John Bell’s Take on Measurement in Quantum Mechanics

The claim that “for an atom, no attributes even exist unless it is brought into interaction with a classical measuring device”
is central to the Copenhagen interpretation of quantum mechanics. But John Bell—one of the most influential
physicists of the 20th century—challenged this view deeply and directly.

1. Bell’s Critique of Measurement-Only Reality

Bell found the measurement-centered view of quantum mechanics unsatisfying and ambiguous:

“Either the wavefunction, as given by the Schrödinger equation, is not everything, or it is not right.”

— J.S. Bell, “Against ‘Measurement’”

He was frustrated by the idea that quantum properties “don’t exist” until measured, viewing it as a philosophically weak stance
and potentially bordering on solipsism.

2. Bell’s Theorem and Hidden Variables

Bell’s famous 1964 theorem asked whether it was possible to build a theory in which physical properties exist
independently of observation. He demonstrated that any local hidden variable theory must obey certain constraints—
Bell inequalities—which quantum mechanics violates.

Experimental results support quantum mechanics, but Bell’s aim wasn’t to disprove realism—it was to test the limits of
locality and show that Copenhagen-style interpretations aren’t inevitable.

3. Bell on the Classical–Quantum Divide

Bell criticized the unclear boundary between classical and quantum systems in the Copenhagen view:

“What exactly qualifies some physical systems to play the role of ‘measurer’? Was the wavefunction of the world waiting to jump
for thousands of millions of years until a single-celled living creature appeared? Or did it have to wait a little longer
for some better qualified system … with a Ph.D.?”

— J.S. Bell, “Against ‘Measurement’” (1990)

He viewed this as a fundamental vagueness that undermined the coherence of the theory.

4. Beables vs Observables

Bell introduced the term “beables”—as opposed to “observables”—to refer to elements of a theory that
exist independently of measurement:

“The beables of the theory are those elements which might correspond to elements of reality, to things which exist.”

— J.S. Bell

He favored interpretations like Bohmian mechanics, which provide a realist account of particles and trajectories,
guided by the wavefunction.

Conclusion

Bell firmly rejected the idea that “attributes don’t exist unless measured.” He saw it not as a necessity, but as a flaw of
the Copenhagen interpretation—a sign that quantum mechanics, in its standard form, might be incomplete.

For Bell, the goal was to restore a kind of objective reality to physics—a world in which things exist whether or not
anyone is looking.