The quantum measurement problem, which could be a poison pill for objective reality

Imagine a physicist observing a quantum system that behaves similarly to a coin toss: it could have a heads or tails outcome. So at this point, can they be sure that their result is an objective, absolute, and indisputable fact about the world? If the coin is just the kind we see in our everyday experience, then the result of the toss will be the same for everyone: it could all be portraits! But like most things in quantum physics, the result of a quantum coin flip would be a more complex "it depends on ......" .

In a recent preprint, the trio proved a theorem that shows why certain theories (such as quantum mechanics) have measurement problems in the first place, and how one can develop alternative theories to avoid it, thus maintaining the "absoluteness" of any observed event. Such a theory would, for example, expel the possibility of a coin flip, i.e., heads for one observer and tails for another.

But their work also shows that the cost of maintaining such absolutes is one that many physicists find unaffordable. "It's a demonstration that there is no painless solution to this problem," Ormrod said, "and that if we could restore absolutes, then we would have to give up some of the physical principles that we really care about."

Eric Cavalcanti of Griffith University in Australia said, "The paper by Ormrod, Venkatesh and Barrett addresses the question of which classes of theories are incompatible with the absolute nature of observed events - whether in some theories it is possible to maintain absolutes, and other desirable properties."

It turns out that insisting on the absoluteness of observed events may mean that the quantum world is even stranger than we know.

The "Which Theories Have Measurement Problems?

In a recent preprint, the trio proved a theorem that shows why certain theories (such as quantum mechanics) have measurement problems in the first place, and how one can develop alternative theories to avoid it, thus maintaining the "absoluteness" of any observed event. Such a theory would, for example, expel the possibility of a coin flip, i.e., heads for one observer and tails for another.

But their work also shows that the cost of maintaining such absolutes is one that many physicists find unaffordable. "It's a demonstration that there is no painless solution to this problem," Ormrod said, "and that if we could restore absolutes, then we would have to give up some of the physical principles that we really care about."

Eric Cavalcanti of Griffith University in Australia said, "The paper by Ormrod, Venkatesh and Barrett addresses the question of which classes of theories are incompatible with the absolute nature of observed events - whether in some theories it is possible to maintain absolutes, and other desirable properties."

It turns out that insisting on the absoluteness of observed events may mean that the quantum world is even stranger than we know.

A visual summary of the argument, with different colors representing different frames of reference. Combined with quantum predictions for measurements made simultaneously in different frames, this means that there cannot be a single observation in every run of the experiment.

In textbook quantum theory, prior to collapse, the system is in a superposition of two states, described by a mathematical structure called the "wave function", which evolves in time and space. This evolution is both deterministic and reversible: given an initial wave function, one can predict what it will look like at some future time, and in principle one can run backwards in evolution to recover the previous state. However, mathematically speaking, measuring the wave function causes it to collapse; thus, in our example, the system is shown as either head or tail.

This collapse-induced process is the dark source of the measurement problem: it is an irreversible, once-only affair, and no one even knows what defines the process or boundaries of measurement. What is a "measurement" or an "observer"? Are there physical constraints on these things, such as minimum or maximum dimensions? Must they also be subject to various quantum effects, or can they be considered immune to such complications? There are no simple, accepted answers to any of these questions.

Considering the system of this example, there is a model that preserves the absoluteness of the observed events. That is, it is either heads or tails for all observers - this is the Ghirardi-Rimini-Weber theory (GRW). In GRW, quantum systems can exist in superpositions of various states until they reach some as yet undetermined size; at this point, the superposition states collapse spontaneously and randomly, independent of the observer. Whatever the outcome (positive or negative in our case) will hold for all observers.

However, GRW, which belongs to the broader theory of "spontaneous collapse", seems to violate a long-standing physical principle: the preservation of information. Just as a burned book can in principle be read by reassembling its pages from the ashes (ignoring, for simplicity, the initial thermal radiation emitted by the burned book), preservation of information means that the evolution of a quantum system through time should allow its previous state to be known. By assuming random collapse, GRW theory destroys the possibility of leading to a collapsed state; by most accounts, this means that information about the system prior to its transition would be irrecoverably lost. "(GRW) would be a model that abandons information preservation, thus preserving the absolute nature of the event." Venkatesh said.

A counter-example that allows for the non-absoluteness of observed events is the "multiple worlds" interpretation of quantum mechanics. In this view, our example wave function would be divided into multiple simultaneous realities: for example, in one "world," the system would appear positive, while in another world it would be negative. In this concept, there is no collapse, Ormrod says: "So the question of what happens is not absolute; it is relative to a world." Of course, to avoid the measurement problems caused by collapse, the many-worlds interpretation introduces the headache of branching wave functions and the runaway proliferation of worlds at every fork in the quantum road.

However, the many-worlds interpretation is an example of "perspectival theory," in which the outcome of a measurement depends on the perspective of the observer.

The visual summary of the argument is that different colors represent different frames of reference. Combining quantum predictions for measurements made simultaneously in different frames means that there cannot be a single observation in each run of the experiment.

In textbook quantum theory, prior to collapse, the system is in a superposition of two states, described by a mathematical structure called the "wave function", which evolves in time and space. This evolution is both deterministic and reversible: given an initial wave function, one can predict what it will look like at some future time, and in principle one can run backwards in evolution to recover the previous state. However, mathematically speaking, measuring the wave function causes it to collapse; thus, in our example, the system is shown as either head or tail.

This collapse-induced process is the dark source of the measurement problem: it is an irreversible, once-only affair, and no one even knows what defines the process or boundaries of measurement. What is a "measurement" or an "observer"? Are there physical constraints on these things, such as minimum or maximum dimensions? Must they also be subject to various quantum effects, or can they be considered immune to such complications? There are no simple, accepted answers to any of these questions.

Considering the system of this example, there is a model that preserves the absoluteness of the observed events. That is, it is either heads or tails for all observers - this is the Ghirardi-Rimini-Weber theory (GRW). In GRW, quantum systems can exist in superpositions of various states until they reach some as yet undetermined size; at this point, the superposition states collapse spontaneously and randomly, independent of the observer. Whatever the outcome (positive or negative in our case) will hold for all observers.

However, GRW, which belongs to the broader theory of "spontaneous collapse", seems to violate a long-standing physical principle: the preservation of information. Just as a burned book can in principle be read by reassembling its pages from the ashes (ignoring, for simplicity, the initial thermal radiation emitted by the burned book), preserving information means that the evolution of a quantum system through time should allow its previous state to be known. By assuming random collapse, GRW theory destroys the possibility of leading to a collapsed state; by most accounts, this means that information about the system prior to its transition would be irrecoverably lost. "(GRW) would be a model that abandons information preservation, thus preserving the absolute nature of the event." Venkatesh said.

A counter-example that allows for the non-absoluteness of observed events is the "multiple worlds" interpretation of quantum mechanics. In this view, our example wave function would be divided into multiple simultaneous realities: for example, in one "world," the system would appear positive, while in another world it would be negative. In this concept, there is no collapse, Ormrod says: "So the question of what happens is not absolute; it is relative to a world." Of course, to avoid the measurement problems caused by collapse, the many-worlds interpretation introduces the headache of branching wave functions and the runaway proliferation of worlds at every fork in the quantum road.

However, the many-worlds interpretation is an example of "perspectival theory," in which the outcome of a measurement depends on the perspective of the observer.

But GRW, which belongs to the broader "spontaneous collapse" theory, seems to violate a long-standing physical principle: the preservation of information. Just as a burned book can in principle be read by reassembling its pages from the ashes (ignoring, for simplicity, the initial thermal radiation emitted by the burned book), preserving information means that the evolution of a quantum system through time should allow its previous state to be known. By assuming random collapse, GRW theory destroys the possibility of leading to a collapsed state; by most accounts, this means that information about the system prior to its transition would be irrecoverably lost. "(GRW) would be a model that abandons information preservation, thus preserving the absolute nature of the event." Venkatesh said.

A counter-example that allows for the non-absoluteness of observed events is the "multiple worlds" interpretation of quantum mechanics. In this view, our example wave function would be divided into multiple simultaneous realities: for example, in one "world," the system would appear positive, while in another world it would be negative. In this concept, there is no collapse, Ormrod says: "So the question of what happens is not absolute; it is relative to a world." Of course, to avoid the measurement problems caused by collapse, the many-worlds interpretation introduces the headache of branching wave functions and the runaway proliferation of worlds at every fork in the quantum road.

However, the many-worlds interpretation is an example of "perspectival theory," in which the outcome of a measurement depends on the perspective of the observer.

Graphical summary of the theorem

The first property is called Bell nonlocality. It was first identified in 1964 by physicist John Bell in a theorem of the same name, and has been shown to be an indisputable empirical fact about our physical reality. Suppose Alice and Bob each have one of a pair of particles that are described by a single state. alice and Bob make separate measurements on their respective particles and on some similarly prepared pairs of particles. alice is free to choose her type of measurement and is independent of Bob, and vice versa. alice and Bob choose their measurement settings with their own The assumption that Alice and Bob choose their measurement setups of their own free will is an important one. Then, when they finally compare, they will find that their measurements are correlated, which means that the states of the two particles are inseparable: knowing the state of one particle tells us the state of the other - a theory that explains this correlation is called Bell nonlocality.

The second property is the preservation of information. Quantum systems that show deterministic and reversible evolution satisfy this condition, but the requirement is more general. Imagine that you are wearing a green sweater today; in a theory that preserves information, it is still possible in principle to retrieve the color of your sweater 10 years later, even if no one saw you wearing it. But "if the world is not information-preserving, then there may be no way to find out what color sweater I'm wearing in 10 years." Ormrod says.

The third type is called local dynamics. Consider two events in two spatio-temporal regions. If there is a frame of reference in which the two events appear to be occurring simultaneously, then the two spatial regions are called "spatially similarly separated. Local dynamics means that the transition of the system in one of these regions cannot causally affect the transition of the system in the other region at a rate faster than the speed of light, and vice versa. Each subsystem undergoes its own transformations, as does the whole system.

In quantum theory, transformations can be broken down into their constituent parts. "So quantum theory is dynamically separable." In contrast, when two particles share a Bell nonlocal state (that is, when two particles are entangled, according to quantum theory), that state is said to be indivisible, becoming a separate state for the two particles, Ormrod said. If the transformation behaves similarly, i.e., the global transformation cannot be described by the transformation of a single subsystem, then the whole system will be dynamically indivisible.

With all the pieces in place, the results of the trio can now be understood, and the work of Ormrod, Venkatesh and Barrett boils down to a sophisticated analysis of how such "BIL theories" (those that satisfy all three of the above properties) deal with a seemingly simple thought experiment.

Imagine that Alice and Bob each make a measurement of one of a pair of particles in their own labs, and Alice and Bob each make one measurement, and both make exactly the same measurement. For example, they might both measure the spin of their particles in the up-and-down direction.

From the outside Alice and Bob and their lab people are Charlie and Daniela, respectively. in principle, Charlie and Daniela should be able to measure the spins of the same particles (e.g., in the left-right direction). In an information-preserving theory, this should be possible.

Let us take as a concrete example a situation that can happen in a standard quantum theory. For example, Charlie treats Alice, her lab and the measurements she makes as a system which is subject to deterministic, reversible evolution. Assuming that he has complete control over the entire system, Charlie can reverse the process and return the particle to its original state (like a burned book being reconstituted from the ashes); Daniela does the same thing with Bob and his lab. Now, Charlie and Daniela each made different measurements on their respective particles in the left and right directions.

Using this situation, the team demonstrated that any BIL theory prediction of the measurements of the four observers contradicts the absolute nature of the observed events. In other words, "all BIL theories have a measurement problem."

This leaves physicists in an uncomfortable impasse: either accept the non-absoluteness of the observed event or abandon one of the assumptions of the BIL theory.

Venkatesh argues that abandoning the absoluteness of observed events is persuasive. After all, she says, physics has successfully transitioned from a rigid Newtonian framework to a more nuanced and fluid description of Einstein's reality. "We had to adjust some concepts that we thought were absolute. Newton had absolutes of space and time." Venkatesh said. But in Einstein's concept of the universe, space and time are one; and this single space-time is not something absolute, but can be distorted in ways that do not fit into Newton's way of thinking.

On the other hand, a theory that relies on the perspective of the observer creates its own problems. Most prominently, how can one do science within a theory where two observers cannot agree on measurements?Ormrod says, "If we can't come up with predictions for observed events that we think are absolute, it's not clear that science can work the way (it) should work."

So if one insists on the absoluteness of observed events, then something has to be. This would not be Bell's non-local or information preservation: the former has a solid empirical basis, while the latter is considered an important aspect of any theory of reality. The focus shifts to local dynamics - specifically, dynamic separability.

Dynamic separability is "a reductionist assumption," says Ormrod: "You can use these little pieces to explain the big stuff."

Preserving the absoluteness of observed events may mean that this reductionism does not hold: just as Bell's nonlocal states cannot be reduced to some constituent states, so perhaps the dynamics of a system is similarly holistic, adding another kind of nonlocality to the universe. Importantly, dropping it does not make a theory contradict Einstein's theory of relativity, just as physicists argue that Bell nonlocality does not require superluminal or nonlocal causal influences, but only nonseparated states.

Ormrod, Venkatesh and Barrett write in their paper: "Perhaps the lesson of Bell is that the states of distant particles are inseparable, while the lesson of the new ...... theorem's lesson is that the same is true of their dynamics."

"I really like the idea of rejecting dynamical separability, because if it works, then ...... we can have our cake and [also] eat it," Ormrod said, "and we can continue to believe what we think are the most fundamental things about the world: that relativity is true, that information is preserved, and that sort of thing. But we can also believe in the absolute nature of observed events."

Jeffrey Bub, a philosopher of physics and professor emeritus at the University of Maryland, College Park, has said he is willing to swallow some bitter medicine if it means living in an objective universe. I want to insist on the absolute nature of observational events," he said. It seems absurd to me to give that up just because of the measurement problems of quantum mechanics." For that reason, Bub doesn't think a dynamically inseparable universe is a bad idea. He says, "I think I would tentatively agree with the authors that (dynamical) inseparability is the least unpleasant option."

A different concept of dynamical locality

The problem is that no one yet knows how to construct a theory that rejects dynamical separability (assuming it is even possible to be constructed) while insisting on other properties, such as preservation of information and Bell nonlocality.

Howard Wiseman of Griffith University, who is considered a founding father of such theoretical thinking, appreciates the efforts of Ormrod, Venkatesh and Barrett to prove this theorem. It's great that they're pushing in this direction," he says. We can say things more broadly without mentioning quantum mechanics at all."

He notes that the thought experiment used in the analysis does not require Alice, Bob, Charlie and Daniela to make any choices; they always make the same measurements. Thus, the assumptions used to prove the theorem do not explicitly include an assumption about freedom of choice, because no one is exercising such a choice. usually, Wiseman says, the fewer the assumptions, the stronger the proof, but that may not be the case here. This is because the first assumption: that the theory must accommodate Bell nonlocality, requires that agents have free will. Any empirical test of Bell nonlocality involves Alice and Bob choosing the type of measurement they make based on their own free will. Thus, if a theory is Bell nonlocal, it implicitly recognizes the free will of the experimenter.Wiseman says, "What I suspect is that they're sneaking in an assumption of free choice."

This is not to say that the proof is weaker. On the contrary, it would have been stronger if it did not require the assumption of free will. The fact is that free will is still a requirement. Given this, the most profound implication of the theorem may be that the universe is nonlocal in an entirely new way: if so, this nonlocality would be equivalent or comparable to Bell nonlocality, and its understanding paves the way for the development of quantum communication and quantum cryptography. It is impossible for anyone to guess what a new kind of nonlocality (implied by dynamical nonseparability) would mean for our understanding of the universe.

Ultimately, only experiment will point the way to the right theory, and quantum physicists can only prepare for whatever might happen. venkatesh has said: "All of these theories will have to be explored, regardless of one's personal view of which [theory] is better. Ultimately, we will have to look at the experiments we can conduct. It may go one way, it may go another way, and it's good to be prepared."

Reference link:

https://www.scientificamerican.com/article/quantum-theorys-measurement-problem-may-be-a-poison-pill-for-objective-reality/