Is There Extraterrestrial Life in the Universe?
Modifications to the Drake Equation.


In Drake's Equation:

N, the number of technological civilizations in the galaxy.


So given all these things, we can write
N = n fp H fl fi ft


You can observe how the value of N is changed based on different assumptions at the highlighted link.  But that prompted me to alter the question a bit and ask:

What if N = The number of technological civilizations in the galaxy that may communicate with us.  How would/should each of the terms be modified?

n = The number of stars in the galaxy.  Yes, but...
n(e1) = Exclude those stars within the first 10% to 20% of their lifetime. Assume that the planets around these stars have not sufficiently cooled and/or collected the majority of the material tributary to their orbits...much less achieved the conditions where life could flourish.  "e1" = 0.85.
n(e2) = Exclude stars older than 10 billion years. Stars like our sun will become red giants and envelope the orbital region where life could exist.  Using a lifetime of 15 billion years, "e2" = 0.66.
n(e3) = Exclude those stars in regions of space that have been sanitized by previous supernovas and/or cosmic ray burst from dying stars, pulsars, etc.  This really depends on the size of the event and proximity to adjacent star systems.  If we are lucky, the axis of rotation of most stars is oriented at a perpendicular to the galactic plane, so the gamma ray bursts are directed away from the plane of the galaxy.  Make your own guess for "e3" < 1.0.

fp = The fraction of stars that have planets.
I'm leaving this one alone for now.  Until we can accurately detect planets at a distance, it is just a guess.

ne = The fraction of planets that are capable of sustaining life.  Yes, and...
ne(e1) = Exclude planets that are either too close or too far away from the sun to support the presence of liquid water. Also see (fl-a).
ne(e2) = Exclude planets without an iron core.  In the absence of a rotating iron core, the planet lacks a magnetic field and is thus subject to the ravages of the solar winds.  The presence of sufficient iron to form a core depends on the amount of previous star formation in that region of space.  Lord, this many conditions should surely whittle down this factor's magnitude. 
ne(e3) = Exclude planets with insufficient iron in the core to maintain core rotation based on their distance from their stars. This balance is further affected by the total mass of the planet relative to core size.
ne(e4) = Exclude planets with excessive mass. High gravity forces will affect the development of life as well as the environment.

fl = The fraction of planets that will develop life.  But consider...
Term #1 (fl-o) = The fraction of planets that develop life from scratch.  It's the old experiment of placing chemicals in a sealed container and subjecting them to electrical discharge and primitive atmosphere...yielding amino acids.  Sounds like growth medium to me.  Life is a little more complex than that.  But for now, an open mind.
or
Term #2 (fl-a) = The fraction of planets that acquire life (at the very least, the building blocks for life) from the constituents of previous planets that were destroyed in previous supernovas.  Also see ne(e2).  Is life better explained in terms of propagation from an origin point through the galaxy?  Are sharp evolutionary changes the result of the introduction of a superior biotic system?  I prefer this second term.
fl(e1) = Exclude planets with insufficient material diversity.  The life that is present may exhaust the available materials before it can evolve.  What if one essential element is missing?
fl(e2) = Exclude planets with insufficient biological diversity.  One type of organism will consume resources until exhaustion. Only diversity promotes selection and therefore evolution (and later, intelligence).

fi = The fraction of the planets with intelligent life.
fi(f1) = The fraction of planets with terrestrial life. And only a small proportion of terrestrial life develops technology.
fi(f2) = The fraction of planets with intelligent life and open social systems.  It might be argued that rigid or closed systems restrict the advancement of knowledge and thus technology.  Also see "L".

ft = The fraction of planets with intelligent life that develops technology.
ft(e1) = Exclude planets whose communication technology is species-specific, e.g. Chemical communication as from one ant to another.
ft(e2) = Exclude planets whose communication technology is species-limited, e.g. Telepathy as between individuals who have no means of speaking or hearing (or even seeing each other).

Now add:

fc = The fraction of "ft" planets that develop the capability to communicate.
fc(f1) = The fraction of planets where the intelligent life develops language, grammar and syntax that is rule-based and abstract...something that we might be able to learn.  Languages that incorporate gesture (requiring one to view the language being spoken) or cultural reference would not translate well.
fc(f2) = The rate of drift of the alien language. In what length of time does the majority of the language become unusable to successive generations?  The time between the original message and our reply may exceed that period of drift...so who is left to translate the reply?
fc(f3) = The same drift problem applies to the technologies being employed. In what length of time does the communication technology become obsolete?
Aside: This is surely an argument for a mathematical message.


L = The fraction of the planet's lifetime during which "fc" life can communicate with us.
L(e1) = Exclude all civilizations that have determined that off-world contact is unwise.
L(e2) = Exclude all civilizations that do not want their presence noticed.
L(e3) = Exclude all civilizations that are sufficiently advanced so that they find us uninteresting / too primitive.
L(e4) = Exclude all civilizations that are insufficiently advanced so that they cannot understand what we are saying.
L(e5) = Exclude all civilizations that purposely mask their transmissions. Also see L(e1) and L(e2).
 

L' = The fraction of our planet's lifetime during which we are capable of communicating with "fc" life.
For discussion purposes, assume that L=L' and equally matched civilizations.  Use the ratio of 50 years to the age of our planet, then square that figure.  Talk about killing the odds.