Saturday, December 12, 2015

Fascinating work on Galactic Habitability

Evolution of NSurvive in Milky Way
Fig 6 of Forgan et al Evaluating...
A short piece in Science draws my attention to the fascinating work of Duncan Forgan at the University of St Andrews and his colleagues. The paper discussed, "Evaluating Galactic Habitability Using High ResolutionCosmological Simulations of Galaxy Formation" estimates galactic habitability in 3D by analysing high resolution numerical simulations of the formation of the Local Group. Thanks to mergers and the accretion of satellite galaxies, they find that if the GHZ exists, it must be fundamentally asymmetric. They also find that "the probability that an individual planetary system contains habitable worlds increases with distance from the galactic centre, with its peak near the edges of galactic discs. Regions above the midplane are also favoured locations, but the density of stars in these areas reduces the total number of habitable planets."

 They apply their simulations to the Milky Way and also to the M33 Galaxy (which is about 3M Light Years away).  This is potentially another step towards being able to test the MaxHELP hypothesis (which I first proposed in 2011) but I suspect we are at least a decade away - there need to be further major advances in computer power and mathematical/physical understanding.

There is another intriguing paper by Adam Stevens, Duncan Forgan and colleagues called "Observational Signatures of Self-destructive Civilisations" which explores the extent that we could see evidence of various forms of civilizational destruction. This is motivated by the Drake Equation and the Fermi Paradox which at least suggests that one reason why we have so far not seen any communications from other civilisations (as far as we know!) is that L the mean lifetime of civilisations capable of communicating might be very short.

The classic problem is that the level of technological development required to send high-powered signals into interstellar space is pretty much the same as that required to make nuclear weapons. The world came perilously near nuclear destruction at the time of the Cuban Missile Crisis which was only a couple of decades or so after Nuclear Weapons had been discovered, and people now think there was a 20-30% chance of a massive nuclear exchange then. There was also a close call in 1983 when Stanisalv Petrov decided that satellite detection of a first strike attack by the US on Russia was a false alarm. This suggests that the half-life of a civilisation in possession of nuclear weapons may be only a few decades. In addition it is a sobering thought that, if Hitler had not expelled so many of his Jewish scientists, he might have developed nuclear weapons at much the same time as the allies.

They explore 4 scenarios and ask how one might detect evidence of each after the event:
  1. Complete Nuclear Destruction
  2. A chemical or biological agent
  3. A technological disaster such as the grey goo scenario
  4. Excessive pollution of the star, planet or interplanetary environment.
They argue convincingly that each of these would leave some evidence that could in principle be detected by observation over inter-stellar distances, although is some cases such as nuclear exchange the main remote "signatures" would be pretty short-lived. They conclude that: "It is clear that some observational signatures of self destructive civilisations are currently amenable to astrophysical observations, but these will be challenging, and in some cases will require a degree of luck in observing at the correct time. However, these detection techniques are relatively cheap, as they dovetail neatly with current astronomical surveys. In time, the first evidence of extraterrestrial intelligence may come to us from the remains of less prudent civilisations. In doing so, such information will bring us not only knowledge, but wisdom."

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