14 ms·

A similar result for superposition would be huge, I guess, after a century we could finally understand what qualifies as a measurement device, why we do not obs

A similar result for superposition would be huge, I guess, after a century we could finally understand what qualifies as a measurement device, why we do not observe quantum effects at macroscopic scales, what actually saves or kills Schrödinger's cat.

sebzim4500 2y ago
There's a result in quantum information that demonstrating a superposition of two states is at least as hard as switching between those states. So designing an experiment to show that a cat is in a superposition of alive and dead is at least as hard as bringing a dead cat back to life (obviously in real life it would be much harder, this is just a lower bound).
- Obscurity4340 2y ago
  Isn't the cat experiment bullshit outside of illustrating a macro version of this? Like, there's no superposition in reality or the cat is too big to be subject to quantum mechanics where these shenanigans happen or something?
  - deleted 2y ago
    [deleted]
  - hiciu 2y ago
    The cat isn't in some quantum state. Cat is locked in a box, with some poison and a Geiger counter. Poison is released when Geiger counter registers a particle. Now, the process of atom decay that would be registered by a Geiger counter has some interesting properties and that's where the "quantum magic" happens. (at least, that's my mental model of it; ask an expert, I'm not an expert)
    - immibis 2y ago
      The quantum magic properties are thought to transfer to the poison and the cat.
    - rcxdude 2y ago
      the idea is that everything is in some quantum state, because quantum mechanics attempts to describe everything. The thought experiment is basically going from the well-observed (and explained in theory) superposition of a nuclear undergoing decay to the never-observed (but still valid in theory) superposition of a cat being both alive and dead.
      - Obscurity4340 2y ago
        Why should it be valid in theory tho? What is the purported bottleneck in physical reality that is supposed to exist whose presence makes the Schrodinger cat thought experiment realistic? How does sentient observation get the ball rolling and why couldn't that happen in the absence of such observation at this scale?
  - lupusreal 2y ago
    Cat observes itself.
    - IX-103 2y ago
      That defeats the point of the exercise. The whole point was about the state of some isolated system. Otherwise we could say that the detector observes the state of the system without even needing the poison or the car
      - megaloblasto 2y ago
        It doesn't defeat the point of the exercise, it just shows that the exercise is nonsense. The cat is an observer therefore collapses the quantum wave function and is either alive or dead.
        baq 2y ago
        In what sense the cat is an observer? Having eyes and a brain doesn't really mean anything in QM.
        lupusreal 2y ago
        If the particle interacts, that collapses the superposition. Cats aren't any more or less blessed as observers than physicists. If either of them can observe the outcome, then both can.
        murftown 2y ago
        Hmm. Maybe what "counts as an observer" is relative to your reference frame, and so is the concept of "superposition". And thus, though the cat is making observations within its environment, since the environment is sealed off, the cat is not "an observer" from our reference frame because there is no interaction between us and the cat. From the cat's reference frame it is probably not in a superposition - If it is observing anything, it is alive, and if it is dead, it is not observing anything. But if we observe the cat, the wave function is collapsed from our reference frame and the cat is in one or the other state. My understanding of quantum mechanics is pretty shallow so take this with an ocean of salt.
  - ben_w 2y ago
    Schrödinger's thought experiment was supposed to demonstrate that superposition itself (and/or the whole of the Copenhagen interpretation) was a nonsense concept… but every test so far says it's real, that he was wrong, and that the cat would be in both states "until observed". But nobody knows what "observed" even means.
  - gus_massa 2y ago
    The problem is to isolate the box enough. If you can heard the cat meow, then it counts as a "measurement" and the superposition disappears. If you can hear there is no heartbeat of the cat, then it counts as a "measurement" and the superposition disappears. If the cats move and the box wags, then it counts as a "measurement" and the superposition disappears... So in practice the experiment only works with very small systems with a few atoms. Each year there are experiments with bigger systems, but they are usually frozen and very small. Perhaps one day we will be able to do the experiment with tardigrades.
    - Vampiero 2y ago
      Hearing at all implies sound waves. If there are soundwaves there is air inside the box. If there is air inside the box the system can't be in a superposition because it has long since decohered. In other words it's not the act of "listening" that collapses the wave function but the interaction of air molecules that creates the sound in the first place, even if no one is there to listen. A "measurement" or an "observation" doesn't actually require a human, or a living thing. It's just a physical interaction.
      - rvbissell 2y ago
        Catch 22: and if there is no air inside the box, then you are certain the cat is dead.
      - Maxatar 2y ago
        This isn't quite right, wave function collapse is relative so while within the box an observer's wave function of the box will be in a definite eigenstate, the wavefunction of the box for an observer isolated from the box will still be in a superposition of its states. In other words, the fact that an observation takes place from within the box and collapses the wave function for an observer in the box does not mean that the wave function for every observer also collapses. A wave function collapse for one observer does not imply a wave function collapse for any other observer.
        2y ago
  - ahazred8ta 2y ago
    Schrödinger never believed it; he gave that as an example of something that could never happen in real life.
  - sebzim4500 2y ago
    As far as known physics goes, nothing is too big to be subject to quantum mechanics. It is very hard to demonstrate that fact though, the obvious way of doing it is to build a large quantum computer.
Joker_vD 2y ago
> what qualifies as a measurement device Anything, really. There are no "classical devices", after all, all of them are quantum systems, aren't they?
- danbruc 2y ago
  Quantum systems evolve unitarily, measurement devices collapse the wave function which is not unitary. And there is no accepted consensus how those two things fit together, just several ideas like Everett's many worlds which solves the problem by declaring that the wave function never actually collapses but instead splits the cosmos into many worlds.
  - Joker_vD 2y ago
    Everett's idea was merely that the measurement device gets entangled with the thing being measured, so we now the wave-function of the (combined) device-thing system is the superposition of states <thing is in pure state Ai| ⊗ <device is in "measured Ai" state|; but for the measurement device it of course "looks" like it measured a pure state. That's it. That's basically what the decoherence suggests anyway. The "cosmos splitting" was proposed by Bryce DeWitt which IMHO turned a perfectly straight-forward and reasonable interpretation into a needlessly philosophical statement.
    - layer8 2y ago
      Entanglement means that the states become macroscopic in noisy (and not too high-temperature, apparently) environments. Whether you call those states “worlds” or not doesn’t change the philosophical implications.
    - kgwgk 2y ago
      > Everett's idea was merely that the measurement device gets entangled with the thing being measured That idea is almost as old as quantum mechanics itself. (von Neumann, 1932 - if not earlier.)
    - danbruc 2y ago
marginalia_nu 2y ago
We have this result already. Interaction with thermal noise ruins wave coherence, and causes classical physics to emerge. This comes when you in a sense replace waves with their average values. It's not so much a question of scale, but of statistical noisiness. Quantum effects are primarily observable in the low-temperature domain (this is not necessarily "thermometer temperature", but a statistical measure)
- namibj 2y ago
  Even lasers deal with weird temperatures: they're negative.
- danbruc 2y ago
  I am not a physicist, but I thought that it is generally accepted that decoherence does not solve the measurement problem. It explains why quantum effects are not observable in a noisy environment but still does not explain the non-unitarity of measurements.
  - marginalia_nu 2y ago
    It does not solve the measurement problem, but it does explain why you don't see quantum phenomena such as superposition or tunneling at the classical limit.
- ko27 2y ago
  This doesn't answer the question. We know how classical physics emerges from a large number of quantum events through statistics. That was not the point of Schrödinger's cat though experiment! The idea is to link a macroscopic event to a random subatomic event that is in a superposition. When or how does the wave function collapse, does it even collapse?
- ruthmarx 2y ago
  How is thermal noise different from heat?
gus_massa 2y ago
The problem is that you can't "fix" superposition. I [1] like to explain it with the https://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experiment https://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experime... . For some strange reasons [2] it's enough to consider atoms with spin |up> and |down>. But if you put the device sidewide, you would preffer to use |right> and |left> instead. But if you are forced to use |up> and |down> then |right> = ( |up> + |down> ) / sqrt(2) |left> = ( |up> - |down> ) / sqrt(2) (Or with the oposite signs, I never remember.) The same idea of expressing a state using superposition/sums of states in other base is very fundamental to Quantum Mechanics and appears everywhere. [1] The idea of using Stern–Gerlach experiment is stolen from Feynman, not a brilliant idea of me. [2] Too much algebra and the SU(2) group. Just bear with me.
- danbruc 2y ago
  Sure, if you would know the spin direction before the measurement, you could align your measurement device accordingly and get a predictable answer and spin state would remain unchanged as the particle is not in a superposition for that measurement basis. But if measurement basis and spin direction are not aligned, then the measurement result will be random in proportion to the misalignment and the spin state will be non-unitarily changed by the measurement, projected onto the measurement basis. It the second scenario that lacks proper understanding, the first one seems much less interesting, you measure something that you already know and you get the value that you expect. It is certainly an interesting fact that being in a superposition is not an absolute statement, but it also seems not to uncommon. In additive color mixing magenta is a combination of red and blue, in subtractive color mixing it is one of the base colors.
  - gus_massa 2y ago
    Rhere are a few interesting cases where superposition is inevitable. A) In a molecule with two electrons, the wavefunction of the electrons is written as |aa> + |bb> + ... combining one electron with spin up and one with spin down. [1] [2] It would be nice to choose the base carefully and write only |aa>. That is the Hartree-Fock method [3] and the result is like 95% or 99% accurate, sometimes you get 100% accurate, but it's not usual. To get the small difference, you must allow combinations like Sqrt(99%) |aa> + Sqrt(1%) |bb>. [4] It would be very nice to be able to use the simple form because calculations would be much faster. B) In a different area, IIUC the neutrino osculation is caused because it's impossible to choose the bases for leptons and neutrinos and their transformations in a way there is no superpositions. [1] Actually, in some cases you must use |ab> + |cd> + ... , but the simple version is good enough. [2] And with more electrons it's even worse. [3] https://en.wikipedia.org/wiki/Hartree%E2%80%93Fock_method https://en.wikipedia.org/wiki/Hartree%E2%80%93Fock_method [4] https://en.wikipedia.org/wiki/Electronic_correlation https://en.wikipedia.org/wiki/Electronic_correlation [5] https://en.wikipedia.org/wiki/Neutrino_oscillation https://en.wikipedia.org/wiki/Neutrino_oscillation
Viliam1234 2y ago
> what actually saves or kills Schrödinger's cat If the cat is not definitely dead (or alive) until you look at it, I think the mainstream consensus is that curiosity killed the cat.
riggsdk 2y ago
(not a physicist) One thing that I always wondered about that I never see "debunked" anywhere is any discussion about whether or not entanglement is actually just because the two entangled particles are put into a pretty predictable state (opposite of each other). If one is measured "up" the other will measure "down". To me that just screams "particle physics are predictable(determinant) as long as the particles are shielded from outside noise, not because they are connected/bound together by some mysterious force or law of physics." I suppose a thought experiment to prove/disprove that would be to send one of the entangled particles, particle A, around a black hole to slow it's time down and then afterwards measure if the entangled particles still give opposite results but consistent with the time delay.
- tzs 2y ago
  If all you could measure was "up" and "down" then I think entangled particles would be indistinguishable from unentangled particles that were created as up/down pairs. But particles can be measured in other directions, and that's where the determinism goes away. A nice thought experiment is the CHSH game. It's a two player game where the players (player A and player B) cooperate to beat the house. It is played as follows: 1. Each player is assigned a referee. 2. The players, accompanied by their referee, go to separate rooms. Before going to the separate rooms the players can confer. They may also bring any equipment with them that they want. The rooms are shielded to block any communication between the players during their time in the rooms. You may assume that the communication blocking is 100% effective. 3. Each referee uses a true random number generator to generate a bit, and tells the player the value of that bit. 4. The player then generates a bit, by any means, and tells it to the referee. 5. The referee records the bit they generated and the bit provided by the player. 6. Steps #3-5 are repeated 999 more times. 7. After both players have gone through #3-5 1000 times, the referees confer and check their records. For each round the players win $100 in these two cases: The players generated different bits and the referees both generated 1 The players generated the same bit and at least one referee generated 0 In a classical universe the best strategy for the players is simply to agree on an algorithm that will result in them picking matching bits every round, such as "always pick 0". 75% of the time the referees will generate 00, 01, or 10 and the players will win $100. In a quantum universe the players can do better. They can generate 1000 pairs of entangled particles and each take one particle from each pair. Let's assume that the particles are linearly polarized photons polarized in the up/down direction. When player A is given the referee's bit, A sends their particle from the first pair through a polarizing filter and reports a 1 if the particle makes it through the filter, and a 0 if it is blocked. If the referee's bit was a 0 the player orients their polarizing filter along the up/down axis. If the referee's bit was 1 they orient their filter rotated 45° to the right. Player B does a similar thing, except their filter is rotated 22.5° to the right if they got a 0 from the referee and 22.5° to the left if they got a 1. Here's a diagram of their measurement angles, where X0 means player X got a 0 from the referee and X1 means they got a 1: B1 B0 | | | | | | +--------+--------+--------+ |-22.5 |0 |22.5 |45 | | | | A0 A1 They do this for each round, using the photons from the n'th entangled pair for round n. Note that if either player receives a 0 from the referee the angle they use will be 22.5° apart from the angle the other player uses no matter what bit the other player got from the referee. When the measurements on a pair of entangled particles are taken at an angle θ the results match the result you'd get at a 0° difference cos^2(θ) of the time. For 22.5° that's 85.4% of the time, so when either referee generates a 0 they players will win 85.4% of the time. If both referees generate 1, B measures at -22.5° and A measures at 45°. That's 67.5° apart and the player bits only match 14.6% of the time, but when both referees generate 1 the players want to generate different bits so that's good. They players win 85.4% of the time in this case. That's an 85.4% win rate in all cases, which beats the 75% that they can get in a classical universe. If you try to make some sort of classical-only thing that can take the place of entangled particle pairs you'll find that you can't make it work. You won't get past 75%. Another thought experiment that might be clearer (or might just muddle things) involving two mysterious devices that you are trying to reverse engineer is here [1]. That puts it more mechanical/computational terms which may be easier to play around with. [1] https://news.ycombinator.com/item?id=35905284 https://news.ycombinator.com/item?id=35905284
drdeca 2y ago
[edit: found another comment which you replied to, and now I see that you already knew all this. Oops. Original comment follows] Maybe you know the thing I’m about to say and just consider it to be too pedantic, but: The question to ask of a system is not [whether it is “in a superposition” or “not in a superposition”], but rather, [whether it is “in a superposition of these different states” or “in a specific one of these states”]. Suppose I have a particle which I know is spin up (ignore the position aspect of things, considering only the spin). Is it “in a superposition”? One might be tempted to say “no, it is spin up.”, but the correct response is “A superposition of what? Anything? If so, then trivially yes.” . Because “spin up” can be regarded as a superposition of “spin left” and “spin right”, or as “spin northeast” and “spin southwest”. Likewise, “spin left” can be regarded as a superposition of “spin up” and “spin down”. To usefully ask whether something is “in a superposition”, you have to specify what states you are regarding as the basis, as far as the question is concerned (or, alternatively, with respect to what observables, where “in a superposition with respect to those observables” would mean that if those observables were measured, the result would be random* ). Now, why don’t we observe superpositions of states of macroscopic things which differ according to ordinary observables such as “x coordinate of center of mass”? In don’t know. That may be something still not understood. I don’t really understand these attempts, but I think there are things called “pointer states” and a process known as “einselection” which are supposed to help explain that? (Though it has been argued that this explanation involving einselection may be somewhat circular?)

sebzim4500 2y ago

There's a result in quantum information that demonstrating a superposition of two states is at least as hard as switching between those states. So designing an experiment to show that a cat is in a superposition of alive and dead is at least as hard as bringing a dead cat back to life (obviously in real life it would be much harder, this is just a lower bound).

Obscurity4340 2y ago
Isn't the cat experiment bullshit outside of illustrating a macro version of this? Like, there's no superposition in reality or the cat is too big to be subject to quantum mechanics where these shenanigans happen or something?
- deleted 2y ago
  [deleted]
- hiciu 2y ago
  The cat isn't in some quantum state. Cat is locked in a box, with some poison and a Geiger counter. Poison is released when Geiger counter registers a particle. Now, the process of atom decay that would be registered by a Geiger counter has some interesting properties and that's where the "quantum magic" happens. (at least, that's my mental model of it; ask an expert, I'm not an expert)
  - immibis 2y ago
    The quantum magic properties are thought to transfer to the poison and the cat.
  - rcxdude 2y ago
    the idea is that everything is in some quantum state, because quantum mechanics attempts to describe everything. The thought experiment is basically going from the well-observed (and explained in theory) superposition of a nuclear undergoing decay to the never-observed (but still valid in theory) superposition of a cat being both alive and dead.
    - Obscurity4340 2y ago
      Why should it be valid in theory tho? What is the purported bottleneck in physical reality that is supposed to exist whose presence makes the Schrodinger cat thought experiment realistic? How does sentient observation get the ball rolling and why couldn't that happen in the absence of such observation at this scale?
- lupusreal 2y ago
  Cat observes itself.
  - IX-103 2y ago
    That defeats the point of the exercise. The whole point was about the state of some isolated system. Otherwise we could say that the detector observes the state of the system without even needing the poison or the car
    - megaloblasto 2y ago
      It doesn't defeat the point of the exercise, it just shows that the exercise is nonsense. The cat is an observer therefore collapses the quantum wave function and is either alive or dead.
      - baq 2y ago
        In what sense the cat is an observer? Having eyes and a brain doesn't really mean anything in QM.
        lupusreal 2y ago
        If the particle interacts, that collapses the superposition. Cats aren't any more or less blessed as observers than physicists. If either of them can observe the outcome, then both can.
        murftown 2y ago
        Hmm. Maybe what "counts as an observer" is relative to your reference frame, and so is the concept of "superposition". And thus, though the cat is making observations within its environment, since the environment is sealed off, the cat is not "an observer" from our reference frame because there is no interaction between us and the cat. From the cat's reference frame it is probably not in a superposition - If it is observing anything, it is alive, and if it is dead, it is not observing anything. But if we observe the cat, the wave function is collapsed from our reference frame and the cat is in one or the other state. My understanding of quantum mechanics is pretty shallow so take this with an ocean of salt.
- ben_w 2y ago
  Schrödinger's thought experiment was supposed to demonstrate that superposition itself (and/or the whole of the Copenhagen interpretation) was a nonsense concept… but every test so far says it's real, that he was wrong, and that the cat would be in both states "until observed". But nobody knows what "observed" even means.
- gus_massa 2y ago
  The problem is to isolate the box enough. If you can heard the cat meow, then it counts as a "measurement" and the superposition disappears. If you can hear there is no heartbeat of the cat, then it counts as a "measurement" and the superposition disappears. If the cats move and the box wags, then it counts as a "measurement" and the superposition disappears... So in practice the experiment only works with very small systems with a few atoms. Each year there are experiments with bigger systems, but they are usually frozen and very small. Perhaps one day we will be able to do the experiment with tardigrades.
  - Vampiero 2y ago
    Hearing at all implies sound waves. If there are soundwaves there is air inside the box. If there is air inside the box the system can't be in a superposition because it has long since decohered. In other words it's not the act of "listening" that collapses the wave function but the interaction of air molecules that creates the sound in the first place, even if no one is there to listen. A "measurement" or an "observation" doesn't actually require a human, or a living thing. It's just a physical interaction.
    - rvbissell 2y ago
      Catch 22: and if there is no air inside the box, then you are certain the cat is dead.
    - Maxatar 2y ago
      This isn't quite right, wave function collapse is relative so while within the box an observer's wave function of the box will be in a definite eigenstate, the wavefunction of the box for an observer isolated from the box will still be in a superposition of its states. In other words, the fact that an observation takes place from within the box and collapses the wave function for an observer in the box does not mean that the wave function for every observer also collapses. A wave function collapse for one observer does not imply a wave function collapse for any other observer.
      - 2y ago
- ahazred8ta 2y ago
  Schrödinger never believed it; he gave that as an example of something that could never happen in real life.
- sebzim4500 2y ago
  As far as known physics goes, nothing is too big to be subject to quantum mechanics. It is very hard to demonstrate that fact though, the obvious way of doing it is to build a large quantum computer.

Joker_vD 2y ago

> what qualifies as a measurement device Anything, really. There are no "classical devices", after all, all of them are quantum systems, aren't they?

danbruc 2y ago
Quantum systems evolve unitarily, measurement devices collapse the wave function which is not unitary. And there is no accepted consensus how those two things fit together, just several ideas like Everett's many worlds which solves the problem by declaring that the wave function never actually collapses but instead splits the cosmos into many worlds.
- Joker_vD 2y ago
  Everett's idea was merely that the measurement device gets entangled with the thing being measured, so we now the wave-function of the (combined) device-thing system is the superposition of states <thing is in pure state Ai| ⊗ <device is in "measured Ai" state|; but for the measurement device it of course "looks" like it measured a pure state. That's it. That's basically what the decoherence suggests anyway. The "cosmos splitting" was proposed by Bryce DeWitt which IMHO turned a perfectly straight-forward and reasonable interpretation into a needlessly philosophical statement.
  - layer8 2y ago
    Entanglement means that the states become macroscopic in noisy (and not too high-temperature, apparently) environments. Whether you call those states “worlds” or not doesn’t change the philosophical implications.
  - kgwgk 2y ago
    > Everett's idea was merely that the measurement device gets entangled with the thing being measured That idea is almost as old as quantum mechanics itself. (von Neumann, 1932 - if not earlier.)
  - danbruc 2y ago

marginalia_nu 2y ago

We have this result already. Interaction with thermal noise ruins wave coherence, and causes classical physics to emerge. This comes when you in a sense replace waves with their average values. It's not so much a question of scale, but of statistical noisiness. Quantum effects are primarily observable in the low-temperature domain (this is not necessarily "thermometer temperature", but a statistical measure)

namibj 2y ago
Even lasers deal with weird temperatures: they're negative.
danbruc 2y ago
I am not a physicist, but I thought that it is generally accepted that decoherence does not solve the measurement problem. It explains why quantum effects are not observable in a noisy environment but still does not explain the non-unitarity of measurements.
- marginalia_nu 2y ago
  It does not solve the measurement problem, but it does explain why you don't see quantum phenomena such as superposition or tunneling at the classical limit.
ko27 2y ago
This doesn't answer the question. We know how classical physics emerges from a large number of quantum events through statistics. That was not the point of Schrödinger's cat though experiment! The idea is to link a macroscopic event to a random subatomic event that is in a superposition. When or how does the wave function collapse, does it even collapse?
ruthmarx 2y ago
How is thermal noise different from heat?

gus_massa 2y ago

The problem is that you can't "fix" superposition. I [1] like to explain it with the https://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experiment https://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experime... . For some strange reasons [2] it's enough to consider atoms with spin |up> and |down>. But if you put the device sidewide, you would preffer to use |right> and |left> instead. But if you are forced to use |up> and |down> then |right> = ( |up> + |down> ) / sqrt(2) |left> = ( |up> - |down> ) / sqrt(2) (Or with the oposite signs, I never remember.) The same idea of expressing a state using superposition/sums of states in other base is very fundamental to Quantum Mechanics and appears everywhere. [1] The idea of using Stern–Gerlach experiment is stolen from Feynman, not a brilliant idea of me. [2] Too much algebra and the SU(2) group. Just bear with me.

danbruc 2y ago
Sure, if you would know the spin direction before the measurement, you could align your measurement device accordingly and get a predictable answer and spin state would remain unchanged as the particle is not in a superposition for that measurement basis. But if measurement basis and spin direction are not aligned, then the measurement result will be random in proportion to the misalignment and the spin state will be non-unitarily changed by the measurement, projected onto the measurement basis. It the second scenario that lacks proper understanding, the first one seems much less interesting, you measure something that you already know and you get the value that you expect. It is certainly an interesting fact that being in a superposition is not an absolute statement, but it also seems not to uncommon. In additive color mixing magenta is a combination of red and blue, in subtractive color mixing it is one of the base colors.
- gus_massa 2y ago
  Rhere are a few interesting cases where superposition is inevitable. A) In a molecule with two electrons, the wavefunction of the electrons is written as |aa> + |bb> + ... combining one electron with spin up and one with spin down. [1] [2] It would be nice to choose the base carefully and write only |aa>. That is the Hartree-Fock method [3] and the result is like 95% or 99% accurate, sometimes you get 100% accurate, but it's not usual. To get the small difference, you must allow combinations like Sqrt(99%) |aa> + Sqrt(1%) |bb>. [4] It would be very nice to be able to use the simple form because calculations would be much faster. B) In a different area, IIUC the neutrino osculation is caused because it's impossible to choose the bases for leptons and neutrinos and their transformations in a way there is no superpositions. [1] Actually, in some cases you must use |ab> + |cd> + ... , but the simple version is good enough. [2] And with more electrons it's even worse. [3] https://en.wikipedia.org/wiki/Hartree%E2%80%93Fock_method https://en.wikipedia.org/wiki/Hartree%E2%80%93Fock_method [4] https://en.wikipedia.org/wiki/Electronic_correlation https://en.wikipedia.org/wiki/Electronic_correlation [5] https://en.wikipedia.org/wiki/Neutrino_oscillation https://en.wikipedia.org/wiki/Neutrino_oscillation

Viliam1234 2y ago

> what actually saves or kills Schrödinger's cat If the cat is not definitely dead (or alive) until you look at it, I think the mainstream consensus is that curiosity killed the cat.

riggsdk 2y ago

(not a physicist) One thing that I always wondered about that I never see "debunked" anywhere is any discussion about whether or not entanglement is actually just because the two entangled particles are put into a pretty predictable state (opposite of each other). If one is measured "up" the other will measure "down". To me that just screams "particle physics are predictable(determinant) as long as the particles are shielded from outside noise, not because they are connected/bound together by some mysterious force or law of physics." I suppose a thought experiment to prove/disprove that would be to send one of the entangled particles, particle A, around a black hole to slow it's time down and then afterwards measure if the entangled particles still give opposite results but consistent with the time delay.

tzs 2y ago
If all you could measure was "up" and "down" then I think entangled particles would be indistinguishable from unentangled particles that were created as up/down pairs. But particles can be measured in other directions, and that's where the determinism goes away. A nice thought experiment is the CHSH game. It's a two player game where the players (player A and player B) cooperate to beat the house. It is played as follows: 1. Each player is assigned a referee. 2. The players, accompanied by their referee, go to separate rooms. Before going to the separate rooms the players can confer. They may also bring any equipment with them that they want. The rooms are shielded to block any communication between the players during their time in the rooms. You may assume that the communication blocking is 100% effective. 3. Each referee uses a true random number generator to generate a bit, and tells the player the value of that bit. 4. The player then generates a bit, by any means, and tells it to the referee. 5. The referee records the bit they generated and the bit provided by the player. 6. Steps #3-5 are repeated 999 more times. 7. After both players have gone through #3-5 1000 times, the referees confer and check their records. For each round the players win $100 in these two cases: The players generated different bits and the referees both generated 1 The players generated the same bit and at least one referee generated 0 In a classical universe the best strategy for the players is simply to agree on an algorithm that will result in them picking matching bits every round, such as "always pick 0". 75% of the time the referees will generate 00, 01, or 10 and the players will win $100. In a quantum universe the players can do better. They can generate 1000 pairs of entangled particles and each take one particle from each pair. Let's assume that the particles are linearly polarized photons polarized in the up/down direction. When player A is given the referee's bit, A sends their particle from the first pair through a polarizing filter and reports a 1 if the particle makes it through the filter, and a 0 if it is blocked. If the referee's bit was a 0 the player orients their polarizing filter along the up/down axis. If the referee's bit was 1 they orient their filter rotated 45° to the right. Player B does a similar thing, except their filter is rotated 22.5° to the right if they got a 0 from the referee and 22.5° to the left if they got a 1. Here's a diagram of their measurement angles, where X0 means player X got a 0 from the referee and X1 means they got a 1: B1 B0 | | | | | | +--------+--------+--------+ |-22.5 |0 |22.5 |45 | | | | A0 A1 They do this for each round, using the photons from the n'th entangled pair for round n. Note that if either player receives a 0 from the referee the angle they use will be 22.5° apart from the angle the other player uses no matter what bit the other player got from the referee. When the measurements on a pair of entangled particles are taken at an angle θ the results match the result you'd get at a 0° difference cos^2(θ) of the time. For 22.5° that's 85.4% of the time, so when either referee generates a 0 they players will win 85.4% of the time. If both referees generate 1, B measures at -22.5° and A measures at 45°. That's 67.5° apart and the player bits only match 14.6% of the time, but when both referees generate 1 the players want to generate different bits so that's good. They players win 85.4% of the time in this case. That's an 85.4% win rate in all cases, which beats the 75% that they can get in a classical universe. If you try to make some sort of classical-only thing that can take the place of entangled particle pairs you'll find that you can't make it work. You won't get past 75%. Another thought experiment that might be clearer (or might just muddle things) involving two mysterious devices that you are trying to reverse engineer is here [1]. That puts it more mechanical/computational terms which may be easier to play around with. [1] https://news.ycombinator.com/item?id=35905284 https://news.ycombinator.com/item?id=35905284

drdeca 2y ago

[edit: found another comment which you replied to, and now I see that you already knew all this. Oops. Original comment follows] Maybe you know the thing I’m about to say and just consider it to be too pedantic, but: The question to ask of a system is not [whether it is “in a superposition” or “not in a superposition”], but rather, [whether it is “in a superposition of these different states” or “in a specific one of these states”]. Suppose I have a particle which I know is spin up (ignore the position aspect of things, considering only the spin). Is it “in a superposition”? One might be tempted to say “no, it is spin up.”, but the correct response is “A superposition of what? Anything? If so, then trivially yes.” . Because “spin up” can be regarded as a superposition of “spin left” and “spin right”, or as “spin northeast” and “spin southwest”. Likewise, “spin left” can be regarded as a superposition of “spin up” and “spin down”. To usefully ask whether something is “in a superposition”, you have to specify what states you are regarding as the basis, as far as the question is concerned (or, alternatively, with respect to what observables, where “in a superposition with respect to those observables” would mean that if those observables were measured, the result would be random* ). Now, why don’t we observe superpositions of states of macroscopic things which differ according to ordinary observables such as “x coordinate of center of mass”? In don’t know. That may be something still not understood. I don’t really understand these attempts, but I think there are things called “pointer states” and a process known as “einselection” which are supposed to help explain that? (Though it has been argued that this explanation involving einselection may be somewhat circular?)