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

[PeterDonis]

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.

 

[jartsa]

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)

 

[BvU]

I think the OP wanted to know "why does collapse happen?"

I was referring to the question by

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?

 

[Mr Wolf]

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.

 

[BvU]

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.

 

[platosuniverse]

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 Schrödinger'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

Schrödinger 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 Schrödinger 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 Schrödinger'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 Schrödinger'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.

 

[PeterDonis]

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.

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.

People say Schrödinger'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.

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.

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.

 

[jartsa]

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 Schrödinger'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.

 

[platosuniverse]

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.

 

[PeterDonis]

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.

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.

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

In Schrödinger'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.

 

[platosuniverse]

In Schrödinger'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.

 

[Boing3000]

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 Schrödinger 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)

 

[PeterDonis]

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.

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.

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.

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.

 

[PeterDonis]

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.

 

[David Lewis]

...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.

 

[platosuniverse]

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.

 

[platosuniverse]

The whole point of Schrödinger 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?

 

[PeterDonis]

Thread closed for moderation.