In order to estimate the number of advanced civilizations in the Milky Way Galaxy a number of factors need to be combined. The factors, layed out by famed astronomer Frank Drake, have come to be known as the Drake Equation. His equation written out looks like:
N = N* x fs x Np x fe x fl x fi x (L/LMW)
Which breaks down in meaning as:
N = number of communicating civilizations in our galaxy now
N* = number of stars in the galaxy
fs = fraction of sunlike stars
Np = average number of planets per star
fe = fraction of planets suitable for life
fl = fraction of planets where life actually develops
fi = fraction of planets where intelligent civilizations arise
L = lifetime of civilization with ability to communicate
LMW = lifetime of the Milky Way Galaxy
The number of stars in the galaxy is believed to be about 300 billion (300 x 109), therefore N* = 3 x 1011. The number of sunlike stars (i.e. those that last for about 5 billion years so that life can evolve) is almost half of all stars, so to be conservative we take fs = 0.3. Our own modest solar system has nine planets, so we take Np = 10. Again, if we look at our own solar system, we find one planet (Earth) eminently suitable for life and another (Mars) very close to being suitable. If we assume that one planetary system in every four has a planet suitable for life, and if planetary systems have, on average, 10 planets, then one planet in every 40 should have conditions suitable for life, so fe = 0.025. The fraction of planets with suitable conditions for life, and on which life actually develops, is close to unity, but to be conservative we take fl = 0.5. Likewise, the fraction of planets where intelligent civilizations arise is near to unity as well, so we take fi = 0.75.
The final factor in the equation involves the lifetime of intelligent civilizations and the lifetime of the Milky Way galaxy. The lifetime of the galaxy is believed to be about 10 billion years (1010 years), however estimating the lifetime of a communicating civilization is more difficult. The only example we have is our own civilization, which has achieved the ability for interstellar communication in only the last generation or two. But the real question is how long will we last as a civilization? This question is difficult to answer. If we are very pessimistic, we might put L at a few decades; if we are very optimistic, we might measure it on a geological timescale. We can imagine two extreme cases: a technical society that destroys itself soon after reaching the communicative phase (i.e. L<102 years) and a technical society that learns to live with itself soon after reaching the communicative phase; if the society gets past L=102 years, it is unlikely to destroy itself from then on and its lifetime can be measured on a stellar evolutionary scale (i.e. L>>108 years).
This gives N = (3x1011) x (0.3) x (10) x (0.025) x (0.5) x (0.75) x (L/1010)
N = 0.843 L
Given the uncertainty in the various factors in this equation, we can approximate by saying that N = L.
The number of civilizations in our galaxy with the ability to communicate is equal to the lifetime of such civilizations.
So, depending on the value of L, N can range from about 100 to several billion. Most likely, an intermediate value is closer to the truth, so most scientists take N = 106. Thus, there may be a million or so civilizations within our own galaxy with whom we might communicate.
This number refers to the Milky Way Galaxy only. The universe contains at least as many galaxies as there are stars in the Milky Way. The reason for the restriction to the Milky Way is because intragalactic communication seems far easier than extragalactic communication. The nearest star in our galaxy would require several years for a round-trip message, while the farthest stars require tens of thousands of years. In contrast, to exchange a message with a civilization in the Andromeda galaxy would require about 4 million years, while further galaxies would require several billion years (by which time we might well be extinct). So chances for a real conversation is much better if we limit the equation to only the Milky Way galaxy.
Estimated Probabilty Factors for Determining
the Number of Civilizations in the Milky Way
100 billion 300 billion 300 billion 300 billion Fraction of
1 3/10 1 1/15 Average number
10 10 20 5 Fraction of planets
suitable for life
1/10 1/40 1/3 1/1000 Fraction of planets
where life does
1 1/2 1 1/1,000,000 Fraction of planets
1 3/4 1 1/1000 Ratio of civilization
lifetime to galaxy
L/10 billion L/10 billion L/10 billion L/10 billion Number of
the galaxy now
L/10 0.8L 300L 1/100 billion
"The only available evidence for 'visitors from outer space' has come so far from highly imaginative individuals who wish either to achieve a little notoriety or to promulgate their particular religious theories and ethical concepts. We are told that some of these visitors come from planets of this solar system with the intention of preventing us from blowing up our earth and as a result, disorganizing the whole system. Their advice consists, as it must, of variations on the golden rule. It is doubtful if there are any other living beings in this system.
"In the case of visitors from planets revolving about far distant suns, which seems the only possible place of origin, there seems no other reason for their coming than curiosity and scientific research, unless they have a genuine desire to share their attainments with others."
Speculating on the nature of the origin of the unknown object generally refered to as the Roswell UFO that allegedly crashed on the night of July 4, 1947 near the city of Roswell, New Mexico, some UFOlogists suggest, as reported by the Roswell Incident Updated and elsewhere, that the object's near Earth trajectory may have been related to an interest in the nuclear tests that occurred not far from Roswell toward the end of World War II.
On July 16, 1945, in a location some distance toward the west of Roswell, at 5:29:45 a.m. Mountain War Time, at the Trinity Site, White Sands, New Mexico, the first nuclear explosive device on Earth was set off --- in turn creating a huge brilliant flash of light on the leading edge of the Earth's AM terminator about twelve days after her obital aphelion. The Roswell Incident occurred almost two years to the date later. That would mean the expanding light bubble would have crossed the one light year threshold one year earlier. Taking into consideration such formulas as the Drake Equation above, by most Earth-based reckoning nothing significant lies within that threshold.
Shortly after that first nuclear test and well before Roswell 1947, the sky over the general New Mexico area was inundated by "green fireballs." Theoreticians in the Air Force believed the fireballs were propelled objects and not natural phenomena, bearing a very close similarity to the World War II UFOs called Foo Fighters. Because of same the noted astronomer and meteorite expert Dr. Lincoln La Paz was called in to investigate. La Paz, in agreement with the theoreticians, stated that normal fireballs (meteors) are seldom if ever green, their trajectory is forced on them by gravity, and leave meteorites when they plunge to Earth. The green fireballs sighted over New Mexico did none of these things. Neither did they appear to La Paz to be electrostatic phenomena as they moved too regularly and too fast. To this day no truly satisfactory explanation for the phenomenon has been released to the general public.
Interesting as well, La Paz, who had thousands and thousands of hours of scientific time observing celestial objects himself --- reported on July 10th, 1947, six days after the Roswell crash, seeing a huge eliptical-shaped object flying in the sky near Fort Sumner, New Mexico, while driving by car with his wife and children. He saw a luminous unknown object sort of oscillating beneath the clouds. Its brightness was stronger than the planet Jupiter and its shape regular and elliptical. The nature of this object was unknown to the astronomer.
In a Life Magazine article dated April 7, 1952 La Paz is quoted as saying the object "..exhibited a sort of wobbling motion" and then disappeared behind some clouds. It reappeared and "projected against the dark clouds gave the strongest impression of self-luminosity." The object then moved slowly from south to north and two and a half minutes behind a cloudbank. According to La Paz's calculations, confirmed by his wife, the object was huge, as large or larger than the infamous "Battle of Los Angeles" object as presented in UFO Over Los Angeles seen by thousands in February, 1942, being some 235 feet long and 100 feet thick (NOTE: according to reports as cited in the above link, the Los Angeles UFO was, however, thought to be closer to 800 feet in length). La Paz reported the horizontal speed of the object he observed ranged between 120 and 180 miles per hour and its vertical rise between 600 and 900 miles per hour.
THE RADAR DILEMMA
THE BATTLE OF LOS ANGELES
WHAT IS IT WE FEAR IN VISITORS FROM SPACE?
THAT THEY MIGHT BE LIKE US?
Is it possible to get around the Drake Equation? Many say yes, including the eminent astrophysicist Dr. Bernard Haisch and a team of fellow physicists as well as Dr. Ronald Bracewell with the introduction of his Bracewell Probe postulate.(see)
DR. FRANK DRAKE
The Dirt Before The Dig