What Is The Great Filter?

Imagine the vastness of the universe, filled with billions of galaxies, each containing billions of stars, many of which have planets orbiting them. It seems statistically probable that life, even intelligent life, should have arisen elsewhere. This is the core of the Fermi Paradox, named after physicist Enrico Fermi, who famously asked, “Where is everybody?” If the universe is so conducive to life, why haven’t we detected any signs of extraterrestrial civilizations?

The Great Filter offers one potential answer to this paradox. It posits that some “filter” – a significant hurdle or barrier – prevents most, if not all, potential life from reaching a stage where it becomes easily detectable to us. This filter acts as a bottleneck in the timeline of life’s development.

Economist Robin Hanson introduced the Great Filter hypothesis, which suggests that a nearly insurmountable barrier prevents most civilizations from advancing to an interstellar presence.

Think of it like a race with many potential starting points (the formation of habitable planets) and a finish line (becoming a technologically advanced, space-faring civilization). The fact that we don’t see many (or any) other finishers suggests that there’s a difficult obstacle somewhere along the track that most participants fail to overcome. This obstacle is the Great Filter.

The big question is: Have we passed the filter, or is it still ahead of us?

Where is the Great Filter?

The crucial question then becomes: Where does this filter lie?

There are a few possibilities:

The Filter is in Our Past:

This would mean that the difficult step(s) required for life to arise and evolve into intelligence are behind us. The origin of life (abiogenesis), the development of complex multicellular life (like eukaryotes), or the emergence of intelligence are sporadic events that occurred on Earth. If this is the case, we are fortunate, and the universe mainly lacks other advanced civilizations. This scenario can be exciting (we are exceptional!) and terrifying (we are alone).  

Implication for us: If we discover simple life on Mars or Europa, it suggests that the Great Filter is ahead of us, which is a bad sign.

Our prospects might be bright because we’ve already cleared the primary bottleneck. We might be one of the first, if not the only, intelligent species to reach this stage in our part of the galaxy (or even the universe). This could mean that our potential for growth and expansion is immense, relatively unconstrained by the factors that halt most other potential civilizations. However, it also places a significant responsibility on us, as we might be the custodians of intelligence in our corner of the cosmos.

Implication for SETI: The lack of detected extraterrestrial intelligence would be less surprising. It would suggest that we shouldn’t expect to find many other advanced civilizations because the odds of reaching this stage are so low. A “silent sky” would be consistent with a filter in our past.

The Filter is in Our Present:

This suggests a critical hurdle we are facing or are about to face that will prevent most civilizations from surviving or becoming detectable. This is something like unsustainable technological development leading to self-destruction (e.g., nuclear war, climate catastrophe, biological warfare) or some unknown cosmic threat that advanced civilizations inevitably face. This is a particularly worrisome scenario, implying that our survival is far from guaranteed.

Implication for us: If we are close to interstellar travel but don’t see others who have made it, we are doomed to self-destruction.

This would suggest that there’s a high probability that we, like many other potential civilizations before us, will not make it through the current stage. The challenges we face today, such as climate change, the risk of nuclear war, or the potential to misuse advanced technologies, could be manifestations of this filter. Our immediate priority would be to identify and overcome these threats to ensure our long-term survival.

Implication for SETI: The silence we observe could be caused by most civilizations’ self-destruction or stalling at a similar stage of development. Detecting advanced extraterrestrial intelligence would be rare because such civilizations are inherently short-lived. If we detect one, they might have found a way to navigate the “present” filter, offering us hope and potential lessons.

The Filter is in Our Future:

This implies that the significant challenges for developing advanced civilizations still lie ahead of us. There are fundamental limitations to interstellar travel, or maybe advanced civilizations inevitably collapse for reasons we can’t yet comprehend. This scenario suggests that while life arises relatively often, it rarely progresses to a truly advanced and detectable stage.

In essence, the Great Filter tries to reconcile the clear likelihood of extraterrestrial life with its observed absence by suggesting that some stage in the development of life is far more improbable than it seems. Identifying where this filter lies has profound implications for understanding our place in the universe and our future.  

Implication for us: If no one has expanded across the galaxy, they couldn’t, and neither can we.

Our present achievements might be less unique than in the “filter in our past” scenario. However, it would mean we still have a significant, yet-to-be-encountered hurdle to overcome. Understanding the nature of this future filter would be crucial for our long-term prospects. Perhaps it’s a physical limitation of the universe or an intrinsic sociological or technological barrier that all advanced species eventually face.

Implication for SETI: We might expect that the universe could have harbored many now-extinct or stalled civilizations that never made it past this future filter. Detecting signs of life or even less advanced civilizations might be more likely than detecting truly advanced, space-faring ones. A “silent sky” regarding advanced signals could still be consistent with a universe where life and even rudimentary intelligence are not uncommon.

Arguments for and against the Great Filter

Arguments FOR the Great Filter Being in Our Past:

1. The Complexity of Abiogenesis: The leap from non-living matter to a self-replicating, evolving organism is incredibly complex. We still don’t fully understand how it happened on Earth, and the conditions required might have been particular and rare.
2. The Eukaryotic Transition: The development of eukaryotic cells was a fundamental step in the evolution of complex life. This occurred only once in the history of life on Earth and involved a symbiotic event that might be highly improbable.
3. The Cambrian Explosion: The rapid diversification of multicellular life during the Cambrian period suggests that the conditions and evolutionary pathways leading to such complexity might be unusual.
4. The Uniqueness of Human-Level Intelligence: While evolution favors traits aiding survival, the specific suite of cognitive abilities that led to human intelligence, including abstract thought, language, and advanced tool use, might be a rare evolutionary outcome.
5. The “Hard Steps” Argument: Proponents argue that there are likely one or more “hard steps” in the development of life that are statistically very unlikely, and one of these occurred in our past.

Arguments AGAINST:

1. The Prevalence of Building Blocks: Organic molecules, the precursors to life, seem common in the universe (e.g., found in meteorites and interstellar clouds). This suggests that the raw materials for life are readily available.
2. Early Appearance of Life on Earth: Life appeared relatively quickly after the planet cooled down enough to support it. This might suggest that abiogenesis is relatively easy under the right conditions.
3. Convergent Evolution: The independent evolution of similar traits in different lineages (e.g., eyes, wings) suggests that specific biological solutions are favored and might arise relatively frequently when conditions are right. This could argue against the uniqueness of some evolutionary steps.
4. Our Limited Understanding: Our current understanding of abiogenesis and early evolution is incomplete. We might be underestimating the likelihood of these events occurring elsewhere.

Arguments FOR the Great Filter Being in Our Present:

1. Observable Self-Destructive Tendencies: Humanity faces significant challenges like climate change, nuclear proliferation, and the potential to misuse powerful technologies. These could represent the kinds of self-inflicted wounds that might plague other advanced civilizations.
2. The Lack of Kardashev Type II or III Civilizations: If advanced civilizations routinely overcome their initial technological hurdles, we might expect to see evidence of civilizations that can harness the energy of their entire star (Type II) or galaxy (Type III). The absence of such clear evidence could suggest a bottleneck at the “Type I” stage (a civilization that can harness all the energy available on its planet).
3. Historical Precedents of Civilizational Collapse: Numerous civilizations have risen and fallen on Earth. While not due to interstellar travel limitations, these collapses highlight the fragility of complex societies.

Arguments AGAINST:

1. Humanity’s Increasing Awareness and Mitigation Efforts: We are increasingly aware of the threats we face and are (slowly) taking steps to mitigate them. This suggests that self-destruction might not be inevitable.
2. The Potential for Technological Solutions: Future technologies might solve existential threats, such as advanced climate engineering, asteroid defense systems, or safer energy sources.
3. The Vastness of Space and Time: Even if civilizations self-destruct, they might do so at different times and ways, making it challenging to observe this “present” filter on a cosmic scale. Brief windows of detectability might be missed.
4. The Possibility of Transcendence: Advanced civilizations might find ways to transcend their physical limitations or move beyond planetary dependence, thus avoiding self-destruction.

Arguments FOR the Great Filter Being in Our Future:

1. Fundamental Physical Limits: There might be insurmountable physical barriers to interstellar travel or communication (e.g., the speed of light, energy requirements, and the vast distances involved).
2. Unforeseen Universal Threats: Catastrophic cosmic events that we cannot currently predict or defend against might be common and act as a filter for even advanced civilizations.
3. Intrinsic Limits to Societal Complexity or Longevity: Perhaps advanced civilizations inevitably reach a point of stagnation, lose the drive to expand, or face internal contradictions that lead to their decline over very long timescales.
4. The “Great Silence” Itself: The lack of compelling evidence for advanced extraterrestrial civilizations could be seen as indirect evidence for a future filter that prevents most from becoming truly widespread and detectable.

Arguments AGAINST:

1. Our Limited Understanding of Future Physics and Technology: We cannot definitively predict what future breakthroughs might be possible. Technologies we can’t even imagine might overcome current limitations.
2. The Potential for Artificial Intelligence and Self-Replication: Advanced AI could potentially overcome the limitations of biological life for interstellar travel and colonization. Self-replicating probes could spread throughout the Galaxy.
3. The Time Scales Involved: The universe is vast and old. Perhaps we haven’t been around long enough to witness the emergence of truly advanced galaxy-spanning civilizations, or they are too far away for their signals to have reached us yet.
4. The “Zoo Hypothesis” or Other Explanations for Silence: There might be non-filter reasons for the apparent lack of contact (e.g., advanced civilizations are deliberately avoiding us, or our search methods are inadequate).

Considering these arguments helps to illustrate the complexity and uncertainty surrounding the Great Filter and its potential location. Each scenario has implications and challenges our understanding of life in the universe.

Discoveries Relevant To The Discussion of The Great Filter

The Origin of Life (Abiogenesis):

1. RNA World Hypothesis Support: Recent research continues to strengthen the “RNA world” hypothesis, which suggests that RNA, not DNA, was the primary form of genetic material in early life. RNA can store genetic information and catalyze chemical reactions, making it a plausible candidate for the foundation of life. Discoveries of RNA’s catalytic abilities and its presence in various environments bolster the idea that the building blocks of life could have formed relatively easily. This might suggest that the origin of life, while still mysterious, might not be as improbable as once thought, potentially weakening the argument for abiogenesis as a major Great Filter.
2. Protocell Research: Scientists are making progress in creating artificial protocells—simple, cell-like structures. Some recent experiments have even shown protocells capable of reproduction. These advances help us understand how cell membranes and basic cellular processes could have arisen from non-living matter. If these steps can be replicated in the lab, it might imply that the transition from chemistry to biology isn’t as difficult, making abiogenesis less of a filter.
3. Deep Sea Hydrothermal Vents: Recent discoveries about microbial life thriving in extreme environments, such as deep-sea hydrothermal vents, suggest that life can emerge and persist in conditions very different from those on Earth’s surface. This expands the range of environments where life might be possible, suggesting that the origin of life might not be as constrained by specific planetary conditions.

The Development of Complex Life:

1. Early Evidence of Complex Life: Some studies have suggested that complex life may have appeared on Earth much earlier. For example, some have interpreted discoveries of ancient fossils in places like the Franceville Basin in Gabon as evidence of early multicellular organisms. If complex life emerged relatively early in Earth’s history, it might indicate that this transition is not as tricky or rare as the Great Filter hypothesis might suggest.
2. Asgard Archaea: The discovery of Asgard archaea, a group of microorganisms with genes that are strikingly similar to those found in eukaryotes (the complex cells that make up multicellular organisms), is providing insights into the evolutionary transition from simple to complex cells. These findings suggest a more gradual and potentially less improbable pathway for the evolution of eukaryotic cells, a key step in developing complex life.
3. Genetic Complexity: Advances in genomics reveal the complex genetic changes necessary for the evolution of multicellularity. While these changes are significant, their existence suggests that the evolution of complexity is possible given enough time and the right conditions.

The Emergence of Technology:

1. Exoplanet Discoveries: The discovery of thousands of exoplanets, including many in the habitable zones of their stars, suggests that Earth-like planets are abundant in the universe. This increases the probability that other planets could have developed life, though it doesn’t directly address the likelihood of that life becoming technological. However, more habitable planets mean more chances for life to get started.
2. Artificial Intelligence (AI) Research: The rapid advancement of AI raises questions about the potential for machine intelligence to either facilitate or hinder the development of advanced civilizations. On one hand, AI could help a civilization overcome some of the challenges of interstellar travel or resource management. On the other hand, some theories suggest that AI could pose an existential threat, potentially acting as a Great Filter. Some scientists, like Michael Garrett, have theorized that the development of Artificial Superintelligence (ASI) could be a Great Filter, potentially leading to the extinction of civilizations before they can make contact.

Final Considerations:

• Sample Size of One: It’s crucial to remember that our understanding of life’s origins and evolution is based on a sample size of one: Earth. This makes it difficult to extrapolate to the rest of the universe.
• Ongoing Research: Astrobiology, genomics, and paleontology are constantly evolving. Future discoveries could significantly change our understanding of the likelihood of various life development stages.