The question of whether time travel would have already happened if it were theoretically possible is known as the "grandfather paradox" and is a subject of much debate and speculation. The grandfather paradox arises from the idea that if time travel were possible and someone traveled back in time to change the past, they could inadvertently alter events in a way that would prevent their own existence or make it impossible for them to travel back in time in the first place.
There are several proposed resolutions to the grandfather paradox, including the following:
Novikov self-consistency principle: This principle suggests that any action taken by a time traveler in the past would be self-consistent and not alter the future in any significant way. In other words, if someone were to travel back in time, their actions would already be a part of history, and they would not be able to change the past in a way that creates a paradox.
Many-Worlds Interpretation: According to this interpretation of quantum mechanics, each possible outcome or decision branches off into its own separate universe. Therefore, if a time traveler were to alter the past, they would create a new branch of reality, effectively avoiding the paradox.
Dynamic timeline: In this view, it is suggested that time is flexible and can be rewritten. Any changes made by a time traveler would result in the creation of a new timeline, while the original timeline remains intact.
These proposed resolutions and others attempt to address the paradoxes and logical inconsistencies associated with time travel. However, it is important to note that they are largely speculative and theoretical. As of now, we do not have concrete evidence or a widely accepted theory that confirms the possibility of time travel, and therefore, we cannot definitively say whether it would have already happened.