Transcript

To contrast the principles of Einstein and Newton, you could place a super-accurate photon clock at the very top of a super-tall building, and another one at the very bottom. The bottom clock sends a photon to the top every time it ticks. At the top, we can then compare the arrival of the pulses to the ticking of the top clock. And if Newton is right, those two super-accurate clocks will always be ticking at the same rate as each other. But they’re not! The difference is incredibly tiny, but the clock at the bottom of the building runs slower than the clock at the top. If spacetime were flat, this would be impossible. But in Einstein’s relative world, this makes perfect sense, because he said that spacetime is curved.