Fermi and the “Great Filter”

Credit: NRAO/AUI/NSF; J. Hellerman

Welcome back! If you’re reading this, it’s probably because you know what terms like “Fermi Paradox” and “Great Filter” mean. Kudos to you for that! If not, don’t worry. Those will be explained shortly. But for the most part, I wanted the third installment in this series to be about what is (as I see it) the next logical step in the whole “where are all the aliens?” debate.

To recap, the Fermi Paradox is named for the renowned physicist Enrico Fermi who famously asked, “where is everybody?” (in the context of extraterrestrial civilizations). The fact that humanity has have never heard from or seen evidence of life beyond Earth is perplexing when you consider that the sheer size of the Universe, the number of galaxies, stars, and planets, and that the ingredients for life appear to be everywhere in abundance.

Ergo, if life is (presumably) so likely, then why aren’t we finding any evidence of it out there? Are we alone in the Universe? Are we not looking in the right places (or for the right things)? Or is there plenty of life out there that we (for whatever reason) can’t communicate with? This is the challenge of the Fermi Paradox as we’ve come to know it, which is crying out for resolution!

Enter the Filter

In 1996, economist and research associate Robin Hanson (of the Future of Humanity Institute at Oxford University) offered a new take. In a paper titled “The Great Filter — Are We Almost Past It?” he argued that while basic life may be common, there must be something in the evolutionary process that keeps intelligent life from reaching an advanced stage of development.

In accordance with Hanson’s theory, the evolution of life on Earth can be used to plot an evolutionary timeline based on a series of steps. If life follows a similar course in all parts of the Universe, then we can assume that those steps would be consistent across the board and would look something like this:

  1. A star system with a potentially habitable planet(s) and organic molecules
  2. Reproductive molecules emerge (e.g., RNA)
  3. Single-cell life develops (prokaryotes)
  4. Complex single-cell life develops
  5. Sexual reproduction emerges
  6. Multi-cell life develops (eukaryotes)
  7. Tool-using animals with intelligence emerge
  8. An advanced, technologically-dependent civilization emerges (where humanity is right now)
  9. Interplanetary exploration and settlement (and maybe interstellar)

Since he first proposed it, the Great Filter has achieved widespread popularity and is even considered an integral part of the whole SETI debate. Not long ago, I had the honor of speaking to Robin Hanson for the sake of doing an article on his proposed resolution. In the near future, we plan to chat again to talk about some of the deeper implications and significance of his theory.

Speaking of implications, the Great Filter is loaded with them! Depending on where you think the filter is located (between steps 1 and 9), the results are consistently bad for humanity. It’s simply a question of how bad. Let’s review the possibilities…

Are We Past It?

If we assume that the Great Filter is at an early stage in the development of life, it could mean that the process through which inorganic elements come together to create life (abiogenesis) is extremely temperamental. Or it could be that simple life emerges without difficulty, but it has a hard time reaching stages of greater complexity.

If we use life on Earth as a template again, we can see the logic in this scenario. In accordance with the available evidence, life is theorized to have emerged on Earth roughly 3.7 billion years ago. That’s was pretty quick, considering that Earth had formed from the protoplanetary disk that surrounded our Sun just 800 million years prior.

In that time, the Earth’s surface also cooled, its interior differentiated (forming a mantle, outer core, and inner core), and the oceans formed on its surface. As such, it seems that the first single-celled lifeforms (prokaryotes) didn’t take that long to be born. However, it would take another billion years before the first multi-celled lifeforms emerged (eukaryotes).

Regardless, an early filter essentially means that life is quite rare in the Universe, and any that has emerged (simple or complex) has beaten the odds simply by being here. For reasons that are self-explanatory, the implications this has for humanity are not good!

Does it Lie in Our Future?

Alternately, it could be that the “Filter” is at a more advanced stage of development. A popular version of this argument is that intelligent life will inevitably face what author Ronald Wright described as “a success trap” (in his critically-acclaimed 2004 book A Short History of Progress). This “trap” refers to how civilizations can invariably collapse as a result of growing too large and overburdening their resource base.

Given the challenges and existential threats that life on Earth is facing today, there are many theorists who argue that the filter rightly belongs between steps 8 and 9 — and that we’re facing it! If we were to assume that this is the common challenge facing all intelligent life, then it’s safe to say that as civilizations grow, so too does their dependence on technology and impact on the natural environment.

If a species cannot become interplanetary before their home planet suffers an ecological disaster, then they are doomed to fail. This echoes arguments made by such luminaries as Stephen Hawking, Elon Musk, and astrophysicists J. Richard Gott and Adam Frank. If humanity doesn’t become an interplanetary species, they argue, then we are doomed to extinction.

What Does This Mean?

As noted already, the implications of either scenario are not good for humanity. If the Great Filter lies in an early stage of species development, it means that just about all life on Earth today has really defied the odds, and human beings are a major exception. This means that life as we know it maxed out its luck a long time ago, and we are likely to go extinct sooner other than later.

If the Filter lies at a later stage, it means that the odds have been in our favor up until now. However, that’s blessed period is now over, and humans face the genuine probability of extinction going forward. In short, humanity is on borrowed time, and the likelihood of “going interplanetary” before a cataclysmic fate claims us are poor.

But before anyone thinks this is all doom and gloom, keep in mind that this hypothesis is all about framing possibilities and their implications. It is essentially hypothetical: if life is statistically likely, and if the “Great Silence” is due to an absence of activity, then we can surmise that something is holding life back. On the other hand, who’s to say just how stringent the Filter is?

Is it not possible that there are many other factors involved — such as the limitations of time and space, distance — and that we have barely begun to scratch the surface when it comes to the search for intelligent life? Is the only reason the “Great Silence” is so pervasive is because we haven’t been looking in the right places or for the right “signatures” so far?

Perhaps, and that gives me an idea of what the installment should address: How have we been looking for extraterrestrial intelligence so far? And equally important, should we be casting a wider net (and how)? Stay tuned!



Space/astronomy journalist for Universe Today, SF author, and all around family man!

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Matt Williams

Space/astronomy journalist for Universe Today, SF author, and all around family man!