“…Virtually apropos of nothing Fermi said, ‘Don’t you ever wonder where everybody is?’ Somehow . . . we all knew he meant extra-terrestrials.” Herbert York, 1950

Confronted with a nearly limitless universe billions of years old with an almost infinitely vast number of opportunities for life, the Italian physicist Enrico Fermi, sitting for lunch at Los Alamos with three colleagues in 1950, asked a question that still perplexes everyone who looks up at the night sky: “Where is everybody”?

The Fermi paradox, which assumes that we should have detected intelligent life in the universe by now, remains an all-time most bedeviling lunchtime remark. It relies on four fairly simple assumptions:

1) There’s nothing special about our sun. It’s ordinary and relatively young, and there are billions of stars in our galaxy that are much older.

2) There must be planets like ours revolving around those stars (scientists now believe that there are billions of exoplanets in the Milky Way alone). Because intelligent life developed on Earth and there are billions of Earth-like planets, we have every reason to believe that intelligent life could have developed on those planets as well.

3) After millions of years of technological progress, an alien civilization would likely have the technology to travel to other stars and galaxies. In less than 100 years, after all, humans went from traveling in covered wagons to landing on the moon.

4) Given another few million years, an alien civilization could colonize an entire galaxy.

How exactly would an alien civilization colonize a galaxy? In 1964, the Soviet astronomer Nikolai Kardashev developed what is now called the Kardashev scale, a hypothetical classification system for an alien civilization based on its energy consumption. The physicist Michio Kaku defines the Kardashev scale as:

Type I: This civilization harnesses the energy output of an entire planet. Type II: This civilization harnesses the energy output of a star, and generates about 10 billion times the energy output of a Type I civilization. Type III: This civilization harnesses the energy output of a galaxy, or about 10 billion time the energy output of a Type II civilization.

If human technological growth continues at an average of 3% per year, Kaku posits that “our own civilization may attain Type I status in about 100-200 years, Type II status in a few thousand years, and Type III status in about 100,000 to a million years.”

By the time a civilization reaches Type II and enjoys the energy of an entire star, new forms of travel become possible. According to mathematicians Duncan Forgan and Arwen Nicholson from Edinburgh University, self-replicating spacecraft traveling at one-tenth of the speed of light — admittedly a quick speed — could traverse the entire Milky Way in a mere 10 million years.

Which leads us to Fermi’s paradox: intelligent life has every chance of existing and has had plenty of time to develop beyond anything we can imagine today. So why haven’t we observed signs of intelligent life in our galaxy? Simply put, where is everybody?

Here are six of the most thought-provoking solutions to the Fermi paradox.

1. The Zoo Hypothesis

Like the Federation in Star Trek, super-intelligent alien life may answer to a Prime Directive: don’t interfere. Also known as the Zoo Hypothesis, this theory was formally proposed as a solution to the Fermi Paradox by John Ball at the Harvard-Smithsonian Center for Astrophysics in 1973.

Given the age of the universe, Ball noted, multiple type III civilizations may have appeared millions or even billions of years before intelligent life developed on Earth. With such a vast head start, these first civilizations would have complete control over the universe. They’d be able to destroy or control less advanced forms of life, like humans have done on Earth. They would have to grapple with the same ethical question as the Federation in Star Trek: what’s the right way to interact with less advanced civilizations, some millions of years behind?

On Earth, we’ve decided to create nature reserves and national parks, places where generations of ants may never come in contact with a human. Likewise, entire alien civilizations may have agreed to let us be, to let us mature and evolve naturally, to observe our progress from afar, hiding from us until they decide we’re ready to join the extraterrestrial community.

2. We’re living in a simulated universe

Call them crazy, but renowned scientists and philosophers are developing increasingly creative (some would say delusional) ideas about how a simulated universe would work. Some of their arguments are logical, perhaps even convincing, and they’ve been debated by serious academics in serious journals for years now.

Nick Bostrom, a philosopher at Oxford and director of the Future of Humanity Institute, has developed my favorite argument in a paper called, “Are You Living in a Computer Simulation?” He argues that at least one of the following three statements must be true:

1. Humans are likely to go to extinct before they are “posthuman” and able to create entirely simulated realities (i.e. an entirely simulated universe).

2. A post-human civilization with the ability to run such a simulation would likely not run many simulations (because of the immense amount of resources needed, perhaps, or the ethical considerations of the beings in the simulation).

3. We are currently living in a computer simulation.

Using extremely clever logic and a bit of mathematical fiddling, Bostrom concludes that “the belief that there is a significant chance that we will one day become post-humans who run ancestor-simulations is false, unless we are currently living in a simulation.” Remember, this is a philosopher at Oxford speaking.

How would such a simulation be created? Writers have long speculated that a Type II or III civilization might develop a “Matrioshka brain,” an almost inconceivably huge computer system that runs on the entire energy output of a star. Using a Dyson sphere, a shell around a star that captures all the energy of the star, mathematicians have calculated that a Matrioshka brain could have the computational power to create a simulated universe, with all of us living inside. This idea, believe it or not, doesn’t appear to violate any laws of physics we know about.

3. Aliens are broadcasting, but we don’t know how to listen

SETI (the search for extraterrestrial intelligence) is counting on this solution. With a little imagination, it’s easy to think of a number of reasons why we haven’t been able to communicate with aliens.

Aliens might not be broadcasting with radio waves, or they might be broadcasting on different frequencies than the ones SETI is currently monitoring. They might use neutrinos or lasers to communicate across the galaxy in a kind of “galactic” internet. Complex compression algorithms that we don’t understand, gamma ray bursts that we can’t detect, modulation methods that we haven’t discovered — the list goes on and on.

In 1985, Carl Sagan came up with an especially compelling reason for why we’re missing alien signals. He speculated that aliens may communicating at a rate so slow or fast that we don’t even recognize their efforts. If an alien civilization takes years to broadcast a single sentence, or even a single word, we not even recognize that we’re being contacted.

They may be communicating with Matrioshka brains, or even more advanced technologies that we can’t imagine and may never discover. Humans have been searching for signals from extraterrestrial life for less than 100 years, using techniques that aliens may have abandoned millions of years ago, or perhaps skipped all together. The chance that our method of communication aligns perfectly with an alien civilization’s method begins to look slim indeed.

4. The “Rare Earth” hypothesis

The Rare Earth hypothesis disputes the first and second assumptions of the Fermi paradox, which suppose that the Earth is a typical planet revolving around a typical star. There’s something special, or at least rare, about Earth, proponents of the Rare Earth hypothesis claim, that allowed complex to form and thrive on its surface.

In 1961, Frank Drake, an American astronomer, created a simple equation to estimate the number of intelligent civilizations in our galaxy. Using rough approximations, Drake calculated that there were at least 1000 and perhaps as many as 100 million extraterrestrial civilizations in the Milky Way alone, but these numbers are highly disputed. Although recent NASA missions have found hundreds of exoplanets, or Earth-like planets, revolving around stars in the Milky Way, scientists still don’t know how life originates on a planet.

from xkcd, by Randall Munroe

Perhaps life needs a planetary solar system with gas giants far from the sun and rocky planets closer in. Maybe plate tectonics and volcanoes are necessary to create suitable atmosphere for life. Evolving life may need a large moon that cause tides, a specific temperature or a planet with a stable orbit. With a sample size of 1 (the only thing we know for sure is that life developed on Earth), it’s nearly impossible to determine whether or not life is rare in the universe.

5. The Youngness Paradox

Alan Guth, theoretical physicist at MIT and likely contender for this year’s Nobel Prize, developed his own solution for the Fermi Paradox that stands above the rest in its sheer astounding-ness, and we’re talking about a field replete with astounding ideas.

In a paper called “Eternal Inflation and its Implications,” Guth used the physics of inflation to elegantly show that each universe (by the way, we’re assuming the existence of an infinite number of universes) likely has only one advanced civilization.

Here’s how the argument goes: according to Guth, cosmic inflation spawns an infinite number of pocket universes at an extraordinary rate. In fact, every second number of pocket universes increases by a factor of 10^37. Therefore, Guth argues, we can assume that young pocket universes vastly outnumber older universes. In other words, the average universe is remarkably young.

Guth then postulates that there is a minimum time — let’s call it T — required for intelligent life like ours to develop. Because we’ve already developed, we know that the age of our universe is greater than or equal to T.

Let’s imagine there is another civilization in our universe more advanced then ours, and for the sake of argument, let’s assume that it is one second more advanced. If the minimum time for our civilization to develop is T, the minimum time for this more advanced civilization to develop is greater than or equal to T + 1s.

Because the number of universes that satisfy the T requirement is 10^37 times the number of universes that satisfy the T + 1s requirement, and because we know only that we live in a universe that satisfies the T requirement, it is extremely unlikely that our universe also satisfies the T+1s requirement. It’s unlikely, therefore, that there is an alien civilization in our own universe even one second more advanced than ours.

6. The Great Filter

The Great Filter is the idea that the development of a civilization that can colonize a galaxy is a unique, maybe once in a universe event because of the many exceptional barriers to life along the way. Dr. Robin Hanson, a well known futurist at George Mason University and the Future of Humanity Institute, published the nine steps he believes are required to get from a suitable planet to an interstellar civilization:

1. The right star system (including organics) 2. Reproductive something (e.g. RNA) 3. Simple (prokaryotic) single-cell life 4. Complex (archaeatic & eukaryotic) single-cell life 5. Sexual reproduction 6. Multi-cell life 7. Tool-using animals with big brains 8. Where we are now 9. Colonization explosion

These nine steps to a colonization explosion are the “Great Filter.” Proponents of the Great Filter argue that the transition between one or more “filtering” step (and of course there may be many more steps we don’t know about) may be so uncommon that few civilizations reach step 8 or 9. If each transition is sufficiently uncommon, humans may be the only life-form in the galaxy that ever reaches step 8. Or we may just be the first ones to the party, so to speak, waiting for other civilizations to catch up.

And the original answer: they’re actually Hungarians

After hearing Enrico Fermi utter his now-famous paradox at Los Alamos, the physicist Leo Szilard immediately answered, “they are among us and they call themselves Hungarians.” As good an answer as any, I suppose.