Okay, so now that you understand the basic idea of entanglement, the other concept we need to understand before we look at this infamous double-slit experiment is the idea of light being able to exhibit two different characteristics, both as a wave and a discrete particle called a photon.

Rather than get into a fulsome discussion of waves and particles, waves or particles, or waves only, which is a long-standing, ongoing argument between scientists, it will speed things up if we just accept that for the purpose of the following experiment, we accept Einstein's premise that "it seems as though we must sometimes use the one theory and sometimes the other. While at times we may use either. We are faced with a new kind of difficulty. We have two contradictory pictures of reality; separately, neither of them fully explains the phenomena of light, but together they do."

"The physicists of the period. They could but make the best of it, and went around with woebegone faces sadly complaining that on Mondays, Wednesdays, and Fridays, they must look on light as a wave; on Tuesdays, Thursdays, and Saturdays, as a particle. On Sundays, they simply prayed."
— Banesh Hoffman - The Strange Story of the Quantum

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