Time dilation occurs due to the fundamental principles of Einstein's theory of relativity. To understand why gravity and speed affect time, let's consider each phenomenon separately:
- Time dilation and gravity: According to Einstein's theory of general relativity, gravity is not merely a force but rather the curvature of spacetime caused by massive objects. In regions with stronger gravitational fields, spacetime is more curved. This curvature affects the passage of time.
Imagine a scenario where two observers are in different gravitational environments: one closer to a massive object (e.g., a planet) and the other farther away. The observer closer to the massive object experiences a stronger gravitational field and thus resides in a more curved region of spacetime.
In this curved spacetime, time actually runs slower compared to the observer farther away from the massive object. The effect is similar to being on a lower gravitational potential, where time "stretches" or dilates. This time dilation occurs because clocks in a stronger gravitational field experience a slower passage of time compared to clocks in a weaker gravitational field.
- Time dilation and speed: In addition to gravity, time dilation also occurs due to relative motion, as described by Einstein's theory of special relativity. The theory states that the laws of physics are the same for all observers in uniform motion relative to each other.
When two observers are moving relative to each other at significant speeds, their measurements of time will be different. From the perspective of one observer, time appears to run slower for the other observer. This effect is often referred to as "time dilation due to velocity."
The reason behind this phenomenon lies in the constancy of the speed of light in a vacuum. According to special relativity, the speed of light is the same for all observers, regardless of their relative motion. As a consequence, space and time must adjust to ensure the constancy of the speed of light.
When an object is moving relative to an observer, their space and time coordinates become intertwined, and time dilation occurs. The moving object experiences a slowing down of time from the perspective of the stationary observer.
Regarding astronauts and biological aging in space, the time dilation effects due to both speed (during space travel) and weaker gravity (in space compared to Earth) contribute to their biological clocks running slower. This means that astronauts in space age slightly less compared to their counterparts on Earth, due to the combined effects of both time dilation factors.
While these concepts may be challenging to visualize intuitively, they are precisely what the well-established theories of relativity predict, and extensive experimental evidence supports their validity.