Nobody understands quantum physics?

[Demystifier]

I meant of course real problems in physics as a natural science... Looking for cancer cure is also much more promising using biology and medicine rather than philosophy.

Show me one important paper in theoretical physics that does not contain any philosophy! There is no such paper, some philosophy is always there, at least in Introduction. What bothers you is not some philosophy, but too much philosophy. I don't like too much philosophy either, but my "definition of too much" is different. There is no universal criterion of how much philosophy in a scientific work is OK, and how much is not. It's completely subjective.

 

[Demystifier]

"Therefore, the program of computing what the effect of the [measurement] disturbance was and correcting for it is, in general, impossible. Accordingly, the two basic tenets of the theory of macroscopic measurement are both violated. Either the interactions cannot be made arbitrarily weak because of the phenomenon of atomicity, or if we wish to accept this and correct for it, we cannot do so because we do not have a detailed, deterministic theory of each individual event" --Schwinger

Using QM to resolve the "somehow" in your message above is presumably impossible. We can't use quantum theory to predict in detail what events will occur. We can assign probabilities to possible alternative histories of events during the measurement process, but no dynamics will ever explain why one history actually occurs over other alternatives.

Impossible ... we cannot do ... no dynamics will ever explain. Do you mean by QM as we understand it today, or by any theory that will ever be developed?

 

[haushofer]

Show me one important paper in theoretical physics that does not contain any philosophy! There is no such paper, some philosophy is always there, at least in Introduction. What bothers you is not some philosophy, but too much philosophy. I don't like too much philosophy either, but my "definition of too much" is different. There is no universal criterion of how much philosophy in a scientific work is OK, and how much is not. It's completely subjective.

Take General Relativity; the main motivation behind its development was Einstein's conviction that there should be more behind the equivalence principle than Newton makes us believe.

Other people would shrug their shoulders and tell us to shut up and calculate.

I never understood how people can do science without philosophy.

 

[Fra]

I never understood how people can do science without philosophy.

Self denial. Just like Popper tried to deny all except deduction.

/Fredrik

 

[Fra]

Other people would shrug their shoulders and tell us to shut up and calculate.

The other obvious difference is that application of existing corroborated theories (which are the "fruits" of the scientific process) is easy to do without philosophy. But to think that one would progress science and create new theories/fruits with "shutting up and calculate" attitude seems to lack insight in how creativity and learning works.

This is why "pure interpretations" that does not aspired to eventually progress into a new better theory, are not very interesting for me, I see interpretations as a sign of your own expectations of in which direction we think we will find the next generation of theory.

/Fredrik

 

[vanhees71]

Take General Relativity; the main motivation behind its development was Einstein's conviction that there should be more behind the equivalence principle than Newton makes us believe.

Other people would shrug their shoulders and tell us to shut up and calculate.

I never understood how people can do science without philosophy.

I think Einstein at the time when he started to think about a relativistic description of gravitation was still pretty much down to earth and more physicist than philosopher, although he was always also interested in philosophy and at good terms with the Viennese Circle and Moritz Schlick, but the main motivation was a scientific one, i.e., to describe gravity consistently within relativity. Roughly it started with his review article about relativity in 1907, and then it took him 10 years to finally arrive at the final answer, i.e., GR. I don't think that philosophy helped him much in this creative effort. The difficulty was mainly mathematical, i.e., to understand the meaning of gauge invariance, which in GR is general covariance. The decisive physical fundament was of course clear to Einstein much earlier, i.e., the equivalence principle and the "equivalence" of inertia and sources of the gravitational field.

 

[LittleSchwinger]

Well QM and QFT are stochastic theories. Morbert and vanhees71 are just saying it gives you the probability of an outcome, it doesn't tell you which specific outcome occurs. Nobody has a theory that actually tells you which outcome occurs.

 

[vanhees71]

With all the Bell tests ruling in favor of QM and QFT, I'd say Nature simply behaves stochastically, i.e., observables don't take predetermined values, if the system is not prepared in a state, where this is the case.

 

[gentzen]

Well QM and QFT are stochastic theories. Morbert and vanhees71 are just saying it gives you the probability of an outcome, it doesn't tell you which specific outcome occurs. Nobody has a theory that actually tells you which outcome occurs.

Are you replying to somebody specific? If yes, to whom?

Edit: Oh, your post was nearly at the same time as vanhees71's previous post. So I guess you are replying to Fra. I guess that is the problem with vanhees71's rule that you are replying to the latest post, if nothing else is indicated.

 

[haushofer]

I think Einstein at the time when he started to think about a relativistic description of gravitation was still pretty much down to earth and more physicist than philosopher, although he was always also interested in philosophy and at good terms with the Viennese Circle and Moritz Schlick, but the main motivation was a scientific one, i.e., to describe gravity consistently within relativity. Roughly it started with his review article about relativity in 1907, and then it took him 10 years to finally arrive at the final answer, i.e., GR. I don't think that philosophy helped him much in this creative effort. The difficulty was mainly mathematical, i.e., to understand the meaning of gauge invariance, which in GR is general covariance. The decisive physical fundament was of course clear to Einstein much earlier, i.e., the equivalence principle and the "equivalence" of inertia and sources of the gravitational field.

I don't understand your distinction between being "physicist" or "philosopher". He wanted to understand the "true nature of gravity" in the conviction that you could go beyond Newton's understanding. A similar question would be what the true nature of the wavefunction is. That's physics.

 

[LittleSchwinger]

With all the Bell tests ruling in favor of QM and QFT, I'd say Nature simply behaves stochastically, i.e., observables don't take predetermined values, if the system is not prepared in a state, where this is the case.

Certainly, that's clearly what the actual formalism says and what experiments support.

 

[LittleSchwinger]

Are you replying to somebody specific?

No, just the general conversation.

 

[Fra]

Nature simply behaves stochastically, i.e., observables don't take predetermined values, if the system is not prepared in a state, where this is the case.

Would you say that probability distributions are predetermined? ie. before the observer has acquired and processed enough data?

/Fredrik

 

[vanhees71]

Yes, they are, since they are given by the quantum state at the time of observation (using the Schrödinger picture in this argument), which is determined by the state preparation before the observation by unitary time evolution.

 

[Fra]

which is determined by the state preparation before the observation by unitary time evolution.

For this to hold you must have perfect knowledge of the hamiltonian. How do you infer this hamiltonian at a time scale that does not even allow acqusition and processing of enough data to even experimentally estimate a distribution from statistics of detected events?

/Fredrik

 

[vanhees71]

But this problem you also have in classical physics, i.e., if you don't know the Hamiltonian precisely you cannot precisely calculate the state of the system from the initial conditions.

 

[Fra]

Yes, but in classical physics things are predetermined and the inference refers only to the physicists ignorance. Ie. it has no deeper significance than that.

So how it works in classical mechanics is no argument as the standards of inference in classical mechanics is poor. The ambition here is in my eyes higher and this is the best thing with qm in all its incompleteness. There is no going back.

I am not advocating classical mechanics, i just try to illustrate that QM requires a massive amount of bg ingo to make perfect sense. You seem to entertain the idea that arbitrarily large systems obey QM. I would even want to claim that is philosophy. As it cant be corroborated in less than cosmoloigcal time.

So the conclusion is supposed to be that QM as an effective theory are only and can only be corroborated for small, shortlived subsystems. I think this should be clear??

But I agree there is no absolute scale where this happens. I think the scale is a relative one. Ie relative complexity and lifetime of the observer vs observed. The larger objecta you try to describe by QM, the more information must the observer handle. And how much information can you squeese into any part?

/Fredrik

 

[Simple question]

Nobody has a theory that actually tells you which outcome occurs.

True. But don't you think this a problem, especially for those philosopher that are all about "computing outcomes" ?

I'd say Nature simply behaves stochastically

The widest claim of all. Clearly false. It is the theory that is stochastic. Nature is probed trough clicks and events.
And Bell's proved that the observed randomness is NOT classic (simple), but quantum. So practically the theory treat entangled system as spacialy extended, which is also not "simple".

Six page's into this thread shows at least 5 very different take on QM, so yes, this little experiment showed it is safe to say that nobody-understands-quantum-physics

 

[LittleSchwinger]

True. But don't you think this a problem

No.

 

[Morbert]

Because one of the goals of theoretical physics is to explain observations. We observe definite outcomes, we don't observe all possible outcomes on an equal footing.

The goal of explaining why one outcome occurred instead of possible alternatives might be a personal one, but it cannot be insisted as a goal of theoretical physics. By this I mean a theory concerned with probabilities for alternative possibilities, but not the actualisation of one possibility, is not inherently a problem to theoretical physics even if it motivates particular research programs like Bohmian mechanics.

 

[vanhees71]

Yes, but in classical physics things are predetermined and the inference refers only to the physicists ignorance. Ie. it has no deeper significance than that.

Yes, and QT teaches us that things are not predetermined, and the randomness for the outcome of measurements is an inherent property of Nature and not due to the physicist's ignorance. That's the great result of Bell's theoretical work and the outcome of the corresponding experimental "Bell tests".

 

[martinbn]

Take General Relativity; the main motivation behind its development was Einstein's conviction that there should be more behind the equivalence principle than Newton makes us believe.

Other people would shrug their shoulders and tell us to shut up and calculate.

I never understood how people can do science without philosophy.

I think this is a very misleading example. The development of general relativity was a result of Einstein solving physics motivated problems with hardcore mathematics not philosophy. The philosophy part, like the hole argument, actually slowed him down. Only when he was able to shrug off the philosophy he made progress.

 

[Simple question]

The goal of explaining why one outcome occurred instead of possible alternatives might be a personal one, but it cannot be insisted as a goal of theoretical physics.

Fine, you are happy with a theory that does not match observations, unless you make a thousands of those in highly artificial and fragile setup. Nobody's here is contesting your personal right to stop doing science, stop searching. But I've just learned from you that you cannot insist that science is about searching explanations.
Good motivational speech that I missed at university.

And maybe one day, your fate is going to be decided by one quanta of light going through one slit.
Maybe then it will dawn on you that you would have been interested after all to know how Nature is going to determine where that quanta will end-up.

 

[vanhees71]

Indeed, I think the hole problem was a disgression, but on the other hand it might have helped Einstein to find the answer, i.e., the discovery of the fact that "general covariance" is a "local gauge symmetry" (Noether symmetry of the 2nd kind) rather than a "true symmetry of Nature" (Noether symmetry of the 1st kind). Of course, this was only finally understood by Noether's groundbreaking work, published in 1918 (but already worked out some years earlier).

 

[Morbert]

Fine, you are happy with a theory that does not match observations

Quantum theories are the most experimentally verified theories we have.

 

[LittleSchwinger]

There's no resolving this in this thread. Simple question insists/believes Nature must be deterministic. Morbert and vanhees71 are pointing out that our most successful physical theory is stochastic.
This is no different from demanding a mechanical exposition of the aether for electromagnetism and is scientifically regressive.

 

[Fra]

Yes, and QT teaches us that things are not predetermined, and the randomness for the outcome of measurements is an inherent property of Nature and not due to the physicist's ignorance. That's the great result of Bell's theoretical work and the outcome of the corresponding experimental "Bell tests".

Agreed. However this was not something I questioned. My point doesn't come across your philosophy filter :)

/Fredrik

 

[Demystifier]

The goal of explaining why one outcome occurred instead of possible alternatives might be a personal one, but it cannot be insisted as a goal of theoretical physics. By this I mean a theory concerned with probabilities for alternative possibilities, but not the actualisation of one possibility, is not inherently a problem to theoretical physics even if it motivates particular research programs like Bohmian mechanics.

Is there anything that can be insisted as a goal of theoretical physics? What is inherently a problem to theoretical physics?

My problem with consistent histories interpretation (CHI) is not that it doesn't pick one of the alternatives. My problem with it is that it doesn't even pick one consistent set of the alternatives. I would expect that theoretical physics should at least pick one consistent set (for example the set of all possible particle positions, or some other set), but CHI does not do even that. In a sense, the CHI is not one theory of nature, but a theory of "all" possible consistent theories of nature, where each consistent framework defines one possible consistent theory. CHI is a theory of theories, a meta-theory. Would you suggest that the inherent problem of theoretical physics is to develop a meta-theory, rather than a theory?

 

[Demystifier]

Quantum theories are the most experimentally verified theories we have.

The predictions of QM are spectacular only in the cases where its predictions are deterministic, for example in the case of g-2 in quantum electrodynamics. The probabilistic predictions of QM, on the other hand, are good, but not that spectacular.

Perhaps the only probabilistic prediction of QM which agrees with experiments to a very high precision is the Planck distribution of thermal radiation, but this is a very generic prediction that does not even depend on validity of the Born rule.

 

[Simple question]

There's no resolving this in this thread. Simple question insists/believes Nature must be deterministic.

Well, I've never seen unicorns, and conservation laws seems pretty real to me. Both theoretically and physically.
As to how Nature conserve probability in not only a deterministic fashion, but non-locally, is indeed a nut I would like to see cracked.

Morbert and vanhees71 are pointing out that our most successful physical theory is stochastic.

Do you mean stochastic thermodynamics ? The way some people here are carelessly throwing around "most successful" around here is baffling. QM is a good theory, no doubt. Stochastic theories have an uncanny way to capture truth about Nature. But it is NOT very useful in practice, compared to any other theoretical framework, on top of which civilization was actually build, and still is.

You see, this is not an opinion peace. Success also can be measured. You may prefer counting papers, I prefer counting bridges.

This is no different from demanding a mechanical exposition of the aether for electromagnetism and is scientifically regressive.

This red herring is worn-out to the point of being comical. Fields are fine. Hilbert spaces are fine. Any idea that is useful is fine by me. Any idea that contains glaring self contradiction, weak spot, incompleteness, or down right lack of foundation, needs to be fixed in some way. It is that simple.

QM have a strange intoxicating effect on some folks, that start calling for things nobody orders, like "quantum gravity". Did Einstein called for a GR'ed version of chemistry ?

I personally think that critique is good. But if you start bashing on people like Feynman, I think arguments should be a little less flimsy.

 

[A. Neumaier]

(My emphasis) Modelling the interaction between the particle in spatial superposition and the detector array with a quantum theory will result in a new superposition state that entangles the particle with the detector.

But coarse-grained macroscopic physics has no superpositions. Thus coarse-graining does not explain everything about the detector, since it does not explain the actual outcome - unless one treats the detector as classical and hence can apply Born's rule to select one outcome stochastically. Therefore

Once one jointly claims (as vanhees71 does)

• that there is no split between quantum and classical, and
• that coarse-graining explains everything about the detector,

the measurement problem becomes unavoidable.

My question: Why is this a problem? Why must we explain in particular the definite outcome? Why can't we accept QM as always treating all possible outcomes on equal footing apart from their probabilities?

One must explain it only if one claims the two points mentioned in my previous post.

It is the rate of events that is explained/predicted, as opposed to the particular event of a single run right?

But an event is a classical concept, there are no events in unitary quantum theory. Your argumentation therefore imposes classical concepts upon quantum mechanics of macroscopic bodies.

The goal of explaining why one outcome occurred instead of possible alternatives might be a personal one, but it cannot be insisted as a goal of theoretical physics. By this I mean a theory concerned with probabilities for alternative possibilities,

Only because your personal choice is to define theoretical physics in terms of probabilities.

 

[vanhees71]

The predictions of QM are spectacular only in the cases where its predictions are deterministic, for example in the case of g-2 in quantum electrodynamics. The probabilistic predictions of QM, on the other hand, are good, but not that spectacular.

Perhaps the only probabilistic prediction of QM which agrees with experiments to a very high precision is the Planck distribution of thermal radiation, but this is a very generic prediction that does not even depend on validity of the Born rule.

What do you mean by "deterministic"? $g-2$ of electrons or muons is a parameter that can be predicted by the standard model after defining all the coupling constants of this model and as such can be tested by measuring it in experiment. The measurement of the magnetic moment of particles is as "probabilistic" as it is for any other observable in QT.

The Planck distribution of course is calculated by using Born's rule. How else do you want to interpret the statistical equilibrium operator you calculate for free photons to obtain this result?

 

[vanhees71]

Do you mean stochastic thermodynamics ? The way some people here are carelessly throwing around "most successful" around here is baffling. QM is a good theory, no doubt. Stochastic theories have an uncanny way to capture truth about Nature. But it is NOT very useful in practice, compared to any other theoretical framework, on top of which civilization was actually build, and still is.

This is a joke, isn't it? QT is not only the most comprehensive theory about matter but also it's the most important fundamental theory our modern technology is based on. Sitting behind a laptop, typing in a message that I can send within a few moments to be readable world wide were completely impossible without the use of QT, leading to the development of modern solid-state and semiconductor physics, the transistor, integrated circuits and all that.

QM have a strange intoxicating effect on some folks, that start calling for things nobody orders, like "quantum gravity". Did Einstein called for a GR'ed version of chemistry ?

Einstein called for GR, because there didn't exist a consistent description of the gravitational interaction within relativity theory. What was known was the Newtonian approximation, which is an action-at-a-distance description, which is not compatible with (special) relativistic causality and spacetime structure. The motivation was to find a relativistic theory for the gravitational interaction, and it lead Einstein to the other corner stone of modern physics, i.e., GR. The success is also not only theoretical or academical, but at least the possibility for navigation with the GPS is entirely based on the findings of GR on the spacetime model, particularly the influence of the gravitational interaction on time measurement, whose accuracy is crucial for the GPS to enable us to localize points on Earth at a precision of a few meters needed to navigate from one point to the other.

After as non-relativistic QM has been formualted in 1925/26, because it was necessary to understand the atomistic structure of the matter around us, as well as the equilibrium distribution of radiation, i.e., Planck's radiation law, which started the discovery of QM in 1900, also relativistic QFT was a necessary consequence, because non-relativistic QM was not compatible with the relativistic spacetime model and thus a relativistic QT had to be developed. It lead to another great success in form of the Standard Model of elementary particles.

It is thus a natural consequence to also look for a quantum theory of gravitation to make also the gravitational interaction consistent with relativity. Relativistic QFT is incomplete in the sense that it doesn't include a satisfactory description of the gravitational interaction, and thus one must look for a new theory, which is even more comprehensive than it in also including gravity.

I personally think that critique is good. But if you start bashing on people like Feynman, I think arguments should be a little less flimsy.

Nobody bashes Feynman!

 

[Morbert]

Is there anything that can be insisted as a goal of theoretical physics? What is inherently a problem to theoretical physics?

I just mean the problem of indeterminism in QM is a subjective one, as opposed to an objective problem like e.g. the recovery of the Born rule in the many-worlds interpretation.

My problem with consistent histories interpretation (CHI) is not that it doesn't pick one of the alternatives. My problem with it is that it doesn't even pick one consistent set of the alternatives. I would expect that theoretical physics should at least pick one consistent set (for example the set of all possible particle positions, or some other set), but CHI does not do even that. In a sense, the CHI is not one theory of nature, but a theory of "all" possible consistent theories of nature, where each consistent framework defines one possible consistent theory. CHI is a theory of theories, a meta-theory. Would you suggest that the inherent problem of theoretical physics is to develop a meta-theory, rather than a theory?

What I am defending in this thread is the indeterministic character of QM. A defense of CHI would muddy the waters. For the purposes of this thread we can probably just take sets of histories as suggested procedures for constructing POVMS. I.e. When considering a measured system $s$ and detector array $D$ with a POVM $\{\Delta(\alpha)\}$, we can construct this POVM from any set of histories $\{C_\alpha\}$ where $\Delta(\alpha) = \mathrm{tr}_DC_\alpha^\dagger C_\alpha \rho_D$ holds. Histories are fictitious sequences of events that reproduce detector responses.

The predictions of QM are spectacular only in the cases where its predictions are deterministic, for example in the case of g-2 in quantum electrodynamics. The probabilistic predictions of QM, on the other hand, are good, but not that spectacular.

I'm not sure about the distinction here. E.g. QM is important for determining electrical characteristics in nanoscale devices or reaction rates and reaction pathways in chemical physics, but it can still be indeterministic in the Schwinger sense that precise knowledge of the quantum states does not precisely determine all events.

 

[WernerQH]

But an event is a classical concept, there are no events in unitary quantum theory.

Strange. The emission of a photon is not an event? Quantum theory does not describe this? Do you envision the decay of a neutron as a gradual, continuous process, with the neutron slowly turning into a proton? This doesn't seem to be mainstream physics.