Physics says, spacetime is curved. This sounds mystical, but it’s simple.
Nothing is ever straight because nothing stops moving.
We’re told the shortest distance between two points is a straight line. But if an airplane plotted a course between two cities in a totally “straight” line, it would be a crash course — a straight line between two points on the earth goes underground.
Instead, the airplane flies in a curve, matching the curvature of the earth. Spacetime is like the surface of the earth — looks flat locally, curved globally. Compared to walking, the airplane seems like it can “cheat” the rules, but it still has to follow the curve. Even spaceships and satellites.
Imagine throwing something at a moving target — playing dodgeball, or skeet shooting. You have to aim where your target will be when the projectile gets there, not where it is now. When a projectile is launched at a target, it takes a little more energy to get it there to account for the time that passes.
I wrote before how spacetime means constant motion. There’s no such thing as sitting still. The earth orbits the sun, which orbits the galaxy, which moves relative to other galaxies. Galaxies spin and drift.
The straightest thing should be a beam of light, but even a laser beam will eventtually curve from its path. Light is attracted by gravity, and the force of gravity moves along with the masses it comes from.
Space and time and motion can’t be discussed separately, and neither can gravity and energy.
