According to our current understanding of physics, there are several principles and theories that suggest time travel to the past might not be possible. These include:
The Second Law of Thermodynamics: The second law states that the entropy, or disorder, of a closed system always increases over time. This concept is often associated with the "arrow of time" and suggests that the past has a lower entropy than the future. Time travel to the past could potentially violate this law by reversing the natural flow of entropy.
The Novikov self-consistency principle: Proposed by physicist Igor Novikov, this principle states that the laws of physics must prevent any event or action that would create a paradox. In other words, if time travel were possible, the universe would somehow prevent actions that could alter the past in a way that contradicts the present.
Causality and the Grandfather Paradox: The concept of causality implies that a cause must precede its effect. The famous Grandfather Paradox illustrates a contradiction that could arise from time travel to the past. If someone were to travel back in time and prevent their own grandparents from meeting, it would create a paradox where the time traveler could not exist to go back in time in the first place. This paradox suggests that events that create logical inconsistencies cannot occur.
General Relativity and Closed Timelike Curves: General relativity, Albert Einstein's theory of gravity, allows for the existence of spacetime paths known as closed timelike curves (CTCs). These paths could potentially enable time travel to the past. However, it is still an area of active research, and the nature of CTCs remains uncertain. Various proposed solutions, such as the chronology protection conjecture, suggest that some mechanism or physical principle prevents the formation of CTCs or makes them unstable.
It is important to note that our understanding of physics is continuously evolving, and there are still many unanswered questions regarding time and its properties. Future scientific advancements may lead to new insights and a deeper understanding of the possibilities and limitations of time travel.