Equivalence principle

The equivalence principle states that gravitational mass (how much an object is pulled by gravity) and inertial mass (how much an object resists acceleration) are fundamentally indistinguishable. This means an observer in a sealed box cannot tell the difference between sitting on Earth feeling gravity, or being accelerated upward through space in a rocket.

The principle was famously formulated by Albert Einstein in 1907. He referred to it as his "happiest thought" and used it as the foundational building block to derive his theory of General Relativity.

In classical physics, inertial mass dictates how hard it is to push an object, while gravitational mass dictates how heavy an object is in a gravity field. The equivalence principle declares that, despite their different definitions, experiments always show they are numerically identical.

The principle has been tested extensively using specialized torsion balances and by dropping objects in a vacuum. Modern satellite experiments, such as the MICROSCOPE satellite launched in 2016, have confirmed the equivalence principle to an astonishing precision.