Drop two identical pebbles from a mountain peak: one lands a centimeter to the left, and a minute later they end up in different valleys. A galaxy is the same mountain, only with billions of pebbles and a slope that stretches over millions of years. The authors of a new study simulated a disk of 40 million point-like stars and tested how quickly it forgets where it started.
Turns out, almost instantly. The time for the divergence between two nearly identical galaxies to grow by a factor of several is less than 0.1 million years. For comparison: light from the center of the Milky Way takes 25,000 years to reach us, and the dinosaurs went extinct 66 million years ago. The stellar disk loses its memory of the past faster than a single spiral wave can form.
The most interesting part is the fate of the bar — the elongated stellar structure at the center. Its birth moment doesn't depend on tiny perturbations, but its strength and subsequent life do. The spread between simulations is greatest exactly when the bar is strongest and vanishes when the bar 'breaks'—loses symmetry and thickens. So even in ideal models, the same galaxy can grow a bar of different strengths.
For astronomers, this is a warning: the smooth potentials used to describe individual stellar orbits lie on timescales longer than the chaotic one. The Milky Way forgets its initial conditions faster than we can blink, and that's not a metaphor but a result of a numerical experiment.
The chaos time in a galaxy is less than 0.1 million years: that's like forgetting where you left your keys in one thousandth of a second.