Yes, you are correct. According to the theory of relativity, both the speed of an object and the strength of gravity can affect the passage of time. These effects are indeed added together, and understanding how they work requires a concept known as spacetime.
In special relativity, time dilation occurs when an object moves relative to an observer at a significant fraction of the speed of light. As the object's velocity increases, time appears to pass more slowly for the moving object compared to a stationary observer. This effect is known as "velocity time dilation."
On the other hand, in general relativity, the presence of gravity affects the passage of time. The theory suggests that the curvature of spacetime caused by massive objects, like stars or planets, can influence the flow of time. This effect is called "gravitational time dilation." In stronger gravitational fields, where gravity is more intense, time appears to pass more slowly relative to weaker gravitational fields.
When both effects are present simultaneously, they combine to create what is known as "combined time dilation." This means that an object moving at high speeds within a gravitational field will experience time dilation due to both its velocity and the gravity it encounters.
To understand how these effects add up, we can consider an example. Let's imagine two observers: one on a spaceship traveling at a high speed, and another on a planet with a strong gravitational field. From the perspective of the stationary observer on the planet, both the speed of the spaceship and the planet's gravity will cause time dilation for the observer on the spacecraft. The effects will combine, resulting in a cumulative time dilation.
In practical terms, this means that clocks on the spaceship will appear to run slower relative to the observer on the planet. This has been observed in experiments using atomic clocks on high-speed vehicles or in high-gravity environments, where the time difference between the moving clocks and stationary clocks has been measured.
It's important to note that the effects of time dilation become significant only at speeds approaching the speed of light or in extreme gravitational conditions, such as near black holes. For everyday speeds and gravitational fields encountered on Earth, these effects are negligible and not readily noticeable.
The concept of spacetime in relativity connects the effects of velocity and gravity on the passage of time, providing a framework for understanding their combined influence.