Time dilation is a phenomenon predicted by the theory of relativity, both the special theory of relativity (STR) and the general theory of relativity (GTR). It refers to the difference in the passage of time between two observers moving relative to each other or experiencing different gravitational fields.
In special relativity, time dilation occurs at high speeds, specifically when objects are moving at speeds comparable to the speed of light (299,792,458 meters per second in a vacuum). According to STR, as an object accelerates and approaches the speed of light, time appears to slow down for that object relative to a stationary observer. This means that clocks on the moving object will appear to run slower compared to the clocks of the stationary observer.
For example, imagine two spaceships: Spaceship A and Spaceship B. If Spaceship A is moving at a significant fraction of the speed of light relative to Spaceship B, the astronauts on Spaceship A would experience time passing slower compared to the astronauts on Spaceship B. From the perspective of Spaceship A, time appears to flow normally. However, from the perspective of Spaceship B, time on Spaceship A appears to be moving slower.
In general relativity, time dilation occurs in the presence of strong gravitational fields. According to GTR, gravity affects the curvature of spacetime, causing time to run differently in different gravitational fields. In areas of stronger gravity, time appears to run slower compared to areas of weaker gravity.
For example, near a massive object like a black hole, where the gravitational field is extremely strong, time dilation becomes significant. Clocks near the black hole will appear to run slower compared to clocks farther away from the gravitational influence.
It's worth noting that time dilation effects are not noticeable in everyday situations or at low speeds relative to the speed of light. They become significant only at speeds close to the speed of light or in extreme gravitational environments.