the principle of relativity
The principle of relativity is the sane, almost obvious-sounding half of Einstein's two postulates. It says: the laws of physics look and work exactly the same in every inertial reference frame — every lab that moves at constant velocity without turning or accelerating. Picture yourself in a windowless cabin on a huge, perfectly smooth ship. There is genuinely no experiment you can perform inside — no clever measurement of balls, springs, magnets, or light — that reveals whether the ship is docked or cruising in a straight line at steady speed.
Stated precisely: no inertial frame is special or 'truly at rest'. If frame A moves at constant velocity relative to frame B, the same physical laws — Newton's laws, the laws of electricity and magnetism, everything — hold in both, written with the same equations. This idea is older than Einstein; Galileo described it for mechanics using ships and dropped objects. Einstein's leap was to insist it holds for all of physics, including the behaviour of light, not just for the motion of ordinary objects.
Why it matters: this principle is why 'my velocity' has no absolute meaning — only velocity relative to something else does. It is also a powerful tool. Because the laws must be identical in every inertial frame, you are free to solve a problem in whichever frame makes it easiest, and the answer will be right in all of them.
Pour a cup of coffee on a jet cruising smoothly at 900 km/h and it falls straight into the cup, just as on the ground. The coffee 'knows' only its motion relative to the plane, not the plane's huge speed relative to the Earth.
Inside a smoothly moving cabin, physics behaves exactly as it does at rest — there is no way to feel constant velocity.
The principle covers constant velocity only. The moment the ship turns, brakes, or accelerates, you can feel it — coffee sloshes — because acceleration is physically detectable and such a frame is no longer inertial.