The reason people doubt the possibility of traveling backward in time, while accepting the idea of traveling forward in time, is based on our current understanding of physics and the concepts involved. Here are a few key factors that contribute to this doubt:
Directionality of Time: Our experience of time is inherently directional—we always move forward in time. We perceive events as unfolding in a particular sequence, with cause and effect relationships following a linear progression. This observation aligns with the second law of thermodynamics, which states that entropy (a measure of disorder) tends to increase over time. The arrow of time, as it's often called, is deeply ingrained in our everyday experience and raises questions about the plausibility of reversing that direction.
General Relativity and Causality: General relativity, our current theory of gravity and spacetime, doesn't provide a clear mechanism for traveling freely backward in time. It allows for scenarios like time dilation, where time can pass differently for different observers under certain conditions. However, these scenarios still adhere to the overall arrow of time and maintain causality—cause and effect relationships are preserved.
Causality Paradoxes: Time travel to the past raises significant causality paradoxes, such as the grandfather paradox mentioned earlier. If you were to travel back in time and change something in the past, it could potentially create logical inconsistencies and paradoxes. These paradoxes challenge our understanding of cause and effect, making the concept of backward time travel appear problematic.
Lack of Evidence: To date, we have no empirical evidence of time travelers from the future. If backward time travel were possible, one might expect some form of interaction or evidence from future beings. The absence of such evidence doesn't prove it's impossible, but it does raise doubts about its plausibility.
It's important to note that while we can "travel" forward in time relative to other observers through time dilation effects, this is different from freely moving backward in time. Time dilation occurs when an object moves at a significant fraction of the speed of light or when in the presence of intense gravitational fields, and it results in different rates of time passage for different observers. This phenomenon is supported by experimental evidence and is an accepted aspect of our current understanding of physics.
Overall, the doubt surrounding backward time travel stems from our understanding of the fundamental principles of physics, the observed directionality of time, the preservation of causality, and the absence of empirical evidence supporting the existence of time travelers from the future.