What is the accepted interpretation of the quantum nature of matter?

[hellsteiger]

I'm having difficulty gauging what the accepted interpretation of the quantum nature of matter is.

On the one hand I have been taught that properties of a particle are always definite, but due to the quantum nature of existence we cannot measure several properties to 100% accuracy at once.

On the other hand I have read that the quantum nature of a particle means that it can teleport, which could lead to light speed violation (i know it was disproved, but only by means of averaging).

Can someone provide a brief explanation of which view is correct, or whether both are?

 

[Dr. Courtney]

I'e studied a lot of quantum mechanics, and teleporting is not a quantum property. Did you mean to say tunnelling?

 

[hellsteiger]

I'e studied a lot of quantum mechanics, and teleporting is not a quantum property. Did you mean to say tunnelling?

Sorry, yes, quantum tunneling, mixed it up with quantum teleportation.

 

[ZapperZ]

On the one hand I have been taught that properties of a particle are always definite, but due to the quantum nature of existence we cannot measure several properties to 100% accuracy at once.

This is not true. QM doesn't really prohibit you to determine the value of any property to arbitrary accuracy. It depends on your instrument. However, QM does indicate your ability and accuracy to determine the value of the corresponding non-commuting observable! Position and momentum are an example of such non-commuting pair. The more accurate you determine the position in a single measurement, the less accurate you are able to predict its momentum, because the momentum spread that it could acquire will be increasingly large.

On the other hand I have read that the quantum nature of a particle means that it can teleport, which could lead to light speed violation (i know it was disproved, but only by means of averaging).

Using the corrected post for "tunneling" instead of "teleporting", I am not sure what issue you have with this, since you didn't exactly asked a question about this. Tunneling phenomena are well-established. We even use it in electronics. The issue of whether a signal can travel faster than c during tunneling is still being debated, and will require plenty more experimental evidence to sort it out.

Zz.

 

[hellsteiger]

Let's focus on this question then - Is it accepted that particles have definite properties?

If a particle has definite properties but we can not measure them simultaneously because they are non-commuting then how is it possible that a particle may tunnel?

 

[ZapperZ]

Let's focus on this question then - Is it accepted that particles have definite properties?

If a particle has definite properties but we can not measure them simultaneously because they are non-commuting then how is it possible that a particle may tunnel?

I do not understand your definition of "definite property". Please note that there is something called "realism", which is that even before measurement, a system is already in some "definite" state. A measurement simply measures such a state. QM does not appear to follow that. If you do a search on realism (even browse through the last few pages of the Noteworthy Papers thread), you will see several discussions and publications on the test of classical realism in quantum systems. All these experiments point to these system as violating "local realism" principle.

But this is more of an issue of quantum superposition, than with "tunneling".

Zz.

 

[stevendaryl]

The formalism of quantum mechanics associates a "state" with a system. This state evolves deterministically with time and allows one to compute probabilities for the results of various measurements. The quantities that we normally think of as "properties" of a particle: its position, its momentum, its energy, its angular momentum, etc., are only probabilistically described by the state.

 

[hellsteiger]

Can you explain to me then - I understand that a wavefunction collapses when it is observed, does it also collapse for a particle when it collides with another particle. (I am trying to gauge the concept of observation now).

 

[ZapperZ]

Can you explain to me then - I understand that a wavefunction collapses when it is observed, does it also collapse for a particle when it collides with another particle. (I am trying to gauge the concept of observation now).

This is not that easy.

First of all, the "collapse" of the wavefunction is for a specific observable. So if you measure, say, the angular momentum along a certain direction (call it the z-direction), then the value or projection of the angular momentum in that direction will be determined. However, the angular momentum in the perpendicular direction (xy-plane) still remains undetermined and continue to be in a superposition. So just because you made ONE measurement and "collapsed" the wavefunction into a particular definite state, it doesn't mean that ALL other observables are also equally determined. So the wavefunction doesn't collapse for all other observables automatically.

Secondly, if the particle collided and THEN you make a measurement of something on the second particle that tells you a property of the first, then yes, you have made a measurement. If both particle remained in the system and there were no measurement made, then you have a 2-particle system that is still undetermined. Remember, a conductor consists of gazillion of electrons continually colliding with each other. That entire system is quantum mechanical, despite the numerous collisions that it undergoes.

Zz.

 

[hellsteiger]

Thanks, I guess what I'm getting at is it seems that for something to be observed it requires an entity with intent to do so for it to happen, which seems odd. Then the fact that an observer can pass on information about what they have observed to another individual even though the other they did not observe the collapse also seems odd.

Say that you are outside of a system, is that system in a superposition of states, even if particles in that system are colliding?

 

[Nugatory]

Say that you are outside of a system, is that system in a superposition of states, even if particles in that system are colliding?

You may have missed the significance of what ZapperZ said in post #9 above:

First of all, the "collapse" of the wavefunction is for a specific observable.

If we just consider a single spin-one-half particle, the states "spin up" and "superposition of spin left and spin right" are exactly the same state, just written differently, in the same way that "moving northeast" and "moving northwards and eastwards" are the same vector except written differently. (Any other spin directions will work the same way - for example "spin left" and "spin right" are both superpositions of "spin up" and "spin down").

If we measure the spin of a particle along the vertical axis and we get spin up, we say that the wave function has collapsed into the spin-up state so it's no longer in a superposition of spin-up and spin-down... but it's still in a superposition of spin-left and spin-right, as well as all other (non-commuting) observables except the specific one that that we measured, spin on the vertical axis.

 

[ZapperZ]

Say that you are outside of a system, is that system in a superposition of states, even if particles in that system are colliding?

Yes, that was what I've been telling you with the conduction electron example.

Zz.

 

[bhobba]

Let's focus on this question then - Is it accepted that particles have definite properties?

QM is silent on the issue - in some interpretations they do in others they don't.

If a particle has definite properties but we can not measure them simultaneously because they are non-commuting then how is it possible that a particle may tunnel?

In interpretations where it has definite properties its got to do with lack of knowledge about initial conditions. But to understand it you need to delve into their details eg:
http://arxiv.org/abs/quant-ph/0611032

Thanks
Bill

 

[bhobba]

Thanks, I guess what I'm getting at is it seems that for something to be observed it requires an entity with intent to do so for it to happen, which seems odd.

It doesn't - all an observation is, is an interaction - no intent, conscious observer, information passing etc, required - well in the vast majority of interpretations anyway - there are fringe ones that do.

Basically QM is a theory about observations that occur in a common-sense classical world - its not quite as weird as some popularisations will give the impression it is eg conciousness is not required, particles are not in two places at once - all the really weird stuff you have likely read is just populist sensationalism.

If you want to see what QM is REALLY about see:
http://www.scottaaronson.com/democritus/lec9.html

That's right - it simply an extension of ordinary probability theory. Popularisations won't tell you that however because its rather ho hum.

Thanks
Bill

 

[Derek Potter]

Basically QM is a theory about observations that occur in a common-sense classical world - its not quite as weird as some popularisations will give the impression it is eg conciousness is not required, particles are not in two places at once - all the really weird stuff you have likely read is just populist sensationalism.

Not quite as weird but still weird enough.

 

[themaea]

It doesn't - all an observation is, is an interaction - no intent, conscious observer, information passing etc, required - well in the vast majority of interpretations anyway - there are fringe ones that do.

Basically QM is a theory about observations that occur in a common-sense classical world - its not quite as weird as some popularisations will give the impression it is eg conciousness is not required, particles are not in two places at once - all the really weird stuff you have likely read is just populist sensationalism.

If you want to see what QM is REALLY about see:
http://www.scottaaronson.com/democritus/lec9.html

That's right - it simply an extension of ordinary probability theory. Popularisations won't tell you that however because its rather ho hum.

Thanks
Bill

Is it not like saying:

Basically Special Relativity is a theory about observations that occur in a common-sense classical world - its not quite as weird as some popularisations will give the impression it is eg length contraction is not required, particles do not time dilate - all the really weird stuff you have likely read is just populist sensationalism.

But since in reality length contraction and time dilation occurs.. then why couldn't particles be in two places at once.. etc. also occur in QM?

 

[bhobba]

Basically Special Relativity is a theory about observations that occur in a common-sense classical world

It isn't. Its a theory about space-time geometry implied by space-time symmetries.
http://physics.umd.edu/~yakovenk/teaching/Lorentz.pdf

Unlike QM it objectively exists independent of observation, just like Euclidean geometry objectively exists.

then why couldn't particles be in two places at once.. etc. also occur in QM?

Since its silent on what's going on when not observed you can likely come up with an interpretation where that's true. But what you would gain with such a weird view has me beat.

Thanks
Bill

 

[Ilja]

Is it not like saying:
Basically Special Relativity is a theory about observations that occur in a common-sense classical world - its not quite as weird as some popularisations will give the impression it is eg length contraction is not required, particles do not time dilate - all the really weird stuff you have likely read is just populist sensationalism.
But since in reality length contraction and time dilation occurs..

It, in fact, depends on the interpretation of relativity. If you follow Lorentz, you have no "time" dilation but a distortion of clocks, and not "length" contraction but a distortion of rulers, above things not more strange or weird than the well-known from everyday life distortion of usual meters by temperature.
So, how strange the universe looks depends on the interpretations we give to physical theories. In the case of relativity we, obviously, prefer a quite mystical interpretation where "space" and "time" themself are influenced and become a "curved spacetime".

 

[bhobba]

It, in fact, depends on the interpretation of relativity.

Ilja is correct. Lorentz Aether Theory (LET) is also a valid interpretation of SR not based on space-time geometry. But its still objective.

Don't agree GR in its usual interpretation is 'mystical' - but that is way off topic for this thread.

Thanks
Bill

 

[VALENCIANA]

Bill, you say that all that QM is is an observation which is an interaction.

How do you exactly define "interaction" ?

What interacts in QM?

 

[bhobba]

How do you exactly define "interaction" ?

It's when decoherence occurs.

What interacts in QM?

The Hamiltonian. And if that sounds like gibberish you need to become acquainted with the basics of QM:
https://www.amazon.com/dp/0465062903/?tag=pfamazon01-20

That, or any other good QM text, will explain the detail. The above book however has the advantage of being pitched at the beginner level.

Thanks
Bill

 

[hellsteiger]

It's when decoherence occurs.
The Hamiltonian. And if that sounds like gibberish you need to become acquainted with the basics of QM:
https://www.amazon.com/dp/0465062903/?tag=pfamazon01-20

That, or any other good QM text, will explain the detail. The above book however has the advantage of being pitched at the beginner level.

Thanks
Bill

That's the thing. Decoherence occurs when an observer interacts with particles. But isn't decoherence occurring all the time between particles.

Take the example of a physicist making an experiment and observing the result. He can these pass on that information to anyone else without that person being involved in the decoherence of the system, because he has will. It seems that decoherence, tunneling and other effects depend only on the fact that the system is observed by a sentient being and has little do do with a universal interpretation of nature.

 

[bhobba]

Decoherence occurs when an observer interacts with particles. But isn't decoherence occurring all the time between particles.

Decoherence does not require an observer which is its advantage.

It seems that decoherence, tunneling and other effects depend only on the fact that the system is observed by a sentient being and has little do do with a universal interpretation of nature.

Its the exact opposite. With decoherencce a superposition is converted to a mixed state. A mixed state can be considered to be in the outcome without actually being observed. In that way an objective common-sense classical world exists independent of if a sentient being is involved or not.

That's the modern decoherence account. But even good old Copenhagen resolves it. In Copenhagen QM is a theory about observations that appear in an assumed common-sense classical world. Part of the common-sense is those observations exist independent of sentience being involved or not. The blemish here that decoherence fixes is QM in Copenhagen can't explain the classical world it assumes.

Thanks
Bill

 

[hellsteiger]

Decoherence does not require an observer which is its advantage.
Its the exact opposite. With decoherencce a superposition is converted to a mixed state. A mixed state can be considered to be in the outcome without actually being observed. In that way an objective common-sense classical world exists independent of if a sentient being is involved or not.

That's the modern decoherence account. But even good old Copenhagen resolves it. In Copenhagen QM is a theory about observations that appear in an assumed common-sense classical world. Part of the common-sense is those observations exist independent of sentience being involved or not. The blemish here that decoherence fixes is QM in Copenhagen can't explain the classical world it assumes.

Thanks
Bill

Seems like the only difference between superposition and a mixed state is the presence of an observer, that is a mixed state is only a description for use in a classical world.

Also, assuming common sense in a theory does not necessarily make it correct. Seems like a bit of a bandaid to me - we have the theory now why should anyone argue, it works in our frame of understanding and therefore it is the epitome of common sense and reality. Kind of after the fact, do you think?

Please explain to me further if there is a difference between a superposition and mixed state (without and observer, or interaction - that interaction typically being how an observation is carried out).

 

[bhobba]

Seems like the only difference between superposition and a mixed state is the presence of an observer, that is a mixed state is only a description for use in a classical world.

Before making comments like that it would be wise learning the difference:
http://pages.uoregon.edu/svanenk/solutions/Mixed_states.pdf

Thanks
Bill

 

[hellsteiger]

Before making comments like that it would be wise learning the difference:
http://pages.uoregon.edu/svanenk/solutions/Mixed_states.pdf

Thanks
Bill

Yeah, thanks, Bill. I had done so already. And reviewing your paper leaves me with the same question. The difference in formalism and application is obvious. As for realistic interpretations, the difference still seems trivial to me, with respect, once again, to what might be called a universal description of nature.

Perhaps in a future comment you may outline the difference in a more formal way, if it is so clear in your mind.

 

[bhobba]

Perhaps in a future comment you may outline the difference in a more formal way, if it is so clear in your mind.

Sorry, but I can't follow your exact issue. Happy to elaborate if you can be a bit clearer.

Thanks
Bill

 

[stevendaryl]

Please explain to me further if there is a difference between a superposition and mixed state (without and observer, or interaction - that interaction typically being how an observation is carried out).

The big difference between a superposition and a mixed state is the presence of "interference terms".

Suppose you have a system that has 3 states of interest: [itex]A, B, C[/itex]. You start the system in either state [itex]A[/itex] or [itex]B[/itex], and later check to see whether it is now in state [itex]C[/itex]. (Assume that states [itex]A[/itex] and [itex]B[/itex] are orthogonal)

Define:

1. [itex]P_A[/itex] is the probability of the system starting in state [itex]A[/itex].
2. [itex]P_B[/itex] is the probability of the system starting in state [itex]B[/itex].
3. [itex]P_{AC}[/itex] is the probability of the system winding up in state [itex]C[/itex], given that it started in state [itex]A[/itex].
4. [itex]P_{BC}[/itex] is the probability of the system winding up in state [itex]C[/itex], given that it started in state [itex]B[/itex].

Then if the system starts in a mixture of states [itex]A[/itex] and [itex]B[/itex], then the probability that the system will end up in state [itex]C[/itex] is simply:

[itex]P_C = P_A P_{AC} + P_B P_{BC}[/itex]

On the other hand, if the system starts in a pure state that is a superposition of states [itex]A[/itex] and [itex]B[/itex], then the probability will be different:

[itex]P_C = P_A P_{AC} + P_B P_{BC} + 2 \sqrt{P_A P_{AC} P_B P_{BC}} cos(\phi)[/itex]

where [itex]\phi[/itex] is a phase depending on the exact superposition you started with, and the nature of the interaction leading to state [itex]C[/itex]

The term involving the phase [itex]\phi[/itex] is an "interference" term that only appears when you have a superposition, rather than a mixture.

The significance of the difference is that a mixture can always be explained using the "ignorance" interpretation: The system is really in state [itex]A[/itex], or state [itex]B[/itex], we just don't know which. The interference term cannot be explained (at least not straight-forwardly) as ignorance about the true state of the system.

Decoherence makes the interference terms negligible, so after decoherence, we can pretend that we have a mixture, and pretend that the system is in one state or the other, and we just don't know which. Prior to decoherence, we can't consistently pretend that's true.

 

[VALENCIANA]

Regarding the double-slit experiment, I wonder why Richard Feynman had this concept:

From Wikipedia https://en.wikipedia.org/wiki/Double-slit_experiment

"Richard Feynman called it "a phenomenon which is impossible […] to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery [of quantum mechanics]."[3]Feynman was fond of saying that all of quantum mechanics can be gleaned from carefully thinking through the implications of this single experiment.[16]"

 

[Feeble Wonk]

Its the exact opposite. With decoherencce a superposition is converted to a mixed state. A mixed state can be considered to be in the outcome without actually being observed. In that way an objective common-sense classical world exists independent of if a sentient being is involved or not.

Seems like the only difference between superposition and a mixed state is the presence of an observer, that is a mixed state is only a description for use in a classical world.

Also, assuming common sense in a theory does not necessarily make it correct. Seems like a bit of a bandaid to me - we have the theory now why should anyone argue, it works in our frame of understanding and therefore it is the epitome of common sense and reality. Kind of after the fact, do you think?

Please explain to me further if there is a difference between a superposition and mixed state (without and observer, or interaction - that interaction typically being how an observation is carried out).

I'd like to make sure that I clearly understand what the debate is here. Could you please define in layman's terms what is meant by a "mixed" state/ensemble versus a "proper" state/ensemble?

 

[DrChinese]

Say that you are outside of a system, is that system in a superposition of states, even if particles in that system are colliding?

All quantum systems are in a superposition of states at all times. But not all properties of that system are necessarily in a superposition. It is possible that the basis of the superposition can change from one property to another. However, that can normally only occur upon some kind of interaction with the outside environment.

As has been mentioned in posts by several mentors, collisions may or may not cause various types of collapse. Don't forget that a closed system can have virtual interactions, effectively meaning that you can't make a strong statement about what is going on inside it.

 

[Feeble Wonk]

I'd like to make sure that I clearly understand what the debate is here. Could you please define in layman's terms what is meant by a "mixed" state/ensemble versus a "proper" state/ensemble?

I suppose I should also ask if the "mixed/pure" designation is equivalent to the "proper/improper" ensemble designation.

 

[stevendaryl]

I suppose I should also ask if the "mixed/pure" designation is equivalent to the "proper/improper" ensemble designation.

An "improper" mixed state is the result of what's called "tracing out" unobservable degrees of freedom. Basically, it's a way of converting a pure state into an effective mixed state. The system is not really in a mixed state, but can be treated as if it were.

A "proper" mixed state is one that arises from ignorance about the true pure state. For example, you flip a coin. If it's heads, you put the system in state [itex]A[/itex]. If it's tails, you put the system in state [itex]B[/itex]. If I don't get to see the result of the coin toss, then I would describe the state as a 50/50 mixture of the two states. It's really in one state or the other, I just don't know which.

Something that adds another level of complexity in thinking about these things is that in general, a system will be in a mixed state of superpositions. So both kinds of "mixing" are involved. There is no way by testing to uniquely sort out which aspects of the probabilities are due to mixing and what aspects are due to superpositions.

 

[bhobba]

I'd like to make sure that I clearly understand what the debate is here. Could you please define in layman's terms what is meant by a "mixed" state/ensemble versus a "proper" state/ensemble?

It has been bought up in many threads where those terms have been used. Unfortunately, unless you know enough of the technicalities to understand the link I gave, then its impossible to explain. At a certain point in discussing these things in lay terms the jig is up - and unfortunately it is here.

However the following book does explain it:
https://www.amazon.com/dp/0465062903/?tag=pfamazon01-20

If you really want to understand QM - not just hand-wavey lay stuff - then the above is a must.

Thanks
Bill

 

[hellsteiger]

Seems to me there is some sort of relativity here, that to someone outside of a system, it is seen as being in a pure state, regardless of whether decoherence is occurring within that system for whatever reasons.

If this outside observer where to then interact with the system it could be put into a mixed state.

Or should I interpret it more this way: a state is pure unless collisions occur within the system that cause it to decohere regardless of whether the system is closed or not.