The dual slit experiment seems like...well...BS!

[Layman]

I don't think there's any odd about the dual slit experiment. Take a look at the physicsworld breakthroughs article from 2011, and you can see Jeff Lundeen mentioned in second place. When you look at his web site you see things like this:

"With weak measurements, it’s possible to learn something about the wavefunction without completely destroying it. As the measurement becomes very weak, you learn very little about the wavefunction, but leave it largely unchanged. This is the technique that we’ve used in our experiment. We have developed a methodology for measuring the wavefunction directly, by repeating many weak measurements on a group of systems that have been prepared with identical wavefunctions. By repeating the measurements, the knowledge of the wavefunction accumulates to the point where high precision can be restored. So what does this mean? We hope that the scientific community can now improve upon the Copenhagen Interpretation, and redefine the wavefunction so that it is no longer just a mathematical tool, but rather something that can be directly measured in the laboratory".

The key phrase is, "you learn little about the wavefunction". A detector can be off and interact with the electron just the same as if it were on. The key to all of this is how much you learn about the speed and position of the particle. The more you learn about one aspect, the less you can learn about the other aspect. Scientist are smart. They have thought that it might be caused by the interaction the detector plays with the particle itself. A "weak measurement" doesn't just interact with the electron less. It is just "weak" in the sense that they didn't really measure anything.

I almost forgot about these kinds of experiments, but if the Copenhagen Interpretation is correct, then it could be possible to send a binary code back in time faster than light by simply finding out if the wavefunction collapsed or not; that would tell if the detector was on or not in its future worldline. It is really more complicated bs than you think it would. This is really the beginning of most pop physics books we have today.

 

[Layman]

Sure. 2 slit experiment with lasers.

How would lasers show wavelike behavior when it is already a small beam? Wouldn't the laser beam just go through a slit? Then I would find it hard to see how the wavefuction of light would collapse. Then this doesn't mention anything about that aspect of the experiment. I don't see any dates or names of people that have performed this. I know it is really nit picking, but I try to be careful when it comes to things on the internet. In all the books I have read, it is always done with electrons. Anyone could have created that web page and just posted their own drawings on it.

For the particle to be changed from an electron to a photon, every other aspect of the equipment used in the experiment would have to be different. The detector would have to be different, and the board in the back wouldn't react the same way to photons as it would to light. I just find it odd that in books they always use the electron in an example, and everyone wants to say it is photons on the internet. Don't rainbows still look like rainbows when you look at them? We already know that light always travels at the same constant speed. I don't see how there could be much uncertainty about that... How would they collapse the wavefuction of light?

 

[Fednis48]

I don't think there's any odd about the dual slit experiment. Take a look at the physicsworld breakthroughs article from 2011, and you can see Jeff Lundeen mentioned in second place. When you look at his web site you see things like this:

"With weak measurements, it’s possible to learn something about the wavefunction without completely destroying it. As the measurement becomes very weak, you learn very little about the wavefunction, but leave it largely unchanged. This is the technique that we’ve used in our experiment. We have developed a methodology for measuring the wavefunction directly, by repeating many weak measurements on a group of systems that have been prepared with identical wavefunctions. By repeating the measurements, the knowledge of the wavefunction accumulates to the point where high precision can be restored. So what does this mean? We hope that the scientific community can now improve upon the Copenhagen Interpretation, and redefine the wavefunction so that it is no longer just a mathematical tool, but rather something that can be directly measured in the laboratory".

Weak measurements are intriguing and subtle things. In my opinion, much of the recent work with them has been misguided. In the case of Jeff Lundeen's work, it is simply not correct to say that weak measurements allow us to "directly" measure the wavefunction. Weak measurements allow us to "spread out" the wavefunction collapse over multiple measurements, giving us partial information about several of a system's variables instead of complete information about one of them. A simple example would be measuring both a particle's position and momentum to finite accuracy, so that we know something about both variables but not enough about either to violate the uncertainty principle. No matter how you slice it up, though, wavefunction collapse (or "measurement backaction" as its sometimes called) is an inevitable consequence of measurement, and it will always add up before one can completely map out the wavefunction. To actually get a system's full wavefunction, you need to measure multiple copies of it and perform statistical analysis on the results. That way, once you've collapsed one wavefunction, you can grab a fresh copy and continue gaining information about it. This process is called quantum state tomography. QST is what Dr. Lundeed does, and from a technical standpoint, his work is impressive. But one can do the same thing with strong measurements.

Weak measurement is a demonstration of an interaction that doesn't occur in discrete lumps. As Lundeen said, as the measurement becomes very weak, you learn very little about the wavefunction, but leave it largely unchanged.

This statement I agree with. The intro-level description of measurement always involves complete collapse of the wavefunction, but in reality there's a whole spectrum of measurement strengths between zero and maximum.

I almost forgot about these kinds of experiments, but if the Copenhagen Interpretation is correct, then it could be possible to send a binary code back in time faster than light by simply finding out if the wavefunction collapsed or not; that would tell if the detector was on or not in its future worldline. It is really more complicated bs than you think it would. This is really the beginning of most pop physics books we have today.

This isn't possible, because there's no way to tell whether a wavefunction has collapsed just by measuring it. In Bell inequality-type experiments, they infer the collapse of the wavefunction by comparing the measurement statistics of two systems; before taking those correlations into account, each individual system's statistics are the same before vs. after the collapse. In fact, this is an important reason why Dr. Lundeen's interpretation above has to be misleading; if it was possible to map out a wavefunction using weak measurements, we could determine whether or not a given wavefunction had collapsed, and that would allow superluminal communication.

How would lasers show wavelike behavior when it is already a small beam? Wouldn't the laser beam just go through a slit? Then I would find it hard to see how the wavefuction of light would collapse. Then this doesn't mention anything about that aspect of the experiment. I don't see any dates or names of people that have performed this. I know it is really nit picking, but I try to be careful when it comes to things on the internet. In all the books I have read, it is always done with electrons. Anyone could have created that web page and just posted their own drawings on it.

For the particle to be changed from an electron to a photon, every other aspect of the equipment used in the experiment would have to be different. The detector would have to be different, and the board in the back wouldn't react the same way to photons as it would to light. I just find it odd that in books they always use the electron in an example, and everyone wants to say it is photons on the internet. Don't rainbows still look like rainbows when you look at them? We already know that light always travels at the same constant speed. I don't see how there could be much uncertainty about that... How would they collapse the wavefuction of light?

To be honest, I'm kind of amazed you didn't know the double-slit experiment could be done with photons; it was done that way long before it was done with electrons, or even before the advent of quantum mechanics. I don't have a reference for this, but I have personally performed the double-slit experiment with a lamp, and it works just fine. I have not performed the version where you turn the intensity way down and watch one photon pop up at a time (that version takes a more elaborate detector), but you'll find it mentioned in any intro quantum mechanics textbook.

To answer your question about the laser: for the double slit experiment to work, the slits need to be very close together -- closer than the beam width of a cheap laser. If the slits are too far apart for the laser to shine on both at once, you're right that the interference effect will not appear.

 

[Layman]

Weak measurement is a demonstration of an interaction that doesn't occur in discrete lumps. As Lundeen said, as the measurement becomes very weak, you learn very little about the wavefunction, but leave it largely unchanged.

It something that is fairly new in experimental research. The wiki on weak measurement states, "
As stated above Busch's theorem prevents a free lunch: there can be no information gain without disturbance. However the tradeoff between information gain and disturbance has been characterized by many authors including Fuchs and Peres;[22] Fuchs;[23] Fuchs and Jacobs;[24] and Banaszek.[25]

Recently the information gain disturbance tradeoff relation has been examined in the context of what is called the "Gentle measurement lemma".[6][26]"

 

[origin]

How would lasers show wavelike behavior when it is already a small beam? Wouldn't the laser beam just go through a slit? Then I would find it hard to see how the wavefuction of light would collapse. Then this doesn't mention anything about that aspect of the experiment. I don't see any dates or names of people that have performed this. I know it is really nit picking, but I try to be careful when it comes to things on the internet. In all the books I have read, it is always done with electrons. Anyone could have created that web page and just posted their own drawings on it.

Geeze, just google 2 slit experiment. Most of the 2 slit work is done with photons! Hell, I did a 2 slit experiment in college with a laser way back in 1986!

 

[Layman]

This isn't possible, because there's no way to tell whether a wavefunction has collapsed just by measuring it. In Bell inequality-type experiments, they infer the collapse of the wavefunction by comparing the measurement statistics of two systems; before taking those correlations into account, each individual system's statistics are the same before vs. after the collapse. In fact, this is an important reason why Dr. Lundeen's interpretation above has to be misleading; if it was possible to map out a wavefunction using weak measurements, we could determine whether or not a given wavefunction had collapsed, and that would allow superluminal communication.

That was what I was trying to say earlier. It wouldn't be possible, because there is some type of chronological protection. The collapse of the wavefunction itself could serve this type of purpose. I just don't know much about weak measurements. Most of what I have studied were basically history lessons on the original experiments when it was first discovered. I don't know if a weak measurement would allow someone to only see if a wavefunction collapsed or not without eventually collapsing it from the weak measurement itself.

To be honest, I'm kind of amazed you didn't know the double-slit experiment could be done with photons; it was done that way long before it was done with electrons, or even before the advent of quantum mechanics. I don't have a reference for this, but I have personally performed the double-slit experiment with a lamp, and it works just fine. I have not performed the version where you turn the intensity way down and watch one photon pop up at a time (that version takes a more elaborate detector), but you'll find it mentioned in any intro quantum mechanics textbook.

To answer your question about the laser: for the double slit experiment to work, the slits need to be very close together -- closer than the beam width of a cheap laser. If the slits are too far apart for the laser to shine on both at once, you're right that the interference effect will not appear.

I don't think you could collapse a wavefunction of the photon. Wouldn't it appear to make a wavelike pattern rather if you detected the light going through the slits or not? A photon without a wave is no photon at all.

 

[Layman]

Geeze, just google 2 slit experiment. Most of the 2 slit work is done with photons! Hell, I did a 2 slit experiment in college with a laser way back in 1986!

Did they ever show you the photons particle like nature from this experiment?

 

[origin]

Did they ever show you the photons particle like nature from this experiment?

I don't remember. But the photoelectric effect is a pretty good indication of the particle nature of photons.

Edit to add: I'm happy I can remember anything from 1986.

Edit to add: Here is the wiki article about the double slit experiment and it starts off with photons and moves to electrons.

 

[Confused2]

Did they ever show you the photons particle like nature from this experiment?

Here is a shool lab double slit apparatus:- http://www.teachspin.com/two-slit-interference--one-photon-at-a-time.html
See especially Figure 4: Photon Counting Data for Single and Double Slit Configuration

 

[JJM]

can light be geometrically altered by the field tensors of the atomic layer s' of the slits?

 

[Layman]

Here is a shool lab double slit apparatus:- http://www.teachspin.com/two-slit-interference--one-photon-at-a-time.html
See especially Figure 4: Photon Counting Data for Single and Double Slit Configuration

I watched the video, and he was not able to show the particle like nature of the photon by using acts of observation. Instead, he covered up one of the slits with an index card. Then I guess only half of the experiment done with an electron can be done with a photon. In this way, electrons and photons can behave differently. You can observe the light in the experiment, and it still acts like a wave. That doesn't occur in the electron experiment.

 

[Farsight]

...This statement I agree with. The intro-level description of measurement always involves complete collapse of the wavefunction, but in reality there's a whole spectrum of measurement strengths between zero and maximum.

I have this mental picture of a mega-huge creature detecting a seismic wave with a pointed stick the size of a mountain range. His stick is so big is that it absorbs all the energy of the seismic wave, and the creature talks about wavefunction collapse and declares the seismic wave to be pointlike. But then comes a little man with a little stick mapping out repeated identical seismic waves, none of which are much affected by his little stick and his weak measurement.

This isn't possible, because there's no way to tell whether a wavefunction has collapsed just by measuring it. In Bell inequality-type experiments, they infer the collapse of the wavefunction by comparing the measurement statistics of two systems; before taking those correlations into account, each individual system's statistics are the same before vs. after the collapse. In fact, this is an important reason why Dr. Lundeen's interpretation above has to be misleading; if it was possible to map out a wavefunction using weak measurements, we could determine whether or not a given wavefunction had collapsed, and that would allow superluminal communication.

I'm sorry Fednis, but I disagree.

 

[danshawen]

I have this mental picture of a mega-huge creature detecting a seismic wave with a pointed stick the size of a mountain range. His stick is so big is that it absorbs all the energy of the seismic wave, and the creature talks about wavefunction collapse and declares the seismic wave to be pointlike. But then comes a little man with a little stick mapping out repeated identical seismic waves, none of which are much affected by his little stick and his weak measurement.

I'm sorry Fednis, but I disagree.

Both sticks (the enormous one and the tiny one) cannot possibly both be resonant to the same seismic wave frequencies. The larger one would be better at resonating with the lower frequencies.

 

[Fednis48]

I watched the video, and he was not able to show the particle like nature of the photon by using acts of observation. Instead, he covered up one of the slits with an index card. Then I guess only half of the experiment done with an electron can be done with a photon. In this way, electrons and photons can behave differently. You can observe the light in the experiment, and it still acts like a wave. That doesn't occur in the electron experiment.

Honestly, if you're genuinely skeptical about the particle nature of photons, the double-slit isn't the best experiment to be considering. Like origin said, the photoelectric effect is a much better demonstration of the quantized nature of light. If you're willing to get a little more technical, you should also check out photon antibunching. (Incidentally, I wonder how Farsight would explain either of these effects.)

I'm sorry Fednis, but I disagree.

Which part do you disagree with? Do you disagree that Bell inequality experiments require classical comparison between the statistics of two systems? Do you disagree that being able to map out the wavefunction of a single particle could tell us whether or not its wavefunction had collapsed? Or do you disagree that measuring wavefunction collapse would allow for superluminal communication?

 

[Farsight]

Honestly, if you're genuinely skeptical about the particle nature of photons, the double-slit isn't the best experiment to be considering. Like origin said, the photoelectric effect is a much better demonstration of the quantized nature of light. If you're willing to get a little more technical, you should also check out photon antibunching. (Incidentally, I wonder how Farsight would explain either of these effects.)

I'm not sceptical about the quantum nature of light. But I don't see this as being anything that says the photon is some kind of billiard-ball thing. It has an E=hf wave nature.

Which part do you disagree with? Do you disagree that Bell inequality experiments require classical comparison between the statistics of two systems? Do you disagree that being able to map out the wavefunction of a single particle could tell us whether or not its wavefunction had collapsed? Or do you disagree that measuring wavefunction collapse would allow for superluminal communication?

The latter. Electromagnetic phenomena propagate at c.

 

[goniahedron]

When it comes to the two-slit experiment being performed with photons (light) I always remember reading somewhere online (quite a few years ago now) someone's belief that the reason for the mega-saga generated by the conventional quantum understanding behind the experiment is due to two major factors: one, because the current understanding about the being and nature of photons is fundamentally wrong; and two, because in addition to the previous reason we have chosen too hastily to believe that the Michelson-Morley experiment implied that there is categorically no frame of reference of any possible sort at absolute rest. In effect, according to the author (whose name unfortunately I can no longer remember), the nature of photons is basically the same as that of the water waves--meaning that instead of picturing a photon as being emitted at a point A and then travelling to a point Z (through successive points B, C, D...) we should consider that a source of light creates a photon in space at point A, then another at point B, another still at C, and so on to Z. Now, regardless of what one may think about the idea it is hard not to be seduced by the fact of how easily the results of the two-slit experiment then would instantly become trivial (accommodated by common-sense).

 

[Fednis48]

I'm not sceptical about the quantum nature of light. But I don't see this as being anything that says the photon is some kind of billiard-ball thing. It has an E=hf wave nature.

It's one thing to say you're not skeptical about the quantum nature of light, but I don't see how that's compatible with your model. For instance, quantum mechanics tells us that if we create one photon and let its wavefunction expand for a while before (strongly) measuring it, we'll only ever find the photon in one location. This holds true even if the wavefunction is larger than the individual detectors, so it wouldn't make any sense to say that one detector can "soak up" the whole wave. How can you explain this without invoking either measurement collapse or some kind of particle nature for the photon?

The latter. Electromagnetic phenomena propagate at c.

If we can map out wavefunctions, then we can do superluminal communication via the following protocol.

1. Create a two-electron superposition state, $$|\psi_0\rangle=(|p,x_0\rangle\otimes|-p,x_0\rangle+|p',x_0\rangle\otimes|-p',x_0\rangle)$$ where $$|P,X\rangle$$ is a momentum-squeezed state with mean momentum P and mean position X. Each electron thus has extremely well-defined momentum and somewhat well-defined position. In words, we've created an entangled state in which two electrons are moving apart with equal and opposite momenta, but the magnitude of their momenta can be either p or p'.

2. Let the electrons travel for a long time t, until t*(p-p') is much larger than the electrons' initial position uncertainty. Each electron's wavefunction is now a two-peaked distribution, with its distance from the source depending on its initial momentum. The wavefunction is now $$|\psi_t\rangle=(|p,x\rangle\otimes|-p,-x\rangle+|p',x'\rangle\otimes|-p',-x'\rangle)$$ where x=tp/m and x'=tp'/m.

3. Two scientists, Alice and Bob, each grab one of the electrons. If Alice wishes to send a 1, she strongly measures her electron's position. Depending on the result of the measurement, the wavefunction becomes either $$|p,x\rangle\otimes|-p,-x\rangle$$ or $$|p',x'\rangle\otimes|-p',-x'\rangle$$. Either way, it is a one-peaked distribution. If Alice wishes to send a 0, she does nothing, and the distribution remains two-peaked.

4. Bob maps out his electron's wavefunction, by whatever means. If it is one-peaked, he knows Alice sent a 1. If it is two-peaked, he knows Alice sent a 0. This works no matter how far away the two parties are, so a bit has been transmitted faster than light.

As a final note, if your response is that Bob's wavefunction does not change when Alice performs her measurement, that your model can't generate Bell inequality violations, so it contradicts experiment.

 

[Layman]

Honestly, if you're genuinely skeptical about the particle nature of photons, the double-slit isn't the best experiment to be considering. Like origin said, the photoelectric effect is a much better demonstration of the quantized nature of light. If you're willing to get a little more technical, you should also check out photon antibunching. (Incidentally, I wonder how Farsight would explain either of these effects.)

You are putting words into my mouth. I don't doubt the particle nature of light. I only doubt that light can be used to show the two slit experiment in its entirety. I am simply stating that I believe that if someone said "photon" instead of "electron" describing this experiment, that they would only be 100% correct in describing the first half of the experiment. The photon example wouldn't apply to the second half of it's description, being the collapse of the wavefunction.

The particle nature of light is mostly accredited to particles absorbing photon in discrete packets. I do not doubt that. I am just saying that an act of observation on a photon does not have the same affects as it would on an electron or other particle. Otherwise, we wouldn't ever be able to see. I have read that mathematically a photon cannot even exist without its wavelike properties. You can look at a rainbow coming from a prism, and it will still look like a rainbow. That is true rather you use your eyes or a detector.

 

[Fednis48]

You are putting words into my mouth. I don't doubt the particle nature of light. I only doubt that light can be used to show the two slit experiment in its entirety. I am simply stating that I believe that if someone said "photon" instead of "electron" describing this experiment, that they would only be 100% correct in describing the first half of the experiment. The photon example wouldn't apply to the second half of it's description, being the collapse of the wavefunction.

Consider the following thought experiment. Say we have a detector screen which consists of a grid of identical atoms. If we shine light on the screen, it may excite some of the atoms, and we can look at which atoms were excited to get some idea where the light was shining. Now say we fire one photon at a time at the screen. Whenever the screen detects one of these photons, only one atom at a time will excite, no matter how spatially extensive the photon's wavefunction is. That means we've localized the photon in space.

The particle nature of light is mostly accredited to particles absorbing photon in discrete packets. I do not doubt that. I am just saying that an act of observation on a photon does not have the same affects as it would on an electron or other particle. Otherwise, we wouldn't ever be able to see. I have read that mathematically a photon cannot even exist without its wavelike properties. You can look at a rainbow coming from a prism, and it will still look like a rainbow. That is true rather you use your eyes or a detector.

I don't understand this argument. Your eyes are detectors, and seeing a photon collapses its wavefunction. If you looked at a single photon coming through a prism, it would look like a dot of a single color; light through a prism only looks like a smooth rainbow because there are lots of photons all coming through at once.

 

[Layman]

I don't understand this argument. Your eyes are detectors, and seeing a photon collapses its wavefunction. If you looked at a single photon coming through a prism, it would look like a dot of a single color; light through a prism only looks like a smooth rainbow because there are lots of photons all coming through at once.

It has to do with what a "wavefunction" actually is. A particle has a wavefunction, because it has only a statistical probability of having a multitude of speeds and locations. That is the definition I have read about how it is defined in books written for laymen. That is what it is supposed to technically mean. The local speed of light is always supposed to be constant, unless you consider the claims of the recent discovery of gravitational waves. Then the location of a photon is every location it could possibly be at, at the same time, according to the Copenhagen Interpretation. If the wavefunction collapsed, it would mean that an observer had measured it to be only at precisely one of those locations. Therefore, light wouldn't be shining everywhere, and it would only shine at one location. Then this is contrary to what is seen in experiments. Light will always exhibit it's wavelike nature, rather it is observed or not. It will always act like it is at every location at once, and it will always show only wavelike properties in a two slit experiment, unless someone manually covers up one of the slits. When your eyes see a photon, it does not collapse the wavefunction. The photon is just absorbed, which shows its particle like nature.