Event horizon is spherical outer boundary of black hole. Gravitational influence prevents light from escaping at event horizon. Objects approaching horizon appear reddening and fading. Event horizon contains singularity where all mass collapsed
Einstein first demonstrated gravitational waves in 1916 as spacetime ripples. First indirect evidence came from Hulse-Taylor pulsar binary in 1974. First direct detection occurred in 2015 from black hole merger. Nobel Prize awarded to Weiss, Thorne, and Barish in 2017
BBC and HBO produced drama featuring David Tennant as Eddington and Andy Serkis as Einstein. Filmed at Hungarian Academy of Sciences, Cambridge and Croatia. Written by Peter Moffat and directed by Philip Martin
Gravitational acceleration is the steady speed gain in free fall without drag. All bodies accelerate at same rate in vacuum regardless of mass. Earth's gravity ranges from 9.764 to 9.834 m/s² depending on location. Standard value is 9.80665 m/s² (about 32.1740 ft/s²)
Free fall occurs when gravity is the only force acting on a body. Objects can be in free fall even when moving upwards or at rest. Weightlessness occurs when there are no other forces acting
Every particle attracts every other particle with force proportional to masses and inversely proportional to distance squared. Law published in Principia Mathematica in 1687, first great unification of gravity and astronomy. Equation: F = Gm1m2r, where G is gravitational constant (6.674×10⁻¹¹ m³⋅kg⁻¹⋅s⁻²)