Enrico Fermi, an Italian-American nuclear physicist, asked his colleagues a question in a lunchtime conversation 70 years ago. Why can't we see evidence of life out there (aka. Where is everyone? Seventy-years later, the same question has prompted just as many resolutions about how extraterrestrial intelligences (ETIs), could be common yet remain unnoticed by our instruments.
There are several possibilities. One possibility is that humanity may be the only one in the Universe. Another is that it might be early to the party or not yet in a position where they can notice them. Robin Hanson, creator of the Great Filter, and an inter-disciplinary team have created a new model to predict when aliens will arrive. Their study shows that humanity is in the early stages of the Universe, and will be able to meet other species within 200 million to 2 miliarde years.
Robin Hanson is an associate at the Future of Humanity Institute at Oxford University and a professor of economics. He was joined by Durham University's Centre for Particle Theory, the Department of Mathematical Sciences and Carnegie Mellon University's Machine Learning Department. Jump Trading is an international trading company.
The grabby aliens model, which is simple to understand, assumes that civilizations are created in a sequence of steps similar to the biological evolution of life on Earth. Hanson and his coworkers refer to these civilizations as grabby civilizations. They will expand at a similar rate to other civilizations and alter the space they occupy. This will prevent technologically advanced civilisations (similar to what we have today) from emerging in these volumes. The three parameters of the model are:
Expansion speed (s), based on the fact that there are no loud alien volumes visible in our sky.
The history of significant events that shaped the evolution of life on Earth's power (n)
Constant (k), by presuming that our date is a random selection from their appearance dates.
The model assumes that alien civilizations are expanding at a rapid rate. This is based on the fact we (13.8 billion year after the Big Bang), do not know their existence, and on the time required for advanced life to evolve. It also considers the assumption that humans' location in time and space is not unusual relative to the appearance advanced and expanding civilisations (similar to Copernican Principle).
Hanson and his team were able to draw conclusions from this. Hanson and his coworkers also produced estimates of where the GCs are located in the Universe, how much space they have occupied, and how long before we meet them.
Where is Everyone?
The first parameter (s), harkens back the Fermi Paradox as originally framed by Michael Hart & Frank Tipler. This refers to an apparent disparity in the statistical likelihood that intelligent life exists in our Universe and the lack of evidence to support it. Scientists are now forced to come up with explanations for intelligent life's omnipresence, but it has been largely omitted from human instruments.
This has led to many resolutions being proposed over the last few decades. The timeline of the Universe's evolution and Earth's origin of life are two of the most important issues. According to current estimates, the Universe is approximately 13.8 billion years old (40 million years), and the Solar System formed about 4.5 billion years ago. According to fossilized evidence, the earliest forms of life were discovered between 4.2 billion and 3.8 billion years ago.
Humanity has been around for only 200,000 years and has had a technological advancement that permits SETI surveys for only 70 years. Many scientists believe that this disparity is simply anthropocentrism, as it assumes that humanity might be the most advanced intelligence (or worse) in the Universe.
Some argue that intelligent species could have emerged millions to billions of years ago before humans existed. Would they not have continued to occupy large swathes of the visible Universe? Is it not possible to see any GCs in the night sky, indicating that there is no one out there?
Some still believe that the 4.5 billion-year evolutionary timeline of stars and planets means only long-lived ones could sustain life. These stars have been known to live for trillions of years, and they are still in their main sequence phase. Recent exosolar planet surveys suggest that these stars are the best place to find rocky worlds orbiting within their habitable zone (HZs). Universe Today received Hanson's email explaining this to Universe Today:
95% of the planets orbiting longer-lived stars are more than ours and many live for over a trillion years. Advanced life such as ours should be seen at the end of a planet's life span, since life must first go through many stages. We are far earlier than we would expect advanced life to emerge.
Our planet has existed for only 30% of the Universe's history, but our evolutionary timeline corresponds with 1% of long-lived planets. This means that 99 percent of advanced lifeforms in the Universe will be present after today. The fact that there is no evidence of alien civilizations in the majority of the cosmos, which is something that increases with time, makes it clear that humanity is on its way.
It's not easy to be a good person!
The second parameter (n), is based upon the idea that biological evolution can only be modelled using a variety of steps. The Anthropic Principle was created by Brandon Carter, an Australian physicist who is also a Fellow of the Royal Society (FRS). This principle was created in response to the Copernican Principle's overextension in cosmology. It states that the Universe is conducive for intelligent life.
Carter created a statistical model that showed how simple dead matter could lead to civilizations such as ours in a 1983 study titled The Anthropic Theory and Its Implications For Biological Evolution. Many scholars, including Hanson, have built upon his model. Hanson published an essay entitled The Great Filter are We Almost Past it? in 1996. He suggested that the Fermi Paradox might be due to one or more of these steps being impossible.
Hanson used Earth's history as a model to show that there are eight steps between the origin of life and today. A ninth step represents our future. These are:
Habitable star system (organics, habitable planets), Reproductive molecules (e.g. RNA) Single-cell prokaryotic life Eukaryotic life Single-cell sexual reproduction Multi-cell animal life Industrial civilization Large-scale colonization
Hanson explained this situation using a lockpicking analogy. With each step, the chance of failure increases. Imagine that you have a number of locks you need to pick in order to meet a deadline. Each lock has a different level of difficulty. You have a power law that says a change in one quantity will result in a proportional increase in another. This is how you can pick all locks before the deadline expires.
Hanson and his coworkers reexamined the steps for the purpose of this study. They also took into consideration that some steps may be more difficult than others. They called the combination of these steps the hard steps power law. Each step impacts whether or not a species can advance enough before another GC takes their space and suppresses them. Hanson explained:
Hanson stated that the timing of events in Earth's history suggests that life had to endure 3-9 difficult steps to reach its current level. Most planets on Earth, like ours, never reach this level before their window closes for life. Advanced life is very rare. It is also rare because there are no other living things that can make visible and significant impacts on the universe at the advanced level. We know that there is a powerful filter between basic dead matter and the expansion of lasting life. This filter's numerical magnitude can be estimated using our new analysis. Advanced life at grabby aliens levels appears approximately once every million galaxies prior to the grabby-aliens deadline.
This means that advanced life must be developed and become more complex before it is overtaken by a more advanced and ancient species. Humanity is not the only species in the Universe. The possibility of humanity arriving early suggests that there are many GCs, as well those that have yet to reach advanced stages of development.
Diagrams showing a sample stochastic outcome of a GC model in one (1D), and two (2D). spatial dimensions. Credit: Hanson (et. al.
Hanson said that if alien civilizations appear randomly and then spread out to remake the universe then there is no place left for life to evolve to our level. Grabby aliens set a deadline for advanced life to appear. The deadline is in a few billion year's time. We are not too far away from that deadline.
Go Loud!
The last parameter (k), is based upon the assumption that the space and time we occupy are representative for the norm (as already noted, the Copernican Hypothesis). This is due to a selection effect in which advanced alien life eventually expands to fill the Universe, according to the GC model. Hanson and his team also considered the last aspect, which was how less-developed civilisations can make the transition to becoming GCs. From being quiet to being noisy.
Loud civilizations are those that increase their volume (of area), alter their appearances (show signs and produce technosignatures), or both. Quiet civilizations do not alter or increase their volume, and this effectively describes our current level in development. If they survive, quiet civilizations will grow to the point where they will be louder, provided that they do so before the deadline.
Hanson and his collaborators defined these parameters and simulated how changes in the expansion speed and time for life to evolve (n), would produce different results. These include how many GCs are currently active in our Universe and how much they have occupied. We might also encounter a GC. These variables were represented in the form of 1D and 2D diagrams (shown here) and a 3D animation below.
This parameter is particularly important because faster-expanding aliens will be harder to spot before they reach our doorstep. Because of the speed of light any activity within an occupied volume space would take thousands upon thousands of years to reach us. If a GC expands rapidly enough, they will continue to generate light when they grow. Hanson said it this way:
About half the universe was filled with large visible alien civilisations at the time of an unknown civilization's origin. If they grew slowly, the sky would be filled with them, large circles in the sky that are larger than the full moon. If they grew at the speed light, we would not be able to see them until they arrived. If they grow at a very rapid rate, like over half the speed of light then most places where we can see them would be areas they have colonized and transformed. If we could see them, they would most likely be there instead of us. We wouldn't exist in this case.
How do we meet them?
The results Hans and his team achieved indicated that there were many possibilities.
At a rate of around one per million galaxies, GCs (or loud civilisations) emerge from still ones.
They expand and grow at half the speed light.
They currently control between 40-50% and 40% of the universe's volume
Each GC will eventually have control of 105 3 x 107 galaxies (100,000 to 30,000,000).
They also estimated that humanity will likely encounter the closest GC in 200 million to 2 miliarde years. Their modeling shows that there are very low chances of humans detecting technological activity (aka. technosignatures) is very low. Hanson explained that this could spell doom for people involved in the Search for Extraterrestrial Intelligence.
One per million galaxies is extremely rare. If grabby aliens were all that SETI could see, then there would be very few chances of SETI seeing any aliens near them. It is possible that there are many more quiet alien civilisations. The closer you are to quiet alien civilizations, the higher your ratio of quiet to grabby aliens civilisations.
Illustration of the selection effect. When expansion speeds (s), are close to lightspeed c, it is probable that a GC will have overtaken us before we see. Credit: Hanson (et. al.
In contrast, the less quiet civilizations (relatively to GCs) out there, the better our future chances of becoming one. This prospect, however, also reduces our chances of detecting and observing alien civilisations in our galaxy. Hanson and his collaborators created a model that predicts that the quiet to grabby ratio must be greater than 10,000 to 1. This allows us to expect that any quiet civilization will have ever been active in our galaxy's history (ca. 13.5 billion years.
To expect any alien civilizations that have lived for more than a million years to be active in our galaxy, this ratio must be at least 10 million to 1. These results are not encouraging for SETI researchers. However, the research team points out that it is possible that the volumes of space occupied GCs are less obvious and their expansion speed slower. They believe that they can predict signs in the night sky.
This research also shows that modeling is possible. While early SETI efforts relied on conjectures that were highly uncertain (e.g. the Drake Equation), now we have enough data about the types of stars, exoplanets and other objects in the Universe to make educated inferences.
Hanson said, "It's thrilling that they are here now." We are not speculating anymore about aliens. We are fairly certain they exist and can tell where they are in spacetime. A simple statistical model can tell us where they are, how they are doing it, and where we might meet them.
Additional Reading: Grabby Aliens arXiv
