# Is the QM axiom for measurements misleading?

1. Aug 8, 2006

### lalbatros

Was the QM postulate for measurements misleading?

Interactions and their understanding is the central topic of QM. So it is for CM too.
The interaction between a QM system and a (macroscopic) Classical system is for sure an extremely interresting subject. Particularly when we want to describe it with the languages of both QM or CM.

However, I have the feeling that the axiom for the wave packet reduction is not only useless but also very misleading, specially in its wording. It has -maybe- been introduced to make a clean presentation of QM. But so many have stick to the letter of this axiom that it has been the source for interpretation and re-interpretation for QM. Just as if some hidden truth had to be discovered. Of course, interpretation has been useless till now. And fortunately few people believe that the human brain has a special relationship with atoms.

What are your ideas?

Michel

Last edited: Aug 9, 2006
2. Aug 8, 2006

3. Aug 9, 2006

### lalbatros

kvantti,

Sorry, but really I cannot be satisfied by a fantasy solution to a non-existing problem.
Why indroducing this useless MWI?

The evolution equation is valid from microscopic to macroscopic. The wave packet reduction can be understood on this basis. What more is needed?

Maybe one would like to avoid randomness, but that's not possible (as far as we know). So, in the most optimistic point of view, the MWI describes the randomness that we are unable to avoid, a sort of catalog of what might be possible. But giving a reality to this catalog of "the possibilities" is a useless abstraction.

The measurement process is nothing more that an interaction and would proceed even if the experiment was build on purpose or by mere chance. What is called a "measurement process" when it occurs in a lab is still a measurement process when it occurs in the chlorophyll of a leaf, in my eye, or anywhere. Who could pretend that Stern & Gerlach sort of interactions can only occur in a laboratory? "Measurement process" is really a bad wording that should be replaced by interaction (between microscopic and macroscopic). This vocabulary is really the source of the debate.

I would like to introduce a comparison. Let's consider the well-know "http://en.wikipedia.org/wiki/Safety_engineering" [Broken]" analysis. This methodology is much used in nuclear engineering to create a list of possible defects for a nuclear reactor and their probability. The analysis considers all possible failure and their eventual accumulation.
My question is then: why should the nuclear engineers introduce a kind of MWI theory? Why should they consider parallel worlds where Tchernobyl doesn't happen or where it happens with a worse outcome? And what would be the use of the theory and what could it explain about nuclear safety?

So, in summary, I find it interresting to study the interaction between quantum systems and macroscopic system, but I really see no use for interpretations, on the contrary.

Michel

Last edited by a moderator: May 2, 2017
4. Aug 9, 2006

### Farsight

lalbatros: I think "particle" is the cause of a lot of problems here.

5. Aug 9, 2006

### kvantti

lalbatros:

http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm

How yould you interpret the results of this experiment? In my point of view, the MWI is the only choice to describe this experiment physically.

You can always go with the "shut up and calculate", but then you're missing a big part of the mystery of QM.

The mathematical formulation of QM makes amazingly accurate predictions, and yet you really can't explain it physically; theres just different interpretations.

Btw. the "fantasy solution" is considered to be THE solution by most (but not all) physicist for explaining quantum mechanics. Copenhagen interpretation was the first one, but it doesn't have to be the right one. It could be that the MWI isn't the right one either, but it sure does get rid of the "paradoxes" of QM...

6. Aug 9, 2006

### lalbatros

kvantti,

My preoccupation is not to discuss the MWI vs the CoI or another attitude toward the strangeness of QM. My preoccupation goes to the reasons why this debate occured and if it really has a meaning. And also, maybe, the criteria to decide about this question.

My interrest in this question started really in the 80's. I was very excited by reading the famous compilation "Quantum Theory and Measurement, by JA Wheeler" (and also the essay by Bernard d'Espagnat). A few years earlier I had to learn the postulates of quantum mechanics. The "measurement postulate" proved very convenient to avoid lengthy explanations about the interaction between microscopic and macroscopic system (kind of shut up!). But this postulate can be dropped easily, as I realised only much later (reading Landau & Lifchits) (kind of nothing to say!). I also come close to conclude that this debate started from a misundertanding around the word "measurement" (and also the word "observable" !!!). What is left then for discussion and interpretation?

I admit however a difficulty. The wave function is the irreductible object of QM. This is to be contrasted to statistical physics where the probabilities are derived from more fundamental objects (say the coordinates). The irreductibility of the wave function is difficult to swallow for the "classical beings" that we are.

Finally, I guess that all interpretations agree on the on the outcome of all QM experience. So why the hype then?

I don't want to convince, but to learn and to test my opinion.

Michel

Last edited: Aug 9, 2006
7. Aug 9, 2006

### kvantti

You can forget the "wave packet" and stick to particles only. The "wave packet" is just a way to calculate probabilities. But you can also calculate the probabilities the following way:

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

The path integral formulation or "the sum over histories" formulation plays a central part in the formulation of quantum field theory, which is the fundamental theory of reality.

If you want to study quantum interactions or "measurements", start with decoherence:

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

8. Aug 10, 2006

### Careful

Come on, what a terrible question . There are plenty of people who have given this lots and lots of braintime (see papers of Schrodinger, Wigner, Barut, 't Hooft, Toffoli....) and they all come up with different stories. The latter is normal since you are asking questions about processes you cannot directly acces through experiment. So, my answer is yes, R should be taken with the right amount of salt'' but any decent alternative is physics for the future.

Careful

9. Aug 10, 2006

### lalbatros

kvantti,

I like the path integral picture and I find studying the decoherence very interresting. These are the kind of path I would favour instead of any kind of interpretation (and specially MWI, I confess). Looking at QM from other point of view or in new situation is really the right job for physicists, I believe.

In addition, path integral theory has yield a high return already and can really be considered a a fruitful point of view? Similarly, the analysis of decoherence is directly related to experience as well as understanding QM and one can only hope the best from such topics. could we say the same from the MWI ? for the CoI, we can at least give it the credit of history as well as some pedagogical utility.

Careful,

I agree totally with you. I think it is not a wrong direction to try reducing QM to something more decent. Hidden variable was a valuable attempt indeed. I guess this way is not closed.

Michel

10. Aug 11, 2006

### kvantti

Yes, the path integral formulation is mostly thought to be the formulation of QM; so many things just get easier with it (especially with the quantum field theory).

The thing with the path integral formulation is that, in my point of view, it avoidably leads to the MWI. Why else would you have to calculate all the possible paths of a particle if the particle wouldn't actually travel along all those paths?

According to the MWI, the particle actually does travel along all the possible paths; one path in one universe.

I'm sorry that I bring up the MWI all the time, but I just recently understood its reasoning and found it to be logical. Before that I hated to think that our universe wouldn't be the only one... but instead of one universe, we have one multiverse... according to the MWI, that is.

Heres another site about the sum over histories formulation:

http://www.einstein-online.info/en/spotlights/path_integrals/index.html [Broken]

Last edited by a moderator: May 2, 2017
11. Aug 11, 2006

### lalbatros

kvantti,

I would like to understand, but unfortunately I don't see what the MWI explains and how it could tested experimentally and how it could lead to anything new in the quantum theory.

We can contrast that with the Hidden Variable Theory. The main success of this HVT attempt were:
that it could be tested experimentally,
that it failed to contradict QM,
it allowed us to reduce the number of possible interpretations
that it restricts the directions for other interpretations
that it was a first operational model to try an explanation for QM​

Now, let me try a comparison with electromagnetism. The fathers of EM had complicated mental pictures to represent the electromagnetic fields. These pictures were essentially analogies with fluid mechanics and led to the famous and lengthy discussions on the nature of "ether". Today, the EM fields are taken as irreductible objects. Nobody feels a need to discuss an explanation altough a more general theory can be considered.

Compare this situation in EM theory with that of elementary QM. Assume that the wave function (or state vector) is the irreductible object needed to describe the world at the microscopic level. I think that nothing else would be needed. In particular, the dark discussions about the postulate for the "reduction of the wavepacket" is totally useless. It is enough to study the interaction between a microscopic system and a macroscopic system (eventually a measurement device) to get the at the rule stated in the postulate. Why then would we need anything else and specially the MWI (purely formal physics I think).

I admit however that QM might be reduced to something more convenient for our human minds and that it is interresting to think about it. It could also be reduced to something less convenient for our minds. Both options might bring new physics. Finally, it may well be that physics cannot be reduced to something more confortable, then we need to cope with it.

Michel

12. Aug 11, 2006

Staff Emeritus
Feynmann "The particle follows all the paths and you add up all the amplitudes and everything cancels out except the classical path". Cancels out. No real paths in sidebar realities.

13. Aug 14, 2006

### kvantti

Well, David Deutch claims that the only way to explain the operations made by a quantum computer physically is the MWI. No one has put a serious argument against him.

In another https://www.physicsforums.com/showthread.php?t=123610" setAI wrote:

Check out the video for more info on quantum computing. As you can notice, David Deutch explains what happens in quantum computer using the multiverse concept. Theres no pro-Copenhagenist who has done the same... as far as I know.

Yes, that is the most probable path of the particle, but it isn't the only one that is observed. And according to the MWI, the paths cancel out because the particle interferes with itself in the multiverse. And that is what is being calculated: the particle (or wavefunction of the particle) interfering with itself.

Last edited by a moderator: Apr 22, 2017
14. Aug 14, 2006

### CarlB

The heart of the problem is the comment in the above link:

The error that the author is making is to divide the experiment up in the middle. This is not physical. All that we can be certain is that before the experiment, when the events are all in the future, we have an initial condition that we can control. After the experiment, when the events are all in the past, we can examine the final state that was measured. We cannot halt time and examine the state of the system between time 2 and time 4.

The MWI purports to provide a physical explanation for the state of the system between time 2 and time 4. While it is true that the Copenhagen interpretation refuses to give a physical interpretation to this intermediate state, it is not true that MWI is the only version of QM that gives an interpretation. Bohmian mechanics, for example, also gives one.

The only way we can "halt" time and examine the states between time 2 and time 4 is to make measurements of the state at some time. We do this by making a measurement, that is, we arrange for a sequence of events which will result in a set of final states that we can distinguish. But that activity in itself is a measurement and doing this would change the whole system. No, the only physical thing we can do with experiments is set them up and much much later, examine the results. For this reason, the initial and final states are the only states that it makes any sense to relate to physical conditions.

We known almost nothing about how time really works. To assume that there is a continuous sequence of states between the initial and final states is to make assumptions about time that we have not and cannot prove. It is quite possible that some other, simpler and more natural, method of describing how this experiment will arise, one that does not need the fantasy of the MWI. I believe that such a description does exist, and arises from making time a much more complicated thing than it usually is. MWI instead assumes a very simple model for time.

I agree with you here, but I would be remiss if I didn't say that the reason we are told to "shut up and calculate" is because the alternative appears to be a waste of time. The MWI has not explained anything about the universe that was not already known. The MWI does not give a calculation for the masses of the neutrinos, or for the weak mixing angle, or for any other physically measurement. So far, MWI has not proved itself useful. Then again, Bohmian mechanics has not proved itself useful either. Shut up and calculate.

I've heard this unlikely statement made before. There must have been some obscure poll taken over a very small number of physicists. MWI is barely even mentioned in the textbooks and no one has earned a Nobel prize from it.

Carl

Last edited: Aug 14, 2006
15. Aug 14, 2006

### Hurkyl

Staff Emeritus
Probably because it's trivial.

The computer begins in a quantum state. It evolves to another quantum state. End of story. The only difference between the Copenhagen and MW interpretations here is what happens after the computer's done running.

To a Copenhagenist, all a functioning quantum computer proves is that no measurement occured during the computation.

Last edited: Aug 14, 2006
16. Aug 15, 2006

### Tipi

Hi,

But then, QM can not describe an individual system? In fact, the reality of the catalog is given by an ensemble of measurements. And the theory gives no reality to a single measurement... I will not ask Why we obtain this result in that single measurement?, but I will ask : Why not to try to explain the result of an individual measurement, why?

How we explain interference without giving a certain reality to the catalog?

Is MWI really talk about particles on paths in particular universe of the multiverse? Your word's sound like Bohm's...

What do you think of that:

http://arxiv.org/abs/quant-ph/0406173

I'm new here, so, nice to meet you guys!

Tipi

17. Aug 16, 2006

### CarlB

The paper gives a Bohmian interpretation with trajectories for the Klein-Gordon equatoin. I think that the paper is brilliant and have saved it to my pile of papers that I am going to reference or reread someday. Thanks for bringing it up.

A question addressed is what happens when you insert a measurement in the middle of a propagation of the Klein-Gordon equation. This reads directly on the topic we've been discussing here, namely the MWI interpretation and the question of the existence of the interim states in between quantum measurements. What the author points out is that if you arrange for the particle to be measured at some time t, then you can't very well expect it to keep going forward in time, then come backwards in time back through time t, and then go forwards in time through time t again. Therefore you can't measure the same particle three times at one time t. He ties this up with the negative probability density problem for the Klein-Gordon equation beautifully.

The paper is a beautiful exposition, but I don't see how it will find a difference with standard QM. What my intuition says is that you will end up wanting to write the measurment problem in field theory terms and then his interpretation will give the same results as usual.

I should mention that despite my pointing out that Bohmian mechanics doesn't give anything over the usual interpretation, I am a great admirer and sort of believer in Bohmian mechanics. I think Bohmian mechanics is closer to the truth than the standard Copenhagen version which is only slightly less silly than MWI. Given the three theories at this time, as long as I were calculating, I would use the Copenhagen version. For my own purposes, I believe in a modification of Bohmian mechanics where the particle and wave are the past and future of the particle, where "past" and "future" are with respect to an observer. The addition to Bohmian mechanics I have to add is that I think that there is a continuous mapping from the wave to the particle trajectories. To explain this is long and difficult because on the face of it it appears to be incompatible with multiple particle wave functions, etc.

One of the interesting parts about the paper is how he looks at Bohmian mechanics' insistence on a preferred frame of reference from a relativistic point of view. I happen to not believe in relativity, except as a calculational convenience, an extremely accurate approximation, so in this case I prefer the usual Bohmian mechanics version. It turns out that not being a "true believer" in relativity is equivalent to a virulent form of leprosy as far as your relations with other physicists go, so I've been working on quantum mechanics written in finite degrees of freedom where the issue of the structure of spacetime can be avoided. But you can only go so far with this sort of retreat. I'm writing an introduction to quantum mechanics from the density matrix point of view (i.e. with spinors derived from density matrices instead of vice versa), but I'm restricting myself to finite degrees of freedom just to avoid the reference frame issue. Later, I figure on writing a follow up that deals with the more general case, and I'll be reading and referencing papers by Hrvoje Nikolic.

As in the case with relativity, I think that QM is not the truth, but instead is an extremely accurate method of making calculations. But not being a "true believer" in quantum mechanics is a fairly mild disease, maybe equivalent to a bad case of acne.

Carl

Last edited: Aug 16, 2006
18. Aug 16, 2006

### kvantti

What is a "measurement" then? How does a "measurement" differ from any interaction that collapses the wavefunction? Wouldn't the wavefunction of a photon collapse when interacting with a beamsplitter?

And what do Copenhagenists have to say about http://xxx.lanl.gov/abs/quant-ph/9906007" paper? Deutsch claims, and shows, that if we chop something with the Occam's razor, quantum physics becomes a local theory. If this is true, wouldn't all non-local interpretations (including Bohm's and Copenhagen) be unphysical?

Last edited by a moderator: Apr 22, 2017
19. Aug 20, 2006

### lalbatros

kvanti,

The beamsplitter in an interferometer experiment plays exactly the same role as the two-slit-screen in the Young's two slit experiment. For the latest, I guess, you would not say the (interaction with) screen makes the collapse happen. It is rather a photon detector -of any kind- (including an obstacle on one slit) that performs the measurement.

Similarly, the quantum computer never collapses any wave function since it would then behave -at best- as a classical computer. A measurement would occur only when some result is read in some way, with a printer or -more realistically- with photon detector. And you certainly know all the funny things that follow, like to possibility to detect spying for example.

Michel

Postscriptum
Of course, the comparison with a screen is not totally perfect. With a screen you could always add more slits by removing some part of the screen (a part that is truly performing a measurement and really constrains the experiment).

But of course this is irrelevant since we are discussing what happens behind the screen.

Last edited: Aug 20, 2006
20. Aug 20, 2006