Intro - what is an SEP?

The Hitchhiker's Guide to the Galaxy

An SEP field is a generated energy field which affects perception. Entities within the field will be perceived by an outside observer as "Somebody Else's Problem", and will therefore be effectively invisible unless the observer is specifically looking for the entity. This effect is greatly heightened if the entity within the field is already unexpected or out of place. The primary example of this was given in the third book Life, the Universe and Everything, when a spaceship built to look like an up-ended Italian bistro utilizes a SEP field to land unobserved in the middle of Lord's Cricket Ground. Another example occurs when the aforementioned ship's field is extended so that the characters fail to notice the fact that they can't breathe or the fact that the asteroid that they are standing on doesn't have enough gravitational force to hold them down, and thus are able to breathe and stay grounded. It should be noted that a SEP field won't render an object invisible if it's expected to be there, and an SEP-cloaked object may be noticed out of the corner of the eye.

The SEP field requires much less energy than a normal invisibility field (a single torch battery can run it for over a hundred years) due to the natural propensity of people to see things as Somebody Else's Problem. This is very close to the idea suggested by Terry Pratchett (who has often been compared to Douglas Adams): People don't see whatever they are sure can't be there.

In the novel, there is a character mentioned to try and create an invisibility field, and given a year, (with a bet on his life) he could render an entire mountain invisible. It's said he spent so much time fiddling with an invisibility field which never worked, that at the very last possible second, he did the most drastic last-minute work ever seen by the known Universe, and by the deadline, the mountain was gone from vision. However, he lost the bet shortly afterwards because there was a curious new moon discovered around the same time. Had he instead painted the mountain pink and erected an SEP field around it, he would probably have won, as most people don't expect to see a pink mountain, especially when someone claims it invisible (or absent).