[ExI] The Great Silence again

[Anders Sandberg]

spike wrote:
> If so, some of you young physics hipsters step up to the plate right now,
> for I am very busy now with some family crises.  First assignment: given a
> sunlike star, with 1 sun mass and luminosity, assume an MBrain reflects 99%
> of the momentum eastward.  What is the star system's westward acceleration?
> I calculated this about a decade ago, and it is in my green notebooks
> somewhere, but I haven't the time to find it.  As I vaguely recall, it was a
> few meters per square year, but that seems high to me now.  Hipsters, have
> it done by this time tomorrow, and I will do it independently as a check.
>   
Hmm, with momentum p=h/lambda and energy E=hc/lambda for each photon, if 
there is I Watts of energy at that wavelength I get lambda I/hc photons 
with a total momentum of I/c... great, no need to get into Planck's 
radiation law. So the total momentum per second if the luminosity is L 
and we have efficiency eta is eta L/c.

For the sun and 99% efficiency, that is 1.27*10^18 kg m/s^2. Putting 
that into F=ma gives us the whooping acceleration 6.39*10^-13 m/s^2. It 
takes about 2.8 million years to move a lightyear this way... 
surprisingly fast, actually! This way you could get out of the galactic 
plane within a galactic rotation, even if you didn't use a stellar 
gravity assist.

-- 
Anders Sandberg,
Future of Humanity Institute 
James Martin 21st Century School 
Philosophy Faculty 
Oxford University