Re: opposite of relativistic time contraction

On 2008-03-16, Ben Crowell <crowell07@[EMAIL PROTECTED]
> wrote:
> Mmm...I would consider diffraction to be a fundamental physical
> limitation, not a minor engineering problem. If the laser is
> at rest with respect to the galaxy, then it has to operate over
> distances of light-years.

You want to accelerate a 10^17 kg black hole at 100g over distances of
at least a couple of light-years?  Ouch.  The project then needs at
least 10^37 J of energy available.


> In any case, there's another issue that seems more fundamental,
> which is that 10^29 W converts via E=mc2 to 10^12 kg/s, which is
> enough to double the mass of our black hole in hours.

Oh, if you need more than that, then don't feed the beam into the
hole: just let the hole's gravity bend it, say 150 degrees or so.  As
a bonus, you get nearly double the momentum transfer.  Of course the
red****ft will be a problem in any event since the gamma factor will go
past 200.

On the plus side, the diffraction problem isn't as bad as you think:
an on-site collimator can direct the laser to the correct location
near the hole.


> Good point. Well, the exhaust velocity can be a small fraction of c
> -- you want it to be a small fraction of c in order to get a decent
> thrust-to-power ratio.

Absolutely not: the mass/payload ratio is exponential in
delta-V/exhaust and for 100g acceleration over 2 light years the
delta-V is at least 6c.  If converting mass directly into energy at
arbitrary rates is a problem, then the whole project is doomed.


> Maybe the magnet is on the end of a 10 km superconducting cable
> trailing behind the thrust module.

Heh, the cable is subjected to a tension of 10^20 N.  I think ordinary
matter is out of the question.


- Tim