[ExI] Physical limits of electromagnetic launchers

[Anders Sandberg]

On 04/06/2012 08:00, Eugen Leitl wrote:
> On Mon, Jun 04, 2012 at 12:50:37AM +0100, Anders Sandberg wrote:
>
>> Basically I am trying to estimate the limits of how quickly a single
>> Dysoned star could spam the universe with colony probes. Most of the
>> analysis has been done already, but I would like to be certain that the
>> launch systems are feasible.
> Launch is easy, braking is hard.
>

Indeed. We spend far more of the paper looking at that. (which is why I 
sent the question to the list - I suddenly got worried that I had 
glossed over some really important aspects).

Basically, we are supposing a retro-rocket to slow things down. Being 
launched backwards from a mass driver also works, but is equivalent to a 
slightly unusual retro-rocket where the reaction mass is on the outside. 
In any case the rocket equation bites hard. Using the expansion of the 
universe to slow is *neat*, but only feasible for very long-range probes.

Generally, I wonder about scaling down rockets. While Freitas has shown 
that one can power nanosmall systems using radioactive decay, I seem to 
recall that fission and fusion reactors do not scale down well (once you 
get below the mean free path of particles containment becomes hopeless). 
So it would be interesting to consider the smallest possible rocket. I 
imagine it would be a pencil-sized Orion rocket using antimatter pellets 
- antimatter at least scales perfectly, at least when it comes to 
exploding. But the shield still needs to be dense and thick enough to 
catch the kinetic energy of the photons, pions and muons.

-- 
Anders Sandberg,
Future of Humanity Institute
Oxford Martin School
Faculty of Philosophy
Oxford University