QUANTUM PHYSICS FUNDAMENTALS - Chapter 9, Exercise 2 Solution
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Conceptual: Why Bell's Theorem Was a Genuine Turning Point

PROBLEM
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Explain, in your own words, why John Bell's real 1964 theorem was a
genuine turning point compared to the original 1935 EPR/Bohr
disagreement, even though Bell himself did not run any experiment.

SOLUTION
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The original 1935 disagreement between Einstein, Podolsky, and Rosen
on one side and Bohr on the other was, for nearly thirty real years,
a genuine philosophical stalemate. Both positions were logically
self-consistent, and crucially, both were equally compatible with
every real experimental result available at the time - there was no
known experiment that could distinguish between "quantum mechanics is
complete" and "quantum mechanics is incomplete, and some real hidden
variable determines the outcome, we just don't know what it is yet."

Bell's real 1964 achievement was not running an experiment himself,
but proving something genuinely new mathematically: that these two
positions are NOT, in fact, equally compatible with every possible
experimental result. He showed that any theory built on local hidden
variables - the kind of explanation Einstein favoured - must obey a
specific, real, numerical bound (a Bell inequality), while quantum
mechanics predicts that entangled particles can violate that same
bound under the right conditions.

ANSWER: Bell's theorem was a genuine turning point because it
converted a purely philosophical disagreement into a real, falsifiable
experimental question with a specific, checkable numerical prediction
on each side. Before Bell, there was no experiment that could settle
the debate; after Bell, physicists knew exactly what result to look
for that would rule out local hidden variables for good.

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WHY THIS WORKS AS AN ANSWER

This is precisely why the real experimental program that followed -
Clauser's 1972 test, Aspect's 1982 experiments, Zeilinger's 1998
work, and the three independent 2015 loophole-free tests - was even
possible in the first place, and why it eventually earned Clauser,
Aspect, and Zeilinger the real 2022 Nobel Prize. Bell did not resolve
the EPR/Bohr debate himself; he did something arguably more valuable
to real science - he showed exactly how it COULD be resolved, and
handed experimentalists a real, specific number to go out and measure.