Schematic diagram of loophole-free Bell test, with time on the vertical axis and space on the horizontal axis. Two entangled photons are generated by a source. Each proceeds on a separate path to different detector destinations (Alice and Bob respectively) placed several hundred meters apart. At some time between each photon's generation and its arrival at the detector, a random choice of measurement basis (e.g., polarization) is made. That choice affects the result when each photon hits its detector. The blue lines represent light cones: the space-time volume within which one object can interact with another. The position of the lines indicates that it is impossible for information about the basis choice made on Alice's photon — even if it is traveling at the speed of light — to get to Bob's photon before it is detected, and vice versa. This arrangement closes the "locality" or "light-cone" loophole.