We ended the last chapter with the quantum eraser experiment, which seems to indicate that the photon “knows” when we—the observers*—are watching. Indeed, the photon behaves very differently when we—the observers—can say (at least in principle) which path it has traveled.
The thought that an objective reality does not exist independently of an observer troubled Einstein very much. In opposition to Bohr’s group in Copenhagen, Einstein believed that the fact that quantum mechanics could only provide an answer in terms of probability meant the theory was incomplete. This was a lively discussion he maintained with Bohr for many years.†
In 1935, Einstein refined the philosophical discussion into a physical argument. At Princeton University, Einstein and his assistants Boris Podolsky and Nathan Rosen authored a paper titled “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?”40 This now-famous paper, better known simply as “the EPR paper” (for Einstein–Podolsky–Rosen), proposed a thought experiment they felt revealed an underlying, objective reality independent of measurement. The EPR argument seemed to completely contradict the Copenhagen Interpretation.
At the heart of the EPR thought experiment is a particle source that produces pairs of particles that have some property that is forever linked. For example, Figure 136a shows what happens when a subatomic particle called a pi meson (also known as a pion) decays into an electron and ...