Total Perspective Vortex
The Total Perspective Vortex is allegedly the most horrible torture device to which a sentient being can be subjected.
When you are put into the Vortex you are given just one momentary glimpse of the entire unimaginable infinity of creation, and somewhere in it a tiny little mark, a microscopic dot on a microscopic dot, which says, "You are here."
Located on Frogstar World B, the machine was originally invented by one Trin Tragula in order to annoy his wife. Because she was forever nagging him for having no sense of proportion, he decided to invent something that would show her what having a sense of proportion really meant. Unfortunately the shock of being placed in the Vortex destroyed her brain, but Trin Tragula's grief was tempered by the knowledge that he had been right and she had been wrong. In Adams's words, the Total Perspective Vortex illustrated that "In an infinite universe, the one thing sentient life cannot afford to have is a sense of proportion."
The machine produces a virtual reality model of the entire universe by means of the axiom that any piece of matter is affected by all other matter. The Vortex reconstructs the universe through computer processing of a high-resolution scan ("extrapolated matter analysis") of a piece of fairy cake. In the words of the Hitchhiker's Guide,
...since every piece of matter in the Universe is in someway affected by every other piece of matter in the Universe, it is in theory possible to extrapolate the whole of creation - every Galaxy, every sun, every planet, their orbits, their composition, and their economic and social history from, say, one small piece of fairy cake.
Only Zaphod Beeblebrox is reported to have survived the Vortex unscathed (and then to have eaten the small piece of fairy cake). When it showed him the "You Are Here" marker, Zaphod correctly interpreted the Vortex as simply telling him that he was the most important being in the universe. This is due to the fact that he entered the Vortex in an artificial universe, which had been specially created for his benefit (thus making him the most important being in it) by Zarniwoop. After emerging from the artificial universe's Total Perspective Vortex, Zaphod ate the piece of fairy cake, saying "If I told you how much I needed this, I wouldn't have time to eat it."