Einstein's Two Postulates

In 1905, Albert Einstein published a paper that overturned our understanding of space and time. The starting point was neither a new particle nor a new instrument, but a demand for consistency between two areas of physics that seemed to contradict each other: Newton's mechanics and Maxwell's electromagnetism. Einstein then laid down two deceptively simple postulates.

The first postulate, called the principle of relativity, states that the laws of physics take the same form in all inertial frames of reference, that is, all frames that undergo no acceleration. Concretely, no experiment carried out inside a closed spaceship moving at constant velocity can tell whether the ship is "at rest" or "in motion": these notions lose their absolute meaning. This postulate extends an idea already present in Galileo's work, but Einstein applies it without exception, including to electromagnetism.

The second postulate is far more radical: the speed of light in a vacuum, denoted c, has the same value for all inertial observers, regardless of the speed of the source emitting the light or of the observer receiving it. This statement flatly contradicts the intuition of classical mechanics, where velocities simply add up. If you run after a light ray, you will never catch it: it will keep moving away from you at exactly the speed c.

Taken together, these two postulates force us to abandon the idea of a universal, absolute time, identical throughout the universe. The diagram below introduces the Minkowski spacetime diagram, the central geometric tool of special relativity: the vertical axis represents time multiplied by c (written ct), the horizontal axis represents space (x), and the light cone shows the possible paths of a light ray emitted at the origin, always inclined at 45 degrees.

                   ct
                    ^
            \       |       /
             \ future cone /
              \     |     /
               \    |    /
                \   |   /
                 \  |  /
                  \ | /
                   \|/
--------------------O---------> x
                   /|\
                  / | \
                 /  |  \
                /   |   \
               /    |    \
              /     |     \
             /  past cone  \
            /       |       \

(each light ray emitted at O follows a line at 45 degrees: x = ct or x = -ct)
(any event in the future cone can be reached from O without exceeding c)

Any event located inside the future cone can be reached from the origin without exceeding c; any event outside the cone would require a speed greater than that of light, which is impossible according to the second postulate. Remember this point: the cone structure is not decoration, it directly encodes the causality of the universe.