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.<\/p>\n\n\n\n
In his book What Is Real?<\/em>, astrophysicist Adam Becker writes that physicists have proposed wildly different solutions to the measurement problem in quantum mechanics\u2014from parallel universes to hidden variables to consciousness-collapsing reality. Read more to learn about this fundamental mystery and why it still matters today.<\/p>\n\n\n\n Becker explains that quantum mechanics seems to demand two different sets of physical laws for identical particles<\/strong>, and which laws apply depends on whether anyone\u2019s watching, as in the double-slit experiment<\/a>. Physicists call this the \u201cmeasurement problem\u201d\u2014the 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?<\/p>\n\n\n\n (Shortform note: What constitutes a \u201cmeasurement\u201d\u2014and who qualifies as an observer<\/a>\u2014in quantum mechanics? Measurement requires an interaction<\/a> that conveys information about the quantum system, and this interaction forces the system to choose<\/a> definite states. For example, in the double-slit experiment, the detectors are what interacts with the electrons, revealing which slit<\/a> 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\u2019t know; they haven\u2019t yet defined where the boundary lies<\/a> between the quantum world and the classical world.)<\/p>\n\n\n\n Schr\u00f6dinger 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\u2014triggering 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 \u201cchoose\u201d definite states. Schr\u00f6dinger 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<\/strong>.<\/p>\n\n\n\n (Shortform note: Schr\u00f6dinger\u2019s thought experiment was largely ignored for decades after he published it in 1935, as scientists and philosophers were troubled by the uncertainty<\/a> it revealed. Writer Ursula K. Le Guin rediscovered it around 1972 and was fascinated\u2014her 1974 short story \u201cSchr\u00f6dinger\u2019s Cat\u201d<\/a> 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<\/a>. Le Guin argued that fantasy and science share a fundamental willingness to question whether things have to be the way they are<\/a>.)<\/p>\n\n\n\n Becker explains that physicists developed three answers to the problem. Einstein and other realists<\/em> insisted that quantum mechanics must be incomplete: that particles have properties the theory fails to describe. Bohr and the anti-realists<\/em> suggested that particles don\u2019t have properties until measured, which makes questions about unmeasured reality meaningless. Heisenberg, also an anti-realist, argued particles exist as \u201cpotentialities\u201d 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.<\/p>\n\n\n\n (Shortform note: Becker discusses the debate between realists and anti-realists<\/a> over whether science describes reality or just organizes our observations. Yet QBism, a radical interpretation<\/a> of quantum mechanics, suggests that this debate misses the point. In the same way that expressionist artists abandoned literal representation<\/a> around the time that quantum mechanics developed<\/a>\u2014shifting from depicting objects as they appeared to expressing subjective encounters<\/a> with those objects\u2014QBIsm suggests that quantum mechanics may describe our relationship with nature<\/em> rather than nature itself. This implies that we engage with the world through interaction and interpretation<\/a>, not as an independent third-person observer.)<\/p>\n\n\n\n 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\u2014the 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.<\/p>\n\n\n\n Unlike Einstein, physicist Niels Bohr abandoned the idea that physics should describe objective reality. His interpretation of quantum mechanics included two key ideas:<\/p>\n\n\n\n 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\u2014they could simply predict outcomes mathematically without asking what it meant about the nature of reality.<\/p>\n\n\n\n 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\u2014not due to flawed equipment, but because of fundamental quantum mechanical constraints.<\/p>\n\n\n\n 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.<\/p>\n\n\n\n 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?<\/p>\n\n\n\n Despite their philosophical differences, both Bohr and Heisenberg ultimately concluded that asking what particles do between measurements is a meaningless question.<\/p>\n\n\n\n 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\u2014there 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.<\/p>\n\n\n\n By the 1960s, the physics community had largely abandoned foundational questions about quantum mechanics, mistaking this intellectual retreat for scientific progress.<\/p>\n\n\n\nTable of Contents<\/h2>
The Measurement Problem Explained<\/h2>\n\n\n\n
Three Responses to the Measurement Problem<\/h2>\n\n\n\n
Response #1: Quantum Mechanics Must Be Incomplete<\/h3>\n\n\n\n
Response #2: Questions About Unmeasured Reality Are Meaningless<\/h3>\n\n\n\n
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Response #3: Uncertainty<\/h3>\n\n\n\n
The Response That Won the Debate<\/h3>\n\n\n\n