[somethingroma]

"Does the external world exist, as it so concretely seems?"

I know this is not the direction of the paper (or question) but physics already gives us an astounding 'No!'.

[Strilanc]

Only if you subscribe to some pretty tortured interpretations of quantum mechanics. QM still describes an objective state of the external world, and exactly how it changes over time.

[Quekid5]

I don't think that's what the poster meant. I think it was probably more along the lines of how you experience e.g. 'light' vs. what it actually is:

   https://www.youtube.com/watch?v=FjHJ7FmV0M4

(The vast majority of light is effectively invisible to you.)