These are worlds identical or nearly identical to a point in Homeline's past.
Important Note[]
"Infinity names echoes based on the number of years the echo’s local present lags behind Homeline" (GURPS Infinite Worlds pg 85)
For example "An echo with a local present date of 1776 would be Echo 251, or 251 Minus (both names get used interchangeably), since 1776 is 251 years behind Homeline’s present date of 2027." (GURPS Infinite Worlds pg 85)
So something like "Echo -222" would NOT be an Echo of 222 BCE but rather an Echo of whatever is 222 years behind Homeline’s present date which likely is c 2038 making it an Echo of c 1816.
All items (28)