# Pushed around by stars

## 4 thoughts on “Pushed around by stars”

1. Kurt Thomas says:

Dear Professor,

I doubt that a mass that is moved outside the solar system has any effect on anything on earth within a second, even on the path of molecules. Or does the gravitational pull manifest with superluminal speed?

Kurt Thomas

2. It’s a good question, but as I wrote: “Technically, it would take years for the gravitational perturbation to reach the Earth, which Borel was unaware of, but that’s irrelevant for the sub-second timescale of our gas being stirred up by the distant mass.” If the mass is 4 light years away, it means the motion 4 years ago (since gravity propagates at the speed of light) scrambles molecular trajectories now, and it makes them “unpredictable” on for timescales well within a second.

3. Fantastic post! Similar-but-different (right?) to the beloved “butterfly effect” in meteorology, i.e. because the atmosphere is a chaotic system, tiny differences in initial conditions rapidly grow, rendering accurate weather forecasts beyond roughly a ~2 week timescale impossible *in principle*, even if we had a perfect model.

Ed Lorenz 1963, http://journals.ametsoc.org/doi/abs/10.1175/1520-0469%281963%29020%3C0130%3ADNF%3E2.0.CO%3B2, gets the credit for this in atmospheric circles at least. Should Emile Borel get partial credit too?