The concept of time travel, especially with regards to changing the past, is a topic of much speculation and remains purely hypothetical based on our current understanding of physics. However, there are several proposed explanations and theories that attempt to address the question of why changing the past might not be possible, even if time travel were to exist.
One explanation is based on the idea of self-consistency or the principle of causality. According to this principle, events in the universe are causally linked, meaning that they occur in a specific order and are influenced by preceding events. If someone were to travel back in time and change an event in the past, it could lead to a contradiction or paradox.
For example, the famous "grandfather paradox" arises when a person travels back in time and prevents their own birth by causing harm to their grandparents. This situation creates a paradox because if the person was never born, they couldn't have traveled back in time to cause the harm in the first place.
To resolve this paradox and maintain consistency, several theories propose that the timeline would somehow be self-correcting. Any actions taken in the past would either be futile, as they would not change the present, or they would inadvertently contribute to the events that led to the present state. In other words, the outcome would be predetermined, and attempts to change the past would be futile.
Additionally, some theories, such as the Novikov self-consistency principle, suggest that if time travel were possible, any actions taken in the past would already be incorporated into the timeline. This means that any attempt to change the past would result in events unfolding exactly as they did before, ensuring a consistent timeline.
It's important to note that these explanations and theories are purely speculative and theoretical. Time travel, especially with the ability to alter the past, remains largely in the realm of science fiction, and our current understanding of physics does not provide a concrete answer to the question of whether it is possible or how it would work.