A New Drake Equation: The Probability of Life in the Multiverse

by Max Andrews

The Drake equation is advocated by SETI (Search for Extra Terrestrial Intelligence) to show that the probability for ETI has a high probability (N).


  • N* Number of stars in the Milky Way
  • fp Number of habitable planets in each system
  • ne number of planets that are Earth like
  • fl number of planets that emerge from inorganic matter or organic precursors
  • fi fraction of those planets on which intelligent beings also evolve
  • fc fraction of those planets on which sufficient communications technology arises
  • fL fraction of average planet lifetime

The problem with the equation is that each f is a number between 0 and 1, the product of the equation will be vastly lower than the total number of suitable stars in the galaxy N*.  Many variables are unknown.  So, the numbers that are brought in depend profoundly on the assumptions we bring into the problem.

There has been a relatively recent paper (February 2010), which argues the use of a modified Drake equation for multiverse scenarios.

{ccosmo} ⊕ {castro} ⊕ {clife} ⊕ {ccomplex life}

Let the direct sum symbol, ⊕, denote ‘together with.’

Concerning {ccosmo}, few parameters are: vacuum energy density, matter-antimatter asymmetry, dark matter density, and perturbations for the formation of large-scale structure. To these, one adds the couplings of the four interactions and the masses of quarks and leptons so that hadrons and then nuclei can form after electroweak symmetry breaking.

{castro} includes constraints in galactic morphology and stellar ages and types. In order to retain heavy elements, galaxies must have masses above a certain value. Numerical results suggest that galaxies with M ≥ 109M (M denotes the mass of a star) are able to retain about ≥ 30% of their heavy elements.

{clife} includes the planetary and chemical constraints for the existence of life. Of the planets in the habitable zone, only a fraction will have the right preconditions for life: liquid water and the elements C, O, H, N, and the less abundant but no less needed P, S, Fe, Ca, Na, Cl, etc. Apart from water, other simple molecules are also supposed to be present, although the specifics may differ (CH4, CO2, NH3…).

{ccomplex life} includes planetary constraints that would essentially ensure the continuity of fairly stable environmental conditions thought to be needed for complex, multicellular life to evolve. To begin, terrestrial  life’s complexity seems to be intrinsically linked to the rise of atmospheric oxygen, courtesy of photosynthetic bluegreen algae. Anoxic environments don’t seem very conducive to multicellular life, although there is much we don’t know of Earth’s biota. However, we do know that complex life is more fragile and requires more efficient energy-processing metabolism to be sustained: planetary platforms able to harbor complex life-forms must thus satisfy extra requirements.

Notice that each factor is a condition for the one following.  For example, {clife} must be met in order for {ccomplex life}.

One of the obvious problems with the multiverse equation, like the original Drake equation (which has been changed several times by SETI), is that the values are largely unknown and are leaping guesses.  Marcelo Gleiser, the author of the paper, discusses one of the problems in applying a probability equation like this one to a multiverse is that there are several different multiverse scenarios.  If the values of the laws of physics vary from universe to universe then there has to be some type of selection effect to account for the variance.  In the end, I don’t think any probability argument like the Drake equation will be ultimately sufficient.  There are too many unknown factors.  While each factor is true and a condition for intelligent life, the values are simply unknown.  I prefer fine-tuning arguments, which consider the values of the laws of nature and initial conditions only (the Drake equation isn’t an argument in and of itself, that’s important, nor do I think it should be used).  My preference is to argue the fine-tuning of the multiverse scenarios and mechanisms involved for the production of the universes (Einstein’s field equations, for example) and variance in the values of physics (super string theory).  Environmental factors, like the galactic and stellar habitable zones, merely add more weight to the arguments after the main argument has been established.

(The paper and data I’ve referenced is taken from Gleiser’s “Drake Equation for the Multiverse: From the String Landscape to Complex Life”).


2 Responses to “A New Drake Equation: The Probability of Life in the Multiverse”

  1. There needs to be a new anthropic principle then, taking the multiverse into account!

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