Re: Arrow of Spacetime?
From: The Ghost In The Machine (ewill_at_sirius.athghost7038suus.net)
Date: 12/23/04
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Date: Thu, 23 Dec 2004 14:00:09 GMT
In sci.physics, tj Frazir
<GravityPhysics@webtv.net>
wrote
on Wed, 22 Dec 2004 21:52:43 -0500
<26918-41CA32FB-195@storefull-3215.bay.webtv.net>:
> If a fire cracker explodes 1 light year away andd you see it ,,you cant
> go get it and it exploded a year ago.
> Idiot
>
That fire cracker won't be all that visible anyway.
The divergence of a laser fired through a telescope apparently
reaches 7 km in diameter on the Moon. Assuming that the
Moon's orbit is 3.85 * 10^8 m, the divergence of a similar
laser from 1 light year (9.467 * 10^15 m) would be
about 1.721 * 10^8 m in diameter. This is a little less than
half the moon's orbit in size.
http://www.lpi.usra.edu/expmoon/Apollo11/A11_Experiments_LRRR.html
An eerie picture suggests that the Galileo Spacecraft
had no trouble seeing the laser from 6 * 10^9 m out, but
it was far more impressive nearby -- assuming that the
beam wasn't simply an artifact of the time exposure.
However, they were using 40 megawatt pulses (10 ns in width,
or 400 millijoules.
http://www.w7ftt.net/gopex1.html
The frequency of the laser is apparently 514.5 nm -- which looks
about right colorwise judging from the photos. This translates
into a per-photon energy of
6.626 * 10^-34 J s * 2.99792458 * 10^8 m/s / 5.145 * 10^-7 m
= 3.861*10^-19 J.
Therefore each pulse is about 1.036 * 10^18 photons.
The scatter area would be about 2.326 * 10^16 m^2.
A 10-m diameter telescope such as Keck translates to an area of
78.54 m^2, and therefore Keck might pick up 3500 photons from
this pulse.
Might be doable, but barely. (There is admittedly the issue of
atmospheric refraction, which may make the 7 km figure suspect;
the beam might actually flicker as seen from the Moon -- we won't
know unless we send people up there again and shoot continuous
wave beams at them.)
-- #191, ewill3@earthlink.net It's still legal to go .sigless.
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