Measuring the distances between Worlds in Probability has always been problematic. As beings limited to perceiving reality in only four dimensions, direct measurement of the fifth is simply not possible. Instead proxy methods have to be deployed, the most common of which defaults to the amount of energy required to either open Gates or move matter directly between Worlds.
The best established measurement is the Wyrymyan ghəι (anglicized as “weyr“). The exact genesis of the weyr is lost to history, although it is noted it is very close to the amount of artonic energy required to directly (ie: without the use of a Gate) transport one ixh* of matter between Wyrymya and the ancient Wyrm colony World of Hfren, leading many to assume ancient travel between these two Worlds as its ultimate origin.
(* One ixh is equal to one 1728th of the weight of a cylinder of Wyrymyan sea-water with a height and radius of 12 ghi – one ghi measuring 6.875 cm. Bizarrely this works out to just over 1kg – 1.069kg to be precise.)
The traditional measurement of Probability among the Zurvár is the kâd, which is the minimum distance traversable via Gate in ancient Zurvár society. One kâd is equal to 16.8 weyr, which provides some commentary on the relative sophistication of Probatial travel in the ancient Zurvár and Wyrm cultures. The kâd has become the standard measure of Probatial distance across local Probability, with the weyr generally reserved for scientific usage.
(The Metaphysical Society of 19th century London created their own measurement of Probatial distance, the “Palmerston”. One Palmerston equals 77.4 kâd (or 1300.32 weyr), and is the distance between Earth and Neanderthan, the first World discovered by the Society. The Palmerston was abandoned shortly after contact was established with the Wyrms.)
The record for minimum Probatial distance traversable currently stands at 0.43 kâd (7.224 weyr) which was achieved at the Werinos Physics Institute in 2007. This is well above the theoretical limit of 0.14 kâd predicted by Probatial Resonance Theory. It is believed by many that the Goatsuckers have been capable of reaching this limit for centuries – if not millennia – or possibly even exceeding it, although the later would raise uncomfortable questions about their motivation in providing so much assistance in the development of an inaccurate theory.