Acceleration is relative, meaning it depends on the reference frame of the observer. This concept is a fundamental principle of Einstein's theory of special relativity.
In classical mechanics (Newtonian physics), acceleration is treated as an absolute quantity, which means that all observers in different reference frames would agree on the magnitude of an object's acceleration. However, when dealing with high speeds or situations involving relativistic effects, this classical notion breaks down.
According to special relativity, the laws of physics, including those governing acceleration, are the same in all inertial reference frames. An inertial reference frame is a coordinate system where an object at rest or moving with constant velocity will experience no acceleration unless acted upon by an external force. If an observer is in an inertial reference frame and sees an object accelerating, then observers in other inertial frames will also agree that the object is accelerating.
However, if an observer is in a non-inertial reference frame (accelerating or rotating), they will experience fictitious forces, such as the centrifugal force, which can create the illusion of acceleration. In this case, the acceleration experienced by an object in the non-inertial frame will be different from the acceleration measured by an observer in an inertial frame.
In summary, acceleration is relative and depends on the observer's reference frame. Observers in different inertial frames will generally agree on whether an object is accelerating, but observers in non-inertial frames may measure different accelerations due to fictitious forces. Special relativity provides a consistent framework for understanding these relative aspects of acceleration and other physical phenomena at high speeds or in accelerating reference frames.