Causality in quantum mechanics

[tenchotomic]

Can someone please elaborate these lines:

"Causality applies only to a system which is
left undisturbed. If a system is small, we cannot observe it without
producing a serious disturbance and hence we cannot expect to find
any causal connexion between the results of our observations. "

(Reference:Dirac-Principles of quantum mechanics)

 

[DrChinese]

Can someone please elaborate these lines:

"Causality applies only to a system which is
left undisturbed. If a system is small, we cannot observe it without
producing a serious disturbance and hence we cannot expect to find
any causal connexion between the results of our observations. "

(Reference:Dirac-Principles of quantum mechanics)

This is an old-fashioned description, and is not in popular usage any longer. Uncertainty is not a result of "disturbance" in the sense implied here.

 

[tenchotomic]

I just want to understand what causality has to do with a system being disturbed or not.

 

[DrChinese]

I just want to understand what causality has to do with a system being disturbed or not.

Causality is not lost because an observer "pokes" a system in the process of making a measurement.

It is sometimes said that the wave function of a closed system evolves deterministically; however, nothing in that eliminates the element of chance in the outcome of any measurement.

 

[Trifis]

My view on the issue is that causality with reference to the classical Newtonian cause and effect is indeed violated in the quantic scale. One cannot predict the outcome of an action, not even if such an outcome occurs in the first place.

I don't understand why Dirac's view is considered anachronistic. Or maybe is just a matter of term's confusion. I think the concepts of Cause&Effect, Causality and Determinism got tangled up a little bit here.

 

[DrChinese]

I don't understand why Dirac's view is considered anachronistic. Or maybe is just a matter of term's confusion. I think the concepts of Cause&Effect, Causality and Determinism got tangled up a little bit here.

To the extent that Dirac's statement is seen as saying "disturbance causes uncertainty, and uncertainty implies indeterminism"... that is old fashioned.

More like "disturbance triggers collapse, collapse triggers indeterminism, indeterminism shows up as uncertainty" where these things are essentially part and parcel of a single idea anyway.

My essential point being: the observer does not "inject" some kind of unknown/uncertain force into the system under observation, which then acts to produce an uncertain outcome. For if that were true, entangled particle pairs could not produce the observed statistics (perfect correlations, for example). Because the outcomes would reflect the uncertainty injected by the observers (Alice, Bob) and that doesn't happen. The only variable is the context, i.e. the *difference* between Alice and Bob's settings... which is known.

 

[yuiop]

It occurs to me that one interesting aspect of the interpretation that "non local" or ftl signalling is used to explain the observed results of entangled photons experiments is that cause and effect are not clearly defined in any claimed entangled ftl interaction. For example, let's us say we have a pair of polarisers, A and B that are far apart with filter A slightly nearer than filter B. We can posit that when entangled photon A passes through filter A that it causes the state of entangled photon B to change instantaneously before photon B passes through filter B. Now if we switch to a different reference frame we can find a frame where photon B appears to pass through filter B before photon A passes through filter A. In this new reference frame, the event photon B passing through filter B appears to be the cause and photon A changing state is the effect which is a complete reversal of the cause and effect sequence observed in the original frame. The interesting aspect is that for any normal cause and effect relationship between non quantum systems, the cause is very distinct from the effect and and any ftl link between cause and effect would be very noticeable, while in the entangled photon case, the reversal is completely indistinguishable. Could it be that nature only forbids an ftl link between cause and effect if the reversal of the temporal order is clearly distinguishable?

Just to elaborate a little, in SR, for any ftl transmission in a given reference frame, we can always find a reference frame where the transmission goes backwards in time and this gives rise to paradoxical situations. In the entangled photons case, the fact that cause and effect are indistinguishable, means that any ftl link can always be interpreted as a forward in time link in any reference frame and thus those paradoxical situations are avoided.

 

[DrChinese]

It occurs to me that one interesting aspect of the interpretation that "non local" or ftl signalling is used to explain the observed results of entangled photons experiments is that cause and effect are not clearly defined in any claimed entangled ftl interaction. For example, let's us say we have a pair of polarisers, A and B that are far apart with filter A slightly nearer than filter B. We can posit that when entangled photon A passes through filter A that it causes the state of entangled photon B to change instantaneously before photon B passes through filter B. Now if we switch to a different reference frame we can find a frame where photon B appears to pass through filter B before photon A passes through filter A. In this new reference frame, the event photon B passing through filter B appears to be the cause and photon A changing state is the effect which is a complete reversal of the cause and effect sequence observed in the original frame. The interesting aspect is that for any normal cause and effect relationship between non quantum systems, the cause is very distinct from the effect and and any ftl link between cause and effect would be very noticeable, while in the entangled photon case, the reversal is completely indistinguishable. Could it be that nature only forbids an ftl link between cause and effect if the reversal of the temporal order is clearly distinguishable?

Just to elaborate a little, in SR, for any ftl transmission in a given reference frame, we can always find a reference frame where the transmission goes backwards in time and this gives rise to paradoxical situations. In the entangled photons case, the fact that cause and effect are indistinguishable, means that any ftl link can always be interpreted as a forward in time link in any reference frame and thus those paradoxical situations are avoided.

Keep in mind that there are also published experiments in which particles are entangled AFTER they have already been detected. You can even entangle (conceptually at least, experiment has yet to be performed) particles that never even existed at the same point in time (in any reference frame).

 

[askhetan]

For example, let's us say we have a pair of polarisers, A and B that are far apart with filter A slightly nearer than filter B. We can posit that when entangled photon A passes through filter A that it causes the state of entangled photon B to change instantaneously before photon B passes through filter B. Now if we switch to a different reference frame we can find a frame where photon B appears to pass through filter B before photon A passes through filter A. In this new reference frame, the event photon B passing through filter B appears to be the cause and photon A changing state is the effect which is a complete reversal of the cause and effect sequence observed in the original frame.

I'm not challenging the conclusion of your thought experiment, however the very possibility of this experiment is a big question to me. Keeping in line with special relativity, is it even possible to find a reference frame such that we will observe that B passes through the polariser earlier than A ? If yes, can you please give an example?

 

[Naty1]

By the OP:

I just want to understand what causality has to do with a system being disturbed or not.

...we cannot expect to find any causal connexion between the results of our observations. "

I'm reading the quote a bit differently than the above posters. I read the quote to imply that two different measurements might not be causally connected due to the disturbance caused by each measurement. That makes sense.

However, the causality of the system itself has not been disturbed...so I do agree with posts such as:

Uncertainty is not a result of "disturbance" in the sense implied here

Causality is not lost because an observer "pokes" a system in the process of making a measurement.

It is sometimes said that the wave function of a closed system evolves deterministically; however, nothing in that eliminates the element of chance in the outcome of any measurement.

 

[Trifis]

More like "disturbance triggers collapse, collapse triggers indeterminism, indeterminism shows up as uncertainty" where these things are essentially part and parcel of a single idea anyway.

My essential point being: the observer does not "inject" some kind of unknown/uncertain force into the system under observation, which then acts to produce an uncertain outcome.

The injected force is not unknown but it does trigger collapse to the system, doesn't it? If the observation itself effects the outcome of an action in an unpredictable way, I really don't understand why it shouldn't be linked directly to the its uncertainity.

Once again I think there is a confusion with the terminology. Does "system causality" implies the deterministic behaviour of our equations? But are the equations themselves not of probabilistic nature to begin with?
If cause and effect cannot be discerned in this level either due to the principles of quantomechanics or due to our apparent involvement during the measuring process, then we need new definitions.

 

[San K]

nice post yuiop. at the edge...

The interesting aspect is that for any normal cause and effect relationship between non quantum systems, the cause is very distinct from the effect and and any ftl link between cause and effect would be very noticeable, while in the entangled photon case, the reversal is completely indistinguishable.

how is cause and effect indistinguishable in QM?

cause ---> if we try to observe/detect the position of a photon,
effect ---> we collapse it's wave function.

In the entangled photons case, the fact that cause and effect are indistinguishable, means that any ftl link can always be interpreted as a forward in time link in any reference frame and thus those paradoxical situations are avoided.

in entangled photons

effect --> the entanglement is broken when
cause --> one of the particles is detected/measured

in what sense is cause and effect indistinguishable in QM?

 

[Naty1]

Trifis:

If the observation itself effects the outcome of an action in an unpredictable way, I really don't understand why it shouldn't be linked directly to the its uncertainity.

Once again I think there is a confusion with the terminology.

Some will agree some will disagree. You can read extensive discussions in these forums debating those perspectives. Different people interpret the mathematics differently. The language is sometimes confusing, but more often it seems different people have different interpretations of the same terminology AND different interpretations of the mathematics AND different interpretations of the physical causalities.

[One great discussion is about Heisenberg Uncertainty Principle that went on for several hundreds of posts. I think this is it: https://www.physicsforums.com/showthread.php?t=516224 ... what is it about position and momentum that forbids knowing both quantities at once? ...And a professional paper by Ballentine is also dissected there...be prepared for several days reading and thinking!]

Check out these descriptions: [I am posting these for perspective,as ONE set of descriptions, not because everybody will agree with them. In these forums, no matter what you say about QM many 'experts' will disagree and often times disagree with each other. ]

Albert Messiah, in Quantum Mechanics, says:

...During the process of observation the measured system can not be considered as separate from the observed phenomena. The ensemble of the system and the observing instrument forms an indivisible whole and so no causal relation ship can exist between the measured system (itself) before and after measurement. The intervention of the measuring instrument destroys all causal connection between the state of the system before and after the measurement; this explains why one cannot in general predict with certainty in what state the system will be found after the measurement.

[so a system seems different before and after a measurement. ]

We postulate that the wave function completely defines the dynamical state of the system under consideration. In contrast to classical theory, the dynamical variables of the system cannot in general be defined at each instant with infinite precision. The results of measurement follow a certain probability law and that law must be completely determined upon specifying the wave function.

[the dynamical state is completely defined yet the measurement variables are probabistic]

Once a measurement is completed, a system can again be described by a wave function involving only its own dynamical variables. This wave function is different from the wave function immediately before measurement because of an uncontrollable perturbation of the quantum system by the measuring device. This non causal uncontrollable perturbation of the system is distinct from modifications of the system during measurement by the measurement device which should be exactly calculable. Its effect is roughly to make the complementary variable to the one measured less well defined. An ideal measurement is one where only the uncontrollable perturbation of the system occurs during measurement: [199]the measuring device acts as a perfect filter….and passes without distortion the measured eigenvalue.

[Sounds like a measurement screws up a system in multiple ways: One we can minimize, one we can't.]

 

[San K]

Keep in mind that there are also published experiments in which particles are entangled AFTER they have already been detected.

is the detection/measurement partial?

once the particles are detected the entanglement is broken, on that factor/state/property

is the entanglement (after detection) being done on some other property/axis?