Physicist Etienne Klein traces the history of quantum physics and shows how it gradually forced scientists to question seemingly established notions in classical physics. In particular, classical physics distinguishes two clearly separated categories of objects: particles and waves, whereas a quantum object may behave alternatively as a wave or as a particle.
One of the controversial theoretical consequences of quantum physics is the assumed existence of entangled particles, i.e. particles that, although separate, behave as a single entity.
Alain Aspect continues after Etienne Klein at this point to tell us that these entangled particles exist. His team was able to create pairs of entangled photons in 1982, that is, photons that show strong correlations, even when separated by long distances.