see..the force of gravity does not completely vanish inside the earth.
take it like this
the particles of dust ball will b atracted to each other if no pressure acts inside them.this will continue until all the partcles come close enough n shrink to point size.
this is bcz gravity is force...
we know that the cladding material is of lower refractive index than the inner core's refractive index.but what if instead of outer cladding glass material, air is used(which too has a lower refractive index)?would it also not cause TIR inside the core.