breakdown of Newtonian gravity
Newton's law of gravity is staggeringly good — good enough to send spacecraft to the outer planets and back. So why do we say it is wrong? The honest answer is that it is an approximation: it works beautifully when gravity is weak and speeds are slow, but it quietly fails in the extremes, where a deeper theory, Einstein's general relativity, takes over. Understanding where the cracks appear is understanding the edge of Newton's world.
Newtonian gravity assumes gravity is an instant force acting across space and that space and time are a fixed, unchanging stage. General relativity replaces this with a radically different picture: mass and energy curve spacetime itself, and gravity is what we feel as objects follow the straightest possible paths through that curved geometry. In everyday conditions the two agree to many decimal places — that is why Newton is still taught and used. The difference only becomes visible when gravity is strong (near very dense, massive objects) or when speeds approach that of light.
Concrete cracks mark the boundary. The extra 43 arcseconds per century in Mercury's orbital precession, the bending of starlight grazing the Sun (twice what Newton predicts), the slowing of clocks deep in a gravitational well (GPS satellites must correct for it), gravitational waves rippling out from merging black holes, and the existence of event horizons — none of these fits Newton, and all of them match general relativity. Newton is not so much false as incomplete: a superb low-energy, weak-field limit of a grander theory.
GPS satellites must correct their clocks for general relativity: clocks aboard tick faster than ground clocks because they sit higher in Earth's gravity, and ignoring this Newton-defying effect would throw navigation off by kilometers within a day.
Newton's gravity is not enough even for the satellites in your phone's map app.
General relativity does not overturn Newton; it contains him. In weak fields and at slow speeds Einstein's equations reduce exactly to Newton's, which is why Newtonian gravity remains the right tool for almost all of everyday astronomy and engineering.