Explain SMBC — the wiki for Saturday Morning Breakfast Cereal

This comic offers a humorous explanation for the Fermi Paradox -- the question of why we have not detected signals from alien civilizations despite the vast number of stars and planets in the universe. The comic proposes that all intelligent species eventually discover science and then quickly either destroy themselves or become so absorbed in technology (such as virtual reality) that they stop caring about the outside world. Because of this, the window of time during which any civilization is broadcasting wireless signals into space is extremely short, producing only a thin expanding shell of radio waves.

The comic then points out that the odds of another civilization being both capable of receiving those signals and existing at the right time to intercept the thin shell are terrible. As a bonus joke, it lists what Earth's own signal shell would contain: Hitler's first broadcast, early amateur radio users shouting profanity into space, crappy TV, less crappy TV, a hypothetical diplomatic incident, and gamma radiation (implying humanity's eventual self-destruction). The alien civilization depicted is humorously primitive, building pyramids and too preoccupied to notice any incoming signal.