Why did Bohr and Einstein disagree about whether measurement determines quantum reality?

The disagreement between Niels Bohr and Albert Einstein regarding quantum reality centered on the nature of physical existence and the role of the observer.

Niels Bohr, a primary architect of the Copenhagen interpretation, argued that quantum mechanics is a complete description of nature, but one that is inherently probabilistic. Central to his view was the complementarity principle, which posits that physical systems possess properties—such as wave or particle behavior—that are mutually exclusive but both necessary for a full description. For Bohr, reality is not independent of the measurement process; rather, a physical property only gains definite status through the act of measurement.12

Albert Einstein rejected this view, famously asserting that "God does not play dice". He maintained that the physical world must possess objective reality independent of whether it is observed. Einstein argued that if quantum mechanics could not predict properties with certainty without measurement, the theory was "incomplete". He and his colleagues proposed that there must be underlying "elements of reality" that exist regardless of our measurements, suggesting that the randomness in quantum mechanics was a limitation of our knowledge rather than a fundamental feature of the universe.123