Re: Running away

From: Dani Eder (
Date: Tue Sep 28 2004 - 15:55:43 MDT

Since I am a rocket scientist, I'd like to address
some issues about 'running away':

(1) Eliezer wrote:
> Suppose you pack your bags and run away at .99c.

If you travel at a high fraction of lightspeed, the
following will become serious issues:

- the cosmic background radiation will be blue-shifted
ahead of you from 3 Kelvins up to a higher
This will generate thermal problems at some point, and
differential light pressure will tend to limit your

- Interstellar gas atoms will begin to act like
a high energy particle beam, causing radiation
problems and drag.

- Interstellar dust particles will act like tiny
high speed bullets. At 0.99c, the particle flux
will deposit 20KW/m^2 in thermal energy if stopped.
That alone will heat a leading edge to 875F.
Mechanical pitting of the leading edge from impacts
will likely be severe.

The only propulsion methods I know of that can get
to a substantial fraction of c would still have a
hard time getting to 0.99c:

- Matter-Antimatter annihiliation (due to mass
overhead to store the antimatter and focus the
annihilation products to get thrust)
- Laser energy source (allows arbitrarily high
performance since the energy is external to the
vehicle, but the laser will be redshifted which
limits velocity)
- Quantum black hole (100% mass-energy conversion
if you feed the hole at the same rate it emits
Hawking radiation, but you still have mass overhead
to make an engine and the hole itself will be heavy.

So absent some new ideas (which is always possible),
running away at 0.99c is not likely feasible.

(2) It can be shown that the energy to send the
description of an object atom by atom is much less
than the energy to send the object itself at a high
fraction of c. Given nanotech level capabilities,
the fastest way to get from here to there is to
send a small fast probe to a destination, then
have it build a receiver/assember factory. Then
you FAX yourself to the destination.

(3) If your back end is a cold (<3K) conical
mirror, you will look like another patch of
cosmic background to someone trying to chase you.
If you can get some uncertainty in direction
when leaving, you will be damn hard to find.

The faster
> you run, the less subjective time you have on board
> the ship before someone
> catches up with you, owing to lightspeed effects.

At 0.99c time dilation is 7x, so stellar flybys will
occur at ~6 month intervals ship time. Each stellar
flyby is a chance to increase your positional

> Suppose it doesn't run after you. In that case, if
> more than one group
> escapes, say, 10 groups, then any one of them can
> also potentially create
> an UFAI that will chase after you at .995c.

For each year head start, in the galactic rest frame,
it will still only be gaining on you at 0.005c, so
it will take 200 years to catch up. Onboard ship,
the time time will be ~30 years.

Another way to look at it is as seen from the ship,
the Earth experiences a time dilation of 7x (it's
all relative you see). So it takes that much
longer for an AI to get built and start chasing
you, from your perspective.

> I also dispute that you would have .99c-capable
> escape vehicles
> *immediately* after nanotech is developed.

Given that you can send information to the escape
vehicle, and it has sufficiently general purpose
materials and equipment, you can launch with what
you have now, and send it upgrades as technology

There used to be an argument that given an assumed
rate of technology improvement, you can calculate
a minimum in the curve of (waiting for better
technology) + (travel time), such that there was
no point in building an interstellar ship earlier
than the optimum time.

With the assumption of a singularity, with a
hyperbolic technology curve, and the idea of sending
upgraded engine designs en-route, I don't know of
anyone who has worked out the optimal strategy
of when to build your ship.


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