Anders Sandberg (
Mon, 16 Dec 1996 16:27:47 +0100 (MET)

On Sun, 15 Dec 1996, John K Clark wrote:

> Well OK, Tipler found a solution in General Relativity that shows that an
> infinitely long, extremely dense cylinder made of Neutronium (the stuff of
> Neutron Stars) and spinning at almost the speed of light would be able to tip
> a light cone enough to act as a time machine, but there are two very important
> problems.
> 1) Nobody knows if a cylinder of finite length would work also, the math is
> too difficult to figure out.

This is true, but there are other FTL or time travel (CTC) solutions of
GR, such as the Alcubierre warp drive, passing cosmic strings or
traversable wormholes. Of the allowable manifolds, a quite sizeable subset
have such beasties (anybody who knows if the measure of CTC manifolds in
the space of all possible GR manifolds is zero, one or somewhere

> 2) The very rapid rotation would cause the cylinder to fly apart. This is
> much more than just an Engineering difficulty, no known force in Physics
> would be strong enough to hold the cylinder together, not even the strong
> nuclear force. For this to work new Physics would need to be found, which
> is just another way of saying that as far as we know now it's impossible.

Well, if the cylinder blows up, doesn't there exist a tiny window of time
where you can move backwards in time before the shock front reaches you or
the metric change destroys the CTC properties?

My guess is that general FTL (as opposed to "tame" FTL which doesn't
violate causality) and general time travel will not be possible either

1. due to quantum effects, such as the Visser virtual particle blowup that
ought to occur (at least in a first approximation, if I have understood
his papers right) when two wormhole ends get too close to form a CTC. This
mechanism ought to occur in other potential causality violations such as
the cosmic string trick and might

2. due to that the universe is always consistent, so any time travelling
will be constrained by its past (you can "change" it, but only so that it
becomes identical with your own past). See the papers by Novikov about
this fun area (I love his time-travelling billiard trick, which Forward
used in _Timemaster_).

On the other hand, I think "tame" FTL and time travel is possible, as
constrained by the results of Khatsymovsky (you can do FTL, but only
inside your future light-cone). I.e. you can build wormhole empires and do
apparent FTL, but not violate causality or build generalized time

Anders Sandberg Towards Ascension!
GCS/M/S/O d++ -p+ c++++ !l u+ e++ m++ s+/+ n--- h+/* f+ g+ w++ t+ r+ !y