I Please help me understand the double slit experiment and conclusion

[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.

 

[PeterDonis]

That you need to justify though

The justification is the many, many experiments that the quantum model makes accurate predictions about, and the classical models don't.

You say you've studied quantum mechanics; if so, you should already know this.

 

[PeterDonis]

I was assuming that space is not empty.

Which, as you have now been told multiple times now, is not a good model in practice--"empty" is not exactly true, but it's a good enough approximation to make accurate predictions for the experiments we're talking about.

You really need to stop repeating things that we've already responded to. It's making this discussion less and less useful.

So the particles would bounce off the medium

Here again your understanding of classical physics is wrong. If you model what's in the chamber as a "medium", particles don't "bounce off" it. A medium is a continuous thing. If you want your particles in your model to "bounce" off something in the chamber, when there is no barrier with slits in it, then you need to have other particles in the chamber, which can collide with the particles you're shooting in.

Again, if you want to improve your understanding of classical physics, you need to start a new thread in the classical physics forum.

 

[PeterDonis]

That is also what a Brownian motion particle would do.

The correct model of Brownian motion is not to consider the particle of interest as being suspended in a "medium". It is to consider a model in which the particle of interest is being bombarded with other particles that are colliding with it, and to do statistics on what behavior that produces when the number of such collisions per unit time is very large. That's how Einstein used Brownian motion to argue for the existence of atoms.

 

[Nugatory]

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?

Neither. “Going through both slits and is some form of superposition” is a misrepresentation of what QM actually says, just the best we can do without the math. (To get a sense of how misleading it is, it would be just as accurate to say that it goes through neither slit).

Prove a particle is in a superposition. Or can you not?

Of course I can’t because it’s not accurate …. It’s how we talk about what the math says when we’re using natural language. An accurate statement would be something like “after the particle has interacted with the barrier, the state of the quantum particle is described by a vector in an abstract mathematical vector space with infinite complex dimensions; this vector is a sum of other vectors in that vector space; and I can accurately calculate the interference pattern from that vector”.
(and now you understand the tendency to use the simple inaccurate language)

And of course I can’t prove the truth of that statement either, but then again I can’t prove the truth of Newton’s laws either. In both cases all I have is that I can use them to accurately calculate how the universe behaves.

 

[PeterDonis]

Thread closed for moderation.

 

[PeterDonis]

After moderator review, the thread will remain closed, as the quantum mechanical predictions about the double slit experiment have now been described thoroughly enough. Thanks to all who participated.