My top two choices (they are pretty much tied for first) are: a) Habitable planets (or more specifically planets which intelligent life can form on) are rarer than we think (#1). One idea we discussed is that everything leading up to the appearance of humans was necessary for the development of humans. If any of those events did not happen, humans would not have evolved. This makes it seem very unlikely for intelligent life to form. b) Dangerous natural forces (category 6, #’s 17-19). In class, we learned about how the entire surface of the Earth could have been sterilized multiple times during its formation due to massive impacts. It seems like it is relatively easy to completely eliminate life on a planet, so even if an intelligent civilization did develop, it could easily be wiped out by some kind of natural disaster. My third choice is that other life forms may have “graduated to other realms” (#6). If a species has been around enough, they are probably focused on what they would consider much more interesting things than finding other life forms. They have already done it enough and have moved onto more advanced things. (Jamie) Category 1: Solitude Habitable planets are not as common as we might expect. Earth-like life (or “intelligent” life as we’ve defined it) is likely incredibly unique and we will struggle to find other life we are able to communicate with outside of our own. The few large “steps” in the evolution of life on a planet all the way to intelligent life (prokaryote to eukaryote, etc.) might possibly be more restricting than we think. This means that even if life were to be sparked into existence on other habitable planets, they may be bottlenecked by a few limiting evolutionary steps, instead of inevitably evolving to intelligent life as many think. Category 2: Gaps in Technology/Timing Life on Earth has only been “intelligent” for under 10,000 years. We have only had the technology for interstellar communication for less than 100. Considering that not all solar systems and planetary systems began at the same time as ours, and that we only have a sample size of one for how long evolution to intelligence takes, it could very well be that we just aren’t lined up with a proper timeline to be able to communicate with extraterrestrial life if it has or will ever exist. (Reece) 1) Habitable planets may be very rare There are too many requirements 2) Some spark may have been needed to get life started We don't know how hard it is for life to start The next two are not in the reading but I think they are the most likely. I am surprised the reading didn't mention them. A) The universe is just too vast and we are unlikely to have evidence of life because life would be very unlikely to travel thousands or millions of light years and reach us. the universe may be teeming with life but it is in another galaxy. B) I think intelligent life is doomed to destroy itself. We are already so good at developing weapons of mass death. All it may take is a few nukes. In a thousand years we may have weapons orders of magnitudes stronger. (Clark) I think one of the most compelling reasons for the silence is that habitable planets may be rarer than it seems. This seems plausible because we have found no evidence of intelligent life on the exoplanets. Other reasons I think are very valid include: alien aversion to spaceflight, the potential danger of the Milky Way, and inevitable self-destruction. (Gretchen) 1. #21 The self-extinction model - Because of the sheer number of stars and likely planets in our universe, it seems unlikely that no other intelligent species would ever have lived. However, with all the chaos in our world, it is very possible that all intelligent species destroy themselves eventually. 2. #4 Other species aren't as curious as humans and #8 they may become less aggressive - It is naive to think that other intelligent species will have anything in common psychologically with humans. Other life could be totally different at least in this regard if not biologically as well. Therefore, these theories that explain our lack of contact based on different basic inclinations seem very plausible. 3. #9 Creating simulated worlds negates desires for space travel - With human's virtual reality technology being where it is today, we aren't necessarily far off from creating a simulated world that all people may choose to live in. We could get to the point where there would be no one left in the outside world to travel into space. There's no reason the same wouldn't happen to other species. (Calvin) 1. Reason 3 2. Reason 2 3. Reason 18 (Colton) I think that the reasons involving the general lack of intelligent life and habitable planets are the most reasonable explanations but also the most boring in terms of science fiction. My choices for the most reasonable explanations generally follow a pattern of assuming that aliens, if they exist, are not infinitely more technologically advanced than us. For example, all the theoretical stuff about crazy particles and spiral arms are much higher than the computer-mind telepathy hivemind. That being said, invert the list and you’ll find which I’d be most excited to read. An alternative explanation for the “Great Silence” is that intelligent alien species are incredibly fragile, only able to exist because of an extremely stable, unique home-world. Humans’ ability to adapt and spread is intimidating to the fragile aliens and thus they won’t contact us unless absolutely necessary because they’re afraid we’ll spread and destroy their civilization. Either that or humans are the most technologically advanced intelligent life so when we eventually do start sending space crafts out to explore we’ll find developing worlds. The information we’d give them would be equivalent to Aliens teaching the Egyptians how to build pyramids. Finally, Earth could be the only planet that is in the right time to support life. That is, it’s mature enough, close enough to the sun which is also mature enough, and the surrounding space is calm enough to support life whereas the exoplanets still can’t or can’t anymore. (Michael) (1) Habitable planets are rarer than we think (6) Other civilizations may have moved so far past us that they find space travel or trying to contact us frivolous (11) Other civilizations may be awaiting mankind's social maturity/have quarantined us as dangerous. (Negatwa) Generally I believed that Earth was a pretty rare phenomena, and developing intelligent life is extremely difficult. I also ranked disasters as being pretty likely for wiping out life, and less likely is the idea that there is intelligent life, they know of us, and choose not to engage us whatsoever. 1 Habitable planets are rarer than astronomers now believe 18 Nemesis wreaks havoc every 26 million years 7 Aversion to spaceflight due to risk (Jay) 1: 21 2: 23 3: 19 (Bethany)