Undergrad Please help me understand the double slit experiment and conclusion

[PeterDonis]

those particles that manage to get through one slit will simply propagate straight to the screen and form a band of exactly the same width as the slit.

But classically, there will also be some particles that get to the screen after hitting one side or the other of the slit, so the band you get on the screen behind the slit will be somewhat wider than the slit itself, and will gradually fade out to either side of the image of the slit.

What you will not get, classically, is an image on the screen when two slits are open, that is any different from a simple sum of the images you get when each slit is open by itself. But quantum mechanically, you will.

 

[jackjack2025]

A ball is a classical object. You can't use classical objects to understand quantum mechanics. It won't work.

Again, you need to read a basic textbook on quantum mechanics.

A wave is a classical thing, you can't use that to understand Quantum Mechanics. Errr...oh wait

I am not incapable of reading a book. I have studied Quantum mechanics and have a PhD. My issue is not that I do not understand how to read. There are great physicists and great books. It is that I do not understand the conclusion in a basic way and so I ask for help. I may be a bad student, but I am not stupid, and just need it explained in a convincing way. If the answer is 'read a book with some assumptions' which presents the standard theory, fine, but I wanted to understand and not just accept standard theory. Please accept my approach to learning, even if it is not yours.

 

[jackjack2025]

No, it's not. It's how classical objects behave (with some idealizations, which I'll address in a response to @renormalize in my next post).

Apparently you need to read a basic textbook on Newtonian physics, as well as one on quantum mechanics.

Oh dear. It is an assumption. Newton was brilliant, he made 3 assumptions and drew some amazing stuff from them. But those assumptions were not all correct in terms of real life.

I don't want to get into this because it is a distraction from what I am asking.

Logically, you understand that in classical mechanics, there are assumptions. I hope you would agree.

 

[PeroK]

In terms of classical mechanics, an object has a well-defined width. That puts a limit on how small you can make a slit and still have the object pass through.

In any case, there is no evidence of any natural limit to the precision with which an object can be aimed at a gap or slit.

A bullet emerges from the barrel of a rifle with high precision relative to the size of the bullet. There's no evidence of diffraction of a bullet emerging from an aperture barely wider than the bullet.

This does not apply to light or electrons. Eventually, as the aperture becomes too narrow the light or electrons diffract. And, among other things, we see nature placing a limit on the precision with which you can focus something.

The double-slit phenomenon is another level of non-classical behaviour, as that also demonstrates particle interference.

 

[jackjack2025]

In terms of classical mechanics, an object has a well-defined width. That puts a limit on how small you can make a slit and still have the object pass through.

In any case, there is no evidence of any natural limit to the precision with which an object can be aimed at a gap or slit.

A bullet emerges from the barrel of a rifle with high precision relative to the size of the bullet. There's no evidence of diffraction of a bullet emerging from an aperture barely wider than the bullet.

This does not apply to light or electrons. Eventually, as the aperture becomes too narrow the light or electrons diffract. And, among other things, we see nature placing a limit on the precision with which you can focus something.

The double-slit phenomenon is another level of non-classical behaviour, as that also demonstrates interference.

Agreed and agreed. But :) sorry a bullet is not affected much by its atmoshpere in which is it fired. Maybe if fired into water the trajectory will be off or whatever. We are talking about very tiny particles, and they are travelling through something, and we don't know if they are affected by that or not? Or do we?

 

[PeterDonis]

A wave is a classical thing, you can't use that to understand Quantum Mechanics. Errr...oh wait

No, you're quite right; you can't use classical wave mechanics to understand QM either. QM is not classical wave mechanics any more than it's classical particle mechanics.

just need it explained in a convincing way.

This is going to sound blunt, but Nature does not care whether you're convinced or not. Countless experiments have shown that things like light and electrons obey the laws of quantum mechanics, not the laws of classical particles or classical waves. You say you've studied QM, but you don't show any signs of grasping that fundamental point, which anyone who has studied QM should know.

In other words, your whole approach is backwards. You're saying, here are the concepts I understand--classical particles and classical waves--now give me an explanation of QM that makes sense in terms of those concepts. And there isn't one. That's the brutal fact that physicists had to cope with when QM was first worked out--they had to retrain their intuitions to accept a new set of concepts, because the old ones simply failed to account for the experimental data. As Feynman once remarked, "Quantum mechanics was not wished upon us by theorists." It wasn't that somebody came up with this brand new concept and then told everyone to use it. It was that experiments started showing things that simply didn't make any kind of sense, and physicists had to be dragged kicking and screaming to a new kind of conceptual model.

Please accept my approach to learning, even if it is not yours.

Again, you have this backwards. You're trying to say, this is how I can understand things, now make QM conform to that. And that doesn't work. You're not the first or even the millionth person to find that out the hard way. The question isn't whether I, or anyone else here, "accepts" your approach to learning. The question is whether your approach to learning will let you understand QM or not. Right now it seems like the answer is "not". You can't fix that by demanding that people accept your approach to learning. You have to change your approach to accept the way Nature actually is.

 

[PeterDonis]

Logically, you understand that in classical mechanics, there are assumptions

Of course there are; every theoretical model is built on assumptions.

But not the ones you're claiming.

 

[PeterDonis]

a bullet is not affected much by its atmoshpere in which is it fired

But it certainly can ricochet off solid objects. That's the equivalent of a particle in the double slit experiment getting through the slit but hitting one of its edges and being deflected so that it lands somewhere on the screen that isn't quite behind the slit.

In any case, focusing on that issue is missing the crucial point. The crucial point is the one I gave in post #31: with classical particles, if you have two slits, the pattern you get on the screen when both slits are open will be the simple sum of the patterns you get when each slit is open by itself. But with quantum objects like electrons, it won't.

The issue with using classical wave mechanics is different. With classical wave mechanics, you can indeed get an interference pattern on the screen when both slits are open, a pattern which is not the sum of the patterns you would get when each slit is open by itself (which are just images of each slit with some diffraction at the edges). But with classical wave mechanics, when you turn the intensity of the source lower and lower, the pattern on the screen just gets fainter and fainter, but it's still all there at once. With quantum objects, that's not what happens: instead, what happens with a very low intensity source is that you start seeing individual dots on the screen--particle impacts. But over time, the dots build up the interference pattern.

Before QM was fully understood, terms like "wave-particle duality" were used to describe this conundrum, that quantum objects sometimes seem to act like particles and sometimes seem to act like waves. But now we know that even that's not really correct. Quantum objects are quantum--the full quantum behavior includes things that aren't like any classical concept. In certain idealized cases, you can get behavior that looks similar to classical particles in some respects, and similar to classical waves in others. But those are special cases and you can't understand QM in terms of them. You have to understand QM as it is, as a separate model of its own, with its own set of concepts that aren't classical concepts.

 

[PeterDonis]

There's no evidence of diffraction of a bullet emerging from an aperture barely wider than the bullet.

Not diffraction, but bullets can ricochet off the edges of slits that are just a little wider than the bullet, if the bullet isn't aimed precisely at the center of the slit.

However, that's a side issue that's not the crucial point, as I explained in post #38.

 

[Nugatory]

That seems to be an assumption. The assumption is that the particles travel through the slits and nothing affects them. Therefore they end up in a roughly straight line and you get two bands. But that is an assumption.

Experiments with electrons and other subatomic particles are done in a hard vacuum so there isn’t anything there to affect their trajectory except the barrier with slits. And of course we verify that if the barriers and slits aren’t there the particles do travel in a straight line so we know (not assume) that our vacuum is good and there’s nothing in there affecting the particles except our barrier/slit.

 

[javisot]

The uncertainty principle is fundamental in quantum mechanics.

If you throw particles at a double slit arranged in a specific way, the uncertainty principle tells you that one particle is passing through two slits simultaneously. This is counterintuitive to classical logic (classically, it will pass through one slit or the other, but not both simultaneously).

At that point, you can assume that what we emitted wasn't a particle but a classical wave, but that doesn't explain everything. If you imagine the set of all possible configurations and outcomes in the double-slit experiment, the most complete description to explain all those results is the wave-particle description.

 

[PeterDonis]

If you throw particles at a double slit arranged in a specific way, the uncertainty principle tells you that one particle is passing through two slits simultaneously.

No, that has nothing to do with the uncertainty principle.

Please do not clutter this thread with misinformation.

 

[jackjack2025]

I don't mind you being blunt and thanks for the very good post PeterDonis.

I am fine with being wrong. This is learning. I don't accept that I shouldn't 'try' to understand based on real results or based on classical thinking. It may be wrong, but we have to try. My way of thinking needs to be changed? Good. But I have to try and we all start where we start. You may know more than me, and that is why I came to the forum.

I do not see anything in the double slit experiment, that can't be the result of deterministic things. There are other experiments and results in QM that might blow my mind, but I started with a basic well known double slit experiment.

 

[jackjack2025]

Experiments with electrons and other subatomic particles are done in a hard vacuum so there isn’t anything there to affect their trajectory except the barrier with slits. And of course we verify that if the barriers and slits aren’t there the particles do travel in a straight line so we know (not assume) that our vacuum is good and there’s nothing in there affecting the particles except our barrier/slit.

Assumption. A "hard vacuum... nothing there". Assumption. Do you see what I mean or not?

 

[Nugatory]

I do not see anything in the double slit experiment, that can't be the result of deterministic things.

There are formulations of quantum mechanics that explain the observed results of the double slit experiment deterministically, but they are not classical. They still have the property that the path of a particle cannot be calculated without considering possible paths through both slits, and the path of a particle going through one slit will be different according to whether the other seemingly irrelevant slit is open or not.

There are other experiments and results in QM that might blow my mind

For that you want entanglement and Bell’s Theorem. But do that in a new thread, and only after you’ve done more reading.

There’s no substitute for a real textbook, but Giancarlo Ghirardi’s “Sneaking a look at God’s cards” is a pretty good layman-friendly presentation of QM, certainly less misleading than the stuff you’ve been reading.

 

[jackjack2025]

But it certainly can ricochet off solid objects. That's the equivalent of a particle in the double slit experiment getting through the slit but hitting one of its edges and being deflected so that it lands somewhere on the screen that isn't quite behind the slit.

In any case, focusing on that issue is missing the crucial point. The crucial point is the one I gave in post #31: with classical particles, if you have two slits, the pattern you get on the screen when both slits are open will be the simple sum of the patterns you get when each slit is open by itself. But with quantum objects like electrons, it won't.

The issue with using classical wave mechanics is different. With classical wave mechanics, you can indeed get an interference pattern on the screen when both slits are open, a pattern which is not the sum of the patterns you would get when each slit is open by itself (which are just images of each slit with some diffraction at the edges). But with classical wave mechanics, when you turn the intensity of the source lower and lower, the pattern on the screen just gets fainter and fainter, but it's still all there at once. With quantum objects, that's not what happens: instead, what happens with a very low intensity source is that you start seeing individual dots on the screen--particle impacts. But over time, the dots build up the interference pattern.

Before QM was fully understood, terms like "wave-particle duality" were used to describe this conundrum, that quantum objects sometimes seem to act like particles and sometimes seem to act like waves. But now we know that even that's not really correct. Quantum objects are quantum--the full quantum behavior includes things that aren't like any classical concept. In certain idealized cases, you can get behavior that looks similar to classical particles in some respects, and similar to classical waves in others. But those are special cases and you can't understand QM in terms of them. You have to understand QM as it is, as a separate model of its own, with its own set of concepts that aren't classical concepts.

Yes. I am not a big fan of this wave particle duality thing as you probably guessed, as it sounds a bit wishy-washy to me. But as you said, it isn't about whether it makes sense to me, it just is.

Can I ask you about very basics on what you mentioned:
"a pattern which is not the sum of the patterns you would get when each slit is open by itself "
I don't classically expect the sum of two slits (alone) to be the sum result when two slits are open. Because I am not assuming that particles are firing through in an empty vacuum without interference.

 

[PeterDonis]

I do not see anything in the double slit experiment, that can't be the result of deterministic things.

Whether or not QM is deterministic depends on what interpretation of QM you adopt; in some interpretations, like the MWI, there is no randomness at all, and everything is deterministic.

However, discussion of interpretations is off topic in this forum; it would require a separate thread in the interpretations subforum.

I also think the issue of determinism vs. randomness is not the crucial point in this thread either, however, even apart from any interpretation discussion. I think the crucial point is that experiments say quantum objects behave a certain way, and you can't understand why they behave that way--and as I've already said, you're not going to fix that by repeating what you've been saying up to now. Nature simply doesn't care what your approach to learning is, or what you do or do not see. Quantum mechanics is a theoretical model that makes accurate predictions in its domain. That's true regardless of whether you can understand why it works. Indeed, Feynman once said that nobody understands quantum mechanics--by which he meant that nobody has a nice intuitive model in their head that explains why QM's predictions are accurate, why Nature behaves that way. They can use the model to make accurate predictions, but that's all.

If Feynman was right, you're really not any worse off than Nobel Prize winning physicists as far as "understanding" goes.

 

[PeterDonis]

I don't classically expect the sum of two slits (alone) to be the sum result when two slits are open.

You should. If you don't, then your understanding of classical mechanics is wrong. That would need to be addressed in a separate thread in the appropriate forum.

 

[javisot]

They still have the property that the path of a particle cannot be calculated without considering possible paths through both slits, and the path of a particle going through one slit will be different according to whether the other seemingly irrelevant slit is open or not.

and what is the reason for that?

 

[PeterDonis]

I am not assuming that particles are firing through in an empty vacuum without interference.

You are implicitly making two different claims here. The first is technically true (though irrelevant in practice, as we'll see below), but the second is false.

Your first implicit claim is that the particles aren't actually traveling through an "empty vacuum". That is technically true--but "empty vacuum" is still an extremely good approximation to actual experimental conditions. As @Nugatory has already told you, we can and have experimentally verified that, if we just send quantum objects through our experimental vacuum chambers, with no slits or barriers, they travel in straight lines. That's very good evidence that, to a good approximation, "empty vacuum" is an appropriate model for what's inside the chamber, apart from any barriers with slits in them. So while it's true that the chamber is not absolutely empty of particles, assuming that it is still makes very accurate predictions--so physicists don't care that it's only an approximation. It works.

Your second implicit claim is that, in a classical model, if we drop the first assumption and assume that the particles we are firing through the chamber have non-negligible effects on their motion due to collisions with other particles in the chamber, while still being treatable as distinguishable particles separate from those that are already in the chamber, that can somehow produce an interference pattern like the ones we see with classical waves. That assumption is false. And, as I've said, if you want to discuss that further, it belongs in a separate thread in the appropriate forum, where you can improve your understanding of classical physics. It's off topic here.

 

[Nugatory]

Assumption. A "hard vacuum... nothing there". Assumption. Do you see what I mean or not?

you may have missed this part:
“ And of course we verify that if the barriers and slits aren’t there the particles do travel in a straight line so we know (not assume) that our vacuum is good and there’s nothing in there affecting the particles except our barrier/slit.”

So yes, the assumption is there but it’s a pretty good one: any change between the barrier-present behavior of the electrons and the barrier-absent behavior is due to the presence or absence of the barrier.

 

[jackjack2025]

There are formulations of quantum mechanics that explain the observed results of the double slit experiment deterministically, but they are not classical. They still have the property that the path of a particle cannot be calculated without considering possible paths through both slits, and the path of a particle going through one slit will be different according to whether the other seemingly irrelevant slit is open or not.
For that you want entanglement and Bell’s Theorem. But do that in a new thread, and only after you’ve done more reading.

There’s no substitute for a real textbook, but Giancarlo Ghirardi’s “Sneaking a look at God’s cards” is a pretty good layman-friendly presentation of QM, certainly less misleading than the stuff you’ve been reading.

Nugatory, thanks and please understand my response in the right way, which is that, I can read all the theory books. I have read some. I did study Quantum Mechanics in Uni. It is a wonderful theory. And actually I don't know nearly all of it, but I know some. But it is a theory. It might be right. It might not. Seems to be right mostly, but I want to understand more. I work from a very logical basis, which is you make some assumptions, you conclude some results. They maybe match what happens in the real world, they maybe are close, they maybe are not.

So probably what you and PeterDonis have is practical experience and you get to test these things and play around and understand. But I don't, I just get the theory and am missing the pieces. Is that ok I ask some questions?

So you talk about a particle and possible paths going through the slits. Can it go through both slits and is some sort of superposition, or is this all just a mistake of measurement?

Please be patient. I am not disregarding what you say, I am challenging because I want to understand better. I think there are very useful posts in this thread, maybe not from me. Prove a particle is in a superposition. Or can you not?

 

[Nugatory]

and what is the reason for that?

The reason why the deterministic formulation has that property? Or the reason why theory predicts and experiment confirms different behavior when one slit is open or two?

For the first question, that formulation (Bohmian mechanics)was constructed to include contributions from both slits because otherwise it failed to match observation. Note that in this formulation the particle unambiguously goes through one or the other slit, but experiment shows that moving the other, seemingly irrelevant slit changes the pattern, so any theory that does not include the other slit is dead on arrival.

The answer to the second question is that we don’t know why nature behaves this way, just that it does. QM is a theory about how the universe behaves, not a suggested behind the scenes process that makes it behave that way.

 

[PeterDonis]

Can it go through both slits and is some sort of superposition, or is this all just a mistake of measurement?

Neither. It's a quantum object, and quantum objects simply don't play by the rules you are implicitly using when you frame your questions.

 

[PeterDonis]

I work from a very logical basis, which is you make some assumptions, you conclude some results.

Guess what? This is exactly what quantum mechanics does. But the assumptions of QM are very different from the ones you're using. And the QM assumptions have the great advantage that when you build a model based on them and use it to make predictions, the predictions are accurate.

 

[jackjack2025]

You are implicitly making two different claims here. The first is technically true (though irrelevant in practice, as we'll see below), but the second is false.

Your first implicit claim is that the particles aren't actually traveling through an "empty vacuum". That is technically true--but "empty vacuum" is still an extremely good approximation to actual experimental conditions. As @Nugatory has already told you, we can and have experimentally verified that, if we just send quantum objects through our experimental vacuum chambers, with no slits or barriers, they travel in straight lines. That's very good evidence that, to a good approximation, "empty vacuum" is an appropriate model for what's inside the chamber, apart from any barriers with slits in them. So while it's true that the chamber is not absolutely empty of particles, assuming that it is still makes very accurate predictions--so physicists don't care that it's only an c

Your second implicit claim is that, in a classical model, if we drop the first assumption and assume that the particles we are firing through the chamber have non-negligible effects on their motion due to collisions with other particles in the chamber, while still being treatable as distinguishable particles separate from those that are already in the chamber, that can somehow produce an interference pattern like the ones we see with classical waves. That assumption is false. And, as I've said, if you want to discuss that further, it belongs in a separate thread in the appropriate forum, where you can improve your understanding of classical physics. It's off topic here.

Thanks PeterDonis, I do want to discuss it further, I didn't realise I placed this in a wrong a forum. Sorry. New to the forum. Can someone move it to where it needs to go?

"if we just send quantum objects through our experimental vacuum chambers, with no slits or barriers, they travel in straight lines."
But that is what you would expect classically. I think I misunderstood the point you are making here.

"...approximation. It works."

So you accept that?

 

[jackjack2025]

you may have missed this part:
“ And of course we verify that if the barriers and slits aren’t there the particles do travel in a straight line so we know (not assume) that our vacuum is good and there’s nothing in there affecting the particles except our barrier/slit.”

So yes, the assumption is there but it’s a pretty good one: any change between the barrier-present behavior of the electrons and the barrier-absent behavior is due to the presence or absence of the barrier.

No I wasn't assuming things bouncing off the barriers. I was assuming that space is not empty. So the particles would bounce off the medium, and basically end up where you would expect, with one slit, two slits etc. If you fire a particle through a medium with one slit, I would expect it to end up somewhere in the 'straight line' you mention. That is also what a Brownian motion particle would do. But it wouldn't be exact, there would be some randomness, +- some standard deviation, you get my point I hope

 

[PeterDonis]

I do want to discuss it further

The classical question? Just start a new thread of your own in the Classical Physics forum.

 

[jackjack2025]

Neither. It's a quantum object, and quantum objects simply don't play by the rules you are implicitly using when you frame your questions.

That you need to justify though

 

[PeterDonis]

"if we just send quantum objects through our experimental vacuum chambers, with no slits or barriers, they travel in straight lines."
But that is what you would expect classically.

Remember, that comment of mine was addressing your first implicit claim, that the experimental chamber is not actually "empty vacuum". That's technically true, but irrelevant as far as making predictions; "empty vacuum" is a good enough approximation to make accurate predictions.

It's true that, if there's no barrier or anything else inside the chamber, it's just an empty chamber, then it's impossible to tell whether the objects you're shooting through the chamber are classical particles or quantum objects--both models make the same simple, boring prediction that the objects will travel in straight lines. You have to add something more to the experiment to distinguish the two models.