principle of relativity

The second assumption is a little more troubling, especially since it seems to contradict the principle of relativity.

In 1907, while still at the patent office, he started to think about extending the principle of relativity from uniform to arbitrary motion through a new theory of gravity.

For example, Max Planck commented about the evolution of relativity theory in a 1910 article, “The Mechanical Theory of Nature”: “The principle of relativity, despite its youth, appears very promising.”

The principle of relativity, though, is more fundamental than our experience of space and time, and the differences between moving observers are inevitable, once you properly understand what it means to measure space and time.

He demonstrated his way of making fundamental science in the notion “movement in vacuum” that resulted in the law of inertia, the principle of relativity, and, most importantly, the law of free fall.