Special relativity: when time stops being absolute

In 1905, Albert Einstein published a paper that reconstructed all of mechanics from two postulates:

1. The laws of physics are the same in all inertial frames of reference.
2. The speed of light in a vacuum is the same for all observers, regardless of the motion of the source or the observer.

The second postulate sounds harmless but defies everyday intuition. If you run after a beam of light, you still measure it moving at exactly speed c relative to you.

Consequences

Time dilation. A clock moving relative to you will run slower than your own clock, according to the Lorentz factor γ = 1/√(1 − v²/c²).

Length contraction. A moving object shortens along the direction of motion by the same factor.

Relativity of simultaneity. Two events that occur at the same time for one person may not be simultaneous for another person who is in motion.

E = mc². Mass and energy are two sides of the same quantity.

Not just theory

GPS systems must calibrate for both special relativity effects (satellites move fast, so clocks run slow by about 7 μs/day) and general relativity effects (being further from Earth, the gravitational field is weaker, so clocks run fast by about 45 μs/day). The total is about 38 μs/day — if ignored, positioning errors would accumulate by about 10 km per day.

Muons produced in the upper atmosphere are also direct evidence: their average lifespan is too short to reach the ground, yet they still do — because from our frame of reference, their clocks run slow.

Further reading

• Gravitational waves and gravity waves: one word apart, two unrelated phenomena
• The Sun: the reactor that holds the entire planetary system