A null-gradient condition rules out closed time loops in certain spacetime models.

Zhang’s lightlike thread blocks one kind of time loop. Nature may permit the past only where spacetime has no single road through it.

A traveler following a timelike path must keep changing Zhang’s global lightlike function, so the path cannot return to where it began. That clean test rules out loops in certain spacetime models, including cases once unsettled in gyratonic waves. But general relativity still permits other loops. The real mystery is which global shape nature allows.

• What it could changePhysicists could reject whole families of time-machine models with one geometric test.
• How it touches todayThe search for time travel already depends on what global structure our universe has.
• Our futureThe past could stay reachable only in the parts of spacetime where no single thread keeps time moving one way.

• The theorem is logically independent of stable causality, offering a distinct geometric obstruction rather than merely restating the usual time-function criterion.arxiv
• A 2004 survey concluded that no conclusive no-go theorem against time machines existed, underscoring the conditional nature of Zhang’s advance.plato
• Other proposals use energy conditions: some analyses argue that wormholes and closed timelike curves require problematic violations of null-energy assumptions.stevehsu +1