time dilation
Time dilation is a physical phenomenon predicted by Einstein's theory of relativity. Special relativity shows that time slows down for objects moving at high velocities (velocity-based time dilation), while general relativity shows that time also slows in stronger gravitational fields (gravitational time dilation). Both effects have been confirmed experimentally and are accepted physical facts.
GPS is the most prominent real-world system that actively corrects for time dilation. At an orbital altitude of about 20,000 km, weaker gravity causes satellite clocks to run approximately 45.8 microseconds per day fast (general relativity), while their orbital speed of roughly 3.87 km/s causes them to lose about 7.2 microseconds per day (special relativity). The net correction of 38.6 microseconds per day is essential; without it, positioning errors would accumulate at about 11.6 km per day.
At everyday speeds and altitudes, time dilation is extraordinarily small. A bullet train at 300 km/h experiences roughly 0.1 picoseconds of dilation per second, and the difference between the top of Tokyo Skytree (450 m) and ground level is about 4 nanoseconds per day. These differences are too small to detect without optical lattice clocks, yet they are physically real and cannot be ignored in ultra-precise timekeeping.
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atomic clock
An atomic clock measures time by using the quantum-mechanical transition frequency of atoms as its reference, achieving accuracies on the order of one second in hundreds of millions of years and forming the backbone of modern timekeeping.
gps time
GPS Time (GPST) is the continuous time scale used internally by the Global Positioning System, originating on January 6, 1980 and never incorporating leap seconds.
utc
The primary time standard by which the world regulates clocks and time, serving as the basis for civil timekeeping globally.
Einstein's theories of relativity show that time is not absolute. This article explains time dilation due to speed and gravity through concrete examples: GPS corrections, the twin paradox, the Tokyo Skytree experiment, and the extreme slowdowns near black holes.
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