# Black Holes, Tides, and Curved Spacetime Understanding Gravity Course

## 24 episodes in this series

Episode 1 The Strangest Force
Begin your exploration of gravity with Isaac Newton and the famous story of the apple. Why was it such a breakthrough to connect a falling apple with the faraway moon?…
Episode 2 Free Fall and Inertia
Review three great discoveries by the "grandfather" of gravity research, Galileo Galilei. His most famous experiment may never have happened, but his principle of inertia, law of free fall, and…
Episode 3 Revolution in the Heavens
Drawing on ideas and observations of Nicolaus Copernicus and Tycho Brahe, Johannes Kepler achieved a great insight about gravity by discovering three laws of planetary motion, relating to the mathematics…
Episode 4 Universal Gravitation
See how Newton was able to finish Kepler's revolution by formulating the law of universal gravitation, which says that every object exerts an attractive force on every other object. Also…
Episode 5 The Art of Experiment
Learn how distances in the solar system were first determined. Then chart Henry Cavendish's historic experiment that found the value of Newton's gravitational constant. Cavendish's work allows almost everything in…
Episode 6 Escape Velocity, Energy, and Rotation
Begin the first of several episodes that dig deeper into Newton's laws than Newton himself was able to go. In this episode, apply the key concepts of energy and angular…
Episode 7 Stars in Their Courses—Orbital Mechanics
Newton was the first to realize that objects could, in theory, be sent into orbit around Earth. Explore how this works in practice, using the ideas of energy and angular…
Episode 8 What Are Tides? Earth and Beyond
Trace the origin of tides to the simple fact that gravity varies from point to point in space. This leads not just to the rise and fall of the ocean,…
Episode 9 Nudge—Perturbations of Orbits
For the next three episodes, study the effects of gravity on the motions of more than two bodies. Here, see how even very small orbital changes--small perturbations--are significant. Such effects…
Episode 10 Resonance—Surprises in the Intricate Dance
Resonance happens whenever a small periodic force produces a large effect on a periodic motion--for example, when you push a child on a swing. Learn how resonance due to gravitational…
Episode 11 The Million-Body Problem
Consider the problem of gravitational interactions between millions of bodies, such as the countless stars in a galaxy. Amazingly, mathematics can reveal useful information even in these complicated cases. Discover…
Episode 12 The Billion-Year Battle
Explore the physics of stars, which are balls of gas in a billion-year battle between the inward pull of gravity and the outward pressure produced by nuclear fusion. Follow this…
Episode 13 From Forces to Fields
For the rest of the course, focus on the revolutionary view of gravitation launched by Albert Einstein. Review new ideas about fields that allowed physics to extend beyond Newtonian mechanics.…
Episode 14 The Falling Laboratory
Einstein focused on gravity in his general theory of relativity. Hear about his "happiest thought"--the realization that a man in free fall perceives gravity as zero. This simple insight resolved…
Episode 15 Spacetime in Zero Gravity
In an influential interpretation of relativity, Einstein's former mathematics professor Hermann Minkowski reformulated the theory in terms of four-dimensional geometry, which he called spacetime. Learn how to plot events in…
Episode 16 Spacetime Tells Matter How to Move
See how gravity affects Minkowski's spacetime geometry, discovering that motion in a gravitational field follows the straightest path in curved spacetime. The curvature in spacetime is not caused by gravity;…
Episode 17 Matter Tells Spacetime How to Curve
The curvature of spacetime depends upon matter--and tidal effects. In this episode, see how ordinary tidal effects reveal a simplified form of Einstein's greatest discovery: the equation governing the curvature…
Episode 18 Light in Curved Spacetime
See how Einstein's general theory of relativity predicts the bending of light in a gravitational field, famously confirmed in 1919 by the British scientist Arthur Eddington. Learn how this phenomenon…
Episode 19 Gravitomagnetism and Gravitational Waves
The general theory of relativity predicts new phenomena of gravity analogous to those of electromagnetism. Discover how ultra-sensitive experiments have detected the gravitomagnetism of the Earth, and follow the search…
Episode 20 Gravity’s Horizon—Anatomy of a Black Hole
Plunge into the subject of black holes, which are massive objects that have collapsed completely under their own gravity. Learn how black holes distort spacetime and explore the supermassive black…
Episode 21 Which Universe Is Ours?
Investigate what Einstein called his "greatest mistake"--his rejection of his own theory's prediction that spacetime should be dynamic and evolving. Chart the work of a group of scientists, including Alexander…
Episode 22 Cosmic Antigravity—Inflation and Dark Energy
In this episode, investigate cosmic antigravity, starting with cosmic inflation, a phenomenon that exponentially increased the size of the universe during the big bang. Then, learn why dark matter cannot…
Episode 23 The Force of Creation
Use a black hole to test the laws of thermodynamics, taking a deeper look at the capacity of gravity to pull matter together and increase entropy at the same time.…
Episode 24 The Next Revolution
Survey the greatest unsolved problem in theoretical physics: the search for a quantum theory of gravity. Examine string theory, loop quantum gravity, and also entropic gravity, which suggests a revolutionary…

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