- #211
ftr
- 624
- 47
A. Neumaier said:No, you asserted that:
Yes I should have said "isn't that ..."
A. Neumaier said:No, you asserted that:
A. Neumaier said:But even asking questions above your level of grasping things is unproductive.
The point is that while your understanding is limited you should assume that the answers you get from better informed people is reasonable, and adapt your mental picture rather than spitting out your momentary thoughts. Usually you need to do in parallel some background reading that let's you understand why the answer is meaningful. You cannot get an understanding of quantum field theory from just taking part in dialogues...ftr said:Theoretically that is true, however in practice it is very hard to know exactly how much, so it is hit and miss( I do stay away from thing I don't understand fully).
Well, physics (and all the other sciences) are so well funded because of this very pragmatic approach. It describes what's observed via real-world devices and it's not primary about some fundamental philosophical questions about fictions. That's the reason why the outcome of the fundamental sciences can be successfully used "to control the world" in the sense of applied sciences like engineering. That's what made the semiconductor revolution possible and that's why we can sit in front of a little box and discuss very efficiently in this forum. Quantum theory is well-enough understood to describe semiconductors to create transistors and ICs.A. Neumaier said:But not primarily. if this were the true goal of physics, work in physics would not be funded, and would not be of interest to society. Physics is about understanding the properties of matter and radiation to an extent that it can be used for understanding and controlling the world at large. Observations and measurements are tools to ensure that the models we form to do this are indeed adequate.
No. Only knowing the detailed state is impossible. But its existence - and an interpretation of what it means - must be assumed even to apply the methods of statistical mechanics:Thus unless quantum mechanics has intrinsic limitations it must be possible - as in Laplace's classical clockwork universe - to phrase quantum mechanics such that it applies to (and models) everything in the universe, no matter how big or complex it is.
Because it is needed nowhere in classical physics, although there the same limitations you mention apply. Having measurement in the foundations makes the foundations depend on human activities. But physical laws must also apply for physical systems never measured by humans, like distant galaxies of which we only measure very little light, or the early stages of the solar system, of which we can measure nothing but only infer information by assuming the validity of physical laws.
Let's stick to the single-photon example, because it's well-defined and perfect to make the point we discuss. I still think "the state of the entire universe" is an empty phrase, and it's hard to discuss about such fictions on a sound basis.A. Neumaier said:It is not a fiction but simply something partially unknown, as anything in physics that has more than a few discrete possible values.
According to your reasoning, the state of a single photon (a general uncharged 1-particle state of QED) is also in principle unobservable, since even when assumed pure (in reality it is never pure) the state is parameterized by the solutions of the free Maxwell equations, and one can measure or prepare only a crude approximation of it.
Fact is that most of what physics models theoretically is unobservable in this sense. But we nevertheless assume that these unobservable states actually exist since only then we can talk about approximating them with the things we use to approach our knowledge about them.
The state of the universe is approachable similarly. First of all, it has a huge algebra of q-observables localized on Earth and hence susceptible to observation. Each of these observations reveals something about the state of the universe. All this combined gives us quite a good (though quite coarse) knowledge about the universe, not only on Earth but even far away - where we can infer what happens because we assume (and find that we can assume consistently) that the q-observables of the universe that refer to other regions of the universe follow the same laws as we know them from Earth. We can then use the maximum entropy principle to get an approximate state of the universe based on the q-expectations we believe to know (primarily smeared values of various effective fields) and get an approximate hydrodynamic 1PI description of the state of the universe. In this approximation it looks essentially classical except very close to the big bang and hence can be (and is) described by classical physics.
Nowhere any fiction, everywhere only the usual approximations we know from the study of all real physical systems.
A discussion of the single photon (which I gave in Subsections 3.4 and 3.5 of Part III) does not make at all the point I am trying to get across.vanhees71 said:Let's stick to the single-photon example, because it's well-defined and perfect to make the point we discuss. I still think "the state of the entire universe" is an empty phrase, and it's hard to discuss about such fictions on a sound basis.
We know of these problems (thoroughly discussed, e.g., in the book by Calcagni, Classical and quantum cosmology, Springer 2017) only because of cosmological models - for the observation of tiny systems everything is already consistent with treating gravitation as an external classical potential and ignore dark matter and dark energy. These cosmological models use at present primarily semiclassical approximations, but there can be no doubt that more accurate models should be fully quantum. Seevanhees71 said:The real problems are [...] the open unsolved questions of contemporary physics, which are
- a consistent quantum description of the gravitational interaction [...]
- the nature of what's dubbed "Dark Energy" and "Dark Matter"
See also Hartle's The quantum mechanics of cosmology.Martin Bojowald said:Quantum cosmology is based on the idea that quantum physics should apply to anything in nature, including the whole universe.
Unlike the Copenhagen or statistical interpretation, the thermal interpretation provides a sound basis for their discussion, without the weird features of the many-worlds interpretation that needs to be invoked by Calcagni at several places (pp.188,197,391) and (in a many-histories variation called post-Everett) by Hartle (pp.5,17,89).James Hartle said:It is an inescapable inference from the physics of the last sixty years that we live in a quantum mechanical universe — a world in which the basic laws of physics conform to that framework for prediction we call quantum mechanics. If this inference is correct, then there must be a description of the universe as a whole and everything in it in quantum mechanical terms. The nature of this description and its observable consequences are the subject of quantum cosmology.[...]
The “Copenhagen” frameworks for quantum mechanics [...] are inadequate for quantum cosmology [...] these formulations characteristically assumed a possible division of the world into “observer” and “observed”, assumed that “measurements” are the primary focus of scientific statements and, in effect, posited the existence of an external “classical domain”. [...]
Measurements and observers cannot be fundamental notions in a theory that seeks to describe the early universe when neither existed.
A. Neumaier said:The point is that while your understanding is limited you should assume that the answers you get from better informed people is reasonable, and adapt your mental picture rather than spitting out your momentary thoughts. Usually you need to do in parallel some background reading that let's you understand why the answer is meaningful. You cannot get an understanding of quantum field theory from just taking part in dialogues...
Until you realize that - as the thermal interpretation asserts - the same holds for everything in quantum physics, including all q-expectations, not only the ones in 2PI calculations. Comparison with experiment (and hence the empirically testable meaning) concerns only a minute fraction of the q-expectations manipulated, primarily:vanhees71 said:How often should I emphasize that Green's functions are not directly describing observable facts but are calculational tools enabling one to calculate them?
They are the stuff that is manipulated by theory, until one ends up with one of the quantities in the above three groups and interprets them in the way appropriate for these groups.vanhees71 said:So what are your "q-expectations" if it is not allowed to use the very intuitive probabilistic interpretation for the outcome of measurement?
Well, already Laplace's demon was a pure fiction, even taken only the then known "universe". In the lab there are no "density operators on some universal Hilbert space" but real-world devices (in HEP something like a silicon chip, calorimeters, RICH detectors etc. etc. with some read-out electronics). It's the relation of the "density operators on some universal Hilbert space" with the corresponding observations with these real-world devices which makes an interpretation. You might say the minimal statistical interpretation is unsatisfactory for the one or other philosophical reason, but at least it gives a clear concept to make this relation between real-world measurement devices and the formalism.A. Neumaier said:A discussion of the single photon (which I gave in Subsections 3.4 and 3.5 of Part III) does not make at all the point I am trying to get across.
The state of the universe is indeed a fiction in the statistical interpretation (and in the Copenhagen interpretation). But in the thermal interpretation, the state of the universe is no less a fiction than it was in Laplace's classical theory ("Ie vrai systeme du monde"). It is no longer an empty phrase but a density operator on some universal Hilbert space. This universal Hilbert space carries a representation of quantum fields - those of the standard model plus gravity, in whatever form it will be made definite when we know how to model it correctly. This state contains all the information about anything we can observe in the universe, and hence it contains all of physics. It is the state modeled in some coarse-grained approximation by cosmologists.
It is the very basis upon which one must discuss what you call the real problems of quantum physics:
We know of these problems (thoroughly discussed, e.g., in the book by Calcagni, Classical and quantum cosmology, Springer 2017) only because of cosmological models - for the observation of tiny systems everything is already consistent with treating gravitation as an external classical potential and ignore dark matter and dark energy. These cosmological models use at present primarily semiclassical approximations, but there can be no doubt that more accurate models should be fully quantum. See
M. Bojowald, Quantum cosmology: a review, Rep. Prog. Phys. 78 (2015), 023901.
Unlike the statistical interpretation, the thermal interpretation provides a sound basis for their discussion, without the weird features of the many-worlds interpretation that needs to be invoked by Calcagni at several places (p.188,197,391).
For me all the three bullets are clearly explained by the standard probabilistic interpretation of QT. Bullet 1 is indeed what comes closest to a "Gibbs-ensemble view". Bullet 2, spectroscopy (spectral lines and widths) are also generic averages/integrations of many "elementary photon-detection events". The same holds for bullet 3, where the detector response already provides enough coarse-graining/integrating/averaging over many microscopic degrees of freedom to lead to a macroscopic observable. It's not that we have to resolve all these single microscopic events, but the device is directly providing the relevant coarse-grained observables.A. Neumaier said:Until you realize that - as the thermal interpretation asserts - the same holds for everything in quantum physics, including all q-expectations, not only the ones in 2PI calculations. Comparison with experiment (and hence the empirically testable meaning) concerns only a minute fraction of the q-expectations manipulated, primarily:
Thus the statistical interpretation is secondary, and restricted to experiments of the first group.
- those that describe cross sections or impact rates, which are experimentally determined by statistics over many events,
- those that describe spectroscopic information (spectral lines and widths), which are experimentally determined by nonstatistical brightness measurements, and
- those that describe smeared field expectations and linear or nonlinear response functions to small external stimuli, which are experimentally determined by a few measurements, not involving any statistics, too.
They are the stuff that is manipulated by theory, until one ends up with one of the quantities in the above three groups and interprets them in the way appropriate for these groups.
It is classical; so you seem to advocate that physics at large-scales is necessarily classical. But at which size to draw the borderline? You have the standard Heisenberg cut of the Copenhagen interpretation, and move it where you think current observational limits allow it to be placed without an observable contradiction. This may be enough for the practitioner...vanhees71 said:Let's rather look at cosmology which we really have today.
Yes. Which microscopic events? You cannot even point to them at the theoretical level, except for ideal gases, let alone measure them experimentally. Therefore - by your stated standards that what cannot be measured is fiction - these events are pure fiction, at least as much as the state of the universe. Only the q-expectation of the coarse-grained field whose q-expectation is used for comparison has an experimental (nonstatistical) meaning.vanhees71 said:It's not that we have to resolve all these single microscopic events, but the device is directly providing the relevant coarse-grained observables.
Because you allow yourself extreme liberties in what to call measurement, and don't care to be precise in your arguments. Thus for you, everything is in full order, the philosophical quibbles (which arise when looking at the details) are irrelevant, and you have no empathy for all those who have higher standards of consistency.vanhees71 said:For me all the three bullets are clearly explained by the standard probabilistic interpretation of QT
You cannot point to his age of 80 when publishing the book, for the whole book is written in an excellent style showing no signs of senility.Steven Weinberg said:There is nothing absurd or inconsistent about [...] the general idea that the state vector serves only as a predictor of probabilities, not as a complete description of a physical system. Nevertheless, it would be disappointing if we had to give up the “realist” goal of finding complete descriptions of physical systems, and of using this description to derive the Born rule, rather than just assuming it. We can live with the idea that the state of a physical system is described by a vector in Hilbert space rather than by numerical values of the positions and momenta of all the particles in the system, but it is hard to live with no description of physical states at all, only an algorithm for calculating probabilities. My own conclusion (not universally shared) is that today there is no interpretation of quantum mechanics that does not have serious flaws
A. Neumaier said:content to interpret the final q-expectations, as the slightly uncertain values measured by some experiment.
It would be close to the nucleus, not significantly further away than the atom radius.ftr said:What do you mean by slightly? are you saying that measuring the electron position in hydrogen atom will always give you a very close value to the expectation.
No. Superpositions are pure states, hence generally idealizations, except when only very few degrees of freedom are involved. Thus they don't deserve the attention they traditionally get. Most states encountered in Nature are mixed states, and are treated as such in real experiments.ftr said:Also, you seem to deny superposition in general, is that correct?
A. Neumaier said:not significantly further
Not extremely. In any case, it would be very difficult to measure, and whatever measurement is made, it would by definition have at least this uncertainty.ftr said:Then would you say that a distance of two Bohr radius is extremely unlikely.
Obviously QT is too difficult for us to come to a common understanding :-(.A. Neumaier said:Yes. Which microscopic events? You cannot even point to them at the theoretical level, except for ideal gases, let alone measure them experimentally. Therefore - by your stated standards that what cannot be measured is fiction - these events are pure fiction, at least as much as the state of the universe. Only the q-expectation of the coarse-grained field whose q-expectation is used for comparison has an experimental (nonstatistical) meaning.
The thermal interpretation does not interpret these fictitious microevents at all but is content to interpret the final q-expectations, as the slightly uncertain values measured by some experiment.
Well, I'm a big fan of Weinberg and particularly his textbooks. The trouble is that, as he says in the quoted paragraph, on the one hand today there's no other description of the observations than standard QT, including Born's rule as one of the independent postulates of the formalism (he carefully analyses in this very concise chapter on "interpretation" that it cannot be derived from the other postulates), but on the other hand he's obviously also no alternative theory or interpretation overcoming what he (and obviously many others) considers a problem, namely the probabilistic meaning of the state.A. Neumaier said:Because you allow yourself extreme liberties in what to call measurement, and don't care to be precise in your arguments. Thus for you, everything is in full order, the philosophical quibbles (which arise when looking at the details) are irrelevant, and you have no empathy for all those who have higher standards of consistency.
Even living Nobel prize winners such as Steven Weinberg (and he is not the only one) are discontent with the statistical interpretation. Weinberg has the same theoretical background as you and the same information about experimental practice. He discusses (after the Copenhagen interpretation) the statistical interpretation on pp.92-95 of his 2013 textbook Lectures on Quantum Mechanics and concludes:
You cannot point to his age of 80 when publishing the book, for the whole book is written in an excellent style showing no signs of senility.
The thermal interpretation, on the other hand, presents such a realist description.
vanhees71 said:For me the only difference between the classical picture, which is in contradiction with very common phenomena as the stability of the matter around us, and the quantum picture is that on a fundamental ("microscopic") level there's (within standard QT) no deterministic description, but only a probabilistic one.
But I have one. The thermal interpretation makes quantum physics as deterministic as classical physics, and explains all probabilistic quantum effects as resulting from coarse-graining, in the same way as you did for the classical case in your previous post #229. In particular, Born's probabilistic interpretation follows, where it applies, from the deterministic rules and coarse-graining.vanhees71 said:he's obviously also no alternative theory or interpretation overcoming what he (and obviously many others) considers a problem, namely the probabilistic meaning of the state.
A measurable density is the q-expectation of the 0-component of a current - an effective relativistic vector field associated to some property distributed in space.ftr said:I think what is not clear to you is similar to my problem when I ask the expectation value of "density" of what physical things. Is that correct.
I think so. My main problem is to make sense of the word "expectation value" if there's no probability theory behind it. So far I don't see, what motivates the formal rules or makes them at least plausible as referring to physics rather than a purely axiomatic "game" of math, but that's what "interpretation" is all about, i.e., how to make sense of the formalism in its application to real-world observations (and measurements are just refined quantitative observations!).ftr said:I think what is not clear to you is similar to my problem when I ask the expectation value of "density" of what physical things. Is that correct.
This is a circular definition. In the standard interpretation it's clear that density is a spatially coarse-grained observable in the sense of an average over many microscopic degrees of freedom. What is this mysterious "q-expectation" if it's not such an average in the usual probabilistic sense? That's the key issue preventing a physical understanding of the proposed "thermal interpretation"!A. Neumaier said:A measurable density is the q-expectation of the 0-component of a current - an effective relativistic vector field associated to some property distributed in space.
No. Like you did in the classical case, I nowhere assume anything probabilistic in the quantum case. Randomness appears as in classical physics by breaking metastability through effects of the deterministic noise neglected in coarse-graining. Of course it will be in agreement with the standard interpretation where the latter is based on actual measurement.vanhees71 said:everything is just put such that the standard interpretation at the end results, but it's already implicitly assumed but just not stated.
vanhees71 said:I still don't see any difference between your "thermal interpretation" and the "minimal statistical interpretation", despite the fact that you substitute the clear probabilistic meaning of the standard formalism with some other concept that you have not yet made clear to me.
A. Neumaier said:A measurable density is the q-expectation of the 0-component of a current - an effective relativistic vector field associated to some property distributed in space.
It is the trace of the product of the q-observable ##j_0(h):=\int_\Omega h(x)j_0(x)dx## with the density operator, ##\langle j(h)\rangle:=Tr~\rho j(h)##, where ##\Omega## is the region in which the coarse-grained current is observed and ##h(x)## is an appropriate smearing function determined by the sensitivity of the measuring instrument or coarse-graining. This is mathematically well-defined; no mystery and no circularity is present (unless you impose your interpretation in addition to mine). The result can be compared with experiment in a single reading, without any statistics involved, giving an operational definition of the meaning of this q-expectation. The average taken is not over microscopic degrees of freedom but over a small spacetime region where the measurement is performed (needed to turn the distribution-valued operator current into a well-defined q-observable,vanhees71 said:This is a circular definition. In the standard interpretation it's clear that density is a spatially coarse-grained observable in the sense of an average over many microscopic degrees of freedom. What is this mysterious "q-expectation" if it's not such an average in the usual probabilistic sense? That's the key issue preventing a physical understanding of the proposed "thermal interpretation"!
vanhees71 said:I think so. My main problem is to make sense of the word "expectation value" if there's no probability theory behind it. So far I don't see, what motivates the formal rules or makes them at least plausible as referring to physics rather than a purely axiomatic "game" of math, but that's what "interpretation" is all about, i.e., how to make sense of the formalism in its application to real-world observations (and measurements are just refined quantitative observations!).
It's also clear that physics provides never a "final answer" to the question of interpretation of a given theory (not even that any given theory is the "final answer" to the quest for ever better descriptions of what we can objectively observe about nature). "Today's signal is tomorrow's background!"
It doesn't matter how one calls it. Tradition calls it expectation value and denotes it by pointed brackets, even in situations like:vanhees71 said:My main problem is to make sense of the word "expectation value" if there's no probability theory behind it.
where it is very clear (and where you agree) that it cannot have this meaning:A. Neumaier said:the terms in (3.1.34) of your lecture notes on Nonequilibrium Relativistic Quantum Many-Body Theory from August 16, 2017, where - against your minimal interpretation - you, like everyone in the field, refer to expectation values of operator products that are not Hermitian, let alone self-adjoint!
Therefore the right way is to regard all these as calculational tools, as you emphasized in this particular case. I call them q-expectations to emphasize that they are always calculational tools and not expectations values in the statistical sense, but using any other name (e.g., ''reference values'', as I did very early in the development of the thermal interpretation) would not change anything.vanhees71 said:How often should I emphasize that Green's functions are not directly describing observable facts but are calculational tools enabling one to calculate them?
(concerning: "an analogous statement about a free relativistic particle somehow prepared at time t in a small region of spacetime suffers the same problem."akhmeteli said:Could you please give a reference?