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" ... This is the big idea of General Relativity, and why it's such an upgraded idea from Special Relativity. Yes, space and time are still stitched together into a unified entity: spacetime. Yes, all massless particles travel at the speed of light relative to all observers, and all massive particles can never attain that speed. Instead, they move through the Universe seeing lengths contract, times dilating and — in an upgrade from Special to General Relativity — seeing novel gravitational phenomena that wouldn't appear otherwise. ... "