I Please help me understand the double slit experiment and conclusion

[jackjack2025]

TL;DR Summary

help on the basics of particle wave duality

I am not an expert in Physics but do have a big interest and would like some people that know more than I do to explain some things, and clear up some holes in my logic.

If you fire particles through two slits, you might expect to get only two lines and not an interference pattern. But if those particles are going through and bouncing off 'something', perhaps in a Brownian motion type way, you would expect to see an interference pattern. Nothing to do with 'waves'. And if you adjusted the size of the slits, that would also affect the interference pattern distribution, which I believe is also backed up by experiment. So why is there an assumption of wave like behaviour? I am not saying it is wrong, just trying to understand.

There is also the concept of 'Observation' affecting the outcome and the Observation problem. Can someone please explain to me how the observation is done in practice and why we think that the measurement isn't actually affecting something that is deterministic. For example, I have read and seen that if you measure which slit the particle goes through, suddenly you will see a change from the interference pattern, to just seeing 'particle-like' behaviour and two strips. But what is the measurement here. Is it really not affecting anything?

Help, thanks

 

[PeroK]

TL;DR Summary: help on the basics of particle wave duality

I am not an expert in Physics but do have a big interest and would like some people that know more than I do to explain some things, and clear up some holes in my logic.

If you fire particles through two slits, you might expect to get only two lines and not an interference pattern. But if those particles are going through and bouncing off 'something', perhaps in a Brownian motion type way, you would expect to see an interference pattern. Nothing to do with 'waves'. And if you adjusted the size of the slits, that would also affect the interference pattern distribution, which I believe is also backed up by experiment. So why is there an assumption of wave like behaviour? I am not saying it is wrong, just trying to understand.

There is also the concept of 'Observation' affecting the outcome and the Observation problem. Can someone please explain to me how the observation is done in practice and why we think that the measurement isn't actually affecting something that is deterministic. For example, I have read and seen that if you measure which slit the particle goes through, suddenly you will see a change from the interference pattern, to just seeing 'particle-like' behaviour and two strips. But what is the measurement here. Is it really not affecting anything?

Help, thanks

If you fire electrons through a narrow slit, then you can detect a diffraction pattern that matches that of classical.waves. this was first observed experimentally in the 1920s. This is something that undergraduate students would do nowadays as part of the lab work.

As electrons are particles, this led to the concept of wave-particle duality. This was explained by quantum mechanics in the 1930s.

Moreover, if you have two slits close to each other, then electrons can produce the double-slit interference pattern. If you fire the electrons through one at a time, then the pattern builds up statistically point by point.

If you find a way to determine that each electron goes through one slit or the other, then the interference pattern is replaced by two diffraction patterns. The electrons in this case still behave like a wave by diffraction.

 

[PeroK]

PS the conclusion is that the electron is neither a classical particle nor a classical wave. It's a quantum object, which has the ability to produce both classical wavelike and classical particle-like behaviour.

 

[jackjack2025]

If you fire electrons through a narrow slit, then you can detect a diffraction pattern that matches that of classical.waves. this was first observed experimentally in the 1920s. This is something that undergraduate students would do nowadays as part of the lab work.

As electrons are particles, this led to the concept of wave-particle duality. This was explained by quantum mechanics in the 1930s.

Moreover, if you have two slits close to each other, then electrons can produce the double-slit interference pattern. If you fire the electrons through one at a time, then the pattern builds up statistically point by point.

If you find a way to determine that each electron goes through one slit or the other, then the interference pattern is replaced by two diffraction patterns. The electrons in this case still behave like a wave by diffraction.

Thank you for the response. But I don't understand. The thing I don't understand is why some conclusions have been drawn from the experiment.

If you have a single slit, I would expect a particle going through to resemble a sort of Normal distribution.

Once you have two slits, I would then expect a result of either, two 'hills' or Bell shaped curves around the slits (if the slits are far apart) or several 'hills' with the biggest 'hill' in the middle of the slits. An interference pattern. This to me is nothing to do with waves or anything that is unexpected or wave like. This is what you would expect from a particle. Isn't this what we see in experiment?

 

[PeroK]

Thank you for the response. But I don't understand. The thing I don't understand is why some conclusions have been drawn from the experiment.

If you have a single slit, I would expect a particle going through to resemble a sort of Normal distribution.

That's what you get if the slit is wide enough. If the slit is sufficiently narrow, then the pattern spreads out into a diffraction pattern of light and dark patches.

This is what you would expect from a classical wave.

Note that when the slit is sufficiently narrow, the uncertainty principle comes into play.

 

[PeroK]

Once you have two slits, I would then expect a result of either, two 'hills' or Bell shaped curves around the slits (if the slits are far apart) or several 'hills' with the biggest 'hill' in the middle of the slits. An interference pattern. This to me is nothing to do with waves or anything that is unexpected or wave like. This is what you would expect from a particle. Isn't this what we see in experiment?

This is not what you see. Again, the slits need to be sufficiently narrow and close enough together. You then get the two diffraction patterns overlapping and, crucially, interference is seen. What you see is not the simple addition of the two diffraction patterns. Instead, you see constructive and destructive interference.

This matches the pattern predicted by classical waves in a double-slit. The question is how a particle can interfere with itself? Quantum Mechanics provides the answer to that.

 

[jackjack2025]

That's what you get if the slit is wide enough. If the slit is sufficiently narrow, then the pattern spreads out into a diffraction pattern of light and dark patches.

This is what you would expect from a classical wave.

Note that when the slit is sufficiently narrow, the uncertainty principle comes into play.

Thanks again for your time. But waves in the sea are just particles and deterministic. I understand they exhibit a behaviour that Quantum Physics has called wave like behaviour, but that doesn't fundamentally change the underlying nature.

And I believe I am wrong on this, but I just don't see where.

 

[PeroK]

Thanks again for your time. But waves in the sea are just particles and deterministic. I understand they exhibit a behaviour that Quantum Physics has called wave like behaviour...

Waves were called waves before quantum physics. The cover photograph of the physics book we used at school showed water diffracting round the edge of a sea wall.

 

[jackjack2025]

This is not what you see. Again, the slits need to be sufficiently narrow and close enough together. You then get the two diffraction patterns overlapping and, crucially, interference is seen. What you see is not the simple addition of the two diffraction patterns. Instead, you see constructive and destructive interference.

This matches the pattern predicted by classical waves in a double-slit. The question is how a particle can interfere with itself? Quantum Mechanics provides the answer to that.

Can you please elaborate on what you mean by interference?

 

[PeroK]

Can you please elaborate on what you mean by interference?

Suppose a particle that goes through slit 1 ( assume slit 2 is closed) sometimes hits point X on the detector screen. And, likewise, if slit 1 is closed and the particle goes through slit 2, then it also sometimes hits point X.

However, when both slits are open the particle may never be detected at point X. That's called destructive interference.

That's what happens in the double-slit experiment.

 

[PeroK]

PS watch Feynman's lecture on the Quantum Mechanical view of nature.

https://www.feynmanlectures.caltech.edu/fml.html#6

 

[jackjack2025]

Suppose a particle that goes through slit 1 ( assume slit 2 is closed) sometimes hits point X on the detector screen. And, likewise, if slit 1 is closed and the particle goes through slit 2, then it also sometimes hits point X.

However, when both slits are open the particle may never be detected at point X. That's called destructive interference.

That's what happens in the double-slit experiment.

--------- ---------
xXx

------- ----------- ---------
xXx xXx

----------- - ---------
xXxXXXxXx

Sorry for the poor way of representing

 

[PeterDonis]

if those particles are going through and bouncing off 'something', perhaps in a Brownian motion type way, you would expect to see an interference pattern.

No, you wouldn't. Classical particles don't exhibit interference. Only classical waves do. That's why people naturally interpreted seeing interference patterns in double slit experiments as evidence of wave behavior.

For light that was not an issue, since light was already believed to be waves; but when experiments began to see such behavior for things like electrons, it created a problem, because electrons were believed to be particles, not waves.

Eventually physicists realized that none of these things are classical particles or classical waves; they are quantum objects that obey the laws of quantum mechanics, which are not the same as the laws of either classical particles or classical waves. So any reasoning you do about quantum objects based on either of those classical concepts--meaning all the reasoning you are trying to do in this thread--is simply wrong.

 

[jackjack2025]

Suppose a particle that goes through slit 1 ( assume slit 2 is closed) sometimes hits point X on the detector screen. And, likewise, if slit 1 is closed and the particle goes through slit 2, then it also sometimes hits point X.

However, when both slits are open the particle may never be detected at point X. That's called destructive interference.

That's what happens in the double-slit experiment.

Is this correct? Or is it that relative to other points that are hit, the X is not hit very often? Probability low but not zero?

 

[PeterDonis]

Is this correct?

Yes.

Or is it that relative to other points that are hit, the X is not hit very often? Probability low but not zero?

No.

You need to read a basic textbook on quantum mechanics.

 

[PeroK]

Is this correct? Or is it that relative to other points that are hit, the X is not hit very often? Probability low but not zero?

In total destructive interference, the probability is zero. But, interference has a variable quality.

Watch the Feynman video!

 

[jackjack2025]

No, you wouldn't. Classical particles don't exhibit interference. Only classical waves do. That's why people naturally interpreted seeing interference patterns in double slit experiments as evidence of wave behavior.

For light that was not an issue, since light was already believed to be waves; but when experiments began to see such behavior for things like electrons, it created a problem, because electrons were believed to be particles, not waves.

Eventually physicists realized that none of these things are classical particles or classical waves; they are quantum objects that obey the laws of quantum mechanics, which are not the same as the laws of either classical particles or classical waves. So any reasoning you do about quantum objects based on either of those classical concepts--meaning all the reasoning you are trying to do in this thread--is simply wrong.

Let's say I have a ball moving down and it bounces to the left or right with 0.5 probability. And it does that as a random walk. What you would see by probability is that of course mostly it ends up towards the middle (0), and it has some normal sort of distribution.

Lets drop that ball now with one slit, same result, centred around the slit. Let's drop it with two slits, if the slits are far apart, you will get two normal distributions centred around those slits, if the slits are close, you will actually get some combination in the middle which will look like an interference pattern.

I think you guys are telling me that there is some destructive quality so that it can't land. How is that shown?

 

[jackjack2025]

In total destructive interference, the probability is zero. But, interference has a variable quality.

Watch the Feynman video!

Watching. Feynman is a genius, but again I am trying to understand how it got to here, and not the end theory.

 

[PeroK]

I think you guys are telling me that there is some destructive quality so that it can't land. How is that shown?

An electron is not a ball.

Watch the Feynman video!

 

[jackjack2025]

Yes.


No.

You need to read a basic textbook on quantum mechanics.

I have studied Quantum mechanics. I am sorry my question seems illiterate or something. It isn't.

When you study a theory, it is just that. Someone presents some assumptions and makes a case and backs it up with some evidence or proof. Awesome. Draw some conclusions and advance.

I do not understand the conclusions that have been drawn. I know it may seem stupid to you or others, but I would prefer the resolution to be based on results.

 

[renormalize]

Once you have two slits, I would then expect a result of either, two 'hills' or Bell shaped curves around the slits (if the slits are far apart) or several 'hills' with the biggest 'hill' in the middle of the slits. An interference pattern. This to me is nothing to do with waves or anything that is unexpected or wave like. This is what you would expect from a particle. Isn't this what we see in experiment?

No, your expectation is wrong. A collimated beam of particles that obey classical mechanics would form only two bands on the screen behind the slits as per the left-side of the diagram below; i.e., there is no interference classically. But real particles obey quantum mechanics and exhibit many bands on the screen, as shown below on the right. That is quantum interference.

 

[jackjack2025]

An electron is not a ball.

Watch the Feynman video!

I don't think it is a ball either. But that wasn't my point.

"it's not any deeper, only wider" - Feynman video

I appreciate you giving me your time and knowledge, sorry if my responses come across in a bad way. Still need some help understanding though

 

[jackjack2025]

No, your expectation is wrong. A collimated beam of particles that obey classical mechanics would form only two bands on the screen behind the slits as per the left-side of the diagram below; i.e., there is no interference classically. But real particles obey quantum mechanics and exhibit many bands on the screen, as shown below on the right. That is quantum interference.
View attachment 359792

Why?

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.

 

[renormalize]

Why?
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.

It's certainly not an assumption, it's just Newton's first (classical!) law: a moving particle travels in a straight line with constant velocity if it's not subjected to a force. That means that 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. And two slits means exactly two such bands (and no more) on the screen.

 

[jackjack2025]

No, your expectation is wrong. A collimated beam of particles that obey classical mechanics would form only two bands on the screen behind the slits as per the left-side of the diagram below; i.e., there is no interference classically. But real particles obey quantum mechanics and exhibit many bands on the screen, as shown below on the right. That is quantum interference.
View attachment 359792

I really appreciate responses, it may seem like I am not learning anything :) but I am.

In classical mechanics you would not expect those two bands. You would expect a single line, unless the particles you sent through are in some sense 'random'. Classical mechanics is deterministic and does not expect these sort of random particles hitting a screen in different ways. So the model is already wrong, correct?

 

[PeterDonis]

Let's say I have a ball

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.

 

[jackjack2025]

It's certainly not an assumption, it's just Newton's first (classical!) law: a moving particle travels in a straight line with constant velocity if it's not subjected to a force. That means that 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. And two slits means exactly two such bands (and no more) on the screen.

That IS indeed an assumption

 

[PeterDonis]

So the model is already wrong, correct?

Everything you have said in this thread is wrong. First, you are trying to use classical objects to understand quantum mechanics, which doesn't work. Second, as @PeroK has pointed out, you aren't even correct about how classical objects behave.

 

[renormalize]

That IS indeed an assumption

Please explicitly state what you see as an assumption. Don't make us guess!

 

[PeterDonis]

That IS indeed an assumption

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.

 

[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.