Seventy years ago, Italian-American nuclear physicist Enrico Fermi asked his colleagues a question during a lunchtime conversation. If life is common in our Universe, why can’t we see any evidence of its activity out there (aka. “where is everybody?”) Seventy years later, this question has launched just as many proposed resolutions as to how extraterrestrial intelligence (ETIs) could be common, yet go unnoticed by our instruments.
Some possibilities that have been considered are that humanity might be alone in the Universe, early to the party, or is not in a position to notice any yet. But in a recent study, Robin Hanson (creator of the Great Filter) and an interdisciplinary team offer a new model for determining when the aliens will get here. According to their study, humanity is early to the Universe and will meet others in 200 million to 2 billion years from now.
In addition to being an associate with the Future of Humanity Institute (FHI) at Oxford University, Robin Hanson is also a professor of economics at George Mason University. He was joined by colleagues from Durham University’s Centre for Particle Theory and the Department of Mathematical Sciences, Carnegie Mellon University’s Machine Learning Department, and the international trading firm Jump Trading.
To break it down succinctly, the “grabby aliens model” assumes that civilizations are born according to a series of steps similar to what we see with the biological evolution of life here on Earth. These civilizations, which Hanson and his colleagues refer to as “grabby civilizations” (GCs), will then expand at a common rate, alter the volume of space they occupy, and prevent technologically advanced civilizations (similar to where humanity is today) from arising in these volumes. The model has three parameters, consisting of:
Expansion speed (s) from the fact that we don’t see loud alien volumes in our sky, Power (n) from the history of significant events in the evolution of life on Earth, Constant (k) by assuming our date is a random sample from their appearance dates.The model assumes that the expansion speed of alien civilizations can be estimated based on the fact that we (13.8 billion years after the Big Bang) do not detect the presence of them at this time, the amount of time it takes for advanced life to evolve (based on) and the assumption that humanity’s location in space and time is not unusual, relative to the appearance of advanced and expanding civilizations (similar to the Copernican Principle).
From this, Hanson and his team Moreover, Hanson and his colleagues were able to produce estimates on where the GCs are in our Universe, how much of the Universe they have occupied so far, and how long it will be before we encounter them.
“Where is Everybody?”
The first parameter (s) harkens back to the Fermi Paradox, as initially framed by Michael Hart and Frank Tipler), which refers to the apparent disparity between the statistical likelihood of intelligent life in our Universe and the absence of evidence for it. Within this theoretical framework, scientists are forced to find explanations for how intelligent life could be ubiquitous but has remained invisible to human instruments until now.
As noted, this has spawned various proposed resolutions over the past few decades. Some key considerations include the timeline of the Universe and the evolution of life on Earth. Current estimates indicate that the Universe is 13.8 billion years old (± 40 million years), while the Solar System and planet Earth formed roughly 4.5 billion years ago. Based on the most recent fossilized evidence, the earliest lifeforms are believed to have emerged between 4.2 and 3.8 billion years ago.
Meanwhile, humanity has only existed for the last 200,000 years of Earth’s history and has only enjoyed a level of technological development that allows for SETI surveys for about 70 years. Given the disparity between these numbers, many scientists argue that it is simple anthropocentrism to assume that humanity could be the most advanced intelligence (or worse, alone) in the Universe.
On the other hand, some argue that if intelligent species had emerged millions or billions of years before humans even existed, would they not have gone on to occupy the visible Universe to a significant extent? Does the fact that we don’t see any GCs when we look up into the night sky not support the notion that no one is out there, or at least not in a position to communicate with us yet?
Others still have argued that a 4.5 billion evolutionary timeline means that only longer-lived stars and planets could support life, such as M-type (red dwarfs). These stars are known to have incredibly long lifespans, remaining in their main sequence phase for up to trillions of years. At the same time, recent exosolar planet surveys have suggested that they are the most likely place to find rocky planets orbiting within their habitable zones (HZs). As Hanson explained to Universe Today via email:
“95% of planets are around longer-lived stars than ours, and most live longer than a trillion years. Furthermore, advanced life like us should appear toward the end of a planet’s life, as life needs to first evolve through many stages. So we are quite early compared to when we’d expect advanced life to appear.”
So while our planet has only existed during the past 30% of the Universe, our evolutionary timeline corresponds to 1% of the lifespans of long-lived planets. Essentially, this means that 99% of advanced lifeforms in our Universe will appear after today. Add to that the fact that we do not see evidence of alien civilizations occupying the majority of the cosmos (something that becomes more likely with time), and one is left with the foregone conclusion that humanity is an “early arrival.”
Stepping Hard!
The second parameter (n) is based on the notion that biological evolution can be modeled based on a number of steps. This concept was introduced by Australian physicist and Fellow of the Royal Society (FRS) Brandon Carter, renowned for having coined the Anthropic Principle. In response to what he saw as the overextension of the Copernican Principle in cosmology, this principle states that the very existence of intelligent life indicates that the Universe itself is conducive to its creation.
In a 1983 study titled “The Anthropic Principle and its Implications for Biological Evolution,” Carter presented a statistical model of how civilizations like ours might arise from simple dead matter via a series of intermediate steps. Since then, many scholars have built on his model, which includes Hanson himself. In 1996, Hanson published an essay titled “The Great Filter – Are We Almost Past It?” wherein he proposed that the Fermi Paradox could be the result of one or more of these steps being improbable.
Using life on Earth as a template, Hanson argued that there were eight possible steps between the earliest known life forms and where humanity is today, with a ninth step representing our possible future. These consist of:
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 colonizationWith every step, the probability for failure increases, a situation that Hanson summarized using a lockpicking analogy. Imagine you have a series of locks that you need to pick before a deadline, and they have different levels of difficulty. The odds of picking all the locks before the deadline is up is a power law, where a change in one quantity gives rise to a proportional rise in another quantity.
For the sake of this study, Hanson and his colleagues reconsidered these steps, taking into account that some may take longer to achieve than others (what they group as “easy” or “hard” steps). The combination of these steps is what they referred to as the “hard steps power law,” where each step has an impact on whether or not a species could advance sufficiently before another GC occupied their space and suppressed them. As Hanson explained it:
“The timing of events in the history of life on Earth suggests that there were 3-9 hard steps that life had to go through to reach our level and that most planets like ours never achieve our level before the window for life on that planet closes,” said Hanson. “Thus, advanced life like ours is rare. We can also see that it is rare because we don’t see any life out there at the more advanced level of making big visible impacts on the universe.
“Thus, we know there is a “great filter” standing between simple dead matter and expanding lasting life. Our new analysis allows us to estimate the numerical magnitude of this filter. Advanced life at the grabby aliens level appears roughly once per million galaxies before the grabby aliens deadline.”
In other words, there is a deadline for advanced life in the Universe, where it must emerge and reach complexity before a more ancient and advanced species overtakes it. Far from placing humanity alone in the Universe, the prospect of humanity being an early arrival suggests that are plenty of GCs out there, as well as ones that have not yet reached an advanced stage of development.
Diagrams showing a sample stochastic outcome from a GC model in one (1D) and two (2D) spatial dimensions. Credit: Hanson (et al.)
“If alien civilizations randomly appear that then expand out to remake the universe, then once all of the Universe is filled with such aliens, there aren’t any places left for life to evolve toward our level,” Hanson added. “That is, ‘grabby aliens’ create a deadline by which advanced life must appear. This deadline is within a few billion years from now. Relative to that deadline, we are not early.”
Going Loud!
The final parameter (k) is based on the assumption that the time and space we occupy are representative of the norm (as noted already, the Copernican Hypothesis). According to the GC model, this is the result of a selection effect whereby advanced alien life will eventually expand to fill up the Universe. This raises the final aspect that Hanson and his team considered, which is how less-developed civilizations make the transition to become GCs – aka. go from being “quiet” to being “loud.”
Loud civilizations are so-called because they increase their volume (of space), change their volumes’ appearances (show signs of activity produce technosignatures). Quiet civilizations are those that do not increase their volumes or alter them, which effectively describes our current level of development. Given time, quiet civilizations (if they survive) will advance to the point that they too will become loud, provided they do so before the deadline passes.
With these parameters defined, Hanson and his colleagues simulated how variations in the expansion speed of GCs (s) and the time it takes for life to evolve (n) would yield different results on how many GCs were currently active in our Universe, how much of it they had come to occupy, and (as a result) when we might encounter a GC. These variables were visualized in terms of 1D and 2D diagrams (shown above) and a 3D animation (shown below).
The s parameter is especially significant since faster-expanding aliens would be more difficult to detect before they reached our doorstep. Due to the speed of light, any activity in an occupied volume of space would take thousands of years to reach us. If a GC is expanding rapidly enough, the light they generated when they first began expanding will not arrive before they do. As Hanson put it:
“At the origin date of a random civilization, about half of the universe is filled with very big visible alien civilizations. If these grew very slowly, then the sky would be full of them, huge circles in the sky, far bigger than the full moon. However, if they grew at the speed of light, then we wouldn’t see them until they got here.
“If they grew very fast, such as at over half of the speed of light, then most of the places that could see them would be places where they had arrived and colonized and changed. That is, if we could see them, then they would likely be here instead of us. In which case, we would not exist.”
When Do We Meet Them?
Ultimately, the results Hans and his team obtained indicated the following range of possibilities:
GCs (or “loud” civilizations) emerge from quiet ones at a rate of about once per million galaxies They expand and increase their volume at about half the speed of light They currently control 40-50% of the Universe’s volume Each GC will eventually control 105 – 3 x 107 (100,000 to 30 million) galaxiesLast but certainly not least, they estimated that humanity is likely to encounter the nearest GC roughly 200 million to 2 billion years from now. In the meantime, their modeling also indicates that the odds of humanity detecting signs of technological activity (aka. “technosignatures”) are very low. As Hanson explained, this could be bad news for those engaged in the Search for Extraterrestrial Intelligence (SETI).
“Once per million galaxies is very rare, and if grabby aliens were the only kinds to see, then the chances for SETI to see any aliens nearby would be very low,” he said. “However, it may be that there are many times more “quiet” alien civilizations out there. The higher the ratio of quiet to grabby aliens civilizations, the closer might be the nearest quiet aliens to be found.”
Illustration of the selection effect, where expansion speeds (s) are near lightspeed c, a GC is likely to have overtaken us by the time we see. Credit: Hanson (et al.)
Conversely, the fewer quiet civilizations out there (relative to GCs) right now, the higher our future chances of becoming a GC ourselves. Alas, this prospect also lowers the odds of us detecting and observing alien civilizations in our galaxy. In fact, the model Hanson and his colleagues created predicts that the “quiet-to-grabby ratio” needs to be over 10,000 to 1 for us to realistically expect that even one quiet civilization has ever been active in the history of our galaxy (ca. 13.5 billion years).
That ratio needs to be as high as 10 million to 1 for us to expect that any alien civilizations with a million-year lifetime are active right now in our galaxy. While none of these outcomes are particularly encouraging for SETI researchers, the research team notes that it’s possible that the volumes of space occupied by GCs are more subtle in appearance and that their expansion speed is slower. In this case, they estimate that we can predict there being signs in the night sky.
Another positive takeaway from this research is the fact that this sort of modeling is now possible. Whereas early SETI efforts were guided by conjectures that were subject to a lot of uncertainty (like the Drake Equation), we now have enough data on the types of stars and exoplanets in our Universe that we can make educated inferences.
“It’s exciting that we’re here now,” said Hanson. “We’re no longer speculating about aliens; we are reasonably sure they exist, and we can say where they are in spacetime. We have a simple statistical model that says where they are, what they are doing, and where we might see or meet them.”
Further Reading: Grabby Aliens, arXiv