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

[Hoku]

I'm thinking about the wave/particle duality and I have a few questions about it.

Question: How do we know a particle is also a wave if we can't measure it as one?
Answer: The double slit experiment, for example, allows us to INFERE the wave nature of particles because of the way multiple particles display an interference pattern on the screen. This interference pattern developes even if we shoot only one particle at a time.

Question: If each particle is being fired only one...at...a...time... then, after each particle hits the screen, it is no longer in the chamber, right? So, how does the NEXT particle interfere with the previous ones, if the previous ones are no longer there to interfere with??

 

[LostConjugate]

I'm thinking about the wave/particle duality and I have a few questions about it.

Question: How do we know a particle is also a wave if we can't measure it as one?
Answer: The double slit experiment, for example, allows us to INFERE the wave nature of particles because of the way multiple particles display an interference pattern on the screen. This interference pattern developes even if we shoot only one particle at a time.

Question: If each particle is being fired only one...at...a...time... then, after each particle hits the screen, it is no longer in the chamber, right? So, how does the NEXT particle interfere with the previous ones, if the previous ones are no longer there to interfere with??

The particle must be interfering with itself. They can't interfere with each other. Keep in mind that the particle is not a wave, wave is just a word that helps explain part of it's nature.

It is more accurate to say that the particle has an oscillatory function (a Fourier transform) associated with it and this function interferes with itself. The complication lies in this association, there is no explanation for it in spoken words. The function is not observable in any way. We [Humans] found it through a lot of hard work in the field of mathematics and experimentation.

 

[Hoku]

But the interference pattern on the screen is made up of single dots, each given by a single particle. Each particle only makes one dot. A single dot is not the result of interference. To be honest, at this point I'm not convinced any interference is happening at all. They just happen to randomly amount to an image similar to waves of interference. I know this isn't correct, but I need some better evidence of a single particle being wavelike.

Would it help to look at the uncertainty principle? In it, you can know speed or position but not both. If you measure speed are you measuring the wave, while measuring position is measuring the particle? If you measure the speed of a wave, you should be able to do so at numerous different locations simultaneously. This would be good proof of a wave for me.

 

[SpectraCat]

But the interference pattern on the screen is made up of single dots, each given by a single particle. Each particle only makes one dot. A single dot is not the result of interference. To be honest, at this point I'm not convinced any interference is happening at all. They just happen to randomly amount to an image similar to waves of interference. I know this isn't correct, but I need some better evidence of a single particle being wavelike.

Ok .. so what is your explanation for the interference pattern then? It definitely exists and can be measured for only single particles passing through the slit. If they were propagating like classical particles, then there is no way for them to produce the measured behavior, right? So it seems there has to be some wave-like character to the indivudual particles, right?

My point is that the available experimental evidence has convinced the physics community that particles have wave-like characteristic, and we have a theory that predicts this (QM), so things look pretty consistent. You can't demand "other evidence" unless you can explain away the experimentally observed effects some other way. (Well, you can, but no one will take you seriously.)

Would it help to look at the uncertainty principle? In it, you can know speed or position but not both. If you measure speed are you measuring the wave, while measuring position is measuring the particle? If you measure the speed of a wave, you should be able to do so at numerous different locations simultaneously. This would be good proof of a wave for me.

I am uncertain (pun intended) what you are trying to say with the above ... it sounds a little like you are saying the wave-like nature of the particles should cause its probability density to be delocalized, according to the HUP. That much is correct, and has been verified experimentally. It's the specification of simultaneity that is troubling me ... I don't understand why it is necessary. As far as I know, such a measurement is impossible according to standard QM ... the probability density of a particle can be delocalized over an arbitrarily large area, however once you make a position measurement, the wavefunction resolves itself to a single point in space. Thus, it should be fundamentally impossible to measure a discrete particle in two places at the same instant.

 

[Hoku]

Thanks for the reply, SpetraCat. I definitely need some help here.

It definitely exists and can be measured for only single particles passing through the slit.

Yesterday, that's what I thought. But after reading through different threads here and looking at the Wikipedia description of the DSE, I've been led to conclude that the interference pattern is NOT displayed from just one particle. It's displayed collectively from numerous particles. I'm just having trouble understanding where the actual "interference" is. If a single particle doesn't display an interference pattern, then why would we say it interferes with itself? The interference pattern is between one particle and others, yet all of those particles are not in the chamber to interfere with each other. Do you understand my confusion?

You can't demand "other evidence" unless you can explain away the experimentally observed effects some other way.

Aren't the only "observed effects" a pattern that is displayed on a screen? The pattern may be predictable, but why does that mean it's the result of "interference"? Again, I don't understand where the interference is coming from.

It's the specification of simultaneity that is troubling me...

From what I understand, a "wave" encompasses a wide area, consisting of several congruent points in space. So if a "wave" can be measured without collapsing the function, then why can't we measure it in the same way at more than one location? Maybe with two screens at 120 deg. angles to each other? But this point is minor. I'm just trying to find ways of understanding the duality. My point isn't to come up with new experimental ideas.

 

[Antiphon]

Each particle interferes with itself. The central mystery of QM is why this happens. Nobody knows.

Its not enough to say "they're really particles but there is a wave-like behavior to them". It's just as true that quanta are only waves that appear to localize depending on what they interact with.

 

[SpectraCat]

Each particle interferes with itself. The central mystery of QM is why this happens. Nobody knows.

Ok, that certainly seems wrong. All QM theories assume some postulates to be true ... and working from these postulates it is possible to predict the observed "self-interference" of quantum particles. This can be done in SQM, dBB, MWI, whatever. One might better say that any quantum theory that *cannot* explain the two-slit experiment is wrong.

Now, if what you meant to say is that "No one knows why the fundamental postulates we take to be true in the theory seem to hold in real life.", then I would tend to agree with that.

Its not enough to say "they're really particles but there is a wave-like behavior to them". It's just as true that quanta are only waves that appear to localize depending on what they interact with.

I think I know what you are trying to say, but to nitpick a little, the phrase in bold makes no sense in terms of QM .. in fact, most of that doesn't seem to make sense as written.

I guess what you were trying to get across is that the classical notions of waves and particles are inadequate to describe quantum phenomena, right?

 

[Hoku]

Hi, Antiphon! What I'm trying to find out is how we know that it interferes with itself if it doesn't display that pattern on the screen. It is said that, if a particle is fired at a barrier in which two slits are cut, it can go through both slits simultaneously. How do we know this is happening if there is no interference pattern displayed?

I know that a quantum "particle" is both a particle AND a wave. I know it, I believe it, no issue there. To me, the dot on the screen is evidence of its particle nature. Now, I need to understand evidence of it's wave nature. I used to think that evidence was displayed as interference on the screen. But now it seems that such interference is not displayed from just one particle.

 

[SpectraCat]

Hi, Antiphon! What I'm trying to find out is how we know that it interferes with itself if it doesn't display that pattern on the screen. It is said that, if a particle is fired at a barrier in which two slits are cut, it can go through both slits simultaneously. How do we know this is happening if there is no interference pattern displayed?

I know that a quantum "particle" is both a particle AND a wave. I know it, I believe it, no issue there. To me, the dot on the screen is evidence of its particle nature. Now, I need to understand evidence of it's wave nature. I used to think that evidence was displayed as interference on the screen. But now it seems that such interference is not displayed from just one particle.

You are correct, any single particle is detected at a discrete location .. a dot. However, if we shoot a large number of particles through the slit one-by-one, with space-like separations between them so we can be certain they don't interact, then we observe that the collective pattern of dots is an interference pattern.

So, let's think about how that can arise. We can safely assume that the test particles come from a stream of identical particles with some spread of transverse velocities, so that if there were no slits present, you would see a circular spot with a roughly gaussian radial intensity profile centered around the beam axis, ok? Now, assume you put a single slit in the way of the beam .. for classical particles, they would either hit the screen or be transmitted, and the observed pattern would be a the subset of the original gaussian profile, windowed by the slit .. also ok? Now put another identical slit quite closely spaced to the first slit. You will see the windowed profile described above from each slit .. they may overlap a bit, in which case the intensities will add normally. In other words, there is no interference pattern.

Thus, the fact that we observe an interference pattern (again, for repeated experiments) *must* indicate that there is something non-classical going on .. hopefully you are convinced of that. Luckily we have "the most successful theory in physics" (i.e. quantum mechanics) available, which predicts this behavior, and explains the observed non-classical pattern as arising from the wave-like character of the quantum particles.

That last paragraph sums up why it is somewhat hard for me to address your question directly ... to us (i.e. physicists), the wave-like properties are clear as day ... no further evidence or experiments is needed. The fact that you are asking for them tends to indicate that you don't fully understand the theory ... in which case you are in the right place to get help, but perhaps we could go about things a little differently. Do you understand how QM predicts the interference pattern?

Just as an FYI, we have been focusing on the interference pattern aspect of the two-slit experiment, which is fine, but that is not even the coolest part of that experiment ... the cool part happens when you try to figure out which slit a given particle goes through ... the interference pattern disappears! There are lots of threads in this forum describing that experiment in detail, so I won't rehash it here. I suggest that you read some of those threads, and come back here with questions (or ask in those threads if it seems appropriate).

 

[lisab]

Right, the particle is detected on the screen as a "dot" - it's the pattern of a large number of particles that is so curious.

To see what this looks like, check out a video of it. Go to http://www.hitachi.com/rd/research/em/movie.html" and click on "doubleslite.wmv".

Hoku, have you seen Feynman's Messenger lecture on this? I recommend it, and I'll look for a link.

Edit: oops, it seems the lecture I was looking for is no longer available - sorry. The Messenger lectures are available in print, though.

 

[Hoku]

Thanks for the video clip lisab! It's nice to have an actual video to look at and this one was good. I'll look up the Messenger lecture, also.

Just as an FYI, we have been focusing on the interference pattern aspect of the two-slit experiment, which is fine, but that is not even the coolest part of that experiment ... the cool part happens when you try to figure out which slit a given particle goes through ... the interference pattern disappears!

Believe it or not, this "FYI" is EXACTLY what I'm trying to get at. It's really hard when you're a layman because you lack the knowledge to make a confusion clear and easily move a question forward.

I was content with my understanding of this phenomena before I posted on PF yesterday. But yesterday (including my PF searches on DSE) completely tore down any understanding I thought I had, so I tried to reconstruct a better picture from the ground up. NOW I see that the whole thing was just a big misunderstanding and I think I was fine to begin with.

The interference pattern shows the wave aspect, and the "not" interference pattern shows the particle aspect, right? Right. *sigh*. That whole concept, which I understood before, was torn down for me and I'm relieved to see it is still there. One of the impressions I was under in the past 24+ hours, was that the only "observation" in effect was the screen detecting the particles. But, obviously, there is ANOTHER observation in play to take the interference pattern away. So, I'll be content to wrap this thread up with only one other question to ask:

What is the other observing apparatus that changes the state of the particles? One observing apparatus is the screen, what is the other??

 

[Gary Boothe]

Hoku: Richard Feynman said, "The basic element of quantum theory is the double slit experiment." Indeed, if you really want to understand quantum theory, you need to thoroughly understand the double slit experiment and its implications. I believe I can help you understand it. First of all, the remakable feature of the experiment is not the interference pattern. Forget the interference pattern. The important fact is that photons or particles such as electrons go through the slits in a different way depending on if one slit is open or both slits are open. Let's say that behind the two slits you placed a photographic film. If you shine a light on the two slits and developed the film, you would see bands of lighter and darker areas (the interference pattern). If you block one of the slits, shine the light again, and devleop another film, you see one bright line behind the open slit, an image of the slit. To Thomas Young, who first did this experiment, this proved beyond any doubt that light was a wave. But Young didn't have the capability of shooting single photons or electrons through the slits one at a time. When physicists achieved this capabilty, a remakable thing was observed; something that shook the foundations of physics. When one slit is open, single photons and electrons pass through that slit and form an image of the slit, as expected. But when both slits are open and single photons or electrons go through either slit, THEY GO THROUGH THE SLIT IN A DIFFERENT WAY to form an interference pattern on the film! But wait! Why should a single photon or electron go through a slit a different way depending on whether or not an adjacent slit was open or blocked? It's as if the single photon or electron KNOWS whether of not the adjacent slit is open or blocked! Impossible! A photon or electron doesn't have a mind! It can't KNOW anything! I'm using too many exclamation marks here! Well, this is the real paradox regarding the double slit experiment, and this paradox led to quantum theory and wave-particle duality. If you want to discuss this explanation in more detail, send me an e-mail (gboothe@charter.net) and I will gladly respound. I hope this was helpful.

 

[Hoku]

Gary, thanks for "respounding" on this thread. I found it educational and got a good chuckle, too (you can call EMA for help. That's, "Exclamation Marks Anonomous").

It sounds like you're saying the "non-interference" pattern comes about when you block one of the slits. This is confusing because I thought it was only when we tried to see WHICH slit it went through, that the interference dissappeared. In other words, I thought we were still giving the particle options to see what it "chooses". But there's no choice if one slit is blocked. And if one slit IS blocked, it seems natural that the particles would go through the only open one. And since it's not being "separated" it wouldn't display interference. ?

 

[LostConjugate]

And since it's not being "separated" it wouldn't display interference. ?

You agree that the particle is being separated as it goes through the slits?

 

[DrChinese]

It sounds like you're saying the "non-interference" pattern comes about when you block one of the slits. This is confusing because I thought it was only when we tried to see WHICH slit it went through, that the interference dissappeared. In other words, I thought we were still giving the particle options to see what it "chooses". But there's no choice if one slit is blocked. And if one slit IS blocked, it seems natural that the particles would go through the only open one. And since it's not being "separated" it wouldn't display interference. ?

You can see that IF you know which-slit - because it is blocked - the pattern is different. There are other ways to detect which-slit as well, and those lead to the same results. There is another video - don't have the link in front of me - which shows the effect of placing polarizers in front of the slits. By varying their relative angle from 0 to 90 degrees - which corresponds with gaining which-slit knowledge - the pattern changes as well.

 

[DrChinese]

To make this clear, consider the following two setups A and B, which are identical except for the settings of the polarizers in front of each slit. In each case, the source beam is polarized to 0 degress and the L (left) slit has a +45 degree polarizer. The R slit has a ++45 degree polarizer in the A setup but has a -45 degree polarizer in the B setup. Sorry for the crude drawing...

A. Inteference IS seen
...======
... Source
...== | ==
... |
... V
== /L/ = /R/ ==

.===Screen===


B. NO Interference seen
...======
... Source
...== | ==
... |
... V
== /L/ = \R\ ==

===Screen===

In both cases A and B above, the amount of light that is detected on the screen is the same. And that amount is half of the light that would go through if there were no polarizers in front of the L and R slits. In other words, the polarizers filter out half the light. Quantum mechanics explains the A results based on a superposition of states which gives rise to interference terms. But there are no interference terms when the polarizers are crossed as per B. That is when the which-slit information is available. So QM relies more on a mathematical formalism rather than an "intuitive" description.

 

[LostConjugate]

That is interesting.

It would be impossible for a single photon to pass through both slits though. It can't be simultaneously 100% polarized in x and 100% polarized in y. It is not so lucid as closing a slit however it is easy to see we are forcing any photons that pass through to only choose one slit.

 

[DrChinese]

It would be impossible for a single photon to pass through both slits though. It can't be simultaneously 100% polarized in x and 100% polarized in y.

Well sort of (and not simultaneously 100%, as you say). Clearly, if the slit polarizers are anything LESS than 90 degrees apart they can pass both to a degree - which depends on the relative angle.

What if the source photons are at 0 degrees and the polarizers are at +45 degrees and -45 degrees relative? How does the part at one side "know" to only allow the other to get through? Predetermination? And yet all photons that are polarized at 0 degrees are otherwise indistinguishable!

 

[LostConjugate]

What if the source photons are at 0 degrees and the polarizers are at +45 degrees and -45 degrees relative? How does the part at one side "know" to only allow the other to get through? Predetermination? And yet all photons that are polarized at 0 degrees are otherwise indistinguishable!

How about this. As an EM wave reaches a grating (polarizer) the component of polarization that is blocked is done so by another EM wave released from the free electrons in the grating that is exactly out of phase.

So this should apply to a single photon. As the photon reaches the grating, the x component is removed because of another photon polarized in x (released by a free electron) that is out of phase with the x component of the original photon.

Now we have two photons heading for the plate. The original one, and one that is working to block out the x polarization of the original. The plate will never pick up the second photon because the original one is our of phase with it and they will reach the plate at the same time.

We have the exact opposite thing happening when a photon goes through the opposite grating.

Now I am fuzzy in this part, but because p_x and p_y do not commute with each other these extra polarized photons that are generated are not exactly along one axis, but more of in a cone, and since the cones are opposite couldn't that destroy the interference pattern?

 

[DrChinese]

How about this. As an EM wave reaches a grating (polarizer) the component of polarization that is blocked is done so by another EM wave released from the free electrons in the grating that is exactly out of phase...

Are you saying that each slit sees "half" a wave? Or an entire wave? Because no matter how you answer, the math doesn't work - eventually.

 

[LostConjugate]

Well in the case of a photon let's just say that it only passes through one slit at a time. So each slit sees 1 photon.

Edit: I am not sure what you meant, actually. What I am saying is not that half of the wave is blocked out, just one component. A wave at 0 degrees has a two components if you transform your coordinates to make the polarizer 0 degrees.

 

[yoda jedi]

From what I understand, a "wave" encompasses a wide area, consisting of several congruent points in space.

Metaphysics.

 

[Hoku]

You agree that the particle is being separated as it goes through the slits?

I actually had to try explaining it to someone even more "lay" than I in order to get the momentum I needed to figure it out. Fortunately, I DID figure it out and I'm entirely confident with it now. And, as SpectraCat had said, it really is as "clear as day".

You can see that IF you know which-slit - because it is blocked - the pattern is different.

Yes, but this doesn't seem very unusual, noteworthy or "cool". When a wave doesn't have another wave to interfere with (which it doesn't through only one slit) then, or course, no interference pattern will be displayed. This seems naturally physical.

[...]don't have the link in front of me - which shows the effect of placing polarizers in front of the slits. By varying their relative angle from 0 to 90 degrees - which corresponds with gaining which-slit knowledge - the pattern changes as well.

I have to admit, I'm a little stuck on the polarization stuff, but it doesn't seem any more "unnaturally cool" to me than blocking one slit does. In your setup B, the particles are heading away from each other so their point of intersection is further away, possibly beyond the screen that they hit? If the waves don't "interfere" before they hit the screen then I wouldn't expect to see the interference pattern. But I think I'm missing something here...

 

[LostConjugate]

But I think I'm missing something here...

The direction of the slashes was just a representation of the configuration of the polarizer. The slits are not angled in any way.

Polarization is fun! I think you would very much enjoy the following courses.

http://ocw.mit.edu/OcwWeb/Physics/8-02Electricity-and-MagnetismSpring2002/CourseHome/index.htm


http://ocw.mit.edu/OcwWeb/Physics/8-03Fall-2004/CourseHome/

 

[Hoku]

I know the slits aren't actually angled like that but doesn't the polarization change the direction of the particles, as though they were angled?? I'll check out those vidoes you sent. I'm sure they'll help. I just hope my computer will let me watch them!

 

[LostConjugate]

A polarizer's job is to cancel a single component of the electric wave by reflecting it. not change the direction of the wave.

 

[Gary Boothe]

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.

 

[Hoku]

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".

 

[Dozent100]

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?

 

[Hoku]

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...

 

[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.