In quantum mechanics, does the observation preceed the event?

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Offline PeteJ

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Hi folks. My first post here. I have a question, most probably a naïve one...

In QM, whether we favour MW or CI as an interpretation, there is the idea that an observation causes an event, either a 'collapse' or a splitting of worlds.

Does the observation preceed the event, or is it simultaneous with the event?   

Or something else..?

Thanks.




« Last Edit: 08/05/2015 15:50:49 by chris »

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Offline alancalverd

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Re: A Measurement Problem Question
« Reply #1 on: 08/05/2015 12:06:20 »
An observation cannot logically precede the observed event, and given the finite speed of light (which defines the limiting speed for transmission of information) it cannot even be simultaneous.

It is important to distinguish between the process of observation, which could indeed precipitate an event (say by a photonuclear interaction), and the resulting observation itself (an emitted electron or whatever).  Bouncing hypothetical photons off particles does indeed lead to Heisenberg's indeterminacy equation but isn't the cause of indeterminacy.

Beware of confusing a mathematical model (wave function collapse, world splitting, whatever) with reality. The model is merely our best shot at predicting the outcome of a future event.
helping to stem the tide of ignorance

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Offline PeteJ

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Re: A Measurement Problem Question
« Reply #2 on: 08/05/2015 12:12:07 »
Thanks Alan, very clear.

It seems to me that you're right. But it doesn't quite answer my question.

If the observation is not prior to the event and not simultaneous, then what?

How can the process of observation be different from the observation?

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Offline alancalverd

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Re: A Measurement Problem Question
« Reply #3 on: 08/05/2015 12:22:58 »
Process of observation: kick it

Observation: landmine explodes
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Offline PeteJ

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Re: A Measurement Problem Question
« Reply #4 on: 08/05/2015 13:28:48 »
Hmm. That does not help. The landmine explodes well before we observe the explosion, and if we sense the kick connecting then that is an observation.

I know my question is naïve but it's proving difficult to find an answer. I've tried elsewhere but had no luck.

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Offline David Cooper

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The following is speculation:-

What I suspect happens is that the wavefunction is only able to be sustained up to a certain level of complexity beyond which it must burst (simplify). By creating data in a brain or computer to represent what is observed, the complexity increases because the computer has to be maintained in multiple states to maintain the data in multiple states, so that is where the simplification is most likely to be forced to take place, after which a matching simplification will be forced to take place in the original object being observed to match up to the data which describes it.

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Offline lightarrow

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If the observation is not prior to the event and not simultaneous, then what?
Usually with "event" in this context, in the sense of "measure event", we mean the physical interaction with a measure device. "Observation" is the subjective act of take awareness of the event from a sentient being.
The first can come without the (unnecessary) second.
Someone has speculated on the possibility that the act of observation could modify the wavefunction, but it's not the main idea between physicists.
Physics, in my opinion, shouldn't be affected by subjective acts, so only the physical measure has a meaning.

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lightarrow
« Last Edit: 08/05/2015 18:07:13 by lightarrow »

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Offline chiralSPO

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I agree with lightarrow here--I do not think (at least it hasn't been shown) that there is any requirement for a human or computer, or any other entity to "observe" a system for decoherence (wavefunction collapse) to occur. Only an interaction (often called measurement, but doesn't have to involve any type of measurement) is required.

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Offline David Cooper

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When are measurements made without the data ever being processed by a computer or mind? Up until the point where they are processed, the wavefunction could continue to represent multiple possible realities without simplifying. If you then look at the results and that forces a simplification, how are you going to know if you caused that collapse or if it happened back at the time of the measurement? Indeed, it may be that a computer or brain isn't always adding sufficient complexity to cause the collapse, so it might maintain two possible states of that computer/brain which both believe there has been a collapse of the wavefunction when it has not actually occurred yet.

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Offline PeteJ

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Great. This is getting interesting.

David - I like your idea and haven't come across it before,. Are you suggesting that there is a law of conservation of complexity? .

Lightarrow - I get the picture and do not see it differently. But I don't think the question of whether only the measurement has meaning is a free decision for physicists. To say that only unobserved events have meaning and then they lose it when they are observed would be a bit odd, and not the usual meaning of 'meaning'.

The thing is, your description leaves a finite time between the event and the observation, and this what I'm exploring. But then...

chiralSPO - You say that an observation is unnecessary, that the event (the detection by the detector) is enough for decoherence. This is not my understanding of things. It would answer my question if it is true, but is it true? It doesn't seem to match with the mainstream ideas that I read about. 

Is it widely agreed that no sentient observation is necessary for decoherence? 
   

   


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Offline chiralSPO

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When are measurements made without the data ever being processed by a computer or mind? Up until the point where they are processed, the wavefunction could continue to represent multiple possible realities without simplifying. If you then look at the results and that forces a simplification, how are you going to know if you caused that collapse or if it happened back at the time of the measurement? Indeed, it may be that a computer or brain isn't always adding sufficient complexity to cause the collapse, so it might maintain two possible states of that computer/brain which both believe there has been a collapse of the wavefunction when it has not actually occurred yet.

I have to dig around to find it, but I'm pretty sure that there have been experiments that show, very explicitly, that a wavefunction has collapsed by looking at successive interactions of a particle--for the first few, it behaves as a wave, and then, after a sufficiently disruptive interaction, it behaves as a particle...

Also, recall that A) nuclear magnetic resonance spectroscopy relies on the half life of coherent quantum states of populations of particles; and B) one of the most difficult obstacles to practical quantum computing is spontaneous decoherence/collapse.

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Offline PmbPhy

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Quote from: PeteJ
Hi folks. My first post here. I have a question, most probably a naïve one...
Welcome to the forum.

Quote from: PeteJ
In QM, whether we favour MW or CI as an interpretation, there is the idea that an observation causes an event, either a 'collapse' or a splitting of worlds.
Where did you get that notion from? When you make a measurement of a system which is in an eigenstate of the system. E.g. if the system is in an eigenstate of energy and you measure the energy then all you get is a measurement of the energy, period. Nothing happens to the system. If the system is not in an eigenstate of the observable you are measuring then when you measure that observable you'll get one of the eigenvalues of the observable. That's all. You didn't cause anything physical to happen. If you write down the expression for the system in an arbitrary quantum state and then measure an observable then the mathematical expression for the system will change to an eigenstate of the observable that you measured. But again, nothing physical happens to the system other than it now being in an eigenstate. But you can't observe the quantum state. It's simply a mathematical expression which represents the system. It's a misconception to think that something physical happens when the system collapses. That's merely mathematical lingo to describe what's going on with the mathematical description of the probabilities of measuring observables.

Quote from: PeteJ
Does the observation preceed the event, or is it simultaneous with the event?
Carefully study what I wrote above and you'll understand that this question is meaningless.

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Offline alancalverd

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If you could obtain information about an event before it occurred, you could prevent or modify it, thus invalidating the information you used, ad infintum.....
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Offline evan_au

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Quote from: ChiralSPO
the event (the detection by the detector) is enough for decoherence.
Quote from: PeteJ
This is not my understanding of things.
I understand that one of the barriers to quantum communication is tiny imperfections in the optical fiber, which result in the entangled photons (rarely) interacting with one of these imperfections.

Nobody is observing all the imperfections in the fiber, and nobody (and no test equipment) has seen that the photon has struck a particular imperfection, but the photon is scattered, and coherence is lost.

So decoherence can be triggered by interaction with the environment.

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Offline PeteJ

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Okay. Thanks. Things are becoming more clear. I'm confused as to how the subject changed to a question that isn't mine but no matter.

PmbPhy - You may be giving the answer to my question but I must admit I don't fully understand it. You seem to be saying that the measurement has no effect on anything except our knowledge of the system.
Is that it?   

Evan-au - says that decoherence can be triggered by the environment, but if decoherence is only about  our knowledge of the system then how can this be?   

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Offline PmbPhy

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Quote from: PeteJ
PmbPhy - You may be giving the answer to my question but I must admit I don't fully understand it. You seem to be saying that the measurement has no effect on anything except our knowledge of the system.
Is that it?
Sort of. If you're confused then I'd have to say - Join the club! [;D]

Here's what is referred to as the orthodox position: Suppose you have a system which is an particular quantum state which isn't an eigenstate of any observable. Before a measurement of, for example, the position of a particle, is made then The particle wasn't really anywhere.

As Pascual Jordan put it: “Observations not only disturb what has to be measured, they produce it….We compel [the electron]
to assume a definite position…. We ourselves produce the results of measurements.”

If a particle assumes a specific position in space then it might be said that the measurement process "caused" it to happen. If you wish to look at it that way then, in this particular example, cause and effect are simultaneous.

The quote above was taken from the article called Is the Moon There When Nobody is Looking? by N. David Mermin, Physics Today. It's available online at: http://maltoni.web.cern.ch/maltoni/PHY1222/mermin_moon.pdf

You might want to read it. It might help you to understand what you're seeking to learn.

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Offline David Cooper

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David - I like your idea and haven't come across it before,. Are you suggesting that there is a law of conservation of complexity? .

I remind you that what I said is pure speculation, and I'm not suggesting anything about conserving complexity beyond the idea that there may be some limit to how much complexity a wavefunction can maintain before it collapses, but that detecting where that limit is reached may not be possible. If we take Schrödinger's cat as an example, it has not been ruled out that the wavefunction of the cat may be compatible with it being dead and alive. A collapse of the wavefunction could simplify it to eliminate one of those two possibilities, and it may be that the cat is sufficiently complex to force that collapse all by itself, long before anyone looks in the box to see if the cat's dead or alive. When a person looks in the box, again there's the question of whether that person can force the collapse of the wavefunction - it may be that (s)he can't, in which case the wavefunction of the person may have to remain compatible with two contradictory outcomes, one of which registers the cat as alive while the other registers it as dead. At some later point, maintaining the two possibilities may no longer be possible due to there being more complexity than the universe can handle, at which point one of the possibilities must be junked and the cat is then either dead or alive, no longer maintained in an uncertain state where it may yet become either, and at that point the wavefunction of the human will simplify to match (if it didn't simplify first, which is probably more likely), leaving us with a human which knows whether the cat is alive or dead rather than incorrectly thinking (s)he knows while actually maintaining both contradictory beliefs. What we certainly don't see is a human opening the box and finding a cat that (s)he determines is both alive and dead at the same time, but that does not guarantee that the wavefunction has collapsed at that point.

What we do see though are experiments where matter can be seen to be maintaining two contradictory states at the same time, though they aren't really contradictory as the matter hasn't actually been forced to simplify what it's doing. That really is like looking at a cat and seeing it as both dead and alive at the same time, only it hasn't yet been done with anything as complex as a cat. The particular experiment I have in mind involved an item made out of several hundred atoms which vibrated from side to side, but it did so by moving from left to right while at the same time moving from right to left, etc. That experiment showed the effect in something much more complex than a single particle and it opened up the possibility of doing the same thing with bigger, more complex items which are doing more complex things. Perhaps it can be extended to a cat, but perhaps not, and we may never know.

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Offline chiralSPO

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Schrödinger's cat is a terrible thought experiment, and I wish it were not so overused. Schrödinger originally proposed this experiment to expose how ridiculous QM logic is on a macroscopic scale.

Forget about whether or not the cat is complex enough to disrupt the wavefunction, whatever device measures whether or not the quantum event has happened and tells the poison to be released or not has already collapsed the wavefunction.

There are different interpretations of what the wavefunction actually represents, if it is a real thing, or just a useful model etc. Ultimately QM tells us what is knowable and what is unknowable (given our current understanding, but it has yet to fail us)--but this does not mean that we can actually measure anything that is theoretically knowable... Unfortunately the question of whether QM mechanisms are actually deterministic or probabilistic is fundamentally unknowable and cannot be probed.

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Offline PeteJ

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I wonder why the thread question has been changed. The OP said nothing about the event preceeding the cause. Not sure I'm happy about this. 

Quote from: PeteJ
PmbPhy - You may be giving the answer to my question but I must admit I don't fully understand it. You seem to be saying that the measurement has no effect on anything except our knowledge of the system.
Is that it?
Sort of. If you're confused then I'd have to say - Join the club! [;D]

Here's what is referred to as the orthodox position: Suppose you have a system which is an particular quantum state which isn't an eigenstate of any observable. Before a measurement of, for example, the position of a particle, is made then The particle wasn't really anywhere.

As Pascual Jordan put it: “Observations not only disturb what has to be measured, they produce it….We compel [the electron]
to assume a definite position…. We ourselves produce the results of measurements.”

If a particle assumes a specific position in space then it might be said that the measurement process "caused" it to happen. If you wish to look at it that way then, in this particular example, cause and effect are simultaneous.

The quote above was taken from the article called Is the Moon There When Nobody is Looking? by N. David Mermin, Physics Today. It's available online at: http://maltoni.web.cern.ch/maltoni/PHY1222/mermin_moon.pdf

You might want to read it. It might help you to understand what you're seeking to learn.
Your summary here is just fine and very helpful. I'm not really trying to learn QM but, rather, what physicists think about QM. I think you've answered my question. If it is allowable that the measured particle is not really anywhere until the measurement and is 'reified' by the measurement then the problem I was investigating evaporates. I had thought that physics rejected this idea since it would lend credence to idealism of some sort and also mad ideas like David's  [;D]

... Which I suspect has some truth in it. It is not quite the same idea, but it seems to make some sense that stable complexity requires a material substrate. I.e that mind requires matter. It's a pet theory. 

No need to go there though. Many thanks for all the excellent replies. On one science forum I know this discussion would have descended into a raging argument within five posts.     
 


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Offline PmbPhy

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Quote from: PeteJ
Your summary here is just fine and very helpful.
Wonderful. I'm so glad to hear that you've got your question answered to your satisfaction.  [:D]

Quote from: PeteJ
On one science forum I know this discussion would have descended into a raging argument within five posts.     
That's why when I opened my own company I added physics forums to my companies website. Membership is by invitation only. I wait until I get to know a member in one of the forums I frequent and if they have what it takes to be in my forum I send them an invitation. It's simple. I do my best to try to predict whether the person will be able to respect and follow the forum rules. If the person objects to the rules then they're not invited. If they're invited and they join then we can discuss whether any changes should be made to the forum rules. I'm very good about that and am always looking for input from members. Problems sometimes arise when a member doesn't like a rule and then quits because of it. The problem with that is that it was their responsibility to discuss their objections to the rules until we're both satisfied. It's an ongoing process to, not a one time thing. So if I enforce a rule and the member convinces me and other members of the forum that the rule was wrong or didn't apply then I undo the enforcement. The rules are what one typically finds in all forums except that I strictly enforce them.

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Offline Atomic-S

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As Pascual Jordan put it: “Observations not only disturb what has to be measured, they produce it….We compel [the electron]
to assume a definite position…. We ourselves produce the results of measurements.”
.
I find this an interesting concept, which brings up the following question regarding the double-slit experiment. As it is typically conducted, a particle is sent through two parallel slits, behaving wave-like, during which, I suppose, it has no position.  Then it hits a screen as, for example, photographic film, is detected at a specific position, and is deemed to be a particle having that position.  I have wondered if there is any way of detecting it that does not demand that it assume a definite position, and if so, that would be a significant experimental breakthrough showing that a particle indeed not need be in a definite position to be a particle.  It would appear that the nature of the detector is significant in this regard. When photographic film is placed there, it becomes part of the experiment. What if we used something else?  We would have to be clever about what it was in order to elicit the desired result. We want something that would indicate the presence of the particle, but in a way that did not demand that it assume a definite position.  If we were able to do that, it suggests the possibility that the particle could continue to travel beyond there still following much of its original wave configuration, that would become evident in a second detection of some kind. With the conventional double-slit experiment, particles hit the detector with a distribition governed by their wave nature up to that point, but whemn each hits, it is recorded in a specific place, and as such, now has a state function that is different, so that (assuming the particle has enough energy to continue on from that point), its subsequent behavior should be different than its original wave distribution indicated. The point is t hat the first detection, by requiring it to assume definite position, has altered its subsequent behavior.  So the objective would be to show that if we could detect its presence in a way that did not require it to assume a definite position, then its subsequent behavior would be much less disrupted and would largely be the same as its earlier wave function would indicate.  Can such an experiment be done?  The first thing that comes to mind as a possible way of doing so is to change the chemistry of the detector so that it is characterized by large molecules having electron orbits that run throughout their entire length and thus do not have narrowly localized locations. You would also want to try to set this up so that the molecules had a dearth of closely-confined electron orbits that would be sensitive to the particular particle to be detected.  The result would be that when the particle interacted, an electron from a wide-ranging orbit would be kicked up into another higher-energy wide-ranging orbit, which could triger other phenomena that would be recorded.  But because the process did not pin the detected particle to a particular location, its wave function would not have "collapsed" as much as would ordinarily be the case, allowing the particle, with such residual energy as it had, to proceed further on and be detected a second time by a conventional detector. The result of that would be, executed on many particles, a diffraction pattern more closely resembling what would have come out of the slits (probably some weak fringes), rather than what would have come out of a narrow-position detector (probably diffuse). 
 

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Offline PmbPhy

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Quote from: Atomic-S
I find this an interesting concept, ...
And it might be interesting to read if you didn't write it as one long sentence. It's easier to read when you break things up into paragraphs. If you choose to do so I'd like to read what you wrote. Otherwise it's a strain on my eyes.

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Offline Atomic-S

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To make a long story short, we do the double slit experiment as follows: There are the slits, and there is a conventional detection screen. It produces an indication of where each particle hits. Between the slits and the screen we place another detector.  It is to detect the passage of the particle without detecting its position.  When the particle passes through it, its presence is recorded but its position is not. Because its position was not, it remains indefinite.  The purpose is to test to see if, by leaving it indefinite, the wave characteristics that emerged from the slits remain in force.  We find the answer to that by allowing the particle to strike the final detection screen and record a position. We repeat the experiment with many particles. If the statistical pattern on the final detection screen is consistent with the diffraction expected from the slits, then we would know that the wavelike behavior was not nullified by the earlier detection as a particle.  That would be interesting because this experiment would have demonstrated an object possessing both particle-like and wave-like properties at the same time.

The detector to be first encountered might be constructed of a semiconductor having two conductive energy bands separated by a small energy difference.  Because such energy bands are characterized by electrons free to wander extensively,  their electrons are of indefinite position. If the energy difference is substantially less that that of the incident photon (for example), the amount of energy lost from the photon when it kicked an electron from the lower band to the upper band would not greatly affect it, so that as it proceeded from that point on toward the final screen, the photon's wavelike character would undergo only limited change. This change would probably appear as an increase in wavelength. The result of that could be that the diffraction pattern would widen a little.  But the experiment should still be doable.

 

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Offline PmbPhy

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Quote from: Atomic-S
To make a long story short, we do the double slit experiment as follows: There are the slits, and there is a conventional detection screen. It produces an indication of where each particle hits.
So far so good.

Quote from: Atomic-S
Between the slits and the screen we place another detector.  It is to detect the passage of the particle without detecting its position.
What you just said is impossible. One can't merely make such statements and expect them to be taken as a valid statement. You have to state exactly how such a thing is possible by stating at least one way on how to do it. And detecting a passage of something means that you've taken a position measurement. You don't appear to understand that those two things are identically the same thing.

Quote from: Atomic-S
  When the particle passes through it, its presence is recorded but its position is not.
Again, impossible. To demonstrate what I mean please draw a diagram of what you're saying and the actual physics of how it would be accomplished.

Things like this are the main problems with ideas in this forum. The ideas presented are so vague as to appear possible but so vacuous that it can't be accomplished in practice.

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Offline Atomic-S

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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.
 

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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 »
 

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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.

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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.

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

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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??

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Offline alancalverd

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The evil twin of an autobra.
helping to stem the tide of ignorance

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Offline Bill S

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

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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??
[;D]Yes, 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.
 

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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.
 

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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.
 

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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.

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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.

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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!
helping to stem the tide of ignorance

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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.

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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 »
 

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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?

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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 »
 

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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.   
 

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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.[attachment=19749]
 

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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.
[attachment=19751]
 

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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.
[attachment=19753]
 

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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.
[attachment=19755]
 

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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
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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.