i. Where are they? Or the Fermi Paradox
Arguably, some of the questions that seem most interesting to kids seem to involve aliens and intergalactic space travel. At the very least that has been the case for many years, and it hopefully continues with the recent advancments made in our ability to travel in space, i.e SpaceX’s as well as NASA’s recent achievements.
The questions of intelligent life outside of earth has not only been discussed by the general populace since the Star Wars movies, but the scientific community has also been pondering these questions in quite some detail.
One of the main questions that have boggling us for decades is — “Where are they?”
This question is a version of the famous Fermi Paradox. The paradox lies in the apparent contradiction between the absence of evidence for extraterrestrial life and the seemingly high probability for such extraterrestrial life. In general, when someone reasons about the probability of extraterrestrial life, the argument goes like this following fictional conversation between Samuel and Tobias:
S: “The universe is insanely large, and the number of stars contained in the observable universe is immense considering that the average number of stars per galaxy is 100 billion!!”
T: “Additionally, it seems likely that at least some of these stars have Earth-like planets in orbit.”
S: “True. There are also many stars that are way older than our star, meaning that there are Earth-like planets that are way older than our planet. Even if the probability of it happening is very low, some of these Earth-like planets have likely produced some intelligent lifeform long ago.”
T: “Also, if the intelligent lifeform becomes intelligent enough, it can become capable of interstellar travel.”
T, again: “Even if the pace of interstellar travel is low, large parts of our galaxy would be traversed in just a few million years. Which is a rather small amount of time on a cosmological scale.”
S: “Since some stars are billions of years older, there should be some evidence of extraterrestrial life by now. Either through encountering the aliens themselves or their probes.”
T: “I think this is correct. But where are they?!”
In light of the lack of evidence, the above argument really does warrant the question “where are they?”
In many ways the Fermi Paradox is a fun and telling mental exercise in reasoning with only one data point and highly uncertain assumptions. It highlights important points about assumptions, reasoning, and the absence of evidence.
ii. The Great Filter
As an add-on to the Fermi paradox, Robin Hanson wrote a paper in 1996 titled “The Great Filter – Are We Almost Past It?”. In it, Hanson argues, based on the one important data point we have — that we have seen no evidence of extraterrestrial life, for a great filter that all lifeforms must pass through on the path to galactic colonization.
Consider Hanson’s best-guess evolutionary path to an explosion which leads to visible colonization of most of the visible universe:
- The right star system (including organics)
- Reproductive something (e.g. RNA)
- Simple (prokaryotic) single-cell life
- Complex (archaeatic & eukaryotic) single-cell life
- Sexual reproduction
- Multi-cell life
- Tool-using animals with big brains
- Where we are now
- Colonization explosion
All of what we have seen so far, our only data point, is nothing, which suggests that at least one of the above steps, or some other step, is very, very improbable. Or in other terms, one of the above steps is a filter for intelligent life.
The concept of a Great Filter has been productive in that it has spurred discussions on where the most improbable steps lies and so on. It does, however, not give us an answer to where they are.
iii. Grabby Aliens
Instead, we can turn to one of Hanson’s most recent papers titled “A Simple Model of Grabby Aliens”. In the paper, Hanson and his coauthors argue that the puzzling earliness of humanity can be explained by the introduction of “grabby civilizations”. The grabby civilizations are defined as lifeforms that expand rapidly, never die alone, change the appearance of the volumes they control, and who are not born within other GC-controlled volumes.
These grabby civilizations set an early deadline, which can explain why we have not seen them yet. Additionally, if our descendants might become a grabby civilization, then today is near a sample origin date of such a GC.
The model used in the paper only has three free model parameters.
- The rate at which grabby civilizations are born. The assumption here is that we have a non-zero chance of becoming a grabby civilization and that our chance of becoming one is space-time representative. These assumptions combined with the fact that we have no evidence of being in changed alien volume allows the authors to estimate this parameter to withing a factor of two.
- The speed at which grabby civilizations expand. The model predicts that at typical grabby origin dates, a third to a half of the universe is within grabby-controlled volumes. Which means that if the expansion speed were low, grabby-controlled volumes would be noticeable from Earth. If the expansion speed instead were within ~25% of lightspeed, then a selection effect implies that we would be more likely to not see than to see any such volumes.
- The number of hard steps in the Great Filter. The chance of the entire process completing within a time duration goes as that duration raised to the power of the number of hard (i.e take-very-long) steps in that evolutionary process. Using data on Earth history durations, a literature estimates an Earth-duration-based power to lie roughly in the range 3-9.
Through this model the authors show that humans do appear early in the universe. This is further shown in another model in which the hard-steps power law (see pt. 3 above) is applied to planets, while allowing stars to form at different dates and to last for different durations. That model produces time distributions for when advanced life should appear, and according to the model less than 10% of this distribution appears before today’s date.
The model thus resolves the question of where the aliens are by embodying the assumption that grabby aliens have an effect on the chances that others are born at later dates. Hence, a selection effect appears. Grabby aliens will grab all the universe volume, which sets a deadline by which others must be born, if they are not to be born in an alien volume.
This last part is rather confusing, so it can be illuminated by an example from our times. If we imagine that the internet will exist for a couple of centuries and that Google could appear at any random time of those centuries. Then, why did Google appear so early? Now, the answer is more obvious and intuitive. If Google had not appeared, then some other company would have taken its place and Google would never appear.
The same effect is what the grabby aliens model predicts, if we can become grabby then there could be few other times in which we could exist.
iv. Reasoning in the Absence of Evidence
The above two models, Grabby Aliens and the Great Filter, are two great examples of the knowledge that can be gained from very few or no datapoints. So far, the only datapoint we have in terms of intelligent life is the life here on earth.
Yet, the model of Grabby Aliens, even though it might be completely wrong, gives a plausible explanation for why we seem to be alone thus far.
This illuminates the power of thinking about what the absence of data can tell us. The aphorism goes “Absence of evidence is not evidence of absence”, and I would like to urge you all to think deeply about what the absence of evidence can tell you.
An example that is closer to our current daily lives than extraterrestrial life is the debate about using masks in Sweden. For a long time, the official stance of the FoHM was that masks do not work. This position was mostly based on the absence of evidence, they argued that there was no evidence that masks protected against covid-19. Since then, FoHM has changed their stance on masks, now they recommend everyone to use them while being in public.
In this case, a more rationalistic approach would have been to urge people to use masks until there was evidence of either masks being effective against transmitting covid-19 or the opposite. Why? First, the absence of data does not tell us that masks are useless, it tells us that it might be the case that they work, and it might be the case that they do not work. Additionally, a reasonable prior when considering a virus that is mainly transmitted through droplets is that masks have some effect when it comes to reducing transmission. Perhaps lives were lost due to this reasoning mistake from FoHM.
Hopefully, I have now convinced you all of the importance about thinking about what the absence of evidence means and how incorporating that kind of reasoning into your decision making can make you better at making informed decisions. It can also help us understand the nature of our reality, as evidenced by the Great Filter and the Grabby Aliens model.