Why haven't we found intelligent alien civilizations? There may be a 'universal limit to t. 




If intelligent entities throughout the cosmos have limitless technological potential, why haven’t we detected signs of their existence?

In less than seventy years, humanity advanced from no flight technology to landing on the moon. In just over a century, we progressed from the first rudimentary computer to handheld devices providing instant access to almost all human knowledge. Given this rapid technological development, many assume that our capabilities are boundless.

This idea, combined with the discovery that habitable planets are abundant in the cosmos, has shaped a question that has puzzled scientists and thinkers for decades: "Why is the universe so silent?" Known as the Fermi paradox, after physicist Enrico Fermi who supposedly raised it in 1950, the question asks: If our solar system is relatively young compared to the rest of the universe, and if humans might one day achieve interstellar travel, why haven’t we seen evidence of other intelligent civilizations spreading across the cosmos? In other words, where are the aliens?


It’s possible we haven’t encountered alien civilizations because there may be a "universal limit to technological development" (ULTD) that applies to all intelligent species in the universe. This limit, according to Antonio Gelis-Filho, a researcher in public policy at the Getúlio Vargas Foundation at the School of Business Administration (FGV EAESP) in Brazil, could be well below the level needed for a civilization to colonize an entire galaxy. Gelis-Filho proposed this idea in a recent paper published in the journal *Futures*.




"If the ULTD hypothesis is correct, there has never been, is not, and will never be anything like an interstellar civilization or an 'interstellar conversation,'" Gelis-Filho explained in an email to Space.com.

Drawing on the history of the rise and fall of human civilizations, the challenges of sustaining scientific endeavors that advance our knowledge and technology, and the apparent absence of technological intelligence elsewhere in the universe, Gelis-Filho argues that we should be cautious in assuming that the technological capabilities of humans or other intelligent species are limitless.



The "uncrossable gap"

Nobel Prize-winning physicist Richard Feynman once said, "What I cannot create, I do not understand." This suggests that our technological abilities — what we can build — are limited by our understanding.

There are natural boundaries to human technology. For instance, we can't travel faster than the speed of light. Similarly, there may be natural limits to human knowledge—truths about the universe that remain beyond our reach due to the limitations of our biology. While we’ve developed technology to extend our senses and cognition—microscopes to explore the microscopic world, telescopes to observe the vastness of space, and computers to process data beyond human capacity—these tools still operate within certain limits.

However, the technologies and experiments that drive our expanding knowledge are becoming increasingly expensive. Projects like the Large Hadron Collider at CERN (which cost $4.75 billion to build and $286 million annually to operate), the International Space Station (costing $3 billion per year), and the international effort to achieve nuclear fusion at ITER (with construction costs estimated between $18 billion and $20 billion) show that pushing the boundaries of science requires more and more energy and resources.

"As a matter of fact, the last major fundamental breakthroughs in our understanding of the universe—both at the macro and micro levels, in cosmology and quantum mechanics—are nearly a century old," Gelis-Filho noted.

While we understand phenomena like black holes much better today than a hundred years ago, Gelis-Filho argues that these insights haven’t had the same transformative impact on technology as relativity and quantum mechanics did.

"Compare the scientific progress between 1830 (before the theories of evolution or electromagnetism) and 1930 (when relativity and quantum mechanics had already emerged) with the progress from 1930 to 2024 (still no unified theory), and you can see that the pace of advancement is slowing," he said. "The low-hanging fruit has already been picked, and what's left seems to be beyond our reach, hanging from impossibly high branches."

The rising cost of exploring the frontiers of human knowledge could eventually make us decide the price is too steep. For example, the European Commission recently scrapped plans for several billion-euro flagship research projects, including efforts to convert solar and wind energy into fuels and advance cell and gene therapies in clinical settings. If this trend continues, the development of new technologies based on breakthroughs in our understanding of reality could come to a halt—along with our aspirations of becoming an interstellar civilization.

According to Gelis-Filho, any intelligent civilization in the cosmos would face the same dilemma. At some point, no matter how advanced they become, they must decide: Do we construct a particle accelerator the size of the Milky Way to test our new unified theory, or do we prioritize building infrastructure essential for the survival of our civilization?

The ULTD hypothesis argues that even if a civilization chose to build such a machine, they would find that the energy required to make the next leap in scientific knowledge does not increase in a straightforward, linear fashion. Eventually, their current technology would be insufficient to bridge the gap between one level of understanding and the next.

"Since the laws of physics are universal, every civilization will inevitably face that 'uncrossable gap,'" Gelis-Filho explained.

The cost of increasing societal complexity

Gelis-Filho believes that insights from the rise and fall of human civilizations can also be applied to this astrobiological context. As complex societies grow, they add layers of complexity to generate more "energy" to sustain their expansion. However, beyond a certain threshold, this added complexity no longer "pays for itself," leading to diminishing returns.