When you think about it, there is something peculiar about the universe we belong to.
Its name, universe, comes from uni (‘one’) and verse (‘turned into’), so its name basically means ‘turned into one’, highlighting from the started a very particular problem it raises.
Any experiment made within our universe can be repeated many times over. Want to check Newton’s law of gravitation on Earth? Shoot an arrow. Not sure you got it right? Shoot another one. Again and again.
With patience you will see that from the knowledge of its initial position, angle and velocity, you can predict where the arrow will land. That is what ballistics is all about. And it works. Bows and arrows would have been abandoned long ago otherwise, and England would be French.
So, with a law and an initial condition, you can, it has been proved, predict where an arrow lands and defend a whole country. For the universe as a whole, it is a bit more difficult. Even if you had a law that explained everything, that applied everywhere, how would you make it work? How would you use it to predict how the universe we live in today became the way it is? You’d need an initial condition for that. Which you don’t have.
But you could try to outsmart nature. Starting today, running time backwards, yo could maybe reach an initial event that occurred a long time ago. That is what scientists have done, and they reached a Planck wall. Which is a pretty good start I guess, since it corresponds to when space and time became what they today are.
But that doesn’t remove the frustrating fact that, unlike with your experiment with the arrows, you only have one universe to play with. You can’t have a go at creating another one, with different initial conditions, and check what comes out of it. Not in the lab anyway.
But if ours isn’t the only universe? What if we were part of yet another type of multiverse? Could our reality be only one of a myriad of possible realities, all having different beginnings, maybe even different laws, and hence very different presents?