A Implications of quantum foundations on interpretations of relativity

[Demystifier]

TL;DR Summary

If the Bell theorem is interpreted as nonlocality of nature, then what does it tell us about the meaning of Einstein theory of relativity?

Physicists often discuss interpretations of quantum mechanics (QM), but they rarely discuss interpretations of relativity. Which is strange, because the interpretations of quantum non-locality are closely related to interpretations of relativity.

The field of interpretations of relativity is not so rich as the field of quantum interpretations. As far as I am aware, basically there are 4 major interpretations of relativity.

1. Operational interpretation. According to this interpretation, relativity is basically about how the appearance of space, time and some related physical quantities depends on motion (and current position) of the observer. Essentially this is how Einstein originally interpreted relativity in 1905.

2. Spacetime interpretation. According to this interpretation, relativity is not so much about the appearance of space and time to observers, as it is about the 4-dimensional spacetime that does not depend on the observer. This interpretation was first proposed by Minkowski. Einstein didn't like it in the beginning, but later he embraced it in his formulation of general theory of relativity. The spacetime interpretation naturally leads to the block-universe interpretation of the world, according to which time does not flow, meaning that the past, the presence and the future exist on an equal footing.

3. Ether interpretation. This is not really one interpretation but a wide class of different physical theories. One simple version of the ether theory was developed by Lorentz, before Einstein developed his theory of relativity in 1905. According to ether theories, there are absolute space and absolute time, but under certain approximations some physical phenomena obey effective laws of motion that look as if absolute space and time did not exist. The original Lorentz version of ether theory was ruled out by the Michelson-Morley experiment, but some more sophisticated versions of ether theory are still alive.

4. Spacetime+foliation interpretation. This interpretation posits that in addition to spacetime, there is some timelike vector field $n^{\mu}(x)$ that defines a preferred foliation of spacetime, such that $n^{\mu}(x)$ is orthogonal to the spacelike hypersurfaces of the foliation. This preferred foliation defines a preferred notion of simultaneity.

What different interpretations of QM can tell us about those interpretations of relativity? Which interpretations of relativity seem natural from the perspective of which interpretations of QM?

 

[DarMM]

Well a simple example might be Copenhagen and the Blockworld, what you call the spacetime interpretation. In Copenhagen measurement results don't exist prior to the measurement (made more concrete by things like the Kochen Specker theorem) which can be hard to square with the view that all of four dimensional history "already exists" in some sense.

 

[Demystifier]

Well a simple example might be Copenhagen and the Blockworld, what you call the spacetime interpretation. In Copenhagen measurement results don't exist prior to the measurement (made more concrete by things like the Kochen Specker theorem) which can be hard to square with the view that all of four dimensional history "already exists" in some sense.

The operational interpretation of relativity is very much in spirit of Copenhagen interpretation of QM. But still, there is no direct contradiction between blockworld interpretation of relativity and the idea that a measurement result doesn't exist prior to the measurement. One can simply say that the measurement result exists at the spacetime point at which the measurement is performed.

An amusing historical fact is that Einstein, who was so much against Copenhagen interpretation of QM, embraced (at least in the beginning in 1905) the operational "Copenhagen-like" interpretation of relativity. Indeed, the categorical statement that "there is no ether" is very much in spirit of the categorical statement that "there are no hidden variables".

 

[Demystifier]

One related subquestion is this. The Bell's famous book "Speakable and Unspeakable in Quantum Mechanics" contains the chapter called "How to teach special relativity". What's the point of this chapter in the context of the whole book?

 

[DarMM]

The operational interpretation of relativity is very much in spirit of Copenhagen interpretation of QM.

David Mermin has a bit on this in his "Why Quark rhymes with Pork" essay collection.

But still, there is no direct contradiction between blockworld interpretation of relativity and the idea that a measurement result doesn't exist prior to the measurement. One can simply say that the measurement result exists at the spacetime point at which the measurement is performed.

Very true. Christopher Timpson says a bit about this in this paper:
https://arxiv.org/abs/0804.2047
He basically says that one can consider 4D spacetime to simply be laid out in advance, but events in each 3D slice don't follow dynamically from those on the previous 3D slice. So from the perspective of the prior slice events in the future are utterly unpredictable. Of course one must still add something like contextuality as otherwise it would simply be a classical stochastic process. Essentially a contextual adynamical view somewhat like @RUTA 's relational blockworld.

An amusing historical fact is that Einstein, who was so much against Copenhagen interpretation of QM, embraced (at least in the beginning in 1905) the operational "Copenhagen-like" interpretation of relativity. Indeed, the categorical statement that "there is no ether" is very much in spirit of the categorical statement that "there are no hidden variables".

Again I believe Mermin points this out as well.

 

[atyy]

An amusing historical fact is that Einstein, who was so much against Copenhagen interpretation of QM, embraced (at least in the beginning in 1905) the operational "Copenhagen-like" interpretation of relativity. Indeed, the categorical statement that "there is no ether" is very much in spirit of the categorical statement that "there are no hidden variables".

Again I believe Mermin points this out as well.

Heisenberg & Einstein's conversation
https://www.informationphilosopher.com/solutions/scientists/heisenberg/talk_with_einstein.html
https://physicstoday.scitation.org/doi/pdf/10.1063/1.1292474
"But you don't seriously believe," Einstein protested, "that none but observable magnitudes must go into a physical theory?" "Isn't that precisely what you have done with relativity?" I asked in some surprise. "After all, you did stress the fact that it is impermissible to speak of absolute time, simply because absolute time cannot be observed; that only clock readings, be it in the moving reference system or the system at rest, are relevant to the determination of time."

"Possibly I did use this kind of reasoning," Einstein admitted, "but it is nonsense all the same. Perhaps I could put it more diplomatically by saying that it may be heuristically useful to keep in mind what one has actually observed. But on principle, it is quite wrong to try founding a theory on observable magnitudes alone. In reality the very opposite happens. It is the theory which decides what we can observe.

 

[EPR]

Summary: If the Bell theorem is interpreted as nonlocality of nature, then what does it tell us about the meaning of Einstein theory of relativity?

That we can only talk meaningfully about observed(measured) quantities. The Copenhagen interpratation connects well with relativity some assumptions notwithstanding.

 

[atyy]

What different interpretations of QM can tell us about those interpretations of relativity? Which interpretations of relativity seem natural from the perspective of which interpretations of QM?

All of the above? From "State vector reduction in relativistic quantum mechanics: An introduction" by Breuer and Petruccione https://doi.org/10.1007/BFb0104397:

"There is a second possibility which has been first proposed by Aharonov and Albert [8]. This possibility consists in the assumption that the state vector reduction takes place instantaneously in all inertial flames. ...

Covariance requires only the equivalence of all inertial systems, hence the independence of all physical statements from a special coordinate system. And exactly this point has been made obvious in what we have discussed above: Performing a Lorentz transformation from an inertial system to another one has to transform the states of the quantum system as well as the observer who ascribes a history of state vectors to his equal-time hypersurfaces."

 

[Demystifier]

"Possibly I did use this kind of reasoning," Einstein admitted, "but it is nonsense all the same."

 

[PGaccount]

There are very close analogies between relativity and complementarity in their epistemological aspects. In relativity, properties of objects like position and velocity are only defined with respect to a frame of reference. This is analogous to the fact that in quantum mechanics, a 'phenomenon' is definable only in the context of a particular experimental arrangement. Because of this fact, we can treat the measuring bodies in an idealization in which their mass is so large compared to the electrons or other quantum mechanical systems that their velocities are not affected by collisions with the electrons. The point is that this idealization is the only basis for the definition of a phenomenon.

This is similar to the fact that in thermodynamics, the use of the concept of temperature is only possible in an idealization in which the second law of thermodynamics is exactly valid. The idea that the second law is only statistically valid does not contradict classical thermodyanmics because of this fact.

The analog of this in relativity is the fact that the reference frame is defined by objects which must be considered rigid, i.e. they are not subject to the lorentz contractions. This means that there is a sharp separation between space and time in the interpretation of measurements, similar to the fact that in quantum theory, we can attach meaning to measurements only by using classical concepts. In relativity, a phenomenon such as length contraction has to be interpreted in terms of the forces within the rod. For example, take a rubber band. The energy density of the band is its tension. If the rubber band is moving, its mass increases due to its kinetic energy (m = KE/c²), and therefore contracts because of the greater tension.

Imagine sending a light signal to an event at time -t and receiving the reflected signal at time t'. The event is then characterized by the coordinates (t, t'). We can define an inertial frame (x, T) by

2x = t + t'
2T = t - t'

Under a lorentz transformation,

t' → (1/f(v)) t'
t → f(v) t

the invariant distance squared between (0,0) and (t,t') is s² = tt' = T² - x². The idea of a light-signal seems to be more basic than the idea of x and T. the measurement of the speed of light therefore must consist of comparision of measurements obtained by different observers, similar to the fact the the measurement of plank's constant implies the comparision of measurements obtained under mutually exclusive arrangements.

 

[Prathyush]

In relativity, a phenomenon such as length contraction has to be interpreted in terms of the forces within the rod. For example, take a rubber band. The energy density of the band is its tension. If the rubber band is moving, its mass increases due to its kinetic energy (m = KE/c2), and therefore contracts because of the greater tension.

Length contraction/time dilation in relativity is a purely kinematic phenomenon(in the sense that it only defined in terms of transformation of co-ordinates and their relationships) and does not have a dynamic interpretation in any obvious sense.

The rest mass of a rubber band can be a completely independent of the tension. Or alternatively tensile strength of a material can also be completely independent of its weight.

I don't think it would be correct to think about greater tension because of mass/energy increase. Forces in a rubber band are electromagnetic and follow lorentz force law and hence co-variant. The idea that dynamical law is co-variant will imply what ever force that applied in frame 1 must transform in way that it would give identical behavior upto transformation in frame 2 also.

 

[PGaccount]

See chapter 15 of the Feynman lectures, volume one.

The whole content of relativity as far as the description of phenomena in a single frame goes is contained in the statement that the mass of a body is given by

m = m₀/√1 - v²

This is the only change needed, and Newton's laws remain

F = dp/dt

p = mv

Therefore all relativistic effects are derivable from, or attributed to the increase of mass with velocity, or in other words, the equivalence between mass and energy. Another way to say it is that the momentum is

pc = Ev/c

(the "flux of energy"), where E = mc².

If you substitute this in the first formula, you get

(p/v)² - p² = m₀²

and (p/v)² = E². This is just the familiar formula E² - p² = m₀²

 

[weirdoguy]

The whole content of relativity as far as the description of phenomena in a single frame goes is contained in the statement that the mass of a body is given by

m = m0/√1 - v2

And this is not true - relativistic mass is not used in physics for ages. See this insight article:
https://www.physicsforums.com/insights/what-is-relativistic-mass-and-why-it-is-not-used-much/

 

[Demystifier]

relativistic mass is not used in physics for ages.

I think it's related to the fact that the interpretation 2. (in the first post on this thread) is today much more popular than the interpretation 1.

What I find surprising is that today most adherents of Copenhagen interpretation of QM are not at the same time adherents of the interpretation 1. of relativity. Does anyone has idea why is that? Perhaps we still wait for someone who will do for QM what Minkowski has done for relativity?

 

[Prathyush]

The whole content of relativity as far as the description of phenomena in a single frame goes is contained in the statement that the mass of a body is given by

m = m0/√1 - v2

This is the only change needed, and Newton's laws remain

It is true that all consequences of relativity(dynamical aspects) can be inferred from the mass changes however its more naturally formulated in the 4 vector language. And how the mass changes is obvious in this picture by defining 3 momentum from 4 momenta.

$$p^\mu = m \frac{\partial u^\mu}{\partial \tau}$$
$$f^\mu = \frac{\partial p^\mu}{\partial \tau}$$

This is a natural generalization of Newton's force law. This is all you need apart from natural kinematic concepts that follow constancy of speed of light. I don't think Feynman was implying something along the lines of what you are suggesting.

For example, take a rubber band. The energy density of the band is its tension. If the rubber band is moving, its mass increases due to its kinetic energy (m = KE/c2), and therefore contracts because of the greater tension.

Also this point remains unexplained.

and (p/v)2 = E2. This is just the familiar formula E2 - p2 = m02

How?
Edit:
3 momentum $p_i = \gamma m v$ which is just $p_i = E v_i$ . I don't see why anything else is needed.

 

[PGaccount]

In relativity, forces are not derived from scalar potentials F = -dU, but from a vector potential F = dA. This is because in relativity, forces can depend not just on the positions, but also on the velocities. The reason for this is that scalar potentials like U cannot be used to describe gravity in relativity, because of the equivalence principle. In relativity, gravity is described by a tensor potential g. The gravitational poisson equation

dδU = 4πGρ

is replaced by an equation of the form δF = j in the linear approximation. If p is the 4-momentum, then the covariant version of F = dp/dt is of the form

dp_a/dτ = eF_abv^b

This is the force law of electrodynamics dp/dt = e(E + v ∧ B) where E and B are the electric and magnetic fields.

 

[atyy]

And this is not true - relativistic mass is not used in physics for ages. See this insight article:
https://www.physicsforums.com/insights/what-is-relativistic-mass-and-why-it-is-not-used-much/

What you said is not correct. The article you linked to disagrees with what you claimed. @PrashantGokaraju referenced the Feynman lectures in his post, which does give correct physics on this point.

 

[martinbn]

What you said is not correct. The article you linked to disagrees with what you claimed. @PrashantGokaraju referenced the Feynman lectures in his post, which does give correct physics on this point.

The Feynman lectures were written ages ago.

 

[weirdoguy]

What you said is not correct.

So can you give any example of paper on HEP, relativity or cosmology written in last, say, 30 years that explicitly uses relativistic mass? I know that some people use it in teaching, but teaching physics should prepare students to use physics in their work and I see no point in teaching relativistic mass if barely anyone use it in their scientific work.

 

[vanhees71]

The Feynman lectures were written ages ago.

It's still an enigma for me, why Feynman used the "relativistic mass" concept. Though among the best textbooks on physics ever written, not everything is perfect!

 

[Demystifier]

It's still an enigma for me, why Feynman used the "relativistic mass" concept. Though among the best textbooks on physics ever written, not everything is perfect!

Perhaps it is not such a nonsense even from a modern point of view. Consider a big object at rest made of many small objects moving relatively to the center of mass of the big object. (For instance, the small objects can be atoms in thermal motion.) The mass of the big object is larger than the sum of individual invariant masses of its constituents, which can be hard to understand without introducing the concept of relativistic mass for each of the constituents.

 

[atyy]

So can you give any example of paper on HEP, relativity or cosmology written in last, say, 30 years that explicitly uses relativistic mass? I know that some people use it in teaching, but teaching physics should prepare students to use physics in their work and I see no point in teaching relativistic mass if barely anyone use it in their scientific work.

You claimed correct physics was wrong - that was the point I was disputing. Infrequent usage does not mean wrong. Anyway, if you wish to see an example in the literature, there are many, as Jaramillo and Gourhoulhon note: "In the literature, references are found where the term ADM mass actually refers to this length of the ADM 4-momentum and other references where it refers to its time component, that we have named here as the ADM energy. These differences somehow reflect traditional usages in Special Relativity where the term mass is sometimes reserved to refer to the Poincar´e invariant (rest-mass) quantity, and in other occasions is used to denote the boost-dependent time component of the energy-momentum." https://arxiv.org/abs/1001.5429

 

[martinbn]

You claimed correct physics was wrong - that was the point I was disputing. Infrequent usage does not mean wrong. Anyway, if you wish to see an example in the literature, there are many, as Jaramillo and Gourhoulhon note: "In the literature, references are found where the term ADM mass actually refers to this length of the ADM 4-momentum and other references where it refers to its time component, that we have named here as the ADM energy. These differences somehow reflect traditional usages in Special Relativity where the term mass is sometimes reserved to refer to the Poincar´e invariant (rest-mass) quantity, and in other occasions is used to denote the boost-dependent time component of the energy-momentum." https://arxiv.org/abs/1001.5429

I don't think that this is relevant. If anything it shows that people are a bit behind on the convention for the ADM mass and energy. That doesn't mean that the convention for invariant mass and relativistic mass currently in use is not a good one.

 

[vanhees71]

Perhaps it is not such a nonsense even from a modern point of view. Consider a big object at rest made of many small objects moving relatively to the center of mass of the big object. (For instance, the small objects can be atoms in thermal motion.) The mass of the big object is larger than the sum of individual invariant masses of its constituents, which can be hard to understand without introducing the concept of relativistic mass for each of the constituents.

The point is that you define mass as a Lorentz scalar. If you have a closed composed system it's its energy measured in its center-momentum frame (divided by $c^2$).

If you introduce relativistic mass, then you should be honest and define it as a function of the velocity of the object and the relative angle between this velocity and its acceleration. It's utmost complicated. Writing the equations of motion down in a manifestly covariant way, using manifestly covariant definitions of intrinsic properties like mass, charge, temperature, pressure/stress, etc. makes everything much more simple.

 

[martinbn]

May be people lost interest in this thread, but I will say what my opinion is anyway. First, I don't think that relativity needs any interpretation. It doesn't have anything remotely similar to the measurement (and related) problems. Second, I disagree that what you wrote are interpretations.

For me the fact that space-time has the structure of Minkowski space is not an interpretation but a consequence. No matter what your preferred formulations of special relativity is, it implies that space-time is Minkowskian. Just as in classical physics the space-time has a specific structure that is implied by the laws of classical physics.

So 1) is just the original formulation. 2) is also not an interpretation, but a better understanding of the colloraries. 3) you, yourself say that these are a class of different theories, so not interpretations either. 4) is a bit strange, but not and interpretation. It just adds something additional, that is completely unnecessary. You may as well pick a point in the universe and call it the centre, and claim that the universe isn’t homogeneous. Completely unnecessary and equally silly.

I also must say that I am puzzled by the very first sentence. You say "If the Bell theorem is interpreted as nonlocality of nature...", well what if it isn’t?

 

[Demystifier]

4) is a bit strange, but not and interpretation. It just adds something additional, that is completely unnecessary.

Maybe it's unnecessary within classical physics, but it appears in some versions of relativistic Bohmian mechanics.

You say "If the Bell theorem is interpreted as nonlocality of nature...", well what if it isn’t?

Then 1. is the most natural formulation of relativity.

 

[Demystifier]

For me the fact that space-time has the structure of Minkowski space is not an interpretation but a consequence.

What about block universe? Is that a consequence or an interpretation?

 

[martinbn]

Maybe it's unnecessary within classical physics, but it appears in some versions of relativistic Bohmian mechanics.

Then it is unrelated. In celestial mechanics it may be convenient to choose coordinates centered at the sun, but that is not an interpretation of classical mechanics.

Then 1. is the most natural formulation of relativity.

I don't understand this.

 

[martinbn]

What about block universe? Is that a consequence or an interpretation?

I've seen different people to mean different things by block universe. What do you take it to mean?

 

[Demystifier]

I've seen different people to mean different things by block universe. What do you take it to mean?

The past, presence and future exist on an equal footing.

 

[Demystifier]

I don't understand this.

See the second paragraph in #3.

 

[A. Neumaier]

For me the fact that space-time has the structure of Minkowski space is not an interpretation but a consequence.

But this alleged fact is already false in general relativity...

 

[Demystifier]

But this alleged fact is already false in general relativity...

He said consequence of special relativity.

 

[Elias1960]

Summary: If the Bell theorem is interpreted as nonlocality of nature, then what does it tell us about the meaning of Einstein theory of relativity?
According to ether theories, there are absolute space and absolute time, but under certain approximations some physical phenomena obey effective laws of motion that look as if absolute space and time did not exist. The original Lorentz version of ether theory was ruled out by the Michelson-Morley experiment, but some more sophisticated versions of ether theory are still alive.

Sorry, but what is known as the Lorentz ether is simply equivalent to SR (and therefore an interpretation of SR) and therefore not ruled out by the Michelson-Morley experiment. And which versions you think about?

4. Spacetime+foliation interpretation. This interpretation posits that in addition to spacetime, there is some timelike vector field nμ(x)nμ(x) that defines a preferred foliation of spacetime, such that nμ(x)nμ(x) is orthogonal to the spacelike hypersurfaces of the foliation. This preferred foliation defines a preferred notion of simultaneity.

The Lorentz ether is here only a particular case, where the foliation is defined by a preferred inertial frame.

What different interpretations of QM can tell us about those interpretations of relativity? Which interpretations of relativity seem natural from the perspective of which interpretations of QM?

There is a quite simple general answer: All realistic as well as all causal interpretations require a preferred foliation. Here, "realistic" means that the EPR criterion of reality holds, and "causal" means a notion of causality which includes Reichenbach's common cause principle. This follows from variants of Bell's theorem, which use, beyond Einstein causality, only EPR realism resp. Reichenbach's common cause principle.

 

[martinbn]

The past, presence and future exist on an equal footing.

How is that specific to relativity? It seems like a general philosophical position. In fact it seems very non-relativistic in spirit. What is present in relativity? A choice of simultaneity convention? Which one?

 

[Elias1960]

What about block universe? Is that a consequence or an interpretation?

An interpretation. In interpretations with a preferred frame, that preferred frame also defines the presence objectively, and the relativity of simultaneity is reduced to an impossibility to identify the preferred frame by local observations.

How is that specific to relativity? It seems like a general philosophical position. In fact it seems very non-relativistic in spirit. What is present in relativity? A choice of simultaneity convention? Which one?

A philosophical position that assumes a block universe exists too, it is named fatalism. In fatalism, the future is predefined, thus, already existing in the same way as the present. In what I would simply name common sense, the future, as well as the past, have a different status, only what is present exists.

This difference is an objective one, a property of the world, not of observations of the world. Once the preferred frame cannot be identified by observation, it cannot be a choice by an observer. The observer can only guess which is the correct preferred frame (and the CMBR frame gives a quite plausible guess).

The preferred frame interpretations are, indeed, very non-relativistic in spirit. Relativistic symmetry holds only for some observable effects, it is not a fundamental symmetry, and in particular not a symmetry of space and time. This is what makes them much better compatible with similarly non-relativistic interpretations of quantum theory.

A class of interpretations of QT which depends on a preferred frame for extensions into the relativistic domain can be easily identified: If we look at the Schrödinger equation in the configuration space, it gives a continuity equation for the density $\rho(q)$:
$$\partial_t \rho(q,t) + \partial_i ( \rho(q,t)v^i(q,t)) = 0. $$

All one needs is to give the corresponding $\rho(q,t)v^i(q,t)$ a physical interpretation, as a probability flow.

 

[Michael Price]

What about block universe? Is that a consequence or an interpretation?

A consequence. The block universe has always seemed, to me, a consequence of pre-Minkowski classical physics, which describes time as a fourth dimension. Nothing in SR (or GR) changes this.

I don't get your point 4, though. There is no preferred foliation for time after Einstein and Minkowski, and no preferred foliation problem, just as there is no preferred basis problem in quantum mechanics.

 

[Michael Price]

How is that specific to relativity? It seems like a general philosophical position. In fact it seems very non-relativistic in spirit. What is present in relativity? A choice of simultaneity convention? Which one?

Just as there is no preferred position, so there is no preferred time. Seems entirely relativistic to me.

 

[vanhees71]

Bell is also famous for the discovery of anomalies in relativistic quantum field theories (particularly the $\mathrm{U}(1)_{\text{A}}$ anomaly, known as the Adler-Bell-Jackiw anomaly).

 

[vanhees71]

I don't think that it makes much of a difference whether you use photons or massive particles to check Bell's local deterministic HV result against QT results. Entanglement is a very universal phenomenon, for which it doesn't make a lot of a difference in which concrete way you realize it. That there are so many Bell tests using photons is simply, because it's technically easier to prepare entangled states with photon Fock states and keep them unperturbed by interactions with "the environment".

Concerning the other questions, it's clear that one must be careful how to think about photons. First of all you cannot divide photons. Of course you can have a process like parametric down conversion were a laser photon interacting with a BBO crystal splits into an entangled photon pair but that's not the split of the original photon but you get two photons with about half the frequency and corresponding wave numbers (fulfilling the phase-matching condition for the wanted preparation of a biphotonic Bell state).

Photons have no wave functions since they do not have a position observable. A photon is by definition a single-quantum Fock state of the electromagnetic field. A true photon state must also be normalizable, i.e., it has a finite width in energy and momentum.

What happens with a photon in an experiment depends of course on its setup. E.g., if you want to make a polarization measurement you can just use a polarization filter and a photodetector behind it. The (ideal(ized)) polarization filter either absorbs the photon or let's it through, with probabilities depending on the polarization state of the incoming photon and the direction of the polarization filter, given by Born's rule. The photons coming through have the corresponding linear polarization determined by the orientation of the polarization filter. The (idealized) polarization filter is in this case described by a corresponding projection operator, which you can interpret in a FAPP sense as a "collapse". I'd simply call it filtering ;-).

Another possibility is to use a (idealized non-absorbing) birefringent crystal. Then the photon is deflected in different directions with probabilities again given by the incoming photon state and the orientation of the crystal, preparing states which are a superposition of the two possible outcomes leading to an entanglement between the polarization and the momentum of the photon. Here the birefringent crystal can be formally described by a unitary operator. Here I think most collapse proponents would not call this a collapse, because what's prepared is a superposition and which polarization state and momentum an individual photon has taken when going through the crystal must be subsequently measured with photodetectors placed at positions to measure the momentum of the photon, and the collapse proponents then call this a collapse, though of course you don't have a photon left, because it's simply absorbed by the photodetector.

 

[Buzz Bloom]

the past, the presence and the future exist on an equal footing.

I would much appeciate your explaining the quote above in some detail. It seems to be quite ambiguous regarding the role of an observer.

 

[Demystifier]

I would much appeciate your explaining the quote above in some detail. It seems to be quite ambiguous regarding the role of an observer.

Why do you think so?

 

[PeterDonis]

Bell's "theoroms" only applies to particles with spin

No, Bell's theorem says nothing about spin. The particular example Bell used to show that QM's predictions violate Bell's theorem used the spin of a spin-1/2 particle, but that does not mean the proof of the theorem itself involves spin. It doesn't. It is much more general than that.

his theorems and his derived inequalities do not capture all of classical physics

They do in the only way that matters for the theorem: every classical theory of physics satisfies the premises of the theorem.

and collapse to "theories" when applied to classical theories that reject the integrity of the photon

First, I have no idea what you mean by "collapse to theories" here. Bell's theorem is not a theory of physics. It is a mathematical theorem that puts a limitation on the predictions of any theory of physics that satisfies its premises.

Second, of course there is no such thing as a "photon" in classical physics. That has nothing whatever to do with what Bell's theorem says about the possible predictions of any classical physics theory.

95 % of experiments do not use Bell inequalities

If you mean they don't use the particular form of the inequalities that Bell put in his paper, yes, this is true. Other forms of the inequalities turn out to be easier to compare with experimental data. But all such inequalities are still derived from the general form of Bell's theorem.

One interpretation of these results is that the the integrity of the photon should be questioned.

Another interpretation is that photon detectors in those earlier experiments were not accurate enough to give a meaningful test of whether the relevant inequalities were violated or not. As detectors become more accurate, the experiments give better tests, and those tests are making it clearer and clearer that the predictions of QM are valid and that the relevant inequalities are violated.

(Note, btw, that the objections to the term "photon" in the Lamb paper you reference, while they are worth considering--@vanhees71, for example, has expressed similar concerns in this thread as well as many other threads here on PF--have nothing to do with Bell inequality tests. Bell inequality tests are about observables, such as clicks in photodetectors; you don't have to adopt a "photon" interpretation of the underlying theory in order to evaluate those observables and how their measured values in experiments compare to Bell-type inequalities.)

 

[Buzz Bloom]

Demystifier said:the past, the presence and the future exist on an equal footing.

It seems to be quite ambiguous regarding the role of an observer.

Why do you think so?

When I try to guess what you mean, I come up with the following.

Since the past is fixed, all events are facts. No conceptually possible alternative facts exist as a part of the past. I am guessing tha you mean that for the future to be on an equal footing there can be no alternative possibilities actually occurring in the future. That is, the events of future are completely deterministic.

The ambiguity which confuses me is that I also think you do not mean this because of the randomness of QM influencing alternative possibilities becoming the measurements of the future. For example, assuming the multi-world interpretation, when a measurement is made the observer exists in only one of two (or more) possible future worlds depending on the actual value of the measurement observed.

I can offer some other examples of the ambiguity if you think that would be helpful to your understanding of my confusion regarding what you intend.

Regards,
Buzz

 

[PeterDonis]

assuming the multi-world interpretation

This is not a good choice for your argument since the MWI is deterministic; there is no randomness at all in the MWI.

 

[Demystifier]

Since the past is fixed, all events are facts.

The point is that future events are also fixed facts, according to the block-universe interpretation. And it doesn't require determinism, probabilistic laws are also compatible with that. In the lack of determinism, we cannot compute the future events from the present ones. But it doesn't change the fact that the future event will be what it will be. If in the future a random event A will happen, then it is a fact that A will happen. It will happen randomly, but if it will happen, then it will happen. I don't know if it makes sense to you, but that's the idea of block-universe interpretation. It's up to you to decide whether you like this interpretation or not.

 

[vanhees71]

(Note, btw, that the objections to the term "photon" in the Lamb paper you reference, while they are worth considering--@vanhees71, for example, has expressed similar concerns in this thread as well as many other threads here on PF--have nothing to do with Bell inequality tests. Bell inequality tests are about observables, such as clicks in photodetectors; you don't have to adopt a "photon" interpretation of the underlying theory in order to evaluate those observables and how their measured values in experiments compare to Bell-type inequalities.)

To clarify my point of view: I don't object against the use of photons. I only object against the bad habit to sell it in terms of "old quantum theory", i.e., Einstein's flawed point of view that photons can be qualitatively understood as if they were massless point-like particles. Einstein himself was very critical against his own "heuristic viewpoint", and as we know today, he was right in being sceptical against this mishmash of quantum and classical ideas.

A photon is a well-defined concept within modern relativistic local quantum field theory. It's part of the Standard model of elementary particle physics and as such has withstood many attempts to disprove it (to the dismay of many HEP physicists who look for "physics beyond the standard model", because it seems pretty clear that it's incomplete; at least it's likely that there are more particles, explaining the nature "dark matter", and some additional mechanism of CP violation to explain our very existence).

Also almost all tree-level results of QED (like the photoeffect, Compton scattering) are identical with the semiclassical approximation (electrons/charged particles quantized; em. field classical). The most simple effect that really needs the quantization of the em. field is spontaneous emission (discovered by Einstein in 1917 when rederiving Planck's black-body radiation law from the kinetic-theory viewpoint).

To give historical justice one should mention that already Jordan in the famous "Dreimännerarbeit" quantized the electromagnetic field within the scheme of "matrix quantum mechanics". At this very early time, however, most physicists disregarded the need for quantizing the em. field, mostly due to the fact that you come very far with the semiclassical theory. Usually today one quotes Dirac as the discoverer of field quantization and spontaneous emission, but he was somewhat later.

 

[vanhees71]

So what? We all agree that classical Maxwell electrodynamics works fine as an approximation. The 95% of experiments are those within the domain where that approximation works. The other 5% are not. And if we're talking about quantum foundations, as we are in this thread, approximations are irrelevant. Your theory needs to explain all the experimental results, not just 95% of them.

Indeed, and particularly everything related to entanglement and the violation of Bell's inequalities and all that cannot be explained by the semiclassical theory (charges quantized, em. field classical). Other examples is the HOM experiment and quantum beats. For a very good and pedagogical discussion, see

J. Garrison and R. Chiao, Quantum optics, Oxford University
Press, New York (2008),
https://doi.org/10.1093/acprof:oso/9780198508861.001.0001

 

[fresh_42]

I had to delete some posts. Please remain focused on provable facts rather than opinions.

Also please refresh your browser. You are answering to posts which aren't visible anymore!

 

[Buzz Bloom]

This is not a good choice for your argument since the MWI is deterministic; there is no randomness at all in the MWI.

Hi Peter:

I was not trying to make any argument. I am trying to describe my confusion regarding the post in which @Demystifier said "the past, the presence and the future exist on an equal footing." I just searched the entire thread, and I can not find the post in which Demystifier said this. The quote seems to have vanished, perhaps due to some recent editing.

I am now also confused by what you posted: "there is no randomness at all in the MWI." I may have misunderstood what I read in Wikipedia.

https://en.wikipedia.org/wiki/Many-worlds_interpretation​

The many-worlds interpretation (MWI) is an interpretation of quantum mechanics that asserts that the universal wavefunction is objectively real, and that there is no wavefunction collapse.[2] This implies that all possible outcomes of quantum measurements are physically realized in some "world" or universe.​

I interpret this to mean that an observer in one of the many worlds who makes a measurement (which has several or many possible values) will become a corresponding multiple of himself, each in a different world corresponding to a particular value being the result of the measurement. Therefore, each observer in one of the post-measurement produced worlds will be in a randomely chosen world of the many possibilities. If this is incorrect, would you please explain the correction.

Regards,
Buzz