General Relativity
Albert Einstein · 1915
"Gravity isn't a force pulling objects together — it's the curvature of spacetime itself caused by mass and energy, and objects simply follow the straightest possible path through that curved geometry."
A decade after special relativity, Einstein extended the theory to acceleration and gravity, replacing Newton's invisible pulling force with something stranger: mass literally bends the shape of space and time around it.
Einstein's field equations describe how matter and energy determine the curvature of four-dimensional spacetime, and how that curvature in turn determines how matter moves through it — famously summarized by physicist John Wheeler as 'spacetime tells matter how to move; matter tells spacetime how to curve.' A planet orbits the sun not because it's being pulled by a force, but because it's traveling in a straight line through spacetime that the sun's mass has curved into a well. The theory made testable predictions Newton's gravity couldn't — including the exact bending of starlight around the sun, confirmed in a famous 1919 eclipse expedition — and it remains the foundation for modern cosmology, including how the Big Bang and an expanding universe are modeled mathematically.
How does general relativity's explanation of gravity differ from Newton's?
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The explanation above is written with AI assistance. These are the originals — go to them to check it.
- The Field Equations of Gravitation (1915)Prussian Academy of Sciences
Special Relativity
"Space and time aren't fixed backdrops — they stretch and compress depending on how fast you're moving relative to what you're measuring."