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Author Topic: In quantum mechanics, does the observation preceed the event?  (Read 16808 times)

Offline Atomic-S

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As for the first detection itself, I turn to the antenna principle to attempt to discuss it in a bit of detail.  The antenna principle attempts to understand the absorption and emission of radiation on the basis of the behavior of radio antennas, which, although it may seem inapplicable to a quantum situation, actually is if you examine the mathematics closely.

  The state function of an electron in a specific energy state is a function of x, y, z, and t in a way such that the absolute square of the function is everywhere time-independent, although the function itself varies with time according to the factor exp(iω1t), ω1 being determined by the energy level.  If the electron moves to a different orbit with a different different energy level, its state function is again of x, y, z, and t in like manner, except that the dependence on x, y, and z will in general be different, and that  the time dependency will be exp(iω2t), with ω2 being determined by the different energy of that state.  But as before, the time dependency of the absolute square will be constant.  The absolute square is associated with the probability that a process to find an electron at a given point would find it there, and thus can be thought of as equivalent to a classical charge density, and what this all says is that when the electron is definitely in one or the other state, its charge-density distribution is independent of time, which corresponds to a classical electrostatic situation.  (Note that this is true despite the kinetic energy the electron possesses).

 When, however, the electron is absorbing or emitting EM radiation, it will be, for a short time, in both states at once.  Now if you add the two states, you will find that the absolute square is no longer time-independent, but varies at a frequency 2π(ω2 - ω1), a frequency that corresponds to the frequency of the emitted or absorbed radiation.  During this time, the charge probability density is varying with time, very much like the currents in a classical antenna. As such, this gives us a method of calculating the radiation pattern, i.e., the probability that the photon will be found in the various possible directions of flight. For the purposes of this experiment, therefore, the task is to devise a first detector that will have such available electron orbits so that when the photon encounters it, the electron that it excites will conform itself to its wave pattern during the transition to the higher energy level, in such a manner as to replicate that wave pattern as nearly as the loss of energy will permit. 

Of course, when so doing, the photon must not lose all its energy to the transition process, but only a small part of it.  Thus we must contrive that the electron when being energized by the photon not simply be ejected from the detector as in a phototube.  Achieving this will likely require that the detector be carefully designed. It must absorb the photon, but then retransmit as much of it as possible on nearly the same frequency and with as near to the original wave pattern as possible.  Properly designed, the detector would be able to respond to any wave pattern (within the wavelength range of interest) that were to impinge upon it, retaining that wave pattern and re-emitting the photon with slightly less energy to continue its journey. I am not able to describe the exact details of such a detector, but believe it can be done through a system of wide-ranging electron orbits made available through a correctly structured system of conduction bands that give us electrons that have no definite position in the transverse directions, as well as slight energy gaps that can be crossed using substantially less energy than in the incident photon.  It may be that more than two such energy bands would have to be provided, and that the detector may have to have layers of differing material that would allow these processes to be carried out in sequence. It may be necessary, for example, to have an intermediate energy band whose energy is high enough to absorb most of the photon energy (so that the electron is not ejected from the material), from which the electron would then transition into the final energy band that would be just a little different than the incident energy.  But the key to everything is that all this takes place in spatial phase, so that the original wave pattern is maintained.  Doing all this may require cooling the material to a very low temperature to depopulate the higher energy levels and make them available to clearly receive the activated electron. With a first detector build on these general principles, the process should be possible.
« Last Edit: 20/05/2015 06:39:56 by Atomic-S »
 

Offline SciencyGummyWorms

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Re: A Measurement Problem Question
« Reply #26 on: 21/05/2015 01:22:51 »
Process of observation: kick it

Observation: landmine explodes

Very well said.
 

Offline lightarrow

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The position of the particle has three components.  The component in the direction from the source to the final target is indeed recorded to the precision of the position of the central detector, but the position in the other two dimensions is not.  It is those transverse measurements that matter in this experiment.
In practice you say that the particle, after the slits, crosses a plane parallel to the screen before this one, without interacting in a specific point of this plane but making the generation of a signal when it crosses it. Correct? I don't know if what you say is actually possible.

Assuming it is possible, measuring the position x of the particle means to put its state in an autoket eigenvector of position so its momentum Px (x component of momentum) is then completely undetermined in this new state.

As a result, I think, you would have an interference pattern which should be the overlap of infinite patterns of different momentum, that is colours (now I assume to use photons) each, that is, a white diffuse area on the screen.
But it's just a thought, not sure of it.

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« Last Edit: 24/05/2015 22:15:22 by lightarrow »
 

Offline PmbPhy

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Quote from: lightarrow
Assuming it is possible, measuring the position x of the particle means to put its state in an autoket of position so its momentum px is then completely indetermined in this new state.
What in God's name is anautoket??
 

Offline alancalverd

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The evil twin of an autobra.
 

Offline Bill S

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It's something you snort on your own; or possibly in your car.
 

Offline lightarrow

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Quote from: lightarrow
Assuming it is possible, measuring the position x of the particle means to put its state in an autoket of position so its momentum px is then completely indetermined in this new state.
What in God's name is anautoket??
;DYes, you're right. For some mysterious reason I made a mix between italian language and Dirac denomination! I intended "eigenvector".

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Offline lightarrow

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The evil twin of an autobra.
LOL 😂

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Offline Atomic-S

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In practice you say that the particle, after the slits, crosses a plane parallel to the screen before this one, without interacting in a specific point of this plane but making the generation of a signal when it crosses it. Correct?
Yes.
 

Offline Atomic-S

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I think, you would have an interference pattern which should be the overlap of infinite patterns of different momentum, that is colours (now I assume to use photons) each, that is, a white diffuse area on the screen.
But it's just a thought, not sure of it.
I believe your analysis is correct, as pertains to what it analyzes. However, I believe I have not described the experiment clearly enough. The purpose of the experiment is not to measure the position in the direction of propagation, although doing so to within certain margins of error would be an inevitable consequence, in the sense that if the intermediate detector detects the particle, then the particle has necessarily been found within it, hence its position has been measured. However we also need to take the time factor into account. We have absolutely no need of finding the particle's X position except to the extent of verifying that it is a single particle.  The existence, upon exiting the slits, of a well-formed diffraction pattern implies that the particle has a narrowly-constrained wavelength, and therefor a very vague position.  What we do not want to do is change that situation in the first detector.  Can we detect the particle's presence in the first detector without altering its wavelength to something highly indefinite?  Yes, if we do not insist on knowing just when the particle passes through the first detector.  Therefore, the nature of the first detector's operation will be such that we are unable to say with any precision just when the particle passes through it, but we can still affirm that a particle, and only one particle, has passed through it if the original source is very weak.  For this experiment to work, we need a source that has a narrow wavelength band but also a very low photon rate so that the probability of having more than one photon between the slits and the final detector is negligible. Under these conditions, when the first detector records action, we know a photon has passed, even though we don't have a good idea of exactly when, in relation to the recorded signal, it did so. (And the recorded signal could itself occupy many times the time required for transit from slits to final screen). The temporal uncertainty of the transit of the first detector would probably be associated with the times required for the agitated electron to jump through three energy levels.
 

Offline Atomic-S

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Assuming it is possible, measuring the position x of the particle means to put its state in an autoket eigenvector of position so its momentum Px (x component of momentum) is then completely undetermined in this new state.
No, because the time at which the position occurs remains highly indefinite.
 

Offline PmbPhy

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Quote from: alancalverd
The evil twin of an autobra.
Why can't you be serious? You waste a lot of my time with these silly remarks.
 

Offline PmbPhy

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Quote from: Atomic-S
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Assuming it is possible, measuring the position x of the particle means to put its state in an autoket eigenvector of position so its momentum Px (x component of momentum) is then completely undetermined in this new state.
No, because the time at which the position occurs remains highly indefinite.
That's incorrect and probably based on a misconception of the time-energy uncertainty principle. There is no such thing as " time at which the position occurs remains highly indefinite."  Position is measured at a specific time. That means when you measure the position of a particle you simultaneously look at the clock and record what it reads. Take a photon hitting an array of photon detectors or CCD for example. When the photon hits a "pixel" which the array of photon detectors or the CCD is composed of the time is recorded. Time is a parameter in QM, not an observable.
 

Offline lightarrow

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I think, you would have an interference pattern which should be the overlap of infinite patterns of different momentum, that is colours (now I assume to use photons) each, that is, a white diffuse area on the screen.
But it's just a thought, not sure of it.
  Can we detect the particle's presence in the first detector without altering its wavelength to something highly indefinite?  Yes, if we do not insist on knowing just when the particle passes through the first detector.
If you don't know when the particle passes, I could even say that it hasn't passed at all, yet! So, how can you say that you have measured its position?  :)

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Offline alancalverd

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Quote from: alancalverd
The evil twin of an autobra.
Why can't you be serious? You waste a lot of my time with these silly remarks.

Come on, even Dirac had a sense of humor - he nearly offered me a job!
 

Offline PmbPhy

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Quote from: alancalverd
The evil twin of an autobra.
Why can't you be serious? You waste a lot of my time with these silly remarks.

Come on, even Dirac had a sense of humor - he nearly offered me a job!
That's not merely humor. It's sarcasm, the lowest form of humor. And there's a huge difference between having a sense of humor and using it to disguise whether you're being insulting or not.
 

Offline Atomic-S

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There is no such thing as " time at which the position occurs remains highly indefinite."  Position is measured at a specific time. That means when you measure the position of a particle you simultaneously look at the clock and record what it reads.
Let's see if we can get a clearer grip on the question by attempting to be more detailed in analysis. How would the middle detector operate?  I envision three energy levels, or more exactly, bands of closely spaced levels, having electrons in the lowest. The others will be assumed to be unoccupied.  The photon comes along, hits the middle detector broadside and energizes an electron in the lowest band that most closely matches the photon's waveform. The electron is kicked into the highest band by a process that replicates that waveform spatially (in the transverse plane), temporarily absorbing the photon. Then the electron drops into the intermediate band, again replicating the wave distribution in the transverse directions, a process which re-emits the photon with the same transverse wave pattern but at a slightly different frequency.  At some time after entering the intermediate band, the detected electron is recorded by other processes, which need not take place before the photon finally reaches the final detector. During the time the photon is engaged with the electron, it is detained in the detector and thus its X position is "measured"; however if I am correct, doing so under these conditions does not permanently efface the shape of its wave function in any major way, and also does not establish with accuracy when the photon was in the detector. The fact that the photon re-emerges in substantially its original form, particularly as to still having a narrow bandwidth, has to do with the nature of the absorbtion and re-emission processes, which are assumed to take place between electron states having narrowly-defined energies. But to have such energies, the electron states have narrowly-defined frequencies, meaning that the lifetimes of the states must be long compared to one cycle of the photon.  But that means that the photon will be trapped within the detector for a while, not being re-emitted instantly. In this way, the photon's having a narrowly defined x coordinate is reconciled with its retaining its narrow bandwith and being re-emitted with close to its original wavelength, made possible by the indefiniteness of the time of its presence in the detector that is created by the accurately defined frequencies of it and the electron.
« Last Edit: 29/05/2015 05:16:33 by Atomic-S »
 

Offline PmbPhy

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Quote from: Atomic-S
How would the middle detector operate?
What middle detector? Your writing is extremely dense and as such very hard to read and recall. Therefore you can't expect people to recall what it is that you're referring to days after they've made their last post. We're carrying on several conversations at once in multiple forums so please don't expect everyone to recall exactly what your set up is. Please provide a simple description of the set u before you start the explanation, okay? Thank you my friend. :)

I read what you wrote below and it's all confused. It appears as if you have several misconceptions about quantum mechanics. First of all you said "The photon comes along, hits the middle detector " but you didn't tell us anything about what this detector is and why you refer to it as the "middle" one. Then you started talking about electrons in bands and photon waveforms. What are these "bands" that you're referring to? It sounds like you're referring to the band theory of solids. Is that it? And apparently you're under the wrong impression that photons have a wavefunction.
See: http://en.wikipedia.org/wiki/Photon

And the term "waveform" is never used in quantum mechanics so I don't know what you mean by it.

Quote from: Atomic-S
The electron is kicked into the highest band by a process that replicates that waveform spatially (in the transverse plane), temporarily absorbing the photon.
What exactly do you mean here? Electrons don't absorb photons. Only a system of charges like a solid or an atom/molecule can do that. Is that what you're referring to?

Quote from: Atomic-S
Then the electron drops into the intermediate band, again replicating the wave distribution in the transverse directions, a process which re-emits the photon with the same transverse wave pattern but at a slightly different frequency.  At some time after entering the intermediate band, the detected electron is recorded by other processes, which need not take place before the photon finally reaches the final detector.
All of that makes no sense. Photons are never "re-emitted". A photon is either emitted or absorbed. Nothing in between. It can never meaningfully be said that the same photon is re-emitted after it's absorbed. And what is this wave form that you're referring to? Waveform of what?

Quote from: Atomic-S
During the time the photon is engaged with the electron, it is detained in the detector and thus its X position is "measured";
A photon is never "engaged" with an electron for any period of time. Also photons cannot be detained.

Quote from: Atomic-S
The fact that the photon re-emerges in substantially its original form, ..
I'm sorry but when you say things like this it only tells me that you don't know quantum mechanics in the sense that your knowledge is seriously and almost hopelessly flawed. Where/how did you come to know what you do about quantum mechanics?
 

Offline Atomic-S

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Further explanation:

Figure 1:  The photon has passed through the slits (located at the left and out of the picture), and is described by its "waveform", by which I mean its state function Ψ(x,y,z,t) whose absolute square at any point is the probability that a photon would be found there, which is represented by the orange wavefronts having come from the slits, and is travelling toward the middle, or preliminary, detector (which sits between the slit plate and the final screen or film), which can be thought of as a wafer that contains three energy bands.  In the diagram, the detector is represented in cross-section so that it appears as lines, though in practice it would be  a plate in which the three energy bands would be effected by three physical layers, and that is the way it is represented in the diagram.  The layer nearest the incoming photon has the lowest energy level, and is occupied by electrons. The middle layer has the highest potential energy level (the energy that an electron would acquire if present) but is empty. The final layer is likewise empty but has a potential energy that falls between the other two.



« Last Edit: 16/06/2015 06:47:37 by Atomic-S »
 

Offline Atomic-S

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Figure 2:

The photon now interacts with the intermediate detector through its state function, shown in orange, entering the first layer. There it finds the electron that it most closely matches, in terms of the way the electron's state function lays the electron out spatially compared to the spatial distribution  of the photon's state function.  Because of the agreement in spatial layout between the photon's and the electron's state functions, the electron is excited and begins transitioning into the middle layer by raising its own (electron's) frequency (the temporal part of its state function) by an amount that equals the frequency of the photon.  In so doing, the electron effectively becomes an antenna that absorbs the photon, having an oscillating charge-probability density made possible by being in the combined state that consists of its initial state in the first layer and its final state in the second layer. An essential feature of the process is that the electron occupies both layers simultaneously for a short while.   
 

Offline Atomic-S

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Figure 3:

Eventually the electron has transitioned to the middle layer. Its state in the middle layer replicates the state it had in the first (left) layer except that the temporal part of the function is at a higher frequency, because the energy is higher by the amount absorbed from the photon. That is important, because by being such, the state preserves the original spatial information, from which the diffraction pattern will be recovered. That is, the absorbtion of the photon by this process did not collapse its wave function in such a way as to localize it in the transverse plane.

 The electron will then begin a second transition, into the right layer, dropping in energy. Actually, this process may well begin before the absorbtion process is fully completed, so that for a brief moment there could be some photon energy still present within the system, represented by the orange and red lines.  Whether in this form or in a fully excited electron in the middle plane, the energy is contained within the wafer and thus the x position of the photon can be said to have been determined to within the thickness of the spacing between layers.  But note that this does not mean that the momentum, and therefore wavelength, of the emitted photon will be indeterminate. The reason it won't be is because the excited electron has a well defined frequency of state-function oscillation, from which a definite wavelength will be recovered.
 

Offline Atomic-S

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Figure 4:

The electron then transitions into the exit (right) layer by a process similar to the earlier absorbtion, by entering a mixed state in which it  temporarily has significant probability to be found in either the middle or right layer. The right layer has a lower potential energy, causing, again, a beat frequency between the two states, and therefore an oscillating charge-probability density. Correspondingly, a photon wave function corresponding to that, as per the antenna principle, is created, and the photon is sent on its way (red wavefronts). The spatial information is preserved as before, causing the photon state function to have the original pattern it did when it had arrived at the wafer, and therefore the interference information is preserved intact.
 

Offline Atomic-S

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Figure 5:

The photon, represented by its state function having the diffraction information (red), continues on to the conventional screen, forms a probability pattern based on the fringes, and is detected as a specific spot (bronze color). That detection in itself does not prove that the fringe patterns are present, but we will repeat this experiment many times and establish the fring pattern by statistics.
 

Offline Atomic-S

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Figure 6:

After the one specific photon in question has been detected, the right layer of the wafer is accessed and the electron, if present, is read out.  The fact that the electron was found to be present demonstrates that the photon passed through the wafer.  If no electron had been found on read-out, that would tell us that no photon had passed through.  Note that the verification that the photon was in the wafer does not occur at a specific instant of time, and therefore has nothing to do with uncertainties as to its energy and frequency.  Indeed, as noted with respect to Figure 3, the photon or its equivalent energy is expected to dwell for a short while in the wafer, which is how a definite x position for it (at that time) can be reconciled with preserving a definite exit wavelength.
 

Offline sunshaker

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Atomic-s I've only skimmed trough thread, is this the experiment you are trying to explain?

http://www.sciencealert.com/reality-doesn-t-exist-until-we-measure-it-quantum-experiment-confirms?utm_content=bufferdc7d6&utm_medium=social&utm_source=twitter.com&utm_campaign=buffer [nofollow]
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They then randomly added a second grating that recombined the paths, but only after the atom had already passed the first grating.

When this second grating was added, it led to constructive or destructive interference, which is what you'd expect if the atom had travelled both paths, like a wave would. But when the second grating was not added, no interference was observed, as if the atom chose only one path.

The fact that this second grating was only added after the atom passed through the first crossroads suggests that the atom hadn't yet determined its nature before being measured a second time.
 

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