100 years on from the birth of quantum mechanics, philosopher of physics Emily Adlam argues that the quantum measurement problem remains in urgent need of a solution. It continues to raise fundamental questions about why measurements yield definite outcomes, meaningful probabilities, and shared evidence. Adlam argues that the leading interpretations of quantum mechanics still fail to explain these basic features of measurement—threatening the whole edifice of scientific method and theory.

The year 2025 marked the official 100th anniversary of quantum mechanics, commemorating Heisenberg’s 1925 paper “On the Quantum-theoretical reinterpretation of kinematic and mechanical relations.” The theory has been extremely successful in those 100 years—and yet, even after 100 years, there is still no consensus on what it is actually telling us about reality. 

The origin of the puzzle is what is known as the “measurement problem,” which refers to the fact that in the standard formulation of quantum mechanics, it has two distinct parts. On the one hand, quantum systems left to their own devices evolve linearly according to the Schrödinger equation; on the other hand, when someone performs a measurement on a quantum system, its wavefunction undergoes a non-linear “collapse” which takes the quantum state from a superposition of all possible outcomes to a state in which the observer ultimately witnesses one particular outcome. And yet this seems very hard to understand. Measurements are physical processes, so shouldn’t they be governed by the same physical laws as anything else? What is this extra “collapse” process, and what causes it? We will not fully understand the theory until we can answer these questions.

Now, the measurement problem is often presented in a way that gives the impression it is purely a matter of intellectual curiosity—we want to solve the measurement problem because we would like to know what is going on “underneath” quantum mechanics. Described thus, it seems that solving the measurement problem would merely be a nice bonus, rather than any particular imperative.

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If we can’t give a sensible account of how quantum measurements could provide reliable information about the world, then we also can’t give any clear argument to demonstrate that classical measurements escape the weirdness—and thus all of our empirical knowledge is in danger.

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However, this way of stating the measurement problem does not do justice to the central importance of measurement as an element of a theory. Measurement is a physical process, of course, but it is not just any physical process; it is the place where the theory makes contact with the evidence for it. And thus it is essential to give an account of the physical nature of measurement, which shows clearly how it is that measurement can reliably connect us to the theoretical structures we are supposed to have learned about by measurement.