Shulou(Shulou.com)11/24 Report--

This article comes from the official account of Wechat: back to Park (ID:fanpu2019), author: N. D. Mermin (Professor Emeritus, Department of Physics, Cornell University), translator | 1Universe 137

The problem of quantum measurement is an unavoidable problem in the development of quantum mechanics, which involves the essence of quantum mechanics, and the related debate still exists. Physicists have different views, and the author N. D. Mermin believes that there is no measurement problem in quantum mechanics: there is no "real" quantum state in physical systems, which depends on the choice and calculation of physicists. Of course, for many physicists, the attitude towards quantum mechanics interpretation is "Shut up and calculate!" Interestingly, this famous saying, which is always attached to Feynman, was first written by N. D. Mermin.

The view that wave function collapse is a physical process stems from the misunderstanding of probability and its role in quantum mechanics.

-- author

There are three types of quantum physicists: (1) they believe that quantum mechanics is destroyed by the so-called measurement problem; (2) like me, they believe that there is no measurement problem; and (3) they think that this problem is not worthy of serious consideration. You can find the different views of the first two categories of 17 physicists and philosophers in Chapter 7 of Maximilian Maximilian Schlosshauer's elegance and Mystery (Elegance and Enigma).

Most of these three categories will agree with the following view: quantum mechanics completely describes the physical system with the help of quantum state (states); state is the program of the probability of all possible answers to all possible problems in the system; quantum mechanics is statistical in nature; and no deeper theory can give a more comprehensive description.

The state of a system changes over time in two ways. If no measurement is made for a system [1], its state will evolve with certainty over time: continuously and according to a given rule. If you ask a question about the system-- called making a measurement-- then when the system responds, the state discontinuously becomes a state, depending both on the state before the measurement and on the system's specific response to the measurement. The second process is called collapse of the state of states. Collapses are usually sudden, discontinuous, and random.

A physical system and another physical system that makes specific measurements-a measuring device (apparatus)-can be treated by quantum mechanics as a single composite system. If it is not measured, quantum mechanics gives a deterministic time evolution of the state assigned to the system. However, if, on the contrary, the whole composite system is measured, the state of the composite system determines the probability; this probability correlates the possible answers given to the states of the original system with the states of the measuring devices that indicate these possible answers. These correlation probabilities are the probabilities given by quantum mechanics to the original individual system. Therefore, in terms of probability, it makes no difference whether quantum mechanics is used in the original system alone or in the composite system of the original system + (measurement) device.

Many physicists in category (2) will add that the distribution of quantum states has no consequences except for all the probabilities. Although many (and perhaps most) physicists regard probability as an objective feature of the world, most probabilists or statisticians do not think so. As the famous probability scientist Bruno de Bruno de Finetti said in 1931, "give up on Phlogiston, cosmic ether, absolute space and time." Or the superstition that fairies and witches exist is an important step on the road to scientific thinking. Probability is the same. If we regard it as something that exists objectively, it is also a misleading fallacy, an illusory attempt to exteriorize or materialize our actual belief in probability.

Physicists who materialize their probabilistic beliefs must also materialize quantum states, which are merely catalogs of this information. However, a physicist who regards probability as a personal judgment is bound to regard his or her assigned quantum state as a catalogue of his or her own judgment. At the turn of the century, Carlton Caves, Christopher Fuchs and Rudiger Schack stressed that the quantum state of the system only expresses the belief that the quantum state is given to the specific physicist of the system, which is the key to the interpretation of quantum mechanics.

The problem of quantum measurement comes from two different ways of examining the measurement: the system itself or the system + measuring device. If the system itself is measured, its state will collapse. However, if the measuring device is not checked, then the state of the composite system + measuring device does not collapse. Which description is correct? Which is the real state?

The answer given by physicists in category (2) is that there is no real state in the physical system. What people choose as a physical system and what state they assign to it depends on the judgment of the specific physicists who measure the system, who use quantum mechanics to calculate the probability of the answer.

The interaction between continuous and random time evolution is also a feature of the usual classical probability. When statisticians give probabilities to the answers to system questions, the laws of the variation of these probabilities with time are given by the stationary time evolution rules of isolated, undetected systems. But these probabilities also depend on further information about the system that statisticians get from any other source. The update of probability is a sudden and discontinuous part of the classical process. No one has ever worried about the classic measurement problem.

If the whole content of a quantum state is a catalogue of the probabilities it produces, then every physicist who uses quantum mechanics plays the role of a statistician. A physicist can obtain further information-- whether by reading the display of measuring instruments, or by communicating with other physicists, or simply by rethinking what he already knows-- can lead to sudden changes in these probabilities, thus updating the quantum states used to represent them. There is no quantum measurement problem.

The first category of physicists deal with their measurement problems in different ways:

In their excellent work on quantum mechanics in all respects, Landau and Lifshitz insist that quantum mechanics should not be seen as a tool in the observer's mind. This led them to claim that the measurement was an interaction between quantum and classical objects. How to distinguish between the two types-which they have never explained-is their (unexplained) measurement problem.

Others exclude the existence of the observer by introducing a special physical noise, which only interacts significantly with subsystems with multiple degrees of freedom macroscopically. This special noise is designed to provide a physical mechanism for the objective collapse of the objective state. They solve the measurement problem by introducing a new physical process.

Others remove the personal judgment of each physicist by completely eliminating collapse. They use quantum states to describe an incredibly large, continuously bifurcated universe (that is, a multi-world explanation [2]), which contains all possible results of all possible measurements.

These solutions all believe that quantum states are the objective properties of the physical systems they describe, rather than a catalogue of personal judgments made by each individual user of quantum mechanics on these physical systems.

Make scientists "in this mountain" [3] Why do we have to understand the laws of science objectively? Science is a kind of human activity, and its laws are expressed in human language. As empiricists, most scientists believe that their understanding of the world is based on their personal experience. Why should I insist on my interpretation of science-- I use it to understand my experience? The existence of the "quantum measurement problem" is either unsolved or there are many incompatible solutions, which strongly proves that the experience of scientists does play an important role in understanding quantum theory, just as the experience of statisticians plays an important role in understanding the usual theory of probability.

Many physicists refute this view, arguing that quantum states collapsed in the early universe long before there were physicists. I wonder if they also believe that probability was updated in the early universe long before there were statisticians.

Niels Bohr never mentioned the problem of quantum measurement. Finally, I would like to conclude with one of his statements, which succinctly expresses the above view that there is no such (quantum measurement) problem, as long as the two occurrences of "we" are not understood as the collective of all of us, but as individuals of each of us: "the purpose of our description of nature is not to reveal the true nature of phenomena." But to track the relationship between all aspects of our experience as much as possible. " I believe that the fuzziness of this unconfirmed first-person plural is behind many misunderstandings that still haunt the interpretation of quantum mechanics.

Further information

1.   M. Schlosshauer, ed., Elegance and Enigma: The Quantum Interviews, Springer, 2011, Chap. 7.

2. B. De Finetti, Theory of Probability, Interscience, 1990, Preface. (Translation of Probabilismo, Logos 14 (Napoli) 163219 (1931).

3. C. A. Fuchs and R. Schack, Quantum-Bayesian Coherence, Reviews of Modern Physics 85, 1693 (2013).

4. N. D. Mermin, Making Better Sense of Quantum Mechanics, Reports on Progress in Physics, 82, 012002 (2019).

5. N. Bohr, Atomic Theory and the Description of Nature, Cambridge U. Press, 1934, p. 18.

Annotation

[1] the original text is "If no question is asked of a system".

[2] Many-worlds interpretation.

[3] the original text is Keep the scientist in the science.

This article was published in "back to Park" authorized by the author N. D. Mermin and translated from arXiv:2206.10741. The original title is A note on the quantum measurement problem;. The original author published under the title "There is no quantum measurement problem" in Physics Today 75, 6, 62 (2022); https://doi.org/ 10.1063 / PT.3.5027.

Product: science Popularization China-Star Project

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.