Understanding duality

  • Thread starter Hoku
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  • #26
A polarizer's job is to cancel a single component of the electric wave by reflecting it. not change the direction of the wave.
 
  • #27
Hoku and Lost Conjuate: When photons or electrons go through any slit ONE AT A TIME, there cannot be interference because a single particle can't interfere with itself, the particle doesn't split (what would half of an electron be like?) and furthermore particles do not form interference patterns because that is something a wave does, where highs and lows of the wave amplitude either tend to cancel out or reinforce each other. I include electrons in a discussion of the double slit experiment because electrons are definitely particles, yet they act the same as light or other radiation which is classically considered to be a wave. The fact that an interference pattern is observed when both slits are open AND when only single particles at a time go through one slit or the other is a paradox in itself, because there SHOULD NOT BE ANY SUCH WAVE PHENOMENA. Another "zinger" associated with the double slit experiment is that if we detect which slit a particle goes though, then we observe no interference pattern. Knowing which slit a particle goes through destoys the pattern which isn't supposed to be there in the first place! When this was discovered, many physicists began pulling their hair out in large clumps, and some jumped off bridges. Others, like me, drank lots of beer. I still drink lots of beer. Anyway, the multiple paradoxes about the double slit experiment are: When electrons or photons go through the slits ONE AT A TIME, the particles act like particles when one slit is blocked; they build up a wave-like interference pattern when both slits are open and consequently the particles seem to "know" if the adjacent slit is blocked or open; and if measurements are performed to determine which slit is being used, the interference pattern magically disappears. So...what could possibly explain all this and also predict the results of different experiments, such as the polarization experiment mentioned by Dr Chinese? I would like a day to think about how to explain away these paradoxes, and then I would love to present my humble explanation. It's what I do. Not very well maybe, but it's fun to try. I will post tomorrow. I also have many questions about modern physics, including duality, that I would like to post. I certainly have more questions than answers, but I get mostly shrugs from physicists in my address book. I'm hoping PF will be different.
 
  • #28
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Polarization is fun! I think you would very much enjoy the following courses.
It seems to be YouTube videos that my computer has trouble with, so, sadly, I was unable to watch the lectures. I noticed there were transcriptions of the lectures, but they are difficult to follow since he is making references to things I can't see.
Hoku and Lost Conjuate: When photons or electrons go through any slit ONE AT A TIME, there cannot be interference because a single particle can't interfere with itself[...]
Evidence says that it can and does. That's part of the mystery of QM and one of the reasons it's "physics" is separated from the classical physics. What "can't" happen in one of these two branches of physics can happen in the other.
Another "zinger" associated with the double slit experiment is that if we detect which slit a particle goes though, then we observe no interference pattern. and if measurements are performed to determine which slit is being used, the interference pattern magically disappears.
What I'm trying to discover is what measurements are we performing to determine "which" slit is being used. "Forcing" a particle through one slit or the other doesn't make sense as an answer to this.
[...]the particles seem to "know" if the adjacent slit is blocked or open;
If only one slit is open the particle doesn't need to "know" anything. It just goes forward as a wave and, consequently, goes through whatever is available. Obviously it won't go through something that's blocked, right? This doesn't seem like any sort of paradox to me. It is a wave until it hits the screen, which means it would go through 8 open slits, if they were there, or just one open slit if that's all that's available. Right?
I would like a day to think about how to explain away these paradoxes, and then I would love to present my humble explanation. [...] I get mostly shrugs from physicists in my address book. I'm hoping PF will be different.
You do need to be careful in these forums with stuff like this. If it seems like you're "creating theories" then you can be banned from the site. They're mostly just trying to keep the forums simplified and focused. Personal theories can get out of hand and confuse many people looking for answers. That's why the rule is in place and important. I think the best way to approach it is from a philosophical point of view and then begin a thread about it in the "philosophy" section of Physics Forums. As far as THIS thread goes, I'm really not looking for interpretations or ways to explain away oddities. I'm just trying to understand how things are measured or "observed".
 
  • #29
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That is why they are sometimes called wavicles! Having characteristics of both waves and particles.
If there is only one slit available, there will be no interference pattern, because there is not another wave present to interfere with. It is not that there are no wave characteristics generated by a single slit, there is just nothing for them to interact with. Stick a reflector off to the side of the slit and the screen, and you will see interference as the wave front is bent so it encounters the rest of the same wave or the one before or behind. It all depends on the angle of the reflector.

For the two slit problem, QM postulates that you can know which slit a particle passes, but not when a particle passes each slit, so the actual interference pattern can only be observed. By the same token, you can know when a particle passes a slit, but you can't determine exactly which slit. You can only calculate based on a hypothesis of which slit at which time to determine one possible interference pattern. You can't KNOW, because the range of probabilities is almost infinite, per Heisenberg

Polarization simply adds another indeterminate variable or two, further complicating the calculation of a "Real World" effect. You can model it, again by hypothesis, but you can't really KNOW much of anything beyond the generalization of a solution. I can postulate that any particle can chose to pass through either slit, and then be polarized in either direction, I do know that only one particle can pass through either slit at any time (Paulie's law) and that there can not be two particles passing through both slits simultaneously. But which particle will pass through which slit at which time devolves into a maze of probability calculations, Even if I can specify that the particles are emitted at a constant rate, which is again unlikely as most models assume that particles are the result of natural decay, it doesn't really help as again, I KNOW when, but don't know which.
If the particles are polarized in different directions, then yes that will effect the interference pattern to the point that at 90 degrees, the waves may not interact at all. But what does that accomplish? If the "wavicles are out of phase and don't interact, that can't be compared to the one slit solution.

Why? Simply because you have made the interference impossible. Not because the wave characteristics cease to exist, but because the interference is now physically impossible (or almost so since there is no such thing as perfect polarization.)
I don't really see the purpose of the thought problem, unless it is to lay out what I have said above. Or am I missing the point?
 
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  • #30
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I appreciate you're input, however, I'm inclined to think you're "missing the point".
If there is only one slit available, there will be no interference pattern, because there is not another wave present to interfere with. [...] If the particles are polarized in different directions, then yes that will effect the interference pattern to the point that at 90 degrees, the waves may not interact at all. But what does that accomplish? If the "wavicles are out of phase and don't interact,[...] Why? Simply because you have made the interference impossible. Not because the wave characteristics cease to exist, but because the interference is now physically impossible (or almost so since there is no such thing as perfect polarization.)
These points you're making are ones that I've already brought up in a couple different posts in this thread.
I don't really see the purpose of the thought problem[...]
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...
 
  • #31
SpectraCat
Science Advisor
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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 (pL), and one for the particle passing through the right slit (pR). 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 pL and pR 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 pL OR pR 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!
 
  • #32
DrChinese
Science Advisor
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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.
 
  • #33
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.
 
  • #34
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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!:smile:
 
  • #35
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.
 
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  • #36
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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!
 
  • #37
Hoku; Glad you finally understand the double slit experiment. I will go on to other things.
 

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