[ExI] Interstellar FedEx

Anders Sandberg asa at nada.kth.se
Wed Sep 30 01:22:24 UTC 2009

A bit more thinking (nightly walks home helps clear the mind):

1: What is happening in the tube model is that energy gets redistributed
between the degrees of freedom: the first spring, the payload, the tube
and the second spring. When the payload is fired the first spring will
wobble, and force transmitted along the tube will make the second spring
wobble. In theory one could time the speed so that all (?) energy gets
collected into the second spring by having all wobbling coincide when the
payload arrives, but this will not be generic for all payload speeds.

2: When the payload is fired, the whole tube will start to move. The
center of mass stays the same, so as the payload traverses the tube the
tube will also move a bit in the other direction. When the payload stops,
the tube will have moved mL/(M+m) meters, where m is the payload mass and
M is the tube mass and L its length. So we do get one of those unwanted
displacements that would need correction if we want to keep the tube in
exactly the same place.

So we seem to have two effects here, a more direct entropy effect where
the degrees of freedom start to move, and a displacement effect due to
reaction. The first one is irreversible, the second one could be reversed
if the first one was reversible - but it isn't.

Anders Sandberg,
Future of Humanity Institute
Philosophy Faculty of Oxford University

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