When physicists say that time is relative to the observer, they are referring to the concept of time dilation in the theory of relativity. According to the theory of special relativity developed by Albert Einstein, time is not an absolute quantity that ticks away uniformly for all observers. Instead, it is influenced by the relative motion between observers and the gravitational field they experience.
There are two main aspects of time dilation: relative motion time dilation (kinematic time dilation) and gravitational time dilation.
Relative Motion Time Dilation: According to special relativity, the passage of time is perceived differently by observers who are moving relative to each other. When two observers are in relative motion, with one moving at a significant fraction of the speed of light relative to the other, they will measure different elapsed times between the same events. This effect is known as time dilation, and it means that time appears to "slow down" for the moving observer compared to the stationary observer.
Gravitational Time Dilation: According to general relativity, the presence of a gravitational field also affects the flow of time. In regions with stronger gravitational fields, time runs slower compared to regions with weaker gravitational fields. This means that an observer closer to a massive object, such as a planet or a black hole, will experience time passing more slowly than an observer located farther away.
These concepts of time dilation have been verified through various experiments and observations, including high-precision atomic clocks, the behavior of cosmic rays, and the study of gravitational fields near massive objects.
In summary, when physicists say that time is relative to the observer, they mean that the passage of time can be different for different observers depending on their relative motion and the gravitational field they are in. Time is not a universally constant quantity but rather depends on the specific conditions of the observer's frame of reference.