How to politely ask my childhood friend to shut the f*ck up

So my friend is the type of guy to get very passionate about random subjects that come across his life. He’s also very heavily influenced by articles, social media, etc. and is sometimes severy lacking in critical thinking. He also LOVES talking about himself and his visions/goals. (Good trait to have, but to a certain degree if you ask me).

That’s not really the issue, I just let him believe whatever he believes. The issue is when he starts yapping about said subjects. Most of the time it’s the most uninteresting subject aswell and when he starts talking about his own vision on something, he might talk for 25 minutes non stop, without even giving a chance to possibly change the subject or say anything in between. And when I say non stop, it literally is non stop. He transforms into this overdrive word printing machine.

We could be having a normal fun convo about childhood memories for example. Then, all of a sudden this memory has made him come to some life revolutionizing discovery and he starts yapping about it for 15+ minutes and how it has changed his perspective on certain things. And this happens ALOT.

Another example was how we were talking about the theory of relativity. Over the span of 20 minutes of non stop talking, he somehow managed to change that subject into the 2008 U.S. financial crisis (we’re european & were too young at that time to notice any influence in our life). AKA I could not care less.

I know this doesn’t sound too bad, but when it’s almost daily and on multiple occasions, it gets very exhausting. I love talking to him and he’s a dear friend, but I’m at my limit.