Question came up in another forum which lead me to a possible theory. We know you can measure thickness of an object using a caliper, but what if the object is too large to get a caliper around it?
Say you want to measure the wall thickness of a large object. Maybe it's only an inch thick but you'd need like a 6' long caliper to reach around both sides due to the shape. Or simply, imagine an infinite wall expanding forever in length and height.
My idea is to use two magnets; two separate objects that can be place on either side of the wall without requiring any rigid connection. How could I measure the magnetic force of one of the magnets? They'll have the greatest force when touching unobstructed, equally 0mm between them. As the magnets are pulled farther apart, that force of attraction decreases and I believe should be measurable to convert into an actual distance.
It's all just a crazy idea at this point, but thought it seemed like a cool project to look into.
Or maybe using radio waves to measure distance, but I feel like the structure of the object might affect that more.