Chaos, where the initial conditions of a system can no longer be recovered from a system’s final state, is known to be present in many-body interactions. For the first time, researchers have demonstrated this fundamental time-irreversibility in a system with only three realistic masses. (EUROPEAN SOUTHERN OBSERVATORY)

Ask Ethan: Why Can’t Time Be Reversed For Three-Body Systems?

In principle, the laws of physics are the same forwards and backwards. But in practice, time only runs in one direction.

Most of the laws of physics are the same whether you run the clock forwards or backwards. A planet that revolves around a star appears to obey the same rules as it would if you recorded it and played the recording backwards. This is true for any gravitational, electromagnetic, or strong nuclear interaction between two particles: they are time-reversal invariant. Normally, we only see the non-reversible effects of chaos and thermodynamics with extremely large systems, but a recent paper claims to have shown this for the gravitational interaction with only three masses. Jonathan Belew wants to know what this means, asking:

Does this have any relevancy to the n-body problem, time-reversal symmetry as a whole, or implications for cosmology? Or is it theoretical and only apply to a small subset of cases that do not make up a significant portion of the observed universe?

Let’s unpack what it all means.