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[Delayed posting]<br>
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On 2015-08-29 21:04, Rafal Smigrodzki wrote:<br>
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cite="mid:CAAc1gFg9HZnHA2dcbhJfQgbnXDKHYNxUvAa_OrTnF32c=x92LA@mail.gmail.com"
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<div class="gmail_quote">On Sat, Aug 29, 2015 at 1:24 PM,
Keith Henson <span dir="ltr"><<a
moz-do-not-send="true"
href="mailto:hkeithhenson@gmail.com" target="_blank"><a class="moz-txt-link-abbreviated" href="mailto:hkeithhenson@gmail.com">hkeithhenson@gmail.com</a></a>></span>
wrote:
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex"> <br>
*IF* a species was inclined to spread out to the stars,
the obvious<br>
way to travel is light sails pushed by laser cannons
themselves<br>
powered by stars. The spill around the light sails
would be visible<br>
clear across the visible universe. We don't see such or
at least have<br>
not yet.</blockquote>
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<div>### This image gives me the goosebumps: Sailing
through the darkest night on pillars of burning light to
claim the farthest shores in the name of life - what
could be more poetic?</div>
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<br>
It is beautiful. <br>
<br>
However, I think the light overspill Keith mentions may not be
present. Eric and me worked out a laser-propelled probe that has
an extensible "optical power cord cord". Imagine it trailing an
optical fiber (somehow) delivering the laser light for launch: no
diffraction-limit caused overspill, you can maintain a tight focus
on the probe until the fiber cannot be extended more. But an
optical fiber is just glass and emptiness: it can be replaced with
a series of thin lenses separated by space, and this arrangement
can e expended quite far. The final step is to realize that it is
all about interference patterns for your monochromatic light, and
they can be Fresnel structures. So in the end the probe gets
launched with little overspill at a huge acceleration, coasting
quietly. It is braking that can be visible, but if the
acceleration is decent the chance of seeing anything is low. <br>
<br>
<br>
<pre class="moz-signature" cols="72">--
Anders Sandberg
Future of Humanity Institute
Oxford Martin School
Oxford University</pre>
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