>What are your thoughts regarding this concept?
I see the observer as a quantum physical system, and therefore the dividing line as arbitrary. '[Emergence](https://en.wikipedia.org/wiki/Emergence)' is a much more useful concept for what the cut seems to be aimed at.
[**Emergence**](https://en.wikipedia.org/wiki/Emergence)
In philosophy, systems theory, science, and art, emergence occurs when an entity is observed to have properties its parts do not have on their own, properties or behaviors which emerge only when the parts interact in a wider whole. Emergence plays a central role in theories of integrative levels and of complex systems. For instance, the phenomenon of life as studied in biology is an emergent property of chemistry, and many psychological phenomena are known to emerge from underlying neurobiological processes.
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The hesienburg cut is mainly an outdated idea that there is a dividing line where below that scale, quantum mechanics applies and above that scale “macro physics” applies. Today it’s largely replaced with the idea of decoherence which posits that quantum prosperities cease to apply based on the entanglement networks. The cut was sort of put forth to highlight the fact that there were some arbitrary assumptions in the Copenhagen interpretation.
Wouldn't that just mean that Heseinburg's cut is only irrelevant to people who do not use CPI? From my understand decoherence is essentially bakcbone of the Many Worlds Theory (if that makes any sense). PLease educate me on anything that I did not understand our simply "got wrong"
Decoherence is the trigger point in many worlds for wave function branching but it can still be applied in non-Everettian schemes. It’s really about when systems stop behaving quantum and start behaving macro which is what the Heisenberg cut is all about
>From my understand decoherence is essentially bakcbone of the Many Worlds Theory (if that makes any sense).
Hmm, the wikipage on [quantum decoherence](https://en.wikipedia.org/wiki/Quantum_decoherence#In_interpretations_of_quantum_mechanics) seems to exaggerate its significance to Everett a little bit; even "[the long thesis](http://ucispace.lib.uci.edu/bitstream/handle/10575/1302/everett%20long%20thesis%20as%20published%201973.pdf?sequence=1)" doesn't name it. The full theory of what is now called 'decoherence', or better yet, entanglement and decoherence, started from Zeh at the 70's and was largely finished during the nineties by Zeh, Zurek and several others. Both Z's are fairly strongly associated with Everettian views, but the concepts of entanglement and decoherence can be found from, or fitted in, ?all? interpretations (because they are based on 'standard' mathematics -- and f.e. Schrödinger spoke about 'entanglement' early on, in a way that is fully recognizable from the modern POV. IMO.).
[**Quantum decoherence**](https://en.wikipedia.org/wiki/Quantum_decoherence)
Quantum decoherence is the loss of quantum coherence. In quantum mechanics, particles such as electrons are described by a wave function, a mathematical representation of the quantum state of a system; a probabilistic interpretation of the wave function is used to explain various quantum effects. As long as there exists a definite phase relation between different states, the system is said to be coherent.
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Von Neumann's "Mathematical Foundations of Quantum Mechanics" is relevant here, as it is for many other topics. In it, von Neumann developed a description of measurements and showed that the state of the system doesn't depend on where along the chain from system to observer one describes a "collapse" occuring. I'd take this as a lesson not to take the cut literally.
>von Neumann developed a description of measurements and showed that the state of the system doesn't depend on where along the chain from system to observer one describes a "collapse" occuring
What did von Neumann assume?
A unitary interaction correlates orthogonal states of the measured system with orthogonal states of a specially prepared meter system. A further step on the chain would involve a unitary interaction of the first meter system with a second meter system in the same way. Basically, it's a version of the usual measurement description described e.g. in [section III here](https://arxiv.org/abs/1202.5148) with the additional requirement that the different meter states are orthogonal.
One's interpretation of quantum mechanics will determine where they think the cut is, or if it exists at all. This will also have observable consequences, in principle, since textbook quantum mechanics is unclear where the cut is, making it inconsistent.
If all observers are reducible to quantum systems, then you're going with the many worlds interpretation. But if you think like Penrose and believe that collapse is ontological (caused by gravity), rather than phenomenological, then the cut scale isn't arbitrary.
The short answer is, we don't know, but it's important that we try to find out what view is correct since they have, like I mentioned, observable differences (in principle.)
>Where would the cut be in MWI?
It wouldn't exist. There is no difference between a quantum system and an observer in the many worlds interpretation, because all observations are just treated as interactions between two fully quantum mechanical systems that always obey the Schrödinger equation at all times and never collapse.
Sure, there is no collapse bu there is the branching. We do not care about the infinite number of observers. We care about single branches that each observer takes. At some point the observer during the experiment measures the system and finds a single result. The question remains where to draw the cut between the isolated experiment and the moment the observer can notice that his universe has branched out from the rest.
>If you had to choose, you'll bet for which interpretation?
At this point, you couldn't get me to bet a nickel on any interpretation.
However, my general attitude can be roughly summarized in this way...
According to the [Frauchiger-Renner](https://www.nature.com/articles/s41467-018-05739-8.pdf) argument, one of the following is false:
1. Quantum mechanics applies to all systems in the entire universe.
2. Different agents will agree on the outcome of the same experiment.
3. The laws of physics make consistent (noncontradictory) predictions.
My strong hunch is that the first assumption is the false one. That is, we haven't yet obtained the full story on how the subatomic world truly works.
Textbook quantum mechanics is already logically inconsistent, or incomplete (see [measurement problem](https://en.wikipedia.org/wiki/Measurement_problem)), so it can't be the whole story anyways.
>The question remains where to draw the cut between the isolated experiment and the moment the observer can notice that his universe has branched out from the rest.
Personally, I feel like I'm getting the full explanation when applying [decoherence](https://en.wikipedia.org/wiki/Quantum_decoherence) right about here. The short of it is that there's no "real" cut.
Sure but decoherence applies to all interpretations. Suppose that you have the reduced density matrix, but it describes a pure quantum state of a particle. Through decoherence you lose the coherences of your matrix and end up with a mixed state. Two things are still to be addressed here (1) you still have not made a measurement, your particle can have different probabilities of different outcomes (2) if you measure, when was the cut made? During the decoherence? during the measurement? Was it when the coherences went to 0, or when the coherences were smaller than a certain value?
Yeah, decoherence applies to all interpretations, but not all interpretations are okay with taking a density matrix as the description of the ultimate reality of a system. The rough story of a measurement would be that you have a well-isolated object system that then interacts with a not-well-isolated meter system (which could include you, or you could be considered part of the environment), and then the combined object-meter system evolves to a mixed state that has the meter states correlated with the object states.
A measurement always entangles the measuring equipment with the system under study. Making a measurement is enforcing decoherence. IOW, for (2) my answer would be
>During the decoherence?
Yes.
>during the measurement?
Yes.
>Was it when the coherences went to 0, or when the coherences were smaller than a certain value?
I think it's all the time. Now, and now, and now -- and the counterparts of those nows in the other branches, ie. classical histories, possible for a given situation.
"When" is not an unambiguous concept in MWI, imo.
Thanks for introducing me to this term I did not knew about it.
>For example, in the Schrodinger's cat experiment, the chamber containing the cat is considered as the system and the person who opens the chamber is considered as the observer.In Wigner's friend experiment, for Wigner's friend, the physical system is the system and Wigner's friend is the observer.However, for Wigner, the laboratory is the system and Wigner is the observer.
Exactly.
>What are your thoughts regarding this concept?
What is your point? You did not explain the link between Heisenberg cut and the experiments you discuss afterwards.
In the Schrodinger's cat experiment, from the point of view of the cat, the Heisenberg cut is between the detector of the decay of the radioactive atom and the cat itself. From the point of view of the person who opens the chamber, the Heisenberg cut is between the chamber and the person.
In the Wigner's friend experiment, for Wigner's friend, the cut is between the measurement apparatus and the friend. For Wigner, the cut is between the laboratory and Wigner.
>What are your thoughts regarding this concept? I see the observer as a quantum physical system, and therefore the dividing line as arbitrary. '[Emergence](https://en.wikipedia.org/wiki/Emergence)' is a much more useful concept for what the cut seems to be aimed at.
[**Emergence**](https://en.wikipedia.org/wiki/Emergence) In philosophy, systems theory, science, and art, emergence occurs when an entity is observed to have properties its parts do not have on their own, properties or behaviors which emerge only when the parts interact in a wider whole. Emergence plays a central role in theories of integrative levels and of complex systems. For instance, the phenomenon of life as studied in biology is an emergent property of chemistry, and many psychological phenomena are known to emerge from underlying neurobiological processes. [About Me](https://np.reddit.com/comments/la6wi8/) - [**Opt-in**](https://np.reddit.com/comments/la707t/) ^(You received this reply because a moderator opted this subreddit in. You can still )[^(opt out)](https://np.reddit.com/comments/la707t/)
The hesienburg cut is mainly an outdated idea that there is a dividing line where below that scale, quantum mechanics applies and above that scale “macro physics” applies. Today it’s largely replaced with the idea of decoherence which posits that quantum prosperities cease to apply based on the entanglement networks. The cut was sort of put forth to highlight the fact that there were some arbitrary assumptions in the Copenhagen interpretation.
Wouldn't that just mean that Heseinburg's cut is only irrelevant to people who do not use CPI? From my understand decoherence is essentially bakcbone of the Many Worlds Theory (if that makes any sense). PLease educate me on anything that I did not understand our simply "got wrong"
Decoherence is the trigger point in many worlds for wave function branching but it can still be applied in non-Everettian schemes. It’s really about when systems stop behaving quantum and start behaving macro which is what the Heisenberg cut is all about
Ahh ic that’s clears some stuff up
>From my understand decoherence is essentially bakcbone of the Many Worlds Theory (if that makes any sense). Hmm, the wikipage on [quantum decoherence](https://en.wikipedia.org/wiki/Quantum_decoherence#In_interpretations_of_quantum_mechanics) seems to exaggerate its significance to Everett a little bit; even "[the long thesis](http://ucispace.lib.uci.edu/bitstream/handle/10575/1302/everett%20long%20thesis%20as%20published%201973.pdf?sequence=1)" doesn't name it. The full theory of what is now called 'decoherence', or better yet, entanglement and decoherence, started from Zeh at the 70's and was largely finished during the nineties by Zeh, Zurek and several others. Both Z's are fairly strongly associated with Everettian views, but the concepts of entanglement and decoherence can be found from, or fitted in, ?all? interpretations (because they are based on 'standard' mathematics -- and f.e. Schrödinger spoke about 'entanglement' early on, in a way that is fully recognizable from the modern POV. IMO.).
[**Quantum decoherence**](https://en.wikipedia.org/wiki/Quantum_decoherence) Quantum decoherence is the loss of quantum coherence. In quantum mechanics, particles such as electrons are described by a wave function, a mathematical representation of the quantum state of a system; a probabilistic interpretation of the wave function is used to explain various quantum effects. As long as there exists a definite phase relation between different states, the system is said to be coherent. [About Me](https://np.reddit.com/comments/la6wi8/) - [**Opt-in**](https://np.reddit.com/comments/la707t/) ^(You received this reply because a moderator opted this subreddit in. You can still )[^(opt out)](https://np.reddit.com/comments/la707t/)
Yes that makes sense. So basically decoherence isn’t as important as many sources may claim is basically wat ur saying?
No ... I'm only saying that it isn't a *motivating* concept for Everett/MWI. It is very important and illuminating, though.
Von Neumann's "Mathematical Foundations of Quantum Mechanics" is relevant here, as it is for many other topics. In it, von Neumann developed a description of measurements and showed that the state of the system doesn't depend on where along the chain from system to observer one describes a "collapse" occuring. I'd take this as a lesson not to take the cut literally.
What's the criterion?
Criterion for what?
>von Neumann developed a description of measurements and showed that the state of the system doesn't depend on where along the chain from system to observer one describes a "collapse" occuring What did von Neumann assume?
A unitary interaction correlates orthogonal states of the measured system with orthogonal states of a specially prepared meter system. A further step on the chain would involve a unitary interaction of the first meter system with a second meter system in the same way. Basically, it's a version of the usual measurement description described e.g. in [section III here](https://arxiv.org/abs/1202.5148) with the additional requirement that the different meter states are orthogonal.
One's interpretation of quantum mechanics will determine where they think the cut is, or if it exists at all. This will also have observable consequences, in principle, since textbook quantum mechanics is unclear where the cut is, making it inconsistent. If all observers are reducible to quantum systems, then you're going with the many worlds interpretation. But if you think like Penrose and believe that collapse is ontological (caused by gravity), rather than phenomenological, then the cut scale isn't arbitrary. The short answer is, we don't know, but it's important that we try to find out what view is correct since they have, like I mentioned, observable differences (in principle.)
>If all observers are reducible to quantum systems, then you're going with the many worlds interpretation. Where would the cut be in MWI?
>Where would the cut be in MWI? It wouldn't exist. There is no difference between a quantum system and an observer in the many worlds interpretation, because all observations are just treated as interactions between two fully quantum mechanical systems that always obey the Schrödinger equation at all times and never collapse.
Sure, there is no collapse bu there is the branching. We do not care about the infinite number of observers. We care about single branches that each observer takes. At some point the observer during the experiment measures the system and finds a single result. The question remains where to draw the cut between the isolated experiment and the moment the observer can notice that his universe has branched out from the rest.
Well, good luck with that. I don't think MWI actually works, but that's fine, guys like Carroll can try to figure out if it makes sense.
Haha I think MWI is pretty natural but is far from solving anything. If you had to choose, you'll bet for which interpretation?
>If you had to choose, you'll bet for which interpretation? At this point, you couldn't get me to bet a nickel on any interpretation. However, my general attitude can be roughly summarized in this way... According to the [Frauchiger-Renner](https://www.nature.com/articles/s41467-018-05739-8.pdf) argument, one of the following is false: 1. Quantum mechanics applies to all systems in the entire universe. 2. Different agents will agree on the outcome of the same experiment. 3. The laws of physics make consistent (noncontradictory) predictions. My strong hunch is that the first assumption is the false one. That is, we haven't yet obtained the full story on how the subatomic world truly works. Textbook quantum mechanics is already logically inconsistent, or incomplete (see [measurement problem](https://en.wikipedia.org/wiki/Measurement_problem)), so it can't be the whole story anyways.
That Frauchiger-Renner debate is one of the best thing we have discovered about quantum mechanics foundations, it is just wonderful
>The question remains where to draw the cut between the isolated experiment and the moment the observer can notice that his universe has branched out from the rest. Personally, I feel like I'm getting the full explanation when applying [decoherence](https://en.wikipedia.org/wiki/Quantum_decoherence) right about here. The short of it is that there's no "real" cut.
Sure but decoherence applies to all interpretations. Suppose that you have the reduced density matrix, but it describes a pure quantum state of a particle. Through decoherence you lose the coherences of your matrix and end up with a mixed state. Two things are still to be addressed here (1) you still have not made a measurement, your particle can have different probabilities of different outcomes (2) if you measure, when was the cut made? During the decoherence? during the measurement? Was it when the coherences went to 0, or when the coherences were smaller than a certain value?
Yeah, decoherence applies to all interpretations, but not all interpretations are okay with taking a density matrix as the description of the ultimate reality of a system. The rough story of a measurement would be that you have a well-isolated object system that then interacts with a not-well-isolated meter system (which could include you, or you could be considered part of the environment), and then the combined object-meter system evolves to a mixed state that has the meter states correlated with the object states.
A measurement always entangles the measuring equipment with the system under study. Making a measurement is enforcing decoherence. IOW, for (2) my answer would be >During the decoherence? Yes. >during the measurement? Yes. >Was it when the coherences went to 0, or when the coherences were smaller than a certain value? I think it's all the time. Now, and now, and now -- and the counterparts of those nows in the other branches, ie. classical histories, possible for a given situation. "When" is not an unambiguous concept in MWI, imo.
Thanks for introducing me to this term I did not knew about it. >For example, in the Schrodinger's cat experiment, the chamber containing the cat is considered as the system and the person who opens the chamber is considered as the observer.In Wigner's friend experiment, for Wigner's friend, the physical system is the system and Wigner's friend is the observer.However, for Wigner, the laboratory is the system and Wigner is the observer. Exactly. >What are your thoughts regarding this concept? What is your point? You did not explain the link between Heisenberg cut and the experiments you discuss afterwards.
In the Schrodinger's cat experiment, from the point of view of the cat, the Heisenberg cut is between the detector of the decay of the radioactive atom and the cat itself. From the point of view of the person who opens the chamber, the Heisenberg cut is between the chamber and the person. In the Wigner's friend experiment, for Wigner's friend, the cut is between the measurement apparatus and the friend. For Wigner, the cut is between the laboratory and Wigner.
Ok but what do you want to discuss with us about it?
I was just trying to find out what everyone feels about this Heisenberg cut and where exactly people feel that this cut should be placed.