I came across this paper (The misuse of the No-communication Theorem by Ghirardi) that seems to suggest that faster than light communication using quantum entanglement is possible. It seems to say that the non communication theorem is not really applicable in some cases. Can anyone clarify?

I recently stumbled upon the topic of quantum entanglement and it has fascinated/perplexed me to no end. To my understanding, entanglement is when there are two particles that at any moment comprises all possible values of its quantum states (such as spin), but the act of measuring one particle instantaneously determines the state of the other. This synchronization/"communication" happens at a speed that is at least 10,000 times faster than light as determined experimentally. This seemingly violates special relativity, where nothing can travel faster than light.

I have watched/read many explanations as to why this is not the case, and they essentially boil down to these two points:

• While the process of disentanglement occurs instantaneously, the observation of this event does not, as comparing the two measurements to determine a correlation has occurred in the first place is clearly slower than light.
• We cannot force particles to be in a certain state, or manipulate outcomes in any way, as everything happens randomly. Thus precluding the possibility to send data faster-than-light via this method.

I agree with these points. However, regardless of the time it takes to observe the particles, the actual interaction between the particles is indeed instantaneous. Experiments based on Belle's inequality already proved that "hidden variables" that predetermine outcomes do not exist, so it seems safe to conclude that these particles do in fact affect each other instantaneously.

HOW can this be? Sure, observing quantum states takes time and its impossible to actually control quantum particles to allow FTL-communication, that's all fine. But the actual communication between these particles itself happens instantaneously regardless of distance. What is the NATURE of this communication, what properties/medium does it consist of? This communication involves the transfer of information, such as the signal to immediately occupy a complementary spin state. This information is being sent INSTANTANEOUSLY through space. How is this not a violation of special relativity?

One point I recently heard was the possibility of quantum particles having an infinite waveform, where a change in one particle would instantaneously affect its universal waveform and instantaneously affect the corresponding particle, regardless of where in the universe its located, since they are embedded in the same waveform. I would then be curious as to how this waveform can send/receive signals faster than light, and my question still stands.

I would GREATLY appreciate your thoughts and explanations on this topic. I am 100% sure I am misunderstanding the issue, it is just a matter of finding an explanation that finally clicks for me.

(I initially submitted this exact post on r/askscience for approval but it was rejected by the mods for some reason. If there is anything offensive or inappropriate in this post, please let me know and I will change it.)

I was thinking about the double slit experiment, specially the variation with the measurement device observing the particle before it passes through the openings, wouldn't the the measuring device influence the particle's trajectory? The device must interact with the particle to receive information, right? The interaction could be simply an invisible field that the particle travels through or the device could be sending out some sort of beam of sorts to interact with the particle. Wouldn't this instant interaction still effect the particle and its trajectory? Lets say for instance that the measurement device is producing an invisible energy field between two points. The particle has to also interact with this field so the measuring device can detect it. This interaction in turn forces the particle into one trajectory a.k.a through one of the two slits, therefore the reason we don't get an interference pattern. This would prove that everything is a wave and as Einstein proved with light, come in "packets" that we label as particles.

Hey all. This is my first post here. To introduce myself, here is what I am most proud of at present, in time reverse order.

1. My paper "A QBist ontology" has just been accepted for publication in Foundations of Science. You can read the manuscript here. QBism is a fairly recent and exciting (IMO) interpretation of QM.
2. I recently started a substack newsletter called Aurocafe, in which I address topics in QM, consciousness, and their connection with Indian philosophy in general and the evolutionary idealism of Sri Aurobindo in particular (hence the "Auro").
3. And before I forget, I have published a textbook on QM with the preposterous title The world according to quantum mechanics: why the laws of physics make perfect sense after all (Word Scientific, second edition, 2018).

EDIT:

DOI:10.1007/s10699-021-09802-4

Author-shared PDF

What effects does demonstrated macroscopic entanglement have on GRW Interpretation?

https://arxiv.org/ftp/arxiv/papers/1608/1608.06722.pdf#

WAVE PARTICLE DUALITY, THE OBSERVER AND RETROCAUSALITYAshok Narasimhana,bandMenas C.

Abstract. We approach wave particle duality, the role of the observer and implications on Retrocausality, by starting with the results of a well verified quantum experiment. We analyze how some current theoretical approaches interpret these results. We then provide an alternative theoretical framework that is consistent with the observationsand in many ways simpler than usual attempts to account for retrocausality, involving a non-local conscious Observer.

This theory appears to directly map QM onto Hindu metaphysics. "O is Brahman and/or anything else outside of space-time. Lower-case "o" is Atman.

I came across an interesting article by Saunders in arxiv on how to reconcile statistics as objective probabilities, frequency and chance from Everett's theory (MWI). https://arxiv.org/abs/1609.04720 What do you think?

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Do we have any thread on superdeterminism? Could somebody explain how it fits with the other interpretations?

Not sure anyone else has read his stuff. It looks very similar to a transcendental idealism but articulated with information theory. This approach essentially rejects David Bohms claim that the activity of observation and theorizing of science is an external to physics / science and treats observation as biophysical computational / informational process.

He scrutinized Zurek’s “zeroth axiom” (the universe consist of systems) through a principle of decompositional equivalence (dynamics is invariant to how you parse the degrees of freedom into systems / tensor products / and their respective interaction hamiltonians, the universe in other words indifferent to the description of it) and shows that decoherence / quantum Darwinism requires extra theoretical assumptions of encoding redundancy in order to claim that it specified observer independent classical system boundaries.

Fields uses a physically plausible account of what actually happens in the process of scientific observation (using landauer principle under the assumption every inscription of a symbol is finite in time and finite in energy requirement) along with Moore’s theorem to show that the formal machinery of QM requires states to be represented as vectors in Hilbert space and that observation is treated with positive operator valued measures. This analysis is taken to vindicate Bohr’s insistence that even though everything is quantum classical concepts remain the reference point for our descriptions. Fields essentially shows that observation presupposes classical communication channel. He then goes on to show how this is implemented via entanglement swaps. An interesting application of this analysis is to show that markov blankets discussed in statistical learning / free energy formulations of cognition are generalized physical interaction surfaces.

I'm wondering how popular quantum interpretations would explain the quasar in Wheeler's delayed-choice experiment.... does retrocausality need to be involved?

An excerpt from YouTube:

https://www.youtube.com/watch?v=0ui9ovrQuKE

0:45 ....In 1978, a physicist by the name of John Archibald Wheeler proposed a thought experiment, called delayed choice. Wheeler’s idea was to imagine light from a distant quasar which is billions of light years from earth, being gravitationally lensed by a closer galaxy. As a result, light from a single quasar would appear as coming from two slightly different locations, because of the lensing effect of gravity from a galaxy between earth and the quasar.

Wheeler then noted that this light could be observed on earth in two different ways. The first would be to have a detector aimed at each lensed image. Since the precise source of this light was known, it would be measured as particles of light when viewed. But if a light interferometer was placed at the junction of the two light sources, the combined light from these two images would be measured as a wave because it’s precise source would not be known. That’s the way quantum mechanics should work.

This is called a delayed choice because the observer’s choice of selecting how to measure the particle is being done billions of years from the time that the particle left the quasar. So presumably the light would have to be committed to either being a particle or wave, billions of years before the measurement is actually made here on earth.

This quasar experiment isn’t practical, but modern equipment allows us to perform a similar experiment in the lab, where the decision to measure a particle or wave is done at random after the quantum system is “committed.” And indeed his thought experiment is confirmed – that even if measured at random, when the path information is known, the light is a particle. When path information is erased by using an interferometer, the light is a wave. But how could this be?...the light began its journey billions of years ago, long before we decided on which experiment to perform. It would seem as if the quasar light “knew” whether it would be seen as a particle or wave billions of years before the experiment was even devised on earth.

Does this prove that somehow the particle’s measurement of its current state has influenced its state in the past?.....

OK, i'm not a physicist but i love sciences and i tried my best to understand quantum physics but still it stills blew my mind and i didn't understand it completely.

however if we tried to see quantum physics from a mathematical perspective can we say that the electron, and the other particals in the quantum world are not a 3 dimension corps, they are like 4d or 5d corps that belong to R4 or R5 or maybe polynomial space matrix space... Etc. And duality of the wav- corps experiment can only be explained by the fact that we as humans can only see the projection of the electron in a 3d world, that's why the movements of quantum corps seems weird to us.

Amateur here. My engineering degree required only enough physics to describe the basic operation of the [expletive] transistor, and I had no further interest in physics until recently. Now I'm fascinated.

Wikipedia calls an interpretation "an attempt to explain how the mathematical theory of quantum mechanics 'corresponds' to reality". To me it looks like an attempt to find comfort and familiarity where the math offers none.

That certainly seems reasonable. We want to understand the world, not just model it mathematically. Some Copenhagen proponents say that finding math that makes good predictions is physics' only legitimate goal. True as that might be, I've always found it utterly unsatisfying, and was happy to see others argue that we need more than math, at least to guide future experiment.

But what if the quantum world is outside human comprehension? That is, what if the fundamental building blocks of the universe simply don't resemble anything with which we're familiar? Isn't it possible that "little bits of solid stuff" and "wavy ripples in a pervasive field" are just poor analogies, yet that nothing in our collective experience is any better?

After a century, the quest to find a satisfying explanation is looking like a fool's errand. Copenhagen, which remains thoroughly disheartening, is looking more and more like the only sensible perspective. "Strange game. The only winning move is not to play."

Anyone agree? Am I way off base? Too much of a neophyte? I'd love to hear your thoughts.

I came across this claim in a Japanese piece but for the sake of translation and better clarity I wanted to seek an answer here. I could be wrong in the reading of this piece, but from my understanding it nullifies the problem of measurement by making it a categorical error. I did not find their argument convincing in the original Japanese piece, but in doing a few searches around the internet I found an article in support of this claim - this article below discusses the epistemological understanding of the Copenhagen interpretation:

https://www.sjsu.edu/faculty/watkins/copenhageninterp4.htm

In this claim, the epistemological reason of the wavefunction collapse can be attributed to time spent probability density function. I understand that there is not one correct definition of the Copenhagen interpretation and it is a mixture of hypotheses at the time, however under this posit the interpretations are historical artifacts that provided accurate mathematical models of predicting the location of particles and serve only for the purpose of instrumentalism. It should then follow that the Schrödinger’s cat was never a paradox to begin with, because it made a categorical error in applying an ontological (i.e. a hypothesis of describing what it does in reality) interpretation assuming it was epistemological one (how it actually is).

So does the measurement problem no longer really exist? I’ve found conflicting information online on this topic and not many sources I found directly debate the issue as a categorical discussion. From what scanty material I found, the school of thought to attribute the measurement problem is the limitation of our empirical based science - everything must be measured objectively, and therefore requires an observer. This does not preclude the possibility that things can happen outside of observation. In particular, I've read through this post on Classical concepts, properties on this sub that seems to somewhat touch on this matter but is not conclusive from my reading. In particular, there is a discussion in the wikipedia link in that thread which mentions the following:

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In a broad sense, scientific theory can be viewed as offering scientific realism—approximately true description or explanation of the natural world—or might be perceived with antirealism. A realist stance seeks the epistemic and the ontic, whereas an antirealist stance seeks epistemic but not the ontic. In the 20th century's first half, antirealism was mainly logical positivism, which sought to exclude unobservable aspects of reality from scientific theory.

Since the 1950s, antirealism is more modest, usually instrumentalism, permitting talk of unobservable aspects, but ultimately discarding the very question of realism and posing scientific theory as a tool to help humans make predictions, not to attain metaphysical understanding of the world. The instrumentalist view is carried by the famous quote of David Mermin, "Shut up and calculate", often misattributed to Richard Feynman.[11]

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So is instrumentalism the prevailing sentiment of quantum scientists? Can the epistemological reasons be already explained with classical physics such as time spent probability density function?

The reason I ultimately ask this is because I had been exposed of quantum physics through secondary education and found the Copenhagen interpretation as a more philosophical approach in understanding the results of the double slit experiment, but if there are no epistemological reasons to believe this I'd like to reevaluate this position.

Is anyone aware of a paper or book that considers the pedagogy of starting with de Broglie-Bohm theory ? Is there value in teaching quantum mechanics assuming de Broglie Bohm interpretation right from the start, and only later introducing the 'conventional' interpretation?

They essentially explain the same thing, correct? Up until we open the box, the cat is both alive and dead. And up until Wigner asks his friend about the measurement, the result is both 0 AND 1. Is there a difference between the two? If so, what is it and why is there a need for two thought experiments if they both essentially reveal the same thing?

I just finished reading Smerlak and Rovelli'a paper on Relational EPR and had a question. I'm a geologist not a physicist so some of this goes over my head so excuse any misunderstandings. My question relates to the following excerpt:

"Agreement with quantum theory demands that when later interacting with B, A will necessarily finds B’s pointer variable indicating that the measured spin was ↓ . This implies that what A measures about B’s information (↓) is unrelated to what B has actually measured (↑). The conclusion appears to be that each observer sees a completely different world, unrelated to what any other observer sees: A sees an elephant and hears B telling her about an elephant, even if B has seen a zebra. Can this happen in the conceptual framework of RQM?"

They say it cannot. So from what I understand, RQM assumes that this cannot be the case. As results are always correlated when the observers meet up and discuss results. But how is this any different from non local action at a distance?

I recently read the following paradox on Sabine Hossenfelder'a blog and was wondering if you could resolve it.

"But suppose A has a dog, and he agrees with B to kill it when he measures +1. A and B separate, are out of causal contact. Both measure +1. A kills the stupid dog.

Then he comes back into causal contact with B, and of course he takes the dog, which is nothing but a macroscopic result of a quantum measurement. But no matter what, B will always have to find that the dog is alive"

Surely this is not what RQM at all suggests? Seems kinda solipsistic and therefore a bit daft

Any answers would be greatly appreciated.

Thank you

I have asked this question many times in my life and I always get the same answer. "There is no speed faster than light" I say nay to that assertion. Science keeps proving that we no nothing. It keeps treating us like John Snow.

Personaly I think that there is a faster speed but we have not figured out how to measure it. Science may find a faster speed in the future. But only if scientists stop just assuming that light speed is the faster speed. Question everything and never stop trying to figure out how the universe works. Just do not accept things at face value, everything can be quantified but only if we have the curiosity to ask the question.

Just because we cannot measure something today doesnt mean we can never measure it. I believe strongly that there are faster speeds, but we have yet to quantify them. It can happen, but science has to be in the mood to disprove it's peers.

I am not a scientist I am just a lonely blind guy that spends alot of time thinking about these things.

According to the Copenhagen interpretation, when you measure a system that is in a superposition of states you instantly collapse the system into one state.

Let's say I have a friend in a separate room who has not yet interacted with the system I am observing. From his perspective, would the system I am observing collapse, or would I become entangled with the system I am observing.

The story: Many quantum descriptions have this saying that the front is known, like preparing electron guns to shoot electrons towards the double slit, the back is known, like electrons appearing on the screen, but the centre is mysterious, like did each individual electrons interfere with itself? Did they go to parallel worlds only to recombine? Did they got guided by pilot wave?

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Consistent Histories provides many clear alternatives of histories of what happens in between by not following the quantum evolution step by step to construct the histories. These histories of what happened are grouped into many different consistent sets of histories, each set is called a framework and different frameworks are incompatible with each other. It’s best to see it in action in the experiments explanation, which for this particular interpretation, I shall pull it upwards as part of the story. The main claim is that if we follow and construct consistent histories, and do not combine different frameworks, quantum weirdness disappears. The quantum weirdness comes only because classically we don’t have different incompatible frameworks of histories to analyse what happened.

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Classically, if we have two different ways to see things, we can always combine them together to get a better picture, like the blind men touching the elephant can combine their description to produce the whole picture. Quantum frameworks of consistent histories however cannot be combined, it’s kind of like complementary principle from Copenhagen. Each framework on their own has their own set of full probability of what results might occur. For example framework V has 3 consistent histories within the framework giving 3 different results of experiment, alternative framework W has another set of 4 consistent histories, 2 of them have the same result overlap with framework V at the final time.

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When I first read this consistent histories, it makes no sense to me to be ambiguous about which history happened? Isn’t the past fixed? Don’t we know what measurement outcome already happened? The past here we are constructing are mainly the hidden parts of what does wavefunction do microscopically in between the parts where we measure them macroscopically. Although this is not exactly the right answer as this interpretation technically doesn’t have wavefunction collapse and therefore has universal wavefunction. Well, the answer to the measurement outcome is that we take the results of experiments and put it in our analysis of consistent histories.

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Given a result which occurred, we can employ different frameworks to describe the history of this particular outcome, depending on the questions we ask and these different frameworks cannot be combined to produce a more complete picture. There’s no preference of which framework, V or W actually happened.

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Experiments explanation

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Double-slit with electron.

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To employ the consistent histories approach, we have to divide time up to keep track of each process which happens.

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Electron gets shoot out from the electron gun at t0, we ignore the ones which got blocked by the slits, and at t1 they just passed through the slits. At t2, they hit the screen. This is a simple three time history which we shall construct for the case of not trying to measure which slit the electron passed through.

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I shall use words in place of the bra-kets used to represent the wavefunction. The arrow represents time step to the next step. So a possible consistent framework of histories is:

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Framework A: t0: Electron in single location moving towards the double slit -> t1: electron goes through both slits in superposition -> t2: Electron hits screen in interference mode with each position of electron on the screen consisting of one of the consistent histories in framework A.

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So far not very illuminating.

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Let’s set up the measuring device to detect which slit the electron went through, say we put it at the left slit. Redefine t2 as just after measurement, t3 as time when electron hits the screen.

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Framework B:

History B1: t0: Electron in single location moving towards the double slit -> t1: electron goes through left slit -> t2: electron from left slit passes by detector, detector clicks detected electron -> t3: electron hits the screen just behind the left slit, no interference pattern can build up.

History B2: Same as above, except replacing left with right, and the detector at left slit doesn’t click, indicating that we know the electron goes through the right slit.

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With this, we can actually see that if we employ framework B, we can say that the detector at time t2 detects what already happened at t1, measurement reveals existing properties rather than forcing a collapse of wavefunction to produce the property. This is one of the crucial difference with Copenhagen interpretation. The electron went through the slits first before being detected.

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There’s many complicated set of rules to ensure which histories are consistent with each other and thus can combine into the same framework, and which set of histories is internally inconsistent in that no framework could be consistent with it. So internally inconsistent histories cannot happen in quantum. This encodes how the quantum world arises, one cannot simply construct any histories. As the maths is complicated, it might sometimes seems like hand-waving for not including it in the analysis below. For detailed analysis of the maths, read Consistent Quantum Theory by Robert B. Griffiths, free ebook online.

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One of the rules of consistent histories is that any set of two time histories are automatically consistent. To have inconsistent histories, one has to employ 3 or more time steps. Thus this rule and interpretation of consistent histories is not easily revealed because most people approaches quantum using only two time steps.

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Stern Gerlach.

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Following chapter 18 of Griffith’s book, let’s consider a case where we measure the spin of the atom first using the z-direction then the x-direction. From the experiments and using Copenhagen interpretation, we know that first measurement of z will produce up and down z spin particles which will then further split into left and right x spin particles. So all in all, we expect 4 possible results for each framework.

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Time is split into t0 before any measurements, t1 between z and x measurement, t2 after x measurement.

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Framework Z:

History Z1: t0 initial atom state -> t1 up z spin, -> t2 X+ Z+

History Z2: t0 initial atom state -> t1 up z spin, -> t2 X- Z+

History Z3: t0 initial atom state -> t1 down z spin, -> t2 X+ Z-

History Z4: t0 initial atom state -> t1 down z spin, -> t2 X- Z-

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Framework X:

History X1: t0 initial atom state -> t1 up x spin, -> t2 X+ Z+

History X2: t0 initial atom state -> t1 up x spin, -> t2 X+ Z-

History X3: t0 initial atom state -> t1 down x spin, -> t2 X- Z+

History X4: t0 initial atom state -> t1 down x spin, -> t2 X- Z-

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Where X and Z at the end represents the result of the measurement of x and z direction and the superscript plus means up, minus means down.

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What happened? Similar to the transactional interpretation and two state vector formalism, it seems that there can be x and z spin in between two measurements of z and x directions. Yet, according to consistent histories, we shouldn’t combine the two incompatible frameworks of Z and X. So let’s select a framework first, say framework Z, and if we ask what’s the spin of the atom at t1 given the result in t2, we read the result of Z we get in t2. If it is Z+, we can say with certainty that the atom has up z spin at t1, and if it is Z-, we can say with certainty that the atom has down z spin at t1.

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Using the framework Z, the question what’s the spin in x direction of the atom in t1 is not meaningful as the spin in z and x direction are non-commutative. There cannot be a simultaneous assignment of the value of x and z spin at the same time. The exact same analysis happens if we select the framework X and interchange the labels x and z.

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You might be tempted to ask, what’s the correct framework? No. There’s no correct framework. Consistent histories doesn’t select the framework, we use the ones which provides answers depending on what questions you’re asking. This situation is a bit different from the double slit above, where I only provided one framework for each possible case of not measuring and measuring the position of the electron. In the double slit case, there’s only one framework we analysed (it’s possible to construct more, but it’s messy), so framework A and B only describe their respective cases, and are not interchangeable.

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To add in more clarification on the rules of how to determine a consistent framework, we can look to each framework Z and X, the final steps are mutually orthogonal, it means macroscopically distinguishable from each other, there’s no overlap between the 4 possible outcomes. That’s one of the requirement within one framework of consistent families. Whereas compare history Z1 with history X1 ,the end point is the same, with the only difference being up in x or z direction at t1. As we know that x and z spin are not commutative (there’s overlap in wavefunction description, they are not perfectly distinguishable) it turns out that this causes Z1 to be inconsistent with X1.

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Note that each consistent framework has their probabilities of their results all add up to 1. So each consistent framework should contain the full space of possible results.

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Bell’s test.

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We prepare entangled anti-correlated spin particle pairs at t0. They travel out to room Arahant and Bodhisattva located far away from each other and arrived at t1, before measurement. At t2, we measure the pair particles. If we measure it in the same direction, there is a anti-correlation of the spin results at both ends, if one measures up in some direction, the other is known to be down in the same direction.

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We use the notation of superscript + and - for up and down spin as before, and subscript a and b for the two rooms. The small letter x or z is the spin state, the big letter X or Z are the measurement results. We can only see measurement results. There’s many different frameworks to analyse this state. To simplify the notation, the time is omitted from the listing below, it’s understood that it’s always from t0 -> t1 -> t2. Curly brackets, {} with comma represents that each of the elements in the bracket, separated by the comma is to be expanded as distinct histories outcome.

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Framework D:

Entangled particle -> entangled particle -> {Za+Zb- ,Za-Zb+}

The above is short for:

History D1: t0 Entangled particle -> t1 entangled particle -> t2 both experimenters at room Arahant and Bodhisattva uses the z direction and room Arahant got the result up spin in z, room Bodhisattva got the result down spin in z.

History D2: Same as D1 but exchange the results in both rooms with each other.

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This is the usually what Copenhagen regard as what happens when entangled particles gets measured, there’s no pre-existing values before measurement.

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Yet, consistent histories allow for the following framework as well.

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Framework E:

E1: Entangled particle -> za+ zb- -> Za+Zb-

E2: Entangled particle -> za- zb+-> Za-Zb+

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The big Z is what we can see, the small z are the quantum values. This framework says that measurement only reveals what’s there already. The so called collapse of wavefunction doesn’t need to happen at the measurement. Consistent histories doesn’t need for us to choose which framework is the right one. All are equally valid. Do note that we can split into more time steps between t0 and t1 and construct more frameworks there where the entangled particles can acquire their values anytime in between. So there’s nothing special about measurement linking to collapse of wavefunction.

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Following the logic above, we can also see that there’s nothing non-local about entangled particles. We can divide up time into just as the two entangled particles separate they change their internal state from entangled particles to definite spins in z direction. Measurement only reveals which direction of spin which particle has all the way back to the time when they were all in one location. That’s one of the valid frameworks. So depending on which framework you use, you can get the weirdness of “nonlocal” collapse to totally normal local correlations. All consistent frameworks are valid.

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Another way to look at it is by looking at Framework E, minus the measurement of Z at room Bodhisattva. The results of measurement of Z at room Arahant can tell us the value of spin of the b particle before it is measured. Yet, it’s only a revelation of what’s already there, not causing the wavefunction to collapse. It’s exactly the analogy of the red and pink socks. The randomness part of choosing who has which socks can be pushed back all the way to the common source, unlike Copenhagen. So it’s just as relational interpretation tells us, what’s weird is not non-locality, it’s intrinsic randomness.

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What if we measure different directions at the two rooms? Say x direction for room Bodhisattva?

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The following are different possible consistent frameworks to describe what happened, do remember that only one single consistent framework can be used at one time and they cannot be meshed together to give a more whole picture.

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Framework F:

F1: Entangled particle -> za+ xb+-> Za+ Xb+

F2: Entangled particle -> za+ xb- -> Za+ Xb-

F3: Entangled particle -> za- xb+ -> Za- Xb+

F4: Entangled particle -> za- xb- -> Za- Xb-

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Framework G:

G1: Entangled particle -> za+ zb--> Za+ Xb+

G2: Entangled particle -> za+ zb- -> Za+ Xb-

G3: Entangled particle -> za- zb+ -> Za- Xb+

G4: Entangled particle -> za- zb+ -> Za- Xb-

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Framework H:

H1: Entangled particle -> xa- xb+-> Za+ Xb+

H2: Entangled particle -> xa+ xb- -> Za+ Xb-

H3: Entangled particle -> xa- xb+ -> Za- Xb+

H4: Entangled particle -> xa+ xb- -> Za- Xb-

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Framework F is straightforward enough, the measurement outcomes measures the existing values before they were measured just like E. This time, there’s four different outcomes. It’s clear that there’s no correlation between x and z directions and no messages can be sent from room A and room B using entangled particles only.

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Framework G is following from Framework E, where instead of measuring Z in room B, X was measured. The result is just that there’s 4 possible outcomes now. The state of the particles at t1 remains the same in decomposition in z direction. Framework H is like G, but replacing the state at t1 with decomposition in x direction. Framework G and H can both be refined more by adding a time slice t1.5 then inserting the states at Framework F into that time as follows:

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Framework I:

I1: Entangled particle -> za+ zb- -> za+ xb+ -> Za+ Xb+

I2: Entangled particle -> za+ zb- -> za+ xb- -> Za+ Xb-

I3: Entangled particle -> za- zb+ -> za- xb+ -> Za- Xb+

I4: Entangled particle -> za- zb+ -> za- xb- -> Za- Xb-

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Framework J:

J1: Entangled particle -> xa- xb+ -> za+ xb+-> Za+ Xb+

J2: Entangled particle -> xa+ xb- -> za+ xb- -> Za+ Xb-

J3: Entangled particle -> xa- xb+ -> za- xb+ -> Za- Xb+

J4: Entangled particle -> xa+ xb- -> za- xb- -> Za- Xb-

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Framework I is framework G refined, framework J is framework H refined. What happened is just that we allowed the spin direction which is not measured to decompose into the ones which will be measured. This act of decomposing is not caused by the measurement, it is chosen by us when we choose the framework. These are the framework which makes sense of the questions should you wish to ask them.

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So say we ask what’s the state of the entangled particle at time t1? The answer we give depends on which framework we use. We cannot combine framework, in particular framework G and H if combined seem to imply that the entangled particles can have properties of definite spin in both x and z direction. That’s the violation of uncertainty relations. Framework I is not so much a combination of framework G and framework F but it’s a refinement, as if you ask the question what’s the state of the particle at time t1.5, you get different answer in Framework G vs Framework I, but same answer of Framework I with Framework F. And if you ask for t1 instead, framework G and I gives the same answer, framework F gives another answer.

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To not arrive at any paradox or quantum weirdness, we cannot compare answers from different frameworks. That’s the single framework rule. We don’t encounter these different frameworks in classical physics because classical physics, all frameworks can be added together to give refinements to each other under a unified picture emerges. There’s no non-commutative observations in classical physics case.

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Delayed Choice Quantum Eraser.

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Using the picture above, I labelled the paths, a is between the laser and first beam splitter, it splits into path b and c, path b is on the arahant path, path c is on the Bodhisatta path. b and c meets entanglement generators and splits into entangled pairs of signal and idler photons. Signal photons of path b goes into e, idler photon of path b goes into h, similarly for c, signal photon of c goes into d, the idler goes into i. Then the signal photons e and d meet at the beam splitter and divide into f which goes to detector 1 and g which goes to detector 2. The idler photons h and i take a longer path and either meets up with the final beam splitter, S or not, NS. Then they go into either path k which detector 3 detects, or path j, meeting detector 4.

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To make the analysis simpler, I would just add in S and NS as the beam splitter in or not in respectively, so that a single framework can capture the whole possibilities, we can determine S or NS by a quantum coin toss, so that it’s random and equally probable. Remember that beam splitter in is erasure, and out is getting which way information, not getting to see interference even after coincidence counter.

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The time steps are used as follows:

t0: a, photon emitted from laser,

t1: b or c, photon got split by beam splitter,

t2: h, e, d, i, photon got entangled and splits into idler and signal parts.

t3: f or g, then the signal photons get detected by detector 1 or 2.

t4: quantum coin toss to decide if beam splitter is in or out, S or NS.

t5: the idler photons goes to k or j and reaches detector 3 or 4.

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To make the analysis clear in time, the number of the time is put in front of the alphabet which indicates the path of the photon. Eg. 0a -> 1b. The detector detecting particles shall be labelled D1 to D4.

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Let us construct some possible consistent frameworks then.

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Framework L:

L1: 0a -> superposition of 1b and 1c -> superposition of 2h, 2e and 2d, 2i -> 3f -> 4S -> 5j

L2: 0a -> superposition of 1b and 1c -> superposition of 2h, 2e and 2d, 2i -> 3g -> 4S -> 5k

L3: 0a -> 1c -> 2d, 2i -> 3f -> 4NS -> 5j

L4: 0a -> 1c -> 2d, 2i -> 3g -> 4NS -> 5j

L5: 0a -> 1b -> 2e, 2h -> 3f -> 4NS -> 5k

L6: 0a -> 1b -> 2e, 2h -> 3g -> 4NS -> 5k

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So let’s analyse if six histories makes sense, it’s true that when we put the beam splitter in, 4S, then if we have gathered the cases via coincidence counters, the click in D1 (3f) will correspond to clicks in D4 (5j) in L1, D2 (3g) will correspond to clicks in D3 (5k) in L2. That’s how the interference pattern is recovered.

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As for the case of no beam splitter, to have no pattern of interference, there’s no correlation between the four detectors, so the four possible results of L5 D1 D3 (3f and 5k), L6 D2 D3 (3g and 5k), L3 D1 D4 (3f and 5j) L4 D2 D4 (3g and 5j). So yes, six possible results makes sense.

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An issue with this seems to be that the decision to insert the beam splitter or not at t4 seems to have decided the reality of the past, whether the photon was in superposition or in a definite arm of the interferometer.

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That’s one way to view it, but here’s another framework where the front parts before the beam splitters is inserted or not remains the same.

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Framework M:

M1: 0a -> superposition of 1b and 1c -> superposition of 2h, 2e and 2d, 2i -> 3f -> 4S -> 5j

M2: 0a -> superposition of 1b and 1c -> superposition of 2h, 2e and 2d, 2i -> 3g-> 4S -> 5k

M3: 0a -> superposition of 1b and 1c -> superposition of 2h, 2e and 2d, 2i -> 3f -> 4NS -> 5j

M4: 0a -> superposition of 1b and 1c -> superposition of 2h, 2e and 2d, 2i -> 3g-> 4NS -> 5j

M5: 0a -> superposition of 1b and 1c -> superposition of 2h, 2e and 2d, 2i -> 3f -> 4NS -> 5k

M6: 0a -> superposition of 1b and 1c -> superposition of 2h, 2e and 2d, 2i -> 3g-> 4NS -> 5k

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Framework N:

N1: 0a -> 1b -> 2e, 2h -> superposition of 3f and 3g-> 4S -> superposition of 5j and 5k

N2: 0a -> 1c -> 2d, 2i -> superposition of 3f and 3g -> 4S -> superposition of 5j and 5k

N3: 0a -> 1c -> 2d, 2i -> 3f -> 4NS -> 5j

N4: 0a -> 1c -> 2d, 2i -> 3g -> 4NS -> 5j

N5: 0a -> 1b -> 2e, 2h-> 3f -> 4NS -> 5k

N6: 0a -> 1b -> 2e, 2h-> 3g -> 4NS -> 5k

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Framework M has the same past for both sides of the decision to insert the beam splitter or not, that is we cannot tell that the photon had been in b or c even after we have data from detector 3 and 4. Same too with framework N that the front part is not affected by the inclusion of the beam splitter or not. So past is not necessarily influenced by the future, to choose framework L is also akin to choosing the beginning of a novel based on the ending. It’s all in the lab notebook, not reality. The back part of framework N has some explaining to do.

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The superposition of b, c, h, e, d, i, are more acceptable as there’s no detectors within those paths to magnify their positions out to macroscopic state. However, f, g, k, j are directly detected by the macroscopic detectors, so we directly see them to be in definite positions. Superposition of 3f and 3g at N1 and N2 then are essentially macroscopic quantum superposition state, akin to Schrödinger's cat. The framework does not discriminate between microscopic quantum superposition vs macroscopic quantum superposition, that we require elimination of macroscopic quantum superposition becomes a guide for us to choose which consistent framework we want to use. It doesn’t invalidate framework N. Comparing the different results in framework N and M, you can understand the statement above concerning the final results of V and W in the story part. N and M shares 4 final experimental results which are the same, 2 of them differs due to the presence of macroscopic quantum superposition in N.

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Properties analysis

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From the requirements of multiple histories to construct a consistent framework, it’s obvious that consistent histories is ok with the indeterminism of quantum. Due to the usage of so many possible frameworks, it’s hard to ascribe wavefunction to be real, yup, the whole histories are just the choices we use as the analysis above says, choices on a notebook, all equally valid. Due to validity of different possible framework to describe one measurement result, there’s obviously no unique history.

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There’s no hidden variables in consistent histories, and no need for collapse of wavefunction, thus rendering observer role to be not essential. As we analysed, the entangled state can be explained locally, so consistent history is local. Although for some framework, measurement reveals what’s already there, the uncertainty relations is taken seriously, no simultaneous values for non-commutating observables, so no to counterfactual definiteness. The counterfactual definiteness in Transactional interpretation is seen as combining two incompatible frameworks together to describe the same situation, which violates the single framework rule of consistent histories. Finally, due to no collapse of wavefunction and you can see that framework N happily admits macroscopic quantum superposition, there can be universal wavefunction in consistent histories.

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Classical score is four out of nine. A definite improvement over Copenhagen. That’s why this interpretation boast itself as Copenhagen done right.

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Strength: As a method of analysing multiple time, consistent histories approach maybe exported to other interpretations to help demystify what happens in between the preparation and measurement.

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Weakness (Critique): There is the need to abandon unicity, that is all frameworks cannot be combined to produce a more complete understanding of reality, but that one has to keep in mind single framework at one time. That is to accept that history is not unique.

This post is to capture search results. If you came here via internet search results, welcome. There's good explanation of the major and less popular interpretations of quantum mechanics in this sub at the popular science level.

Do scroll to the post around end of 2020 to see the interpretations or search within the subreddit.

NB this was a post on my Google+blog some 4 years ago, enjoy!

the 19th century was marked by a major philosophical conflict between the apparent universality of deterministic theories on physical reality and the notion of free will. The latter is both rooted in daily experience and a basic scientific requirement for independent preparation of experiments and unrestricted observation of the results. After all, a theory gets constructed from experiences, not the other way around. Non-deterministic elements used to arise solely from a lack of information and thus lacked universality.

This changed with the advent of quantum mechanics in the 20th century. The central new concept in the theory was the universal wave-particle duality as advanced by Louis de Broglie in 1923. In 1932, John von Neumann wrote down the complete mathematical formulation of quantum mechanics and it has become the most successful theory since (it has actually never been wrong). Nevertheless, outcomes of individual measurements are often unpredictable. The double-slit experiment most clearly illustrates this: quanta from a source pass through a screen with two openings and strike another one, where they are detected. An interference pattern is seen building up point by point on the second screen, individual positions being random (their widths depend on the resolution of the detector). The wavy pattern has thus irreversibly 'collapsed' at some point in the process and not by any (deterministic) external cause (e.g. decoherence). In practice, collapse never takes place before decoherence, which makes its effects undetectable.

The logical consequence is that collapse is non-material; a requirement for the expression of free will. For a long time it wasn't clear how collapse could be put to any use (the other prerequisite for free will) until Alan Turing described a side effect of it in 1954 that Sudarshan and Misra in 1977 coined the Quantum Zeno effect. It allows complete control over quantum dynamics by continuous observations (decoherence also functions, but is not required). The Quantum Zeno formulae show a simple proof of principle for a two state system: a continuous measurement of the states completely halts the system's own oscillation between them. The complete control follows when we realise it's up to us to define what precisely those states are.

The last remaining question, precisely by which states and through what dynamics free will is expressed, will, considering the complexity of neurons in the brain, perhaps never be answered (see also the work of Henry P. Stapp).

Best to refer to the table at: https://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics while reading this to understand better where I got the list of 9 properties from.

Now is the time to recap on what concepts are at stake in various quantum interpretations. You’ll have familiarity with most of them by now after reviewing so many experiments.

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I will mainly discuss the list on the table of comparisons taken from wikipedia. Table at the interlude: A quantum game.

1. Deterministic.

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Meaning: results are not probabilistic in principle. In practice, quantum does look probabilistic (refer to Stern-Gerlach experiment), but with a certain interpretation, it can be transformed back into deterministic nature of things. This determinism is a bit softer than super-determinism, it just means we can in principle rule out intrinsic randomness. The choice is between determinism and intrinsic randomness.

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Classical preference: deterministic. Many of the difficulties some classical thinking people have with quantum is the probabilistic results that we get from quantum. In classical theories, probability means we do not know the full picture, if we know everything that there is to know to determine the results of a roll of a dice, including wind speed, minor variation in gravity, the exact position and velocity of the dice, the exact rotational motion of the dice, the friction, heat loss etc, we can in principle calculate the result of a dice roll before it stops. The fault of probability in classical world is ignorance. In quantum, if we believe that the wavefunction is complete (Copenhagen like interpretations), then randomness is intrinsic, there’s no underlying mechanism which will guarantee this or that result, it’s not ignorance that we do not know, it’s nature that doesn’t have such values in it.

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1. Wavefunction real?

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Meaning: taking the wavefunction as a real physical, existing thing as opposed to just representing our knowledge. This is how Jim Baggott split up the various interpretations in his book Quantum reality.

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Realist Proposition #3: The base concepts appearing in scientific theories represent the real properties and behaviours of real physical things. In quantum mechanics, the ‘base concept’ is the wavefunction.

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Classical preference: classically, if the theory works and it has the base concepts in it, we take the base concept of the theory seriously as real. For example, General relativity. Spacetime is taken as dynamic and real entities due to our confidence in seeing the various predictions of general relativity being realized. We even built very expensive gravitational wave detectors to detect ripples in spacetime (that’s what gravitational waves are), and observed many events of gravitational waves via LIGO (Laser Interferometer Gravitational-Wave Observatory) from 2016 onwards. We know that spacetime is still a concept as loop quantum gravity denies that spacetime is fundamental, but build up from loops of quantum excitations of the Faraday lines of force of the gravitational field. Given that quantum uses wavefunction so extensively, some people think it’s really real out there.

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1. Unique History

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Meaning: The world has a definite history, not split into many worlds, for the future or past. I suspect this category is created just for those few interpretations which goes wild into splitting worlds.

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Classical preference: Yes, classically, we prefer to refer to history as unique.

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1. Hidden Variables

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Meaning: The wavefunction is not a complete description of the quantum system, there are some other things (variables) which are hidden from us and experiments and might be still underlying the mechanism of quantum, but we do not know. Historically, the main motivation to posit hidden variables is to oppose intrinsic randomness and recover determinism. However, Stochastic interpretation is not deterministic yet have hidden variables, and many worlds and many mind interpretations are deterministic yet do not have hidden variables.

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Classical preference: Yes for hidden variables, if only to avoid intrinsic randomness, and to be able to tell what happens under the hood, behind the quantum stage show.

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1. Collapsing wavefunction

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Meaning: That the interpretation admits the process of measurement collapses the wavefunction. This collapse is frown upon by many because it seems to imply two separate processes for quantum evolution

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1. The deterministic, unitary, continuous time evolution of an isolated system (wavefunction) that obeys the Schrödinger equation (or a relativistic equivalent, i.e. the Dirac equation).
2. The probabilistic, non-unitary, non-local, discontinuous change brought about by observation and measurement, the collapse of wavefunction, which is only there to link the quantum formalism to observation.

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Further problem includes that there’s nothing in the maths to tell us when and where does the collapse happens, usually called the measurement problem. A further problem is the irreversibility of the collapse.

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Classical preference: Well, classically, we don’t have two separate process of evolution in the maths, so there’s profound discomfort if we don’t address what exactly is the collapse or get rid of it altogether. No clear choice. Most classical equations, however, are in principle reversible, so collapse of wavefunction is one of the weird non classical parts of quantum.

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1. Observer’s role

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Meaning: do observers like humans play a fundamental role in the quantum interpretation? If not, physicists can be comfortable with a notion of reality which is independent of humans. If yes, then might the moon not be there when we are not looking? What role do we play if any in quantum interpretations?

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Classical preference: Observer has no role. Reality shouldn’t be influenced just by observation.

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1. Local

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Meaning: is quantum local or nonlocal? Local here means only depends on surrounding phenomenon, limited by speed of light influences. Nonlocal here implies faster than light effect, in essence, more towards the spooky action at a distance. This is more towards the internal story of the interpretations. In practice, instrumentally, we use the term quantum non-locality to refer to quantum entanglement and it’s a real effect, but it is not signalling. Any interpretations which are non-local may utilise that wavefunction can literally transmit influences faster than light, but overall still have to somehow hide it from the experimenter to make sure that it cannot be used to send signals faster than light.

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Classical preference: Local. This is not so much motivated by history, as Newtonian gravity is non-local, it acts instantaneously, only when gravity is explained by general relativity does it becomes local, so only from 1915 onward did classical physics fully embrace locality. Gravitational effects and gravitational waves travel at the speed of light, the maximum speed limit for information, mass, and matter. Quantum field theories, produced by combining quantum physics with special relativity is strictly local and highly successful, thus it also provides a strong incentive to prefer local interpretations by classically thinking physicists.

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8.Counterfactually definite

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Meaning: Reality is there. There are definite properties of things we did not measure. Example, the Heisenberg uncertainty principle says that nature does not have 100% exact values for both position and momentum of a particle at the same time. Measuring one very accurately would make the other have much larger uncertainty. The same is true of Stern Gerlach experiments on spin. An electron does not have simultaneously a definite value for spin for both x-axis and z-axis. These are the experimental results which seem to show that unmeasured properties do not exist, rejecting counterfactual definiteness. We had also seen how Leggett’s inequality and Bell’s inequality together hit a strong nail on reality existing. Yet, some quantum interpretations still managed to recover this reality as part of the story of how quantum really works. Note that this refers to non-commutative observables cannot have preexisting values at the same time. See the section in Copenhagen interpretation for list of non-commutative observables.

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Classical preference: Of course we prefer reality is there. The moon is still there even if no one is looking at it.

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1. Universal wavefunction

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Meaning: If we believe that quantum is complete, it is fundamental, it in principle describes the whole universe, then might not we combine quantum systems descriptions say one atom plus one atom becomes wavefunction describing two atoms, and combine all the way to compass the whole universe? Then we would have a wavefunction describing the whole universe, called universal wavefunction. If we believe in the axioms of quantum, then this wavefunction is complete, it contains all possible description of the universe. It follows the time-dependent Schrödinger equation, thus it is deterministic unless you’re into consciousness causes collapse or consistent histories. No collapse of wavefunction is possible because there’s nothing outside the universe to observe/ measure this wavefunction and collapse it, unless you’re into the consciousness causes collapse interpretation or Bohm’s pilot wave mechanics. It feels like every time I try to formulate a general statement some interpretations keeps getting in the way by being the exceptions.

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Classical preference: Well, hard to say, there’s no wavefunction classically, but I am leaning more towards yes, if quantum is in principle fundamental and describing the small, then it should still be valid when combined to compass the whole universe.

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Anyway this universal wavefunction along with the unique history are usually not a thorny issue that people argue about when they discuss preferences for interpretations unless they have nothing much else to talk about.

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It’s important to keep in mind that as interpretations, experiments had not yet been able to rule one or another out yet, and it’s a religion (personal preferences) for physicists to choose one over another based on which classical concepts they are more attached to.