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Quantum Mechanics
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What is the Copenhagen Interpretation?
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The Pioneers
The central ideas of the Copenhagen interpretation were developed by a core group of quantum physics pioneers centered around Niels Bohr's Copenhagen Institute through the 1920s, driving an interpretation of the quantum wavefunction that has become the default conception taught in quantum physics courses.
One of the key elements of this interpretation is that the Schrodinger equation represents the probability of observing a particular outcome when an experiment is performed. In his book The Hidden Reality, physicist Brian Greene explains it as follows:
"The standard approach to quantum mechanics, developed by Bohr and his group, and called the Copenhagen interpretation in their honor, envisions that whenever you try to see a probability wave, the very act of observation thwarts your attempt."
The problem is that we only ever observe any physical phenomena at the macroscopic level, so the actual quantum behavior at the microscopic level is not directly available to us. As described in the book Quantum Enigma:
"There is no 'official' Copenhagen interpretation. But every version grabs the bull by the horns and asserts that an observation produces the property observed. The tricky word here is 'observation.'...
"The Copenhagen interpretation considers two realms: there is the macroscopic, classical realm of our measuring instruments governed by Newton's laws; and there is the microscopic, quantum realm of atoms and other small things governed by the Schrodinger equation. It argues that we never deal directly with the quantum objects of the microscopic realm. We therefore need not worry about their physical reality, or their lack of it. An 'existence' that allows the calculation of their effects on our macroscopic instruments is enough for us to consider."
The lack of an official Copenhagen interpretation is problematic, making the exact details of the interpretation difficult to nail down. As explained by John G. Cramer in an article entitled "The Transactional Interpretation of Quantum Mechanics":
"Despite an extensive literature which refers to, discusses, and criticizes the Copenhagen interpretation of quantum mechanics, nowhere does there seem to be any concise statement which defines the full Copenhagen interpretation."
Cramer goes on to try to define some of the central ideas that are consistently applied when speaking of the Copenhagen interpretation, arriving at the following list:
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The uncertainty principle: Developed by Werner Heisenberg in 1927, this indicates that there exist pairs of conjugate variables that cannot both be measured to an arbitrary level of accuracy. In other words, there is an absolute cap imposed by quantum physics on how accurately certain pairs of measurements can be made, most commonly the measurements of position and momentum at the same time.
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The statistical interpretation: Developed by Max Born in 1926, this interprets the Schrodinger wave function as yielding the probability of an outcome in any given state. The mathematical process for doing this is known as the Born rule.
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The complementarity concept: Developed by Niels Bohr in 1928, this includes the idea of wave-particle duality and that the wave function collapse is linked to the act of making a measurement.
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Identification of the state vector with "knowledge of the system": The Schrodinger equation contains a series of state vectors, and these vectors change over time and with observations to represent the knowledge of a system at any given time.
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The positivism of Heisenberg: This represents an emphasis on discussing solely the observable outcomes of the experiments, rather than on the "meaning" or underlying "reality". This is an implicit (and sometimes explicit) acceptance of the philosophical concept of instrumentalism.
This seems like a pretty comprehensive list of the key points behind the Copenhagen interpretation, but the interpretation is not without some fairly serious problems and has sparked many criticisms ... which are worth addressing on their own individually.
Origin of the Phrase "Copenhagen Interpretation"
As mentioned above, the exact nature of the Copenhagen interpretation has always been a bit nebulous. One of the earliest references to the idea of this was in Werner Heisenberg's 1930 book The Physical Principles of the Quantum Theory, wherein he referenced "the Copenhagen spirit of quantum theory." But at that time it was also really the only interpretation of quantum mechanics (even though there were some differences between its adherents), so there was no need to distinguish it with its own name.
It only began to be referred to as "the Copenhagen interpretation" when alternative approaches, such as David Bohm's hidden-variables approach and Hugh Everett's Many Worlds Interpretation, arose to challenge the established interpretation. The term "Copenhagen interpretation" is generally attributed to Werner Heisenberg when he was speaking in the 1950s against these alternative interpretations. Lectures using the phrase "Copenhagen Interpretation" appeared in Heisenberg's 1958 collection of essays, Physics and Philosophy.
from https://www.askamathematician.com/2011/06/q-what-is-a-measurement-in-quantum-mechanics/
The Copenhagen interpretation of quantum mechanics (which comes in a couple different flavors) is generally stated as “a thing is in a super-position of states until it’s measured”. Some people (including very few physicists) have come to the conclusion that “measurement” means “measured by something conscious, and also we’re all part of the same energy field, so we’re psychic, and modern science is only now coming to understand what eastern philosophers have known for millennia”.
Just to be clear, the Copenhagen interpretation is a bottomless font of problems and paradoxes, of which the “measurement problem” is one of the more interesting (but still: one of many). Luckily, since Copenhagen is based on an assumption (“things are in many states until measured”) that never needed to be made, isn’t well-defined, and is in no way supported by any kind of evidence, it can be abandoned giving rise to the Many Worlds Interpretation. Sorrowfully, it’s often found unabandoned (particularly in new age literature).
http://www.quantumphysicslady.org/glossary/measurement-problem/
The original interpretation of quantum mechanics and, to this day, one of the most accepted, is the Copenhagen Interpretation. This is the interpretation most often taught in universities. Within the Copenhagen Interpretation, the Measurement Problem creates riddles with no solutions. The riddles are: 1) What triggers the wave to collapse down to a particle? What is the exact mechanism of collapse? And how can it be described mathematically? 2) What causes the particle to choose a particular property, such as a particular position in a superposition of positions, rather than another? Other interpretations, such as the Transactional Interpretation, solve part of these riddles. And some interpretations, like the De Broglie-Bohmian and Many Worlds, maintain that wave function collapse does not even occur.
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