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?
The story: One of the major criticism on Copenhagen interpretation is that it doesn’t define where the Heisenberg cut is. In principle, if quantum is applicable to atoms and we are all made of atoms, all the way up to the whole universe, quantum should be able to describe the whole universe. So why don’t we see superposition in daily life? Where does the wavefunction collapses? One of the mysteries to physics is the nature of the mind or consciousness (here I’ll use them interchangeably). The mind might not be physical, thus not subjected to the rules of quantum, it is outside of the quantum system to look into it and collapses the wavefunction to give rise to classical notions of definite positions of particles instead of superposition of positions.
Properties analysis.
So from above, we know that in principle, a universal wavefunction can exist, collapsing wavefunction is in the theory, as well as the observer (mind) role is to collapse the wavefunction. To have the wavefunction really collapse, it is deemed as real, not just a tool for calculation as in Copenhagen. Since the wavefunction is real, collapse happens, it cannot be a local theory, as the wavefunction of an electron can in principle be extended to say the orbit of Jupiter, but once we detected it in our lab by seeing it with our eyes, the wavefunction of the electron everywhere else collapses to update the universe that there’s zero probability to find the electron anywhere else but there.
The other four properties of no determinism, yes to unique history (only one world), no hidden variable and no counterfactual definiteness follows from Copenhagen’s interpretation as there’s nothing much added except to insist that collapse happens when a mind observe quantum results.
Let’s see how classical this interpretation is. Only three out of nine properties lean towards classical preferences. A bit better than Copenhagen.
Experiments explanation
Overall speaking because it is very similar to Copenhagen, there’s little difference from the standard view, except that the wavefunction is regarded as a real thing here and superposition extends to measuring devices until it meets a conscious being.
Double-slit with electron.
Without trying to discover which slit the electron goes through, the wavefunction of the electron hits the screen, it is still not collapsed there yet, the screen goes into superposition of all possible position the electron might appear, then light from these superposition travels to the eye of the observer then to the brain, then to the mind, wherein only one of the light, corresponding to only one electron position becomes real from the collapse.
If we try to discover which slit the electron goes through, the measuring device looking on the left slit may detect or not detect the electron, and stays in that superposition of electron going through the left slit and electron going through the right slit. The superposition only collapse to give classical answer when we look into the result of the measuring device.
Stern Gerlach.
The silver atoms goes into the z measurement, then x measurement then z… the atoms are in superposition of all possible results until the signal reaches our brain and then our minds.
Bell’s test
Entanglement is real and truly, weirdly non-local. Any quantum system which interacts with one another are entangled, considered to be one quantum system. Being coherent, the quantum wavefunction maintains entanglement even as the two particles move apart. Once measurement of one of the entangled particles is made and result is read by the mind, the collapse of wavefunction on one side of the particle means the other side also collapse their wavefunction and have their property 100% predictable based on the result we have here (which is randomly obtained) and the correlation between the two particles. Example, for two electrons, if they are anti correlated, one will be spin up when the other is spin down, but the results of which will get spin up or down is not determined until measurement happens. So measuring one side in the z direction and getting the result down means we know for certain the other side is spin up in z direction. Spooky action at a distance is tolerated because we cannot use it to send signals faster than light anyway.
Delayed Choice Quantum Eraser.
A lot of people[Yu S., Nikolić D. (2011). "Quantum mechanics needs no consciousness" (PDF). Annalen der Physik. 523 (11): 931–938. Bibcode:2011AnP...523..931Y. doi:10.1002/andp.201100078.] tends to want to use this experiment to disprove this interpretation. Yet, many others[de Barros, J., Oas, G. (2017). "Can we falsify the consciousness-causes-collapse hypothesis in quantum mechanics?". Foundations of Physics. 47 (10): 1294–1308.
Andrew Knight (2020). "Quantum mechanics may need consciousness". arXiv:2005.13317.] shows that the original objection is not valid for failing to account for the need for coincidence counter to see the interference pattern.
The result of detector clicking in 1 or 2 for each individual signal photon is in superposition until it reaches the mind of an observer. The choice of erasure or not does not impact upon the wavefunction collapse, but merely chooses between particle and wave nature of the photon. Same thing for wavefunction collapse for the idler photon is to choose which detectors of 3 or 4 do the photons choose to be detected in. There’s no significant difference from the Copenhagen’s case. See the bottom example of the fiction novel for more clarity between the choice of particle and wave nature of the photon.
De Barros also proposed in his paper that it’s basically impossible to falsify this interpretation as it would require the conscious observer to be in quantum coherence to test for this interpretation, that is being very cold or isolated from the environment.
Additional example, thought experiment Wigner’s friend.
Wigner’s friend is an extension of the Schrödinger's cat. Eugene Wigner, one of the originators of this interpretation came out with this thought experiment to show support for consciousness causes collapse. Wigner and his friend, say Alice are in a lab. Alice does a simple Stern Gerlach quantum experiment and got either the result spin up or down. Wigner does not directly see the result of the experiment that Alice did, he asked Alice instead what’s the result and got it from Alice.
Before Wigner asked Alice, his model for the wavefunction of alice is alice sees up, experiment shows up in superposition with alice sees down, experiment shows down. It has not yet collapsed. Whereas Alice having already done the measurement, got the definite result of spin down electron. So they disagree on the wavefunction.
If the wavefunction is to be real, it should be agreed by different observers, so clearly the wavefunction should already be collapsed by any conscious observer, so Wigner cannot say that Alice is in a superposition state just because of his classical ignorance of the quantum result. The wavefunction was collapsed by Alice. That’s it. Be prepared for a radically different way of seeing this thought experiment in other interpretations.
Strength: Having the mind as a nonphysical entity to collapse wavefunction resolves the in principle everything physical should be subject to quantum. It allows for a universal wavefunction and thus a theory of quantum gravity. If this interpretation is true, it might allow physics some foothold into investigating consciousness using the tools of physics like maths, experiments, etc.
Weakness (Critique): A lot of people who critique this basically is uncomfortable with what they deem as putting two mysteries together. The mystery of quantum and the mystery of consciousness. Some are basically materialists, and having the mind being something special outside of physical world to be able to collapse the wavefunction, clearly does not gel with their belief that the mind is basically physical (brain).
Interesting cases: What happens to the universe before sentient beings appear in the universe? Can early universe process still happens with uncollapsed wavefunction? This is one of the critique too. This leads to some people proposing panpsychism (everything has some small degree of consciousness), or Integrated Information Theory (integrated information is a measure of consciousness).
In fiction: This interpretation is well known enough, and interesting enough to seep into science fiction works. For the book The Flicker Men by Ted Kosmatka, the author specifically make use of this interpretation as one of the key plot points in the story. The experimental set up is the double-slit experiment wherein if there’s a consciousness trying to see which slit did the particle goes through, the interference pattern disappears. This is used in the novel to first test between people who are blind vs people who can see and they found that blind people do not cause the interference to disappear. Then they found that some people even with good eyesight do not cause the interference to disappear, indicating that they do not have real consciousness or mind, that they are mere robots or programmes.
If true, it would be a very useful test to directly test for this interpretation. Yet, obviously no one in the physics community took it seriously. Why? Because there’s a misinterpretation of the double slit experiment by the author. The wave-particle duality transformation is different from collapse of wavefunction. Whether the electron behaves like a particle or wave in the double slit depends on experimental set up, that is, is it possible to detect which way the electron goes through? Once we put the measuring device near to the double slit to try to detect the electrons, they become particles. The experimental set up itself chooses the wave or particle behaviour. Once the set up is done, as analysed above, the only collapse of the wavefunction is to determine if the electron had gone to the left or right side of the slit, the screen shows only two vertical lines of electrons passing through two slits where we know which slits each electrons passes through.
This can illuminate the delayed choice quantum eraser explanation above too.
A recent paper[Narasimhan, A., Chopra, D. & Kafatos, M.C. The Nature of the Heisenberg-von Neumann Cut: Enhanced Orthodox Interpretation of Quantum Mechanics. Act Nerv Super 61, 12–17 (2019). https://doi.org/10.1007/s41470-019-00048-x] (2019) argues that delayed choice quantum eraser may imply an extension of this interpretation to include observers from outside of space and time. The delayed choice quantum eraser according to them shows that the collapse of wavefunction is outside of time, much like entanglement shows that it is also outside of space. So if wavefunction collapse is due to conscious observers, then it should be outside of spacetime and called the universal observer. They call their interpretation as the Enhanced Orthodox Interpretation. Their paper misidentify the consciousness causes collapse as part of Copenhagen interpretation and regard Copenhagen as the Orthodox interpretation hence their name. This goes to show the need for an popular book categorising the various interpretations of quantum. I wouldn’t honour their interpretation with a new page yet, as it’s very new and doesn’t seem to worth the effort to distinguish it much from this consciousness causes collapse interpretation.
There’s a second fiction book which touches upon these concepts, Stephen Baxter already had the notion of the ultimate observer in his book Timelike Infinity which was published in 1992. The reasoning is that using the Wigner’s friend example, if everyone’s wavefunction can be represented by someone else, going all the way up, wouldn’t there need to be someone ultimate at the end of time (Timelike infinity) to observe everything and collapse all the wavefunctions to the past and actualise reality? The friends of Wigner became a religious cult which was bent on sending a message to this hypothetical observer at the end of time instead of using their time travel to the past to help the past humans to defend against the future invasion of humankind by aliens.
Everyone likes to posit very bizarre and exotic ways to interpret QM, but to me it seems rather intuitive just to take it as a form of global statistical mechanics. Every "paradox" is given a trivial solution in that framework without believing that particles spread out as waves when you look at them and collapse when you look, without believing in multiverses, without even having to modify the theory, without calling into question the very existence of objective reality, etc. By global I mean that the statistical dynamics do not depend upon what the particle directly interacts with, but the entire experimental context, and thus it is a non-local form of statistical mechanics.
I have never seen a good argument against this position, yet it remains unpopular, despite it being rather conceptually simple. Any good arguments you can think of against it?
Quantum physics, popularly known as quantum mechanics is widely reputed to be not understood by anyone. For one thing, the term mechanics is a misnomer, which is why I am using the term quantum physics in this book. Mechanics, as in the classical sense implies that we know the underlying structure and how things link to cause from one thing to another in a very nice matter which we can explain, picture in our heads and use intuition to predict what happens next. Not so in quantum physics.
Before you get confused and think since no one understands quantum physics, “I will not even get the popular version of its explanation, so I also don’t understand quantum physics”, let me clarify by what physicist meant by “understanding”.
Understanding here I split into three levels.
I. Ontology (Reality): The underlying reality of things, the mechanics of which you can form a mental picture and then use intuition and basic principles to predict what happens next. This part is the one which is referred to as no one understands quantum physics.
II. Epistemology (Knowledge): The mathematical structure of quantum physics which allows us to predict many experimental values, probabilities of results, and is the reason we have electronics, nuclear physics, particle physics and so forth. The bread and butter of physicists which can be worked with as long as they follow the rules of calculations and has no clear mapping onto the ontology. This part is understood by any good physicists worth their degree.
III. Interpretation (Belief): This is the exciting field of interpreting what does the mathematics of quantum physics means. Some link it to the underlying structure, of which some commonly held assumption about the world has to be abandoned, some think the epistemology is the ontology, there is no deeper reality, some thinks a lot more weird stuff. Most of these differences either has no different prediction from the usual epistemology of quantum physics, or the prediction is still too hard to test. Which lead to some physicists to think that this is all philosophical, not worth pursuing. Yet, the mistake had been done before of not noticing non-locality sooner, thus the age of quantum entanglement came relatively late after the discovery of quantum physics more than half a century ago. So, most physicists nowadays have at least one favourite interpretation of quantum physics, which you can think of as their religion. This is because no one can prove that they have the right interpretation, at least not yet. So based on which interpretation you believe, you can say a myriad of things about quantum physics, including whether you have understood it fully or not.
So you can now confidently say physicists understand the knowledge of quantum physics but disagree on what is the reality of it, if any, based on their belief. Now, I shall attempt to make clear what is the epistemology of quantum physics or the mathematical structure of it without using equations. The following chapter follows up on the various interpretations which are out there in the market, oops, I mean the speculative field of cutting edge research realm of physics literature
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?
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.
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.
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.
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.
Experiments explanation
Double-slit with electron.
To employ the consistent histories approach, we have to divide time up to keep track of each process which happens.
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.
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:
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.
So far not very illuminating.
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.
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.
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.
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.
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.
Stern Gerlach.
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.
Time is split into t0 before any measurements, t1 between z and x measurement, t2 after x measurement.
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-
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-
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.
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.
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.
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.
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.
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.
Bell’s test.
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.
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.
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.
This is the usually what Copenhagen regard as what happens when entangled particles gets measured, there’s no pre-existing values before measurement.
Yet, consistent histories allow for the following framework as well.
Framework E:
E1: Entangled particle -> za+ zb- -> Za+Zb-
E2: Entangled particle -> za- zb+-> Za-Zb+
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.
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.
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.
What if we measure different directions at the two rooms? Say x direction for room Bodhisattva?
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.
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-
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-
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-
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.
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:
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.
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.
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.
Delayed Choice Quantum Eraser.

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.
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.
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.
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.
Let us construct some possible consistent frameworks then.
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
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.
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.
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.
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.
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
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
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.
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.
Properties analysis
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.
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.
Classical score is four out of nine. A definite improvement over Copenhagen. That’s why this interpretation boast itself as Copenhagen done right.
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.
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.
I'm not a physicist, so please forgive me if I'm mixing concepts that don't actually belong together. I'm just genuinely curious and trying to understand this intuitively.
I've been thinking about the Many-Worlds interpretation, and I got stuck on something.
Let's say there is a quantum event and the universe branches into A and B. In both branches there is a version of me. Up until that point, they have the same memories and the same past.
Why am I aware of only one of them?
What actually makes “me” experience branch A rather than B? Is there even a meaningful sense in which one of them is the “original” me?
And what happens to personal identity after the split? If both versions remember being me, are they both equally the same person up until the branching point, and then become two different people?
Another thing I've been wondering about: could two branches ever interact or interfere with each other again? If they could, would they have to meet at the same moment in time, or could something like this happen at different points along their timelines?
This also made me think about the block universe. If spacetime is a four-dimensional structure rather than something that is literally “flowing”, could the different branches be thought of as different paths within a larger structure?
And finally, if what we call “me” is not really the individual atoms in my body, but rather a continuously maintained pattern of information, structure and brain processes, does that change how we should think about identity across different branches?
Am I making a meaningful connection between quantum states, decoherence, Many-Worlds, the block universe and personal identity, or am I putting together concepts that shouldn't really be connected?
I don't necessarily believe Many-Worlds is true — I'm just curious whether this way of thinking about it makes physical sense.
I'd really appreciate an intuitive explanation rather than a highly mathematical one. I'm mostly interested in understanding what the physics actually says versus what I'm imagining.
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:
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:
In a broad sense, scientific theory can be viewed as offeringscientific 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 mainlylogical positivism, which sought to exclude unobservable aspects of reality from scientific theory.
Since the 1950s, antirealism is more modest, usuallyinstrumentalism, 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 attainmetaphysicalunderstanding of the world. The instrumentalist view is carried by the famous quote ofDavid Mermin, "Shut up and calculate", often misattributed toRichard Feynman.[11]
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.
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.
Background in physics: bachelor, phd, public, working in academia etc.
Expertise in which interpretation.
Reason for joining this subreddit: to learn, to share etc.
Also, once you had read the posts about the respective interpretations and chosen one due to your personal preferences, you can apply a user flair on which interpretation you currently believe in.
Change in your belief is allowed and is part of the fun of science and philosophy.
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.
I'm wondering how popular quantum interpretations would explain the quasar in Wheeler's delayed-choice experiment.... does retrocausality need to be involved?
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?.....
To better understand the maths, let’s get familiar with at least one experiment first to get a picture in the mind.
Young's double-slit experiment with electrons
The set up is just to put a traditional double slit in the path of an electron beam, shot out from an electron gun to see if there would be interference in the results or not.

Picture from Wikipedia
Historically, the issue of waves vs particle nature of things started all the way back to Newton. Newton thought that lights are particles, perhaps due to geometrical optics where you can trace the path of light through lenses by just drawing straight lines. There is also a common-sense answer (which ignores how small light's wavelengths are) that if light is a wave, how can our shadows be so sharp instead of blurry?
Thomas Young back in 1800s first did the double-slit experiment on light. It's basically the same set up as the picture above, just replace the electron gun with a light from a lamp, which is focused via a small hole. Laser hasn't been invented yet then. As light passes through the double slit, if it is made out of particles, we should only see two slits of light at the screen, yet we see an interference pattern!
Wait a minute you might say. You go get a torchlight, cut out two slits out of a cardboard and shine the torchlight through the slits, you see two slits of light shining through. Where is the interference pattern? The caveat for the double-slit is that the size of the slit and the distance between the slit should be roughly around the wavelength of whatever waves you wish to pass through it. And the wavelength of light is around 400 to 700 nanometres. For comparison, the size of a bacteria is about 1000 nanometres. The enlarged slits in the picture are merely for illustration purposes, it's not to scale.
What can produce an interference pattern? Waves. Observe the gif below. Waves can meet with each other and if they happen to be in phase at the position where they meet the screen, constructive interference happens, the amplitudes add up and you see light-gathering there. If they happen to have opposite amplitude at another position, destructive interference happens and you are left with a dark region. Destructive interference is also what happens when you use noise-cancelling headphones.

gif from wikipedia
So Thomas Young settled that lights are waves after all, with wavelengths being very small, thus our shadows seem sharp. Next up, Maxwell showed that light is electromagnetic waves with a calculable theoretical speed. Thus it was with great difficulty to accept again that light maybe particles in some other situations. That's why Planck didn't believe the mathematical trick he did had a physical significance. And Einstein was pretty much didn't get much support when he took the idea of photon (light as particles) seriously.
Louis de Broglie had some idea that if waves have particle-like properties, might not particles also behave like waves? It took a long time, but finally, the proper experiment was done using electron beams fired from electron guns towards the double slit only to find (to no one's surprise by then) that yes, electrons exhibit interference pattern too.
What's so hard for classical thinking and expectations to accept is that a thing is either a particle or a wave. How can it exhibit particle-like behaviour in some cases and wave behaviour in other cases just for the convenience of explaining what happens in certain cases? Quantum thinking would have to accept a certain relaxation of this criterion that a thing must be either a particle or a wave. So it could be that they have both properties which are real (as advocated by Bohm's interpretation), or that they behave like wave or particles depending on how we set up the experiment (Copenhagen interpretation). Or some other possibilities. It's a common practice to not be too concerned with our language to say it's a particle-wave. Usually, we just use the term particle and the wave properties are understood to be there when needed.
Let's take a breath here to reflect that you might not find the results so far as strange at all. I had to point out what kind of thinking (classical) would make these results weird. If you had at all heard that quantum physics upends a lot of classical notions, you would have already come in, prepared to have an open mind and not be attached to classical thinking. So you readily see nothing weird about quantum physics, just a different set of rules. You might be gradually be used to the quantum logic pathway to make sense of quantum, which are called the modal interpretations.
Continuing on the double-slit experiments, there are quite a few additions to the basic experiment to exhibit some other properties of quantum systems.
First, the experiment can be done with single particles. A single photon, or single electrons or other particles. Single as in the particles gets shoot through the slit one by one. If it passes the slit, we use a super-sensitive detector, capable of detecting one particle at a time and also recording the position of where is the particle detected. Over time, the interference pattern can be seen to be build up again. One by one, the particles somehow knows where to land in order to rebuild that interference pattern.
It gives a creepy feeling for people to think that somehow a single particle has to use its wave properties to feel both slits in order to land at the positions which is consistent with the interference pattern. So a particle can interfere with itself! Different interpretations will give different pictures of this phenomenon. So don't be attached to the first two sentences of this paragraph!
Second variation, we can try to observe which path did the particle took on its way to the screen. There are many subtle details and recent developments in this bit, elaborated more later on when we discuss wave-particle duality.
For now, the simplified version is if we put a measurement device to detect if the particles would go through one slit or another. As long as we can have the information of which path, left slit or right slit was taken by the individual particles as they pass, we see no interference pattern; the particles make a pattern of two slits on the detector.
The act of observing things (with or without consciousness involved is interpretation dependent) changes what happens to the thing you observe. Do take note that the observation need not necessarily involve consciousness and the most important thing is the measuring device is present. Also, we shall see this property that measurement changes quantum systems even in the Stern-Gerlach experiment later.
Perhaps the most important take away is that do not place all your eggs onto one interpretation yet. Have some patience and an open mind to keep on reading and participate in the analysis. An interpretation of quantum mechanics means it currently has no way to experimentally distinguish itself from other interpretations, or the experiments done to do so had not been thought of yet, or it is not yet technically feasible, or it was done but not universally conclusive and persuasive yet. So no point to attach to one viewpoint (interpretation) based on the notion: it agrees with my view.
We shall move on to the mathematical structure of quantum and includes introducing the axiom of quantum as taught to physics undergraduates even now. Many of the terms are repeated there, so don’t worry. You’ll get a better picture of the maths there.
Let us start by appreciating the history first as this will be the basis of your mental picture of what quantum physics is before it gets very abstract in the mathematical structure.
Light in Newton’s days was considered to be particles, but Thomas Young with his famous double-slit experiment showed that light interferes with each other if the distance between the two slits is close to the light’s wavelength, thus light became a wave. This notion became solidified when Maxwell came out with the speed of light from the electromagnetic equations, showing that light is an electromagnetic wave, travelling at the speed of light. Thus we have the picture that electromagnetic waves unite all these radiations as one, just differing by their frequencies. From the shortest frequency to highest, we have radio waves, microwave, infrared, visible light from red to violet (following the rainbow colour arrangement), ultraviolet, X-rays and finally gamma rays. It is based on this wave theory of light which got us into the ultraviolet catastrophe.
The first sign of quantum is when Max Planck used the Planck’s constant, h to fit in the data for the black body radiation in 1900. Basically, classical theories cannot explain how light interacts with matter, predicting that as light gets to a higher frequency, and lower wavelength, there will be more ways for energy to be emitted from the matter (like when the matter is heated up). When it goes further up the ultraviolet frequency, there should be even more amount of energy emitted. This is in contrast with the experimental fact where the most common frequency of a hot body peaks depending on its temperature. Thus you see fire changes colour from red to blue as it gets hotter, and not like spontaneously releasing unlimited gamma rays. Physicists called the failure of classical theories in this area as the ultraviolet catastrophe. The X-rays and Gamma rays haven’t been discovered and named yet, or else it would be called the gamma catastrophe, which would bring about the mental image of the Hulk in most people’s mind nowadays. Maybe it is fortunate naming because this has nothing to do with the Hulk.
Planck just helped to hack the system by fitting the data in by making sure energy exchanged between light and matter happens in the form of discrete amount of energy, proportional to its frequency, linked by Planck’s constant. This is instead of splitting the energy between modes of lights which increases with the square of frequency, and allowing continuous exchange of energy between matter and light as the classical theory assumed. Planck did felt that his fitting was a mathematical trick and do not believe what the equations told him about the nature of light. That it is quantised. Hence the word quantum in quantum physics came about.
Albert Einstein then in 1905 provided the physical interpretation of this usual behaviour by suggesting that lights are particles. We call them photons. Photons as particles carry a discrete amount of energy depending on its frequency. This also explains the photoelectric effect where light only kicks out electrons from metal if its frequency goes high enough (hence enough energy per photon to kick out the electrons), regardless of its intensity (amount of photon). The electrons need a preset amount of energy to be kicked free from the metal, weak low-frequency photons can bump onto the metal all they want, but cannot combine their energy to kick out the electrons. Thus light is no longer considered as continuous wave containing continuous energy, but as photons, particles of light containing quantised energy. By the way, this is the reason Einstein got that Nobel Prize of his, not his general relativity.
This was the beginning of the crisis of interpretation.
How can a particle explain the double-slit experiment? If we assume that many photons go through the slit then maybe the particles interfere with each other. However, experiments had gone to the point where we can send individual photons to the double-slit and still after collecting enough data, the interference pattern emerges! Did the particles somehow split into two and interferes with itself? Did it interacted with a split parallel universe version of itself and recombined to form the interference? Did the particle travel through time and go through both slits at once interfere with itself and came back to the present to land on the screen? Mental pictures of the quantum world are starting to break down as we insist on using classical concepts onto the quantum particle. Weirder still, try to find out which slits did the photon goes through, then once we know which slit and cannot erase the information, the interference is gone. We get two slits of light for light going through two slits. Light behaves like a particle when information about which slit it goes through is revealed and cannot be erased away without any copies of that information. So it seems that observation changes the outcome, something totally alien to the classical world of physics where it is assumed that the observer can observe and do not affect the observed system. You might have heard of this phenomenon is called wave-particle duality. Light behaves like a wave or particle depending on our decision to observe or not to observe which path it had taken.
It seems magical now, the nature or properties of light changes depending on what we do! Some take it as there is no underlying mechanics (reality/ nature) of quantum, some disagree, this becomes a matter of interpretation. Keep in mind that the experiments and ideas which physicists came out with helped them to develop the mathematical structure of quantum theory and step by step lead them away from having a classical mental picture of reality. However, those mathematics can be used to explain and predict experimental results, because it is developed mainly to fit in with experimental results.
Next came Niels Bohr, who in 1913 introduced the atomic model which explains how atoms can be stable and the emission lines of the hydrogen atom. According to classical electromagnetic theory, if the atom is to behave like our solar system, with the nucleus of the atom in the middle like the sun and the electrons orbiting it like planets, then the electron is undergoing acceleration. Yet the electron is a charged particle, accelerating charged particle according to classical electromagnetic theory emits electromagnetic radiation. This is how radio and TV waves can be transmitted and received with the antenna. So if the electron is radiating electromagnetic waves, it must be losing energy and very soon sucked into the positively charged nucleus and the atom is destabilised. If the electrons do not move, then it will be attracted into the nucleus anyway. So it is an utter mystery how atoms which subparts of positive and negative charged particles, and the positive ones in the middle can exist at all.
Bohr suggests that electrons can only occupy some orbits, the ones which respect discrete angular momentum. Angular momentum is like momentum, spinning objects tend to remain spinning without outside forces (or torque in this case). Thus if the electrons are at the lowest orbit, it means that it cannot fall into a smaller orbit. Its angular momentum is at the lowest and cannot be reduced. There are no in-between orbits between two lowest orbits, thus angular momentum is quantised, or discretised. This, by the way, is the origin of the concept: quantum jump. As electrons cannot be found in between orbits, but jump from one to another. This is in very much contrast with our usual notion of classical motion as there is no smallest unit of jump or movement unlike in quantum systems.
In 1924, Louis de Broglie proposed that since light can behave like particles, might not particles like electron can behave like waves? The de Broglie wavelength for particles is Planck's constant over the momentum of the particle. So for very massive objects, our wavelengths are far too small for quantum effects to manifest. However, for small objects, their momentum means that their wavelength can be calculated and we can put electrons to the double-slit experiment and see that it interferes as light does. Electrons do show wave properties!
In 1925 and 1926, two different ways of getting the basic equations of quantum mechanics correct were discovered, first the matrix mechanics by Heisenberg, then the wave mechanics by Schrödinger. Both are shown to be equivalent to each other, that is different ways of expressing the same thing.
Both concepts have the concept of a state of the quantum system and an observable. The state of a quantum system is this abstract concept not directly accessible to us. What we see from experiments are the observables. Both have a system of evolution which can tell how change happens. In Heisenberg picture, the state remains constant and it is the observable that changes in time; whereas the opposite happens in the Schrödinger picture. We can call this the stage one of the quantum mechanics calculation: evolution equations. This is about the equivalent of any classical physics evolution in which time is part of the equation that tells how everything else in the equation changes or remain constant in time.
After seeing how the evolution happens, we want to know what we can observe. In classical physics, the things we can observe are obvious. Position, velocity, acceleration, force etc. Yet, state is not directly observable to us. So in quantum physics, we have to use Born's rule to translate the results of stage one of quantum mechanics to do stage two, the probabilistic part. Born's rule tells us that from the results of stage one, we can get the probability amplitude of the system. One for each possible results we can observe. Square the probability amplitude and we can get the probability density of finding each results of the experiments. And strange enough, that accurately describes all sorts of quantum experiments we care to do.
Now it is worth it to pause here and link this presentation to the usual ones you might have read in many popular physics books. If this is your first popular physics book, then just go along for the ride to recognise the terms on your second popular physics book which talks about the basic quantum theory.
Usually, the presentation uses only the Schrödinger’s picture. It's using an equation which is more familiar to physicists in the early 1900s. Wave equations. At that time, wave had united electromagnetism, optics, sound, linking to many dynamics and kinematics equations, have close relationship with the simple harmonic motion and so on. So physicists were very glad to see this familiar old friend in an unfamiliar new theory. At least for a while.
In the Schrödinger picture, quantum systems have their own wavefunction, which is the state stated above. In the Copenhagen interpretation of quantum mechanics, the wavefunction contains all possible information for whatever questions or observable you wish to ask or measure on the system. In practice, we just write the wavefunction according to the relevant observable we are interested in.
The observables can be position, momentum, energy and so on. It's the usual quantities classical physics can make sense of. So we can apply the wavefunction to the Schrödinger's equation, which roughly means how the total energy evolution of the system evolves for this particular state. The evolution here is deterministic, the same wavefunction going through the same Schrödinger's equation will yield the same resultant wavefunction to any time you care to set to. This is still stage one.
In stage two we apply the observables unto the wavefunctions to get the respective probability amplitudes for each possible results of the observable. Eg. If I want to find the position of an electron in free motion, I apply no potential energy at the Schrödinger's equation, evolve its initial wavefunction to the one I want at a certain time. Stage one completed, stage two follows. Then measure the position at that time by applying the position observable unto the wavefunction, obtaining the probability density of the position of the electrons.
If you are not mathematically inclined or had never studied quantum physics with its maths before, the above might sound gibberish to you. And it sure is very much so to many physicists in a different way. To us, we can compare it to how do you find the position of a ball in free motion. Use Newton's first law. If the ball is at rest, there is no external force on it, it remains at rest. If it is in motion, without fiction, then it will continue to be in motion.
The difference is that the evolution equation operates at stage one in quantum, a stage which is mysterious, hidden from us and all we see is the probabilistic results of stage two. There is no stage one stage two in classical physics, the evolution is clear and visible to us.
And that folks, is quantum mechanics proper. Just the maths. The story of what it means is down to the interpretations. Here lies the mystery of the quantum. Why is there two stages in the calculation? What story, if any, can we give to why is stage two probabilistic, is nature inherently non-deterministic or is it some information is hidden in stage one which we cannot know even in principle?
When Richard Feynman said, "I can safely say nobody understands quantum mechanics", he was not referring to the maths side. He is referring to the story side. With the maths side, we have the knowledge and capability to calculate and predict the probability distributions of the experimental results and so far experiments had been on the side of quantum mechanics. The calculation of molecular bonds in theoretical chemistry rely on solving super complicated equations of quantum mechanics. We can do all of these if we understand how to use the maths, even if it is super complicated.
The surprising thing is, even without knowing the underlying story of the two stages of quantum calculations, the maths still works well, predictions can be made. Nature does not seem to care if humans demand for a story.
Without that story, for you, the general layperson to predict anything in quantum systems, you would have to learn the maths. Yet, there are a few general guidelines developed in the Copenhagen interpretation, not all of which is adopted by other interpretations. Some of it you might have heard of: wave-particle duality, complementarity, superposition of states, Heisenberg uncertainty principle, inherent randomness.
We will go through them later on so as not to overly bias you towards the Copenhagen interpretation.
Why is the story important? Notice that when I used the classical ball example, I can just quote one law (Newtonian mechanics), then we can predict how the ball will behave. That's because the classical laws directly paint an obvious story for us to see and once we internalise the story, we can use it to do predictions of what will happen. In other words, it gives us power. To understand how nature works. But haven't we already know how to do predictions with quantum mechanics? What's the difference? The difference is in the intuition. The world does not behave in a quantum behaviour in our everyday experience. So as we have the intuition of how classical physics works, we would like to see if there is any underlying mechanism behind the two stages.
Brian Greene uses a theatre performance as an analogy in his book: The Fabric of Reality. In the theatre, we see the front stage, that's the probability density calculated in stage two of the quantum calculation.
Yet there is also a backstage, the place where actors change clothes really fast, where the spotlights are directed, where special effects and props are prepared, hidden until it is used. That's the stage one of the quantum calculations, the state of the quantum systems, the wavefunction. Hidden from the audiences, we do not even know to consider them real quantities in the world, or just reflections of our understanding for us to do the maths. In classical physics, the backstage is clear to us, for example, general relativity we say mass-energy curves spacetime, spacetime tells mass-energy how to move.
To make such a simple statement (or more likely, paragraphs of statements) for quantum physics means selecting one of the interpretations.
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.
I will mainly discuss the list on the table of comparisons taken from wikipedia. Table at the interlude: A quantum game.
Deterministic.
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.
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.
2. Wavefunction real?
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.
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.
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.
3. Unique History
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.
Classical preference: Yes, classically, we prefer to refer to history as unique.
4. Hidden Variables
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.
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.
5. Collapsing wavefunction
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
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).
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.
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.
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.
6. Observer’s role
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?
Classical preference: Observer has no role. Reality shouldn’t be influenced just by observation.
7. Local
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.
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.
8.Counterfactually definite
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.
Classical preference: Of course we prefer reality is there. The moon is still there even if no one is looking at it.
9. Universal wavefunction
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.
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.
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.
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.
Jim Baggott in his book Quantum reality does nicely list out what do we mean when we say real.
Realist Proposition #1: The Moon is still there when nobody looks at it (or thinks about it). There is such a thing as objective reality.
Realist Proposition #2: If you can spray them, then they are real. Invisible entities such as photons and electrons really do exist.
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.
Realist Proposition #4: Scientific theories provide insight and understanding, enabling us to do some things that we might otherwise not have considered or thought possible. This is the ‘active’ proposition. When deciding whether a theory or interpretation is realist or anti-realist, we ask ourselves what it encourages us to do.
Many quantum interpretations reject Realist proposition no. 3, not so much no. 1 which a lot of people misunderstood.
For many years, quantum interpretations have been suppressed in physics, first from the Copenhagen interpretation which results from physicists having education in philosophy back before world war two. Then after the second world war, many fundings going into physics treat physicists as pragmatic tools of war, for the cold war. Pragmatism and specialisation made a lot of physicists have a negative view of philosophy. Today (in the year 2020), the subreddit r/quantum outright banned posts on quantum interpretations, but allow for quantum foundational posts like Bell’s theorem. I created r/quantuminterpretation to give a platform for all to learn and discuss this interesting aspect of physics.
Many physicists nowadays in the research of quantum foundations do cling onto the pragmatic attitude of instrumentalism. Copenhagen was influenced by the logical positivist philosophy which that philosophy had been brutally beaten down in the 1970s.
For many years, even after Bell’s theorem has been popularised, there has been a niche left unfilled in the popular physics books. Many books which introduce quantum mainly base their presentation on Copenhagen interpretation. While there are some interpretations like many worlds and pilot-wave theory which became relatively popular, there are astonishingly more than 15 different interpretations out there.
I always wanted to search for a book for quantum interpretations. If there were so many interpretations and physicist cannot rule them out yet, why is there no fair, impartial manner in which they are introduced? Why should the public and physicist be exposed to some bias in their philosophy to prefer one or another interpretation that they are personally exposed to first?
I highly doubt that when the questionnaire of which interpretation they prefer is given to physicists, that the questionnaire is fairly answered because I don’t think that most physicists had been exposed to understanding the various interpretations. In particular, during our quantum mechanics classes in undergraduate physics courses, we certainly had not to need to learn through all the interpretations, we mainly only get the gist of Copenhagen and the shut up and calculate attitude.
It’s only recently that more and more popular books on quantum interpretations appear in the market. This is one of them, as an offshoot as I write a bigger book on Physics and Buddhism. As such I am distilling out the Buddhist elements from the writing for the general audience, but certain terms are left behind as it’s not likely to hinder the reader’s understanding of the texts. Like label of A and B, I use Buddhist terms of Arahant and Bodhisattva.
It’s my hope that this book can be widely read by all physicists to complete their understanding of what might quantum mean. As for general readers, this is the book with the most interpretations I had seen and they are expanded upon enough that you can get the gist of their stories. Once you had read this, your knowledge of quantum interpretations is likely to rival any professional physicists and maybe even surpasses them, except in the maths part.
whether entropy has any deeper connection with philosophy, particularly life, time, existence, and the concepts of order and chaos. I'd love to hear different perspectives beyond just the scientific explanation.
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?
