A conductor shields its inside from its outside but not vice-versa

[thecommexokid]

A point charge outside a hollow conducting sphere will produce no field in the hollow interior: the metal shields the interior. But a point charge inside the hollow interior will produce a field outside: the same metal ceases to act as a shield. Why is this?

(In particular, aren’t “inside” and “outside” just matters of perspective? Why can’t I think of the sphere as enclosing the rest of the Universe, instead of the small spherical region?)

 

[mfb]

The field "far away" is always given by the total charge of your system - shielding cannot change this total charge, and far away is not a matter of perspective, you can measure it.
Shielding inside works as this is just a local change in the field geometry, something that can happen via local rearrangements of charges.

(In a hypothetical closed universe (with a finite volume), shielding the outside region could work if you want to wait hundreds of billions of years. Most experiments don't wait that long.)