Why Does the Measurement Problem Terrify Physicists?

[Anonym]

I think I'm missing something here concerning Schrödinger's cat. At some moment in time the cat will die. If one person was watching the cat with a camera inside the box then it wouldn't be a paradox for him as he'd see the cat die. But if someone did not see inside the box - it would be a paradox for him.

You are right. If one person was watching the cat with a camera inside the box then it wouldn't be a paradox for him as he will not see the cat: he will die together with him. But if someone will see outside the box - it would be a paradox for him: the cat should disappear exactly as described by L. Carroll, but that is not what happens.

Therefore, the Cat can’t be described quantum mechanically (microscopically). But if the Cat described macroscopically (belong to the Classical World) he is observed only in the two states: either life or dead. That what the experimentalist always see as the result of his experiments. It is not predicted or explained by the Classical Physics. That must be explained by the physical theory. That is the answer to your first question:” What is the measurement problem?”

Now try E. Schrödinger, “THE PRESENT SITUATION IN QUANTUM MECHANICS” (1935) in W&Z. It is extremely clear paper about the measurement problem and the theory of measurements (if you know and understand QM).

The complete solution of the measurement problem requires the formulation of the theory of measurements. Wait to the end of our discussion and you will see what needed to do that and why it is such a problem.

Regards, Dany.

 

[tommyburgey]

yeh that's exactly what i think <<<<(concerning previous post)

 

[jostpuur]

yeh that's exactly what i think <<<<(concerning previous post)

I'll assume it was response to me, since I asked if it was what you were thinking.

Have you done some exercises in quantum mechanics? Do you know particle in one dimensional infinite potential well? Plane waves? Harmonic oscillator?

Anyway, you have probably at least heard about a wave function. You should not think that a wave function, that describes a location of a particle, is just some field like classical fields are. When you have a wave function \Psi:\mathbb{R}^3\to\mathbb{C} for a particle, it means that the particle has complex amplitude \Psi(x_1) to be in a position x_1, and amplitude \Psi(x_2) to be in a position x_2, and so on. Classically the particle has one position, but in quantum mechanics the particle has amplitudes for being in different positions. And we say that it is in superposition of being in different places.

It is not useless philosophy to emphasize that the wave function is not a classical field. The principle of superposition does not apply only to positions of particles, but to states of any system.

So your idea that the poison either gets released into the box or doesn't isn't correct when you think about it quantum mechanically. Suppose that the mechanism uses a single particle to release the poison. If a particle hits certain "button", the poison gets released, and if the particle does not hit the button, then the poison remains in a container. But the particle is not going to hit or not hit it. There is an amplitude \psi_{\textrm{hit}} for the particle to hit the button, and an amplitude \psi_{\textrm{miss}} for the particle to not hit the button. An equation |\psi_{\textrm{hit}}|^2 + |\psi_{\textrm{miss}}|^2=1 is true here.

After this the poison has an amplitude to get released, and an amplitude to remain in the container. So the poison is in superposition of being released, and being still in the container. Then the cat gets into superposition of being dead and being alive.

If you still don't feel like there's a paradox, then I would suggest also thinking about other problems in QM. For example, do you know how QM explains interference patterns that are got in a double slit experiment using individual particles that fly through the slits without interacting with each others? Popular explanations tell something mystical about individual particles "behaving as waves and particles at the same time", but the real QM explanation is in this superposition and wave function collapse stuff.

 

[Anonym]

The fact that a human outside doesn't know what's happening inside brings no paradox.

Sure. But that human is not a physicist. The experimentalist exists to tell us what happening inside:

I must not be a physicist then, because the "measurement problem(?)" strikes no fear in my eyes - and I'm an experimentalist too and I measure things for a living!

Moreover, every one of them in every other laboratory will tell us the same result. That what the objective reality means and that is the physics all about.Every one of them is the element of Poincare group.

I remember, that when I first read about Shrodinger's paradox.

You did mistake. Instead read about Schrödinger’s paradox (bbc4 or something like that), you should read E. Schrödinger:

“It is typical of these cases that an indeterminacy originally restricted to the atomic domain becomes transformed into macroscopic indeterminacy, which can then be resolved by direct observation. That prevents us from so naively accepting as valid a "blurred model" for representing reality. In itself it would not embody anything unclear or contradictory. There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks.”

Regards, Dany.

 

[reilly]

Could someone explain to me why Schrodinger's Cat has anything to do with QM?
Regards,
Reilly Atkinson

 

[Anonym]

Could someone explain to me why Schrodinger's Cat has anything to do with QM?

My people usually add in such cases: le zaazel!

Yes. The Quantum Physics must be naturally connected with the Classical Physics.

Regards, Dany.

 

[Anonym]

What is the measurement problem and why is it such a problem?

Now let me go two steps further. The isolated measurement or the collection of the measurements is only the data base. What we intend to do with it? How we define what the knowledge and the process of the acquisition of knowledge is? How the evolution of the knowledge is described?

You should not misinterpret me. I have no background in philosophy and it is last thing that I have in my mind. All that interested me is how the collection of facts may be organized using the suitable mathematical languages. I suggest reading more about the process of the acquisition of knowledge in E.P.Wigner “The problem of Measurement” (AJP, 31, 6 (1963), W&Z, p.324).

Here I would like to say more about the evolution of the knowledge. Indeed we do not know the laws of it yet. But we must use it at least intuitively similarly as we use equations of motion. For this reason I insist to use history of physics and was so rude in my post # 16. If “all conceptual problems of QM are already solved a long time ago”, we understand everything today. It is obviously wrong statement. Therefore some inaccuracies were made in the past. They should be identified and corrected. This is the only way to progress I know. And even after the more adequate theory is formulated, it is clear that something will remain wrong, the additional measurements will be required and further adjustment will be made.

I have year long history of discussions with Reilly here in PF. I was so glad to read his post that even suggested adding:” Could someone explain to me why Schrodinger's Cat has anything to do with QM, le zaazel!?”
Perhaps it is wishful thinking, but Reilly did not reject my statements. I will explain to you what do they mean:

1)the def of the objective reality: entire rejection of M. Born statistical interpretation of QM;

2)the statement that the third Newton law governs the measurement procedure: entire rejection of J. von Neumann theory of measurements, namely, the state of the system under test and the measurement apparatus is not described by the direct product of the individual states;

3)the statement that the physical theory must explain the collapse: entire rejection it’s treatment as a postulate (it was pointed out by J. von Neumann that it can’t be treated as a postulate, but later “philosophers” did).

4)“Could someone explain to me why Schrödinger’s Cat has anything to do with QM?”: entire reformulation (generalization) of all of the Classical Physics (Newtonian mechanics, Maxwell ED and Einstein gravitation) according to W.R. Hamilton and E. Schrödinger conjecture.

I have no doubt that this should be done and will be done. Now you understand what is said: "If you want to strike fear into the eyes a physicist; mention the measurement problem".

Regards, Dany.

 

[JDługosz]

What about decoherance? I would think that practical experiments in maintaining entanglement in anticipation of developing quantum computing technology would begin to give them a handle on the problem of "when the wavefunction collapses".

—John

 

[jostpuur]

What about decoherance? I would think that practical experiments in maintaining entanglement in anticipation of developing quantum computing technology would begin to give them a handle on the problem of "when the wavefunction collapses".

—John

I believe that the correct answer to the problem "when collapse occurs" is that it doesn't occur in Copenhagenian sense at some instant, but instead decoherence makes it appear as if the collapse had occured. If this is what you meant, then I agree.

But if somebody attempts to explain that there is no problems with collapse, it is still appropriate to ask the "when" question. It at least directs the conversation a little bit.

Still, the decoherence does not solve all collapse problems. Eventually the experimenter gets some measurement results, so some kind of real collapse must occur somewhere somehow in the experiment.

 

[genneth]

The question of *when* a measurement happens is subtle. Here's one take: http://arxiv.org/abs/gr-qc/0203056v2

 

[Anonym]

if you don't know what the quantum mechanical measurement is, you can pretend that you don't have any problems with it, and nothing will force you to admit that you have no idea what it is.

Equally well, you can pretend that you have problems with it, and nothing will force you to admit that you have no idea what it is.

I believe that the correct answer to the problem "when collapse occurs" is that it doesn't occur in Copenhagenian sense at some instant, but instead decoherence makes it appear as if the collapse had occured. If this is what you meant, then I agree.

Pity. The dispersion in/of time is the same property of the quantum world as the dispersion in/of space. If you accept the HUP and the special relativity you should understand that your question and the question where the electron is located in the hydrogen ground state is the same question. The measurements allow to “see” that properties of the QM systems. The collapse have nothing to do with it, the collapse is the property of the measurement equipment. You ask the wrong question. Mathematically, all you need to know are the properties of 2X2 or 4X4 matrices.

I don’t think that the problems are with the physics or with the math. There is some deep psychological trouble with QM in spite that our brain provides the every day practical example how it works. Consider for example what the referred by genneth (http://arxiv.org/abs/quant-ph/0205039, p.49) writer quoted without ref.:

“Look at classical physics, how nice it is: We can measure a particle’s position and momentum with as much accuracy as we would like. How limiting quantum theory is instead. We are stuck with

Delta(x)*Delta(p) >=h/2

And there is nothing we can do about it. The task of physics is to sober up to this state of affairs and make best of it.”

In QM delta(x)*delta (p) may have any real value from h/2 to infinity, but in classical physics delta(x)*delta (p) = 0 only. It is enough to participate intelligently at the first year of math at base school to know that 2>1, not necessary to attend meetings on the foundations of physics and to quote Winston Churchill.

Regards, Dany.

 

[genneth]

My preferred position is to view time as on an equal footing with any other observable, i.e. there's clock time, but no absolute time. In classical mechanics this is hardly a problem, see de Donder-Weyl mechanics or what Rovelli calls relativistic mechanics. Essentially, instead of calculating the correlation between "dependent" variables and the "independent" one called time, we just calculate correlations between observables. In quantum, we've actually still not got a problem, as far as we can derive a propagator, which basically gives the correlation between observing one event and another. We can actually come up with quantum models which lack a time variable. However, the situation with multiple events is more complicated. Hartle has his generalised, history-based quantum mechanics (see http://arxiv.org/abs/gr-qc/0602013), and Rovelli (and his bunch) proposes instead that event ordering is emergent from the dynamics (see http://arxiv.org/abs/gr-qc/0610140). I personally find the latter to be easier to understand, and better physically motivated.

However, all of this should be counterbalanced by the fact that we've never come across an experiment in quantum mechanics which violates our theory. Whenever we've properly taken into account the equipment we're measuring with, and the precise set up, we can always calculate the relevant probabilities. This is why experimentalists never worry about the measurement problem: they just do it! Theoretically, we'd (or some) like to have a physical theory which doesn't force the introduction of the details of measuring equipment -- which is why volumes have been written on this subject.

 

[Anonym]

I personally find the latter to be easier to understand, and better physically motivated.

I personally find all four papers referred by you have nothing to do with physics. There are no refs on A.Einstein, E. Schrödinger, W. Heisenberg, P.A.M.Dirac, E.P.Wigner and C.N.Yang. It is not possible to formulate the physical theory of measurements without well defined notion of the objective reality and I don’t know how it may be done without language of the theory of the continuous groups.

You try to switch the discussion to the nature of time in QM and problems of QG. PF rules suggest not repeating the same statements under different names in the different sessions. The questions related to time in QM are discussed in “nature of time” session. I consider any attempt to formulate the adequate theory of QG before relativistic QM hopeless. If you are interesting, my POV is presented in “How to explain the benefits of string theory to my grandmother...” session.

Regards, Dany.

 

[Demystifier]

... see de Donder-Weyl mechanics ...

I personally find all four papers referred by you have nothing to do with physics. It is not possible to formulate the physical theory of measurements without well defined notion of the objective reality ...

Perhaps some will be interested in my results
http://xxx.lanl.gov/abs/hep-th/0407228
http://xxx.lanl.gov/abs/hep-th/0512186
http://xxx.lanl.gov/abs/hep-th/0601027
showing that quantization based on de Donder-Weyl mechanics automatically determines what objective reality is: the Bohmian reality.

 

[ZapperZ]

I hate to interrupt (but I will) this festivities. Please use, as far as you can, peer-reviewed citations. Unless this is a speech, conference proceedings, or lecture notes of prominent physicists, we much prefer published work rather than ArXiv uploads. If they have been published, please give a full reference alongside the ArXiv links. If they have not been published, why not?

Zz.

 

[genneth]

ZapperZ: an exceedingly good idea, I usually mean to, though sometime forget to check, esp. if I consider the authors to be good authorities.

EPR in relational interpretation: http://www.arxiv.org/abs/quant-ph/0604064 in Found.Phys. 37 (2007) 427-445

Dany: That paper cites some of authors in your list.

 

[Demystifier]

I hate to interrupt (but I will) this festivities. Please use, as far as you can, peer-reviewed citations. Unless this is a speech, conference proceedings, or lecture notes of prominent physicists, we much prefer published work rather than ArXiv uploads. If they have been published, please give a full reference alongside the ArXiv links. If they have not been published, why not?

The journal references are also written down on these links, as can be easily seen. I think it is more practical to give the arXiv links because it makes easier to download that papers by those who are interested. It is actually a common practice in high-energy physics community, and I do not see why other physicists should not adopt this beautiful practice. Anyway, if you still think that I should write the journal references in the posts themselves, I can do that as well.

 

[lightarrow]

The journal references are also written down on these links, as can be easily seen. I think it is more practical to give the arXiv links because it makes easier to download that papers by those who are interested. It is actually a common practice in high-energy physics community, and I do not see why other physicists should not adopt this beautiful practice. Anyway, if you still think that I should write the journal references in the posts themselves, I can do that as well.

Yes, otherwise, the forum will be restricted only to those few people who have access to the journal references (maybe this is what ZapperZ wishes?).

 

[ZapperZ]

The journal references are also written down on these links, as can be easily seen. I think it is more practical to give the arXiv links because it makes easier to download that papers by those who are interested. It is actually a common practice in high-energy physics community, and I do not see why other physicists should not adopt this beautiful practice. Anyway, if you still think that I should write the journal references in the posts themselves, I can do that as well.

That is why I requested that the actual reference to the published work be cited along side the Arxiv link. And yes, I'm aware that HEP community often cites ArXiv papers (since I'm surrounded by high energy physicists at work). While we allow some of that in the particle physics and BTSM forum, we would like to try to use published work in the remaining physics sub-forums.

Yes, otherwise, the forum will be restricted only to those few people who have access to the journal references (maybe this is what ZapperZ wishes?).

I'm not even going to dignify that with a response.

Zz.

 

[tommyburgey]

maybe vacuum energy causes the interferance pattern! (i'm excited)

someone tell me if I'm a genius

 

[Demystifier]

That is why I requested that the actual reference to the published work be cited along side the Arxiv link. And yes, I'm aware that HEP community often cites ArXiv papers (since I'm surrounded by high energy physicists at work). While we allow some of that in the particle physics and BTSM forum, we would like to try to use published work in the remaining physics sub-forums.

OK, I will try to obey this rule in the future.