I am looking for a book title and author of a story set in the Jurassic or similar period, with dinosaurs, but also some kind of aliens that resemble ghost. In the story it seems that in the Earth in that period the gravity is lower than today.
It looks like "End of an Era" by Robert J. Sawyer.
Archaeologist Brandon Thackery and his rival Miles 'Klicks' Jordan fulfill a dinosaur lover's dream with history's first time-travel jaunt to the late Mesozoic. Hoping to solve the extinction mystery, they find Earth's gravity is only half its 21st century value and dinosaurs that behave very strangely. Could the slimey blue creatures from Mars have something to do with both?
Look like ghosts: not quite, the Het are virus-based, jelly-like (Hal Clement's Eye of the Needle anyone?) and invade mammalian bodies to use them as locomotion and manipulation.
Lower gravity: check (and I think this is the clincher), thanks to Het gravity-manipulation satellites.