How Does the Double Slit Experiment Demonstrate Particle-Wave Duality?

[SpectraCat]

I appreciate you're input, however, I'm inclined to think you're "missing the point". These points you're making are ones that I've already brought up in a couple different posts in this thread. This quote is the biggest reason I think you've missed the point. This thread isn't based on a "thought problem". I'm just trying to obtain a few simple facts about the DSE.
Even though I'm certain I've made my questions clear, I'll try presenting them one more time.

People say that when you try to see which slit the particle passes through, it interrupts the interference pattern. So, my questions are:
1) Does it interrupt the interference pattern simply because we've made it impossible for interference to occur? For example, making only one slit available thus having no waves to interfere with or altering one wave to be out of sync with the other.
If this is the case, then the big mystery of it "changing states" when we try to see which slit it goes through, seems to be a lot of hype for nothing particularly unusual.
2) If the interruption occurs but there is no logical reason why - in other words, it SHOULD still display interference - then I'm trying to find out exactly what the observational tool is that makes it change states.

I hope, I hope, I hope this makes my questions clear. And I hope even MORE that someone can help me answer them...

It depends on how you define impossible. Certainly you do not have to block one of the slits to observe the destruction of the interference pattern. If your experiment can detect "which path" information in any way, even with both slits always open, then the interference pattern will be destroyed.

This experiment, as with many (all?) QM experiments, is about measuring probabilities of events, and what matters is the context of the experiment at the moment of detection. You can think about the pattern that you observe as ALWAYS representing the interference of two probability waves: one for the particle passing through the left slit (p_L), and one for the particle passing through the right slit (p_R). If you set up your experiment so that, at the point of detection, there is an equal probability that the particle has passed through either slit (i.e. there is no "which path" information available), then p_L and p_R have equal magnitudes, and you observe the interference pattern. If on the other hand, at the moment of detection, it is possible for you to determine with certainty which path the particle took, then either p_L OR p_R will be 1, and the other will be zero, so you won't see any interference.

A couple of related points:

1) you cannot "trick" the experiment by starting the experiment in one configuration, and changing it suddenly just before detection. This has been proven in the Delayed Choice Quantum Eraser (DCQE) experiments ... definitely worth a read if you haven't seen them. There are some very detailed threads here explaining that experiment, so I won't rehash it here.

2) The fact that you are ALWAYS observing an interference pattern is supported by the theory of "weak" measurements. If you bias the experiment, so that at the time of detection, you know that there is a 75% chance that the particle went through one slit, and a 25% chance that it went through the other (instead of 50-50, or 100-0, as discussed above), then you see a reduction in the intensity of the interference pattern, and a build-up of intensity of the "one-slit" pattern for the most likely slit. In other words, you observe a hybrid of the 50-50 and 100-0 cases. Furthermore, by adjusting the bias of the experiment, you can "tune" the result smoothly between the interference pattern and the "which path" result.

Cool huh!

 

[DrChinese]

This experiment, as with many (all?) QM experiments, is about measuring probabilities of events, and what matters is the context of the experiment at the moment of detection.

A very minor quibble with the idea of "moment of detection" (and this is something of a nod to RUTA):

Your point is well taken that what happens before and after the detection (at the detector) is not as relevant as what happens at detection.

However, there can be elements of the context which are not completely specified at that point in spacetime. A detection "here" means there is no detection "there". And technically, "there" is a part of the context. In most cases, "there" can be safely ignored - regardless of "when" that is. But there are other cases in which "there" figures into the total context - and the "when" associated with that where can be in the future. Delayed choice setups being an example both going the other way as well as supporting your statement. Clearly: with any delayed choice setup, the definition of the "moment of detection" gets very muddy as there are at least 2 such moments.

Again, this is only a quibble as your point about the total context is right on.

 

[LostConjugate]

definitely particles,

I have to ask what you mean when you say "particle". Are you talking about a hard solid object such as a billiard ball? What is your "particle" made out of?

Electron's are particle-waves, not bouncy balls.

 

[Hoku]

I have to ask what you mean when you say "particle". Are you talking about a hard solid object such as a billiard ball? What is your "particle" made out of? Electron's are particle-waves, not bouncy balls.

I'm fairly certain he just means that electron's have mass, which makes them objects, or "particles", in a way that other things like photons are not. He seems to be having the same problem with a wave having mass as I did.

If your experiment can detect "which path" information in any way, even with both slits always open, then the interference pattern will be destroyed.

I'm just curious to know by what means we are able to do this detection. Can we detect it with laser beams that particles can "trip" as they pass through either slit? Can we put on some sort of 3-D like glasses to detect the presence of the particles? Like maybe night goggles or something?

Cool huh!

Indeed!

 

[LostConjugate]

Mass is just a word we use to describe the energy an object has. It's ability to resist acceleration.

An EM wave has mass and it increases as the frequency increases.

A water wave has mass and it increases as the frequency increases.

There is more volume in a wave with a higher frequency. The integral of cos(kx) is kcos(kx) hence higher k, higher volume per unit area.

Einstein proved that mass is energy in his equation E = M (in proper units)

Edit: opps, integral of cos is sine just for the record. Point remains.

 

[Hoku]

Doing "searches" for topics is very useful, but it does have limitations. I had already tried two different ways of searching for info on this topic using the PF search engine. Today, I did a PF search for "delayed choice quantum eraser" as SpectraCat suggested, and I found a thread that was begun just 2-weeks ago. This thread gives exactly the information that satisfies all my questions for this thread! Why didn't this thread come up when I was TRYING to find the info? Oh well. The thread title was, "How does one type of detector determine path of photon?" https://www.physicsforums.com/showthread.php?t=392774&highlight=Delayed+Choice+Quantum+Eraser
Any additional ideas are welcome, but I am satisfied with the answers I've found. Thanks to everyone!

 

[Gary Boothe]

Hoku; Glad you finally understand the double slit experiment. I will go on to other things.