Our everyday lives are limited compared to the speeds, masses, and lengths that we experience. What we find is that reality has unexpected features which manifest outside our everyday experience. In this module we looked at nature at extreme mass.

Looking back to what Newton proposed for understanding gravity, we find that some conundrums were left unresolved. In particular, Newton could not explain any mechanism for gravity. Moreover, (and in part a result of that point) gravity seemed to act instantaneously, and so outrun the speed of light. What Einstein posited is that the mechanism for gravity is understood better as geometry. This mechanism and Einstein’s equations led to the prediction that gravity was not instantaneous, but instead moved at the speed of light. Detailed takeaways include:

• General Relativity: Einstein’s theory in which gravity is described in terms of warps and curves in spacetime.
  • Agrees with Newton’s theory in familiar contexts, differs otherwise.
• Gravitational Waves: A consequence of general relativity. If spacetime is jostled, ripples can be sent traveling outward — at the speed of light.
• Black Holes: A consequence of general relativity. If mass M is squeezed smaller than size R = 2 G M/c², turns into a black hole.
• Expanding Universe: A consequence of general relativity. Universe is expanding and has been since “the bang” 13.8 billion years ago.