gravity
From New Scientist, July 1, 2020: Fermilab theorist Gordan Krnjaic and Fermilab postdoc Daniel Carney of the University of Maryland are quoted in this article on efforts to sense its minuscule gravitational force as it brushes past us.
In recent years, scientists have found ways to study black holes, listening to the gravitational waves they unleash when they collide and even creating an image of one by combining information from radio telescopes around the world. But our knowledge of black holes remains limited. So scientists are figuring out how to make do with substitutes — analogs to black holes that may hold answers to mysteries about gravity and quantum mechanics.
The inability of scientists to create a theory of quantum gravity arises from long-standing tensions between general relativity and quantum mechanics. There have been few approaches with any success. Don Lincoln explains one of the few promising ideas, called loop quantum gravity, in this 9-minute video.