The West fears that Iran is using its nuclear drive to try to build atomic weapons, a charge Tehran has consistently denied, saying it is aimed at generating electricity.
But what still stands in the way is a complicated dance: Maintaining uranium enrichment while addressing Western fears that Iran could move toward atomic weapons a charge it denies.
It is defined by gravity because that is the only way that it interacts it has no electric charge, and does not feel the strong and weak forces that bind atomic nuclei together.