According to Einstein's theory of general relativity, gravity and velocity can both cause time dilation, which is the slowing down of time relative to an observer in a different gravitational field or reference frame.
Gravity and spacetime curvature: In the presence of a massive object, such as a planet or a star, spacetime is curved. This curvature is responsible for the force we perceive as gravity. When you are closer to a massive object, you experience a stronger gravitational field, which means that the curvature of spacetime is more significant.
According to general relativity, the curvature of spacetime affects the flow of time. In regions with stronger gravitational fields, time flows more slowly compared to regions with weaker gravitational fields. This effect is known as gravitational time dilation. Essentially, the stronger the gravity, the slower time passes.
For example, if you were to observe someone standing near a massive object from a distance, you would see time passing more slowly for them compared to your own perception of time. This means that if you could somehow measure the time intervals experienced by the observer near the massive object and compare them to your own measurements, you would find that their time appears to be dilated or stretched out.
Velocity and relative motion: According to the theory of special relativity, as formulated by Einstein, time dilation also occurs when an object or observer is moving at a significant fraction of the speed of light relative to another object or observer.
When an object moves at high velocities, close to the speed of light, the relativistic effects come into play. One of these effects is time dilation. As an object's velocity approaches the speed of light, time slows down for that object relative to a stationary observer.
This effect has been observed in experiments with high-speed particles, such as those in particle accelerators. The particles' lifetimes are extended due to their high velocities, as measured by an observer at rest.
In summary, both gravity and velocity can cause time dilation according to Einstein's theories of relativity. Gravity is associated with the curvature of spacetime, while velocity leads to time dilation due to the relativistic effects of high-speed motion.