So I read that at age 23 Newton went about inventing calculus, I've read different stories on who invented calculus but I'm not going into that here. So my question is Calculus is considered for the most part the most difficult form of math right? So how can a 23 year old invent it? I mean it's difficult enough for people to learn it at uni with qualified professional teachers and this guy in his early 20's, with no books, internet or anyone to guide him invented and taught himself calculus is just mind boggling.

I've never studied calculus but when I think about Galileo inventing algebra, well it doesn't match anywhere near to calculus. I mean I can see how algebra is a pretty simple subject to invent and it's easy to prove with a pen and pad. But when you're dealing with orbiting planets, motion of epic proportions how can you ever prove that your theory i correct?

I also read that even today, Newton's calculations are so precise that when we shoot a probe right passed the rings of saturn, we use exactly the same equations that Newton unravelled in the 1600's. But how he have possibly calculated for planets, suns ect?

I also have the same problem when it comes to people who invented string theory and partical physics etc... how can you possible write an equation for something so tiny you can't even see it? How would you even go about inventing some equation? I could sit in my room and just make up some random equations for string theory and they'd be just as random as the equations being taught at universities...