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Watson received an electronic signal that the buzzers were active. The human contestants have to time it via Trebek's voice or the light.

An electronic signal and a photonic signal ("the light") both travel at the same speed.

An incandescent light might take a nontrivial amount of time (tenth of a second?) to heat up and light after power is applied.

But a computer's detection and response to a signal is probably faster than a human's reaction time.