Imagine that we're living in a universe where there might be other intelligent alien civilizations out there. These aliens could be either "loud" or "quiet." Loud aliens are really noticeable because they expand quickly, stick around for a long time, and make big changes to their environments.In this particular study, the researchers came up with a model for loud aliens that they called "grabby". They also found that, unless you assume that there are very few conditions that have to be met for advanced life to evolve and that the lifetime of habitable planets (planets with conditions suitable for life) is very short, it seems implausibly early in the history of the universe for advanced life like humans to have evolved.
Loud aliens, which fill the universe early, can help explain why we evolved so early because they set a deadline for when advanced life needs to appear in order to see an empty universe like we do.Finally, the researchers considered the idea that grabby civilizations might evolve from more common "non-grabby" civilizations. They found that, in order to estimate that even one non-grabby civilization has ever existed in our past or present, we'd have to assume a very low probability of a non-grabby civilization transitioning to a grabby one.
If that probability is too low, it might not bode well for humanity's future.1. They expand the volumes of space that they control at a common speed.2. They alter the appearance of the volumes they control, making them distinct from uncontrolled volumes.3. They are born according to a power law over time, but only outside of the controlled volumes of other such civilizations.4. They do not die unless displaced by other grabby civilizations.1. The rate at which these civilizations are born - The model assumes that humans have a non-zero chance of giving birth to a grabby civilization and that this chance is representative of the rest of the universe.
The model estimates that the overall grabby birth rate can be determined to within a factor of 2, at least for powers of 3 or higher.2. The expansion speed of these civilizations - The model predicts that on average, at the time of their origin, a third to a half of the universe is within grabby-controlled volumes. If the expansion speed were low, these volumes would be large and noticeable in the sky. However, if the expansion speed were close to the speed of light, a selection effect would make it less likely for us to see such volumes.
Therefore, the model concludes that grabby aliens, if they exist, must expand very fast.3. A power derived from the number of steps required for simple matter to evolve into a grabby civilization - This power is based on the chances of the evolution process completing within a certain time duration and is estimated to be in the range of 3-9.In 1983, Brandon Carter developed a model to explain the process of how advanced civilizations like ours might have evolved from simple, lifeless matter. The model suggests that there are a series of steps that must be completed for an advanced civilization to evolve.
The process begins when a planet becomes habitable and that all steps are completed within a certain period of time, T. It divides the steps into two categories: easy steps, with a duration of T, and hard steps, with a duration longer than T. According to Carter's model, the harder the steps are, the less likely it is that they will be completed within the given time frame. These hard steps may include the creation of self-replicating molecules, the passage from prokaryotic to eukaryotic cells, the development of multicellularity, or the creation of particular combinations of body parts.
The concept of the "great filter" is also mentioned in the text, which refers to the steps or barriers that must be overcome for simple matter to evolve into an advanced civilization. Essentially, the great filter is like a set of checkpoints that a civilization must pass through on its way to becoming advanced, and the harder the checkpoints are, the less likely it is that a civilization will successfully navigate through them.The number of hard steps that have occurred on Earth can be calculated by comparing the remaining duration before Earth becomes uninhabitable for complex life (estimated to be 1.
1 billion years) to the duration from when Earth was first suitable for life to when life actually appeared (estimated to be 0.4 billion years). Using these durations and the assumption that only a certain number of hard steps (e) have occurred on Earth, the study calculates e to be 3.9 or 12.5 (with a range of 5.7 to 26), suggesting a middle estimate of at least 6 hard steps.Imagine you're a space explorer trying to find out where advanced life (like humans) is most likely to exist in the universe.
You know that advanced life needs certain conditions to thrive, like the right kind of star and a suitable planet. To figure out where to look, you decide to create a model that predicts when advanced life is likely to appear on a planet.First, you look at the lifetimes of different stars and how long their planets can support advanced life. You call this information the "habitable SFR" (star formation rate) and the <span class="idk">"wearable lifetime distribution"<span>The wearable lifetime distribution is a term introduced in the text to describe the durations of habitability of planets, or how long they are capable of supporting advanced life.
</span></span>. You then look at previous research on these topics to see what patterns emerge. You find that the SFR curve has a particular shape and peaks at a certain point in time, and that the wearable lifetime distribution is related to the overall lifetime distribution of planets, but not exactly the same.Now that you have your model, you can use it to estimate the probability that a planet like Earth is an "early" planet, meaning that advanced life appeared on it relatively soon after it formed.
You plug in all the numbers and do the calculations, and voila! You have a prediction of how likely it is that advanced life exists on a given planet within a large area of space.Imagine a hypothetical advanced civilization that has the capability to expand rapidly and indefinitely throughout the universe. This civilization might be driven to do so by a variety of factors, such as a desire to colonize new territories and access new resources, or perhaps simply a competitive desire to outdo other civilizations.
These expansion efforts might involve changes to local environments and processes in order to support the expansion, and could potentially have a significant impact on the overall appearance of the universe."Grabby civilizations" (GCs), can spontaneously arise at different locations and times. These civilizations expand and take control of the space around them, making it clear that they are different from the rest of the universe. These GCs are thought to come from "non-grabby civilizations" (NGCs), which are less developed and don't have much impact on the universe.
However, not all NGCs give rise to GCs - in fact, it's thought that only 1 in every R NGCs will give rise to a GC.As the GCs expand, they can potentially come into contact with other GCs, at which point they might either peacefully coexist or fight over territory. It's also possible that a GC might trigger a catastrophic event that destroys the universe soon after its birth, although this would require the GC to expand at the speed of light.According to this model, the chances of a GC forming in a small volume of space at a given time follow a power-law dependence.
This means that the chances of a GC forming increase over time, but at a decreasing rate.Here’s a simulation of the model in an expanding spherical region of space, starting with a diameter of half a billion light-years and containing 400 million galaxies. See these colored regions expanding? They represent the grabby civilizations.In this simulation, they are born at a particular rate, and then they expand at half of the speed of light. When they meet, they simply stop expanding. The model doesn’t tell us anything about what they’ll do after meeting each other.
Civilizations that are born later take up less space, and if they are born very late, they occupy the crevices between bigger civilizations.Conversely, the first civilizations to appear end up controlling larger volumes. At the end of the simulation, there are 87 grabby civilizations, 25 billion years have passed, and the diameter of the spherical region of space has expanded to 25 billion light-years. This is because the universe intrinsically expands.GCs are "grabby civilizations" that are able to spread their influence throughout the galaxy, while NGCs are "normal civilizations" that do not have this ability.
NGCs are more common than GCs, with the ratio of NGCs to GCs being represented by an unknown value R. The estimates of R can affect our expectations for the future of humanity (optimists tend to have low estimates of R) and the success of SETI efforts. The text suggests that the probability of finding NGCs decreases as the region in space and time becomes more distant from us, and that higher estimates of R are required to expect success in SETI in these regions.The probability of finding extraterrestrial civilizations within our past light cone (i.
e. within the part of the universe that we can see) is very low, unless the ratio of NGCs to GCs is extremely high (in the tens of thousands or even millions). This suggests that SETI may have a low chance of success, at least in the short term. The authors also note that other factors, such as the longevity of alien techno-signatures and the volume power of the universe (essentially, how much space is available for civilizations to occupy), can affect the probability of finding extraterrestrial civilizations.