Quantum mechanics has a strange quirk: particles seem to follow different rules depending on whether we’re watching them. This puzzle, called the measurement problem, has sparked decades of debate about what’s actually real at the quantum level.
In his book What Is Real?, astrophysicist Adam Becker writes that physicists have proposed wildly different solutions to the measurement problem in quantum mechanics—from parallel universes to hidden variables to consciousness-collapsing reality. Read more to learn about this fundamental mystery and why it still matters today.
Table of Contents
The Measurement Problem Explained
Becker explains that quantum mechanics seems to demand two different sets of physical laws for identical particles, and which laws apply depends on whether anyone’s watching, as in the double-slit experiment. Physicists call this the “measurement problem”—the act of measurement appears to change the rules that govern particles. This creates a puzzle: Where does the transition between one set of rules and the other occur?
(Shortform note: What constitutes a “measurement”—and who qualifies as an observer—in quantum mechanics? Measurement requires an interaction that conveys information about the quantum system, and this interaction forces the system to choose definite states. For example, in the double-slit experiment, the detectors are what interacts with the electrons, revealing which slit they passed through. But as Becker points out, this creates a puzzle: If measurement devices are also made of quantum particles, why do they behave according to laws of classical physics, producing definite results? Physicists don’t know; they haven’t yet defined where the boundary lies between the quantum world and the classical world.)
Schrödinger responded with a thought experiment: Imagine a cat in a box with a Geiger counter and a radioactive atom that has a 50% chance of decaying—triggering a hammer to break a vial of poison. Quantum mechanics says the radioactive atom exists in superposition, both decayed and not-decayed. If quantum mechanics applies universally, superposition extends to the Geiger counter (triggered and not-triggered), the vial (broken and intact), and the cat (dead and alive). Only when you open the box does everything “choose” definite states. Schrödinger thought this ridiculous: Cats are alive or dead regardless of observation. This exposed that either quantum mechanics was incomplete or reality was stranger than anyone imagined.
(Shortform note: Schrödinger’s thought experiment was largely ignored for decades after he published it in 1935, as scientists and philosophers were troubled by the uncertainty it revealed. Writer Ursula K. Le Guin rediscovered it around 1972 and was fascinated—her 1974 short story “Schrödinger’s Cat” launched the thought experiment into mainstream consciousness. Le Guin saw a connection between fantasy literature and physics: Both require rejecting common sense explanations and embracing a radical, even imaginative, uncertainty about reality. Le Guin argued that fantasy and science share a fundamental willingness to question whether things have to be the way they are.)
Three Responses to the Measurement Problem
Becker explains that physicists developed three answers to the problem. Einstein and other realists insisted that quantum mechanics must be incomplete: that particles have properties the theory fails to describe. Bohr and the anti-realists suggested that particles don’t have properties until measured, which makes questions about unmeasured reality meaningless. Heisenberg, also an anti-realist, argued particles exist as “potentialities” until measurement makes them actual. By 1927, these crystallized into two competing visions: Realists insisted physics must describe an objective world that exists independently of observation, while anti-realists saw quantum mechanics as a tool for organizing experimental results rather than describing reality.
(Shortform note: Becker discusses the debate between realists and anti-realists over whether science describes reality or just organizes our observations. Yet QBism, a radical interpretation of quantum mechanics, suggests that this debate misses the point. In the same way that expressionist artists abandoned literal representation around the time that quantum mechanics developed—shifting from depicting objects as they appeared to expressing subjective encounters with those objects—QBIsm suggests that quantum mechanics may describe our relationship with nature rather than nature itself. This implies that we engage with the world through interaction and interpretation, not as an independent third-person observer.)
Response #1: Quantum Mechanics Must Be Incomplete
Einstein was dissatisfied with quantum mechanics despite contributing to its development. He objected to the idea that reality depends on observation, believing science should describe the world as it objectively exists. Through a thought experiment involving two colliding particles, Einstein demonstrated what he saw as a fundamental problem: measuring one particle instantly determines the other’s properties regardless of distance, yet quantum mechanics treats the unmeasured particle as existing only as a probability wave. This suggested either that quantum mechanics is incomplete (missing real properties like position and momentum) or that nature violates locality—the principle that objects are only influenced by their immediate surroundings. Einstein concluded that quantum mechanics must be incomplete and hoped future discoveries would reveal it to be merely a statistical approximation of a deeper, more complete theory that would restore both locality and objective reality.
Response #2: Questions About Unmeasured Reality Are Meaningless
Unlike Einstein, physicist Niels Bohr abandoned the idea that physics should describe objective reality. His interpretation of quantum mechanics included two key ideas:
- Complementarity: Certain properties (like wave and particle nature) cannot be observed simultaneously, though both descriptions are needed to fully explain phenomena.
- No independent reality: Particles don’t have definite properties until measured, making it meaningless to ask about their state when unobserved.
Bohr’s view created a split between a “classical realm” of real measurement devices and outcomes, and a “quantum realm” that existed only as mathematics, not independent reality. He dismissed philosophical questions about unobserved quantum behavior, arguing physics should focus solely on experimental results. This approach allowed physicists to use quantum mechanics practically without wrestling with its deeper interpretive challenges—they could simply predict outcomes mathematically without asking what it meant about the nature of reality.
Response #3: Uncertainty
Heisenberg addressed the measurement problem through his uncertainty principle, which states that precisely measuring one property of a particle (like position) makes another property (like momentum) less precise—not due to flawed equipment, but because of fundamental quantum mechanical constraints.
Like Bohr, Heisenberg rejected realism, claiming particles lack definite properties until measured. However, he diverged from Bohr by proposing that particles exist between measurements as “potentialities” rather than actualities, whereas Bohr denied any reality exists between measurements at all.
This potentiality concept raised difficult questions: How can particles that only exist as possibilities interact with instruments to produce concrete measurements? How can something without actual properties generate specific results?
Despite their philosophical differences, both Bohr and Heisenberg ultimately concluded that asking what particles do between measurements is a meaningless question.
The Response That Won the Debate
Becker argues that physicists didn’t adopt Bohr’s anti-realist interpretation of quantum mechanics because it was scientifically superior, but due to historical and institutional pressures. The common narrative that physicists reached consensus at the 1927 Solvay Conference is misleading—there was no unified position, only opposition to Einstein’s realism. What later became known as the “Copenhagen interpretation” was actually a collection of different anti-realist views.
By the 1960s, the physics community had largely abandoned foundational questions about quantum mechanics, mistaking this intellectual retreat for scientific progress.
| How Scientific and Moral Reasoning Intersected Other experts agree with Becker that the Copenhagen interpretation was less a coherent position on what quantum mechanics meant and more a coalition of opposition to Einstein’s realism. Beyond rejecting Einstein’s realism, physicists disagreed on basic issues: Some physicists emphasized that consciousness must play a role in measurement, while others rejected this idea. Jim Baggott (Quantum Drama) argues that the dominance of the thinking, despite such disagreements, reflected a culture of indifference toward interpretive questions, especially among American physicists. But, if physicists were pressured to treat questions about what was real as unscientific, might this have also made it easier to avoid asking what was morally right as World War II turned physics into a military operation? American physics culture was pragmatic and anti-philosophical even before the war. During the war, relatively few Manhattan Project scientists interrogated the moral implications of their work. The uncertainty about quantum mechanics’ meaning might have blurred a sense of moral responsibility for the theory’s material consequences. |
3 Alternative Paths Forward
Becker discusses how fundamental questions about quantum mechanics resurfaced after years of institutional suppression. In 1932, mathematician John von Neumann had supposedly proven that Einstein’s realist view (particles have definite properties before measurement) was mathematically impossible. In 1964, John Bell found von Neumann’s proof was flawed and transformed Einstein’s philosophical objections into testable mathematics through “Bell’s inequalities”—constraints that would apply if particles had definite properties before measurement. Bell found that reality itself is nonlocal; quantum effects can occur instantaneously across space.
Bell’s theorem forced physicists to choose between abandoning locality (accepting instant connections), abandoning realism (accepting that properties don’t exist until measured), or rejecting quantum mechanics’ completeness. Various interpretations of quantum mechanics represent different responses to this fundamental choice, particularly regarding what causes wave function collapse when measurement occurs:
- Preserve everything by multiplying universes. The “many-worlds” interpretation keeps both locality and realism by making reality vastly bigger than we perceive—every quantum possibility is real somewhere.
- Accept nonlocality, and restore objective reality. The “pilot-wave” theory posits that particles always have definite positions and follow specific paths guided by pilot waves, so measurements simply reveal pre-existing states rather than collapsing possibilities. This eliminates the measurement problem but requires instant nonlocal connections between distant particles.
- Modify the mathematics. Spontaneous collapse theories suppose that quantum weirdness naturally disappears at large scales through built-in random collapse mechanisms, rather than requiring mysterious measurement processes.
Why the Measurement Problem Matters
Becker argues that we shouldn’t ignore the measurement problem. He contends that it goes to the heart of our best scientific theory. Furthermore, it could be crucial for future breakthroughs—solving it might help us unify quantum mechanics with gravity and develop better cosmological theories. Essentially, Becker believes these aren’t just philosophical quibbles to set aside; they’re potentially critical scientific questions that could unlock the next major advances in physics.
Explore Further
To better understand the measurement problem of quantum mechanics in its broader context, take a look at our full guide to Becker’s book What Is Real?
