Good piece

Posted Nov 30, 2012 7:07 UTC (Fri) by apoelstra (subscriber, #75205)
In reply to: Good piece by Cyberax
Parent article: LCE: Don't play dice with random numbers

> 1) "Space invaders" scenario is perfectly deterministic, they just have a problem with incorrect abstraction. In particular, treating gravity as infinitely fast. If we instead treat gravity as a classical field with finite interaction propagation speed, then this paradox disappears. Ditto for special relativity (it has its own problems with singularities, though).

This isn't the problem with space invaders, though. The problem is that you have an object that disappears to infinity, where it remains for all time. But since classical mechanics is time-reversible, it could just-as-legitimately said that the object "has been at infinity since eternity, then moves to a finite position at time t".

But nothing in classical mechanics predict when "time t" is, hence the indeterminacy.

As you have pointed out, special relativity wrecks up this pathology (though general relativity introduces many more, much worse, ones). But that's irrelevant to whether the claim "classical mechanics is deterministic" is true.

Good piece

Posted Nov 30, 2012 14:07 UTC (Fri) by davidescott (guest, #58580) [Link]

As an aside. SR eliminates infinite velocity singularities in classical mechanics, but there are other non-regular solutions in Classical Mechanics which are finite velocity. To eliminate those you would have to enforce C^2 on all constraint surfaces (and hope that covers all possible singularities). In many ways these surface is C^1 but not C^2 problems are easier to understand than space invaders because you don't have any messy infinities to deal with. So go read Norton's paper.