What does quantum mechanical mean?

[Ken G]

What does "quantum mechanical" mean?

We often hear statements like "an atom is a fundamentally quantum mechanical system" and so forth. But what does that mean? Yes, we can predict its behavior by solving the Schroedinger equation, but what I'm asking is, if we were given a black box that behaved just like an atom, is there any experiment you can do to it to be able to tell if whatever is in that box "is quantum mechanical" or not? So does that descriptor really mean what we tend to imply it means?

 

[khemist]

I tried responding, but asking whether something is "quantum mechanical or not" does not really make any sense to me. Everything has quantum mechanical properties, but once the scale gets too large the method becomes unnecessary, and one can use either classical mechanics or relativity.

Sorry if its a poor response. Maybe you could elaborate a little bit more?

 

[JordanL]

I believe he is asking what scale or type of systems can ONLY be described using QM.

 

[xts]

- we owe this sketch to Wojciech Żurek:
Decoherence and the Transition from Quantum to Classical—Revisited
http://arxiv.org/ftp/quant-ph/papers/0306/0306072.pdf

 

[DrChinese]

Awesome sketch, got to love it!

 

[unusualname]

http://arxiv.org/abs/quant-ph/0306072

nice link xts.

To answer Ken G, if you could determine the outputs from the black box with a deterministic mathematical model then it wouldn't be quantum-mechanical, for example a very accurate (deterministic) statistical model might correctly predict the black box outputs for dozens of years if the black-box had large numbers of particles in it. But you'll never find a model to deterministically predict beta decay of a single atom for example,so that's quantum mechanical.

I mean, basically, quantum mechanical means your observables are probabilistically defined eigenvalues, with no more fundamental description.

 

[JordanL]

http://arxiv.org/abs/quant-ph/0306072

nice link xts.

To answer Ken G, if you could determine the outputs from the black box with a deterministic mathematical model then it wouldn't be quantum-mechanical, for example a very accurate (deterministic) statistical model might correctly predict the black box outputs for dozens of years if the black-box had large numbers of particles in it. But you'll never find a model to deterministically predict beta decay of a single atom for example,so that's quantum mechanical.

I mean, basically, quantum mechanical means your observables are probabilistically defined eigenvalues, with no more fundamental description.

So any part of what we consider to be objective reality that is not mathematically deterministic is quantum mechanical by definition?

 

[xts]

predict beta decay of a single atom

That is something beyond my (mis)understanding of QM...

The question: is it possible (fundamentally, I am not asking for practical realisation) to produce pairs of neutrons entangled somehow such, that their beta decays times are correlated?

 

[xts]

So any part of what we consider to be objective reality that is not mathematically deterministic is quantum mechanical by definition?

Yes, if it is fundamentally indeterministic (as we also have practical indeterminism - where we are not able to posses sufficiently accurate knowledge about current state of the system - that's a case of thermodynamics)

 

[JordanL]

Yes, if it is fundamentally indeterministic (as we also have practical indeterminism - where we are not able to posses sufficiently accurate knowledge about current state of the system - that's a case of thermodynamics)

Wow... I had a fundamentally flawed understanding of what QM was...

That almost makes it sound like we are peering into an entirely different concept of reality than our own that is only loosely related to the reality our consciousness "exists" in or interacts with.

Wouldn't fundamental indeterminism also mean that the concepts of cause and effect would be nearly meaningless? How can the scientific method possibly function in an environment like that?

 

[xts]

That almost makes it sound like we are peering into an entirely different concept of reality than our own that is only loosely related to the reality our consciousness "exists" in or interacts with.

Sure! Quantum "reality" (if we are stubborn to stay along this word) must be pretty much different than common experience reality.

Wouldn't fundamental indeterminism also mean that the concepts of cause and effect would be nearly meaningless?

Not nearly meaningless but rather redefined. Cause and effect relation is no longer 100% deterministic (except of rare cases of eigenvalues), but often only probabilistic. But that is also a common-sense meaning of causality. If our bombers bomb Kadafi's bunker, we aim to have 95% probability of destroying it. If you were in the bunker, you surely wouldn't tell that such bombing is nearly meaningless...

 

[DrChinese]

Wouldn't fundamental indeterminism also mean that the concepts of cause and effect would be nearly meaningless? How can the scientific method possibly function in an environment like that?

Just like always! Hypothesis, followed by experimental investigation!

Keep in mind that useful patterns arise from many chance events. The odds of getting a total of 7 from 2 dice is 1/6 regardless of whether the throws are "truly indeterministic" rather than "causal but unknowable" due to stochastic complexities.

 

[JordanL]

Not nearly meaningless but rather redefined. Cause and effect relation is no longer 100% deterministic (except of rare cases of eigenvalues), but often only probabilistic. But that is also a common-sense meaning of causality. If our bombers bomb Kadafi's bunker, we aim to have 95% probability of destroying it. You won't tell that such bombing is nearly meaningless

Hmmm... your example has to do less with effect and more with intent. Was that on purpose?

As for "reality"... I am beginning to learn the math behind QM and GR. Beginning. I am trying very hard to give myself context in non-mathematical logic so that I can connect the concepts through several thought processes within my brain, so as to create as many axion connections as possible. In that, I am trying where possible to relate the complex concepts down to more fundamental psychological understandings of human mind, such as realism vs. unrealism.

If there is a way you prefer to describe it that is of more precision to what I'm trying to do, I welcome your input. :)

 

[unusualname]

So any part of what we consider to be objective reality that is not mathematically deterministic is quantum mechanical by definition?

er, well we don't have any understanding of "objective reality", we only understand a reality described mathematically by quantum mechanics, the Standard Model is our description of "reality" - anything else is (literally) metaphysics.

I don't think we know of any deterministic reality, that was the old classical paradigm, there is only one deterministic law in nature as far as we know, and it is Schrödinger Evolution (of probabilistic states)

 

[JordanL]

Just like always! Hypothesis, followed by experimental investigation!

Keep in mind that useful patterns arise from many chance events. The odds of getting a total of 7 from 2 dice is 1/6 regardless of whether the throws are "truly indeterministic" rather than "causal but unknowable" due to stochastic complexities.

Hmmm... That's approximately how the science of psychology is conducted... theories are proposed and statistical evaluation is performed to create degrees of certainty about our theories with the understanding that variables we do not know about might be confounding our conclusions.

The difference I see is that QM as a field has created lists of mathematical rules for how to generate conclusions. Is that a fair comparison?

EDIT: Sorry if my questions seem naive or annoying. I'm trying to wrap my head around it. :)

 

[xts]

Hmmm... your example has to do less with effect and more with intent. Was that on purpose?

No, Kadafi is like Schrödinger's cat ;) I should rather use an almost 100 years old example of Verdun, where probabilistics gave statistically valid results.

I am trying very hard to give myself context in non-mathematical logic [...]

You may either adopt an approach of non-classical logic (which - in my opinion - is weird and leads to lots of problems) or use classical logic to describe probabilistic behaviour of the world (like Bayes, Gauss, and other did). I definitely prefer the second approach: 'don't play with logic!'

fundamental psychological understandings of human mind, such as realism vs. unrealism

Yes... That's hard. It is up to your choice - if realism is more important for your Weltanschauung, or logic? That was a great frustration for Einstein: "Does the Moon exist when nobody looks at it?".
As for every metaphysical question I can't give you a ready, definite answer. I may only say that for my taste I am ready to abandon "realism" in order to preserve "logic", "causality", "no influence on past", and avoid other even weirder ideas, you had to adopt to protect "realism" and stay in accordance with an experiments.

 

[DrChinese]

Hmmm... That's approximately how the science of psychology is conducted... theories are proposed and statistical evaluation is performed to create degrees of certainty about our theories with the understanding that variables we do not know about might be confounding our conclusions.

The difference I see is that QM as a field has created lists of mathematical rules for how to generate conclusions. Is that a fair comparison?

The difference between correlations in physics versus psychology is that most human behavior studies seem to highlight spurious correlations. Spurious correlations are generally not useful. On the other hand, many physical correlations are indicative of underlying principles. But there is still fundamental indeterminism (apparently anyway, as it depends on one's interpretation).

There is a mathematical formalism underlying quantum physics. No one really knows why that seems to be a good map to describing things.

 

[JordanL]

No, Kadafi is like Schrödinger's cat ;) I should rather use an almost 100 years old example of Verdun, where probabilistics gave statistically valid results.


You may either adopt an approach of non-classical logic (which - in my opinion - is weird and leads to lots of problems) or use classical logic to describe probabilistic behaviour of the world (like Bayes, Gauss, and other did). I definitely prefer the second approach: 'don't play with logic!'

Yes... That's hard. It is up to your choice - if realism is more important for your Weltanschauung, or logic? That was a great frustration for Einstein: "Does the Moon exist when nobody looks at it?".
As for every metaphysical question I can't give you a ready, definite answer. I may only say that for my taste I am ready to abandon "realism" in order to preserve "causality", "no influence on past", and avoid other even weirder ideas, you had to adopt to protect "realism" and stay in accordance with an experiments.

I actually abandoned "realism" a while ago, but such abandon has always put me at odds with science instruction in the past, or I suppose instructors, and so I have learned that it leads to less confrontation and more learning if I approach "scientific" people by first assuming realism then asking questions until they can no longer provide me with reason to continue holding on to realism.

Or rather, I have learned that I learn faster if I approach my abandon of realism within new scientific ideas by first playing Devil's Advocate to my own ideas.

Generally, philosophically, I agree that it seems more crucial to the only fundamental thing I can prove to myself, that is my own existence, that logic be maintained over realism. (Mostly because logic seems to be the underpinning of awareness, and their own awareness is the only fundamentally "real" thing for any given person.)

 

[xts]

Oh, don't be so submissive to your teachers!

Realism is a pretty good approach for all practical purposes (from hunting mammooths to computing properties of semiconductors), and I, actually, advocate some kind of "common-sense realism". Moon exists even if nobody looks - as it may be detected e.g. by tidal waves (vomits) in Einstein's stomach.

But if you go deeper into QM metaphysics (especially Bell's inequality implications, but even to simple Young's experiment) - realism fails...

My favourite is a common-sense Zeilinger's interpretation of 'reality': real are only acts of interaction (those which may be amplified to degree perciveable by us) - all the rest are just mathematical constructs, helping us to predict outcomes of such experiments.

 

[JordanL]

Oh, don't be so submissive to your teachers!

Realism is a pretty good approach for all practical purposes (from hunting mammooths to computing properties of semiconductors), and I, actually, advocate some kind of "common-sense realism". Moon exists even if nobody looks - as it may be detected e.g. by tidal waves (vomits) in Einstein's stomach.

But if you go deeper into QM metaphysics (especially Bell's inequality implications, but even to simple Young's experiment) - realism fails...

My favourite is a common-sense Zeilinger's interpretation of 'reality': real are only acts of interaction (those which may be amplified to degree perciveable by us) - all the rest are just mathematical constructs, helping us to predict outcomes of such experiments.

It's not so much submission as realization: they have something I want (information) and will make it difficult for me to get if I don't perform certain actions (beliefs) that I can do without conviction or sincerity. Most teachers are not that bad, and the more advanced the knowledge set, the less like this teachers have been, in my experience.

It is mostly an approach to expedite learning.

But back to the actual discussion, common-sense realism not only makes sense, it is depended upon in certain areas of science. So what I meant was not so much that I believe in solipism or nihilism, but that at its core all areas of knowledge seem to depend on self-referential understanding that either has no logic or has no realism.

In that sense, I think Bell's theorems are even more important than many realize: I do not believe they describe the limits of QM or physics, I believe the inequality accurately describes the limits of arbitrarily large or small fields of knowledge. Or rather, they describe the fact that all areas of knowledge have a limit at which more knowledge requires giving up either logic or reality, as you begin to study the fundamental nature of either.

The difference between correlations in physics versus psychology is that most human behavior studies seem to highlight spurious correlations. Spurious correlations are generally not useful. On the other hand, many physical correlations are indicative of underlying principles. But there is still fundamental indeterminism (apparently anyway, as it depends on one's interpretation).

There is a mathematical formalism underlying quantum physics. No one really knows why that seems to be a good map to describing things.

Thanks. That's actually a very helpful answer for me. :)

I personally think that psychological correlations often describe underlying principals as well, but from "further away" if that makes sense. The correlation is several concepts removed, instead of one or two. Or that's what I like to think anyway.

 

[xts]

In my opinion, solipsistic interpretation is pretty valid one. As I refuse to assign any 'reality' to 'collapse', and I see it only in terms of information flow, that is me who is an ultimalte sink of information - as long as I have no knowledge about experiment outcome, it is not 'collapsed' from my perspective, it is still described by (pretty complicated) entangled wavefunction of all devices, assistants, and graduate students, involved in the experiment.

I agree that Bell's don't describe "limits" of physics, or theories. It just describe the real phenomenon, which contradicts our intuitions and which can't be explained in terms of "hidden variables" (worms living in rabbit's intestines).

 

[jfy4]

We often hear statements like "an atom is a fundamentally quantum mechanical system" and so forth. But what does that mean? Yes, we can predict its behavior by solving the Schroedinger equation, but what I'm asking is, if we were given a black box that behaved just like an atom, is there any experiment you can do to it to be able to tell if whatever is in that box "is quantum mechanical" or not? So does that descriptor really mean what we tend to imply it means?

I would say try an experiment that depends on the property of "superposition of the state", since this property is fundamentally "quantum mechanical", i.e. it has no classical analogue. On first thought, this seems to me a reasonable way to determine if a system is "quantum mechanical".