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

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:

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?

#### Attachments

• double_slit_setup.png
154.1 KB · Views: 955

mfb
Mentor
Everything that hits the plate is absorbed (or reflected) and doesn't matter for what happens behind the slits.
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.

PeterDonis
Mentor
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.

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.

Last edited:
Nugatory
Mentor
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.

mfb
Mentor
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).

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

PeterDonis
Mentor
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.

PeterDonis
Mentor
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.

Demystifier
Gold Member
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.

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.

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?

PeterDonis
Mentor
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##.

Nugatory
Mentor
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....

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?

PeterDonis
Mentor
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).

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?

PeterDonis
Mentor
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.

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.

Then you don't like running double slit experiments.

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?

PeterDonis
Mentor
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.

Yes.
OK. Thanks for your time, much appreciated. :)

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?

Nugatory
Mentor
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.

BvU
Homework Helper
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 probaly 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.

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 probaly 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.

Nugatory
Mentor
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
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".

…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.

PeterDonis
Mentor
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.

Mr Wolf
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 :)

PeterDonis
Mentor
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.

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)

Last edited:
BvU
Homework Helper
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?

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