How Many Sentient Life Forms are in the Milky Way?

[N2NH]

Yes, we have a good idea. In fact we have scientifically known for around a half century. It's the SCIENTIFIC formula known as the Drake Formula. It helped to get Star Trek on Television, well, sorta. It's also the way SETI operates and why they listen the way they do.

The Drake equation states that:where:

N = the number of civilizations in our galaxy with which communication might be possible;and

R^* = the average rate of star formation per year in our galaxy
f_p = the fraction of those stars that have planets
n_e = the average number of planets that can potentially support life per star that has planets
f_ℓ = the fraction of the above that actually go on to develop life at some point
f_i = the fraction of the above that actually go on to develop intelligent life
f_c = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L = the length of time for which such civilizations release detectable signals into space.^[3]

It has alternatively been expressed as:

The number of stars in the galaxy now, N^*, is related to the star formation rate R^* bywhere T_g = the age of the galaxy.
Assuming for simplicity that R^* is constant, then
and the Drake equation can be rewritten into an alternate form phrased in terms of the more easily observable value, N^*.^[4]

Regarding SETI (and in the past Carl Sagan):

In 1960, Frank Drake conducted the first search for radio signals from extraterrestrial civilizations at the National Radio Astronomy Observatory in Green Bank, West Virginia. Soon thereafter, the National Academy of Sciences asked Drake to convene a meeting on detecting extraterrestrial intelligence. The meeting was held at the Green Bank facility in 1961. The equation that bears Drake's name arose out of his preparations for the meeting:

As I planned the meeting, I realized a few day[s] ahead of time we needed an agenda. And so I wrote down all the things you needed to know to predict how hard it's going to be to detect extraterrestrial life. And looking at them it became pretty evident that if you multiplied all these together, you got a number, N, which is the number of detectable civilizations in our galaxy. This, of course, was aimed at the radio search, and not to search for primordial or primitive life forms.

— Frank Drake^[1]

This meeting established SETI as a scientific discipline. The meeting's dozen participants — astronomers, physicists, biologists, social scientists, and industry leaders — became known as the "Order of the Dolphin". The Green Bank meeting has been commemorated by a plaque at the site.

As far as Star Trek...

• The equation was cited by Gene Roddenberry as supporting the multiplicity of inhabited planets shown in Star Trek, the television show he created. However, Roddenberry didn't have the equation with him, and he was forced to "invent" it for his original proposal.^[31] The invented equation created by Roddenberry is:

Drake has gently pointed out, however, that a number raised to the first power is merely the number itself. A poster with both versions of the equation was seen in the Star Trek: Voyager episode "Future's End."

One equation, drawn up for one meeting in 1960 and it has changed our world and our view of the universe. In both mass media and scientifically. Well, IMHO, of course. I'm sure some will say this is unscientific, but there you go.

The Drake Equation.

[NQ6U]

The Drake Equation is meaningless. Depending on how you choose the data, you can make it come out however you choose.

[N2NH]

The Drake Equation is meaningless. Depending on how you choose the data, you can make it come out however you choose.

It's a starting point. It implies nothing, merely gives the parameters and was useful in changing the viewpoint of scientists and the general public to the idea that there could be other civilizations on other star systems. It changed the picture of aliens from Little Green Men from Mars and The Blob to Klingons and Starman. A quantum jump in thinking.

It has been modified and extrapolated into other, sometimes more useful functions.

As with anything, if you are a pessimist, you will get an abnormally low number of Sentient Civilizations in the galaxy.

If you are an optimist, you will get an abnormally high number of Sentient Civilizations in the galaxy.

The truth is in-between as with most things.

Now, the real question is, did a pessimist discover fire? If not, then he's unlikely to discover extra-terrestial civilizations also.

[KC2UGV]

The best sign of intelligent life in our galaxy, is that none have tried to contact us...

[N2NH]

The best sign of intelligent life in our galaxy, is that none have tried to contact us...

That we know of. If the Southwestern Indians are to be believed, they have.

[KG4CGC]

42. The answer to the number of sentient life forms is 42.

[KC2UGV]

That we know of. If the Southwestern Indians are to be believed, they have.

If they have tried, then I would rule them out as intelligent.

[kc7jty]

Michele Bachmann says life exists only on earth, and if you continue in your disrespect for God's truth you will burn in eternal hellfire.

[W3WN]

Strictly speaking, of course, when Gene Roddenberry was first pitching Star Trek (TOS), while he had the Drake Equation in mind, he didn't have it with him and had to make up something on the fly.

I also understand that the equation was modified for The Big Bang Theory to extrapolate the odds of the show's protagnists in hooking up with the opposite sex.

A variation of which is now employed by my nephew, as he prepares for his freshman year at college. (Need I explain that his name is Drake?)

[suddenseer]

They probably drive by everyday. "What's that planet?"
"Carbon burning lower life forms, nothing to see here."