Physics says a butterfly triggers a tornado but does not cause it
In physics, a cause must sit inside the past light cone of its effect, reachable by a signal no faster than light. Break that rule and some observers would see messages arrive before they were sent. This single constraint quietly shapes relativity, quantum field theory and even the arguments about time travel.
Special relativity made the old notion of 'at the same time' depend on who is watching. To keep cause and effect in a fixed order, physicists require that every inertial observer agrees the cause came first. That holds only when the two events are separated by a timelike interval, meaning a slower-than-light signal could link them. A faster-than-light signal would cross a spacelike gap and, for some observers, travel backwards in time, which is why relativity rules out such communication. General relativity keeps the same rule even in curved spacetime.
Physicists also distinguish levels. The strong causality principle forbids information moving faster than light. A weaker, microscopic version need not involve any information transfer, and a theory can satisfy the weak version while breaking the strong one. In the algebraic approach to quantum field theory, microscopic causality is an axiom amounting to the claim that simultaneous measurements at different places cannot influence each other. Bell's theorem shows that assuming local causality in entanglement experiments runs into the non-classical correlations quantum mechanics predicts.
A second, older requirement is contiguity: influence should pass continuously through space. Newton's gravity, acting across empty distance between Sun and Earth, offended this principle and displaced Descartes' mechanical vortices. Field theories such as Maxwell's electrodynamics and Einstein's general relativity later restored contiguity more successfully.
Chaos theory adds nuance. Tiny changes in the starting state of a nonlinear system can produce huge long-term differences, yet classical physics generally counts as causes only conditions proportionate to their effects. A ball balanced on a slope rolls because gravity accelerates it; the small nudge that started it is a trigger. The butterfly is treated the same way, with the tornado's energy already present in the atmosphere. Ordering events into causes and effects also helps frame paradoxes like the time traveller who prevents his own grandfather from meeting his grandmother.
Source: Causality (physics)
