A Is quantum theory a microscopic theory?

[PGaccount]

I don't think there is a "why" for the statistical character of quantum physics. You don't find the need to ask "why" classical physics is deterministic, so why here? All you can ask is, what removes the foundation for a deterministic description? A deterministic description requires the existence of an objective 'state' which can determine the future state. This is not possible anymore because quantum physics shows that you cannot have a space-time and energy-momentum description at the same time. For a deterministic description on the classical lines, you need to be able to have p and x at the same time.

 

[Lord Jestocost]

No. Atoms in materials exist. We can image them. Rutherford 'saw' gold atoms deflecting alpha-particles !

What I wanted to say, expresses Paul Davies in his introduction to Werner Heisenberg’s “Physics and Philosophy” in the following words:

“Thus an electron or an atom cannot be regarded as a little thing in the same sense that a billiard ball is a thing. One cannot meaningfully talk about what an electron is doing between observations because it is the observations alone that create the reality of the electron. Thus a measurement of an electron's position creates an electron-with-a-position; a measurement of its momentum creates an electron-with-a-momentum. But neither entity can be considered already to be in existence prior to the measurement being made.”

 

[Jimster41]

I don't think there is a "why" for the statistical character of quantum physics. You don't find the need to ask "why" classical physics is deterministic, so why here? All you can ask is, what removes the foundation for a deterministic description? A deterministic description requires the existence of an objective 'state' which can determine the future state. This is not possible anymore because quantum physics shows that you cannot have a space-time and energy-momentum description at the same time. For a deterministic description on the classical lines, you need to be able to have p and x at the same time.

I like this concise reminder that time, microscopically is ...problemo. Things in the past are definitely in a sense at t,x. But, nothing in the past has momentum...? Is that right? Or at least this is what was bugging me yesterday reading this thread. Is there anything in the past that has momentum? Is there anything that has been measured that now has momentum... which is connected, in my mind, to the question of Cauchy surface conservation. If I measure something and put it at t,x. The momentum of that QM thing is conserved (or the energy involved in it). Some thing(s) got it. but they is all back down in QM? I could draw a network couldn't I to try to account for it, but then on one side of some line of incidence in that drawing there is a set of enumerable events at t's,x's, culminating in, causing, my event. On the other side of that line it's some nightmarish fuzz of Feynman diagrams?

Something is going to pull more events out of that fuzz. My event detector is part of it. But...

 

[PGaccount]

The thing is that, in classical physics, momentum can be defined in terms of space-time pictures

p = m dx/dt

This is an idealization, and is not exactly valid when seen in the light of the correct quantum mechanical description. The definition p = mv is valid in what is called the geometrical optics limit of quantum theory.

 

[Jimster41]

The thing is that, in classical physics, momentum can be defined in terms of space-time pictures

p = m dx/dt

This is an idealization, and is not exactly valid when seen in the light of the correct quantum mechanical description. The definition p = mv is valid in what is called the geometrical optics limit of quantum theory.

v being directional speed being a f(t), is that correct? Just want to make sure I followed that.
There was a big discussion about background time in QM, QFT etc in a recent thread. Definitely made my head spin. It seems tricky to suggest a background time for QM, QFT formulations when there isn't a fully realized QM theory of GR which to my mind is the bar for describing the concept of time.

I will shut up now. Great thread.

 

[Dr. Courtney]

As an atomic experimentalist, most of my measurement experience relates to measuring the wavelength (or frequency) of laser light used for excitation, counting photons resulting from an event (usually with a photomultiplier tube), or counting electrons (usually with a channeltron or microchannel plate). Occasionally ions were counted and velocities determined with time of flight (delay between excitation event - laser pulse - and distant arrival.)

Now I can see how some of these measurements may be characterized as macroscopic - especially when table top optics are involved and there is enough light intensity to use photodiodes. But when one is counting single photons, electrons, or ions, these seems like fundamentally microscopic events - unless you are using a different definition of microscopic than I am.

 

[DarMM]

Now I can see how some of these measurements may be characterized as macroscopic - especially when table top optics are involved and there is enough light intensity to use photodiodes. But when one is counting single photons, electrons, or ions, these seems like fundamentally microscopic events - unless you are using a different definition of microscopic than I am.

The problem is that quantum theory seems to only talk about microscopic events provided they manage to get magnified up to the classical level. It doesn't reference microscopic events in and of themselves when no classical devices are around.

 

[Mentz114]

What I wanted to say, expresses Paul Davies in his introduction to Werner Heisenberg’s “Physics and Philosophy” in the following words:

“Thus an electron or an atom cannot be regarded as a little thing in the same sense that a billiard ball is a thing. One cannot meaningfully talk about what an electron is doing between observations because it is the observations alone that create the reality of the electron. Thus a measurement of an electron's position creates an electron-with-a-position; a measurement of its momentum creates an electron-with-a-momentum. But neither entity can be considered already to be in existence prior to the measurement being made.”

Well, I have read that and similar statements. This statement Thus an electron or an atom cannot be regarded as a little thing in the same sense that a billiard ball is a thing.
is self-evidently true (I believe I said something similar about atoms and baseballs) whereas the assertion But neither entity can be considered already to be in existence prior to the measurement being made is not a deduction from anything, merely speculation.

Furthermore, it is scientifically void because its truth cannot be tested by experiment. I see no reason to believe it.

 

[julcab12]

As an atomic experimentalist, most of my measurement experience relates to measuring the wavelength (or frequency) of laser light used for excitation, counting photons resulting from an event (usually with a photomultiplier tube), or counting electrons (usually with a channeltron or microchannel plate). Occasionally ions were counted and velocities determined with time of flight (delay between excitation event - laser pulse - and distant arrival.)

Now I can see how some of these measurements may be characterized as macroscopic - especially when table top optics are involved and there is enough light intensity to use photodiodes. But when one is counting single photons, electrons, or ions, these seems like fundamentally microscopic events - unless you are using a different definition of microscopic than I am.

The problem is that quantum theory seems to only talk about microscopic events provided they manage to get magnified up to the classical level. It doesn't reference microscopic events in and of themselves when no classical devices are around.

Not only that. Each type of experimenter yield a different reading. The photon that spectroscopy experimenter uses to explain how spectra are connected to the atoms and molecules is a different concept from the photon quantum optics experimenters talk about when explaining their experiments. Those are different from the photon that the high energy experimenters talk about and there are still other photons the high energy theorists talk about. There are probably even more variants (and countless personal modifications) in use. Definition really varies from any setup and how it reacts to different setup. Even in HEP experimenter's concept behind the readings is - a particle that cannot be observed directly, but is something having position, energy and momentum that helps explain interactions of charged particles among themselves and their behavior in external EM field (Compton's effect, pair creation). That the reason some think that 'maybe' the picture/detention is a phenomenon or mirage of a natural dynamics of interacting things.

 

[Dr. Courtney]

I can't see how a definition of microscopic that excludes all single photon and all single electron observations does not also exclude most (or all) observations of things that are traditionally considered microscopic: bacteria, viruses, cells, organelles, etc.

 

[DarMM]

I can't see how a definition of microscopic that excludes all single photon and all single electron observations does not also exclude most (or all) observations of things that are traditionally considered microscopic: bacteria, viruses, cells, organelles, etc.

In the quantum formalism in its standard reading, photons are not spoken of alone in and of themselves when no macro devices are around. It's not so much that something is excluded from being microscopic, it's that the electron in quantum theory is spoken of in terms of micro-macro phenomena.

Viruses, cells, etc have frameworks that discuss them as they are when no microscopes are around and don't suffer from these issues.

 

[vanhees71]

I think I have commented on such question before. Because we believe in science and science tells us that there is a reason for everything. randomness with no reason seems utterly illogical. Now, if it is indeed that way, people want to know why, that's all.

Well, the strength of science tells us first to be open to learn how nature behaves, and that in investigating this with using an interplay of quantiative reproducible observations and experiments and analytical and mathematical reasoning we may find that we have to give up prejudices about what we think to know. Nothing in this process is save against the possibility not to be in need for revision with new discoveries.

This happened indeed twice in the first half of the 20th century: One revision was necessary with the discovery that the electromagnetic phenomena are accurately described by Maxwell's theory of the electromagnetic field, but this theory was inconsistent with the "very sure knowledge" about the mathematical model describing spacetime (or at this time the strictly separated time and space) in terms of the Galilei-Newton model. This was such a "sacrosanct" piece of knowledge that it took about 50 years to resolve the issue finally and it involved the work of some of the most brillant mathematicians and physicists of their time (Poincare, Lorentz, Michelson, and finally Einstein): What had to be revised was not the idea of the special principle of relativity, i.e., the invariance of the physical laws under transformations from one inertial reference frame to another, but the very law of how the transformation had to be done, i.e., instead of Galileo transformations the Lorentz transformations (discovered in some predecessor form already by Voigt in the late 1880ies), implying a new spacetime model with space and time now no longer strictly separated but amalgamated into a pseudo-Euclidean affine spacetime manifold. This implies that also the laws of mechanics had to undergo a revision, and the corresponding revisions by Einstein (and corrections thereof by Planck, von Laue, and Einstein himself thereof) were after some experimental confusion resolved by experiments with the then also newly discovered electrons in gas-discharge tubes.

This turned out, however, to be a pretty harmless revision. Nothing of the considered really fundamental ideas, which you still more than 100 years later insist on, had to be revised. The physical laws still were strictly deterministic, i.e., any possible observable of any system by tacit assumption has always a determined value, and the principle of causality on the fundamental level holds in a very strong (time-local) form: Known the initial values of a complete set of observables (which simply is a set of observables all other observables are functions of) at some initial time $t_0$ their values in principle (and thus that of all possible observables, which are functions thereof) are determined at any later time $t$.

Now this apparently save knowledge had to be revised with the advent of problems concerning phenomena becoming observable with the ever faster progress of technology. First these obstacles were considered minor issues. When Planck asked for advice what to study, a famous physics professor at the university told him, with his brillant grades from high school he shouldn't waste his time with physics, because this is all settled and the "little clouds" on the horizon would be simply resolved by measuring some fundamental constants of ever better precision and small revisions of the laws of classical physics (at the time consisting of classical (still Newtonian) mechanics, Maxwell electrodynamics, and (phenomenological) thermodynamics).

One of the clouds was not so new to begin with: It was the question of the absolute value of entropy and a theoretical understanding of what's now known as the Gibbs phenomenon in statistical physics, which however was already met with quite some skepticism by the more conservative physicists of the time, since they didn't even believe in the existence of the atomistic structure of matter to begin with. With the advent of low-temperature physics (with one milestone being Kammerlingh-Onnes achievement of liquifying Helium in the early 1900s) the issue became very evident: The specific heat of most substances did not behave as expected at low temperatures. Also when it became clear that metals had conduction electrons, but these didn't contribute to the specific heat even at room temperatures, another "cloud" arised at the horizon.

Then there was the problem of "black-body radiation", which was not describable with classical electrodynamics and thermodynamics/statistical physics. Famously this was how the quantum revolution started: When Rubens and Kurlbaum accurately measured the black-body radiation spectrum as a function of temperature, which was first an attempt to provide a better standard for measuring the efficiency of lightning sources (gas and the new electric light bulbs), lead Planck to guess the right law and subsequently with a brillant (but first not really understood) statistical analysis which only worked when assuming that each frequency mode of the electromagnetic field could exchange energy only in lumps of $h \nu=\hbar \omega$. Already this was too much for Planck, but he couldn't find any other way to derive the accurate black-body law, named after him.

The rest of the story is well known. Einstein came with his light-quanta idea in 1905, then Bohr (completed by Sommerfeld) with his ad-hoc idea to explain the hydrogen spectrum from the atomic model enforced on the physics community by Rutherford's findings. Then the very stability of matter, the precise indistinguishability of atoms of each kind has become an enigma for classical physics. Then the ad hoc solution for hydrogen was found to be flawed, because it only worked for hydrogen. Another happy incident has it that it also works for the harmonic oscillator and thus lattice vibrations of solids, which resolved the problem with specific heat but not yet the question, why the conduction electrons in metals do not provide anything to the specific heat, while the model to describe the conduction electrons as a quasi free gas moving in a positively charged background worked very well in explaining Wiedemann-Franz's law of propotionality of electric and heat conductivity.

The up to day "final" resolution was modern quantum mechanics, discovered more or less independently in parallel no less than 3 times by (a) Heisenberg, Born, and Jordan (with some important help by Pauli) in terms of "matrix mechanics", (b) Schrödinger in terms of "wave mechanics", and (c) Dirac in terms of "transformation theory". Very early it was clear that these are just different mathematical expressions of the (so far one and only) full modern quantum theory. Even the idea of Jordan's not only to "quantize" the mechanics of particles but also the electromagnetic field (an idea that had to be rediscovered a few years later by Dirac since it was abandoned first as "overdoing the quantum revolution somwhat") turned out to be necessary to get the correct kinetic explanation of the Planck Law a la Einstein (1917) with the necessity to have not only absorption and induced emission but also spontaneous emission of "light quanta" also within the new theory, and up to today nobody has come with anything better.

Then, indeed there was a unique new issue, namely that of "interpretation", and this was solved (at least in my and that of most physicists' opinion solved) also very early by Born in a footnote in his paper describing the important wave-mechanical treatment of particle scattering: Schrödinger's wave function had to be interpreted probabilistically, ie.., not as a classical field describing a single electron, but as "probability amplitude" for finding the electron at a given position.

The theory thus turned to be perfectly causal, i.e., the quantum states, described by wave functions (and more generally by statstical operators) evolve according to a causal law (e.g., as given by the Schrödinger equation for the wave function), but the meaning of this state description is completely probabilistic, implying that observables like position and momentum (and other observables like energy and angular momentum) were in general not determined but only probabilities could be predicted which value will bemeasured given a state in terms of some preparation procedure, determining the wave function at some initial time (which implies how it has to look at a later time by solving the Schrödinger equation).

In my opinion, after all the decades of hard tests of this conjecture of "irreducible randomness" in the behavior of nature, including some of the most "weird-looking" implications (entanglement), it looks as if nature is indeed "random/indeterministic" on a fundamental level.

 

[vanhees71]

Not only that. Each type of experimenter yield a different reading. The photon that spectroscopy experimenter uses to explain how spectra are connected to the atoms and molecules is a different concept from the photon quantum optics experimenters talk about when explaining their experiments. Those are different from the photon that the high energy experimenters talk about and there are still other photons the high energy theorists talk about. There are probably even more variants (and countless personal modifications) in use. Definition really varies from any setup and how it reacts to different setup. Even in HEP experimenter's concept behind the readings is - a particle that cannot be observed directly, but is something having position, energy and momentum that helps explain interactions of charged particles among themselves and their behavior in external EM field (Compton's effect, pair creation). That the reason some think that 'maybe' the picture/detention is a phenomenon or mirage of a natural dynamics of interacting things.

This is not true at all. A photon is a photon, and it's described by relativistic QFT (applied to QED of course). There's no difference in the notion of a single photon (a one-photon Fock state) between HEP and quantum-optics physicists. Only the emphasis of the theoretical treatment is a bit different, but at the end the measurments are pretty much the same: A photon is registered in the one or other kind of macroscopic detector, be it a CCD cam of your smartphone or some em. calorimeter in one of the big experiments at the LHC.

 

[lucas_]

Well, the strength of science tells us first to be open to learn how nature behaves, and that in investigating this with using an interplay of quantiative reproducible observations and experiments and analytical and mathematical reasoning we may find that we have to give up prejudices about what we think to know. Nothing in this process is save against the possibility not to be in need for revision with new discoveries.

This happened indeed twice in the first half of the 20th century: One revision was necessary with the discovery that the electromagnetic phenomena are accurately described by Maxwell's theory of the electromagnetic field, but this theory was inconsistent with the "very sure knowledge" about the mathematical model describing spacetime (or at this time the strictly separated time and space) in terms of the Galilei-Newton model. This was such a "sacrosanct" piece of knowledge that it took about 50 years to resolve the issue finally and it involved the work of some of the most brillant mathematicians and physicists of their time (Poincare, Lorentz, Michelson, and finally Einstein): What had to be revised was not the idea of the special principle of relativity, i.e., the invariance of the physical laws under transformations from one inertial reference frame to another, but the very law of how the transformation had to be done, i.e., instead of Galileo transformations the Lorentz transformations (discovered in some predecessor form already by Voigt in the late 1880ies), implying a new spacetime model with space and time now no longer strictly separated but amalgamated into a pseudo-Euclidean affine spacetime manifold. This implies that also the laws of mechanics had to undergo a revision, and the corresponding revisions by Einstein (and corrections thereof by Planck, von Laue, and Einstein himself thereof) were after some experimental confusion resolved by experiments with the then also newly discovered electrons in gas-discharge tubes.

This turned out, however, to be a pretty harmless revision. Nothing of the considered really fundamental ideas, which you still more than 100 years later insist on, had to be revised. The physical laws still were strictly deterministic, i.e., any possible observable of any system by tacit assumption has always a determined value, and the principle of causality on the fundamental level holds in a very strong (time-local) form: Known the initial values of a complete set of observables (which simply is a set of observables all other observables are functions of) at some initial time $t_0$ their values in principle (and thus that of all possible observables, which are functions thereof) are determined at any later time $t$.

Now this apparently save knowledge had to be revised with the advent of problems concerning phenomena becoming observable with the ever faster progress of technology. First these obstacles were considered minor issues. When Planck asked for advice what to study, a famous physics professor at the university told him, with his brillant grades from high school he shouldn't waste his time with physics, because this is all settled and the "little clouds" on the horizon would be simply resolved by measuring some fundamental constants of ever better precision and small revisions of the laws of classical physics (at the time consisting of classical (still Newtonian) mechanics, Maxwell electrodynamics, and (phenomenological) thermodynamics).

One of the clouds was not so new to begin with: It was the question of the absolute value of entropy and a theoretical understanding of what's now known as the Gibbs phenomenon in statistical physics, which however was already met with quite some skepticism by the more conservative physicists of the time, since they didn't even believe in the existence of the atomistic structure of matter to begin with. With the advent of low-temperature physics (with one milestone being Kammerlingh-Onnes achievement of liquifying Helium in the early 1900s) the issue became very evident: The specific heat of most substances did not behave as expected at low temperatures. Also when it became clear that metals had conduction electrons, but these didn't contribute to the specific heat even at room temperatures, another "cloud" arised at the horizon.

Then there was the problem of "black-body radiation", which was not describable with classical electrodynamics and thermodynamics/statistical physics. Famously this was how the quantum revolution started: When Rubens and Kurlbaum accurately measured the black-body radiation spectrum as a function of temperature, which was first an attempt to provide a better standard for measuring the efficiency of lightning sources (gas and the new electric light bulbs), lead Planck to guess the right law and subsequently with a brillant (but first not really understood) statistical analysis which only worked when assuming that each frequency mode of the electromagnetic field could exchange energy only in lumps of $h \nu=\hbar \omega$. Already this was too much for Planck, but he couldn't find any other way to derive the accurate black-body law, named after him.

The rest of the story is well known. Einstein came with his light-quanta idea in 1905, then Bohr (completed by Sommerfeld) with his ad-hoc idea to explain the hydrogen spectrum from the atomic model enforced on the physics community by Rutherford's findings. Then the very stability of matter, the precise indistinguishability of atoms of each kind has become an enigma for classical physics. Then the ad hoc solution for hydrogen was found to be flawed, because it only worked for hydrogen. Another happy incident has it that it also works for the harmonic oscillator and thus lattice vibrations of solids, which resolved the problem with specific heat but not yet the question, why the conduction electrons in metals do not provide anything to the specific heat, while the model to describe the conduction electrons as a quasi free gas moving in a positively charged background worked very well in explaining Wiedemann-Franz's law of propotionality of electric and heat conductivity.

The up to day "final" resolution was modern quantum mechanics, discovered more or less independently in parallel no less than 3 times by (a) Heisenberg, Born, and Jordan (with some important help by Pauli) in terms of "matrix mechanics", (b) Schrödinger in terms of "wave mechanics", and (c) Dirac in terms of "transformation theory". Very early it was clear that these are just different mathematical expressions of the (so far one and only) full modern quantum theory. Even the idea of Jordan's not only to "quantize" the mechanics of particles but also the electromagnetic field (an idea that had to be rediscovered a few years later by Dirac since it was abandoned first as "overdoing the quantum revolution somwhat") turned out to be necessary to get the correct kinetic explanation of the Planck Law a la Einstein (1917) with the necessity to have not only absorption and induced emission but also spontaneous emission of "light quanta" also within the new theory, and up to today nobody has come with anything better.

Then, indeed there was a unique new issue, namely that of "interpretation", and this was solved (at least in my and that of most physicists' opinion solved) also very early by Born in a footnote in his paper describing the important wave-mechanical treatment of particle scattering: Schrödinger's wave function had to be interpreted probabilistically, ie.., not as a classical field describing a single electron, but as "probability amplitude" for finding the electron at a given position.

The theory thus turned to be perfectly causal, i.e., the quantum states, described by wave functions (and more generally by statstical operators) evolve according to a causal law (e.g., as given by the Schrödinger equation for the wave function), but the meaning of this state description is completely probabilistic, implying that observables like position and momentum (and other observables like energy and angular momentum) were in general not determined but only probabilities could be predicted which value will bemeasured given a state in terms of some preparation procedure, determining the wave function at some initial time (which implies how it has to look at a later time by solving the Schrödinger equation).

In my opinion, after all the decades of hard tests of this conjecture of "irreducible randomness" in the behavior of nature, including some of the most "weird-looking" implications (entanglement), it looks as if nature is indeed "random/indeterministic" on a fundamental level.

vanheez71, did you write all of the above today and spontaneously?

I think you can be a good chronicler or blogger of the next revolution in physics. We are like in 1899 now before the Planck started the quantum revolution. It's deja vu all over again.

 

[willielandon Refere]

Well... the task you set out in the question details, that is, to define quantum theory "as pertaining to appreciation by the senses", is quite impossible.

You see, quantum theory describes precisely what matter does when it is notbeing sensed (by a human or any other classical instrument). That, really, is the essence of quantum theory: that systems, when they do not interact with classical entities, are in states that have no classical equivalent.

The moment you attempt to make sense of a quantum system using the intuition of classical senses, it ceases to be a quantum system. So while I believe it is possible to develop an intuition for quantum physics, this intuition necessary has to be abstract, not relying on concepts related to our senses.

 

[DarMM]

In my opinion, after all the decades of hard tests of this conjecture of "irreducible randomness" in the behavior of nature, including some of the most "weird-looking" implications (entanglement), it looks as if nature is indeed "random/indeterministic" on a fundamental level

I think we need to add two elements to this as fundamental randomness alone would be satisfied by a normal stochastic process. We have to add incompatibility, i.e. the uncertainty principle. And also the requirement of macro devices as the quantum formalism does not give a probability for a photon to develop a certain spin component say without a classical device measuring it.

 

[vanhees71]

vanheez71, did you write all of the above today and spontaneously?

I think you can be a good chronicler or blogger of the next revolution in physics. We are like in 1899 now before the Planck started the quantum revolution. It's deja vu all over again.

Yes, I wrote this just spontaneously. That's why for sure it's far from being accurate, but that's the beauty of forums like this. You can just exchange some ideas :-)).

 

[vanhees71]

I think we need to add two elements to this as fundamental randomness alone would be satisfied by a normal stochastic process. We have to add incompatibility, i.e. the uncertainty principle. And also the requirement of macro devices as the quantum formalism does not give a probability for a photon to develop a certain spin component say without a classical device measuring it.

But all this IS what's described by QT. It's a kind of probability theory adapted to the real world, discovered by the scientific method of observation and mathematical modeling. I don't know what you mean by a photon is developing a certain spin. I guess you mean how to get specific polarization states? That's not so difficult, as far as photons in the range of visible light is concerned: Just take the well-known optical devices like polaroid foil to get linearly polarized light to get linearly polarized photons and then devices like quarter-wave plates etc. to create any polarization state.

 

[TeethWhitener]

No, I imply that all detections are macroscopic. But the converse is not true, some macro objects may not be detections.

So then do you agree with @atyy ’s definition of micro/macro and the notion that a classical microscopic theory is impossible?

 

[julcab12]

This is not true at all. A photon is a photon, and it's described by relativistic QFT (applied to QED of course). There's no difference in the notion of a single photon (a one-photon Fock state) between HEP and quantum-optics physicists. Only the emphasis of the theoretical treatment is a bit different, but at the end the measurments are pretty much the same: A photon is registered in the one or other kind of macroscopic detector, be it a CCD cam of your smartphone or some em. calorimeter in one of the big experiments at the LHC.

Of course a photon is a photon. For instance, QFT - 'thing' from the mode expansion of free fields, free relativistic field that fulfills the Klein-Gordon equation, a Fourier transform. The word Quantum--Photon in quantum theory-- colloquial and technical. My point is that, there is no ontological cut with that "thing". All the modern variants allow for creation and destruction. We can talk of the same thing but registered different readings with each experimental setup. It's not about lack of consistency here. We can register position and delocalization on a normal basis but to HE experimenter can only read tracks and scattering events.

 

[DarMM]

But all this IS what's described by QT

Of course. I wasn't saying QT didn't describe it, just that fundamental randomness alone doesn't characterise QT. A fundamental classical stochastic process would also be random, so we need to mention extra features to get QM.

I don't know what you mean by a photon is developing a certain spin

Typo, I meant polarization. I know the details of the devices needed, the point is that the device fundamentally is needed unlike a classical stochastic theory.

 

[Jimster41]

So then do you agree with @atyy ’s definition of micro/macro and the notion that a classical microscopic theory is impossible?

I for one would love to see an insights article on Kocken-Specker. I hadn't realized it was so associated with Bell. It feels very Godelian to me... so I'm very curious to have an intuitive sense of some kind as to what it is saying.

 

[DarMM]

I for one would love to see an insights article on Kocken-Specker. I hadn't realized it was so associated with Bell. It feels very Godelian to me... so I'm very curious to have an intuitive sense of some kind as to what it is saying.

The proof for a quantum system with four degrees of freedom is very simple. It basically shows that you can't consider the values of quantum observables to be determined unless you also accept that they are contextual. Contextual meaning the value of an observable depends on what other observables it is measured with.

 

[Alex Torres]

Summary: If quantum theory is nothing but a set or rules to compute the probabilities of macroscopic measurement outcomes, then what is microscopic about it?

since all measurement outcomes are macroscopic events,

...in a double slit experiment the computer zooms in the microscopic spot where the photon is absorbed and computes the probability of hitting that spot, if this is a measurement outcome then it must arise from an specific event (the photon hitting a microscopic spot in the screen) IMO the nature of the events defines the nature of the theory along with any mathematical construct supporting it...i think

 

[haushofer]

To me, the relevant question and thecessence of Copenhagen) is: is the demand of a realistic interpretation, i.e. an "ontology", just a "classical" artefact of our thinking and ill-defined at subatomic lengthscales, or the key to understanding QM better?

 

[Demystifier]

Atoms really do exist !

Perhaps, but the minimal instrumental view of QM says nothing about that.

 

[Demystifier]

If that is the case then QT does tell us something about the microscopic world and the philosophical doubts are proved meaningless.

Perhaps QT does tell us that, but MQT (M is for minimal) doesn't.

 

[Demystifier]

what hope is there?

To go beyond the minimal. More precisely, to adopt some ontic interpretation of QM.

 

[Demystifier]

I mean the "feature" of probabilistic events. Why should nature not behave probabilistically on a fundamental level? I think the main quibbles of philosophers and still even some scientists with QT is the fact that it's indeterministic, i.e., that there is probabilistic/statistical behavior on the fundamental level, i.e., not due to some incomplete knowledge as within the realm of classical theory.

No, the issue of determinism is secondary in most quantum philosophy quibbles. The primary issue is the ontology. The things which are there even if nobody observes it. Minimal QM says almost nothing about ontology, especially about microscopic ontology, and that's what many philosophers (and a substantial number of scientists) find disturbing.

 

[Demystifier]

But when one is counting single photons ... these seems like fundamentally microscopic events

The fact is that you are counting detector clicks. Whether those clicks correspond to single photons, well, that's an miscroscopic interpretation of your macroscopic events. And I'm not saying that such an interpretation is wrong, I am saying that such an interpretation requires going beyond the minimal instrumental view of QM. In effect, you are dealing with a quantum interpretation even if you don't want to. A physicist cannot really avoid dealing with quantum interpretations.

 

[vanhees71]

Of course the things are there if nobody observes them because there are fundamental conservation laws. Read Kant, who clearly defined "substance" as something "persistent", and nothing has changed on that with quantum theory.

Of course, whether or not a photon is still there is just a question whether or not it was absorbed by something from the last observation, but where is the problem? Also the classical electromagnetic field excitations get absorbed all the time. The only things that are persistent are energy, momentum, and angular momentum, which are transferred to the matter the em. field is absorbed from.

The more I listen to these philosophical debates about apparent problems of QT and it's "ontology", the less I understand them. I come more and more to the conclusion that those people who have such problems just cannot accept that nature behaves in another way than thought based on our everyday experience with "classical phenomena", which is however an apparent phenomenon due to a much coarse grained observation of the relevant macroscopic degrees of freedom.

 

[Demystifier]

So then do you agree with @atyy ’s definition of micro/macro and the notion that a classical microscopic theory is impossible?

I agree that classical microscopic theory is impossible. Bohmian mechanics, for instance, is not classical.

 

[ftr]

Well, the strength of science tells us first to be open to learn how nature behaves, ...

you do have a nag for writing nice summaries, maybe a history book.
However, I am sure you know about all the controversies in physics whether in SR, GR, QM, QFT, cosmology ...etc
As I have said many times I think a lot of progress have been achieved, but obviously no coherent picture is there.

 

[Demystifier]

Of course the things are there if nobody observes them because there are fundamental conservation laws.

Conservation laws are not enough. For example, from conservation laws alone you cannot deduce that the Moon is there when nobody observes it. As far as conservation laws are concerned, the Moon could spontaneously turn into a gigantic pink elephant of the same energy-momentum as that of the Moon, whenever it is no longer observed.

 

[vanhees71]

Hm, well, I'm sure if there were no conservation laws forbidding it, this would already have happened ;-)).

 

[Dr. Courtney]

The fact is that you are counting detector clicks. Whether those clicks correspond to single photons, well, that's an miscroscopic interpretation of your macroscopic events. And I'm not saying that such an interpretation is wrong, I am saying that such an interpretation requires going beyond the minimal instrumental view of QM. In effect, you are dealing with a quantum interpretation even if you don't want to. A physicist cannot really avoid dealing with quantum interpretations.

Fair enough. But by this definition of microscopic, nothing is microscopic at the experimental level. Everything humans sense is a microscopic interpretation of macroscopic events. So no theory in science is microscopic by this definition, because humans do not directly observe microscopic events. We only have microscopic interpretations of macroscopic events.

 

[Demystifier]

Fair enough. But by this definition of microscopic, nothing is microscopic at the experimental level. Everything humans sense is a microscopic interpretation of macroscopic events. So no theory in science is microscopic by this definition, because humans do not directly observe microscopic events. We only have microscopic interpretations of macroscopic events.

I agree. But the minimal instrumental version of QM tries to deny it.

 

[DarMM]

Hm, well, I'm sure if there were no conservation laws forbidding it, this would already have happened ;-)).

Conservation of elephants forbids it.

 

[DarMM]

Fair enough. But by this definition of microscopic, nothing is microscopic at the experimental level. Everything humans sense is a microscopic interpretation of macroscopic events. So no theory in science is microscopic by this definition, because humans do not directly observe microscopic events. We only have microscopic interpretations of macroscopic events.

It's a little more difficult than that.

To simplify in the standard quantum formalism a beam of light say could be measured in the photon basis or the field basis. The photon basis would correspond to a machine that clicks like you mentioned, however results for devices measuring in the field basis seem to contradict those from the photon basis. It seems difficult to piece them together as being the result of some underlying picture. You can only consider the beam of light to consist of photons if you measure it in the photon basis.

Thus in the standard reading the macroscopic device can't be detached from your description. There are photons because that is what you are measuring, not because there are photons around when your device is absent.

 

[atyy]

I agree. But the minimal instrumental version of QM tries to deny it.

Not necessarily deny. It leaves it as an open problem.

 

[vanhees71]

The problem here is that you formulate the things in too abstract a way. You cannot simply say, "I measure a beam of light in the photon basis or the field basis". I've no clue what you mean. So I have to guess: The "photon basis" may be the Fock basis, i.e., states of the em. field with a defined total number of photons. I'm a bit at lost how to realize such a measurement. Do you know of any real-world device that measures only photons if they are a prepared in a photon-number eigenstate? I've no clue, how to construct such a device with real-world materials. Also what do you mean by "field basis"? Are these coherent states?

There's a well-developed subfield of relativstic QFT called quantum optics, which clearly defines, what's observed in experiments. They have all kinds of measurement devices. Most of them are based on the photo effect, i.e., an electromagnetic wave (no matter in which state it is prepared, be it a Fock state (not so easy to do, but standard today with parametric down conversion, or a coherent/squeezed state (lasers are your friend), is interacting with electrons which undergo a transition from a bound to a continuum state, and this signal is amplified to make these photoelectrons countable. In this way you can measure correlation functions of the electromagnetic (in the usually sufficient lowest-order dipole approximation the electric) field.

 

[Dr. Courtney]

I agree. But the minimal instrumental version of QM tries to deny it.

I suspect the "denial that every theory is macroscopic" is not unique to the instrumental version of QM.

How would biologists view assertions that cell theory and the germ theory of disease are not really microscopic theories but "microscopic interpretations of macroscopic events"? How would chemists view assertions that atomic theory (from Dalton) and kinetic theory are not really microscopic theories but "microscopic interpretations of macroscopic events"? I expect they can follow the logic, but this is not really how "microscopic" is used in other fields of science.

One could construct a similar logic regarding scientific reconstructions of past events since these are not observed directly but are inferred from modern observations. "The Big Bang theory is not properly a theory of origins, but it is a historical interpretation of modern events." In the same way that one cannot separate microscopic interpretations from the macroscopic things humans actually observe, one cannot separate historical interpretations from the modern events that humans actually observe.

 

[atyy]

I suspect the "denial that every theory is macroscopic" is not unique to the instrumental version of QM.

How would biologists view assertions that cell theory and the germ theory of disease are not really microscopic theories but "microscopic interpretations of macroscopic events"? How would chemists view assertions that atomic theory (from Dalton) and kinetic theory are not really microscopic theories but "microscopic interpretations of macroscopic events"? I expect they can follow the logic, but this is not really how "microscopic" is used in other fields of science.

One could construct a similar logic regarding scientific reconstructions of past events since these are not observed directly but are inferred from modern observations. "The Big Bang theory is not properly a theory of origins, but it is a historical interpretation of modern events." In the same way that one cannot separate microscopic interpretations from the macroscopic things humans actually observe, one cannot separate historical interpretations from the modern events that humans actually observe.

The problem is we would like a theory of reality, but quantum mechanics (as we understand it) is simply not such a theory. Of course it is absurd to say that germs don't really exist or that the measuring apparatus is not made of electrons - but the formalism does not grant us the ability to say these things - it is an open problem of quantum mechanics.

 

[Demystifier]

Conservation of elephants forbids it.

Can this conservation law be derived from a symmetry? Which symmetry?

 

[DarMM]

The problem here is that you formulate the things in too abstract a way

It's the old problem of complimentary between results in different basis. I could make things more precise, but it doesn't really change the point. Why would the details matter? You'll still get complimentary where the statistics in different bases can't be considered as marginals of one set of properties.
It was just an attempt to say why complimentarity makes viewing things in a way that's detached from your device is difficult, it's not intended as a completely accurate rendering of quantum optics.

 

[DarMM]

Can this conservation law be derived from a symmetry? Which symmetry?

Trunkal translations.

 

[Demystifier]

Trunkal translations.

Reference please!

 

[Demystifier]

I suspect the "denial that every theory is macroscopic" is not unique to the instrumental version of QM.

How would biologists view assertions that cell theory and the germ theory of disease are not really microscopic theories but "microscopic interpretations of macroscopic events"? How would chemists view assertions that atomic theory (from Dalton) and kinetic theory are not really microscopic theories but "microscopic interpretations of macroscopic events"? I expect they can follow the logic, but this is not really how "microscopic" is used in other fields of science.

One could construct a similar logic regarding scientific reconstructions of past events since these are not observed directly but are inferred from modern observations. "The Big Bang theory is not properly a theory of origins, but it is a historical interpretation of modern events." In the same way that one cannot separate microscopic interpretations from the macroscopic things humans actually observe, one cannot separate historical interpretations from the modern events that humans actually observe.

In principle one can reason like that in any science, but in reality such reasoning can only be found in quantum foundations. Why is quantum theory different? I think the main reason is that some physicists like to think quantum theory is very close to the final fundamental theory of everything, so they don't like to think that there is some hidden reality on which quantum theory has nothing to say. Instead, they prefer to think that there is no any hidden reality at all. All Copenhagen-like interpretations of QM are nothing but attempts to justify the ideology that QM must somehow be complete.

 

[vanhees71]

It's the old problem of complimentary between results in different basis. I could make things more precise, but it doesn't really change the point. Why would the details matter? You'll still get complimentary where the statistics in different bases can't be considered as marginals of one set of properties.
It was just an attempt to say why complimentarity makes viewing things in a way that's detached from your device is difficult, it's not intended as a completely accurate rendering of quantum optics.

I'm not saying you should make things more precise, but you should make a statement about what you mean by "measuring" in the one or the other basis. Particularly to understand complementarity (one of Bohr's enigmatic terms) right, you need to think about many concrete experiments, i.e., descriptions of real-world measurement devices applied to concrete real-world preparations of measured objects. I don't know of a single example, where this has not resolved apparent quibbles with overly abstract formulations like "measuring in a basis".

Quantum optics is just the example, where the real-world descriptions of measurement and preparation processes become most transparent, because they are usually not that complicated after all. You need just a good grasp of classical optics. To understand all kinds of devices like lenses, polarizers, beam splitters even linear electrodynamics as taught in classical electrodynamics is sufficient. For parametric down conversion and preparation of proper Fock states also some nonlinear optics is useful. The quantization part is also not too hard. Usually you just need a good understanding of the quantization of the free electromagnetic field and some perturbation theory for the interactions with matter. The latter almost always can be described with non-relativistical QM.

 

[DarMM]

I'm not saying you should make things more precise, but you should make a statement about what you mean by "measuring" in the one or the other basis. Particularly to understand complementarity (one of Bohr's enigmatic terms) right, you need to think about many concrete experiments, i.e., descriptions of real-world measurement devices applied to concrete real-world preparations of measured objects. I don't know of a single example, where this has not resolved apparent quibbles with overly abstract formulations like "measuring in a basis".

A good understanding of how concrete experiments work resolves the conceptual difficulties with complementarity? Even if I specified the exact devices and what they measure it wouldn't do this. I can't imagine how it would. It would still reduce to the fact that the statistics for two different measurements don't seem to be marginals of a third more complete measurement as is always the case in classical mechanics.

For example an $x$ measurement and a $p$ measurement on a single particle cannot in general be considered as a coarse graining of some $(x,p)$ measurement, preventing you from considering them as measurements of properties already present.