Every possible action was decided at the moment of the big bang. With a big enough computer the size of a couple of universes you could calculate the exact position of all particles at any moment in time in the future
No. You cannot. There is literally no way.
I can prove it as well through something called entropy and another thing called quantum mechanics.
Now, how much entropy there is in something is essentially how much information it carries.
Now, the 2nd law of thermodynamics states that as time passes, entropy must go up, which means more information is being added to the universe all the time.
This means that, if you were to know all the information in the universe at the time of the big bang, later on when more information has been added you no longer know everything since that information is actually added in randomly: that's where quantum mechanics comes into play.
At the atomic, scale, things stop being like what they were at the classical level. Particles like electrons and photons aren't physical things anymore, they're actually probabilities. You have a certain chance of finding a particle in one spot when you look and a certain chance of finding it in another spot if you look, but it's impossible to predict which it is without actually measuring. There is no formula to predict where it's going to be.
That is where entropy (and hence information) is introduced into the system. Even if you knew all the positions and velocities of all the particles right at the start of the big bang, all the information added would make it utterly impossible to predict where the universe would be later on.
