- "I do hope we have a moment to discuss field density and its relationship to gravity phenomena."
- "Indeed. I would appreciate such a talk. It is an interest of mine."
The field density between the universe and the mirror universe was important in one method to cross over. In 2267, an away team from the USS Enterprise crossed over into the mirror universe. Montgomery Scott used the ISS Enterprise to cross back over, requiring having a large field density. (TOS: "Mirror, Mirror")
In 2269, field density was an interest of Starfleet Commander Spock. It had also been studied by the android Rayna Kapec (along with such other studies like sub-dimensional physics) who had learned the concepts from the immortal Human Flint. (TOS: "Requiem for Methuselah")
The term field density was also used as a unit of measurement for phase coils. Geordi La Forge's warp drive modifications made to the USS Enterprise-D included the phase coils upgraded to 55 field densities. (TNG: "Galaxy's Child")