[Bill S (OP)]

One of the problems with brief, sporadic, visits to the forum is the risk of losing track of threads; so starting a new thread with thoughts I’ve had between visits is always a temptation.

The Copenhagen Interpretation of quantum theory holds that there is no reality at a quantum level, without observation.

Heisenberg, in developing his case for the uncertainty principle, used the example of “observing” an electron by hitting it with a gamma ray photon.  This act established the location of the electron, at the instant of contact, but gave no information about its velocity.

Bohr used this same example in defending complementarity; arguing that it supported the contention that “observation” created reality, in the quantum realm. The reasoning seemed to be that the electron had neither position, nor velocity, until an observation was made.

This must raise the questions: What was the gamma ray photon aimed at?  And, what did it hit, if the electron was not there before it was hit? 

[Bogie_smiles]

I think your understanding of the Copenhagen interpretation is pretty good.

...

This must raise the questions: What was the gamma ray photon aimed at?  And, what did it hit, if the electron was not there before it was hit? 

I would agree with that skepticism. The objective view is that a particle has location and momentum at all times, but that you cannot measure both at the same time. It is good logic to rationalize that a particle does not need to be observed in order to exist.

[jeffreyH]

The biggest problem is that the measuring device cannot be independent of the object that it is measuring. It affects the object it is measuring. You can adjust a wavelength to detect either momentum or position but not both. If you want to determine momentum your wavelength has to be long so that multiple measurements disturb the system as little as possible. Shorter wavelengths measure position. This is a physical fact. Not just some esoteric idea.

[Bill S (OP)]

It is good logic to rationalize that a particle does not need to be observed in order to exist.

Einstein would have been proud of you.

Trouble is that what our natural logic tells us is "obviously" the case, can break down very easily.

[Bill S (OP)]

The biggest problem is that the measuring device cannot be independent of the object that it is measuring. It affects the object it is measuring.

Undoubtedly; but it doesn't say how one could aim at, or hit, an object that doesn't exist until it is hit.

[Bogie_smiles]

Einstein would have been proud of you.

Trouble is that what our natural logic tells us is "obviously" the case, can break down very easily.

Maybe some would see a distinction between good logic that doesn't easily break down, and natural logic that can break down easily?

[jeffreyH]

The biggest problem is that the measuring device cannot be independent of the object that it is measuring. It affects the object it is measuring.

Undoubtedly; but it doesn't say how one could aim at, or hit, an object that doesn't exist until it is hit.

Well then, the object has to exist. An indeterminacy relates more to our lack of understanding than to whether or not a particle exists. Quantum mechanics is a model. It approximates an underlying reality. We are taught to believe what our senses tell us but they are fallible.

[Bogie_smiles]

The biggest problem is that the measuring device cannot be independent of the object that it is measuring. It affects the object it is measuring.

Undoubtedly; but it doesn't say how one could aim at, or hit, an object that doesn't exist until it is hit.

Well then, the object has to exist. An indeterminacy relates more to our lack of understanding than to whether or not a particle exists. Quantum mechanics is a model. It approximates an underlying reality. We are taught to believe what our senses tell us but they are fallible.

Sometimes a Devil's advocate position broadens the discussion, and so, though I don't invoke this thinking, the possible logic for the aiming and hitting an object that isn't there yet, is that there is an expectation that the particle will pass through a given space at a given time, and so the measurement attempt is made.

[jeffreyH]

Well that leads nicely to the concept of an expectation value.
https://en.m.wikipedia.org/wiki/Expectation_value_(quantum_mechanics)

[Bogie_smiles]

Well that leads nicely to the concept of an expectation value.
https://en.m.wikipedia.org/wiki/Expectation_value_(quantum_mechanics)

Yes, and expectation value is certainly useful in quantum mechanics. When I say that under the Copenhagen interpretation that maybe just the expectation of the hit occurring is enough to cause it to occur is where I am playing the Devil's advocate.

[Bill S (OP)]

Well then, the object has to exist. An indeterminacy relates more to our lack of understanding than to whether or not a particle exists. Quantum mechanics is a model. It approximates an underlying reality.

Isn't this what the Copenhagen Interpretation says is not the case?

[chiralSPO]

Heisenberg, in developing his case for the uncertainty principle, used the example of “observing” an electron by hitting it with a gamma ray photon.  This act established the location of the electron, at the instant of contact, but gave no information about its velocity.

Bohr used this same example in defending complementarity; arguing that it supported the contention that “observation” created reality, in the quantum realm. The reasoning seemed to be that the electron had neither position, nor velocity, until an observation was made.

This must raise the questions: What was the gamma ray photon aimed at?  And, what did it hit, if the electron was not there before it was hit? 

As far as I understand it, the problem with this line of reasoning is that electrons aren't actually particles, and neither are gamma rays (and neither of 'em are waves either!) None of these phenomena have any macroscopic analog that offers an intuitive and accurate way of thinking about them--that's how we get into trouble!

Instead of trying to think about a discrete point particles, consider two intersecting infinite (but not evenly distributed) blobs that are going to interact somewhere within their intersection. "Before" the interaction occurs, the best we can do is offer up a probability distribution of where the most likely times and places of the interaction will be, based on information we have about the histories or environments of these blobs. The "time" and "location" of this interaction do not mean that two point particles collided at that precise time and place. No, it merely means that this is when and where the two blobs interacted--no more, no less.

I know this isn't that much more satisfactory than the "shut up and calculate" philosophy, but I think it is very important to remember that subatomic particles aren't mysterious because they are magical--they are mysterious because we are simple creatures that can only understand things by experience, and we really can't experience anything remotely close to the reality of this scale.

Some of QM does kinda boil down to epistemology: we can imagine tiny particles whizzing around at (nearly?) infinite speed, guided by some waveform that only becomes apparent when multiple observations can be made. And maybe this is "actually" how it is. But I doubt it. Even if a ridiculous model ends up making excellent predictions, and lines up well with observations, that doesn't mean that the ridiculous (untestable) parts are true too--those are just there so we have something to imagine.

[Colin2B]

I agree with @chiralSPO it is our attempt to rationalise the quantum world which gets us into trouble. We try to find mental hooks to hang ideas on so we say that because a wave’s energy can be counted in integer units and because it carries momentum it has particle properties (and we end up forgetting that other waves can carry momentum). We also talk about tunneling, but no one suggests that the particle/wave digs a little hole!
I view Copenhagen as a set of rules for using a methodology that gives amazingly accurate answers, but like most analogies and methodologies these rules can be overextended to give even greater confusion.

[Bill S (OP)]

I know this isn't that much more satisfactory than the "shut up and calculate" philosophy,

I have to disagree with that.  It's the best explanation of the (possible) situation I've seen.  People have written books and said less that is meaningful to "hitch-hikers" than that.

Colin's addition is the icing on the cake.

[chiralSPO]

I know this isn't that much more satisfactory than the "shut up and calculate" philosophy,

I have to disagree with that.  It's the best explanation of the (possible) situation I've seen.  People have written books and said less that is meaningful to "hitch-hikers" than that.

Colin's addition is the icing on the cake.

Well, thank you!

[alancalverd]

The Copenhagen Interpretation of quantum theory holds that there is no reality at a quantum level, without observation.

And there's the problem. You start with an undefined and probably undefinable concept "reality" then add an equally undefined "observation". That's OK if you want to build a mathematical model in which the undefined variables cancel each other and you end up with a testable prediction, but you can't take the axioms too seriously. It's a bit like "aether" and "conventional current" - helpful but ultimately meaningless!

It's generally preferable to begin with experimental observations, e.g. light is quantised, and work out how these relate to other experimental observations like atomic spectra.

Heisenberg's indeterminacy is mathematically obvious, and no more than a correct formulation of Zeno's paradox. Speed is Δx/Δt, position is x. The larger we make  Δx and Δt, the more precisely we can measure their ratio, but the less meaning we can assign to either x or t. The clever bit is mixing in mass, then guessing that ΔpΔx ≥ h, so whilst we might have a very good idea of the position of a cannonball at any given moment, it becomes very difficult to locate an atom or an electron as m decreases. Ultimately this explains why the hydrogen atom has a finite (but fuzzy) diameter instead of collapsing, and to everyone's surprise h turns out to be a universal constant.

The key to quantum physics is, I think, to accept and use it as a series of statements about what actually happens and not to attempt to derive or "interpret" it from common sense, philosophy, or classical physics.

[jeffreyH]

I think the idea that 'reality' is a bad starting point is very important. Observation and experiment tell us what happens and not why. Ignore my statement above about an underlying reality. It was just wrong.

[Bill S (OP)]

  It approximates an underlying reality.

If this is what you want us to ignore; I would like to propose, for consideration, the following:

It links a possibly underlying reality, which we cannot perceive directly, to an apparent reality in which we exist, and which we must therefore be able to perceive. 

[alancalverd]

Quantum mechanics is nothing more or less than a mathematical description of what we have observed to date. Its value is in its apparent predictive power and universality.

[Bogie_smiles]

Interpreting the observations and postulates of QM is as fun as it gets in quantum mechanics,