A critical insight made decades later by John Stewart Bell showed that his Bell 's theorem could be used to assess the existence or non-existence of singlet entanglement experimentally .
John Stewart Bell was a strong advocate of Einstein 's locality - first perspective .
A critical insight made decades later by John Stewart Bell , who was a strong advocate of Einstein 's locality - first perspective , showed that his Bell 's theorem could be used to assess the existence or non-existence of singlet entanglement experimentally .