Hi, Art Historian here. First, there was a huge conceptual shift in many forms of art in the early 20th century, not just painting. There's a multitude of theories as to why this shift away from representational art to conceptual/avant garde artforms happened during this period, so condensing it all into a single tidy explanation is nearly impossible (and the subject of literally hundreds of art historical texts). The biggest impetus, however, seems to have been mechanization and advancing technologies (especially in terms of artificial light and widespread, readily available photography) that steered western art culture away from the handmade to the man made. This seemed to unmoor western art from its literal, representational lockdown and allowed for several movements to occur almost simultaneously, effectively creating what's referred to as "modern art". It started a bit earlier than the turn of the 20th c. with the expressionists of the late 19th c. (Van Gough, Cezanne, way early Picasso) but post-1880, there's this rapid escalation in art that pushes the boundaries of reality. You get Surrealism, Dadaism, Cubism, and Futurism all more or less occurring on top of one another in the first 20 years of the 20th century, and each was attempting to respond to the speed at which the culture around them was changing. There's a lot of political ideology wrapped up in these early 20th c movements, and that's also super interesting but a bit too much to get into in this post.

So, jumping forward a bit to post-WW2, we see modern art movements completely rejecting any passing nod to representation and exploring a far more stripped down, "essence" of art. You mention Pollock, and current pop culture loves to shit on him (something he probably would have appreciated in the abstract sense) for basically throwing paint around like an 2-year old could do. Full confession: I am a hardcore classical art historian. I find the vast majority of modern art to be underwhelming. But let me explain why Pollock and his abstract expressionist cohorts deserve a whole lot more respect. You have to go back to what AbEx was originally called, what Pollock himself referred to as "action paintings" to get why what he did was so revolutionary at the time, and in many ways, still is.

See, like Picasso, Jackson Pollock was a classically trained artist. They guy knew how to paint "properly" but chose not to. That's step 1. Step 2 is to look at a Pollock in person. You absolutely cannot appreciate the sheer scale of his canvases by looking at photos online or in a book. These things are massive, overwhelming the space they're occupying with a chaotic energy that is -- even as the classicist in me dies a little to admit it -- incredibly powerful.

The final step is to go online and look up videos of Pollock in action. Again, that word "action". Watching him work, barefoot, with the canvas laid flat on the ground... It's a dance. I would love to know if anyone has ever written about Pollock's movements as analogous to the modern dance movement he was a direct contemporary with, because there's a real clear symmetry to the way he performs his paintings with the likes of Martha Graham.

So, to distill it all down, Pollock was attempting to capture movement and energy using paint, not just fling paint around. He was methodical in action -- seriously, look up videos of him at work. They will change your mind about AbEx as far as its value as a cultural lynchpin is concerned. Photography was just getting to that point in the 1940s-1950s when it was possible to photograph things like infrared light waves, things that had been invisible to the human eye but that we knew existed. Pollock was taking that concept and trying to put it into physical terms using paint.

I'm happy to ramble on about art history any time so feel free to keep asking questions!