# Why cant a particle go through one slit but behave as if it went through both?

1. Jul 19, 2012

### scrappykoala

In all my reading on the dual slit experiment the idea that the particle went through a single slit is never even considered and yet I don't see any reason why this idea should be discounted. If an atom can decide to be in two places at the same time in order to give a certain result then it seems much less physical work for it to simply change its direction slightly in order to give the same result. This would get pretty spooky so I can see why we wouldn't ever want to think the atom could do that but I fail to see why the "spooky" level of it should discount it.

Bottom line you can't see whats happening anyways so you don't have proof its not happening only that maybe its less likely.

Am I missing something is there some law that says an atom can decide to be in two places but it can't decide to change its direction?

2. Jul 19, 2012

### gildomar

What exactly do you mean about it "behaving as it went through both"? Do you mean in terms of trying to determine which slit that it went through, or in terms of the interference pattern that it would create on the screen? Or something else?

And what are you talking about in terms of an atom being in 2 places at once?

3. Jul 19, 2012

### DrChinese

Welcome to PhysicsForums, scrappykoala!

Of course it has been considered whether or not a particle can go through a single slit and still give an interference pattern. You may not have picked it up, but that does not mean it has not been considered.

When you send a particle through a double slit setup, the variable is whether you know which slit it went through (in principle - you don't need to actually know, just that the setup could let you know) or not. It is possible to vary this particular parameter from 0% knowledge to 100% knowledge by adjusting elements of the apparatus. And when you do that, the interference pattern is 100% eliminated when there is 100% certainty in which slit. And 50% eliminated when there is 50% certainty in which slit. Etc.

Given that piece of the puzzle, you can see that it nothing to do with "changing direction". It has everything to do with the propagation of the particle's wave function.

4. Jul 19, 2012

### Staff: Mentor

There are interpretations of quantum mechanics where the particle goes through a single slit only. But even there, it is required to have something going through both - otherwise you just get the single-slit pattern.

5. Jul 19, 2012

### scrappykoala

In terms of the interference pattern. In other words could the particle go through one slit and give the interference pattern by simply changing its direction. Seems to me the experiment shows two possible outcomes either the particle went through both slits at the same time or it went through one and behaved as if it went through both. The implications of the later would mean some serious intelligence being exhibited. And would suggest that the particle is TRYING to show you something.

6. Jul 19, 2012

### scrappykoala

Thats interesting about the 50% vs. 100% certainty havn't heard that yet so I'm new will have to think more on that to wrap my head and figure out what that means to me. But in one of your sentences you say "the variable is whether you know which slit". Seems to me the variable is whether the atom is exhibiting intelligence. I'm not saying the atom is intelligent but that it may be exhibiting it. If it is then its not the test subject you are.

This whole thing may be nothing more complicated then the universe sending out an easter egg to civilizations that have evolved enough over thousands of years that they can even see the egg. Then they ponder over the egg for another hundred years. Maybe the universe is simply saying I'm here and I'm looking back at you while your looking at me. Im aware and have been waiting for you.

7. Jul 19, 2012

### scrappykoala

How do you know its required to go through both in order to get the interference pattern if you cant see whether it goes through one or both? If both slits are open but you cant even figure out which one or both then its possible it went through one but behaved as if it went through both.

8. Jul 19, 2012

### DrChinese

Just a friendly suggestion, scrappykoala, since this is your first set of posts:

Your comment is not really science and certainly has nothing to do with quantum physics. You could explain any phenomena this way, same really as saying it is god's hand. Our forum rules prohibit undue speculation, and this qualifies. Please do not attempt to justify your thinking, because if this is the direction you are intending to take, I can tell you right now that you will be reported and the thread will get locked.

9. Jul 19, 2012

### DrChinese

In the example I mentioned earlier, both slits are open all of the time. Yet I can prevent interference if I simply have a way to know which slit it went through. My technique does not involve anything which changes the particle's path midflight, ie it is completely passive. It ONLY involves being able to determine the slit.

10. Jul 19, 2012

### gildomar

Interference is a purely wave phenomenom; ascribing intelligence to the process is unnecessary. If the experiment is arranged so that the particle definitively goes through one slit, you'll see a dot on the screen, not an interference pattern.

11. Jul 19, 2012

### jfy4

I think a great interpretation that smoothly eliminates the paradox is to take the wave-function seriously as it describes reality very acurately. QM has been remarkable at matching experiment, so why not take the step and say the space of states is physical. Then, we imagine the particle lives on a space defined on $\mathbb{C}^2$ with two basis vectors, one for each slit, and notice that as the wave function evolves, it doesn't move through the points (with the basis fixed) which correspond to either slit, rather it traces out a trajectory that is between those two states. Thus the particle doesn't pass through one or the other, nor both! it is somewhere else on the space of states entirely, neither slits! Issue resolved.

12. Jul 19, 2012

### scrappykoala

Wow didnt mean to get an emotional response or get banned. I don't understand why it is not scientific about determining intelligence. And I most certainly said nothing about god or a god or a hand of god as god has nothing to do with this.

So now that I can no longer ask questions about this I have no idea where to go now. No one I know could care less about QM and other forums they are just silent. Guess I'll go see if I can talk science in the theological section of this board and not get banned?

13. Jul 19, 2012

### Drakkith

Staff Emeritus
You can ask questions. The issue is that you are asking "how do we know this" which would require very in depth answers of the type that would take a long time to answer if you don't know anything about QM. We'd have to discuss the experiments in depth and look at why classical mechanics doesn't work. These things are covered in many basic QM books and articles, including wikipedia, so your reference should be there.

This isn't to discourage you, so please don't take it that way.

14. Jul 19, 2012

### gildomar

scrappykola, I believe what DrChinese is saying is that it sounded like you were trying to describe the experiment/universe in a quasi-mystical/religious way (since you were seemingly ascribing intelligence to the universe or electron), which is not what this forum is about. This forum is for talking about quantifiable physics. The closest manner that you could talk about electrons or the universe having an intelligence within a physics context would possibly be within the philosophy of physics, but that's a stretch. As it is, the experiment can be fully described using just quantum mechanics (without invoking an intelligence), however unintuitive the results may be.

15. Jul 19, 2012

### gildomar

Oh, and double what Drakkith implied about having an inquiring mind and encouraging it. :)

16. Jul 19, 2012

### bohm2

This is not the most popular interpretation but if I'm understanding that view, the closest model that sort of tries to interpret QM in an "objective informational" way is probably Hiley/Bohm when using their concept of "quantum potential" and "active information":
From the Heisenberg Picture to Bohm: a New Perspective on Active Information and its relation to Shannon Information.
http://www.birkbeck.ac.uk/tpru/BasilHiley/Vexjo2001W.pdf

By the same token, this model would see entangled particles as passing a kind of private, instantaneous, hidden signal between them that seemingly transcends space-time:
Quantum nonlocality based on nite-speed causal influences leads to superluminal signaling
http://arxiv.org/pdf/1110.3795v1.pdf

Last edited: Jul 20, 2012
17. Jul 19, 2012

### scrappykoala

I should apologize to DrChinese in that re-reading his post he did not put emotions in to this but I did in my response. I was taken off guard by the possibility of censor for what I can see was a form of ignorance on my part. So DrChinese I apologize to you and you did no wrong and were simply trying to keep things on path here ... that makes sense to me.

Im am not trying to talk about god or religion or even philosophy. I have no axe to grind on any of that but Im looking at something that is making me ask questions. Lets face it ... when your faced with this stuff it is a bit huge. It can change your understanding of things you used to think were pretty concrete. Im starting to be ok with some of it in that I can say ok its possible for this to happen even though it doesn't make sense with what I knew my entire life before this.

So I have a lot of questions and I have no where else to turn. I would totally not want to burn my only bridge to this because there are no other places I know to go to. I live in the ghetto and the one science geek that I tried to get answers from on this stuff I realized was talking to me in a classical physics methodology even as I was talking to him from QM method and so I very kindly began to disengage from him in a polite way so that he wouldn't understand that we were on two different pages (though he probably thought I was a little nuts). But he is not here yet. You guys are.

My family, friends and girl could care less about this stuff so I cant bounce anything off them. Im totally not about to go to college for this. I just think that a life in this universe without trying to figure out how the universe works is a life wasted. And I dont want to do that.

So I plug along alone with no input and try and figure it out on my own.

And its fustrating at times but fascinating none the less.

I have more questions now but Im a bit afraid to ask as the last thing I want is to cut off my only bridge.

I'll go slow and careful and continue on my own and hope for the best. I guess its not the end of the world or even my own if I don't get this or that so no need for violins. Maybe this is how this is supposed to be. Too crazy to talk to friends and family about. I don't know of any other subject like this that is sooooo far out there in its fascination and seemingly appeal and yet so few are interested or can get over some of the quarks. Makes it more interesting I guess.

18. Jul 20, 2012

### DrChinese

I think you will find this a receptive area for discussion. This board consists of a lot of folks with varying backgrounds, many quite like yourself.

The trick is to make sure you at least understand some of the basics and key arguments before trying to insert your own interpretations and ideas. So as you learn the lingo, you will also learn how to ask your next questions. The best way is to ask a few questions (which you have), read the responses, search a bit on your own, and then repeat. Jumping off the cliff to start with is not recommended.

I am called a science advisor. All that means is that I have hung around here for a while and posted comments that certain others consider beneficial. It does not mean I am "right" about anything. Anyone can answer any question, and often the sum of the responses prove much more valuable than any single response. The reason I mention this is that you would do good to re-read some of the standard material along with these responses. If, then, you still have some questions, you can ask them in a manner which evidences the understanding you have gained from your prior work.

For example: Can particle "intelligence" explain the double slit results? The answer is NO, not unless it is non-local. If it is non-local, then it could. This is a scientific assessment based on the results of experiments. Next you have to ask, what would "non-local intelligence" be? The answer to that, for most scientists, would be god. Invoking god, on the other hand, could be used to explain anything: from the big bang to evolution. So really, it explains nothing. More importantly, it is not a useful description of anything and so does not constitute good science. And again, has nothing to do with quantum mechanics.

The point is, you must decide if you are here to learn or tell others something. Sometimes I am here to learn, other times I am here to tell others something. I try to know when I am telling someone something that they might find useful.

PS No apology needed, although I appreciate your re-reading my response. We have all been in the same boat at one point or another.