According to the theory of relativity, time is indeed relative, and its perception can vary depending on the observer's relative motion and gravitational field. However, the notion that the past, present, and future exist simultaneously is not a conclusion derived solely from the relativity theory.
In the context of relativity, the theory describes how different observers moving at different speeds or experiencing different gravitational forces can have different perceptions of the order and duration of events. This can lead to phenomena such as time dilation, where time appears to pass slower for objects in motion or in stronger gravitational fields compared to objects at rest or in weaker gravitational fields.
While relativity challenges our intuitive understanding of time as an absolute and uniform entity, it does not necessarily imply that the past, present, and future exist simultaneously. The theory of relativity deals with the way in which time is experienced and measured by different observers, but it does not address the fundamental nature of time or its relationship to the existence of events.
The concept that the past, present, and future exist simultaneously is more closely associated with certain philosophical and metaphysical perspectives, such as the block universe theory or eternalism. These viewpoints propose that the entire spacetime continuum, encompassing all past, present, and future events, exists as a single entity. In this view, our perception of the flow of time is an illusion, and all moments are equally real and coexist in a timeless framework.
It's important to note that the question of whether the past, present, and future exist simultaneously remains a subject of philosophical and scientific debate. Different interpretations and theories exist, and there is no definitive consensus on the nature of time or the existence of events across time.