Chasing a beam of light.

When Einstein was young, he wondered what it would be like to chase a beam of light. Years later, this insight led him to develop Special Relativity (SR) from two simple postulates:

1. The laws of physics are the same for all observers in uniform motion relative to one another.
2. The speed of light in vacuum is the same for all observers, independent of the relative motion of the source (c = 299.792.458 m/s).

With those two assumptions, he changed the way we understand space and time:

• There's a speed limit: c is the speed limit in the universe. Nothing can travel faster than c.
• Matter moves below c: Nothing with mass can reach c speed (because it would need infinite energy to accelerate).
• Spacetime warps: Objects in motion experience length contraction and time dilation. (It has to be that way if we want to keep the speed of light constant, despite observer's motion.)

Relativity changed our ideas about absolute space and time. Concepts like distance, simultaneity or duration became flexible and dependent on motion.

But it's kind of tragic that the same question that led him to develop SR could hold the key to understand QM.