This is a complete layman question because the topic goes well beyond my head, but I was thinking... There has been a quest of a "grand unified theory", a single theory of physics that would explain spacetime, gravity, quantum mechanics, and everything in between. But what makes scientists so sure that there is such a unified theory? I mean, couldn't it be that gravity (ie. spacetime geometry) and quantum effects are two completely separate and disconnected features of our universe? They are not related, they are independent of each other, they exist as completely separate phenomena, and there is no underlying common ground between them, and trying to search for a unified theory is a completely futile endeavor because there isn't one? It's a bit like the shape and color of an object. Both are independent features of the object, and neither one defines the other, nor is there any third feature that affects both and creates a connection between them. Trying to develop a "unified theory of color and shape" is futile because they simply are not connected. An object can be of any color and any shape, completely independently of each other.