Professor Susskind opens the lecture by examining entanglement and density matrices in more detail. He shows that no action on one part of an entangled system can affect the statistics of the other part. This is the principle of locality and is directly connected to the requirement that systems evolve over time only through unitary operators. Violating locality implies non-local hidden variables which are equivalent to wires that transmit information instantaneously. These would allow true "spooky action at a distance," but they don't exist.