

Yes there is is a frontier between QM and CM but you put it where you want.
You need a classical word in which information escape to explain decoherence.

 

[bhobba]

You need a classical word in which information escape to explain decoherence.

You don't. Decoherence follows directly from tracing over the environment which has nothing to do with a classical world.

Thanks
Bill

 

[Jimster41]

So is it accurate to say there are non probabalistic candidates systems that may account for the existence of the classical world.

I'm just struck by the image of them digging up a (causal) tree to find the roots (what is stuff made of?) then when they find the roots they say the roots wouldn't exist without the tree?

Or the causal flower, if you prefer.

 

[bhobba]

So is it accurate to say there are non probabalistic candidates systems that may account for the existence of the classical world.

Of course. BM is classical and deterministic and accounts for the classical world.

Thanks
Bill

 

[Jimster41]

Doesn't BM also say that world has non-local hidden variables? Or a universal pilot wave of some kind? I can imagine that those terms are somehow considered canonically technically consistent with "classical", but uh...

 

[bhobba]

Doesn't BM also say that world has non-local hidden variables? Or a universal pilot wave of some kind? I can imagine that those terms are somehow considered canonically technically consistent with "classical", but uh...

I think what classical means isn't pinned down exactly but comes from context. Lots of things are like that. But if by classical you mean local and realistic - well there is this theorem by Bell that says you can't have that.

Thanks
Bill

 

[naima]

Do not forget that in classical physics you have statistical mechanics. You use it when you speak of pressure and temperatures. When you describe an apparatus that measure a quantum property you need the language of classical physics.
Can you imagine that the CERN announced they discovered a new particle like that:
We used the tensor product of hilbert spaces (see Appendix A for the complete details of the apparatus) and we (see App B for our hilbert spaces) found that ...
This is the ficticious world the Founders of QM refused.
It seems that bhobba thinks the environment has nothing to do with the classical world. It is his own point of view, it is not the initial Copenhague interpretation. Tracing out is accepting classical world.

 

[bhobba]

When you describe an apparatus that measure a quantum property you need the language of classical physics.

You do not need the language of classical physics to define an observation eg it can be defined as the improper state just after decoherence.

Thanks
Bill

 

[naima]

Do you refuse the point of view of those who discovered QM?

 

[bhobba]

Do you refuse the point of view of those who discovered QM?

Yes. A LOT has been discovered since then.

Thanks
Bill

 

[atyy]

Yes. A LOT has been discovered since then.

Thanks
Bill

But so far it has only been shown that decoherence allows the classical world to rmerge if additional assumptions (eg. Bohmian Mechanics) are introduced to define the environment, choose the preferred basis and say when an observation occurs. No one has shown that this can be done in a minimal interpretation.

For a minimal interpretation, the founders of quantum mechanics were essentially right.

 

[bhobba]

For a minimal interpretation, the founders of quantum mechanics were essentially right.

Add any assumptions you like - the fact remains an observation can be defined independent of the existence of a classical world. If you want to see a rigorous exposition of it you can see Wallace's book on MW.

Thanks
Bill

 

[atyy]

Add any assumptions you like - the fact remains an observation can be defined independent of the existence of a classical world. If you want to see a rigorous exposition of it you can see Wallace's book on MW.

Thanks
Bill

But it is misleading to state it as if one is still using a minimal interpretation. One needs something like BM or MWI. There is no consensus that any interpretation except Copenhagen covers all of quantum mechanics.

Not stating these nontrivial assumptions is like stating Gleason's without highlighting the contextuality assumption.

 

[bhobba]

But it is misleading to state it as if one is still using a minimal interpretation.

I am not claiming that. I am simply claiming an observation can be defined without reference to a classical world. Have the most assumption laden interpretation you can imagine - its not relevant - the only relevant thing is it can be done.

Thanks
Bill

 

[atyy]

I am not claiming that. I am simply claiming an observation can be defined without reference to a classical world. Have the most assumption laden interpretation you can imagine - its not relevant - the only relevant thing is it can be done.

Also, even with additional assumptions, we don't know whether it can be done. The extension of Bohmian Mechanics to all relativistic quantum theories remains a matter of research, and even proponents of Many-Worlds like Wallace and Carroll agree that problems remain.

 

[bhobba]

Also, even with additional assumptions, we don't know whether it can be done

There is a myriad of interpretations. Are you sure everyone has issues? Here is a little known one:
http://arxiv.org/pdf/quant-ph/9508021.pdf

And, as you know, all those things you call issues are rather controversial.

Thanks
Bill

 

[atyy]

There is a myriad of interpretations. Are you sure everyone has issues? Here is a little known one:
http://arxiv.org/pdf/quant-ph/9508021.pdf

And, as you know, all those things you call issues are rather controversial.

But they are controversial the other way - ie. it is not generally agreed that they don't have issues. Even in less controversial realms, there can be errors in work that has not been widely examined. For example, https://www.physicsforums.com/threads/status-of-lattice-standard-model.823860/.

So it is not correct of you to ask me whether I am sure there are issues. One rather should ask you whether you are sure there are no issues.

 

[bhobba]

One rather should ask you whether you are sure there are no issues.

Actually I believe a number don't not just those mentioned here eg GRW and mine. Not wanting to derail this thread if you want to pursue it best to have a new thread. But you are making a very strong claim - every single interpretation that can give meaning to an improper state without referencing classical physics is problematical.

Thanks
Bill

 

[atyy]

Actually I believe my interpretation doesn't. Not wanting to derail the thread if you want to pursue it best to have a new thread.

Your interpretation has issues, because you just wave your hands and say "factorization" can be done objectively, "preferred basis can be done objectively", "decoherence threshold can be done objectively", and it is misleading because you present it as a minimal interpretation.

At the very least it is non-minimal because to define the preferred basis objectively, you need the predictability sieve, which is not part of standard quantum mechanics.

Furthyermore, you simply have no definition of factorization and threshold, so it is ill-defined. And you do not show that there is a way of defining factorization and decoherence threshold such that orthodox quantum mechanics is recovered. For a given threshold, the collapse is a nonlinearity, and the onus is on you to show that it does not show up as a failure of superposition, as it does in GRW and CSL.

 

[bhobba]

Your interpretation has issues, because you just wave your hands and say "factorization" can be done objectively,

And you just wave yours and say assuming you can factor a system into what's doing the observing and what's being observed invalidates it.

But that my last comment in this thread, Start another.

Thanks
Bill

 

[atyy]

And you just wave yours and say assuming you can factor a system into what's doing the observing and what's being observed invalidates it.

That is exactly what I don't do. Copenhagen does have the measurement problem, and this problem is stated immediately, eg. Landau and Lifshitz and Weinberg. This is why Copenhagen is an honest interpretation - it is honest about its problems.

 

[Nugatory]

I think this episode of The Bhobba and Atyy Show is a rerun.

 

[Jimster41]

Do not forget that in classical physics you have statistical mechanics. You use it when you speak of pressure and temperatures. When you describe an apparatus that measure a quantum property you need the language of classical physics.
Can you imagine that the CERN announced they discovered a new particle like that:
We used the tensor product of hilbert spaces (see Appendix A for the complete details of the apparatus) and we (see App B for our hilbert spaces) found that ...
This is the ficticious world the Founders of QM refused.
It seems that bhobba thinks the environment has nothing to do with the classical world. It is his own point of view, it is not the initial Copenhague interpretation. Tracing out is accepting classical world.

So the founders of QM considered the classical description of the world primary? Or did they, and should we, consider it dependent on or emerged from the QM world?

If the latter why isn't it necessary for the LHC to describe things they discover always only at the level of fundamental detail? Why isn't it desirable?

I thought the sentence "tracing out is accepting the classical world" was a good one. My question is, would any intelligent machine have to struggle with the same problem of classical QM boundary interpretation and the notion of wave collapse we do? Couldn't a sufficiently comprehensive machine percieve reality as one big non-stationary probabilistic (or non-local) QM fabric and navigate in the very "fictitious" world the founders refused, one free of classical objects?

If so doesn't that imply the collapse may only be an artifact of our specifically evolved and limited (layered, heirarchical) system of comprehension.