[ExI] Fermi Paradox and GRB bursts

[Keith Henson]

On Thu, Oct 2, 2014 at 11:07 PM,
<extropy-chat-request at lists.extropy.org> wrote:

>> On Thursday, October 2, 2014 2:12 PM, Keith Henson <hkeithhenson at gmail.com> wrote:
>>
>> I agree with Anders and add that GRB are *short,*
>> a few seconds at the most.  So ~half of the planet
>> surface is shielded by the whole planet's mass.
>
>> Even if a burst fried everything on one side, the
>> other side would be untouched and life would
>> rapidly spread back into the damaged half.
>
> Depends on how fast the planet is spinning, but, yah.

Let's put some logic and numbers on this.

Unless the GRB happened right over the equator, then some of the
planet isn't going to be toasted no matter how fast it spins.  If it
came from right over the north or south pole, then half the planet
would be spared.  But consider the right-over-the-equator case.

Given the time for a GRB, to get uniformly toasted, the planet would
need a rotation rate of at least 1 revolution per second.  Using the
earth as an example of a planet with life, it's equatorial rotation
speed is 465.1 m/s at one revolution per day.

It would be close to 670 km/s spinning at 60 RPM.  Low earth orbit is
~8 km/s so this is around 84 times faster.  I.e., physics tells you
that an earth sized planet can't spin anywhere close to fast enough to
get toasted all around by a GRB.

Keith