What is gravity? Newton left that question unanswered when he formulated

his inverse square law of the gravitational force, framing no hypothesis

for what agency transmits gravity, only asserting it was an action

at a distance. Einstein brought gravity into relativity by answering

Newton's unanswered question with his General Relativity, our modern

theory of gravity. In Einstein's formulation, Matter tells spacetime how

to curve, and curved spacetime tells matter how to move. This lecture

presents the basic picture of General Relativity, and introduces some

of its observational consequences. The surprising conclusion is that

instead of space and time being a backdrop for physics in Newton's view,

united into spacetime by Relativity they are understood to be physical

and dynamic. This is important for understanding how the Universe as

a whole works.

Recorded 2006 February 21 in 1008 Evans Laboratory on the Columbus campus

of The Ohio State University.