Einstein comes along and says, space and time can warp and curve, that’s what gravity is. Now string theory comes along and says, yes, gravity, quantum mechanics, electromagnetism — all together in one package, but only if the universe has more dimensions than the ones that we see.

Brian Greene, a theoretical physicist and prominent popularizer of modern physics, often explains the historical arc from Einstein’s general relativity to contemporary attempts at unification such as string theory. This quotation reflects a standard explanatory move in his public lectures and media interviews: first summarizing Einstein’s geometric account of gravity as curved spacetime, then contrasting it with string theory’s claim to unify gravity with quantum mechanics and the other fundamental forces—at the cost of requiring additional, typically hidden, spatial dimensions. The phrasing and cadence suggest an expository, audience-facing setting (talk/interview) rather than a tightly edited passage from a technical paper.

Greene is sketching a lineage of ideas about what gravity “is.” In general relativity, gravity is not a force in the Newtonian sense but the manifestation of spacetime geometry: mass-energy tells spacetime how to curve, and curvature tells matter how to move. String theory, in Greene’s telling, extends this geometric rethinking into a quantum framework by treating particles as vibrational modes of strings, with gravity emerging alongside the other forces in a single mathematical structure. The key caveat—extra dimensions—highlights both the ambition and the speculative burden of string theory: unification is purchased by positing aspects of reality that are not directly visible at everyday scales.