Figure from ROBOT, Moravec, Oxford, 1998, Chapter 6: The Age of Mind, page 181
simple time loops

Simple time loops
Time-travel issues are illustrated by causal loops involving an amplifier (cone) and inverter (cone with knob), with negative time delays (double-cone structures) to cancel their forward delay. The amplifier loop is consistent, while the inverter loop is a simple example of paradoxical time travel. Depending on its representation, the inverter loop's signal can either hover at a value intermediate between 1 and 0, exist in a mixture of 1 and 0, or refuse to appear at all. Or the components of the circuit can mysteriously fail! The paradoxical loop eliminates the possibility that the circuit will work normally, and thus brings to the fore a host of otherwise less likely alternatives.

Examine the figure in 3D.
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