We don’t have to worry about this one at the moment, but eventually we will join the thousands of civilizations that have preceded us in the Milky Way galaxy. Not that we seem to have any chance of ever meeting them.
Seventy years ago, famous physicist Enrico Fermi was having lunch with colleagues (including Edward Teller of H-bomb fame). Conversation wandered, as conversation does, until Fermi burst out with, “Where are they?”
What he meant was, “Where are the aliens?” He then backed up the question with some quick calculations on the probabilities of planets like Earth, of life, and of high technology and concluded that we should have been visited many times. So, “Where are they?”
This is the Fermi Paradox, and if you believe in UFOs and alien abductions and other such myths, all the way back to Ezekiel’s wheel, it’s no mystery. We have been visited, many times, and aliens even live amongst us, as well as in caverns underground and on the dark side of the moon. But, Fermi noted, there is no evidence. Only folklore and fantasy.
So. The Fermi Paradox. It has puzzled many people over the decades since that lunch. In Where Is Everybody? Fifty Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life (Copernicus Books, 2002), Stephen Webb describes Fermi and his paradox and offers a variety of answers that have been suggested–most seriously, some a bit tongue-in-cheek–for why the search for extraterrestrial intelligence (SETI) has not succeeded. John Gribbin, in “Alone in the Milky Way,” Scientific American (September 2018), argues that the conditions that led to our technological civilization would be difficult to match even among the 100 billion other stars of our galaxy.
By 1961, radio astronomer Frank Drake had already been listening for radio signals from other civilizations for 4 years. In that year, he formulated what came to be known as the Drake Equation to calculate N, the number of civilizations in the Milky Way galaxy whose electromagnetic emissions are detectable at the present time:
N = R* x fp x ne x fl x fi x fc x L
R* = the rate of formation of stars suitable for the development of intelligent life;
fp = the fraction of those stars with planetary systems;
ne = the number of planets, per solar system, with an environment suitable for life;
fl = the fraction of suitable planets on which life actually appears;
fi = the fraction of life bearing planets on which intelligent life emerges;
fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space;
L = average length of time such civilizations release detectable signs of their existence into space.
This equation has impressed many people whose eyes widen at the sight of anything that looks mathematical. But it was never intended to be a way to calculate how many ETs are out there waiting for contact. Rather it was a way of defining our ignorance, of saying “This is what we need to know before we can estimate.” At the time, we really knew only one of the terms, R*,the rate of formation of suitable new stars (10 per year). Since then, we’ve found enough stars with planets that we can venture a reasonable guess at fp, the fraction of those stars with planetary systems. But all the rest of the numbers? We have no idea, for we know of only one place–Earth–that has developed life, intelligence, and technology. That is too small a sample to work with.
What about L, the average length of time technological civilizations release detectable signs of their existence into space? In the 1960s, some people put that as low as 100 years. The estimate grew as the Cold War waned and the threat of nuclear annihilation receded. With the advent of communications satellites and fiber optic cable, we realized that even if a civilization survives, technological change may reduce or eliminate leakage of radio signals into space, so the estimate shrank again. Today, with political chaos threatening to engulf the planet, it should perhaps shrink some more, perhaps even back to the Cold War number. And of course a civilization does not have to end in Armageddon. We have problems of overpopulation, global warming, water supply, other resource shortages, and more that could do the job. And then there are asteroid impacts, solar flares, and other catastrophes, only a few of which we can hope to do much about. Planetary civilizations are very unlikely to last forever.
A value of L in the range of centuries rather than millennia or eons does not seem unreasonable.
So what does the Drake Equation look like if we put in some numbers?
N = R* x fp x ne x fl x fi x fc x L
R* = 10 stars suitable for the development of intelligent life per year;
fp = .5 of those stars with planetary systems;
ne = .1 planet, per solar system, with an environment suitable for life;
fl = 1 suitable planet on which life actually appears;
fi = .1 life-bearing planet on which intelligent life emerges;
fc = .1 civilization that develop a technology that releases detectable signs of their existence into space;
L = 100 years during which such civilizations release detectable signs of their existence into space.
N = 10 x .5 x .1 x 1 x .1 x .1 x 100 = .5 civilizations available to contact at this moment in time. 
We’re here, now, so it rather looks like we’re alone (although there may well be technological civilizations that do not use radio technology). There may have been many in the past. There may be many in the future. But very few will ever coincide in time. We need to bear in mind that cosmological time is vast.
You can fiddle the numbers all you want. Go ahead and make L 10,000 years instead of 100. N becomes 50, which is still not impressive to anyone raised on Star Trek and Star Wars. An L of 10,000,000 gives you an N of 50,000. An L of 10,000,000,000 gives you an N of 50,000,000, which would surely make the Trekkies happy. But remember–it’s a game played largely in ignorance.
So are we alone, at least for now?
If not, where are they?
If so, can we somehow stretch out L until we have a chance to find someone?
We can of course do our very best to deal with resource shortages and political upheavals and ward off asteroid impacts. But Mother Nature has a great many ways to produce disaster, and extinction is normal (99 percent of all the species that ever lived are extinct). Our best chance for long-term survival is probably to widen the target zone by getting off the planet. That means space colonies, either in habitats (giant space stations) or on other worlds. Since it seems impossible to travel faster than light, we are not going to be able to go very far outside our solar system—not past a few of the most nearby stars. If the colonies in turn spawn colonies, humanity could spread out into the galaxy, albeit very slowly. So could ETs, of course, so their absence again suggests they aren’t there.
The apparent impossibility of faster than light travel, by the way, also speaks to the lack of ET visitors. If they can’t get here from there, we won’t see visitors even if the galaxy has as many ETs as a fig has seeds. In that case, though, we should be picking up signals. Since we aren’t…
Meanwhile, is it worth our while to continue listening for extraterrestrial radio signals, still the major component of SETI? As science projects go, it’s relatively inexpensive and the potential payoff is large, for we could learn a great deal from another civilization. There is also the point that, even if are alone in the galaxy at this time, there may well have been technological civilizations in the past whose radio signals are still spreading through space. If we ever detect such radio fossils, SETI will become an exercise in xenoarcheology.
But face it—the odds seem slim.
 There have been more than fifty searches for extraterrestrial (ET) radio signals since 1960. The earliest ones were very limited. Later searches have been more ambitious, using multiple telescopes and powerful computers to scan millions of radio frequencies per second. New technologies and techniques continue to make the search more efficient. See Monte Ross, “The New Search for E.T.,” IEEE Spectrum (November 2006).
 S. G. Wallenhorst, “The Drake Equation Reexamined,” Quarterly Journal of the Royal Astronomical Society, Vol. 22, P. 380, 1981. http://adsabs.harvard.edu/full/1981QJRAS..22..380W
 As a theoretical biologist, I’m optimistic that where life is possible, it will happen.
 Frank Drake notes that it takes only one very long-lived civilization to make the average lifetime much larger. That, of course, would throw off the calculation by overestimating N. See “The Drake Equation Revisited, Part I,” Astrobiology Magazine, September 29, 2003. https://www.astrobio.net/alien-life/the-drake-equation-revisited-part-i/