Just three orbiting black holes can break time-reversal symmetry, physicists find

Most of the laws of physics don’t care which direction time is travelling. Forwards, backwards… either way, the laws work exactly the same. Newtonian physics, general relativity – time is irrelevant to the mathematics: This is called time-reversal symmetry.
In the real Universe, things get a bit messier. And now a team of scientists led by astronomer Tjarda Boekholt of the University of Aveiro in Portugal have shown that it takes as few as three gravitationally interacting bodies to break time-reversal symmetry.
“Hitherto, a quantitative relation between chaos in stellar dynamical systems and the level of irreversibility remained undetermined,” they wrote in their paper.
“In this work we study chaotic three-body systems in free fall initially using the accurate and precise n-body code Brutus, which goes beyond standard double-precision arithmetic. We demonstrate that the fraction of irreversible solutions decreases as a power law with numerical accuracy.”
The n-body problem is a famous problem in astrophysics. It arises as you add more bodies to a gravitationally interacting system.
The movements of two bodies of comparable size in orbit around a central point are relatively simple to mathematically predict, according to Newton’s laws of motion and Newton’s law of universal gravitation.
However, once you add another body, things become tricky. The bodies start to gravitationally perturb each others’ orbits, introducing an element of chaos into the interaction. This means that, although solutions exist for special cases, there is no one formula – under Newtonian physics or general relativity – that describes these interactions with complete accuracy.
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Even within the Solar System, which we understand pretty well, we can only predict a few million years into the future. Chaos in the Universe is a feature, not a bug.
When running n-body simulations, physicists sometimes return time-irreversibility in their results – in …
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