[ExI] is a FTL drive a dream without any physics to back it up?

[Eugen Leitl]

On Fri, Dec 16, 2011 at 11:21:26AM +0100, Stefano Vaj wrote:

> OK. So it would not be a matter of subjective time, but a matter of energy
> consumption (which would not apply to, say, a Bussard propulsor or any

No, it's about mass. Relativistic travel is energetically 
expensive, so you want to limit the payload size.

> other renovating source thereof).

If we want to ramp up to >0.9 c within months you'll need
a light craft with a huge sail and a noticeable fraction of
Sun's total output for propulsion. Nothing else really
works with known physics. Even so braking is iffy, as I don't
see how sacrificial sails would work over few lightyears.
 
> In such event, I think the best would be to switch off whatever you send
> altogether, be it biological or digital or a mix thereof, unless and until
> you find the next source of energy.

The beam tracking the sail provides more than enough energy. It's
the mass that we don't have. Even with machine-phase a few grams
of computronium isn't that much.
 
> I think pioneers would travel as seeds, not as full organisms.
> >
> 
> Yes, I always wondered about that. How much do we push the idea, however?
> The "lightest" way to seed AIs at destination might be to send a few
> probiotes likely to inseminate an evolutionary cycle, leading to organisms

In theory you only need a few cubic microns. In practice you'll need
a ~kg craft deccelerated to sufficiently low delta v so that it can
find and latch upon suitable space debris to feed on. The rest is
an exponential growth process.

> able to transfer themselves on synthetic support. But this is not much more
> emotionally satisfying than send the Voyager with a few records of our
> civilisation towards the big Nothing...

You're not going to send the seeds. Something that has a need will
send the seeds. You will contribute to that something with the needs
will come into being. 

It will be definitely not human, so no human motivations apply.
(Apart from making babies, that is).