What qualifies as an observer in quantum mechanics?

[DennisN]

It is a philosophy orientated book, written by a physicist.
https://www.amazon.com/dp/0691095515/?tag=pfamazon01-20

Well, I was confused by the quote since it mentions a "rock"; a rock can not pop in and out of existence like that. That would obviously mean faster-than-light travel, and this is not possible according to special relativity, which you might be aware of. The author may have tried to use a figure of speech, I don't know. But anyway, I do not know if you were quoting or simply recalling those words? Were these the exact words in the book? (By the way, there are no experimentally confirmed violations of special relativity).

 

[meBigGuy]

That's like saying brownian motion can evacuate all the air molecules in front of your face and you suffocate.

Jumping from 1 to 2 can't be instantaneous by relativity.
I have a problem with it though.
Think about it. The rock has to be in a superposition of 1 and 2 while completely entangled with its environment (that is, the environment has to "agree" it could be 1 or 2) and then something happens that says it has to be in 2. But that means it was never really in position 1, but a superposition.

I don't think it can just be in one and suddenly the universe thinks it should have been in 2. The air molecules, photons of reflected light, pressure on the ground, all of that has to be in superposition and then it was never really in 1.

That is, the environment, the universe, has to be entangled with the rock in a way that such a thing would happen.

Word soup --- sorry.

 

[DennisN]

Oops, StevieTNZ, you have to excuse me that I missed that you actually linked to an article in post #30 which was about the experiment I posted about above in post #57. I don't know why I missed that post of yours, it was an excellent link! EDIT: I've added your link to my post.

 

[Prathyush]

I'm not sure what you mean by "the degrees of freedom for the state of the particle become reduced". Do you mean going from a superposition of spin-up and spin-down to definitely spin-up or definitely spin-down? If so, that's what people mean when they say that the wave function has "collapsed". The question is: what caused such a collapse?

What is the physical mechanism by which knowledge is obtained?
The Wavefunction of an election describes the past History a particle. When A Measurement Apparatus Interacts with a particle, It forms an entanglement, which serves as the record. Now what does it mean the particle Has been Recorded onto a Device. It means all future records made will be entangled with the past record.

The State of the record serves to define the State of the particle, for any further experimentation on the particle. Note that any further observation of the record,involves formation of more records by any physical mechanism chosen. These records will also be entangled with the state of the particle.

Is record Permanent? How is it consistent this unitary evolution? Can a record be destroyed and the particle state be restored?
When a record is made, Multiple copies of it are made, each time you observe it and so on. For instance shine light on the record, then the photons are entangled with the state of the Record. It is clear that one can only restore the state, if all the copies of the record are collected and then evolved back using the correct unitary evolution, Usually in practice its not possible but still it has been observed in experiments like the Quantum Eraser experiment.

 

[stevendaryl]

What is the physical mechanism by which knowledge is obtained?

Wave function collapse is not exactly the same as acquiring knowledge. That's the lesson of decoherence. If a particle's state becomes entangled with that of the environment, the quantum effects of interference between alternative states disappears. In effect, the wave function has collapsed. But nobody necessarily acquired any knowledge about the particle.

The Wavefunction of an election describes the past History a particle. When A Measurement Apparatus Interacts with a particle, It forms an entanglement, which serves as the record. Now what does it mean the particle Has been Recorded onto a Device. It means all future records made will be entangled with the past record.

The State of the record serves to define the State of the particle, for any further experimentation on the particle. Note that any further observation of the record,involves formation of more records by any physical mechanism chosen. These records will also be entangled with the state of the particle.

Is record Permanent? How is it consistent this unitary evolution? Can a record be destroyed and the particle state be restored?
When a record is made, Multiple copies of it are made, each time you observe it and so on. For instance shine light on the record, then the photons are entangled with the state of the Record. It is clear that one can only restore the state, if all the copies of the record are collected and then evolved back using the correct unitary evolution, Usually in practice its not possible but still it has been observed in experiments like the Quantum Eraser experiment.

I think that's right. Measurement or memory involves something like entanglement between the past and the present state. As I said in a previous post, I think that this requires irreversibility.

 

[Fiziqs]

Jumping from 1 to 2 can't be instantaneous by relativity.
I have a problem with it though.
Think about it. The rock has to be in a superposition of 1 and 2 while completely entangled with its environment (that is, the environment has to "agree" it could be 1 or 2) and then something happens that says it has to be in 2. But that means it was never really in position 1, but a superposition.

I don't think it can just be in one and suddenly the universe thinks it should have been in 2. The air molecules, photons of reflected light, pressure on the ground, all of that has to be in superposition and then it was never really in 1.

But doesn't HUP say that you can never know both the position and momentum of a particle at the same time? So on a very micro scale isn't the rock always in a superposition of these two states? Does the wave function ever really collapse, or does its uncertainty just get transferred from one property to another? So while on a macro scale it may look like the position and momentum of the rock are pretty darn certain, on a micro scale they're not.

For that matter isn't every particle in the universe in a constant state of superposition? Solid and substantial in appearance, but at the same time ethereal in nature? Is the world really as tangible as it appears?

I realize that this sounds extremely metaphysical, but isn't that an unavoidable consequence of HUP.

Either HUP is wrong, or reality isn't as solid as it appears.

If a particle's state becomes entangled with that of the environment, the quantum effects of interference between alternative states disappears.

Do they?

 

[stevendaryl]

(In response to my statement: "If a particle's state becomes entangled with that of the environment, the quantum effects of interference between alternative states disappears.")

Do they?

Well, a rule of thumb for using quantum mechanics is that you only see interference between two intermediate states if they both lead to the SAME final state. But when a particle has interacted with the environment, the environment is subtly changed by the state of the particle in a way that can't easily be reversed. (The effects spread out at the speed of light and are soon way beyond the boundaries of your experimental setup.) So in these circumstances, there usually cannot be a single final state that is consistent with both alternative intermediate states.

 

[Fiziqs]

Well, a rule of thumb for using quantum mechanics is that you only see interference between two intermediate states if they both lead to the SAME final state. But when a particle has interacted with the environment, the environment is subtly changed by the state of the particle in a way that can't easily be reversed. (The effects spread out at the speed of light and are soon way beyond the boundaries of your experimental setup.) So in these circumstances, there usually cannot be a single final state that is consistent with both alternative intermediate states.

Thank you for the clarification. Nice explanation, with which I wholeheartedly agree. As you point out "you only see interference between two intermediate states if they both lead to the SAME final state." Thus most particles never exhibit quantum effects even if they are in a quantum state.

Again, thanks for the clarification.

 

[Fiziqs]

But when a particle has interacted with the environment, the environment is subtly changed by the state of the particle in a way that can't easily be reversed.

I know that the idea of irreversibility is a popular one, and I do for the most part agree with it, but I'm not sure that irreversibility is the actual determining factor.

Well, a rule of thumb for using quantum mechanics is that you only see interference between two intermediate states if they both lead to the SAME final state.

I kind of think that this is the relevant factor. If the final state is such that it precludes either intermediate state, then only the intermediate state which leads to the final state will be observed. The final state may indeed be reversible, and as such altering the final state may alter the observed state, but I don't think that this is the determining factor. To me the deciding factor is whether the final state precludes one or the other of the intermediate states. Regardless of whether the final state is reversible or not.

 

[StevieTNZ]

Well, I was confused by the quote since it mentions a "rock"; a rock can not pop in and out of existence like that. That would obviously mean faster-than-light travel, and this is not possible according to special relativity, which you might be aware of. The author may have tried to use a figure of speech, I don't know. But anyway, I do not know if you were quoting or simply recalling those words? Were these the exact words in the book? (By the way, there are no experimentally confirmed violations of special relativity).

It is information that cannot travel faster than light, as Brian Cox points out in his book.

According to the Schrodinger equation, at t=0 a micro (and in principle macro) system is at position 1. Then at t=1, it is potentially everywhere else in the universe (until observation occurs). That's why we can observe the rock to be at position 1 at t=1, and then at position 2 at t=2, because at t=2 it is a potentiality to be there.

 

[DennisN]

It is information that cannot travel faster than light, as Brian Cox points out in his book.

According to the Schrodinger equation, at t=0 a micro (and in principle macro) system is at position 1. Then at t=1, it is potentially everywhere else in the universe (until observation occurs). That's why we can observe the rock to be at position 1 at t=1, and then at position 2 at t=2, because at t=2 it is a potentiality to be there.

Ok, it was a quote regarding the wave function (as I suspected). As long as the velocity of any information (incl. the velocity of a "rock" or any massive particle) does not exceed the speed of light, I'm fine, and so is relativity.

 

[meBigGuy]

According to the Schrodinger equation, at t=0 a micro (and in principle macro) system is at position 1. Then at t=1, it is potentially everywhere else in the universe (until observation occurs). That's why we can observe the rock to be at position 1 at t=1, and then at position 2 at t=2, because at t=2 it is a potentiality to be there.

I have trouble with the words here. If at t=1 you observe it at position 1 it is entangled with the observer. It cannot then just appear at 2 unless it (and the observer) agree was in superposition of 1 and 2 in which case it was not at position 1, but potentially at 1 and 2. Or am I missing something.

 

[StevieTNZ]

I have trouble with the words here. If at t=1 you observe it at position 1 it is entangled with the observer. It cannot then just appear at 2 unless it (and the observer) agree was in superposition of 1 and 2 in which case it was not at position 1, but potentially at 1 and 2. Or am I missing something.

It is still in a potentiality to be at position 1 and 2 at t=1, even if its observed to be a position 1 at that time. That's because no collapse of wave function occurs, in accord with the Schrodinger's equation. Therefore at t=2, it can stay at position 1 where it is observed to be, or go to position 2, if the observer is observing that position.

 

[meBigGuy]

It seems it can show up at 1 or 2, but not 1 then 2 if it was observed at 1. How is the entanglement by observation at 1 disentangled or rendered non-existant?

 

[StevieTNZ]

If that was the case, then nothing would move.

 

[meBigGuy]

That's different. The application of other influences changes everything. I don't think that is what was initially being claimed, or at least implied. It won't move from 1 to 2 in absense of a force in conjunction with the environment.

 

[StevieTNZ]

The author of Quantum Philosophy produces that example as it is possible, in principle, according to QM. No force is needed to move the rock from position 1 to 2. Roland claims it is a tunneling effect, with a very small probability (but not zero) of occurring.

I invite the honourable member to read the book, in particular pages 190-193.

 

[Ken G]

I would say that focusing on either decoherence, or even the coupling of two quantum systems, repeated for Avogadro's number of interactions, is still not the issue that separates the interpretations, and really has nothing to do with the measurement problem. As bhobba and stevendaryl pointed out, it's just basic quantum mechanics that a quantum system will become entangled with its environment, and if we average over all the information in the environment that we, the physicists, are simply not choosing to track, then of course we get a mixed-state treatment of the result. The mixed state can then be used to predict experiments, statistically, and we get lovely agreement, because statistical agreement is all we are shooting for anyway.

The measurement problem is something quite different. It is the question, what determines the outcome that we actually perceive in any single experiment? Is it information that actually exists in the environment, that causes that outcome but we just weren't tracking it? (That sounds like an ensemble interpretation, or perhaps Bohmian as well.) Is it that there is no quantum world in the first place that would need to have a particular outcome to be caused, because the methods of physics end in the macro domain and the quantum world behaves statistically because it is basically a figment of our imagination? (That sounds like Copenhagen, which I like because it is the most overtly skeptical about our own theories.) Is it that the entire issue is moot, because all outcomes actually do occur, and it is just an illusion of our minds that we exist in a sector where only one happened? (That sounds like many worlds, yet note how close it is to Wigner's consciousness being responsible for the collapse.) So we need an interpretation not to understand the equations we are writing down, we need it to understand our experience of the experiment. That's why they are so subjective, they are actually explaining a fundamentally subjective aspect of physics.

 

[meBigGuy]

As I learn more I become less able to relate to the interpretations. They seem contrived. Trying to explain things that can't be explained. I always favored relational interpretations, but they aren't really saying anything more than things are relative and related, which is pretty obvious.

There is a thought experiment/analogy I like called the (quantum) spooky socks that makes a strong point (for me, anyway) about what is real and what exists. I'm not allowed to post the link, but I'm interested in how the statement made by spooky socks relates to interpretations. It is epitomized by the difference between "filling the drawers" and "preparing the chest".

 

[bhobba]

As I learn more I become less able to relate to the interpretations. They seem contrived. Trying to explain things that can't be explained. I always favored relational interpretations, but they aren't really saying anything more than things are relative and related, which is pretty obvious.

Now you are starting to understand the truth of the situation - all interpretations suck in their own inimitable and peculiar way. As you think about and compare interpretations you get the feeling all you are doing is mapping bits of the same big elephant and we have a long way to go before we see it whole.

I am an advocate of decoherence as being a big step forward in interpretations, but I think we have a long way to go. I believe string theory will eventually have something very important to say about it - but only time will tell.

Thanks
Bill

 

[Ken G]

Personally, I think there is a silver lining to the unsatisfactory character of the interpretations-- it is trying to tell us that we don't just need a better theory, we need a better idea of what a theory is. Too long we have gotten away with imagining that physics is happening "out there", with no reference to us, and we are just flies on the wall, taking notes. But that's never what physics really was, or is, it's just an idealization that we got away with for a few centuries. Real physics is done by a physicist, we involve ourselves in nature before we try to figure nature out. This is an inescapable aspect of just what physics is, the breakthrough in science where we stopped being flies on the wall (watching the wheels of the cosmos) and started being players. We realized that we can interact with nature, and use that interaction to figure nature out. That's what we call experiments! But there's a price to pay-- it means we have to realize that physics involves a physicist. That's the primary philosophical impact of quantum mechanics, and the source of the dissatisfaction with our interpretations. Let us not miss the opportunity to learn the appropriate lesson-- we must recast physics as something that we are doing to understand nature, we must understand the role of the physicist in what physics has always been. Perhaps the next great theory will have that flavor-- but if so, string theory is not getting that message.

 

[meBigGuy]

Can't we define observation as any action that entangles entities, one of which we might call a measuring device? For example, in the 2 slit experiment, it is the entanglement of a some measurement device (possibly just a particle) with the "test" particle at 1 slit that destroys the interference pattern. If the entanglement (measurement) is weak, the pattern is only partially destroyed. I've always thought that any interaction qualified as observation. The "strength" of the observation depends on the degree of interaction.

 

[bhobba]

Can't we define observation as any action that entangles entities, one of which we might call a measuring device?

Not really - measurement is a kind of entanglement but the converse is not necessarily true.

I think the best way to define it is simply when a system, environment, and measuring apparatus is in a mixed state after decoherence. Interpretations differ on what you can infer from that but the QM formalism is clear - the result must be one of the 'elements' of the mixture and its probability is its proportion in the mixture.

But, as I think Ken was pointing out, pinning it down is a slippery issue.

Thanks
Bill

 

[meBigGuy]

There needs to be a result or at the least a probability of a result? Entanglement in general doesn't mean there is a result (yet). Does that work, or am I just digging myself in deeper.

 

[Ken G]

I think you're pretty close, but we can be more precise. What a "result" means is that some space of mutually exclusive definite states ("eigenstates") are having their mutual correlations completely scrubbed by the interaction with the environment. Not just any environment will do that in regard to any set of definite states, it requires a very purposefully chosen environment to do that.

Consider, say, a momentum measurement. Nature doesn't usually do those on her own, if we want a quantum system, like a particle, to have a definite momentum, we need to do something quite purposeful to that system that just isn't going to happen naturally (though it depends on what kind of precision we will tolerate). That's what I mean by the "role of the physicist", we have designed very special interactions to scrub coherences in a very specific way, such that the state of the quantum system becomes a perfectly mixed state in regard to, say, momentum. That's basically an environmental interaction that must produce states of definite momentum, even though we don't know which one it will produce. So that's a measurement, it's not just any entanglement, it's a very strong entanglement that will produce a perfectly mixed state (no remaining correlations whatsoever between some set of definite states), and even more, it has to be a mixed state with regard to some set of definite states that we actually recognize as something physical (like momentum, or location, etc.).

It has even been said that ultimately, all we ever do are position measurements (the location of some meter, etc.), though it's probably not important to try to be that specific about it. The point is, it has to be an interaction of an extremely special type, a small subclass of the things nature does to its quantum systems. We as physicists can't really create any instrument that nature couldn't make on her own, but still, we go to a lot of trouble selecting those instruments very carefully, until we have the ones that are good enough to be considered measuring devices. By restricting the tools of physics to such a small subclass of natural phenomena, we try to use the processes we understand to try to figure out the ones we don't. But what's hiding in those processes we think we understand? That part we can never get at with physics, because we always have to use something we think we understand, chosen from that special class of interactions, the measurements, to try and understand everything else. And we are surprised there is a "measurement problem"?

 

[bhobba]

And we are surprised there is a "measurement problem"?

Well once you have a fundamental theory concerning what objects (called measurement devices etc etc) 'register', and those things are built from what that theory is supposed to explain, you are bound to have issues.

Thanks
Bill

 

[Ken G]

Exactly. It's amazing it took this long for us to encounter the conundrum, I guess we had to get to a point where we were bridging suitably large gaps between what we already understood and what we wanted to understand. I believe that concept was at the heart of what Bohr was saying.

 

[stevendaryl]

Exactly. It's amazing it took this long for us to encounter the conundrum, I guess we had to get to a point where we were bridging suitably large gaps between what we already understood and what we wanted to understand. I believe that concept was at the heart of what Bohr was saying.

I think that there is some kind of "measurement problem" that is inevitable, regardless of the laws of physics: Any attempt to measure a quantity requires a physical object to do the measurement, and requires an interaction between the measuring object and the measured object, so it can be difficult (or maybe impossible, in some circumstances) to complete disentangle the two. The original understanding of Heisenberg's uncertainty principle was in terms of a measurement disturbing that which is being measured. For example, to accurately determine the position of an object, you have to "see" the object using very short-wavelength light, and the light imparts momentum to the object. So attempting to measure position accurately makes the momentum uncertain.

But I don't think that quantum mechanics, and its "measurement problems" are really explained by such a disturbance model. The EPR experiment shows that. If you create two particles with identical (or complementary) properties such as spin or momentum, then you can find out about one without disturbing it, by measuring the corresponding property of the OTHER particle.

So even though the general discussion of the problem of measurement might seem to make the quantum mechanical situation more understandable, I think that looking at the details shows that quantum mechanics has its own problems that are not the same as the generic measurement problems.

 

[Ken G]

Yes, it's more than just the disturbance issue, complementarity comes from the wave/particle duality. That duality is a classic example of the problem of using one thing that you think you understand to try to figure out the other. When waves are aggregates of particle behavior, like a sound wave is an aggregate of air molecules, you can write the equations for the particle motions and show how the wave equation emerges. But what if the particles already obey a wave equation, how can we understand waves using particles then? The wave behavior cannot be said to emerge from the particle behaviors. Or, if we abandon particles, and just try to understand wave mechanics in its own right, so we understand a two-slit diffraction pattern in the language of interfering waves, then we have trouble saying why we only get one tiny "blip" at a time-- the particle behavior doesn't arise completely from the mechanics of waves.

 

[meBigGuy]

I just stumbled across "Experimental motivation and empirical consistency in minimal no-collapse quantum mechanics" .(schlosshauer) http://arxiv.org/pdf/quant-ph/0506199v3.pdf

Puts it all in perspective for me. Doesn't leave much room for hocus-pocus interpretations.