Posts Tagged ‘Drake equation’

Calculating Doomsday – An interesting but ultimately meaningless probability game

March 21, 2013

A new paper playing probabilistic games – this time about the Doomsday Argument.

Universal Doomsday: Analyzing Our Prospects for Survival, by Austin Gerig, Ken D. Olum, Alexander Vilenkin,  arxiv.org/abs/1303.4676 , Cosmology and Extragalactic Astrophysics (astro-ph.CO)

The full version of the paper (pdf) is here

Doomsday argument Gerig et al

Doomsday argument Gerig et al

The Doomsday Argument is the idea that we can estimate the total number of humans that will ever exist, given the number that have lived so far. The argument goes that since around 100 billion is the number of humans that have ever lived and assuming that there is a 95% probability that we are among the last 95% of humans who will ever live then there is a 95% probability that the number of humans who will ever live will lie between 1.4 and 2.0 trillion. A fairly trivial conclusion since any probability greater than 0 and less than 100% would be valid for the exercise.

In this paper the authors try to formalise the probability calculations and introduce the effect of known existential threats. Just like in Drake’s equation for the number of extra-terrestrial civilisations that may exist in the Milky Way, all the probabilities are unknown and could be assumed to be anything you like. The Doomsday Argument like Drake’s equation is really no more than a probability game, based on nothing at all. But it is fascinating to consider which terms are relevant and necessary in any such game.  And that is what makes these games interesting.

The author’s conclusions could be considered a trifle obvious and almost cliched – but none the less they are perfectly true!! The Earth will surely experience catastrophic events in the future which threaten human existence – whether by earthquake or volcanos or meteors and even if we survive all of these, eventually by the inevitable death of our sun.  In fact you could play another – and equally valid – probability game and calculate how many humans will have lived if humanity continues to survive till the death of our sun. And this probability is surely not zero.

To avoid Doomsday, humanity needs to make sure that asteroids don’t crash into earth and that catastrophic earthquakes, volcano eruptions or the like don’t occur until  such time as humanity has spread into space and  developed colonies on other planets.

From the Conclusions:

With the priors that we considered, the fraction of civilizations that last long enough to become large is not likely to exceed a few percent. If there is a message here for our own civilization, it is that it would be wise to devote considerable resources (i) for developing methods of diverting known existential threats and (ii) for space exploration and colonization. Civilizations that adopt this policy are more likely to be among the lucky few that beat the odds. Somewhat encouragingly, our results indicate that the odds are not as overwhelmingly low as suggested by earlier work. 

Abstract (Submitted on 19 Mar 2013)

Given a sufficiently large universe, numerous civilizations almost surely exist. Some of these civilizations will be short-lived and die out relatively early in their development, i.e., before having the chance to spread to other planets. Others will be long-lived, potentially colonizing their galaxy and becoming enormous in size. What fraction of civilizations in the universe are long-lived? The “universal doomsday” argument states that long-lived civilizations must be rare because if they were not, we should find ourselves living in one. Furthermore, because long-lived civilizations are rare, our civilization’s prospects for long-term survival are poor. Here, we develop the formalism required for universal doomsday calculations and show that while the argument has some force, our future is not as gloomy as the traditional doomsday argument would suggest, at least when the number of early existential threats is small.

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Perhaps there is no life on other planets

August 1, 2011

The fundamental weakness of the Drake Equation is that it starts with the assumption that life on Earth is not unique. After that it merely applies a string of probabilities to derive the number of civilizations in our galaxy with which communication might be possible. Fermi’s paradox is the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations (such as given by the Drake Equation) and the lack of evidence for, or contact with, such civilizations.

The paradox of course is resolved if there are no other such civilisations except on Earth which would be the case if life (as we know it) was unique to Earth.

It might not be what I would want to or like to believe but it is the simplest resolution of the apparent paradox. And if that is so then Drake’s equation starts with a false premise and is irrelevant and invalid. The existence of extraterrestrial life is still in the realms of belief and hope and is not (yet) science.

Now Bob Yirka of PhysOrg reports that

Astrophysicists apply new logic to downplay the probability of extraterrestrial life

David Spiegel and Edwin Turner of Princeton University have submitted a paper to arXiv that turns the Drake equation on its head. Instead of assuming that life would naturally evolve if conditions were similar to that found here on Earth, the two use Bayesian reasoning to show that just because we evolved in such conditions, doesn’t mean that the same occurrence would necessarily happen elsewhere; using evidence of our own existence doesn’t show anything they argue, other than that we are here.

…….. Spiegel and Turner point out, basing our expectations of life existing on other planets, for no better reason that it exists here, is really only proof that we are more than capable of deceiving ourselves into thinking that things are much more likely than they really are. ……

When taken at face value, some might conclude that such arguments hold no more logic than arguments for the existence of God, i.e. it’s more about faith, than science. At any rate, most would agree that the only concrete way to prove whether there is life out there or not is to prove it, by finding it.


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