Symbolism, Imagery, Allegory
We've all seen them, folks. The World's Largest Tomato, World's Longest String, World's Tiniest Car. They're tourist traps found along the side of any sleepy highway you can name. "The Most Photographed Barn in America" is one of them, and it just happens to be a central prop in White Noise, and the perfect location for Murray Siskind to make a point about modern America.
The barn is located just a little way outside Jack's town of Blacksmith, and surprise surprise, it looks like a totally normal barn. But what happens when you've already seen a half-dozen signs telling you it's the most photographed barn?
For Murray, modern American culture has completely lost any connection to an objective reality. To even try to talk about a "real" world outside our own minds is nearly impossible. According to Murray, it's totally impossible for people to look at the barn for what it actually is (duh, a barn) because their eyes have been trained not to see the barn:
Once you've seen the signs about the barn, it becomes impossible to see the barn. (3.19)
What people's eyes see instead is the most photographed barn in America. You've can't actually reach the barn without passing "five signs before [you] [reach] the site" (3.16), which means that you've already had your mind warped by the barn's advertising before you see it.
One funny thing about the logic of this barn is that it might not have been the most photographed barn to begin with. But if someone went out one day and threw up a sign that said it was the most photographed barn, people would start flocking to it to take pictures because they'd want a photo of the most photographed barn in America. In other words, the barn's claims about itself reach your mind before the actual barn does, meaning that reality is always shaped by the claims we make about it. The real world of physical things doesn't shape our language. Our language shapes it.
If you just heard a muffled explosion, rest assured. That wasn't another Airborne Toxic Event. That was your mind going "ka-blam!"