The concept of time travel is largely speculative and still falls within the realm of theoretical physics. While there are various theories and hypothetical scenarios that discuss time travel, it is important to note that they are largely speculative and not yet supported by empirical evidence.
Regarding the question of changing the past, many theories propose that if time travel were possible, it would be subject to certain constraints or limitations. One such idea is the Novikov self-consistency principle, which suggests that any attempt to change the past would be inherently self-defeating and lead to events that would ensure the past remains consistent with what has already happened. In other words, any action taken by a time traveler in the past would already be a part of the established timeline and would not alter it.
The idea of creating a new present separate from our own when traveling to the past is another concept that arises from different theoretical frameworks. One theory is the "many-worlds interpretation" of quantum mechanics, which suggests that every possible outcome or event actually occurs in a separate branch of the universe. According to this theory, when a time traveler alters the past, it would result in the creation of a new branch of reality, distinct from the traveler's original timeline. However, it is important to note that this interpretation remains speculative and lacks empirical confirmation.
It's essential to understand that time travel, especially to the past, is still largely a topic of scientific and philosophical speculation. While there are intriguing theories and thought experiments surrounding it, the practical realization of time travel and its implications are still uncertain and require further scientific advancements and understanding.