Novie questions about the double-slit experiment

[lostprophets]

good mornings
hi sophiecentaur
i have another question.
does the spyin eye spy the slit from top to bottom?
if so
what would happen if the spying eye only measured from the middle down wards or middle to top or from a third from the bottom to a third from the top?
if taken it that they have used the "spy" on a single slit experiment also yes?
and if so did the back screen give the same pattern as the one without the "spy"

 

[VCortex]

But if you can't detect a photon without destroying it, surely then particle-wave duality is moot? How do you even get any change in the interference pattern from observation?
Wouldn't you get an interference pattern build up from the unmonitored photons through the slits, and a blank from the monitored slits, as sure as if the intervening obstruction had no slits at all? Your reply (without a statistical qualifier on detection rate for the monitored slits) seems contrary to every demonstration & explanation of the test that I have seen (& the law of energy conservation?!), unless I'm completely misunderstanding something obvious (hence the questions :)

 

[Meta14]

I've been trying to get answers from Yahoo! Answers, but I haven't really been getting them. Can the double slit experiment be done with bigger objects, say grains of sand, or more? Would there be any difference in the results? I'm already aware that C60 was used, but I don't know if the results were identical to that of a photon or electron, or if there was any differentiation.

 

[sophiecentaur]

good mornings
hi sophiecentaur
i have another question.
does the spyin eye spy the slit from top to bottom?
if so
what would happen if the spying eye only measured from the middle down wards or middle to top or from a third from the bottom to a third from the top?
if taken it that they have used the "spy" on a single slit experiment also yes?
and if so did the back screen give the same pattern as the one without the "spy"

You would have two different experiments - one for the top half and one for the bottom half. The unmonitored half would give a good pattern with a big contrast between light and dark fringes. The half with monitoring would show weak or no fringes

 

[sophiecentaur]

But if you can't detect a photon without destroying it, surely then particle-wave duality is moot? How do you even get any change in the interference pattern from observation?
Wouldn't you get an interference pattern build up from the unmonitored photons through the slits, and a blank from the monitored slits, as sure as if the intervening obstruction had no slits at all? Your reply (without a statistical qualifier on detection rate for the monitored slits) seems contrary to every demonstration & explanation of the test that I have seen (& the law of energy conservation?!), unless I'm completely misunderstanding something obvious (hence the questions :)

All the demonstrations behave as expected from the theory.

 

[VCortex]

You would have two different experiments - one for the top half and one for the bottom half. The unmonitored half would give a good pattern with a big contrast between light and dark fringes. The half with monitoring would show weak or no fringes

Ok, so from this can we say that both halves are exhibiting a wave pattern, with the monitored half showing decreased intensity due to interaction with an 'observation field' of the same intensity & 'material' as the measured unit, that has a statistical detection/destruction(/uncertainty?) threshold which precludes the destruction of all photons through the slit? Is that a sensible way to interpret the result?

 

[Joncon]

But if you can't detect a photon without destroying it, surely then particle-wave duality is moot? How do you even get any change in the interference pattern from observation?
Wouldn't you get an interference pattern build up from the unmonitored photons through the slits, and a blank from the monitored slits, as sure as if the intervening obstruction had no slits at all? Your reply (without a statistical qualifier on detection rate for the monitored slits) seems contrary to every demonstration & explanation of the test that I have seen (& the law of energy conservation?!), unless I'm completely misunderstanding something obvious (hence the questions :)

You can place polarizers behind the slits. A polarizer won't destroy the photon, but any photon which passes through it is now polarized in a certain direction. So, in theory, you could later inspect the polarization of the photon to determine which slit it passed through. The fact that this information is available in principle can be enough to destroy the interference pattern.

I've been trying to get answers from Yahoo! Answers, but I haven't really been getting them. Can the double slit experiment be done with bigger objects, say grains of sand, or more? Would there be any difference in the results? I'm already aware that C60 was used, but I don't know if the results were identical to that of a photon or electron, or if there was any differentiation.

I believe the Zeilinger group have demonstrated diffraction by adding 48 fluorine atoms to a C60 molecule, I can't find a reference for this though. As for a grain of sand - I doubt it. The problem is that, the more massive an object gets, the larger it's momentum is and therefore smaller the wavelength - meaning less chance of seeing any wave effects.

Some drivers (usually drunk) have apparently attempted to diffract their cars around lamp posts, but none have been successful ...

 

[lostprophets]

Some drivers (usually drunk) have apparently attempted to diffract their cars around lamp posts, but none have been successful ...

this is true.but the difference there is the car is hitting the tree and not either side of the tree.
if it were to go by the side of the tree it wouldn't hit the tree .
but
everything traveling with it would hit the tree and go around the other/both sides of the tree.
so if it was emitting a wave the wave would go with the car ,hit the tree and go around the other side of the tree opposite to the side the car went.
so if the car was a photon the same thing would happen yes?

 

[sophiecentaur]

Ok, so from this can we say that both halves are exhibiting a wave pattern, with the monitored half showing decreased intensity due to interaction with an 'observation field' of the same intensity & 'material' as the measured unit, that has a statistical detection/destruction(/uncertainty?) threshold which precludes the destruction of all photons through the slit? Is that a sensible way to interpret the result?

that sums it up, I think. The pattern for the monitored path would be built up with 'resolved' photons from the other slit (no interference) which dilutes the interference pattern due to unresolved photons from both slits.

 

[Meta14]

I believe the Zeilinger group have demonstrated diffraction by adding 48 fluorine atoms to a C60 molecule,

I'm not too proficient in technical terms, but by diffraction, are you referring to the interference pattern?

 

[sophiecentaur]

'Diffraction' covers all these phenomena, aamof

 

[Drakkith]

so if it was emitting a wave the wave would go with the car ,hit the tree and go around the other side of the tree opposite to the side the car went.
so if the car was a photon the same thing would happen yes?

No, the photon would either hit the tree and be absorbed, or it would diffract around it and go off in a random direction that we can calculate the probabilities for. Honestly, thinking about the experiment as one photon doing something is going to be confusing. Instead, think of it as a wave propagates through the slits, interferes with itself as it passes through, and impacts the screen behind the slits. Because energy is only transferred in quanta, aka photons, you will only measure the energy of the wave in specific locations. The probability of detecting this wave at a specific point is random, but follows a pattern determined by the properties of the wave. Where the two waves constructively interfere you will see more detections over time, and where they destructively interfere you will see fewer or no detections over time.

 

[lostprophets]

No, the photon would either hit the tree and be absorbed, or it would diffract around it and go off in a random direction that we can calculate the probabilities for. Honestly, thinking about the experiment as one photon doing something is going to be confusing. Instead, think of it as a wave propagates through the slits, interferes with itself as it passes through, and impacts the screen behind the slits. Because energy is only transferred in quanta, aka photons, you will only measure the energy of the wave in specific locations. The probability of detecting this wave at a specific point is random, but follows a pattern determined by the properties of the wave. Where the two waves constructively interfere you will see more detections over time, and where they destructively interfere you will see fewer or no detections over time.

i do here you.
i kind of imagine the photon , much like a sun and all its light/energy coming of it in every direction.this light/energy i see as the wave and the sun/photon as the particle...so to me the sun/photon is a wave and a particle at the same time.
what i then do is to imagine doing the two slit experiment with the sun.
i imagine it all to scale including the spying eye and i ask myself, would the eye be able to to some how hold the light/energy coming of the sun back from going through the slit,but the sun itself was able to penetrate this and go through.
now obviously the sun and a photon are different.but my point is the photon must be emitting something off itself and this something is being blocked by the eyes energy coming from it.like an aura..
i have a wild imagination

 

[Drakkith]

now obviously the sun and a photon are different.but my point is the photon must be emitting something off itself and this something is being blocked by the eyes energy coming from it.like an aura..
i have a wild imagination

I assure you, photons do not emit anything. Our eyes, as well as all other light sensors, work by detecting photons, which requires that they be absorbed and disappear. So if the photon is absorbed it is gone. Forever. Thus you cannot interact with a photon and measure it, AND have it continue on in your experiment. Furthermore, we cannot use light to "see" other light. IE light does not bounce off of other light back to us. So we have to place something physically in the way of the light to detect it, such as a CCD or CMOS sensor.

 

[sophiecentaur]

i do here you.
i kind of imagine the photon , much like a sun and all its light/energy coming of it in every direction.this light/energy i see as the wave and the sun/photon as the particle...so to me the sun/photon is a wave and a particle at the same time.
what i then do is to imagine doing the two slit experiment with the sun.
i imagine it all to scale including the spying eye and i ask myself, would the eye be able to to some how hold the light/energy coming of the sun back from going through the slit,but the sun itself was able to penetrate this and go through.
now obviously the sun and a photon are different.but my point is the photon must be emitting something off itself and this something is being blocked by the eyes energy coming from it.like an aura..
i have a wild imagination

Your "wild imagination" may need a bit of taming if you really want to have a better understanding about this 'duality' thing. Your mental picture of a photon is just not viable, I'm afraid.
'Imagination' is certainly needed if you want to avoid being tied to a purely classical model (which means you will never 'get' QM) but the imagination still needs to take into account the evidence.

 

[VCortex]

that sums it up, I think. The pattern for the monitored path would be built up with 'resolved' photons from the other slit (no interference) which dilutes the interference pattern due to unresolved photons from both slits.

I'm not sure I understand this interpretation, as it relates to the situation we were discussing (with the two slits simultaneously being partially monitored at the top of each).
It seems like you're trying to say that the monitored halves are interacting with the unmonitored halves somehow? Maybe your wording just seems vague to me.

i do here you.
i kind of imagine the photon , much like a sun and all its light/energy coming of it in every direction.this light/energy i see as the wave and the sun/photon as the particle...so to me the sun/photon is a wave and a particle at the same time.
what i then do is to imagine doing the two slit experiment with the sun.
i imagine it all to scale including the spying eye and i ask myself, would the eye be able to to some how hold the light/energy coming of the sun back from going through the slit,but the sun itself was able to penetrate this and go through.
now obviously the sun and a photon are different.but my point is the photon must be emitting something off itself and this something is being blocked by the eyes energy coming from it.like an aura..
i have a wild imagination

I was trying to think about it like that as well, with little suns gong through the slits, but since on that scale the energy measurements are unquantifiable (if they even exist.. Which I think is possible but not practically measurable right now) it seems like that metaphor is a bit stretched.

Would it be more beneficial to think about the situation like a whistle, where the quantized photon is the whistle ball, the slits are the whistle body & blowing it can represent the photon emitter/uncertainty of photon path (& maybe hypothetical further quantized energy states) through a medium?

So, we blow the whistle, and the ball vibrates within the body. The pitch of the sound we get represents the interference pattern we get (which is a combination of degrees of freedom available to the ball inside the body, and energy imparted to the ball by blowing, and as it represents light we will blow the whistle at a constant pressure :).

Now to describe the quantized photon we must either split the whistle open mid-blow and see where the ball flies, and this would subsequently destroy the interference pattern (or 'pitch' of the whistle) & the photon ball would be scattered randomly wherever its' last bounce within the body sent it;
Or, we could try being less intrusive and try to find out where the photon is without losing it. So we try blowing back into the whistle's exhaust hole and peeping through at the same time, hoping that the ball still has enough energy to sound the whistle (this would be like an 'observation field' parallel to the slits that has a percentage chance of intercepting a photon).
Now, although we can still hear the whistle, the degrees of freedom for the ball are significantly reduced. The higher energy states of freedom that were available before are taken up by our blow-back, and so the ball is more likely to vibrate at the bottom of the whistle body, so although we can hear the whistle and the pitch is the same, the volume is much lower (and so in the real experiment, there is little to no wavelike interference).

Sound ok, anyone?

 

[Drakkith]

I'm not sure I understand this interpretation, as it relates to the situation we were discussing (with the two slits simultaneously being partially monitored at the top of each).
It seems like you're trying to say that the monitored halves are interacting with the unmonitored halves somehow? Maybe your wording just seems vague to me.

You aren't interacting with the slits, but with the photons. I think what he means is that if a photon would have gone through the top of a slit and interfere with itself, but you are monitoring the top half of one slit, it can no longer interfere with itself if it goes through the top of the unmonitored slit, and passed through as if there was only one slit. If it went through the bottom then it can still interfere and will produce an interference pattern.

I was trying to think about it like that as well, with little suns gong through the slits, but since on that scale the energy measurements are unquantifiable (if they even exist.. Which I think is possible but not practically measurable right now) it seems like that metaphor is a bit stretched.

I don't know what you mean here. What energy are you measuring? We have detectors capable of detecing single photons, electrons, etc.

Would it be more beneficial to think about the situation like a whistle, where the quantized photon is the whistle ball, the slits are the whistle body & blowing it can represent the photon emitter/uncertainty of photon path (& maybe hypothetical further quantized energy states) through a medium?

No. There is no way to classically think about this. The photon or electron or whatever you are shooting through the slits travels like a wave and is detected like a particle. Because it travels like a wave it can interfere with itself (like any wave does). If you image it as a little "ball" or "sun" or anything like that it will NOT make sense and you will not understand it.

But if you can't detect a photon without destroying it, surely then particle-wave duality is moot? How do you even get any change in the interference pattern from observation?
Wouldn't you get an interference pattern build up from the unmonitored photons through the slits, and a blank from the monitored slits, as sure as if the intervening obstruction had no slits at all? Your reply (without a statistical qualifier on detection rate for the monitored slits) seems contrary to every demonstration & explanation of the test that I have seen (& the law of energy conservation?!), unless I'm completely misunderstanding something obvious (hence the questions :)

A few things here. If we use electrons instead of photons we CAN bounce light off of them to see where they are at. If we set up our experiment so that we see which slit the electron went through it will NOT produce an interference pattern even though the electrons get through the slits just fine and aren't destroyed. This also works with excited atoms. We can excite them and have them emit light before or after the slits so that we can see which one they went through. Again we find that if we know which slit they pass through then they do not produce an interference pattern. The same atoms, not excited, DO produce an interference pattern.

 

[sophiecentaur]

I'm not sure I understand this interpretation, as it relates to the situation we were discussing (with the two slits simultaneously being partially monitored at the top of each).
It seems like you're trying to say that the monitored halves are interacting with the unmonitored halves somehow? Maybe your wording just seems vague to me.

It seems to me that you are still determined to link all this back to Classical thinking. Doomed to failure, I'm afraid - which is why QM had to be introduced in the first place.
If you are "monitoring" a photon's presence then it no longer can take part in the interference. By eliminating some of the photons that go through the monitored slit, you are, in effect, reducing the number that get through and the classical wave treatment would then say that you will not get total cancellation in the nulls because the amplitudes are not the same. The QM argument would be that you have altered the statistics of the probability of where a photon happens to arrive - again producing an 'impure' interference pattern'.

As for the rest of what you write - it is just not a valid set of ideas. For instance, what is a "quantised photon" supposed to be. A photon IS a quantum of energy. All that stuff about whistles and balls is -well - idle ravings (with respect, of course). Spend some time reading what 'people who know' have to say about the topic (I do not refer to my own input, btw) and you have a chance of coming to a useful understanding. Home produced pictures are pretty much guarantee to let you down.

The idea of monitoring the presence of electrons is an interesting one - because it can be done. However, in order to monitor it, you have to disturb its path in some way. It will change its energy in some undetermined way. So its wavelength will no longer be exactly the same as its 'alternative self' and it can no longer 'interfere with itself on the way through the two slits at the same time' and its 'wave' collapses into that of a single, uniquely identified, electron going past where it was detected. I defy anyone NOT to have a bit of a problem with that but QM is like that at every twist and turn.

 

[VCortex]

1:You aren't interacting with the slits, but with the photons. I think what he means is that if a photon would have gone through the top of a slit and interfere with itself, but you are monitoring the top half of one slit, it can no longer interfere with itself if it goes through the top of the unmonitored slit, and passed through as if there was only one slit. If it went through the bottom then it can still interfere and will produce an interference pattern. 2:I don't know what you mean here. What energy are you measuring? We have detectors capable of detecing single photons, electrons, etc.
3:No. There is no way to classically think about this. The photon or electron or whatever you are shooting through the slits travels like a wave and is detected like a particle. Because it travels like a wave it can interfere with itself (like any wave does). If you image it as a little "ball" or "sun" or anything like that it will NOT make sense and you will not understand it.
4:A few things here. If we use electrons instead of photons we CAN bounce light off of them to see where they are at. If we set up our experiment so that we see which slit the electron went through it will NOT produce an interference pattern even though the electrons get through the slits just fine and aren't destroyed. This also works with excited atoms. We can excite them and have them emit light before or after the slits so that we can see which one they went through. Again we find that if we know which slit they pass through then they do not produce an interference pattern. The same atoms, not excited, DO produce an interference pattern.

1: This wording seems like a more coherent description although it again supposes a different set of apparatus to the one we were previously discussing. I would question whether a photon would rather 'choose' to go through the top of unmonitored slit 2 rather than monitored slit 1, as your interpretation seems to imply.

2: I'm just trying to get a handle on the implications of the experiment without a strictly defined energy unit, so as it stands I suppose anything from photons to C60 & anything inbetween.

3: My analogy took into account both particle & wave features (particle = whistle ball, wave = whistle pitch). I fail to see how trying to think logically is incompatible with a mathematical description like a wavefunction.
I also have no idea how to define a clear mental divide between whatever 'classical' & 'non-classical, contemporary(?)' modes of thinking are supposed to be, let alone accuse someone of thinking either way.

4: This is interesting. How many times can you 'sample' a collapsed path electron's (or other relevant wave/particle thing) position?

5:It seems to me that you are still determined to link all this back to Classical thinking. Doomed to failure, I'm afraid - which is why QM had to be introduced in the first place.
If you are "monitoring" a photon's presence then it no longer can take part in the interference. By eliminating some of the photons that go through the monitored slit, you are, in effect, reducing the number that get through and the classical wave treatment would then say that you will not get total cancellation in the nulls because the amplitudes are not the same. The QM argument would be that you have altered the statistics of the probability of where a photon happens to arrive - again producing an 'impure' interference pattern'.

6:As for the rest of what you write - it is just not a valid set of ideas. For instance, what is a "quantised photon" supposed to be. A photon IS a quantum of energy. All that stuff about whistles and balls is -well - idle ravings (with respect, of course). Spend some time reading what 'people who know' have to say about the topic (I do not refer to my own input, btw) and you have a chance of coming to a useful understanding. Home produced pictures are pretty much guarantee to let you down.

7:The idea of monitoring the presence of electrons is an interesting one - because it can be done. However, in order to monitor it, you have to disturb its path in some way. It will change its energy in some undetermined way. So its wavelength will no longer be exactly the same as its 'alternative self' and it can no longer 'interfere with itself on the way through the two slits at the same time' and its 'wave' collapses into that of a single, uniquely identified, electron going past where it was detected. I defy anyone NOT to have a bit of a problem with that but QM is like that at every twist and turn.

5: See 3. I have not been considering 'purity' of a 'classical' or 'QM' wave &/ particle so far due to the small scales & quantities I had envisaged to be involved! From this statement can I infer that impurity of a 'classical' wave null is taken into account by the QM wavefunction equations!? What is the level of observable 'impurity'?

6: "For instance, what is a "quantised photon" supposed to be. A photon IS a quantum of energy" therefore, a quantised photon = 1 quantum. You are arguing semantics, and ignoring a question.
"Spend some time reading what 'people who know' have to say about the topic (I do not refer to my own input, btw)" Thanks for the insightful suggestion, & I shall henceforth disregard your input :)

7: My analogy did attempt to address this, yet still managed to be met with condescension & ad-hominem remarks. I think you're great.

 

[sophiecentaur]

Sorry for the bit of 'ad hominem' but the whistle and ball thing is really pretty preposterous and I don't see how the analogy can seriously relate to QM. If you knew some QM, you would see that. Rather than taking offence, you should do some serious reading. Whilst there are many unsolved questions, you might at least acknowledge that the questions that you pose and have tried answering for yourself have actually been answered and the answers have been subjected to close scrutiny by people much cleverer than me or you. You seem to be missing on some vital aspects of QM.
Your idea of just taking a look at a photon (/particle) without destroying its wave function goes entirely against the whole QM idea. You detect a photon or you don't detect it. If you detect it, you destroy it and if you don't detect it then it is unchanged and ready to be detected somewhere else - possibly the other side of the slits on a screen. Hence, if you detect some of the photons then you are effectively blocking part of one slit. The resulting pattern will be a good interference pattern (narrow bands) plus the (wide) pattern due to the contribution from one slit. This always happens because no two slits are identical and the pattern never has perfect nulls in it.

I should forget about trying to talk of a 'general particle' first and get down to discussing photons. That is difficult enough. The reason why observing a photon upsets the pattern is much easier to discuss than the effect of merely 'perturbing' a massive particle.

Don't knock Semantics, either. A word that is used out of context or with the wrong meaning can be as bad as if you change your symbols on the way through an algebraic argument without telling people. If you had read and understood the accepted QM stuff then you would be using the right terms because you would see that they are important in any argument.

 

[Drakkith]

1: This wording seems like a more coherent description although it again supposes a different set of apparatus to the one we were previously discussing. I would question whether a photon would rather 'choose' to go through the top of unmonitored slit 2 rather than monitored slit 1, as your interpretation seems to imply.

Photons do not choose anything. It is simple probability. The photon can go through the top of the monitored slit where it is detected by the detector and never makes it to the screen, through the top of the unmonitored slit where it continues through to the screen or detector behind the experiment, or through the bottom of the two slits where it interferes with itself, changing the probability of detecting it at a location on the screen to the pattern you see if you shoot many photons through two unmonitored slits.

3: My analogy took into account both particle & wave features (particle = whistle ball, wave = whistle pitch). I fail to see how trying to think logically is incompatible with a mathematical description like a wavefunction.
I also have no idea how to define a clear mental divide between whatever 'classical' & 'non-classical, contemporary(?)' modes of thinking are supposed to be, let alone accuse someone of thinking either way.

This has nothing to do with trying to think logically and more to do with your analogy simply being a bad one that doesn't make any sense. In a whistle the movement of the ball determines the changing sound that comes out. A photon has nothing to do with how the EM wave works. The EM wave is simply detected as one photon.

4: This is interesting. How many times can you 'sample' a collapsed path electron's (or other relevant wave/particle thing) position?

I'm not sure what you are asking.