What do scientists still need to know in order to harmonize relativity with classical mechanics?

Answer 1

Nothing is needed to harmonize the two. They're one and the same. Relativistic physics is classical physics in it's most developed and most accurate form.

Answer 2

Perhaps you mean Newtonian mechanics when you say "classical". Newtonian Mechanics may be "harmonized" with Relativity as the latter's flat-space low-velocity limit.

Answer 3

Gravity. Big problems with getting gravity into the Unified Theory. They need a quantum theory of gravity.

Answer 4

Nothing

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