I Please help me understand the double slit experiment and conclusion

[jackjack2025]

TL;DR

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.