Stephen Hawking's chronology protection conjecture suggests that quantum effects near closed timelike curves would generate overwhelming radiation, destroying the time machine and preserving causality.

In 1992, Stephen Hawking proposed that while general relativity allows CTCs, quantum field theory in curved spacetime predicts that vacuum fluctuations would become infinitely amplified near the chronology horizon—the boundary where CTCs begin to form. This buildup of energy density would likely warp spacetime so severely that the time machine collapses before it can be used. Hawking argued that this 'chronology protection' mechanism ensures that the laws of physics prevent macroscopic time travel to the past, thereby safeguarding causality. Although the conjecture remains unproven, subsequent studies in semiclassical gravity have supported the idea that quantum effects act as a natural deterrent to CTC formation.