Before being measured, a particle exists in a superposition of all possible states. Observation “collapses” this superposition. What does this mean for the nature of reality? In this article we answer the most fundamental questions about one of the deepest mysteries in physics.
🌊 What is the wave function and why does it matter?
In quantum mechanics, every particle is described by a mathematical entity — the so-called wave function. If ψ represents this function, it can be written as a sum of many possible states:
|ψ⟩ = Σ ci |φi⟩
Each |φi⟩ represents a possible value — for example, the position or angular momentum of an electron. The coefficients ci don't provide certainty — they provide probabilities. Specifically, Max Born proposed in 1926 that the squared modulus |ci|² gives the probability of measuring the corresponding state. He was awarded the 1954 Nobel Prize in Physics for precisely this “statistical interpretation” — the cornerstone of all quantum theory.
The wave function evolves deterministically according to the Schrödinger equation — there is no randomness in its evolution. Randomness appears only at the moment of measurement.
💥 What does wave function “collapse” mean?
Before measurement, a particle can exist in a superposition of many states — simultaneously “here” and “there”, simultaneously spin-up and spin-down. The moment a measuring device registers a result, the superposition vanishes instantaneously. The particle “chooses” one single state.
Werner Heisenberg introduced this idea in his foundational 1927 paper on the uncertainty principle. John von Neumann formalized it mathematically in his book Mathematische Grundlagen der Quantenmechanik (1932), distinguishing two types of evolution:
- Continuous, deterministic evolution according to Schrödinger
- Discontinuous, probabilistic collapse during measurement
This is the so-called "measurement problem": how does the deterministic Schrödinger equation produce random, irreversible results? As Steven Weinberg noted, if observers and their instruments are themselves described by a wave function, why can't we predict exact results instead of probabilities?
👁️ Does collapse require a conscious observer?
One of the most misunderstood points. The short answer is: no. Heisenberg himself clarified:
Wolfgang Pauli emphasized that measurement results can be obtained by “objective registering apparatus”. Collapse does not require consciousness — it requires interaction with a macroscopic system capable of irreversibly recording a result. A photographic plate, a Geiger counter, or even air molecules can “observe” a particle.
🏛️ What is the Copenhagen interpretation?
The Copenhagen interpretation — named by Heisenberg around 1955 — is the oldest and probably the most widespread view of quantum mechanics. It was developed primarily by Niels Bohr, Heisenberg, and Max Born during 1925–1927 at the Bohr Institute in Copenhagen.
Core principles:
- Quantum mechanics is intrinsically indeterministic
- The Born rule gives outcome probabilities
- Bohr's complementarity principle: certain properties (e.g. position and momentum) cannot be defined simultaneously
- The wave function collapses irreversibly upon measurement
It's worth noting that Bohr himself never used the term “collapse” — he emphasized irreversibility as a fundamental characteristic of every observation. A 2013 survey (Schlosshauer, Kofler & Zeilinger) of quantum foundations experts confirmed that the Copenhagen interpretation remains the most popular, while a Nature poll (Gibney, 2025) showed that physicists “disagree wildly” about what quantum mechanics says about reality.
🔀 Are there alternative interpretations?
Collapse is not the only explanation. Three main alternatives:
Many-Worlds interpretation: Hugh Everett proposed in 1957 that during measurement nothing collapses — instead, the universe “branches” into parallel worlds, each realizing one possible outcome. The wave function of the universe never collapses.
De Broglie–Bohm theory: Particles always have definite positions, but their motion is guided by a “pilot wave”. There is no true collapse — interaction with the environment separates wave packets, giving the impression of collapse. The theory is deterministic but non-local.
Objective collapse (GRW): The Ghirardi–Rimini–Weber theory hypothesizes that collapse happens spontaneously, without an observer, at a rate of ~1 time per 108 years per particle. The enormous number of particles in a macroscopic object guarantees that system-level collapse happens almost instantaneously. Experiments are already approaching the point where this prediction can be tested.
🔬 What is quantum decoherence and does it solve the problem?
Starting in the 1970s, H. Dieter Zeh and later Wojciech Zurek developed the theory of quantum decoherence. The basic idea: every quantum system inevitably interacts with its environment. This interaction “erases” quantum interferences extremely rapidly, converting a quantum superposition into a statistical mixture of classical alternatives.
Decoherence explains why we never see macroscopic objects in superposition — why Schrödinger's cat is not truly “both alive and dead” in our world. But it does not fully explain why one particular outcome ultimately emerges. As Schlosshauer noted (Reviews of Modern Physics, 2005), decoherence translates quantum probabilities into classical ones, but doesn't tell us why one single state “wins”.
The problem remains open. Whether collapse is a real physical process or merely an information update constitutes one of the deepest unsolved questions in modern physics. As Fuchs and Peres wrote in Physics Today (2000): "collapse is something that happens in our description of the system, not to the system itself".