But Where Is Everybody? The Fermi Paradox
It was a summer day in Los Alamos, New Mexico. The year was 1950. Physicist Enrico Fermi was walking to lunch with colleagues Edward Teller, Emil Konopinski, and Herbert York. Somehow the conversation had turned to the subject of flying saucers, little green men, and an epidemic of missing garbage cans in New York City.
All three had been the subject of a recent New Yorker cartoon that put forth the unusual but still remotely plausible hypothesis that aliens were stealing municipal trash cans. The physicists lightheartedly agreed, with Fermi noting that it was an elegant solution to the mysteries of flying saucer sightings and the stolen New York Sanitation Department property.
The four sat down to lunch at Fuller Lodge and the conversation drifted on to other topics for a while. Suddenly, according to the recollections of the participants, Enrico Fermi slapped his hand on the table and asked some variant of, "but where is everybody?"
The three startled physicists immediately understood Fermi's non sequitir . It was indeed a very good question, and is now known as the Fermi Paradox.
Given the age of the universe, the number of stars in our galaxy, and the apparent ubiquity of the elemental building blocks of life, other life forms should be out there somewhere. And if they're anywhere, they should theoretically be everywhere - including here.
So where are they?
Death By Von Neumann Probe
At first glance, this seems a rather self-centered way to look at the universe. It is akin to moving to a new house and then assuming your neighborhood to be deserted because nobody has showed up at your door to offer you pie. There is, however, a valid mathematical argument to be made.
The basis was Fermi's outburst was the idea that an intelligent, technologically-advanced species will inevitably feel a sense of interstellar Manifest Destiny and spread out to neighboring systems. Even a civilization using slower-than-light craft would be able to colonize most of the galaxy within 50 million years.
Given the fact that in our stellar neighborhood there are many high-metallicity stars (stars with stuff other than hydrogen and helium, considered likely candidates to have rocky planets) several billion years older than our Sun, it is likely that some other civilization out there should have done this already.
A technologically-advanced species need not even do the exploring themselves. They could, hypothetically, build a group of self-replicating probes that would travel to a new system, gather information, and then mine exo-Oort cloud objects for materials to build and fuel the next generation. This type of probe is known as a von Neumann probe, named for the Hungarian-American physicist of the early 20th Century who spent much of his career studying self-replicating machinery.
Such an object, if technologically feasible, leads the imagination to some frightening hypothetical scenarios. If inexpertly programmed, these machines could replicate like a computer virus, mining deeper and deeper into solar systems until they begin to tear up inhabited planets. They could also, hypothetically, mutate due to a copying error into a more aggressive probe that aims to destroy other von Neumann probes in the fierce competition for resources.
There is no current evidence to suggest that this sort of interstellar technological warfare is happening, and attempts by astronomers and space archaeologists to seek out such devices in our neighborhood have so far turned up negative. Indeed, a counter-argument for the existence of destructive von Neumann probes is that any civilization careless enough to produce such an instrument of destruction would likely wipe themselves out long before they can build the machines to wipe out the rest of us. A brief look at human history provides some fantastic examples of this. The fact that we haven't yet been destroyed yet, then, is cause for a sigh of relief. But it doesn't help to solve Fermi's riddle.
"Space is big. You just won't believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space."
- Douglas Adams
The Great Silence
Perhaps the most likely explanation for the lack of physical ET evidence is scale. Space is big. The distances between stars are simply incomprehensible to our everyday understanding, even when put in terms of, say, ping-pong balls located in Los Angeles and London. Physically travelling such distances is impractical even with highly speculative technologies such as the Alcubierre drive. The more practical alternative is to communicate with neighboring systems remotely rather than in person.
Since 1960's Project Ozma, radio astronomers have been searching the stars for radio transmissions beamed in our direction. To date -- with one notable exception -- they have yet to find anything but background noise. Given the size of the galaxy and the number of planets out there - a number which may exceed the number of stars, according to a new study - this radio silence is a somewhat surprising result. Or lack thereof.
A number of possibilities have been put forth to explain why we've seen no conclusive evidence of exo-intelligence yet. These range from the practical (we haven't been looking long enough, we're using the wrong frequency) to the conspiratorial (they're here among us and it's all a big cover-up) to the downright depressing (we are the only living planet in the galaxy).
We as a species have only been emitting and receiving radio signals for a bit more than a century, and have already begun the transition away from radio to terrestrial cable for communication. And while it is a popular notion that Earthly radio transmissions have reached several dozen light years into deep space, the fact remains that for practical purposes our signal leakage dissipates into the background noise at around 0.3 light years from home.
Apart from a few messages we've beamed into the cosmos, we're not doing much talking. We can't really expect other civilizations to be reaching out to us when we're not making our presence known. From a security standpoint, it may be unwise to advertise ourselves too much.The fact that a civilization has invented interstellar travel is no reason to assume they're nice.
The Mystery Continues
Six decades after Fermi's question, we're still without an answer. Though astrophysicists have filled in some of the unknown variables of the Drake Equation, the question of whether we are alone or not is still an open one. The answer may or may not be discovered in our lifetime.
Same goes for the mystery of the missing trash cans in New York in the 1950s. We might never know the answer.
Sources and More Information
- Jones, Eric, "'Where is everybody?' An account of Fermi's question"
Los Alamos Technical report LA-10311-MS, March, 1985.
- SETI@home FAQ for newsgroups alt.sci.seti and sci.astro.seti.
- INTERSTELLAR PROBES: A NEW APPROACH TO SETI - Robert A. Freitas Jr.
Interstellar transmissions via energy-markers (photons) or matter-markers (probes) appear to be energetically indistinguishable alternatives for advanced technical societies. Since only Type II and Type III civilizations realistically can afford beac
- John Hodgman: Aliens, love -- where are they? | Video on TED.com
TED Talks Humorist John Hodgman rambles through a new story about aliens, physics, time, space and the way all of these somehow contribute to a sweet, perfect memory of falling in love.