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PeterDonis
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@strangerep if you want to continue the MOND subthread it should be moved to its own thread in the appropriate forum.
So your point is that those sequences of 1-photon Fock states or entangled 2-photon Fock states are not beables in the thermal interpretation, hence vanhees71's point does not apply? I tentatively concluded this from your answer, together with your earlier answers to vanhees71, like:A. Neumaier said:Photon particles are (both in the sense of vanhees71 and in my sense) very special states of the quantum field along a beam, namely sequences of 1-photon Fock states, or in the entangled case of 2-photon Fock states generated by parametric down-conversion, for use in Aspect type experiments).
A. Neumaier said:It is about measuring fields, not particles - cf. the subject of the thread.
In the thermal interpretation, particles are not beables; only their probability distributions are. For this is what objectively distinguishes particles prepared in different states; see your answer and my reply here.
Fine. But ... I can't really blame vanhees71 in this specific case for not getting your point. If you want to distinguish between fields and detector events, you have to make that clear. Or rather, you have to make it clearer what you mean when you talk of fields. Apparently some measurable effects occuring in the context of fields in the QFT sense are not included in your notion of measuring fields.A. Neumaier said:Nevertheless a detector event at a photosensitive screen (or a pair of them in Aspect-type experiments) is not at all a measurement of the electromagnetic field at the screen(s). What would be the measured value of E(x) or B(x) at the screen?
If it is, what are the values of E(x) and B(x) obtained through the measurement?vanhees71 said:Of course it is a measurement of the electromagnetic field.
One measures the response of the detector to the field. One needs to use theory of the detector to know how to translate this into a statement about the electric field. The theory tells you the relation after a sufficiently long exposure to a stationary field, but not the relation about the exposure to a single 1-photon state.vanhees71 said:What else do you think you measure with a photoplate exposed by some electromagnetic radiation?
What is the value of the smeared correlation function known once the photon has left its mark? How to get a whole function from a single mark???vanhees71 said:It's also clear that a photon detector measures "smeared" correlation functions
States of the field are beables, but not the traveling photons.gentzen said:So your point is that those sequences of 1-photon Fock states or entangled 2-photon Fock states
I had clearly emphasized it in my answer.gentzen said:If you want to distinguish between fields and detector events, you have to make that clear.
In our case, the fields are the electric field E(x) and the magnetic field B(x).gentzen said:Or rather, you have to make it clearer what you mean when you talk of fields.
Yes. Not every measurable effect is a measurement of its cause. If rain caused your trousers to be wet, did you measure the rain?gentzen said:Apparently some measurable effects occuring in the context of fields in the QFT sense are not included in your notion of measuring fields.
Yes. Thus scanning the plate after long exposure is indeed a measurement of the coarse-grained field intensity.vanhees71 said:The photoplate after repeating a single-photon detection experiment many times depicts the energy density of the em. field (of course, as you stress yourself all the time, "coarse grained" due to finite resolution).
So the observation of the dot is not a measurement of the field or its correlation function.vanhees71 said:A correlation function is a statistical quantity and cannot be empirically studied with just a single experiment. One photon leaves one dot at a random spot, not a distribution.
No, we are at the very basis of our eternal differences!vanhees71 said:I think, we are splitting hairs here.
It might be called a random position measurement of the photon.vanhees71 said:Of course is the detection of a single photon a measurement. How else would you call it?
But it's not a measurement of the field, since, as you have said, such a measurement requires a large number of dots so you can compute statistics from them.vanhees71 said:Of course is the detection of a single photon a measurement
I've thought of this in terms of the moment when an analog signal out of some device is converted to a digital form that we then record. The output current from an Avalanche PhotoDiode or similar device is noisy but near zero, then it is noisy but near some value that is distinctly different from zero, which we commonly say is because of an "avalanche" within the device. Discriminating hardware typically monitors the current and decides what time to record as the time at which the output current became non-zero, an event. Either such hardware is from a trusted manufacturer, off a slightly less trusted storeroom shelf, or else an experimenter has designed and built it. In any case, the experimenter will have to verify that the hardware responds appropriately to the signal it receives, and perhaps debug its operation, presumably as shown on a good oscilloscope that can also measure the current from the device at GHz rates.A. Neumaier said:No, we are at the very basis of our eternal differences!
It might be called a random position measurement of the photon.
But it is certainly not a measurement of a field. The latter would produce approximate values for the field.
The same way you get (or don't get) a whole function from n marks?A. Neumaier said:How to get a whole function from a single mark???
It is a measurement of the current in thewire, not of the electromagnetic field that caused the ejection of an electron. That's quite different!Peter Morgan said:The output current at the input to the hardware discriminator is a proxy for the electromagnetic field in a small region of the wire where it connects to the discriminating hardware. Loosely, that's a smeared measurement of E(x), not in the original device but at the connection to the discriminating hardware.
But for n=1 (as in the case under discussion) you don't get a function but only a point.Fra said:The same way you get (or don't get) a whole function from n marks?
Yes, would you get a "function" for n=100? so the find the "best match" of a function from that one point, perhaps it in practice is some dirc or gaussian, sharply peaking around that point, with a width corresponding to the "size" of the point. The size of a "real point", would be like the size of hte pixel or whatever the sensor element is like.A. Neumaier said:But for n=1 (as in the case under discussion) you don't get a function but only a point.
It is different, but it can at least be thought of as the most direct record we have of a measurement of the electromagnetic field. In particular, such measurements are —I think by construction— the only recorded measurement results. From the totality of all such actually recorded measurement results, we infer what we expect we would have recorded as the results of other measurements.A. Neumaier said:It is a measurement of the current in thewire, not of the electromagnetic field that caused the ejection of an electron. That's quite different!
But here what is measured is inside the measurement apparatus. However, a measurement apparatus is designed to measure something outside it, otherwisePeter Morgan said:It is different, but it can at least be thought of as the most direct record we have of a measurement of the electromagnetic field. In particular, such measurements are —I think by construction— the only recorded measurement results.
vanhees71 claimed that the apparatus measures the electic field intensity impinging from a source to the screen. Thus we should be able to infer approximate values of the electic field intensity. But this is not the case. So whatever is measured in (inferred from) a single detector event, it is not a measurement of the electric field intensity in the beam.Peter Morgan said:From the totality of all such actually recorded measurement results, we infer what we expect we would have recorded as the results of other measurements.
Yes. I am approximately the richest man in the world! And at the same time the poorest!Fra said:Indeed such an approximation is terribly bad.
Makes perfect sense!A. Neumaier said:Yes. I am approximately the richest man in the world! And at the same time the poorest!
You might be interesting in looking at my online bookjoneall said:I'm ok with basic Lie algebra.
Moderator's note: The MOND subthread has now been spun off to a new thread in the BTSM forum:PeterDonis said:@strangerep if you want to continue the MOND subthread it should be moved to its own thread in the appropriate forum.
A. Neumaier said:No, we are at the very basis of our eternal differences!
It might be called a random position measurement of the photon.
But it is certainly not a measurement of a field. The latter would produce approximate values for the field.
This is commonly called an (approximate) position measurement. It measures the transverse position orthogonal to the beam direction. This is represented by a well-defined operator with two commuting components.vanhees71 said:A photon has no position to begin with. A photon is a one-quantum Fock state of the electromagnetic field. There is a probability distribution for detecting it at a given place and a given time (with finite resolution of both of course).
Measuring the probability would therefore be a measurement of the field intensity. But from a single photon one cannot get a probablilty, hence no measurement of the field.vanhees71 said:The probability is given by the (normalized) energy density of the electromagnetic field,
Yes, the observables are the correlation functions but:vanhees71 said:I thought that's your view: the observables are given by correlators of local observable-operators, and indeed the energy density of the em. field is such an observable.
Thus the observation of a single photon (which is what we were discussing) impact does not measure these observables.vanhees71 said:A correlation function is a statistical quantity and cannot be empirically studied with just a single experiment. One photon leaves one dot at a random spot, not a distribution.
Anything that results in an approximate value of the smeared field at some point.vanhees71 said:What else should be a measurement of "a field" than that?
The classical intensity is a field, and observing it at x gives the value of the field averaged near x. The same holds in the quantum case with my definition of measurement, but not with your contrived one.vanhees71 said:Also in classical electrodynamics, what's observable of the field are precisely such things as the "intensity", which also classically is given by the energy density.
A. Neumaier said:Thus the observation of a single photon (which is what we were discussing) impact does not measure these observables.
The position is the position of the detector. There's no position operator for the photon. In relativistic QFT time and position (four-vector) components are parameters with precisely this meaning.A. Neumaier said:This is commonly called an (approximate) position measurement. It measures the transverse position orthogonal to the beam direction. This is represented by a well-defined operator with two commuting components.
As in any QT the state refers to probabilistic properties of ensembles, of course.A. Neumaier said:Measuring the probability would therefore be a measurement of the field intensity. But from a single photon one cannot get a probablilty, hence no measurement of the field.
A photodetector registers a single photon at a given space-time point (within a finite resolution). That's a measurement par excellance as it is defined in standard QT.A. Neumaier said:Yes, the observables are the correlation functions but:
Thus the observation of a single photon (which is what we were discussing) impact does not measure these observables.
This can of course only be achieved by measuring an ensemble (or rather a "statistical sample") of equally prepared systems.A. Neumaier said:Anything that results in an approximate value of the smeared field at some point.
I don't see, where we differ in this respect: the expecation value of a local observable like the electromagnetic field ##(\vec{E}(x),\vec{B}(x)## can of course again only be measured on an ensemble not a single system, and the expectation value as only one of the moments of the corresponding probability distribution only describes a small aspect of the state.A. Neumaier said:The classical intensity is a field, and observing it at x gives the value of the field averaged near x. The same holds in the quantum case with my definition of measurement, but not with your contrived one.
In your view the "statistical samples" approximate the "ensemble". But the ensemble is a fiction in the sense of requiring infinite repeats etc. This is what is the "problem".vanhees71 said:I don't see, where we differ in this respect: the expecation value of a local observable like the electromagnetic field ##(\vec{E}(x),\vec{B}(x)## can of course again only be measured on an ensemble not a single system, and the expectation value as only one of the moments of the corresponding probability distribution only describes a small aspect of the state.
The detector is a screen and has many positions, one of them responds to the photon. The two coordinates of the responding position define the transverse position of the photon measured.vanhees71 said:The position is the position of the detector.
For the photon, in the observer frame, there is no 3-component position operator with commuting components transforming properly under rotations.vanhees71 said:There's no position operator for the photon.
But it is a measurement of a particle, not of a field. If it would measure a field, as you claim the energy intensity, which value do we get for the incident field at the impact point? and which values at non-impact points? (Not seeing a response is also a measurement of photon presence, but not a field measurement.)vanhees71 said:A photodetector registers a single photon at a given space-time point (within a finite resolution). That's a measurement par excellance as it is defined in standard QT.
An engineer measures a local observable like the electromagnetic field ##(\vec{E}(x),\vec{B}(x)## with a single measurement at x, not by statistical means. This works well, although only a single electromagnetic field is prepared, not an ensemble of fields.vanhees71 said:This can of course only be achieved by measuring an ensemble (or rather a "statistical sample") of equally prepared systems.
I don't see, where we differ in this respect: the expecation value of a local observable like the electromagnetic field ##(\vec{E}(x),\vec{B}(x)## can of course again only be measured on an ensemble not a single system, and the expectation value as only one of the moments of the corresponding probability distribution only describes a small aspect of the state.
An engineer records no statistical sample but only one value at any point where a measurement is made.Fra said:If I understand Neumaier, he thinks the "statistical sample" approximates not some fictional ensemble but the value of an actual "real" field (that is defined by accounting for ALL the actual varialbes the in universe, even those the local observer isn't informed about).
In classical physics we prepare one electromagnetic field and can measure it anywhere with a single measurement, provided the intensity of the field is large enough. The smaller the intensity the large the exposure time needed for an accurate measurement.vanhees71 said:it's a problem of all kinds of measurement also within classical physics. You can always only prepare a finite number of systems and measure them.
For a macroscopic measurement (when an engineer measures a field, in particular, for most classical measurements) one rarely takes a sample, one just measures a single time. One needs it only in those case where the measurement results are so noisy that one needs to average a large number of measurement results.vanhees71 said:Independently from the measured system, be it describable with good accuracy within classical physics or be it only describable within QT, you always have to repeat an experiment on a "sample of equally prepared systems" very often to be able to evaluate the statistical and systematical errors.
No, only a longer exposure is needed before a measurement of the field results.vanhees71 said:For very "dim laser light", where in the extreme you can have ##\langle N \rangle <1## this is no longer the case, and the quantum description is needed.
Yes, and that's why you dont have an intensity measurement.vanhees71 said:That's why in this case you'll see the "quantum noise" and the discreteness of the registration processes of single photons.
This is a solution only if stationarity assumptions holds, right? Isn't the stationarity assumption a kind of "repeated preparation" in disguise?A. Neumaier said:The smaller the intensity the large the exposure time needed for an accurate measurement.
Yes, I think is in principle a problem in classical physics too, but since classical physics is non-contextual, in practice, it stays beeing a practical problem of the physicist ignorance...vanhees71 said:There is no problem or if there is a problem it's a problem of all kinds of measurement also within classical physics.
...because the right value of which we have approximations is independent of the measureement. Its just ignorance.vanhees71 said:You can always only prepare a finite number of systems and measure them. In general also both the preparation and the measurement are only approximate etc.
It must be nearly stationary during the time of observation. For less stationary sources with a known evolution law one gets a 1-point function heavily smeared in time.Fra said:This is a solution only if stationarity assumptions holds, right?
If you think only statistically then you need to use this disguise. But If you think in terms of 1-point functions stationarity is not needed and one just has different smearing functions.Fra said:Isn't the stationarity assumption a kind of "repeated preparation" in disguise?
In your interpretation, is this evolution law merely "in principle" knowable by some "omnipresent superobserver" but still assumed objectively determined?A. Neumaier said:For less stationary sources with a known evolution law one
The evolution law of the universe is known only to God. But it is assumed to exist and to be deterministic and observer independent. In this sense it is objective. Observer form their own approximate models of this evolution law.Fra said:In your interpretation, is this evolution law merely "in principle" knowable by some "omnipresent superobserver" but still assumed objectively determined?
Yes. Any collection of observations informs the observer about properties of the universe near its spacetime position. From this information observers construct, based on statistics and subjective plausbility (= prejudice) , their models for prediction.Fra said:How do you put that in terms of process tomography? Do you treat an actual finite observers processing of statistical samples just an "approximation" of something "real".
For the universe, it is given by a classical action for fields defined on spacetime, to be interpreted somehow as a quantum dynamical law.Fra said:Or how is an observer independet definition of law (hamiltonian?) defined for say arbitrary observers in non-inertial frames? (Conceptually that is! as we know there is no full quantum gravity theory yet)
Fra said:/Fredrik