Special Relativity
Special relativity starts from the principle that the speed of light is the same for every observer. As a result, time runs slower for a fast-moving object (time dilation). Seen through a light clock, the light on a moving clock travels a longer diagonal path, but its speed is unchanged, so one tick takes γ = 1/√(1−v²/c²) times longer.
01Special Relativity
Concept at a GlanceThe starting point of special relativity: the speed of light c is the same for every observer (invariance of c). From this single fact, time stretches.
A light clock — one round trip of light between the top and bottom mirrors is one tick. In a stationary clock the light travels straight up and down.
Now the right clock moves at v=0.6c. In the stationary frame, the light takes a longer diagonal path.
The speed of light is unchanged but the path is longer, so one tick of the moving clock takes γ times as long (time dilation).
The faster it goes (v=0.9c), the larger γ grows and the slower the clock runs. Near the speed of light, time seems almost to stop.
So the fast-moving one ages less (the twin paradox). GPS satellites correct for this effect every day.
02 Understand It Simply
For EveryoneThe speed of light is the same c for every observer. Light on a moving frame has to cover a longer path and cannot go faster, so that frame's time runs slower instead.
Because the speed of light never changes, a longer path can only mean more time elapses.
So a moving clock runs γ times slower to a stationary observer.
The closer v is to c, the more sharply γ grows, until time seems almost to stop.
- –GPS satellite clock correction
- –extended particle lifetimes in accelerators
- –the twin paradox
- –understanding time differences in space travel
03 Frequently Asked Questions
FAQWhat is Special Relativity?+
Special relativity starts from the principle that the speed of light is the same for every observer. As a result, time runs slower for a fast-moving object (time dilation). Seen through a light clock, the light on a moving clock travels a longer diagonal path, but its speed is unchanged, so one tick takes γ = 1/√(1−v²/c²) times longer.
Where is Special Relativity used?+
GPS satellite clock correction, extended particle lifetimes in accelerators, the twin paradox, understanding time differences in space travel.
What's a simple analogy for Special Relativity?+
The speed of light is the same c for every observer. Light on a moving frame has to cover a longer path and cannot go faster, so that frame's time runs slower instead.