Milton asks:
“What if earth
Be but the shadow of heaven, and things therein
Each to other like, more than on earth is thought?”
And in the Bible, Hebrew 8:5 mention along the similar lines.
So let’s understand this first. When a three dimensional object cast shadow, it’s in two dimension. That is, one dimension would be less. The shadow of a globe would look like a circle and a cube would look like a square
Similarly, 2d object would cast 1 dimensional shadow on a 1 dimensional surface.
Thinking along similar lines ( note, this is a physics theory) 4d object would cast, 3d shadow, which is what we see on earth.
So what if heaven is, 4d and earth is shadow of that ( that is, 3d)