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**Interpretations of Quantum Mechanics**

Quantum Mechanics, even though accepted by the totality of the Physics community, is widely controversial. However, the controversy lies not so much in the theory itself, but in how to interpret its predictions. There are a number of different interpretations, most of which revolve around the measurement postulate, its necessity and implications. Another source of controversy is QM’s apparent non-locality, a phenomenon closely related to quantum entanglement.

In Quantum Mechanics, the behavior of a particle is described by its state. The state of a particle is a mathematical abstraction which contains all the information about the particle. A state can be viewed as a combination of “basic” states (called eigenstates), each of which gives a possible answer for the quantity being measured. Let’s say, for example, we are looking at the particle’s energy. In Quantum Mechanics, the particle doesn’t need to have a well-defined energy, but it can actually have many possible energies at the same time. We say the particle is in a state which is a linear combination of states. For example, an electron could be in a state with three different energies which we could number 1, 2, and 3.

The fact that the electron is in a state combining these three energies does not mean we do not know which energy the electron actually has: in fact, it means the electron will evolve according to its state, which is a combination of energies 1, 2, and 3. The electron is in the three states at the same time.

Things change when we measure the electron’s energy. If we do so, the electron’s state will stop being a combination of the three different energy and settle into one of the three. Which one it actually goes to is impossible to know, though the amplitudes -numbers multiplying each of the states- give us the probability that the electron will collapse into that particular energy state.

The Copenhagen interpretation was developed while QM was still a theory in its infancy. It was formulated by Bohr, Heisenberg and others in the 1920s. It attempts to explain the reason behind the apparent bizarreness in QM and the collapse of the wave function. It was considered until recently the standard interpretation of QM.

In the Copenhagen interpretation, there is a duality between the Quantum realm and the classical one. Measurement devices are classical devices and measure classical properties: therefore, the quantum properties of a system are lost when performing a measurement on it. The wave-function (the particle’s state) contains the observer’s knowledge of the particle, which is modified after performing the measurement. Before the experiment, the observer can only know the probability of each of the possible outcomes.

This seemed ad-hoc to many physicists at the time, especially given the vague notion of “measurement” and its inapplicability to the universe as a whole. Furthermore, the postulate seemed to create a duality between the quantum world and the “real” world, which is unacceptable if one believes QM is the theory which describes reality. However, the Copenhagen interpretation is still viewed as correct for a great part of the Physics community.

**The many-worlds interpretations**

The many-worlds interpretation was developed by Hugh Everett in the 1950s. However, it was totally ignored for a decade, until Bryce DeWitt popularized it and gave it the name “many-worlds”.

In the many-worlds interpretation, there is no measurement postulate, no reference to observers or measurements. There is only one real entity, the universal wave-function, where each possibility for our universe is realized.

The idea behind the many-worlds interpretation is to take the equations of QM seriously and use them to describe the whole universe, not only the microscopic world or the relationship between observers and measurements. In this view, the observer is also a quantum system which gets entangled with the system under measurement, thus giving rise to a seemingly classical outcome through a process known as decoherence.

What this process seems to create is a number of alternate realities: in one of them, the experimenter will find the value number 1; in another one, the value number 2, and so on. However, there is only one reality, which is specified by the universal wave-function and which contains all the alternate histories. What happens is those stories become isolated from each other through decoherence, therefore seeming like the wave-function has collapsed into one of the values.

Many-worlds interpretation is becoming more and more popular in the Physics community, especially after some related ideas in string theory which go in a very similar direction.

Other sources: Max Tegmark’s homepage, the Wikipedia page on many-worlds.

Short video on the many-worlds interpretation.

**The Instrumentalist view**

This can hardly be called an interpretation, as its point is precisely the lack of interpretation. In the instrumentalist view, theories are tools which predict experimental outcomes, nothing less and nothing more. Some have summarized the instrumentalist view with the quote “shut up and calculate.”

There are many more interpretations of Quantum Mechanics. For more information, visit the very thorough Wikipedia article.

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