this post was submitted on 26 Dec 2024
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I see some people are having issues with the scenario, but it's not as impossible as it seems. The key is that Newtonian mechanics are in principle time-reversible. If a system got to a state one way, it can get back to the state it was by running it backwards, so to speak. A ball going down an inclined plain with a given kinetic energy could be going up that inclined plain up to the top with that same amount of energy.
The problem with these systems is, it's possible to impel the right amount of force on a mobile so that it goes through a path and then stops. But since there is time reversibility, it should be possible for the mobile to spontaneously start moving from that stopping point and draw the same path.
Other weird similar cases are the so-called space invader (particle going to infinity, and therefore spontaneously appearing in reverse) and some strange n-body problem cases.
It's not spontaneously starting to move from rest. There was a force exerted on it to cause it to stop. That force will cause it to start moving in time-reversal.