The question of what happens to the timeline when someone travels to the past and alters events is a topic of speculation and debate in both physics and philosophy. There are different theories and interpretations regarding the nature of time and the consequences of time travel. I'll present a couple of perspectives on this matter:
Novikov's Self-Consistency Principle: This principle, proposed by physicist Igor Novikov, suggests that if time travel were possible, it would be inherently self-consistent. According to this view, any actions taken by a time traveler in the past would already be accounted for in the timeline they originated from. In other words, any attempt to change the past would either be unsuccessful or would cause events to unfold in such a way that the time traveler's actions were always part of the history.
Multiple Timelines and Parallel Universes: Another possibility is that when someone travels to the past and alters events, a new timeline or parallel universe is created. This perspective is often explored in science fiction and involves the concept of branching timelines. In this scenario, the original timeline from which the time traveler came continues to exist alongside the newly created timeline resulting from their actions.
It's important to note that time travel to the past is currently purely speculative and not yet supported by scientific evidence. The theories and concepts related to time travel are still subjects of active research and discussion. As such, definitive conclusions about the consequences of time travel remain uncertain.
Ultimately, the nature of time, causality, and the implications of time travel are complex and still not fully understood. Different theories and interpretations offer various explanations, but until empirical evidence emerges or a consensus is reached among physicists and philosophers, the question of what happens to the original timeline when someone alters the past remains open to speculation and personal interpretation.