# Old Copenhagen Interpretation

## Main Question or Discussion Point

Hi all,

I have read in several places about an "old Copenhagen Interpretation" and I wonder what are the differences between the old one and the current one.

I guess that the old one probably has relation with Bohr's point of view (reality only exists after a measurement, so the Moon is not there meanwhile nobody is watching). Is that right?

Well, any hints about the differences between the old and the new/current one would be appreciated.

P.S.: I've also heard that Copenhagen Interpretation (the new one I suppose) is no longer the orthodox point of view. Then, which interpretation would be instead the today's orthodoxy?

Thanks a lot.

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DrChinese
Gold Member
Welcome to PhysicsForums, computerphys!

Today there exists a mixture of viewpoints, although they typically reduce to mostly the same points. Keeping in mind that the QM formalism is the generally accepted and agreed upon scientific standard: the "old" Copenhagen viewpoint is often identified with Bohr today. Not sure that is fair.

Sometimes Copenhagen is used in a derogatory manner. Of more interest to me is the idea that the general science of 1927 is still mostly in place today. After 80 years, the HUP is still alive and well. Ditto all the key ideas of the day. They had little to work with compared to today, and yet they were able to deduce so much. I consider it impressive in the extreme.

Sometimes the current orthodox view of QM is also identified with Copenhagen. That sometimes shifts over time. Ideas and terminologies come and go. I would say that the majority of physicists don't care too much about the particular "flavor" of QM they advocate. Instead, they focus on the formalism.

Fredrik
Staff Emeritus
Gold Member
I think the Copenhagen interpretation was originally very close to what many people today call "the ensemble interpretation". Most people who use the term "Copenhagen interpretation" now seem to be talking about something else entirely, an interpretation in which the wavefunction represents the properties of the system and a mysterious physical process called "collapse" or "reduction" changes those properties in an uncontrollable and unpredictable way at the time of a measurement.

There's some good stuff in this thread. See in particular the article I linked to in #7. It's about how different people define "the Copenhagen interpretation" very differently, and about what Bohr actually said.

"Is the Moon there when no one looks" is really about whether QM can be said to describe reality even at times between state preparation and measurement. The ensemble interpretation would say no. Many worlds* seem unavoidable if we answer yes. The way I see it, those two are the only possible interpretations of the theory defined by the standard axioms of QM. A different "interpretation" would have to be defined by a different set of axioms.

*) Actually the MWI I'm talking about isn't quite Everett's MWI, which is more of a re-interpretation of the underlying mathematics than an interpretation of QM.

I believe from my experience with friends in this field, that formalism dominates, as Dr. Chinese said. The rest is dressing on the salad, but the nutrition is to be found in the veg. The math of QM raises questions that don't need to be answered with words, but with more math to describe more physics. Maybe the universe is also very strange.

P.S.: I've also heard that Copenhagen Interpretation (the new one I suppose) is no longer the orthodox point of view. Then, which interpretation would be instead the today's orthodoxy?
There is no concensus, but for me one thing is obvious - nobody is talking about the 'wavefunction collapse' seriously. I mean, on the fundamental level. It is very useful for most practical purposes, but I think nobody believes that it happens on the fundamental level.

Make an experiment - try to find proponents of the Copenhagen Interpretation. There are nobody or may be they are hiding.

The conclusions I got from them are:

1.- Quantum interpretation map is probably as fuzzy as the own quantum measurement process ;-)

2.- The old-Copenhangen interpretation would correspond to Bohr's point of view and would also to the ensemble interpretation.

3.- Current orthodoxy would be a mixed state of the following eigenvectors: CI, MWI(decoherence) and the most heavy one, positivism(formalism).

So, my question now would be: Is there any Bohr's idea or conception that now a days would be rejected as invalid?

Or asking that in another way: Is old-Copenhagen interpretation really debunked?

Nice to meet this forum and you all. Thanks!

So, my question now would be: Is there any Bohr's idea or conception that now a days would be rejected as invalid?

Or asking that in another way: Is old-Copenhagen interpretation really debunked?
No
because CI had never provided a clear definition of measurement (or an observer). So nothing to be rejected because the subject is too slippery.

Note that CI usually used a mixture of 2 views: objective (consciousness plays no role... world is objective...) and subjective (wavefunton is not real. It just an observer's knowledge about the system...), and CI proponents tend to switch between these 2 point of view based on the specific case.

Fredrik
Staff Emeritus
Gold Member
No
because CI had never provided a clear definition of measurement (or an observer).
A theory can be defined as a set of statements that tells us how to calculate probabilities of possible results of experiments. There's nothing in that definition that requires "measurement" to be defined as a dynamical process.

I agree with the word "no". I am not aware of anything he said that can be dismissed as invalid (but I'm no expert on what he said).

Frederik, it is hidden
experiment = event.
in CI, photon of visible light travelling thru glass is NOT an event and is not considered as measurement. Gamma photon in the same setup leaves a track and it IS a an event.
So the description of a problem (what is a probability of event X) in CI already implicitly contains a description of a 'collapse'.

So, you cant define an 'experiment' without defining 'measurement'.

Fredrik
Staff Emeritus
Gold Member
Every interpretation of every theory has the problem that measuring devices can only be defined operationally, i.e. by a non-mathematical description of how to build the measuring devices. This is a fundamental limitation of science, not of a version of the CI. You can certainly define "experiment" when you have defined your measuring devices operationally. Note that this is something you always have to do. The operational definition of the measuring devices is an essential part of the definition of the theory, even though no one ever bothers to mention it. What I'm saying is just that there's no reason to require that the theory be able to describe what happens during a measurement. It only needs to be able to tell us the probabilities of the possible results.

The concept of collapse is only needed if we insist on believing both that QM describes reality even at times between state preparation and measurement, and that there's only one "world". I'm pretty sure Bohr would have dismissed the former.

Every interpretation of every theory has the problem that measuring devices can only be defined operationally, i.e. by a non-mathematical description of how to build the measuring devices.
No.

If you define physics in mathematical way (say, axiomatic existence of particles) then experimenters, measurement devices and observers are nothing more then complicated QM systems (= theorems in axiomatic approach).

TOE should be free of any role of 'observation', 'expermenter' etc, because axioms should not depend on the theorems.

Fredrik
Staff Emeritus
Gold Member
You're missing the point Dmitry. The sentence you quoted is 100% true and completely unrelated to what you just started talking about. It's certainly true that measuring devices are quantum systems, but it's also irrelevant. (Perhaps you read the word "defined" as "described"). If you use the theory to define the measurement devices, you have eliminated any chance to test the theory's predictions. In fact, you have eliminated the predictions altogether.

CI usually used a mixture of 2 views: objective (consciousness plays no role... world is objective...) and subjective (wavefunton is not real. It just an observer's knowledge about the system...)
Is it right to say that "objective Copenhagen interpretation" is Schrödinger's point of view and that "subjective Copenhagen interpretation" is Bohr's point of view?

Thanks!

Hi all,

the today's orthodoxy?

Thanks a lot.
and there are the "Objective Collapse" models, mostly Nonlinear Quantum Mechanics (unlike Standard Quantum Mechanics, SQM a linear one).

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Objective collapse theories are Theories, not Interpretations, as they can be verified experimentally.

Is it right to say that "objective Copenhagen interpretation" is Schrödinger's point of view and that "subjective Copenhagen interpretation" is Bohr's point of view?

Thanks!
no Bohr but rather von Neumann's (based on what I know but I am not sure)

Objective collapse theories are Theories, not Interpretations, as they can be verified experimentally.

and who said INTERPRETATION ?

no Bohr but rather von Neumann's
That is interesting. I thought that subjective CI came from Bohr mainly. I also thought that von Neumann made the modern quantum formalism using Hilbert space to explain matrix and wave mechanics at the same time. Can you or anybody tell me what exactly did von Neumann in favor of subjective CI?

Well, I am very interested in these facts because I am trying to make a flowchart showing the major quantum interpretations, the people who advocated them and the main features of every interpretation (kind of realism, ontology of wavefunction, how they deal with collapse and main disadvantages).

Thanks!

Is it right to say that "objective Copenhagen interpretation" is Schrödinger's point of view and that "subjective Copenhagen interpretation" is Bohr's point of view?

Thanks!
When it comes to the so-called "Copenhagen Interpretation," the key word to keep in mind is: complementarity.

Bohr was essentially trying to "save" physics from implosion by way of somehow integrating the classical concept of nature-as-experienced (i.e. as a continuum; wavy) and the modern concept of nature-as-measured (i.e. quantitatively; discrete).

Bohr was simply trying to "keep hope alive," in terms of developing a fully representational theory of the spatio-temporal experience while at the same time allowing for innovation within the context of "measurement theory."

But it seems to have backfired because all attempts to develop Einstein-like "picture theories" are currently confined to fringe status, while the purely pragmatic notion that physical theories are merely supposed to be purely formalistic "handmaidens" to the empirical arts has been on the ascendancy ever since 1927.

Fredrik
Staff Emeritus
Gold Member
That is interesting. I thought that subjective CI came from Bohr mainly. I also thought that von Neumann made the modern quantum formalism using Hilbert space to explain matrix and wave mechanics at the same time. Can you or anybody tell me what exactly did von Neumann in favor of subjective CI?
He introduced the idea that collapse is a physical process caused by consciousness, which is a pretty bizarre idea, and not something you'd expect to see in a long text of mathematically rigorous stuff written by one of the greatest mathematicians ever.

I don't think that makes his interpretation "subjective". I think the collapse was supposed to be objective, even though it was caused by one individual. I think Bohr's view can be described as subjective, because he seemed to think of QM as a set of rules you could use to calculate probabilities of possible results of experiments that you can perform.

Well, I am very interested in these facts because I am trying to make a flowchart showing the major quantum interpretations, the people who advocated them and the main features of every interpretation (kind of realism, ontology of wavefunction, how they deal with collapse and main disadvantages).
Make sure to include the word "bull****" as a main feature of most interpretations. Seriously, most of what's been written about interpretations is garbage in my opinion, or at least very sloppy work. The literature is filled with mistakes, and people are still using those mistakes as references 40 years later.

He introduced the idea that collapse is a physical process caused by consciousness
So, I would say that he is who advocated "von Neumann/Wigner interpretation" but not "subjective Copenhagen interpretation". Right?

Ok. The other idea I am not sure of is if Schrödinger was the most important influence on the "objective Copenhagen interpretation". If anybody could tell me if that is true or false ...

Thanks!

Make sure to include the word "bull****" as a main feature of most interpretations. Seriously, most of what's been written about interpretations is garbage in my opinion, or at least very sloppy work. The literature is filled with mistakes, and people are still using those mistakes as references 40 years later.
When you talk about "interpretation," you are talking about the attempt to correlate a pure mathematical formalism with some kind "happening" that is occurring "out there."

But I do not think of QM as anything other than a purely non-realistic statistical theory. That is, the very idea of calling a Hermitian operator an "observable" is the first step into interpretive "hell." All we should really be saying is that an operator merely operates on a field of possible values by way of probabilistically choosing one of these values, with the statistical effect (i.e. averaged over infinite "operations") being that the expected value has born fruit.

It is for this reason that I propose the banishment of the following terms from all quantum mechanics textbooks:

-particle
-wave
-position
-velocity
-spin
-collapse
-observer/observation
-experiment
-apparatus
-mass
-energy

For, all of these terms have meanings that relate to some sort of "picture" of the natural order. So, if we can only get the "powers that be" to categorize QM as a mathematical art, as opposed to a physical one, then we can finally end all the silliness as far as what QM is "really" supposed to mean.

And finally, instead of calling it "quantum mechanics" (after all, what does the word "mechanics" imply?), lets just call it: the statistical theory of measurement! Ta-da... no more interpretation!

This is the flowchart I was talking about previously. I think here are represented the major trends when understanding Quantum theory. If you find any error or misconception, please, tell me. Thanks in advance.

Code:
[SIZE="1"]World View  => | CLASSICAL REALISM           |  REALISM     |  UNREALISM       | HYPERREALISM   | AGNOSTICISM  |
----------------------------------------------------------------------------------------------------------------
Name        => | Collapse theories, Hid.Vars | Objective CI | Subjective CI    | Multiverse MWI | Positivism   |
WF Ontology => | real Wavefunction           | real Wfunc   | in-mind Wfunc    | real Wfunc     | who cares?   |
Advocates   => | Bhom, Einstein              | Schrödinger  | Bohr, Heisenberg | Everett        | Most of phys |
Problems    => | Debunks QM, Bell's Theorem  | EPR          | No Moon, no tree | dE=0, Occam Rz | Built on sand|[/SIZE]

Fredrik
Staff Emeritus
Gold Member
When you talk about "interpretation," you are talking about the attempt to correlate a pure mathematical formalism with some kind "happening" that is occurring "out there."
Not quite. A theory is defined by a set of statements that tells us how to interpret the mathematics as predictions about possible results of measurements. In other words, we need to interpret the mathematics to turn it into a theory of physics. But I would define QM as a theory of physics, not as some piece of mathematics. So at least by my definition of "QM", it already includes an interpretation of this kind. This means that an "interpretation of QM" must be something else entirely (as long as we use my definition of QM).

An "interpretation of QM" should be an attempt to interpret QM as a description of what's actually happening to the physical system at all times, even at times between state preparation and measurement.

By the way, I have explained this several times before, and I don't think anyone has ever understood what I meant. At least not the first time. Maybe I'm not explaining it well, I don't know. (Hm, now that I think about it, I think Demystifier understood it). I probably won't try to elaborate on this, because discussions about interpretations take absurd amounts of time, and I don't want to get sucked into another one.

Most of the ideas that people call "interpretations of QM" don't satisfy this simple definition of an interpretation of QM. Some of them (like Everett's MWI) are interpretations of the first kind, i.e. attempts to use a different set of axioms to turn the underlying mathematics into a theory of physics. Many other "interpretations" aren't even that. They're just loosely stated ideas about what reality might be like.

But I do not think of QM as anything other than a purely non-realistic statistical theory.
I do too, most of the time. But I used to think that this was the only possibility, so I was quite frustrated when I realized (a couple of years ago, during one of these discussions) that QM might actually describe a physical system. Now I'm torn between those two options (a kind of MWI and the possibility that QM is just a set of rules that tells us how to calculate probabilities of possibilities). One of the reasons why I think the realist interpretation is worth investigating is that I think it might be capable of explaining some things that the anti-realist interpretation isn't.

That is, the very idea of calling a Hermitian operator an "observable" is the first step into interpretive "hell." All we should really be saying is that an operator merely operates on a field of possible values by way of probabilistically choosing one of these values, with the statistical effect (i.e. averaged over infinite "operations") being that the expected value has born fruit.
I like the approach that you can find in some advanced books: "Observables" are defined as equivalence classes of measuring devices (which are defined operationally). Then we take one of the axioms QM to be that observables are represented mathematically by members of some C*-algebra (like the algebra of bounded self-adjoint operators on a complex separable Hilbert space).

It is for this reason that I propose the banishment of the following terms from all quantum mechanics textbooks:

-particle
-wave
-position
-velocity
-spin
-collapse
-observer/observation
-experiment
-apparatus
-mass
-energy
I don't see how we could dispose of any of those, except "collapse".

So, if we can only get the "powers that be" to categorize QM as a mathematical art, as opposed to a physical one, then we can finally end all the silliness as far as what QM is "really" supposed to mean.
Uh...uh... Uh...wh...You want people to think of the best theory in all of science as...mathematics!? If we throw out the best theory, shouldn't we throw out all the others too?

And finally, instead of calling it "quantum mechanics" (after all, what does the word "mechanics" imply?), lets just call it: the statistical theory of measurement! Ta-da... no more interpretation!
"Statistical" implies things too, and so does "theory"...and even "measurement". I guess we're just going to have to find some weird symbol that no one uses and say that it's the new name of the theory formerly known as QM.

Is it right to say that "objective Copenhagen interpretation" is Schrödinger's point of view and that "subjective Copenhagen interpretation" is Bohr's point of view?

Thanks!
It would be more accurate to just avoid using the term "Copenhagen Interpretation" since it doesn't apply to anything in particular except maybe a confused straw man version of things .

What do you mean by Schrodinger's point of view though? One part of the Wikipedia article on the cat paradox gets it right (but a lot of it is wrong):
Schrödinger did not wish to promote the idea of dead-and-alive cats as a serious possibility; quite the reverse, the paradox is a classic reductio ad absurdum.
The original formulation of the cat paradox was actually developed by Einstein in a letter to Schrodinger. Einstein was clearly not a supporter of any objective collapse theories. From Einstein's letter:
The system is a substance in chemically unstable equilibrium, perhaps a charge of gunpowder that, by means of intrinsic forces, can spontaneously combust, and where the average life span of the whole setup is a year. In principle this can quite easily be represented quantum-mechanically. In the beginning the psi-function characterizes a reasonably well-defined macroscopic state. But, according to your equation [i.e., the Schrödinger equation], after the course of a year this is no longer the case. Rather, the psi-function then describes a sort of blend of not-yet and already-exploded systems. Through no art of interpretation can this psi-function be turned into an adequate description of a real state of affairs; in reality there is just no intermediary between exploded and not-exploded.
So Schrodinger's interpretation was probably much closer to Einstein's view than anything else.