- A Quantum System Might be a Superposition of Waves - Photograph by ppdigital
In quantum physics, a ‘measurement’ is an interaction between a quantum system (a physical system on the sub-microscopic scale of atoms or particles) and a measuring device. This interaction leaves the device in a state representing the outcome of the measurement (the result) that can be detected by the observer in the form of, for example, a read-out on a display or the position of a pointer.
The Copenhagen Interpretation and the Collapse of the Quantum Wave Function
The measurable properties of a quantum system are called ‘observables’. From the mathematical description of the state of the system (the ‘wave function’), the possible outcomes of any measurement of an observable – and the probability that each outcome might occur – can be predicted.
The ‘Copenhagen interpretation’ of quantum mechanics was developed by the pioneers of quantum theory Niels Bohr and Werner Heisenberg. It is the most widely accepted description of the nature of the quantum world, and considers the wave function to be the fullest possible description of a quantum system’s state. Probabilities are thus an inherent part of nature, and systems do not possess values of observables until they are measured.
The possible outcomes of the measurement of an observable are known as ‘eigenvalues’. Imagine a system described by a superposition of wave functions; that is, by a combination of many wave functions, each one representing the quantum state in which some observable has a particular eigenvalue. If the observable is measured and a particular result is obtained, the state of the system is then represented by the single wave function corresponding to that eigenvalue.
This change in state, from the superposition of wave functions to a single wave function corresponding to a particular result, is termed the ‘collapse of the wave function’. It can be said that the process of measurement ‘forces’ the system into the state corresponding to the measured result.
The Many Worlds Interpretation and Parallel Universes
In Hugh Everett's ‘many worlds’ view of quantum mechanics, the quantum state of a system can never be independent of the Universe that contains it. For a given state of the quantum system, the rest of the Universe is considered to be in a relative state that is described by a universal wave function.
According to this interpretation, wave functions do not collapse abruptly on the measurement of an observable. Instead, the universal wave function has many branches, each one corresponding to a single, different outcome of the measurement.
The contribution that each of these branches makes to the universal wave function is equivalent to what would be described in the Copenhagen interpretation as the probability of obtaining the given result. In the many worlds scenario, however, each branch of the universal wave function corresponds to a separate universe, and the probability of obtaining a given result reflects the fraction of the total number of universes in which that result occurs. It can thus be said that every event that does not occur in our Universe happens in another.
The implication of the many worlds interpretation is that there is a very large, perhaps infinite, number of universes, all existing parallel to our own.
The Significance of Measurement in Quantum Theory
In the 'real' world of molecules, people and cats, taking a measurement seems a simple act. This is not the case in the quantum world, where the implications must be considered carefully. Does the observable that is being measured have any value at all before that value is observed? Does the very act of measurement 'create' the value? Or is the Universe that we live in only one of a near-infinite number of universes that together encompass every possible event that might have occurred, or might ever occur?
Reference:
Bolton J et al., eds. Quantum Physics: An Introduction. 2nd ed. Milton Keynes: The Open University, 2008.
