QM describes reality?

[blike]

Does quantum mechanics describe reality, or just what we perceive as reality? For example: Does a particle actually have both an absolute position and momentum? Or do these particle properties only solidify when the particle is interacted with.

 

I read that Einstein, Podolsky, and Rosen raised an objection to QM stating that particles possessed definite properties, which is why two widely separated particles with a common origin behave the same. Then I read about Bell and Aspect's data that seemed to put EPR's notion to rest.

 

So far I'm really confused and not sure what to make of it all. Is the current view that particles exist in a probability wave when they're not being interacted with (by anything)? Thats the impression I'm getting. But if that's the case, let's say I have an electron detector in my garage. When I measure an electron and collapse the probability wave, how does the probability wave for that electron 300 million light years away also intantly collapse? By what mechanism?

[Severian]

Quantum mechanics is intrinsically non-local because, as you say, you are determining the outcome of a measurement 300 million light years away. But it is not a problem for relativity because you can't pass information using this mechanism.

[pulkit]

When I measure an electron and collapse the probability wave, how does the probability wave for that electron 300 million light years away also intantly collapse?

Mechanism = The mechanism you use to measure your electron.

Typically you would use light to capture position say, this would instantly cause a huge uncertainity in momentum. So you have acctually fiddled with the wave and not destroyed it. You suddenly give it the electron an energy pulse which will alter your probability wave.

It is like trying to measure an im-measurable, you can never quite have enuf information.

[Aeschylus]

Does quantum mechanics describe reality, or just what we perceive as reality? For example: Does a particle actually have both an absolute position and momentum? Or do these particle properties only solidify when the particle is interacted with.

 

Is there a 'true reality'? How does it differ from the reality we perceive? These are fair questions however they are more in the scope of philosphy than physics. In the most formal sense, quantum mechanics predicts the results of measuremnts in terms of probabilty. The orthodox appraoch to QM assigns no physical meaning to the wavefunction only what it predicts when we make a measuremnt. The principle of complementarity, a feature of the othrodox interpretation says that it is meaningless to talk about a particle with both an absolute postion and momentum; QM particles never behave as if any two complementary observables such as postion and momentum both have absolute values.

 

I read that Einstein, Podolsky, and Rosen raised an objection to QM stating that particles possessed definite properties, which is why two widely separated particles with a common origin behave the same. Then I read about Bell and Aspect's data that seemed to put EPR's notion to rest.

 

What E., P. and R. did was to come up with the EPR paradox which was a thought experiment which seemingly demonstrated, that if QM is corrcet, the instaneous transmission of information between two spatially seperated locations. Now we know that such instaneous transmission is prohibited by special relatvity, so there must be some 'hidden variables' at work and QM is therfore 'incomplete'. What Bell and Aspect showed that even if there were hidden variabbles at work there still must seemingly be instant transmission of information. These days the EPR paradox is not actually interpreted as the instantaneous transmission of information (something re-enforced by the fact it cannot actually be used to instaneously transfer information), but as the nonlocal collapse of the wavefunction, thus QM is not necesarily 'incomplete'.

 

So far I'm really confused and not sure what to make of it all. Is the current view that particles exist in a probability wave when they're not being interacted with (by anything)? Thats the impression I'm getting. But if that's the case, let's say I have an electron detector in my garage. When I measure an electron and collapse the probability wave, how does the probability wave for that electron 300 million light years away also intantly collapse? By what mechanism?

 

As said before, the wavefunction isn't conventionally interpreted as having physical existnace only measuremnts are interpreted that way (so you can see why we don't interpret that colapse of the wavefunction as violating relativity). There is no other mechanism than collapse at work in the situation you describe as far as we know.

[Severian]

What E.' date=' P. and R. did was to come up with the EPR paradox which was a thought experiment which seemingly demonstrated, that if QM is corrcet, the instaneous transmission of information between two spatially seperated locations. Now we know that such instaneous transmission is prohibited by special relatvity, so there must be some 'hidden variables' at work and QM is therfore 'incomplete'. What Bell and Aspect showed that even if there were hidden variabbles at work there still must seemingly be instant transmission of information. These days the EPR paradox is not actually interpreted as the instantaneous transmission of information (something re-enforced by the fact it cannot actually be used to instaneously transfer information), but as the nonlocal collapse of the wavefunction, thus QM is not necesarily 'incomplete'.

[/quote']

 

I agree with most of this but disgree on a few subtleties. First of all, the collapse does not pass information in the sense that you could not use this experiment to pass information faster than the speed of light (because which eigenstate the system collapses into is essentially random). So QM is not violating special relativity. Secondly, although QM is non-local, this non-locality is perfectly acceptable and does not make QM 'incomplete' (QM is incomplete for a different reason - it only allows a fixed number of particles - a flaw which is corrected by quantum field theory).

[Aeschylus]

I agree with most of this but disgree on a few subtleties. First of all, the collapse does not pass information in the sense that you could not use this experiment to pass information faster than the speed of light (because which eigenstate the system collapses into is essentially random). So QM is not violating special relativity. Secondly, although QM is non-local, this non-locality is perfectly acceptable and does not make QM 'incomplete' (QM is incomplete for a different reason - it only allows a fixed number of particles - a flaw which is corrected by quantum field theory).

 

Notice that I was only giving the point of view of those who formulated the EPR paradox, I later said this:

 

These days the EPR paradox is not actually interpreted as the instantaneous transmission of information (something re-enforced by the fact it cannot actually be used to instaneously transfer information), but as the nonlocal collapse of the wavefunction, thus QM is not necesarily 'incomplete'.

[Severian]

Sorry - that will teach me to read these posts a little more completely

[blike]

The orthodox appraoch to QM assigns no physical meaning to the wavefunction only what it predicts when we make a measuremnt.

 

So when people talk about qubits and quantum computers, are qubits actually in that superposition, or is it simply the way we mathematically describe it because we can't know what position it's in? It seems to me like if it were simply a mathematical description, quantum computers wouldn't work. I don't know if I'm articulating well enough what I'm thinking here: I mean, if the particles aren't really in a superposition, how would it behave any different than a binary system. Isn't a functioning quantum computer evidence for particles actually existing in a superposition?

[Severian]

Aeschylus is right that the wavefunction has no 'physical reality' because we can never measure it. A measurement will always collapse it to an eigenstate, so we can never see it away from an eigenstate. One can hold the view that since we cannot observe it, then it doesn't exist - our physical reality is expressed entirely in terms of observations, so only observations are 'real' and any abstract mathematics behind it is just an artificial mechanism for predicting the observations.

 

Personally, I think that is a rather sterile way to view the world, even if it is formally valid. I believe that fields do exist even when we are not looking at them, but that has to be a 'belief' since I can never prove it, and is anyway probably a semantic distinction. What do we mean by 'exist'?

[julien]

I agree with most of this but disgree on a few subtleties. First of all, the collapse does not pass information in the sense that you could not use this experiment to pass information faster than the speed of light (because which eigenstate the system collapses into is essentially random). So QM is not violating special relativity. Secondly, although QM is non-local, this non-locality is perfectly acceptable and does not make QM 'incomplete' (QM is incomplete for a different reason - it only allows a fixed number of particles - a flaw which is corrected by quantum field theory).

 

QM is nonlocal by looking at the correlation function, and Bell's inequalities

 

But since the full correlation is simultaneously local and non-local (superposition) :

 

C(A,B)=<AB>-<A><B>

 

then QM is in a superposition of locality and non-locality...

 

Taking only the non-local part of it (<AB>), not the complete definition of it, leads then naturally to a non-local result

 

In fact one can show that the local part of the correlation gives rise to a spooky variable...hence in agreement, QM is probably complete in that sense too.

[RICHARDBATTY]

I think its because the split particle once occupied the same point in space time and still does according to space time but to an observer the parts seperate. The wierdness barrier is the point at which an object can not be said to occupy one point. Thats where I started to come up with that theory no one seemed to like and so this is only my opinion.

[julien]

Yes...this is espcially true when considering the Bohm version, where there the wavefunction does not depend on space-time....In fact this comes out of the [math]\otimes[/math] product : [math] A\otimes B [/math] is a vector, or operator, which components are at both places, whereas A and B are well defined in space...

 

but I don't know it this is what you mean

[RICHARDBATTY]

Yes...this is espcially true when considering the Bohm version' date=' where there the wavefunction does not depend on space-time....In fact this comes out of the [math']\otimes[/math] product : [math] A\otimes B [/math] is a vector, or operator, which components are at both places, whereas A and B are well defined in space...

 

but I don't know it this is what you mean

Was this for me. If it is you should know I have the math abilty of a dead ferrit. To know what I mean you would have to read my theory of space time and movement. I didn't finish all of it like inertia because no one seemed to like it, but if you understand it you will see the quantum relationship.