Kurzgesagt – In a Nutshell
Sources – Grabby Aliens
Thanks to our expert —
Dr. Robin Hanson
George Mason University
Note: This video summarizes a possible new solution to the Fermi paradox. It is based on the idea presented in the recent paper
#Hanson, Robin et al. (2021): “If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare”. The Astrophysical Journal, Volume 922.
https://iopscience.iop.org/article/10.3847/1538-4357/ac2369#apjac2369s3
The paper is speculative but the conclusions follow from a mathematical model, which we had the chance to discuss with the first author of the paper. We are trying to explain some of the ideas in a more approachable language in this document.
–The universe is magnificent and vast. Hundreds of billions of galaxies, trillions of stars, and even more planets. If even the tiniest fraction are habitable, then the universe should be teeming with life. And yet we see nothing, only vast emptiness. Where is everyone else?
There are about 1022 to 1024 stars in the observable universe, and even if each star has only one planet, that makes for up to a trillion trillion planets.
#ESA: “How many stars are there in the universe?” (retrieved 2023)
https://www.esa.int/Science_Exploration/Space_Science/Herschel/How_many_stars_are_there_in_the_Universe
Quote: “For the Universe, the galaxies are our small representative volumes, and there are something like 1011 to 1012 stars in our Galaxy, and there are perhaps something like 1011 or 1012 galaxies. With this simple calculation you get something like 1022 to 1024 stars in the Universe.”
#Cassan, A. et al. (2012): “One or more bound planets per Milky Way star from microlensing observations”. Nature, vol. 481.
https://www.nature.com/articles/nature10684
https://arxiv.org/abs/1202.0903 (open-access version)
Quote: “Taking the full range of planets that our survey can detect (0.5–10 AU, 5 M⊕ to
10 MJ), we find that on average every star has 1.6+0.72−0.89 planets.”
Which fraction of all those planets are habitable? Within our Milky Way there could be 300 million habitable planets:
#Tavares, Frank (2020): “About Half of Sun-Like Stars Could Host Rocky, Potentially Habitable Planets”, NASA (retrieved 2023).
https://www.nasa.gov/feature/ames/kepler-occurrence-rate
Quote: “Our galaxy holds at least an estimated 300 million of these potentially habitable worlds.”
If that is representative of what we find in all galaxies, then there should be hundreds of millions of trillions of habitable planets in the observable universe.
– To become a starfaring civilization, life as we know it needs to master a number of very hard steps. It starts with dead stuff turning into the building blocks of life. Then it needs to organize into self-contained cells. Those cells have to learn to work together to form multicellular organisms. This keeps going until complex creatures with big brains learn to use tools and language. Civilization has to be formed from cultures that value progress and technological development. And then they need to actually venture out beyond their home planet.
The so-called “hard steps model” was originally used to describe the evolution of cancer – a certain number of specific and nontrivial mutations are required for a cell to become cancerous within a certain deadline (elimination by the immune system):
#Nunney, Leonard (2016): “The multistage model of carcinogenesis, Peto's paradox and evolution”. International Journal of Epidemiology, Volume 45.
https://academic.oup.com/ije/article/45/3/649/2572603?login=false
Quote: “The multistage (or multistep) model of carcinogenesis is the cornerstone of our understanding of how cancer is initiated. This model of sequential mutation driving carcinogenesis is generally considered to have originated with Nordling (1953),1 who presented age-specific incidence data consistent with individual cells becoming cancerous after accumulating about seven mutational hits. [...] In any event, all multistage models are based on the principle that cancer initiates after a single cell has accumulated a series of genetic changes and that these changes are primarily the result of somatic mutation (broadly defined as a genetic, epigenetic or chromosomal change).”
The same model has also been used to describe how life needs to evolve to become complex and, in particular, to create a technological civilization:
#Carter, Brandon (2008): “Five- or six-step scenario for evolution?”. International Journal of Astrobiology, Volume 7.
https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/abs/five-or-sixstep-scenario-for-evolution/841C9AC57BFBD5491756EB5951572B36
https://arxiv.org/abs/0711.1985 (open-access version)
Quote: “The prediction that (due to the limited amount of hydrogen available as fuel in the Sun) the future duration of our favourable terrestrial environment will be short (compared with the present age of the Earth) has been interpreted as evidence for a hard-step scenario. This means that some of the essential steps (such as the development of eukaryotes) in the evolution process leading to the ultimate emergence of intelligent life would have been hard, in the sense of being against the odds in the available time, so that they are unlikely to have been achieved in most of the earth-like planets that may one day be discovered in nearby extrasolar systems.”
The number of hard steps needed to get a technological civilization is not known, but typical figures are within the range 5-10. For example, the earliest life on Earth spawned about 3.7-3.5 billion years ago, and the first multicellular animals appeared about 600 million years ago.
#Smithsonian Institution: “Early Life on Earth – Animal Origins”, National Museum of Natural History (retrieved 2023):
Quote: “The earliest life forms we know of were microscopic organisms (microbes) that left signals of their presence in rocks about 3.7 billion years old. The signals consisted of a type of carbon molecule that is produced by living things. Evidence of microbes was also preserved in the hard structures (“stromatolites”) they made, which date to 3.5 billion years ago.”
#Choi, Charles Q. (2017): “How Did Multicellular Life Evolve?”. Astrobiology at NASA (retrieved 2023).
https://astrobiology.nasa.gov/news/how-did-multicellular-life-evolve/
Quote: “The first known single-celled organisms appeared on Earth about 3.5 billion years ago, roughly a billion years after Earth formed. More complex forms of life took longer to evolve, with the first multicellular animals not appearing until about 600 million years ago.”
One of the first animals ever documented (i.e. a multicellular organism that was an ancestor of the human lineage) is Dickinsonia, from the Ediacaran period (the geological period around 600 million years ago that featured the first unambiguous examples of multicellular life):
#Bobrovskiy, Ilya et al. (2018): “Ancient steroids establish the Ediacaran fossil Dickinsonia as one of the earliest animals”. Science, vol. 361.
https://www.science.org/doi/10.1126/science.aat7228
Quote: “The enigmatic Ediacara biota (571 million to 541 million years ago) represents the first macroscopic complex organisms in the geological record and may hold the key to our understanding of the origin of animals. Ediacaran macrofossils are as “strange as life on another planet” and have evaded taxonomic classification, with interpretations ranging from marine animals or giant single-celled protists to terrestrial lichens. Here, we show that lipid biomarkers extracted from organically preserved Ediacaran macrofossils unambiguously clarify their phylogeny. Dickinsonia and its relatives solely produced cholesteroids, a hallmark of animals. Our results make these iconic members of the Ediacara biota the oldest confirmed macroscopic animals in the rock record, indicating that the appearance of the Ediacara biota was indeed a prelude to the Cambrian explosion of animal life.”
#Vogel, Gretchen (2018): “This fossil is one of the world's earliest animals, according to fat molecules preserved for a half-billion years”. Science News (retrieved 2023)
Hominins appeared 6 or 7 million years ago, while the earliest civilizations can be taken to have emerged about 6000 years ago:
#Pontzer, Herman (2012): “Overview of Hominin Evolution”. Nature Education Knowledge 3(10):8
https://www.nature.com/scitable/knowledge/library/overview-of-hominin-evolution-89010983/
Quote: “The oldest hominins currently known are Sahelanthropus tchadensis from Chad (Brunet et al. 2005) and Orrorin tugenensis from Kenya (Senut et al. 2001). Sahelanthropus, dated to between 6 and 7 mya, is known from a largely complete skull and some other fragmentary remains.”
#National Geographic Society: “Key Components of Civilization” (retrieved 2023)
https://education.nationalgeographic.org/resource/key-components-civilization
Quote: “The earliest civilizations developed between 4000 and 3000 B.C.E., when the rise of agriculture and trade allowed people to have surplus food and economic stability.”
All these examples are significant milestones in the history of life on Earth towards a technological civilization, but the detailed list of hard steps (how many and which ones) is unknown. However, there have been some concrete proposals for such a list:
#Carter, Brandon (2008): “Five- or six-step scenario for evolution?”. International Journal of Astrobiology, Volume 7.
https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/abs/five-or-sixstep-scenario-for-evolution/841C9AC57BFBD5491756EB5951572B36
https://arxiv.org/abs/0711.1985 (open-access version)
Quote: “To start with, the candidate for the status of the 2nd hard step is the emergence of procaryote (simple celled) cyanobacteria about 3.5 Gyr ago; the candidate for the status of the 3rd hard step is the emergence of eukaryotes (with cell nuclei) which were certainly present 1.8 Gyr ago, and for which there is evidence [15] dating back to late Archaean times, roughly 2.5 Gyr ago; the candidate for the status of the 4th hard step is what I call combigenisis, meaning the introduction of sexual gene propagation, about 1.2 Gyr ago; and finally the candidate for the status of the 5th hard step is what might be called macromorphogenesis, meaning the emergence of metazoans (large multicellular animals) about 0.6 Gyr ago. On this basis, the emergence of our own anthropic civilization now would count as the sixth hard step”
According to the list above, the hard steps that led to a technological civilization on Earth would have been the emergence of (estimated times have been rounded off):
1. Self-replicating life ?
2. Simple cells (without nucleus) ~ 4 Gyr ago
3. Eukaryotes (cells with nucleus) ~ 2 Gyr ago
4. Sexual reproduction ~ 1 Gyr ago
5. Large multicellular animals ~ 6 Myr ago
6. Civilization ~ 6,000 years ago
– On Earth life appeared basically as soon as the oceans formed. But then it took two billion years to make the step from single cells to multicellular organisms, and two billion more for us to appear. Culture, civilization and space travel developed super quickly though.
We don’t know how many hard steps are necessary for complex life to emerge, but we can try to estimate the number of hard steps that so far have happened on Earth based on the habitable period of our planet. The start of this period is likely the moment the oceans formed. The end of this period is when the Sun expands and boils the oceans.
The Earth’s surface took some time to cool down into a solid surface after the planet was formed. Then, it is thought that a rain of comets or asteroids brought water to create the oceans. Life appeared relatively quickly after this event, perhaps as little as 200 million years after the origin of the Earth itself:
#Ben K.D. Pearce et al. (2018): “Constraining the Time Interval for the Origin of Life on Earth”, Astrobiology, Vol. 18
https://www.liebertpub.com/doi/abs/10.1089/ast.2017.1674
https://arxiv.org/abs/1808.09460 (open-access version)
Quote: “The habitability boundary could be as early as 4.5 Ga, the earliest possible estimate of the time at which Earth had a stable crust and hydrosphere, or as late as 3.9 Ga, the end of the period of heavy meteorite bombardment. [...]. Evidence from carbon isotope ratios and stromatolite fossils both point to a time close to 3.7 Ga. Life must have emerged in the interval between these two boundaries. The time taken for life to appear could, therefore, be within 200 Myr or as long as 800 Myr.”
Despite appearing so early, it took life a very long time to evolve into more complex forms. Although the exact appearance of multicellular life (not to be confused with animals) is not precisely known, there is evidence dating this transition at about 2 billion years ago:
#El Albani, Abderrazak (2010): “Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago”. Nature, volume 466
https://www.nature.com/articles/nature09166
Quote: “Here we report the discovery of centimetre-sized structures from the 2.1-Gyr-old black shales of the Palaeoproterozoic Francevillian B Formation in Gabon, which we interpret as highly organized and spatially discrete populations of colonial organisms. [...] The Gabon fossils [...] may be seen as ancient representatives of multicellular life, which expanded so rapidly 1.5 Gyr later, in the Cambrian explosion.”
Homo Sapiens only arrived about 300,000 years ago, which means there was a gap of about 2 billion years between the first multicellular life and humans:
#Samithsonian Institution: “Homo sapiens”, National Museum of Natural History (retrieved 2023):
https://humanorigins.si.edu/evidence/human-fossils/species/homo-sapiens
Quote: “The species that you and all other living human beings on this planet belong to is Homo sapiens. During a time of dramatic climate change 300,000 years ago, Homo sapiens evolved in Africa. Like other early humans that were living at this time, they gathered and hunted food, and evolved behaviors that helped them respond to the challenges of survival in unstable environments.”
But the first human launched into space had to wait until less than 100 years ago. This means that, if life’s history was a 24 hour clock, our technological civilization capable of space travel came in the last millisecond.
All this gives us a rough idea about the timespan between hard steps happening so far on Earth – or at least between some plausible hard steps. We don’t know for sure what happened between the emergence of life and the first space travel, but we can try to estimate the total number of hard steps e on Earth using the two data points we are relatively sure of: the time elapsed between the moment at which the Earth became habitable and the emergence of life, and the time remaining between now and the end of Earth’s habitability:
#Hanson, Robin et al. (2021): “If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare”. The Astrophysical Journal, Volume 922.
https://iopscience.iop.org/article/10.3847/1538-4357/ac2369#apjac2369s3
Quote: “A literature tries to estimate the number of (equivalent) hard steps passed so far in Earth's history from key durations. Here is an illustrative calculation. The two most plausibly diagnostic Earth durations seem to be the one remaining after now before Earth becomes uninhabitable for complex life, ∼1.1 Gyr, (Ozaki & Reinhard 2021), and the one from when Earth was first habitable to when life first appeared, ∼0.4 Gyr (range 0.2–0.8 Gyr, Pearce et al. 2018). Assuming that only e hard steps have happened on Earth so far (with no delays or easy steps), the expected value ai for each of these durations should be ∼5.4 Gyr/(e+1). Solving for e using the observed durations of 1.1 and 0.4 Gyr then gives e values of 3.9 and 12.5 (range 5.7–26), suggesting a middle estimate of at least 6.”
For a given civilization to become “grabby” (i.e. venturing into space and controlling the volumes they expand into) we’ll assume a number n of effective hard steps (in general, n can differ from e above).
–Do things always take that long or was this actually exceptionally fast? Also, passing one step does not mean the next one is guaranteed: multicellularity evolved over 25 times independently on Earth, but there’s only one species that builds spaceships.
Multicellularity didn’t develop once, but several times:
#Wegener, Laura et al. (2013): “Multicellularity arose several times in the evolution of eukaryotes”. BioEssays, vol. 35.
https://onlinelibrary.wiley.com/doi/10.1002/bies.201200143
Quote: “Multicellularity has arisen more than 25 times across the eukaryotic tree of life and in all of the major clades (Fig. 1; 12, 13), though the majority of eukaryotic lineages are unicellular in nature 38.”
–We don’t know how many steps life needs to pass and how long they take to give rise to a technological civilization but there are probably many, and it is likely that, on trillions of planets, life has been trying for billions of years. Since we don’t see any other technological civilizations out there, it might well be that we are a rare exception. We might be among the first or even THE first technological civilization in the Milky Way.
More steps means that life appearing at a certain date is less likely. Quantitatively, this is because, if a civilization has a limited time T to arise (say, T = the habitable time-window of the planet), the chance for such a civilization to effectively arise within T will be proportional to T n, where n is the number of hard steps:
#Hanson, Robin et al. (2021): “If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare”. The Astrophysical Journal, Volume 922.
https://iopscience.iop.org/article/10.3847/1538-4357/ac2369#apjac2369s3
Quote: “Carter posited a sequence of required steps i, each of which has a rate 1/ai per unit time of being achieved, given the achievement of its previous step. The average duration ti to achieve step i is ai. Assume that this process starts at t = 0 when a planet first becomes habitable, and that we are interested in the unlikely scenario where all of these steps are completed by time t = T [...] [T]he chance of this unlikely completion is proportional to T n, where n is the number of hard steps.”
Based on the hard step model and a few other simple considerations, this paper derived the following probability density for advanced life to appear at time t:
#Hanson, Robin et al. (2021): “If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare”. The Astrophysical Journal, Volume 922.
https://iopscience.iop.org/article/10.3847/1538-4357/ac2369#apjac2369s3
Quote: “To explore this question, and to estimate human earliness, we now consider a somewhat more realistic model for the timing of the appearance of advanced life. In this model, stars form at different dates, planet lifetimes vary with star lifetimes, and only planets with lifetimes L < L are suitable for advanced life. The probability density function α(t) of advanced life to appear at date t becomes
where b is each star's birth date, ϱ(t) is a star formation rate (SFR), H[L] is a cumulative distribution function (CDF) over planet lifetimes, n is a planet-based hard steps CDF power, and q is a normalization constant.
All the needed data to fill the formula above (essentially, how often stars of different kinds are born in a galaxy as time evolves, and how long planets around those stars will be habitable) can be taken from reasonably well known astrophysical models. The only non-astrophysical parameter in the formula above is n, the number of hard steps (in principle, a planet-dependent quantity).
When one uses the function above to calculate the fraction of advanced life already born in today’s universe (i.e. 13.8 billion years after the Big Bang), one gets the following chart
where the colors indicate various percentages/fractions of currently-existing advanced life as a function of the number n of hard steps (vertical axis) and the maximum lifetime of habitable planets. (The red and orange lines are a visual aid whose meaning is explained below.)
The Earth will only be habitable for a total timespan of about 5 billion years, but this is because the Sun will become a red giant in about 1 billion years, which will put an end to Earth’s habitability. However, if there exist planets that can remain habitable for e.g. trillions of years (around red dwarfs, for example), they will have a much longer time ahead and technological civilizations in them will have a higher chance to be born in the future. This is why, among the total number of advanced life forms that existed or will exist some day in the universe, the fraction of them born today decreases as the maximum planet lifetime increases.
The plot above puts some concrete numbers to a surprising conclusion: unless advanced life needs a very small number of hard steps to arise (n < 3) and unless the maximum planet lifespan is less than 50 billion years (unrealistic if we consider red dwarfs, which are extremely abundant in the Milky Way and can last for trillions of years, as will be explained below), we are among the first 1% of advanced civilizations that will ever be born in the universe. The bounds n = 3 and L = 50 Gyr are indicated by the red lines in the figure above (note that the scales are logarithmic).
Even more extremely, if we take n = 6 (the average value of hard steps deduced above to have happend on Earth so far) and a maximum planet lifetime of a trillion years (orange lines), we find that we are among the first tenth billionth of all advanced life forms.
–The universe is already 13.8 billion years old, but it is unlikely that many other technological civilizations had a chance to appear before us.
The latest estimates of the present age of the universe based on the properties of the cosmic microwave background indicate that the universe is about 13.8 billions years old:
#Planck Collaboration (2020): “Planck 2018 results VI. Cosmological parameters”. Astronomy & Astrophysics, vol. 641
https://www.aanda.org/articles/aa/full_html/2020/09/aa33910-18/aa33910-18.html
–Because in the earlier universe life would have had a pretty hard time to emerge, let alone thrive, because it was such a hostile environment. Early stars constantly blew up, galaxies crashed into each other and supermassive black holes vomited massive amounts of radiation. Enough to sterilize galaxies over and over again.
The (un)inhabitability of the younger universe due to too many supernova explosions and gamma-ray bursts, as well as to too few rocky planets (which need large amounts of heavy chemical elements to be created, but those elements are only synthesized slowly by stars) has been pointed out several times in the literature. In the case of the Milky Way, for example, it has been found that its habitability has indeed generally increased with time:
#Spinelli, R. (2021): “The best place and time to live in the Milky Way”. Astronomy & Astrophysics, vol. 647.
https://www.aanda.org/articles/aa/full_html/2021/03/aa39507-20/aa39507-20.html
Quote: “Until about six billion years ago, the outskirts of the Galaxy were the safest places to live, despite the relatively low density of TPs [terrestrial planets]. In the last about four billion years, regions between 2 and 8 kpc from the center, which had a higher density of TPs, became the best places for a relatively safer biotic life growth. We confirm the hypothesis that one long GRB played a leading role in the late Ordovician mass-extinction event. In the last 500 Myr, the safest neighborhood in the Galaxy was a region at a distance of 2 to 8 kpc from the Galactic center, whereas the MW outskirts were sterilized by two to five long GRBs.”
–Our Sun was born right at the end of this cosmic death show. The universe has never been more welcoming to life than it is now. So humanity has arrived at a very convenient spot in time, maybe the earliest reasonably possible for life to thrive. What about the future?
The most intense period of galactic activity that birthed most of the stars in the Milky Way was 10 billion years ago. The Sun was born much later, but this might have been very good for the chances of a habitable planet like Earth to exist at all. To be born, rocky planets need heavy chemical elements that are only synthesized slowly in the cores of the stars or in cosmic cataclysms like supernova explosions or neutron star mergers. Therefore, the younger the universe, the less heavy elements available and the less rocky planets:
#NASA (2015): “Our Sun Came Late to the Milky Way's Star-Birth Party”. Spitzer Space Telescope,
Quote: “Our Sun, however, is a late "boomer." The Milky Way's star-birthing frenzy peaked 10 billion years ago, but our Sun was late for the party, not forming until roughly 5 billion years ago. By that time the star formation rate in our galaxy had plunged to a trickle.
Missing the party, however, may not have been so bad. The Sun's late appearance may actually have fostered the growth of our solar system's planets. Elements heavier than hydrogen and helium were more abundant later in the star-forming boom as more massive stars ended their lives early and enriched the galaxy with material that served as the building blocks of planets and even life on Earth.”
To evolve from single-celled organisms to complex forms, life on Earth required a few billions of years of biological evolution. Therefore, the best cosmic time for complex life forms could well be today.
–The Sun burns brighter than 90% of the stars in our galaxy and will keep getting brighter. In about a billion years, it will boil all of Earth’s oceans and then become a giant that swallows it whole. So in the galactic context, the Sun is very short-lived.
The Sun looks like an “average” star when its properties are compared to those of other kinds of stars. That is, the Sun would be an “average” star if, among the various types of stars, we took one representative of each type. However, the number of stars that are dimmer than our Sun is much larger than the number of Sunlike stars:
#David Taylor (2012): “Stellar Evolution For Large Stars”. Weinberg College of Arts and Sciences, Northwestern University (retrieved 2023).
https://faculty.wcas.northwestern.edu/infocom/The%20Website/large.html
Quote “The Sun is in the middle of the H-R diagram and in this sense it is an "average" star. But if one takes a census of all the stars in our galaxy, it turns out that most of them are red dwarfs with less than one-half of the Sun's mass, and less than 10% of its luminosity. The Sun may have an "average" position on the H-R diagram, but it is brighter than about 90% of the stars in the Milky Way.”
The Sun is also getting brighter and, within the next few billion years, it will become a red giant:
#David Taylor (2012): “The Sun's Evolution”. Weinberg College of Arts and Sciences, Northwestern University (retrieved 2023).
https://faculty.wcas.northwestern.edu/infocom/The%20Website/evolution.html
Quote: “In short, in the end, the nuclear furnace at the center of every star begins to overheat. To put numbers on this, when the Sun was formed 4.5 billion years ago it was about 30% dimmer than at present. At the end of the next 4.8 billion years, the Sun will be about 67% brighter than it is now. In the 1.6 billion years following that, the Sun's luminosity will rise to a lethal 2.2 Lo. (Lo = present Sun.) The Earth by then will have been roasted to bare rock, its oceans and all its life boiled away by a looming Sun that will be some 60% larger than at present.”
But long before it becomes a red giant, in about one billion years from now, the Sun is expected to evaporate the Earth’s oceans, putting an end to all or to at least most forms of life:
#K.-P. Schröder (2008): “Distant future of the Sun and Earth revisited”. Monthly Notices of the Royal Astronomical Society, Volume 386
https://academic.oup.com/mnras/article/386/1/155/977315
Quote: “Certainly, with the 10 per cent increase of solar luminosity over the next 1 Gyr (see previous section), it is clear that Earth will come to leave the HZ [habitable zone] already in about a billion years time, since the inner (hot side) boundary will then cross 1 au. [... ] What will happen on the Earth itself? Ignoring for the moment the short-time-scale (decades to centuries) problems currently being introduced by climate change, we may expect to have about one billion years before the solar flux has increased by the critical 10 per cent mentioned earlier. At that point, neglecting the effects of solar irradiance changes on the cloud cover, the water vapour content of the atmosphere will increase substantially and the oceans will start to evaporate (Kasting 1988). An initially moist greenhouse effect (Laughlin 2007) will cause runaway evaporation until the oceans have boiled dry.”
According to some models, the Earth may end up being swallowed up by the Sun when it becomes a red giant star.
#K.-P. Schröder (2008): “Distant future of the Sun and Earth revisited”. Monthly Notices of the Royal Astronomical Society, Volume 386
https://academic.oup.com/mnras/article/386/1/155/977315
Quote: “According to these solar evolution models, the closest encounter of planet Earth with the solar cool giant photosphere will occur during the tip-RGB phase. During this critical episode, for each time-step of the evolution model, we consider the loss of orbital angular momentum suffered by planet Earth from tidal interaction with the giant Sun, as well as dynamical drag in the lower chromosphere. As a result of this, we find that planet Earth will not be able to escape engulfment, despite the positive effect of solar mass-loss.”
–Most stars are red dwarfs that can sustain habitable planets for tens of trillions of years! Life on these planets has an incredibly long time window to appear and pass the hard steps. Even knowing nothing about how rare or common life is, this makes it way more likely for technological civilizations to appear some time in the future than in the past.
Red dwarfs are significantly smaller, less massive and less luminous than our Sun. An example is given by Barnard’s star, a red dwarf close to the Sun:
#Daniel Johnson (2019): “Meet Barnard’s Star, Our Red Dwarf Neighbor”. Sky & Telescope (retrieved 2023).
https://skyandtelescope.org/astronomy-news/meet-barnards-star-our-red-dwarf-neighbor/
They are by far the most common type of star in the Milky Way:
#Encyclopaedia Britannica: “Red dwarf star” (retrieved 2023)
https://www.britannica.com/science/red-dwarf-star
Quote: “In the Milky Way Galaxy, about three-fourths of the stars are red dwarfs.”
And they can be extremely long-lived:
#Encyclopaedia Britannica: “Red dwarf star” (retrieved 2023)
https://www.britannica.com/science/red-dwarf-star
Quote: “Red dwarfs are the coolest main-sequence stars, with a spectral type of M and a surface temperature of about 2,000–3,500 K. Because these stars are so cool, spectral lines of molecules such as titanium oxide, which would be disassociated in hotter stars, are quite prominent. Red dwarfs are also the dimmest stars, with luminosities between about 0.0001 and 0.1 times that of the Sun. [...] The heaviest red dwarfs have lifetimes of tens of billions of years; the smallest have lifetimes of trillions of years. By comparison, the universe is only 13.8 billion years old. The dim red dwarfs will be the last stars shining in the universe.”
Despite their small size and lower temperature, red dwarf stars are thought to produce habitable conditions for their planets:
#Wandel, Amri (2020): “Bio-habitability and life on planets of M-G-type stars”. Proceedings of the International Astronomical Union, Volume 14. Symposium S345; Origins: From the Protosun to the First Steps of Life
Quote: “We have show that tidally locked planets of M-type stars may have temperatures suitable for liquid water and complex organic molecules on at least part of their surface for a wide range of atmospheric properties.”
As a consequence of all the above, habitable planets around slow-burning red-dwarfs could remain habitable for up to trillions of years, i.e. far longer than the Earth:
–Because, if civilizations appear at random in the Milky Way within a time window of a trillion years, then very few, if any, would appear before today. Then a couple more arrive in this period of a billion years that we are in, before all starfaring civilizations that could ever exist emerge all together. This weird tsunami-like distribution is the result of both the hard steps model and something else:
When this model is used to perform simulations of how many starfaring technological civilizations have originated at a given time t in the cosmic history, one gets the following kind of plot:
#Hanson, Robin et al. (2021): “If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare”. The Astrophysical Journal, Volume 922.
https://iopscience.iop.org/article/10.3847/1538-4357/ac2369#apjac2369s3
where the horizontal axis indicates the age of the universe in billions of years (t), the vertical axis indicates the cumulative fraction of starfaring civilizations originating at a time less or equal than t, and the color shows the evolution of those distributions for various effective numbers of hard steps (n). Why the time-distribution of starfaring civilizations takes this “tsunami” shape will be explained below.
–A sort of deadline for any spacefaring civilization. Any civilization coming after will find it hard to have room to survive, so all potential life has to cram in before it. Humanity exists now because otherwise we might have missed this deadline.
Such a deadline exists if some advanced life has a non-vanishing probability of becoming spacefaring and rapidly expanding throughout the universe, controlling the volume it occupies and monopolizing its resources.
Once an advanced technological civilization expands and controls a significant volume of space (one galaxy, say), it won’t let other life forms within that volume develop. Here, “missing the deadline” means not being able to become starfaring before we are conquered by a “grabby” alien civilization, and therefore not being able to become starfaring afterwards. Why all grabby civilizations are expected to appear roughly at once will be explained below.
#Hanson, Robin et al. (2021): “If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare”. The Astrophysical Journal, Volume 922.
https://iopscience.iop.org/article/10.3847/1538-4357/ac2369#apjac2369s3
Quote: “We offer this explanation: a deadline is set by loud aliens who are born according to a hard steps power law, expand at a common rate, change their volume appearances, and prevent advanced life like us from appearing in their volumes.”
Quote: “Our grabby alien model resolves this puzzle by denying a key assumption of this appearance model: that the birth of some advanced life has no effect on the chances that others are born at later dates (Ćirković & Vukotić; 2008; Berezin 2018). Our model instead embodies a selection effect: if grabby aliens will soon grab all the universe volume, that sets a deadline by which others must be born, if they are not to be born within an alien-controlled volume.”
–Humans are curious, expansionist and hungry for energy. We have spread over the world and made it our own. Our technology has been improving over time, first slowly, then breathtakingly fast. If these things do not change drastically, and our descendants want to prosper, they will expand into space. We could construct a Dyson swarm for endless energy and transform planets into new homes. We could cross interstellar distances, allowing us to reach for planets around distant stars. If we have the motivation we can become a galactic civilization.
Technological progress is hard to measure on its own, but we can use indicators like computer speeds or the number of objects we’ve launched into space to track it. In most cases, the improvements are exponential:
#Roser, Max et al. (2022): “Technological Change”. Our World in Data (retrieved 2023)
Energy use is another key indicator for the path that human civilization is taking:
#Ritchie, Hannah et al. (2022): “Energy Production and Consumption”. Our World in Data (retrieved 2023).
Even as the source of our energy is changing, we keep consuming more and more. At some point we will not be able to meet our energy needs with the resources available on Earth. That would move humanity past the 1.0 point on the Kardashev scale. Beyond that, we would seek to extract energy directly from the Sun, by means of Dyson swarms.
The idea of the Kardashev scale was devised by astrophysicist Nikolai S. Kardashev and outlined in his 1964 article Transmission of information by Extra-terrestial Civilizations. It is a way to classify civilizations depending on their energy consumption for the purposes of information transmission. Kardashev came up with Type I, II and III civilizations based on their ability to extract and utilize the power in their planet, star and galaxy, respectively.
#Kardashev, N. S. (1964): “Transmission of Information by Extraterrestrial Civilizations”. Soviet Astronomy, Vol. 8
A “Dyson sphere” is a hypothetical device aimed at harvesting the energy of a whole star
#Wee, Alastair (2016): “The Dyson Sphere”. Stanford University (retrieved 2023)
.http://large.stanford.edu/courses/2016/ph240/wee1/
Quote: “A Dyson sphere, as proposed by the physicist Freeman Dyson in 1960, describes an immense artificial construct - or a series thereof - orbiting a star, with the capability to capture its light and convert it to useful forms of energy. [2] Dyson originally envisioned a gargantuan spherical shell that would completely encapsulate the star and soak up all its light. [...] The most realistic and feasible variant of the Dyson sphere is called the Dyson swarm. Such an installation is depicted in Fig. 1; a constellation of man-made satellites in orbit around a star.”
We have covered the topic of Dyson spheres and their more realistic variant, the Dyson swarm, in a previous video:
#Kurzgesagt – In a Nutshell (2018): “How to Build a Dyson Sphere - The Ultimate Megastructure”
–A civilization that does this sort of stuff can be called “loud”, because its activity creates “noise”. Signs that can be detected from far away. Imagine someone in a forest, cutting down trees, starting fires and laying down roads. The more intense their work, the easier they are to notice. An expanding technological civilization would probably be hard to miss. Our telescopes would pick up all that energy and we would clearly identify artificial interference with stars and planets.
Active expanding galactic civilizations are what we call “loud” aliens:
#Hanson, Robin et al. (2021): “If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare”. The Astrophysical Journal, Volume 922.
https://iopscience.iop.org/article/10.3847/1538-4357/ac2369#apjac2369s3
Quote: “To a first approximation, there are two kinds of aliens: quiet and loud. Loud (or expansive) aliens expand fast, last long, and make visible changes to their volumes. Quiet aliens fail to meet at least one of these criteria. As quiet aliens are harder to see, we are forced to accept rather uncertain estimates of their density, via methods like the Drake equation (Drake 1965; Grinspoon 2003; Westby & Conselice 2020). Loud aliens, in contrast, are far more noticeable if they exist at any substantial density (Hart 1975).
Loud aliens are thus much better suited for empirical study via fitting simple models to available data.”
Their behavior also earns them the name “grabby aliens”, as they reach out and grab all planets or resources around them as quickly as possible, and hold onto them tightly. This makes them very hard to miss (point (b) below):
#Hanson, Robin et al. (2021): “If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare”. The Astrophysical Journal, Volume 922.
https://iopscience.iop.org/article/10.3847/1538-4357/ac2369#apjac2369s3
Quote: “Ours is a model of grabby aliens, who by definition (a) expand the volumes they control at a common speed, (b) clearly change the look of their volumes (relative to uncontrolled volumes), (c) are born according to a power law in time except not within other GC volumes, and (d) do not die unless displaced by other GCs.”
–Another consequence of this business is that it is very disruptive to the environment. Clearing a forest means the end of its wildlife. Human activity has left no chance for a squirrel civilization to appear. Not because we hated squirrels, it’s simply that the thought that they might want to do that at some point never crossed our minds and we needed wood.
A key consequence of this model is that loud aliens taking over a chunk of space prevent other starfaring civilizations from appearing in that region.
#Hanson, Robin et al. (2021): “If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare”. The Astrophysical Journal, Volume 922.
https://iopscience.iop.org/article/10.3847/1538-4357/ac2369#apjac2369s3
Quote: “We offer this explanation: a deadline is set by loud aliens who are born according to a hard steps power law, expand at a common rate, change their volume appearances, and prevent advanced life like us from appearing in their volumes.”
The history of biological evolution on Earth’s has been pointed out as an argument supporting this conclusion:
#Hanson, Robin et al. (2021): “If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare”. The Astrophysical Journal, Volume 922.
https://iopscience.iop.org/article/10.3847/1538-4357/ac2369#apjac2369s3
Quote: “In Earth's history, competing species, cultures, and organizations have shown consistent tendencies, when possible, to expand into new territories and niches previously unoccupied by such units. When such new territories offer supporting resources that can aid reproduction, then behaviors that encourage and enable such colonization have often been selected for over repeated episodes of expansion. In addition, expansions that harness resources tend to cause substantial changes to local processes, which induce changed appearances, at least to observers who can sufficiently see key resources and processes. While these two tendencies are hardly immutable laws of nature, they seem common enough to suggest that we consider stochastic models that embody them.”
–Similarly, if loud civilizations were running around the galaxy in the past, terraforming planets or harvesting the energy of stars, they may have prevented our existence. Had aliens started colonizing earth while we were still sludge in the oceans, that sludge would never have turned into humans. This is how loud aliens create a deadline for new civilizations appearing. The galaxy may have trillions of years to create life, but there may only be a short window for it to spread and thrive.
Such a short window for life to become starfaring appears if we assume three things:
1) That humanity has a non-negligible probability for it to become grabby;
2) That, if humanity were to become grabby, not much more than a few million years from now would be needed;
3) That humanity is not special in this respect: Our probability to become grabby and the time required to transition from an advanced civilization to a grabby one is spacetime representative
#Hanson, Robin et al. (2021): “If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare”. The Astrophysical Journal, Volume 922.
https://iopscience.iop.org/article/10.3847/1538-4357/ac2369#apjac2369s3
Quote: “Our model has three parameters, the first of which is the rate at which GCs are born. We assume that we humans have a nonzero chance of giving birth to a GC, and that, if this were to happen, it would happen within roughly 10 Myr. We also assume (with Olson 2016) that this chance is spacetime representative, i.e., we have no good reason to expect our spacetime location to be unusual, relative to other GC origins. Given these assumptions, and the fact that we do not now seem to be within a clearly changed alien volume, our current spacetime event becomes near a sample from the distribution of GC origins. That allows us to estimate the overall grabby birth rate to within roughly a factor of 2 (for its interquartile range), at least for powers of 3 or higher.”
This explains the “tsunami” shape of the plot of emergence of grabby civilizations, and why the upward trend of the curved should take place at about the present age of the universe.
#Hanson, Robin et al. (2021): “If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare”. The Astrophysical Journal, Volume 922.
https://iopscience.iop.org/article/10.3847/1538-4357/ac2369#apjac2369s3
Why the “tsunami” shape? Since grabby civilizations don’t let others become grabby, this is a kind of “winner takes all” model: either you arrive among the first ones or you will never arrive at all. Therefore, once the first grabby civilizations start taking over the universe, the whole of it will be taken by those first grabby civilizations in a relatively short period of time.
Why now? Since humanity exists and it seems reasonable to expect that we have a non negligible possibility of becoming a starfaring civilizations within the next millions of years, unless we are alone or completely unique, the other early-existing civilizations (like us) that will become grabby will take over the universe in a relatively short period of time from now.
An important point here is that, if grabby civilizations exist at all, they are expected to expand very fast in astronomical terms, i.e. they could take over a whole galaxy in just a few million years. The diameter of the Milky Way is about 100,000 light-years:
#Brennan, Pat (2019): “Our Milky Way Galaxy: How Big is Space?”. NASA's Exoplanet Exploration Program (retrieved 2023).
https://exoplanets.nasa.gov/blog/1563/our-milky-way-galaxy-how-big-is-space/
Quote: “Our galaxy probably contains 100 to 400 billion stars, and is about 100,000 light-years across.”
Therefore an advanced civilization that expanded into interstellar space at an average speed of 0.1% of the speed of light (300 km/s) would be able to colonize the entire galaxy in roughly 100 million years. If it managed to do this at 1% or 10% of the speed of light, then it would only only take 10 million or 1 million years. All these are very short times in astronomical terms
– Even if a loud civilization respects planets with naturally-occuring life and expands around them, like humans do with wildlife reserves – any civilization on such a planet would not be able to expand, ever. Trapped forever on a tiny island. But here we are, so Loud aliens were probably never here.
So we arrive at the unsettling conclusion that, unless we are unique or super-special in the universe, the cosmic race to take over the universe has already begun. However, we have not met grabby aliens yet, so our “spacetime” diagram with respect to expanding alien civilizations should look like this:
#Hanson, Robin et al. (2021): “If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare”. The Astrophysical Journal, Volume 922.
https://iopscience.iop.org/article/10.3847/1538-4357/ac2369#apjac2369s3
And that very soon the universe will look like this (where colors represent volumes controlled by different grabby civilizations.
–What about aliens that don’t expand? They would be ‘quiet’ aliens. They’re probably limited to one star system and don’t have a noticeable impact on their cosmic surroundings. Humanity is like this right now. We wouldn’t be able to detect ourselves from the other side of the Milky Way. If they stay quiet forever, maybe because of their culture or abilities, then they are not really a concern for us.
Scientists have conducted thought experiments where they imagine an alien civilization searching for life in the universe and looking at our Solar System to see whether Earth could be detected. In general, we would be detectable from a very tiny fraction of stars in the Milky Way:
#Kaltenegger, L. et al. (2021): “Past, present and future stars that can see Earth as a transiting exoplanet”. Nature, vol. 594.
https://www.nature.com/articles/s41586-021-03596-y
https://arxiv.org/abs/2107.07936 (open-access version)
Quote: “Here we report that 1,715 stars within 100 parsecs from the Sun are in the right position to have spotted life on a transiting Earth since early human civilization (about 5,000 years ago), with an additional 319 stars entering this special vantage point in the next 5,000 years. Among these stars are seven known exoplanet hosts, including Ross-128, which saw Earth transit the Sun in the past, and Teegarden’s Star and Trappist-1, which will start to see it in 29 and 1,642 years, respectively. We found that human-made radio waves have already swept over 75 of the closest stars on our list.”
The study above only focused on transits. Our radio waves would be another clear sign of our existence. However, as of today they have only reached a tiny fraction of our Milky Way’s stars, as illustrated here:
#Lakdawalla, Emily (2012): “This is how far human radio broadcasts have reached into the galaxy”. The Planetary Society (retrieved 2023).
–If we are really early, then eventually, others will catch up with us. Civilizations will emerge all over the place. And these new aliens will look at space, see no signs of life and come to the same conclusion: they exist because Loud civilizations have not yet taken over everything, but it only takes one loud civilization to crowd them out of the entire galaxy.
As emphasized above, if we assume that we have a non-negligible probability of becoming a starfaring civilization within the next few million years, and if we further assume that we are not unique or super special in this respect, it is reasonable to expect a cosmic race among grabby aliens to take over the universe as soon as possible. And such a race could be enhanced by the fact that all those civilizations will in turn reach the same conclusion.
–They, like us, will face an important decision: do they stay quiet, take it easy and tend to their planet for as long as possible, or do they start expanding to take a chunk of the galaxy, before someone else arrives? Meeting others does not necessarily mean war or conflict. But it means that new borders will arise, limits that may persist forever. In the worst case, a civilization could be completely enveloped by the empires of others, eternally doomed to be a galactic backwater, without control over their fate.
By entering specific numbers for some of the parameters of this model (specifically, the expansion speed s of grabby civilizations in terms of the speed of light, and the number of hard steps n) one can derive, among other things, the expected waiting time for two such grabby civilizations to meet for the first time. Such times are typically of about a few hundred million years:
#Hanson, Robin et al. (2021): “If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare”. The Astrophysical Journal, Volume 922.
https://iopscience.iop.org/article/10.3847/1538-4357/ac2369#apjac2369s3
So we’ll probably have a long period ahead of us to prepare for a potential meeting. However, the hundreds of millions of years it is likely to take does not mean that we have a comfortable time ahead of us, since mistakes or delays would have cumulative effects throughout the entire period.
To get an idea of what cumulative effects can imply over long periods of time, let’s assume that –just like until now– human technology Th will continue to improve at a small but approximately constant rate h per year:
Th (next year) = (1 + h)·Th (this year)
Or, equivalently:
dTh = hTh dt
(where Th = Th(t) is some appropriate measure of our technological level as a function of time t expressed in years, h is the yearly growth rate of the technology, and dTh is the technological improvement after a short time dt), then we’ll experience an exponential increase in our technology given by:
Th (t) ~ exp (ht)
But if the first grabby civilization that we’ll meet in the future improves at a yearly rate g just 0.0001% higher than ours:
g = h + 0.000001
then their technological level after 10 million years will be
Tg (107) ~ exp (107g) = exp (107h + 10) = exp (10)·Th (107) ~ 22,000·Th(107)
I.e. tens of thousands higher than ours after 10 million years. At this point one can only speculate about how it will be to suddenly meet an alien civilization that is many orders of magnitude more advanced than us.