Why don't the Slits collapse the wave function?

Click For Summary
The discussion centers on the phenomenon of wave function collapse in quantum mechanics, particularly in the context of the double-slit experiment. Participants explore why the interaction with the slits does not collapse the wave function, while a measurement does, emphasizing that the slits do not randomize the phase of the wave function. The concept of internal degrees of freedom is introduced, suggesting that if the slits or impurities can change state upon interaction, they could disrupt the phase coherence necessary for interference patterns. The conversation also touches on the nature of measurement in quantum mechanics, indicating that it is not clearly defined when or how a measurement occurs. Overall, the dialogue reflects a deep engagement with the complexities of quantum behavior and the implications of measurement.
  • #31
DrChinese said:
Real? Who decides that? And why would a local model be more real than a non-local model anyway?

I'd say that for something to be real it must at least exist in 4D spacetime. This includes the particles and force fields. AFAIK you cannot map the wave function in spacetime, only its squared amplitude, which is a probability distribution.

The evidence we have points to a local universe. Matter/energy and all the known force fields cannot travel faster than c. Until some non-local mechanism (tachyons, other dimensions, etc) is shown to exist we have no reason to assume it. On the other hand, non-local correlations are common, but most of them have proven to be based on a local mechanism (see for example the speed of gravity: infinite in Newtonian gravity, c in GR).

The interference itself is not caused by interaction at the slits (although there are elements of the shape of the interference pattern that are).

How do you know that? And, if the particles do not exchange momentum with the wall, how do you get the momentum conservation along the initial direction of the beam? Let's assume that the average scattering angle of the particles is 45 degrees, right or left, doesn't matter. This means that they arrive at the detector with half of the momentum they left the source. If you perform the experiment in space, the source would accelerate twice as fast as the detector, which is a clear violation of the conservation law.

It is a direct result of interference between the different available paths from the source to the target. (Sum over histories). This is true of ordinary light and can be easily confirmed experimentally. It is well known that light reflected from a mirror receives contributions to its intensity from paths that are different from the traditional straight line path. Placing etches (where there is no reflection) on a mirror at precise spots *increases* the intensity of a beam, for example.

Sum over histories is, according to Feynman, just a method of calculation, nothing more. The mechanism behind the quantum phenomena is unknown and, as I've argued before, there is no reason to assume it is non-local.
 
Physics news on Phys.org
  • #32
RandallB said:
I don’t remember where but I’ve seen this thought experiment worked though with a light weight double slits that could move and be measured. The idea being momentum conservation of the photon winding up say on the right side screen would require one of both slits receiving a ‘kick’ moving them to the left. At the end of it all the resulting movement of the slits required to conserve that momentum was smaller than the uncertainty of any possible measurement of a change in location for the slits. And of course any history of multiple photons going through the slits always average to a even balance between left and right. The conclusion was the idea was not testable.

I think this was a gedankenexperiment proposed by Einstein. Indeed, it can be shown that you cannot determine the particle's path this way. However, the problem is with momentum conservation along the particle’s propagation direction (see my post for DrChinese). In the wall's plane, the average momentum is zero and the uncertainty makes it impossible to detect the momentum exchange.
 
  • #33
RandallB said:
I don’t remember where but I’ve seen this thought experiment worked though with a light weight double slits that could move and be measured. The idea being momentum conservation of the photon winding up say on the right side screen would require one of both slits receiving a ‘kick’ moving them to the left. At the end of it all the resulting movement of the slits required to conserve that momentum was smaller than the uncertainty of any possible measurement of a change in location for the slits. And of course any history of multiple photons going through the slits always average to a even balance between left and right. The conclusion was the idea was not testable.

Interesting, one wonders if the conclusion: that the idea is not testable,
had a practical reason, because of the "lack of sophistication" of the used
equipment, or something else.

One could imagine that some time in the, not so far away, future such an
experiment using Extreme Ultra Violet photons and a nanotechnology slit
plate could do measurements with the required sensitivity.Regards, Hans
 
  • #34
ueit said:
On the other hand, non-local correlations are common, but most of them have proven to be based on a local mechanism (see for example the speed of gravity: infinite in Newtonian gravity, c in GR).
I've never seen a documented example of non-local correlation proven to be based on a local mechanism. At least not a "truly local" mechanism. (I understand there are "local mathematical constructs" such as BM, MWI, etc. that could work in a reality with additional dimensions to allow for guide waves etc.)
You site as an example “the speed of gravity: infinite in Newtonian gravity, c in GR”? – I don’t see how that relates to correlations being based on a true naive Einstein local mechanism. Do you have a link or other resource pertaining to your point here?
 
  • #35
ueit said:
I think this was a gedankenexperiment proposed by Einstein. Indeed, it can be shown that you cannot determine the particle's path this way. However, the problem is with momentum conservation along the particle’s propagation direction (see my post for DrChinese). In the wall's plane, the average momentum is zero and the uncertainty makes it impossible to detect the momentum exchange.

Note that QM uncertainty does not limit the precision of the measurements
we make. It limits the predictability. The conservation laws hold up in any
interaction, even in interactions with virtual particles. We wouldn't be able
to do practical calculations in QED if it wasn't for the delta functions which
preserve Energy/momentum at each interaction.


Regards, Hans.
 
  • #36
Hans de Vries said:
One could imagine that some time in the, not so far away, future such an experiment using Extreme Ultra Violet photons and a nanotechnology slit plate could do measurements with the required sensitivity.
Not in my wildest imagination whould I expect to ever see technology that sensitive. We are looking at the movement of the mass of a wall holding a slit being moved by just a small few photons that by chance happen to all deflect the same way before others cancel the effect by going the other way. I would expect tempature alone to swamp the detail needed.
 
  • #37
RandallB said:
Not in my wildest imagination whould I expect to ever see technology that sensitive. We are looking at the movement of the mass of a wall holding a slit being moved by just a small few photons that by chance happen to all deflect the same way before others cancel the effect by going the other way. I would expect tempature alone to swamp the detail needed.
To stimulate your imagination then: The "wall" would be in the order
of 40 nm wide with nano technology and EUV photons. There are several
ways of measuring forces and/or displacements with extreme precision.

Measuring the effects of single particles is not so exceptional anymore.
The record measurement of the electron's gyromagnetic anomaly was
measured with a precision of 12 digits by observing a single electron
in a Penning trap for a long enough time. Regards, Hans.
 
Last edited:
  • #38
ueit said:
Sum over histories is, according to Feynman, just a method of calculation, nothing more. The mechanism behind the quantum phenomena is unknown and, as I've argued before, there is no reason to assume it is non-local.

If you can test the specific assumption (in this case: sum over histories), and the results are as predicted by theory, how can you say it is not physical?

As for the requirement of 4D spacetime, it is a bit premature to require that there be no additional dimensions. Maybe there are more.
 
  • #39
The interference pattern is real, is it not? I therefore deduce the slit is also real, and the interference pattern is a causal consequence of photon-slit interactions.
 
  • #40
Hans de Vries said:
To stimulate your imagination then: The "wall" would be in the order
of 40 nm wide with nano technology and EUV photons. There are several
ways of measuring forces and/or displacements with extreme precision.

Measuring the effects of single particles is not so exceptional anymore.
The record measurement of the electron's gyromagnetic anomaly was
measured with a precision of 12 digits by observing a single electron
in a Penning trap for a long enough time.
To Bring your imagination Back to Reality then;
consider the total mass of the 40 nm THICK wall holding the slit (we only need one here) that will be moved by the deflection of the one or two photons we detect going to one side.
I can only guess what you might imagine; but do you actually think that the movement of the wall caused by just a couple of photons we be nearly as large as a precision of 12 digits
 
  • #41
Chronos said:
The interference pattern is real, is it not? I therefore deduce the slit is also real, and the interference pattern is a causal consequence of photon-slit interactions.

The interference is not an artifact of interaction with the slit per se. If it was, then a single slit would also produce interference.

That is not say that the *shape* of the interference pattern is not influenced by the shape of the slit. Diamond shaped slits produce diamond shaped patterns.
 
  • #42
DrChinese said:
The interference is not an artifact of interaction with the slit per se. If it was, then a single slit would also produce interference.
But a single slit does give an interferance pattern. The applet at http://www.physics.northwestern.edu/ugrad/vpl/optics/diffraction.html" ; is a nice tool to show how both types of patterns must be considered to define the Complete Pattern that will be produced.
 
Last edited by a moderator:
  • #43
RandallB said:
But a single slit does give an interferance pattern. The applet at http://www.physics.northwestern.edu/ugrad/vpl/optics/diffraction.html" ; is a nice tool to show how both types of patterns must be considered to define the Complete Pattern that will be produced.

Hi RandallB,

I must be an idiot, because I tinkered with that applet and couldn't find that result.

I would guess that there is some interference in a single slit, but that the effect on total intensity would be minimal (and would not be visually evident to the resulting pattern).
 
Last edited by a moderator:
  • #44
DrChinese said:
I must be an idiot, because I tinkered with that applet and couldn't find that result.

I would guess that there is some interference in a single slit, but that the effect on total intensity would be minimal (and would not be visually evident to the resulting pattern).
I find the best viewing comes if you:
Leave wave length at 1
Slits at n = 2
Change slit separation to d=30
Change Slit width between a = 4, 8, & 12 (my favorite is 3.8)

Then alternate which boxes you check you check at the bottom.

The pure single slit plot shows the area that the pure n-slit graph must fit under for the complete pattern.
As you can see in most cases real experiments are done in the center area, being careful not to let the value of “a” slit width get to big. Or the single slit dispersion interference will move toward the center with both a ‘null’ spot getting in the way and a lowering of the relative intensity as compared to dead center. Either affect would ruin the purpose of most experiments.
But I would like to see an experiment looking past the first single slit null at about 45 to 50 degrees using 3.8 width slits in a 2 slit. Maybe I should ask DrDave to do it; but it looks like a particularly difficult test to run for real.

Note: The display does have a code error in that when putting in a slit separation of 30 it actually represents a slit separation of 15. (30 should give 60 peaks from end to end for the pure n-slit not 30 peaks)
 
Last edited:
  • #45
RandallB said:
But a single slit does give an interferance pattern.

The single-slit pattern is qualitatively very different from the double-slit one. How can one account for this difference via interactions with the slit edges?
 
  • #46
kliide said:
One thing I don't understand is how the screen with the slits do not collapse the wave function until there is a measurement. Wouldn't bouncing off the sides of the slit constitute some sort of interaction?

The slits do collapse the wavefunction: into to delta functions located at the slits. This is because the slits measure position, and delta functions are the eigenfunctions of position.
 
  • #47
jtbell said:
The single-slit pattern is qualitatively very different from the double-slit one. How can one account for this difference via interactions with the slit edges?
Single slit diffraction & interference is rather basic stuff just google or start at http://en.wikipedia.org/wiki/Slit_experiment"
 
Last edited by a moderator:
  • #49
RandallB said:
I've never seen a documented example of non-local correlation proven to be based on a local mechanism. At least not a "truly local" mechanism. (I understand there are "local mathematical constructs" such as BM, MWI, etc. that could work in a reality with additional dimensions to allow for guide waves etc.)
You site as an example “the speed of gravity: infinite in Newtonian gravity, c in GR”? – I don’t see how that relates to correlations being based on a true naive Einstein local mechanism. Do you have a link or other resource pertaining to your point here?
From this article:

http://arxiv.org/PS_cache/gr-qc/pdf/9909/9909087.pdf"

It is certainly true, although perhaps not widely enough appreciated, that observations are incompatible with Newtonian gravity with a light-speed propagation delay added in [3,4].
If one begins with a purely central force and puts in a finite propagation speed by hand, the forces in a two-body system no longer point toward the center of mass, and the resulting tangential accelerations make orbits drastically unstable.

As a warm-up, let us first consider electrodynamics. It is well known that if a charged source moves at a constant velocity, the electric field experienced by a test particle points toward the source’s “instantaneous” position rather than its retarded position. Lorentz
invariance demands that this be the case, since one may just as well think of the charge as being at rest while the test particle moves. This effect does not mean that the electric field propagates instantaneously; rather, the field of a moving charge has a velocity-dependent component that cancels the effect of propagation delay to first order

Finally, let us return to the question asked in Ref. [1]: what do experiments say about the speed of gravity? The answer, unfortunately, is that so far they say fairly little. In the absence of direct measurements of propagation speed, observations must be filtered through theory, and different theoretical assumptions lead to different deductions. In particular, while the observed absence of aberration is consistent with instantaneous propagation (with an extra interaction somehow added on to explain the gravitational radiation reaction), it is also consistent with the speed-of-light propagation predicted by general relativity. Within the framework of general relativity, though, observations do give an answer. The Einstein field equations contain a single parameter cg, which describes both the speed of gravitational waves and the “speed of gravity” occurring in the expression for aberration and in the velocity-dependent terms in the interaction. This parameter appears in the
gravitational radiation reaction in the form c−5 g , as in eqn. (3.3), and the success of the theory in explaining the orbital decay of binary pulsars implies that cg = c at the 1% level or better [22].

I think this is a good example of how a local mechanism can produce non-local correlations. If you agree that GR is correct and not just a "local mathematical construct" my point is proven.

I don't understand your question about the "true naive Einstein local mechanism".a
 
Last edited by a moderator:
  • #50
ueit said:
I think this is a good example of how a local mechanism can produce non-local correlations. If you agree that GR is correct and not just a "local mathematical construct" my point is proven.

You need to understand Bell better. The question is NOT whether it is possible to get an apparently "non-local" correlation from a purely local mechanism. Bell states specifically that you cannot get QM's predictions from a LOCAL and REALISTIC theory. That is a completely different issue altogether!

So your point continues to miss the mark wide, and that is why it is not generally accepted science. Your "citations" add nothing to the debate.
 
  • #51
DrChinese said:
You need to understand Bell better. The question is NOT whether it is possible to get an apparently "non-local" correlation from a purely local mechanism. Bell states specifically that you cannot get QM's predictions from a LOCAL and REALISTIC theory. That is a completely different issue altogether!

So your point continues to miss the mark wide, and that is why it is not generally accepted science. Your "citations" add nothing to the debate.

GR is both local and realistic, do you agree?

Gravity appears to be non-local, that is, if I am on Mars and you on Earth we could establish our instantaneous position by using accelerometers. We would find that the directions these instruments show are always correlated (parallel).

Now, let's make Bell's assumption. For whatever reason (say evolutionary adaptation) we strongly believe that the planets are moving because of our will, that is, their positions are just like the detector orientations in EPR experiments. It is easy to see that in this case the only way to make sense of the correlations is to assume non-locality. Sort of Bell theorem for gravity.
 
  • #52
DrChinese said:
If you can test the specific assumption (in this case: sum over histories), and the results are as predicted by theory, how can you say it is not physical?

Sum over histories requires a particle to travel an infinite distance in a finite amount of time, in other words, it has to travel at infinite speed. A particle has mass/energy and mass/energy cannot travel faster than c. Therefore, the assumption is falsified by relativity.
 
  • #53
ueit said:
Sum over histories requires a particle to travel an infinite distance in a finite amount of time, in other words, it has to travel at infinite speed. A particle has mass/energy and mass/energy cannot travel faster than c. Therefore, the assumption is falsified by relativity.


The what with the which, there? :confused:
 
  • #54
ueit said:
GR is both local and realistic, do you agree?

Gravity appears to be non-local,
No it is not, see other threads on indeterminate background of GR.
And as you claim gravity appears to be non-local are you saying GR is not a proper description of gravity or disagreeing with yourself and that GR is non-local.
 
  • #55
RandallB said:
No it is not, see other threads on indeterminate background of GR.
The background is dynamic in GR, I don't see how this is relevant. Although some solutions in GR seem to allow for non-locality, they are entirely speculative. As applied to my example, GR is a local mechanism (each planet follows the local space curvature), explaining an "apparently" non-local force (as required in Newtonian gravity). Isn't this what you’ve asked me for?
Include the "free will" assumption and you have a nice analogy with Bell's theorem.
 
  • #56
ueit said:
Isn't this what you’ve asked me for?
... a nice analogy with Bell's theorem.
No it is not.
Gravity and GR with planets have nothing to do with non-local (or local) correlations. You’ve shown no analogy with Bell's theorem here.
 
  • #57
ueit said:
GR is both local and realistic, do you agree?

Gravity appears to be non-local,...

GR is a local realistic theory, athough it is not part of the domain that is QM.

Gravity has never been found to have a non-local component by any known experiment result. Not sure what any of this has to do with this thread.
 
  • #58
RandallB said:
To Bring your imagination Back to Reality then;
consider the total mass of the 40 nm THICK wall holding the slit (we only need one here) that will be moved by the deflection of the one or two photons

A cube with 40 nm sides can be accelarated to up to 400 nm/s by a single
EUV photon. That's ten times its size per second. Nowadays we can detect
displacements well below an Angstrom ...


Regards, Hans.
 
  • #59
DrChinese said:
GR is a local realistic theory, athough it is not part of the domain that is QM.

Gravity has never been found to have a non-local component by any known experiment result. Not sure what any of this has to do with this thread.
In Newtonian physics the speed of gravity was infinite. If you use a limited speed, let's say c, the planetary orbits become unstable.
In GR this "seemingly" infinite speed of gravity was explained by a local mechanism.
This is an example where a non-local effect in an old theory is known to be the result of a local mechanism in a new theory. This is what I was asked for:

I've never seen a documented example of non-local correlation proven to be based on a local mechanism.

I do not claim that this is what happens in the EPR experiment although it is possible (if a GR-like theory can be created for the EM field). It is only an analogy to point to the fact that, in the absence of direct confirmation of non-locality, it is premature to consider non-local correlations as evidence for non-locality.

P.S.

I'll not be able to post here for about four days, I'll answer then if necessary.
 
  • #60
ueit said:
I'll not be able to post here for about four days, I'll answer then if necessary.
No need to reply - I think it is clear you do not understand the term "correlation" from the origianal question
 

Similar threads

High School The wave function of the electron doesn't collapse when passing through air?
  • · Replies 9 ·
Replies
9
Views
2K
High School Why does anyone think gravity might collapse wave function?
  • · Replies 59 ·
2
Replies
59
Views
7K
High School How is energy transferred from a wave to a photon?
  • · Replies 18 ·
Replies
18
Views
3K
Undergrad Is There A Study Showing The Observer Effect Ignoring Conscious Intent?
  • · Replies 3 ·
Replies
3
Views
4K
Undergrad How non-conscious can an Observer be and cause wave function collapse?
  • · Replies 23 ·
Replies
23
Views
7K
High School Can someone please explain to me the motion of an electron?
  • · Replies 11 ·
Replies
11
Views
2K
High School Why doesn't the electron "wave" collapse at the double-slit?
  • · Replies 28 ·
Replies
28
Views
7K
High School Wave Function Collapse using faulty recording devices.
  • · Replies 7 ·
Replies
7
Views
2K
Graduate Can particles appear and disappear "with" a cause?
  • · Replies 1 ·
Replies
1
Views
2K
Undergrad "Wave-particle duality" and double-slit experiment
  • · Replies 36 ·
2
Replies
36
Views
8K