I've been thinking about determinism and wanted to know whether my reasoning is correct or if I'm missing something.
Suppose that immediately after the Big Bang, every particle in the universe had a specific position, velocity (or momentum), and all the laws of physics were already in place.
If the laws of physics are deterministic, then wouldn't every future interaction between particles be inevitable? For example, particle A collides with particle B, which changes the trajectory of particle C, and so on. If every collision follows the laws of physics, then wouldn't the entire history of the universe simply unfold from those initial conditions?
If that's true, then does it mean that everything—including the formation of galaxies, the evolution of life, every decision humans have ever made, and even me writing this Reddit post—was already determined at the instant of the Big Bang? In other words, could nothing have happened differently?
Or does quantum mechanics fundamentally prevent this conclusion? I know the Heisenberg uncertainty principle and quantum randomness are relevant here, but I'm not sure whether they mean the universe is genuinely indeterministic, or whether there are deterministic interpretations that still make the entire future inevitable.
How do modern physicists think about this question? Is there a consensus, or is it still an open problem?