Welcome back to our ongoing series, a Brief-ish History of SETI. In our previous installments, we looked at the philosophical underpinning of SETI and the earliest experiments. We also examined the first modern SETI project and its lasting legacy, and the big ideas that remain integral to the discipline. Then, we looked at the first attempt at Messaging Extraterrestrial Intelligence (METI) and what is considered the best candidate for a signal detection. This was followed by a retrospective of the first physical messages humanity has sent to space.

Today, we will delve into one of the most daunting questions that continues to haunt SETI researchers. As Fermi famously said, “Where is Everybody?” Answering that question requires that we face some uncomfortable possibilities and address how little we know about life in our Universe. To recap on the “Lunchtime Conversation” we explored in Part I, Fermi’s question was motivated by some salient facts:

1. The Universe is Old: The Universe began roughly 13.8 billion years ago with the Big Bang and has been expanding ever since.
2. The Universe is Huge: The “Observable Universe” measures an estimated 96 billion light-years in diameter, and may be infinite.
3. The Universe is Packed: The most recent estimates indicate that there are over 2 trillion galaxies in the known Universe. The population of each ranges from thousands of stars (in the smallest dwarf galaxies) to over a trillion in larger galaxies.
4. The Universe is Abundant: The basic ingredients for life as we know it – carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur (CHNOPS), and water – are everywhere in abundance.
5. Our Solar System is Young: The Solar System formed ca. 4.6 billion years ago, and humanity has existed for only 200,000 years, making us a late addition to the party.

So… given the amount of time life has had to emerge, the sheer number of stars and planets, and the fact that the ingredients for life are so common, it’s a foregone conclusion that life is quite common too. By extension, it stands to reason that intelligent life would also have had enough time to emerge many times over and to explore the Milky Way galaxy. So why hasn’t humanity seen or heard from any advanced life forms yet?

When Fermi and his colleagues did the math on this question, they found that Earth should have been visited several times already. And yet, there is no definitive evidence of extraterrestrial visitors to Earth, and when our instruments are pointed toward the heavens, we encounter what scientists call the “Great Silence.” This is the essence of what came to be known as “Fermi’s Paradox,” referring to the discrepancy between the assumed likelihood of life and the absence of evidence. This gap has led to multiple proposed resolutions.

They Don’t Exist

The first formal proposals, which also formalized the Fermi Paradox, were published in the 1970s and early 80s by two physicists: Michael Hart and Frank Tipler. In 1975, Hart published a paper titled “Explanation for the Absence of Extraterrestrials on Earth,” where he made a controversial claim. According to Hart, if advanced civilizations had emerged in our galaxy in the past, they would have surely developed the technology for interstellar space travel.

By his estimates, such a civilization would only need two million years to colonize the entire galaxy and would have been to Earth many times. Ergo, the absence of evidence for extraterrestrials on Earth (what he called “Fact A”) implied that intelligent life did not exist beyond Earth. This was followed in 1981 by Tipler’s paper, “Extraterrestrial Intelligent Beings Do Not Exist,” in which he made similar arguments. However, Tipler gave a more liberal estimate, claiming an advanced civilization could colonize the galaxy in 300 million years.

This came to be known as the Hart-Tipler Conjecture. While they make some fatalistic conclusions, these arguments are not without merit. Using humanity as an example, both authors reasoned that aliens would be subject to the same exponential rate of population growth and technological progress. As such, it would not take them very long to develop advanced communications, spacecraft, and self-replicating (Von Neumann) probes.

Also known as “Universal Constructors,” Hungarian-American John Von Neumann proposed this last concept in the 1940s based on his research into the self-replicating nature of DNA. As described in the 1966 book Theory of Self-Reproducing Automata, written by Neumann’s colleague Arthur W. Burks after his death, these machines would be capable of harvesting resources and building exact copies of themselves.

He further reasoned that probes equipped with this ability would be an ideal means for exploring space, as they could proliferate endlessly across star systems. The absence of such machines in our backyard, said Hart and Tipler, proved that there were no advanced civilizations out there.

“Sagan’s Response”

These conclusions prompted the famed astronomer, planetary scientist, and science communicator, Carla Sagan, to draft a formal response. In a paper he co-authored with fellow astrophysicist William Newman in 1983, “The Solipsist Approach to Extraterrestrial Intelligence” (aka. “Sagan’s Response”), Sagan argued that there were countless reasons why humanity has not found evidence of ETCs yet. As they summarized:

Seeking, in effect, a universal principle to explain the apparent absence of extraterrestrial beings on Earth, [Tipler] contends that if extraterrestrial beings exist, their manifestations will be obvious; conversely, since there is no evidence of their presence, they do not exist. But absence of evidence is not evidence of absence.

In particular, Sagan and Newman challenged the inherent assumptions Hart and Tipler made and were critical of the values they employed. For instance, Hart assumed an ETI would spread from one star to the next at a constant rate of 10% the speed of light without any serious pauses to settle new worlds before sending out more ships. Meanwhile, Tipler’s estimate of 300 million years was based on a replication rate of 10,000 probes a year and a modest travel velocity of less than 1% the speed of light.

But as Sagan and Newman pointed out, even if such probes only produced a single copy of themselves every time they replicated themselves, “the entire mass of the Galaxy would be converted into von Neumann machines within a few million years of their invention.” In addition, Hart and Tipler’s arguments assumed that an advanced species would pursue a policy of unlimited expansion and that its colonies, once established, would last for millions or even billions of years. If even one of these assumptions is incorrect, the entire Conjecture falls apart.

This echoed statements made by Sagan and Newman in a 1981 paper titled “Galactic Civilizations: Population Dynamics and Interstellar Diffusion.” Based on how much time and energy it takes to travel between stars, they argued, it was likely that alien signals and probes may not have reached Earth yet.

This response was one of many challenges and counter-proposals to the Hart-Tipler Conjecture, all of which sought special explanations for why humanity has not yet made contact with extraterrestrials.

Where’s the Filter?

One such explanation, which summarized many schools of thought, was the “Great Filter,” proposed by Robin Hanson, an associate professor from George Mason University and a former research associate with Oxford University’s Future of Humanity Institute (FHI). In 1996, he published a paper titled “The Great Filter – Are We Almost Past It?” where he proposed that something may exist in the cosmos that prevents intelligent life from achieving a high level of development on the Kardashev Scale.

Hanson argued that the “Filter” must lie somewhere between the point at which life emerges on a planet (abiogenesis) and the point at which it becomes an interstellar civilization. Using life on Earth and the emergence of humanity as a template, Hanson outlined a nine-step process that life would need to follow to reach the point of becoming a space-faring civilization. These included:

• Habitable star system (organics and habitable planets)
• Reproductive molecules (e.g. RNA)
• *Prokaryotic single-cell life*
• *Eukaryotic single-cell life*
• *Sexual reproduction*
• *Multi-cell life*
• *Animals capable of using tools*
• *Industrial civilization*
• *Wide-scale colonization*

In accordance with Hanson’s hypothesis, at least one of these steps must be improbable, which would constitute the “Filter.” Either life has a difficult time emerging from inorganic materials early on, or the odds of catastrophic failure increase as species become more and more complex and advanced. Examples of the latter include asteroid impacts and other Extinction-Level Events (ELE), which are statistically more likely the longer a planet hosts life, nuclear annihilation, or environmental destruction.

Either of these possibilities, said Hanson, has significant consequences for humanity:

Humanity seems to have a bright future, i.e., a non-trivial chance of expanding to fill the universe with lasting life. But the fact that space near us seems dead now tells us that any given piece of dead matter faces an astronomically low chance of begating such a future. There thus exists a great filter between death and expanding, lasting life, and humanity faces the ominous question: how far along this filter are we?

Nick Bostrom, a philosopher who also hails from the FHI, provided an excellent description of this hypothesis, which he described in his 2008 essay, “Where Are They? Why I Hope the Search for Extraterrestrial Intelligence Finds Nothing.” As he wrote:

The Great Filter can be thought of as a probability barrier. It consists of [one or] more highly improbable evolutionary transitions or steps whose occurrence is required in order for an Earth-like planet to produce an intelligent civilization of a type that would be visible to us with our current observation technology.

By the closing of the 20th century, the field of SETI faced an uncertain future. On the one hand, Congress chose to cancel NASA’s formal SETI program, the High Resolution Microwave Survey (HSMS), in 1993. The move was led by Nevada Senator Richard Bryan, who argued the program was a waste of money, citing the Hart-Tipler Conjecture as the reason. On the other hand, the field had matured thanks to the many insightful ideas and frameworks introduced over the previous decades.

Within a decade and a half, SETI efforts would be revitalized thanks to renewed interest and the growth of public-private partnerships. But before we get into that, there are a few more notable ideas to explore. Stay tuned for those in our next installment!

California and a coalition of other Democratic-led states are suing the Trump administration over new limits on federal borrowing by aspiring nurses, physician’s assistants, therapists, social workers, mental health practitioners and other healthcare workers, arguing the changes will further reduce a struggling but vital workforce.

“This case is about protecting access to education, protecting our healthcare workforce, and protecting patients who rely on these providers every single day,” California Atty. Gen. Rob Bonta said during a virtual news conference Tuesday. “The Trump administration is going out of its way to make it harder and more expensive for students to pursue the advanced degrees necessary to serve their communities and pursue meaningful careers that allow them to support themselves and their families.”

Bonta said the new limits on loans sought by nursing and other healthcare students — which the U.S. Department of Education initiated in response to Republicans passing broader student loan caps as part of last year’s One Big Beautiful Bill Act — was an illegal overreach by the agency that was “deeply shortsighted” and went beyond the scope of the legislation.

“Congress can act,” he said. “But what the Department of Education can’t do is — contrary to law and in an arbitrary and capricious way and in violation of the Administrative Procedure Act — redefine what a professional student is.”

In response to the litigation, Trump administration officials defended the new rules, saying they will help student borrowers in the long run by driving down schooling costs at universities nationwide and preventing them from taking on too much debt.

“After decades of unchecked student loan borrowing that gave schools no reason to control costs, these commonsense loan caps — created by Congress — are already incentivizing colleges and universities to lower tuition,” Under Secretary of Education Nicholas Kent said in a statement to The Times.

Kent said Bonta and his fellow Democratic litigants “are more concerned about institutions’ bottom-line [than] American students and families’ ability to access affordable postsecondary education.” As one example of institutions responding to loan caps by lowering costs, Kent pointed to UC Irvine reducing the costs of its master’s in business programs by up to 38% to keep them below a federal loan cap for such programs.

The One Big Beautiful Bill, passed by Congress in July 2025, placed new limits on student loans, which could previously be sought for the full cost of such degrees. Starting this July, applicants categorized as “graduate students” will be capped at borrowing $20,500 per year and $100,000 in total, while applicants categorized as “professional students” will be allowed to borrow up to $50,000 annually and $200,000 in total.

On May 1, the U.S. Department of Education issued a new rule defining the “professional student” category as including those pursuing degrees to become doctors, pharmacists, dentists, veterinarians, lawyers, various medical specialists, pastors and other religious academics, and excluding those pursuing nursing and other advanced healthcare degrees.

In announcing the change, Kent said it would “simplify our complex student loan repayment system and better align higher education with workforce needs,” “drive a sea change in higher education by holding universities accountable for outcomes and putting significant downward pressure on the cost of tuition,” and “benefit borrowers who will no longer be pushed into insurmountable debt to finance degrees that do not pay off.”

Others fiercely disagreed, including healthcare industry leaders who also had objected to the rule change during a public comment period. Some said the changes would simply increase student reliance on less favorable, private-sector loans.

The American Assn. of Colleges of Nursing, in a statement, said it and its members were “angered by the Department of Education’s failure to support the nursing profession as the demand for patient care services rises.”

Nearly 150 members of Congress — including more than a dozen Republicans — wrote a letter the day after the rule was promulgated expressing “disappointment” over the exclusion of post-baccalaureate nursing degrees.

“At a time when our nation is facing a health care shortage, especially in primary care, now is not the time to cut off the student pipeline to these programs,” the lawmakers argued.

Rachel Zaentz, a spokesperson for the University of California, which is not party to the lawsuit but operates a vast network of public health programs, said in a statement Tuesday that UC “strongly opposed” the administration’s new caps on federal loans for nurses and other health professionals, which she said “will be felt most strongly by lower-income graduate students.”

“UC will continue to do all we can to ensure that cost is not a barrier for anyone who wants to pursue higher education, and we will continue to advocate with our federal partners for the programs and policies that make this possible,” Zaentz said.

Bonta rejected the administration’s argument that the new caps would help students pursuing a dream of a medical career avoid taking on too much debt — calling it “tone deaf.” He said those students are already “struggling with all costs right now” thanks to the Trump administration’s tariffs, war in Iran and lax approach to regulating monopolies and other big business.

He also rejected the idea that the new loan caps would force institutions to reduce costs for students, calling that “wishful thinking.”

The lawsuit is the 68th filed by Bonta’s office against the second Trump administration. Joining Bonta in the lawsuit — which was filed in the U.S. District Court in Maryland — were the attorneys general of Arizona, Colorado, Connecticut, Delaware, the District of Columbia, Hawaii, Illinois, Maine, Maryland, Massachusetts, Michigan, Minnesota, Nevada, New Jersey, New Mexico, New York, North Carolina, Oregon, Rhode Island, Vermont, Virginia, Washington and Wisconsin, as well as the governors of Kentucky and Pennsylvania.

Times staff writer Jaweed Kaleem contributed to this report.