General relativity, what Albert Einstein said was the happiest thought of his life, basically expands upon the idea that “the laws of physics are the same everywhere“. Light and gravity behave in the same way for you as they do for the astronauts on the International Space Station. From LiveScience:
…imagine the spaceship speeds up by firing its thrusters. Those onboard will stick to the ground just as if they were on Earth. Einstein claimed that the force we call gravity is indistinguishable from being in an accelerating ship.
This by itself was not so revolutionary, but when Einstein worked out the complex math (it took him 10 years), he discovered that space and time are curved near a massive object, and this curvature is what we experience as the force of gravity.
It is difficult to picture the curved geometry of general relativity, but if one thinks of space-time as a kind of fabric, then a massive object stretches the surrounding fabric such that anything passing nearby no longer follows a straight line.
From Fusion: General relativity explained in under three minutes by Einstein biographer Walter Isaacson, and below, Feynman Professor of Theoretical Physics Kip Thorne explains why GPS wouldn’t work if we didn’t know about relativity: