Einstein's Doubt of 'Spooky Action at a Distance': The Reality of Quantum Entanglement Proven by Bell's Inequality Experiments

In the history of physics, Einstein is recorded as a figure who contributed to the birth of quantum mechanics and, at the same time, raised questions about its direction of development. When Max Planck proposed the concept that energy is divided and exchanged in fixed units in 1900, Einstein applied this to light and established the concept of the Photon. He explained the photoelectric effect by utilizing the fact that light behaves not only as a continuous wave but also like small lumps of energy, and he received the Nobel Prize in Physics for this research.

Einstein's 'Local Realism' and the EPR Paradox

As quantum mechanics developed based on probabilistic interpretations, Einstein rose in opposition to it. When Heisenberg and Schrödinger created the mathematical framework in the 1920s and Max Bohr began to interpret the results as probabilities, Einstein believed that physical values must be determined even before measurement. In 1935, he published the 'EPR Paper' with Boris Podolsky and Nathan Rosen, questioning whether quantum mechanics fully explains nature.

There are two premises underlying the logic asserted by Einstein. The first is 'locality,' the idea that something happening in Seoul cannot immediately affect New York, and the second is 'realism,' the idea that particles actually possess physical values even before measurement. Together, these are called 'Local Realism.' He expressed his aversion to the phenomenon where two distant particles appear connected as if they were one, using the expression 'Spooky Action at a Distance.'

Bell's Inequality and Experimental Verification Toward 'Hidden Variables'

Schrödinger named the relationship where two distant particles are described by a single state 'Quantum Entanglement.' Entangled particles show an unusually strong correlation depending on the direction of measurement, and Einstein proposed the 'Hidden-Variable Theory,' suggesting that the particles must have had the correct answers predetermined from the moment they departed. David Bohm refined the theoretical framework by simplifying this through the introduction of the Spin concept.

The person who brought this philosophical debate into the realm of experiment was John Bell. In 1964, he discovered 'Bell's Inequality,' a mathematical limit that experimental results could not exceed if, as Einstein claimed, particles existed independently with predetermined answers. Since quantum mechanics predicted results that exceed this limit (up to approximately 2.83), if an experiment produced results exceeding the limit of Bell's Inequality, it could prove the validity of quantum mechanics.

Alain Aspect's Sophisticated Experiment Proves the Correlation of Quantum Entanglement

Since the 1970s, physicists have strived to confirm Bell's ideas through experiments. In 1972, John Clauser and others tested Bell's Inequality using Photons and obtained values closer to the results predicted by quantum mechanics. Later, the French physicist Alain Aspect performed even more sophisticated experiments. He invested five years in developing a device that creates entangled photon pairs by firing lasers at calcium atoms.

The results of Aspect's experiments, conducted throughout 1981 and 1982, showed values that exceeded the limit of Bell's Inequality and supported quantum mechanics. In particular, Aspect utilized an 'acousto-optic modulator' for experiments to change the measurement conditions after the Photon had departed. By rapidly changing the direction of light propagation in units of 10 nanoseconds through ultrasound, he blocked the possibility that the particle could be associated with the measurement conditions at the time of departure, confirming once again that the predictions of quantum mechanics were correct.