Of the four known forces, one of them stands out as different. Gravity is much weaker than the other known forces, and nobody knows why. In this video, Fermilab scientist Don Lincoln describes one possible explanation for the weakness of gravity — the existence of extra dimensions of space.

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.

Despite a century of effort, scientists have had only cursory success in describing the nature of gravity in the quantum realm. Don Lincoln talks about quantum gravity and the need to make advances in this area.