Why couldn't Abraham go over to the rich man in Luke 16?
A great gulf was fixed between the two sides of Sheol, or Hades, separating torment from paradise. That fixed chasm prevented any crossing from Abraham's side to the rich man's side, so Abraham could not come over to dip his finger in water or relieve the rich man's torment.
Les explains that the rich man and Abraham were both in the same place, called Sheol, Hades, or hell, but it was divided. On one side was torment; on the other side was paradise. Abraham told the rich man he could not come over and dip his finger in cold water because a great gulf was fixed between them. That fixed gulf made crossing impossible. Les also notes that Jesus told the thief on the cross, "Today thou shalt be with me in paradise," and that after His atoning blood was shed, Christ spent three days and three nights in paradise, not torment. The context then points to Ephesians 4:8, where Paul says that when He ascended up on high, He led captivity captive.
And beside all this, between us and you there is a great gulf fixed: so that they which would pass from hence to you cannot; neither can they pass to us, that would come from thence.