B Why doesn't the plate interact with the particle in double slit?

platosuniverse
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When particles are shot at the plate/screen in the double slit experiment, why doesn't the particles interact with the screen? Shouldn't the plate act as an observer and "collapse" the wave function into one or the other slit? Why does it take a measuring apparatus to know which slit the particle went through? Here's a pic:

double_slit_setup.png


The screen is a classical object so shouldn't the particles interact with the screen? The detector plate is classical. It's said Schrodinger's cat will end up in one state or the other because of decoherence. Why doesn't that apply to the screen/plate and the detector plate?

So if you had the cat in the box, wouldn't it have to be in both states like the screen/plate until a measuring apparatus or human measured it? Let's replace the cat with a tennis ball. If the atom decays, the ball explodes but if it doesn't the ball is fine.

Without a measuring device, doesn't the ball have to be in two states until a measurement occurs? The screen/plate isn't in a single state. It's not particle through left slit or particle through right slit until measured.

So why would a cat be dead/alive or the tennis ball be exploded/not exploded until a measuring apparatus measured it? The wave function must contain both states of the cat/ball until it's measured. There may be a single cat/ball in the box but do they have an objective existence until a measurement occurs?
 

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Everything that hits the plate is absorbed (or reflected) and doesn't matter for what happens behind the slits.
platosuniverse said:
So why would a cat be dead/alive or the tennis ball be exploded/not exploded until a measuring apparatus measured it?
It is not feasible to keep such a large and especially a biological object in a state without measurement, but if we ignore practical constraints then we can keep a superposition of these states.
 
platosuniverse said:
The screen is a classical object so shouldn't the particles interact with the screen?

Many of them do; as @mfb says, we don't observe those particles because they get absorbed or reflected by the screen and never reach the detector. The only particles we observe at the detector are the ones that don't interact with the screen because they go through the slits.
 
platosuniverse said:
The screen is a classical object so shouldn't the particles interact with the screen? The detector plate is classical. It's said Schrodinger's cat will end up in one state or the other because of decoherence. Why doesn't that apply to the screen/plate and the detector plate?
An electron is going through the slits?

Well, the screen is a an object with quantized states. And the quantized states are such that electron passing by does not cause any state change at all. Or alternatively the screen does detect that an electron passed by, but the resolution at which said screen observes its surroundings is not so high that the screen could know through which hole the electron went.
 
Thanks for the responses.

I think it shows decoherence is something that happens after a measurement occurs and decoherence time explain why we don't see the cat or ball in a mixed state. We see this or that after measurement which means Schrodinger's cat is in a mixed live/dead state until a measurement occurs. There would have to be some magic hidden variable that tells the cat how to decohere into one state or the other before a measurement occurs.It would have to know if a particle is emitted or not before the particle is emitted or not.

Here's a video of the original Thomas Young's double slit experiment and it's done with a box. You have all of these things in the environment plus the classical box yet you still get light acting as a wave. Why didn't the wave function decohere or collapse as it interfered with the classical box?



So I would think that the quantum state is real and not a cat or ball because a physical cat can't be in a live/dead state.This could mean Susskind and Hawking in his last paper are correct about the Holographic Universe. Here's Susskind, paper the World as a Hologram.

https://arxiv.org/abs/hep-th/9409089

So a hypothetical nano being that can perceive the passage of a nanosecond, might see the cat or ball pop out of existence and there's just this cloud of information. It will then watch as the states transition to a live cat in one universe and a dead cat in another universe. We can't see this transition because of decoherence time.

If the universe is holographic, then a cat, ball, moon and everything else might be a 3D projection of 2D information. Subatomic particles might be like pixels that illuminate these quantum states.

This makes sense to me because I don't see how a cat, ball, human or moon can be in one state or the other prior to measurement. After measurement, the outcome is fixed in universe A or universe B because these states can no longer interfere.
 
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platosuniverse said:
Here's a video of the original Thomas Young's double slit experiment and it's done with a box. You have all of these things in the environment plus the classical box yet you still get light acting as a wave. Why didn't the wave function decohere or collapse as it interfered with the classical box?
What wave function? The waves in Young's experiment were (although he didn't know this, because he was working a half-century before the discovery of Maxwell's equations) electromagnetic radiation, oscillations in the electrical and magnetic fields. They aren't the waves of the quantum mechanical wave function and they don't collapse.
 
platosuniverse said:
Why didn't the wave function decohere or collapse as it interfered with the classical box?
It did. It either collapses to "particle interacted with the box" or "particle did not and continues behind the slits" (or you get many worlds with these results, depending on your favorite interpretation).
 
Nugatory said:
What wave function? The waves in Young's experiment were (although he didn't know this, because he was working a half-century before the discovery of Maxwell's equations) electromagnetic radiation, oscillations in the electrical and magnetic fields. They aren't the waves of the quantum mechanical wave function and they don't collapse.

I disagree. Here's a few published papers.

Solutions of the Maxwell equations and photon wave functions

Abstract Properties of six-component electromagnetic field solutions of a matrix form of the Maxwell equations, analogous to the four-component solutions of the Dirac equation, are described. It is shown that the six-component equation, including sources, is invariant under Lorentz transformations. Complete sets of eigenfunctions of the Hamiltonian for the electromagnetic fields, which may be interpreted as photon wave functions, are given both for plane waves and for angular-momentum eigenstates. Rotationally invariant projection operators are used to identify transverse or longitudinal electric and magnetic fields. For plane waves, the velocity transformed transverse wave functions are also transverse, and the velocity transformed longitudinal wave functions include both longitudinal and transverse components. A suitable sum over these eigenfunctions provides a Green function for the matrix Maxwell equation, which can be expressed in the same covariant form as the Green function for the Dirac equation. Radiation from a dipole source and from a Dirac atomic transition current are calculated to illustrate applications of the Maxwell Green function

https://www.nist.gov/sites/default/files/documents/pml/div684/fcdc/photon-wave.pdf

THE PHOTON WAVE FUNCTION

http://www.cft.edu.pl/~birula/publ/CQO7.pdf


The Maxwell wave function of the photon

ABSTRACT James Clerk Maxwell unknowingly discovered a correct relativistic, quantum theory for the light quantum, forty-three years before Einstein postulated the photon’s existence. In this theory, the usual Maxwell field is the quantum wave function for a single photon. When the non-operator Maxwell field of a single photon is second quantized, the standard Dirac theory of quantum optics is obtained. Recently, quantum-state tomography has been applied to experimentally determine photon wave functions.

https://arxiv.org/ftp/quant-ph/papers/0604/0604169.pdf
 
platosuniverse said:
I disagree.

You shouldn't. The papers you reference basically say: quantum mechanical states of single photons are solutions of Maxwell's Equations. But that does not mean that anything that can be described by a solution of Maxwell's Equations must be a quantum mechanical state of a single photon, which is the claim you are making when you say "I disagree" in response to @Nugatory's post.
 
  • #10
platosuniverse said:
Why didn't the wave function decohere or collapse as it interfered with the classical box?

If we rephrase this question to apply to a double slit experiment done with weak enough light that quantum effects become important, then the answer is that it does--but that decoherence corresponds to photons that don't get observed, because they hit the walls of the box instead of going through the slits and getting observed at the detector.
 
  • #11
platosuniverse said:
When particles are shot at the plate/screen in the double slit experiment, why doesn't the particles interact with the screen? Shouldn't the plate act as an observer and "collapse" the wave function into one or the other slit? Why does it take a measuring apparatus to know which slit the particle went through?
The particle either gets absorbed by the screen (with probability ##p##) or passes through the slit (with probability ##1-p##). In the experiment one emits many particles, some which get absorbed while others pass through the slit. In the further analysis of the experiment one ignores the particles which are absorbed by the screen (because they are not interesting) and concentrates only on particles that passed through the slit.
 
  • #12
As the slit through which the particle passes narrows, the particle's momentum becomes less precise, so we might say the screen interacts with the particle in that sense.
 
  • #13
Demystifier said:
The particle either gets absorbed by the screen (with probability ##p##) or passes through the slit (with probability ##1-p##). In the experiment one emits many particles, some which get absorbed while others pass through the slit. In the further analysis of the experiment one ignores the particles which are absorbed by the screen (because they are not interesting) and concentrates only on particles that passed through the slit.

Are the detector electrons (which interact with the incoming particle) loosely bound, i.e. can be readily kicked up to another energy level and cause a signal, whereas the screen electrons are more tightly bound, and thus will not absorb the energy of the incoming particle as readily (basically deflects it)? Thus collapse is less likely to happen at the screen?
 
  • #14
Idunno said:
Thus collapse is less likely to happen at the screen?

You're missing the point. When you talk about particles being absorbed by the screen, these are different particles from the ones that are detected at the detector. For a particle that's absorbed by the screen, the screen collapses the particle. We just don't observe it because the screen is not designed that way. If you wanted to directly observe the particles that hit the screen, you could make the screen out of a material similar to what the detector is made out of, so that particles that hit the screen showed a flash of light, the same way particles that hit the detector do. Then you would find that every time the source emits a particle, you either see a flash on the screen, or a flash on the detector, with relative probabilities ##p## and ##1 - p##.
 
  • #15
David Lewis said:
As the slit through which the particle passes narrows, the particle's momentum becomes less precise, so we might say the screen interacts with the particle in that sense.
Or we might say that the particles with the least precise momentum are the ones that are least likely to interact with the screen and be absorbed...
 
  • #16
PeterDonis said:
You're missing the point. When you talk about particles being absorbed by the screen, these are different particles from the ones that are detected at the detector. For a particle that's absorbed by the screen, the screen collapses the particle. We just don't observe it because the screen is not designed that way. If you wanted to directly observe the particles that hit the screen, you could make the screen out of a material similar to what the detector is made out of, so that particles that hit the screen showed a flash of light, the same way particles that hit the detector do. Then you would find that every time the source emits a particle, you either see a flash on the screen, or a flash on the detector, with relative probabilities ##p## and ##1 - p##.

Sure, that is fine, my question is: can you decrease the probability that the incoming particles will collapse at the screen or not by making the screen out of a material that reflects (deflects? the proper word?) the incoming particles, which I would imagine you do by having a material whose electrons are more tightly bound?
 
  • #17
Idunno said:
can you decrease the probability that the incoming particles will collapse at the screen or not by making the screen out of a material that reflects (deflects? the proper word?) the incoming particles

I'm not sure what you mean. If the screen doesn't collapse particles that hit it, it's not a screen. The whole point of the double slit experiment is to have a screen with two slits, where "screen" means "collapses particles that hit it", and "slits" means "whatever isn't screen". These are the functional specifications of the experiment, not things you can adjust at will.

You could certainly run an experiment where you had a "screen" made out of something that didn't collapse particles, but then it wouldn't be a double slit experiment. It would be some other kind of experiment (I can't say what kind because I would need a much more specific description of what you wanted this modified "screen" to actually do).
 
  • #18
PeterDonis said:
I'm not sure what you mean. If the screen doesn't collapse particles that hit it, it's not a screen. The whole point of the double slit experiment is to have a screen with two slits, where "screen" means "collapses particles that hit it", and "slits" means "whatever isn't screen". These are the functional specifications of the experiment, not things you can adjust at will.

You could certainly run an experiment where you had a "screen" made out of something that didn't collapse particles, but then it wouldn't be a double slit experiment. It would be some other kind of experiment (I can't say what kind because I would need a much more specific description of what you wanted this modified "screen" to actually do).

Well, suppose you're running this experiment, and without the screen with the double slit in place, you're detecting 100 particles per minute. You put the screen in, and now you're getting 1 particle per minute. Most are being absorbed by the screen, it seems. Suppose you don't like that, and want to increase the number getting through, besides doing things like adjusting the beam, is one option to switch the material that the double slit is made from to another material?
 
  • #19
Idunno said:
You put the screen in, and now you're getting 1 particle per minute. Most are being absorbed by the screen, it seems. Suppose you don't like that

Then you don't like running double slit experiments. :wink:

The point of a double slit experiment is to see the interference pattern. To see that, the size of the slits has to be small in comparison with the size of the screen; in other words, there cannot be a very large percentage of area which is slit and not screen. That means it's unavoidable to have only a small percentage of all the particles go through the slits.
 
  • #20
PeterDonis said:
Then you don't like running double slit experiments. :wink:

The point of a double slit experiment is to see the interference pattern. To see that, the size of the slits has to be small in comparison with the size of the screen; in other words, there cannot be a very large percentage of area which is slit and not screen. That means it's unavoidable to have only a small percentage of all the particles go through the slits.
So I cut two narrow slits in a dark plate (something that readily absorbs photons) and put it in front of my low intensity photon beam, and the number of particles hitting the detector per minute drops from 100 per minute to 1 per minute. Then I cut two identical slits in, say, a piece of polished silver (something that readily reflects photons), and put it in front, I will find the same drop in the number hitting the detector, all other things (beam width, position of slits. etc.) being equal?
 
  • #21
Idunno said:
So I cut two narrow slits in a dark plate (something that readily absorbs photons) and put it in front of my low intensity photon beam, and the number of particles hitting the detector per minute drops from 100 per minute to 1 per minute. Then I cut two identical slits in, say, a piece of polished silver (something that readily reflects photons), and put it in front, I will find the same drop in the number hitting the detector, all other things (beam width, position of slits. etc.) being equal?

Yes.
 
  • #22
PeterDonis said:
Yes.
OK. Thanks for your time, much appreciated. :)
 
  • #23
What about when you make the double slit experiment shooting one photon or one electron at the time? Why do the photon or the electron pass through the slits and form an interference pattern, instead of being absorbed by the screen in which the slits are created?
 
  • #24
Mr Wolf said:
What about when you make the double slit experiment shooting one photon or one electron at the time? Why do the photon or the electron pass through the slits and form an interference pattern, instead of being absorbed by the screen in which the slits are created?
Same thing - they don't all make it through. You'll get fewer detections at the screen than emissions at the source.
 
  • #25
Nugatory said:
Same thing - they don't all make it through. You'll get fewer detections at the screen than emissions at the source.
Not so sure this answers wolfie's question ?

In physics, the question 'why' is difficult/impossible to answer. Here I would say the only right answer is: "because that is the way they behave", which is probably not the kind of answer that wolf was looking for.

Perhaps it is instructive to view the first of the 1979 Auckland lectures by Richard Feynman. As far as I remember, he agrees with me.
 
  • #26
BvU said:
Not so sure this answers wolfie's question ?

In physics, the question 'why' is difficult/impossible to answer. Here I would say the only right answer is: "because that is the way they behave", which is probably not the kind of answer that wolf was looking for.

Perhaps it is instructive to view the first of the 1979 Auckland lectures by Richard Feynman. As far as I remember, he agrees with me.
Sorry to be... I don't know what the word is. But, if you get your beam of particles nice and confined and aimed really well and lined up with some very well cut slits, you can increase the percent getting through, right? But you never get 100%. Would it be 100% with a perfect beam and perfect slits?

I think the OP wanted to know "why does collapse happen?" in general. For instance, if you send a single photon through a half silvered mirror, it doesn't collapse the photon, it splits it into two. Similarly, you send single photons through a slit and some of them get through and interfere with themselves, other get absorbed and hence "collapse". So, sometimes particles collapse when you mess with them, sometimes they don't collapse and get all mangled up. For a non expert trying to get a intuitive grasp of QM, it's a lingering question as to when and where will collapse happen, and when it won't.
 
  • #27
Idunno said:
For a non expert trying to get a intuitive grasp of QM, it's a lingering question as to when and where will collapse happen, and when it won't.
It's a lingering problem for everyone:smile:
Looking into quantum decoherence (Google, or David Lindley's "Where does the weirdness go?" is a good layman-friendly introduction) will help a lot. However, there's no completely satisfactory resolution, and this is why quantum mechanics has a "measurement problem".
 
  • #28
Idunno said:
…with some very well cut slits, you can increase the percent getting through, right? But you never get 100%. Would it be 100% with a perfect beam and perfect slits?
It might be possible, but extremely difficult, and I don't see what the benefit would be.
 
  • #29
Idunno said:
if you get your beam of particles nice and confined and aimed really well and lined up with some very well cut slits, you can increase the percent getting through, right?

You could do this, but you would be running a different experiment, and you would not see interference at the detector. The whole point of the standard double slit experiment is that, as the particles come from the source, they are not aimed at any particular place on the screen. The usual approximation that is adopted for analysis is that the wave function is a plane wave when it arrives at the screen, meaning that it has a basically equal probability of striking any point on the screen. That is what makes it the case that only a small percentage of particles get through the slits (basically the ratio of the area of the slits to the area of the entire screen). And that is what makes it the case that you get interference at the detector for particles that get through the slits.

If you change the particle source so that particles are aimed with high accuracy, then the wave function of the particles coming from the source will be very, very different: it will have a very high probability of hitting just one particular spot at the screen (wherever it was aimed at), which means that, if you aim it at a slit, it will go through just that slit. And that will mean there will be no interference at the detector, because the interference depends on there being approximately equal components of the wave function passing through both slits.
 
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Likes Mr Wolf
  • #30
PeterDonis said:
You could do this, but you would be running a different experiment, and you would not see interference at the detector. The whole point of the standard double slit experiment is that, as the particles come from the source, they are not aimed at any particular place on the screen. The usual approximation that is adopted for analysis is that the wave function is a plane wave when it arrives at the screen, meaning that it has a basically equal probability of striking any point on the screen. That is what makes it the case that only a small percentage of particles get through the slits (basically the ratio of the area of the slits to the area of the entire screen). And that is what makes it the case that you get interference at the detector for particles that get through the slits.

If you change the particle source so that particles are aimed with high accuracy, then the wave function of the particles coming from the source will be very, very different: it will have a very high probability of hitting just one particular spot at the screen (wherever it was aimed at), which means that, if you aim it at a slit, it will go through just that slit. And that will mean there will be no interference at the detector, because the interference depends on there being approximately equal components of the wave function passing through both slits.
Ah, I see, due to your position certainty going up, because with a nicely aimed collimated beam your position information is good enough and the wave packet small enough, you can't go through both slits at once. Thank you for the clairificaiton :)
 
  • #31
Idunno said:
I see, due to your position certainty going up, because with a nicely aimed collimated beam your position information is good enough and the wave packet small enough, you can't go through both slits at once.

Exactly.
 
  • #32
Idunno said:
But, if you get your beam of particles nice and confined and aimed really well and lined up with some very well cut slits, you can increase the percent getting through, right? But you never get 100%. Would it be 100% with a perfect beam and perfect slits?

There is some law about aiming waves. The wavefront must be wide compared to the wavelength, only then the wavefront can stay straight and move straight.

Of course we can create a wave in a mirror-lined container, and then squeeze it out through small holes using a mirror-lined piston. (Or just wait for the wave to leak out by itself)
 
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  • #33
Idunno said:
I think the OP wanted to know "why does collapse happen?"
I was referring to the question by
Mr Wolf said:
What about when you make the double slit experiment shooting one photon or one electron at the time? Why do the photon or the electron pass through the slits and form an interference pattern, instead of being absorbed by the screen in which the slits are created?
 
  • #34
Ok, first of all, I think it's one of the most important experiment ever made in Physics. ...Even if I don't understand clearly the "why" and "how", even if I do agree that the "why" is something that goes beyond Physics and Science in general.

Having saing that, as far as I've understood, the single photon or electron behave as a wave, not as particle,, when they are shot against the double slit.
 
  • #35
In the lecture at some point you'll hear Feynman saying (in a heay accent) 'It's a particle' ...

The behaviour is not determined by whatever they are being targeted at.
 
  • #36
Mr Wolf said:
What about when you make the double slit experiment shooting one photon or one electron at the time? Why do the photon or the electron pass through the slits and form an interference pattern, instead of being absorbed by the screen in which the slits are created?
Great question and that's exactly my point.

This goes back to Schrodinger's cat and a lot of people don't read what he said:

One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following device (which must be secured against direct interference by the cat): in a Geiger counter, there is a tiny bit of radioactive substance, so small, that perhaps in the course of the hour one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges and through a relay releases a hammer that shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The first atomic decay would have poisoned it. The psi-function of the entire system would express this by having in it the living and dead cat (pardon the expression) mixed or smeared out in equal parts.

It is typical of these cases that an indeterminacy originally restricted to the atomic domain becomes transformed into macroscopic indeterminacy, which can then be resolved by direct observation. That prevents us from so naively accepting as valid a "blurred model" for representing reality. In itself, it would not embody anything unclear or contradictory. There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks.

https://en.wikipedia.org/wiki/Schrödinger's_cat

Schrodinger didn't like what Quantum Mechanics was saying and his objection was that it's too absurd. He admitted though, there's nothing unclear or contradictory about this. Einstein wrote this letter to Schrodinger in 1950.

You are the only contemporary physicist, besides Laue, who sees that one cannot get around the assumption of reality, if only one is honest. Most of them simply do not see what sort of risky game they are playing with reality—reality as something independent of what is experimentally established. Their interpretation is, however, refuted most elegantly by your system of radioactive atom + amplifier + charge of gun powder + cat in a box, in which the psi-function of the system contains both the cat alive and blown to bits. Nobody really doubts that the presence or absence of the cat is something independent of the act of observation.

https://en.wikipedia.org/wiki/Schrödinger's_cat

Sadly, decoherence can't save Schrodinger's cat and Einstein was wrong on this matter.

This is because there would need to be some hidden variable that tells the cat what state and what universe it should be in before decay occurs or it doesn't occur. You can have a dead cat but there wasn't any decay if decoherence occurs before a measurement. I think a mix of a live cat/dead cat is possible thanks to the great work of people like Susskind, Hooft and Maldacena on the holographic model of the universe.

This is why I asked about the plate in the double slit experiment. Why doesn't the screen/plate or detector plate cause collapse or the appearance of collapse to occur? People say Schrodinger's cat can't be in superposition because it's a classical object but the cat wouldn't need to be. It's in a mixed state until a measurement occurs.

I think all classical objects have to be in a mixed state until a measurement occurs. Maybe I'm missing something though and the cat can decohere to a dead cat in a universe before decay/no decay has occurred.
 
  • #37
platosuniverse said:
This is because there would need to be some hidden variable that tells the cat what state and what universe it should be in before decay occurs or it doesn't occur.

Not in the MWI. In the MWI, both outcomes occur, each one appropriately correlated with all other observables.

platosuniverse said:
Why doesn't the screen/plate or detector plate cause collapse or the appearance of collapse to occur?

You evidently have not read all of the responses in this thread. The short answer is, it does.

platosuniverse said:
People say Schrodinger's cat can't be in superposition because it's a classical object but the cat wouldn't need to be. It's in a mixed state until a measurement occurs.

Not if the cat is treated as a quantum system.

platosuniverse said:
I think all classical objects have to be in a mixed state until a measurement occurs.

If you have a theory that has things in it called "classical objects" that aren't governed by the laws of quantum mechanics, then you have an incomplete theory. Which is precisely the objection many have raised to interpretations of QM that have this dichotomy.

platosuniverse said:
Maybe I'm missing something though and the cat can decohere to a dead cat in a universe before decay/no decay has occurred.

I don't think there is any interpretation of QM that says this.
 
  • #38
platosuniverse said:
This is why I asked about the plate in the double slit experiment. Why doesn't the screen/plate or detector plate cause collapse or the appearance of collapse to occur? People say Schrodinger's cat can't be in superposition because it's a classical object but the cat wouldn't need to be. It's in a mixed state until a measurement occurs.
The plate, which now is black, is measuring, all the time, and the psi-function is executing some super-luminal contortions because of that, when the psi-function meets the plate.

When every part of the plate happens to get the result that the photon is not at this particular area of the plate, then in that case the psi-function goes through the slits ... squeezes itself through the slits so to speak.

Probability of finding a photon in the slits is high, after absence of photon has been measured on the surface of the plate.
 
  • #39
PeterDonis said:
Not in the MWI. In the MWI, both outcomes occur, each one appropriately correlated with all other observables.

I know but this correlation can't occur until a measurement has occurred. Unless there's some magical hidden variable that knows when an atom will decay/not decay before decay even happens, then the cat has to be in a mixed state of live/dead cat.

This is because the state of the cat is also entangled with it's environment. That's the box, the Scientist, the lab, etc. The only orthogonal state is decay/not decay. Those states are correlated but the live cat/dead cat states isn't orthogonal because the cat is entangled with which universe it will be in.

A live/dead cat can't be in universe A or universe B until a measurement has occurred. If you have decay that cat is dead and no decay the cat is alive but even then things start to get mixed up because there's a probability that there's no poison released but the cat dies because it was sick.

So outside of decay/no decay, you have this undefined, uncorrelated state that can't be resolved until a measurement occurs. After the measurement, there will be a decay, dead cat, experimenter and lab in one universe and thew opposite in another universe. This can't be resolved until the orthogonal states of decay/not decay are measured.
 
  • #40
platosuniverse said:
this correlation can't occur until a measurement has occurred

The correlation is just the outcome of unitary evolution. There is no need in the MWI to pick out a "measurement"; it's just like any other interaction.

platosuniverse said:
Unless there's some magical hidden variable that knows when an atom will decay/not decay before decay even happens, then the cat has to be in a mixed state of live/dead cat.

Sorry, but you're just restating your incorrect statement.

platosuniverse said:
This is because the state of the cat is also entangled with it's environment.

In Schrodinger's original formulation, this is only the case after the box is opened. Until the box is opened, the cat, like the poison vial and the radioactive substance inside the box, is treated as an isolated quantum system, not entangled with anything else. If you are saying that's impossible, you are saying, as I said before, that QM is necessarily incomplete: it can only describe "quantum" systems, not "macroscopic" or "classical" systems. As I said, this is one interpretation of QM, but not the only one, and you can't state it as fact, only as one interpretation.
 
  • #41
PeterDonis said:
In Schrodinger's original formulation, this is only the case after the box is opened. Until the box is opened, the cat, like the poison vial and the radioactive substance inside the box, is treated as an isolated quantum system, not entangled with anything else. If you are saying that's impossible, you are saying, as I said before, that QM is necessarily incomplete: it can only describe "quantum" systems, not "macroscopic" or "classical" systems. As I said, this is one interpretation of QM, but not the only one, and you can't state it as fact, only as one interpretation.

Again, this is incorrect. Here's the rest of the Wiki article:

Schrödinger's famous thought experiment poses the question, "when does a quantum system stop existing as a superposition of states and become one or the other?" (More technically, when does the actual quantum state stop being a linear combination of states, each of which resembles different classical states, and instead begin to have a unique classical description?) If the cat survives, it remembers only being alive. But explanations of the EPR experiments that are consistent with standard microscopic quantum mechanics require that macroscopic objects, such as cats and notebooks, do not always have unique classical descriptions. The thought experiment illustrates this apparent paradox. Our intuition says that no observer can be in a mixture of states—yet the cat, it seems from the thought experiment, can be such a mixture. Is the cat required to be an observer, or does its existence in a single well-defined classical state require another external observer? Each alternative seemed absurd to Einstein, who was impressed by the ability of the thought experiment to highlight these issues. In a letter to Schrödinger dated 1950, he wrote:

https://en.wikipedia.org/wiki/Schrödinger's_cat

This is exactly what I'm saying. The cat isn't in a well defined state until a measurement occur. The cat has to be in a mixture of live cat/dead cat. It's like if I had a box of marbles. One side is red and the other blue but I don't know which side is which. If I open one side of the box and I see red marbles then I know the other side is blue and vice versa. The marbles are an example of orthogonal states. The marbles all mixed up are not in a well defined state. This is the state of the cat until a measurement occurs.

Einstein understood this:

You are the only contemporary physicist, besides Laue, who sees that one cannot get around the assumption of reality, if only one is honest. Most of them simply do not see what sort of risky game they are playing with reality—reality as something independent of what is experimentally established. Their interpretation is, however, refuted most elegantly by your system of radioactive atom + amplifier + charge of gun powder + cat in a box, in which the psi-function of the system contains both the cat alive and blown to bits. Nobody really doubts that the presence or absence of the cat is something independent of the act of observation.

Einstein was wrong as far as we know because how can the cat be in a well defined state when it doesn't know which universe it will be in if MWI is correct? You can have a case where you have a dead cat but there was no poison released and no decay if somehow the cat takes a well defined state prior to measurement. The state of the cat is entangled with the outcome of decay/ no decay.
 
  • #42
platosuniverse said:
The cat isn't in a well defined state until a measurement occur. The cat has to be in a mixture of live cat/dead cat.
The whole point of Schrodinger was to illustrate that QM is not about a cat, but about a description/a model of the cat. That description live in a probability space made of zillionth of dimension made of complex scalars. There is never a real-cat being in a mixture of state, even less so in WMI (and that's why this interpretation is liked by some people)
 
  • #43
platosuniverse said:
The cat isn't in a well defined state until a measurement occur.

That's because it's entangled with the radioactive decay source, not because it's entangled with the environment outside the box.

platosuniverse said:
The cat has to be in a mixture of live cat/dead cat.

No, it isn't. It's in a superposition. That's what "isn't in a well defined state" means. A mixture means "it's in a well defined state, we just don't know which one". That is not the same as being in a superposition.

platosuniverse said:
The marbles all mixed up are not in a well defined state.

Yes, they are. Each marble is definitely red or blue, and the location of each marble is specified. Again, this is not the same as any of the marbles being in a quantum superposition.

platosuniverse said:
The state of the cat is entangled with the outcome of decay/ no decay.

Yes, which means that the cat is in a superposition; it is not in a well-defined state. Only the joint system of cat + radioactive decay source is in a well-defined state. And that is, once more, not the same as a mixture.
 
  • #44
platosuniverse said:
Einstein was wrong as far as we know because how can the cat be in a well defined state when it doesn't know which universe it will be in if MWI is correct?

Einstein's view was that QM is incomplete. This interpretation (which, as I've already noted, is a valid one) is incompatible with the MWI, yes, because the MWI states that QM is complete: more precisely, that the entire universe can be described as a pure quantum state which always evolves in time by unitary evolution.

It is also not correct to say that the cat "doesn't know which universe it will be in" under the MWI. There is no splitting of universes. The "cat" is simply not the kind of thing that our classical intuitions say it is (and nor are we ourselves). It is a quantum system which is entangled with other quantum systems, and that means it doesn't even have a definite state by itself. Only the universe as a whole has a definite quantum state. Everything else is in a superposition, entangled with many other things that are also in superpositions.
 
  • #45
Idunno said:
...if you get your beam of particles nice and confined and aimed really well...
Then you would know rather precisely the momentum of the particles.
 
  • #46
PeterDonis said:
No, it isn't. It's in a superposition. That's what "isn't in a well defined state" means. A mixture means "it's in a well defined state, we just don't know which one". That is not the same as being in a superposition.

This is wrong. I can't write equations on here but this lecture spells out the misconception. A mixed state doesn't have well defined properties in any direction. It's a mixture of states.



Watch starting at 6:50 on the video.

The Wiki article even sees this:

Schrödinger's famous thought experiment poses the question, "when does a quantum system stop existing as a superposition of states and become one or the other?" (More technically, when does the actual quantum state stop being a linear combination of states, each of which resembles different classical states, and instead begin to have a unique classical description?) If the cat survives, it remembers only being alive. But explanations of the EPR experiments that are consistent with standard microscopic quantum mechanics require that macroscopic objects, such as cats and notebooks, do not always have unique classical descriptions. The thought experiment illustrates this apparent paradox. Our intuition says that no observer can be in a mixture of states—yet the cat, it seems from the thought experiment, can be such a mixture. Is the cat required to be an observer, or does its existence in a single well-defined classical state require another external observer? Each alternative seemed absurd to Einstein, who was impressed by the ability of the thought experiment to highlight these issues. In a letter to Schrödinger dated 1950, he wrote:

https://en.wikipedia.org/wiki/Schrödinger's_cat

A mixed state isn't well defined.The probabilities than can occur are well defined but the total state of the system isn't well defined as live cat observed/dead cat observed until a measurement occurs.
 
  • #47
Boing3000 said:
The whole point of Schrodinger was to illustrate that QM is not about a cat, but about a description/a model of the cat. That description live in a probability space made of zillionth of dimension made of complex scalars. There is never a real-cat being in a mixture of state, even less so in WMI (and that's why this interpretation is liked by some people)

It has to be in a mixture of states prior to measurement. How can the cat know whether decay/no decay occurred before it occurs?
 
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