Some physicists such as Sean Carroll argue that the entire universe can be described as just a quantum wave function, the basic mathematics underlying quantum mechanics. But LMU Munich philosopher of science, Ali Barzegar, argues that quantum mechanics supports a very different cosmological story, according to which there can be no final description of “the universe as a whole.” For, he argues, quantum mechanics shows that any description of a system is always a description of one system relative to another. But if the universe is everything, then there is no system outside the universe, and so no description of the universe as a whole is possible.

Quantum mechanics is one of the most successful theories in the history of science. The wavefunction is the central element of its mathematical formalism. It is used to calculate the probabilities of obtaining different outcomes when measurements are performed on a physical system. In this sense, the wavefunction can be viewed as a catalogue that encodes the probabilistic predictions for all possible measurements on the system. More precisely, in general a wavefunction represents a superposition of the possible states associated with different measurement outcomes. The coefficients of this superposition determine the probabilities of obtaining those outcomes when a measurement is made.

Now, if a system is left to itself without intervention on our part, its wavefunction will evolve unitarily in a superposition until we make a measurement on the system. Upon the measurement, the old wavefunction suddenly is updated to a new wavefunction corresponding to the outcome of the measurement. This update is called “the collapse or reduction of the wavefunction” upon measurement. However, these two processes are incompatible within the formalism of quantum mechanics. As long as there is unitary evolution of the wavefunction in a superposition there is no collapse and so no measurement outcome. Moreover, there is nothing in the formalism telling us exactly when and where the unitary evolution breaks down and collapse occurs. This is the famous “measurement problem” of quantum mechanics.

Generally, there are two main sorts of interpretative approaches to the measurement problem. As the wavefunction is the central element of quantum mechanical formalism, these two approaches come down to two different views on the nature of the wavefunction.

Does quantum mechanics describe reality?