Quantum Physics again - Measurement

[Humphreys]

Okay, I'm by no means a Quantum Physics expert, but I've read a fair bit. I'm trying to get a straight answer on what exactly counts as a measurement. I know that when not being observed, photons (for example) do not have a specific position in space, and act as waves. Then, a measurement occurs, the wave function is collapsed, and the photon becomes actualized in space, and we can locate it.

What I don't understand is, what is needed for this measurement?

Consciousness?
Just a measuring device?
Something more?

Most people tell me all that is needed is a measuring device, and no observation or consciousness is needed. If that's the case, we should be able to leave a measuring device alone and it will constantly cause a collapse of the wave function. But, why isn't the measuring device itself also in a superposition, acting as a mass of probabilities? What collapses the measuring device and causes it to become actualized allowing it to collapse the wave function of some photon its measuring?

I believe that's why some suggest consciousness is needed. But what counts as consciousness? A human? A monkey? A cat? A rat? A computer?

Does anyone actually know, or is it all guesswork at this stage?

 

[Cecil]

Does anyone actually know, or is it all guesswork at this stage?

From what I know (which is likely the same as you: no formal training but lots of reading), the latter is correct.

 

[Yllanes]

Most people tell me all that is needed is a measuring device, and no observation or consciousness is needed. If that's the case, we should be able to leave a measuring device alone and it will constantly cause a collapse of the wave function. But, why isn't the measuring device itself also in a superposition, acting as a mass of probabilities? What collapses the measuring device and causes it to become actualized allowing it to collapse the wave function of some photon its measuring?

There are measures of the first and second kind. Consider the first system in the attached image (I can't post images yet). We produce a particle travelling in a definite direction (guaranteed by the two grey screen). It is then deflected by a magnetic field, perpendicular to the plane of the figure and registered by a detector. From the radius of curvature we can easily infer its momentum. But the outcome of the measure doesn't allow us to say anything about the momentum after the detection (in fact, the particle may very well have been destroyed). This is a measure of the second kind.

Consider now the second apparatus. Several particles go through the two screens and enter the magnetic field. But now there is no detector, but another screen. After it, we now the momentum of the particles, because only those that give the correct radius may pass the third screen. This is a preparation.

As another example, consider the third setting. A particle with a definite momentum (prepared, for example, with the second apparatus) goes trough a magnet. We now the magnetic moment of the particle is discrete. Assume it can only take two values (as is the case for electrons). Then some of the particles will be deflected upwards and some will be deflected downwards. Only the first group makes it past the screen. This is an easy example of collapse. Initially, a statistical mixture of particles are generated in some kind of furnace. Then, only a particular momentum is selected (second figure). Finally, only one polarisation is selected.

 

[Dancing David]

Okay, I'm by no means a Quantum Physics expert, but I've read a fair bit. I'm trying to get a straight answer on what exactly counts as a measurement. I know that when not being observed, photons (for example) do not have a specific position in space, and act as waves. Then, a measurement occurs, the wave function is collapsed, and the photon becomes actualized in space, and we can locate it.

What I don't understand is, what is needed for this measurement?

Consciousness?
Just a measuring device?
Something more?

Most people tell me all that is needed is a measuring device, and no observation or consciousness is needed. If that's the case, we should be able to leave a measuring device alone and it will constantly cause a collapse of the wave function. But, why isn't the measuring device itself also in a superposition, acting as a mass of probabilities? What collapses the measuring device and causes it to become actualized allowing it to collapse the wave function of some photon its measuring?

I believe that's why some suggest consciousness is needed. But what counts as consciousness? A human? A monkey? A cat? A rat? A computer?

Does anyone actually know, or is it all guesswork at this stage?

I beleive that the laws of QM apply at all times, the wave funcyion doesn't collapse it just interacts with another wave function.

If we have rooms where a quantum experiment is carried out and we randomly choose the rooms that will be observed for results, we will find quantum results in all rooms.

Unless the measuring device is reaaly small it will have a combined macroscopic wave function, which will have consistant quantum predictabiliy, given the size of the device.

People suggest that consciousness is needed for the same reasons they believe is qualia and the deam world. They want to believe that humans are unique and devine.

 

[hammegk]

People suggest that consciousness is needed for the same reasons they believe is qualia and the dream world. They want to believe that humans are unique and devine.

Does decoherence occur without human intervention? I'd say yes, but if it's ever measured some observer will be needed to notice it.

One's definition of consciousness could perhaps be the result of interesting contemplation.

 

[Tez]

Okay, I'm by no means a Quantum Physics expert, but I've read a fair bit. I'm trying to get a straight answer on what exactly counts as a measurement. I know that when not being observed, photons (for example) do not have a specific position in space, and act as waves. Then, a measurement occurs, the wave function is collapsed, and the photon becomes actualized in space, and we can locate it.

What I don't understand is, what is needed for this measurement?

Consciousness?
Just a measuring device?
Something more?

Most people tell me all that is needed is a measuring device, and no observation or consciousness is needed. If that's the case, we should be able to leave a measuring device alone and it will constantly cause a collapse of the wave function. But, why isn't the measuring device itself also in a superposition, acting as a mass of probabilities? What collapses the measuring device and causes it to become actualized allowing it to collapse the wave function of some photon its measuring?

I believe that's why some suggest consciousness is needed. But what counts as consciousness? A human? A monkey? A cat? A rat? A computer?

Does anyone actually know, or is it all guesswork at this stage?

In orthodox quantum mechanics only the registering of an outcome by a human counts as a measurement.

This is not puzzling if you take the view that the quantum state is a "state of knowledge". The same thing happens with the "states of knowledge" of classical physics - which are just probability distributions. Does a rat or a monkey looking at the outcome of a die roll "collapse" the classical probability vector? No - because rats and monkeys have better things to be doing with their time than assigning probability distributions to the outcomes of gambling games! This process is intrinsically one to do with the way that we, as humans, are processing information about the world around us.

There are a number of us who believe this is the correct way to view the quantum state - as a part of the theory equivalent to a classical probability distribution. In this sense then consciousness does play a preferred role, but hardly a controversial one. If one doesn't want to view the quantum state this way then many weird things need to be dealt with in order to obtain strict conformation of one's interpretation with orthodox quantum mechanics. There are some subtle aspects to this "epistemological" view of quantum states too of course, but at its heart I am sure it is correct...

 

[Paul C. Anagnostopoulos]

If, like most of mankind, you have passionate convictions on many such matters, there are ways in which you can make yourself aware of your own bias. If an opinion contrary to your own makes you angry, that is a sign that you are subconsciously aware of having no good reason for thinking as you do.

Then there is never a good reason to get angry about horrific acts suggested by evil people. You must wait until they commit the acts.

Anyhoo, back to the topic. The Wikipedia articles on the interpretation of QM and the measurement problem state that there are problems with the consciousness gambit:

http://en.wikipedia.org/wiki/Interpretation_of_quantum_mechanics#Consciousness_causes_collapse
http://en.wikipedia.org/wiki/Measurement_(quantum_mechanics)

Could you expand on your interpretation, Tez?

~~ Paul

 

[Oolon Colluphid]

Off the wall thought, if humans were needed to collapse the wave function, then there could not have been any photosynthesis before humans evolved. And how would photographic film or cmos/ccd cameras work?

 

[epepke]

The idea of the wavefunction collapse by an observer is not really part of the Copenhagen interpretation, let alone QM per se. The idea seems directly attributable to John von Neumann. Why it has taken such a hold on the popular imagination, I don't understand.

 

[Tez]

The idea of the wavefunction collapse by an observer is not really part of the Copenhagen interpretation, let alone QM per se. The idea seems directly attributable to John von Neumann. Why it has taken such a hold on the popular imagination, I don't understand.

Because it works, and no one has proposed something that works as efficiently and is simpler.

As I stated elsewhere, once you get into a multiple experimenter situation everyone starts thinking about the states various observers assign the same system in terms of the information available to the observer. There are plenty of people, possibly such as yourself, who think they can see how to formulate an observerless quantum mechanics, but the results to date have always been cruddy - much as reformulating an observerless Liouville mechanics in classical physics would also be cruddy...

There is not really a good definition of the copenhagen interpretation, its not what I think of by "orthodox quantum mechanics" (which is better defined as per von Neumann, who at least was clear and rigorous). There is the way Bohr talked about quantum mechanics, and the way (after the war when he was trying to ingratiate himself with the community again) that Heisenberg said he and Bohr talked about quantum mechanics. They are not commensurate - search for "Don Howard", "The myth of the Copenhagen Interpretation" to read a good article by a leading philosophers of physics on this...

 

[Tez]

Off the wall thought, if humans were needed to collapse the wave function, then there could not have been any photosynthesis before humans evolved. And how would photographic film or cmos/ccd cameras work?

Its this sort of stupidity that annoys me (not you personally, I mean the thought that this is a natural consequence of orthodox quantum mechanics, you are not the first).

The role of obervers in a theory (think about standard probability theory applied to incomplete knowledge of the dynamics of classical physical systems) does not dictate that observers are necessary in order for the underlying reality to "exist". If one were stupid enough to insist that the probability distribution itself was an "element of physical reality" then even in classical physics one would have such problems....

 

[Humphreys]

In orthodox quantum mechanics only the registering of an outcome by a human counts as a measurement.

This is not puzzling if you take the view that the quantum state is a "state of knowledge". The same thing happens with the "states of knowledge" of classical physics - which are just probability distributions. Does a rat or a monkey looking at the outcome of a die roll "collapse" the classical probability vector? No - because rats and monkeys have better things to be doing with their time than assigning probability distributions to the outcomes of gambling games! This process is intrinsically one to do with the way that we, as humans, are processing information about the world around us.

Tex, are we possibly differentiating what we need to do to make the mathematics work, and what we believe is actually going on at the Quantum level here?

I'm very interested in what is really going on, rather than making the mathematics fit.

What is really happening? Do you think a photon is literally in all places at once, until a conscious human observer collapses into one specific place, and causes it to become a particle, rather than a wave?

Or am I making the mistake of treating Quantum Physics as more than just mathematics?

 

[Oolon Colluphid]

Its this sort of stupidity that annoys me (not you personally, I mean the thought that this is a natural consequence of orthodox quantum mechanics, you are not the first).

The role of obervers in a theory (think about standard probability theory applied to incomplete knowledge of the dynamics of classical physical systems) does not dictate that observers are necessary in order for the underlying reality to "exist". If one were stupid enough to insist that the probability distribution itself was an "element of physical reality" then even in classical physics one would have such problems....

It was actually meant to point out how absurd it is to think this way. It was just poorly worded. Obviously humans are not necessary for reality to magically reveal itself. Out of necessity borne of zero knowledge on the subject, I've read mostly 'QM For Dummies' type articles, and those did seem to suggest such a scenario, but I couldn't swallow that. The universe got along just fine before we showed up, and it'll continue on it's merry way long after we're consigned to a footnote in evolutionary history.

 

[Tez]

Tex, are we possibly differentiating what we need to do to make the mathematics work, and what we believe is actually going on at the Quantum level here?

I'm very interested in what is really going on, rather than making the mathematics fit.

What is really happening? Do you think a photon is literally in all places at once, until a conscious human observer collapses into one specific place, and causes it to become a particle, rather than a wave?

Or am I making the mistake of treating Quantum Physics as more than just mathematics?

Its very hard to say whats going on in a language that describes the underlying reality - we have a bunch of theorems which say various things about what the nature of such a reality must be (it must be highly "contextual" - a technical term I cant explain easily - and somewhat nonlocal for example). I suspect the language of photons etc is the wrong one which to talk about this reality, and I'm certain that the things we think of as important (locality, spacetime etc) are simply to do with the nature of us as a glorified conglomoration of electromagnetic fields. e.g. A vacuum fluctuation is highly nonlocal in space and local in time - a monkey made of vacuum fluctuations would find us strange!