[tt] [megascale] Fwd: [orions_arm] Re: Galaxy obliteration

Fri Nov 30 07:49:19 UTC 2007

----- Forwarded message from Bryan Bishop <kanzure at gmail.com> -----

From: Bryan Bishop <kanzure at gmail.com>
Date: Thu, 22 Nov 2007 07:23:47 -0600
To: megascale at heybryan.org
Subject: [megascale] Fwd: [orions_arm] Re: Galaxy obliteration
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Subject: [orions_arm] Re: Galaxy obliteration
Date: Wednesday 21 November 2007 22:38
From: "drashner1" <drashner1 at yahoo.com>
To: orions_arm at yahoogroups.com

> That can be worked out using the formula on the new Laser page;
>
>  "The smallest possible spot size to which a beam can be focused can
> be calculated; if the initial beam width is D, the wavelength of the
> light is L, and the distance to the target is R, the smallest spot
> size (S) is given by S = 1.2 R  L/D."
>
> It is possible to work out using this formula how big the aperture
> needs to be to ensure a hit on a planet over a distance of a million
> light years.
> I think it depends on the wavelength used, but the aperture required
> should be smaller than a dyson sphere.

After reading this I pulled up MS Word and my trusty calculator and
ran some numbers. Hopefully they are accurate but please check me. Ok
then:

Given the scale we are discussing here, I set the following values:

Initial beam width (D) = 1E5km = 1E8m

Wavelength (L) = .5micron (probably not what would be used but I
figured why not for a first approximation).

Distance (R) 2.5 million ly (distance to Andromeda)

Putting all this together I got:

Beam diameter at target = 146,880km diameter

Ok, this is actually pretty impressive given the distances involved.
Now, the energy density of the beam.

Taking the 'surface area' covered by the beam at target and inputting
the output of a star like the sun we get an energy density of roughly:

22GW/m2  (assuming 100% efficiency which is unlikely of course. Anyway)

Ok, again very impressive and probably enough to cause considerable
damage at target.  Not sure that this is enough to rapidly 'boil away'
a planet however since with the exception of something like Jupiter,
the majority of the beams energy will miss the planet and pass on into
space. Let's see:

22GW/m x 'beam interception area' of Earth =

1.28E14m2 x 22GW/m2 = 2.8E15GW = 2.8E24W

Dividing this number into total solar output we get roughly 136.
Which is to say it would take about 136x as long to 'take apart' the
planet if it is targeted by the beam. Per the planet disassembly
section of the Matrioshka Brain homepage it would take about 22 days
to take the Earth apart using total solar output at 30 percent
efficiency.  We're presuming 100 percent here so first lets take that
number and divide by 3:

Approx 7 days to take the planet apart.  But we don't have all that
energy. So multiply by 136:

997 days/365 equals: 2.7 years.

Hmm. It might be better to just sweep the planet long enough to cook
the surface and move on.  Assuming you can find the planet of actually
hit it of course:) Because...

Given the distances involved if the aperture aiming is off by even a
very small amount the beam will miss the target by a much larger
amount.  Not sure what the limit is for accurate aiming either with
current tech or theoretically, but based on some rough calcs, it works
out that if the aim is off by even 1 second of arc the beam will miss
its target by *12 light-years*.  Hmm. Challenging.

Ok, getting late.  I guess the biggest issue at this point is just how
accurately you could aim the laser over this sort of distance (I have
doubts).  Assuming I didn't slip up my math somewhere.

Hope this helps,

> There seems to be a way of using the x-rays to propel kinetic
> particles to boost a spacecraft, described by Isaac Kuo over at
> Badastronomy; however there doesn't seem to be a way to decelerate
> such a craft.

Hmm. This might be the easy bit.  The ships could use magsails or
ramscoops to slow down. Or carry fuel onboard and use it to decelerate.

> Attacking across millions of light years using such beams is
> probably a waste of time, as the target will probably change beyond
> recognition over such periods. But where two galaxies are close
> together and actually interacting (as in the case of M81 and M82)
> thes distances are slightly less daunting.

All good points. Although I'm still figuring that this would be a
rather onesided affair in which someone in one galaxy just decides to
take over the other.  I have a hard time imagining exactly what sort
of exchange at lightspeed would lead to two galaxies deciding to go to
war as a matter of mutual distaste.

Todd

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