Presentation on theme: "1 Class events: week 15 This week’s goals: Great Silence (aka Fermi Paradox) - - What is the Great Silence? - Possible resolutions to the Great Silence."— Presentation transcript:
1 Class events: week 15 This week’s goals: Great Silence (aka Fermi Paradox) - - What is the Great Silence? - Possible resolutions to the Great Silence
2 An important take-home message If nothing else… …I hope you leave today understanding why you should never again utter the phrase, “There may be aliens in the galaxy, but I think they just haven’t gotten here yet.” This terribly flawed perspective reveals a basic lack of understanding of our galaxy’s size, age, and history.
3 Timescales of space exploration How long will it take for a spacefaring civilization to explore the galaxy? Consider a colonization model where the exploring species travels to a new planet, resides there long enough to resupply, and then continues on its way. This move-rest-move approach results in an overall average speed of exploration that would be the same as if the explorers travelled the same distance, at a slower velocity. Move-rest-move: Continuous motion at a slower speed: Obviously, an expanding civilization would not explore the galaxy with only a single spaceship—it would create more as it went. This is called the coral model of exploration.
4 Timescales of space exploration Consider two colonization models. In the slow model, spaceships move slowly (0.01c), perhaps as generational arks, which then reside for a long time (5000 years) before moving on. In the fast model, the spaceships are very speedy (0.1 c), and only reside for 150 years before moving on. SlowFast Travel speed:0.01 c0.1 c Travel time:500 y50 y(Stars are about 5 LY apart.) Rest on a planet:5000 y150 y Resulting velocities and timescales V explore : c0.025 c T explore :9.2×10 7 y3.3×10 6 y While these times may seem very long, compared to the age of the galaxy (10 10 years) they are exceedingly short: T explore / Age galaxy :1/1101/3000
5 Galactic differential rotation Our galaxy rotates differentially, so stars slide past each other with time. In effect, even the inhabitants of a stationary solar system explore different star systems, as its neighbors change over time. Because of galactic differential rotation, it will take a sub-light civilization only about million years to completely explore the galaxy, instead of million years. The new time ratio to explore the galaxy is as small as 1/20,000.
6 Timescales of space exploration Von Neumann Probes Self replicating machines that would have simple programming: Find new planets, find raw materials, replicate new machines. Once the probes are ready, they move on. And they do this fast…
7 Exploration by von Neumann Probes Exploration by von Neumann probes is hugely effective: – –No need for bulky life-support → smaller crafts; – –Faster rebuilds, less time between generations; – –Can make far more crafts with limited resources; – –Easier to accelerate to high speed. – –No need for delicate accelerations, when 30g would do!
8 Exploration by von Neumann Probes Recall our biological exploration wave through the galaxy: Robot probes, by any calculation, would explore the galaxy about ten times faster, exceeding even the rate of a fast biological exploration, 0.5 million years—or about 1/20,000 th of the galaxy’s age. Robot exploration, even if it started much later than biological exploration, would expand in a wave that would quickly overtake the biological exploration wave.
9 “They are out there, but they haven’t gotten here yet” Recall that star formation has been occurring in our galaxy for about 10 billion years, making stars (and presumably planets) just like ours for all that time. If there is just one other civilization in our galaxy, it might have formed at any time during those 10 billion years. Indeed, it would be preposterous to expect that its age is within 0.5 million years, 5 million years, or even 50 million years of our own. 0.5 million years (fast robot probes, fastest biological exploration) 5 million years (slow robot probes) 50 million years (slow biological exploration) 10 billion years
10 “They are out there, but they haven’t gotten here yet” 0.5 million years (fast robot probes, fastest biological exploration) 5 million years (slow robot probes) 50 million years (slow biological exploration) 10 billion years Instead, it would be vastly more likely that if even one other civilization EVER occurred in our galaxy, it would have had billions of years to explore the entire galaxy, before we even gelled from the primordial soup. Saying they haven’t gotten here yet, in the case of a 0.5 million year exploration time, would have the same likelihood as spending one day per year in a rental cabin, just to find that someone else—the only other renter all year—arrives within 26 minutes of your arrival. If there is more than one other civilization in the galaxy, the situation is even less likely that “they haven’t gotten here yet.”
11 Two observations in conflict 1) If aliens exist in our galaxy then they should have reached the Earth long, long ago. 2) Daily public experience tells us that we are not surrounded by a constant barrage of aliens. WHERE ARE THEY?? This is the Fermi paradox. WHY CAN’T WE HEAR THEM??This is the Great Silence.
12 Systems in equilibrium In general, science proceeds on the assumption of near-equilibrium. This assumption is valid because changes to systems usually transpire slowly… - -stars evolve, but slowly; - -mountains grow or erode, but slowly; - -trees grow larger over time, but very slowly. Of course, many systems can move rapidly from one state to a dramatically different state. But such systems spend very little time in the actual transition process. In other words, dramatic things CAN happen, but the time between dramatic events is large… - -stars explode, but very rarely; - -earthquakes happen, but between long spaces of time; - -trees fall over, but rarely. The lesson from this is that it is almost always very safe to assume that a system is in a slowly-changing stage, and that it is not in a stage of rapid transition.
13 Equilibrium and the Copernican Principle Recall the Copernican Principle… We cannot assume we are at a special place in our Universe. We are not at… the center of the Earth’s surface (for flat-Earthers); the center of the solar system; the center of our galaxy; the center of the Universe. Nor can we assume that we are in a special moment, such as a transitional time. Such transitional times are rare and short-lived. Reflect: this is really a restatement of the Copernican Principle. Avoiding the assumption of a special time or place is called the Copernican Principle. (In cosmology: The Cosmological Principle.) It is foolish to assume we are at a special time in our galaxy’s history.
14 Equilibrium and galactic evolution Assuming we survive into the future, we (or our von Neumann probes) will explore the galaxy during the next million years. If we are alone in the galaxy, we are about to transform it from an unexplored one, to a fully explored one. This would be a huge, dramatic transition for our galaxy, implying that the galaxy is in a vast state of disequilibrium. This is a huge violation of the Copernican Principle.
15 In case you lost focus for a moment… The Milky Way Galaxy is ancient (10 billion years), but in contrast, the time needed to explore it is but a few million years. Other life forms in the galaxy should have explored it by now. But we don’t see them, nor have we detected them. WHY NOT??? (Fermi Paradox/Great Silence). (Thinking that they evolved simultaneously as us would require an incredibly improbable coincidence.) If other life forms simply don’t exist, we are in the unique position of being the first galactic explorers. In that case, we are about to change the galaxy forever, but the Copernican Principle says that almost certainly isn’t likely to happen. Therefore, something is just about to descend upon us, and stop us in our tracks! (Statement A) (Statement B)
16 Resolutions to the Fermi Paradox We now seek possible resolutions to this quandary. How do we circumvent either… – –Statement A—we don’t see or detect aliens that should be swarming throughout the galaxy! – –Statement B—something is going to stop us from exploring the galaxy. There are many possible explanations. Let us look at some…
17 1:4 Other space-faring civilizations do not exist Rare Earth Hypothesis Other civilizations never formed in the first place! Perhaps developing civilizations need: – –Water – to allow life to form; – –Dry land – to force technology; – –Plate tectonics – to regulate CO 2 and avoid a catastrophic greenhouse effect; – –A magnetic field – to prevent atmospheric stripping; – –A large satellite – to stabilize the planet’s axis; – –An absence of hot Jupiters – inward spiraling giants are devastating; – –A cold Jupiter – to remove comets from the inner solar system. BUT… Is this argument simply rooted in carbon chauvinism? This hypothesis violates the Cosmological Principle by saying we are unique, and does not address the fact that our galaxy is about to change character because of our own exploration.
18 1:4 Other space-faring civilizations do not exist Self Destruction Hypothesis All civilizations destroy themselves before reaching space: – –Nuclear disaster? – –Ecological disaster? – –Technological singularity? BUT… Could these mechanisms really have a 100% kill rate?
19 2:4 Critical barriers to communication exist Unattractive Earth Hypothesis Aliens are widespread, but either the Earth is difficult to reach, or it might not fit the notion of a habitability for alien species. – –Are nearby black holes necessary for space travel by spectacular technologies? – –The Sun is leaving a spiral arm—is this inconvenient for space travel? – –Are all other alien civilizations associated with K-M dwarf stars? BUT…. Von Neumann probes could reach “inconvenient” planets with little delay. It is non-Copernican to expect our civilization is unique in being the only one orbiting a G star.
20 2:4 Critical barriers to communication exist Overwhelming Technology Hypothesis Communication with aliens would be easy, if we knew how to look. – –Exotic communication (neutrino modulation), etc. – –Are we misinterpreting obvious beacons such as pulsars, etc? – –Communication is out there, but it is mostly carefully masked to be energy-efficient. While islanders in a Southeast Asian stone-age culture might communicate with each other via drum beats, they would be unaware that the air was also filled with radio broadcasts. BUT… Wouldn’t aliens be able to dumb down their communications to talk to us?
21 2:4 Critical barriers to communication exist Government Conspiracy Hypothesis Aliens are, or have been, in communication with us, but our government is not letting the truth out.The hypothesis includes… – –In 1947, aliens crashed in Roswell, NM. (“Operation Mogul” is a cover-up.) President Truman started the “Majestic 12” secret committee of scientists. The FBIT denies this. – –Area 51 is a high-security Air Force installation in Nevada (130 km NW of Las Vegas). Its contents are secret, but may contain alien aircraft and bodies. – –In the 1970s, cattle were found in 15 states, surgically and mysteriously mutilated. Even an investigation by the FBI came up with questions about the cause. Aliens might need to experiment on cattle. – –The cover-up is international in scope. BUT… Conspiracy theories are, in general, largely impractical because of the huge financial and celebrity gains associated with revealing them.
22 3:4 Aliens choose to observe us quietly Sneaky UFO Hypothesis UFO-proponents argue that aliens have gone through all their efforts and energy expenditures to visit us secretly—and then accidentally reveal themselves!? Rice #1: If aliens wanted to sneak around, why don’t they just turn off their lights? A: those are their engines, not just their in-flight lights! Rice #2: If those are their engines lights, then why not use a craft that is a little more sneaky? After all, they knew they were coming here to sneak around! A: They are so powerful, they don’t need to sneak around, after all—we can’t threaten them. Rice #3: Then why sneak, and in particular, sneak so poorly?
23 3:4 Aliens choose to observe us quietly Even though UFO reports are generally lame, we cannot discount the possibility that we are being observed. Immaturity Hypothesis We must reach maturity before the aliens make themselves known. – –Prime Directive—Roddenberry, ; – –Codex Galactica—Newman & Sagan, Zoo Hypothesis We are oddities worth watching for obscure reasons. Quarantine Hypothesis Something is wrong with us? Is it our aggressive nature?
24 3:4 Aliens choose to observe us quietly Have astronomers spotted mystery spacecraft? We have filled our sky with countless bits of space debris. Nearly all the pieces of debris are in orbit around the Earth. What about secret military spacecraft? The military is interested in bombing humans, not in creating space probes to explore the solar system. We can expect that secret military debris would only be orbiting the Earth, and not the Earth-Moon system. There is no supportable reason to envision that the military would create weaponry that would escape the Earth-Moon gravity, to enter orbit around the Sun.
25 3:4 Aliens choose to observe us quietly There are a few threads of evidence consistent with our being watched. J002E3 has a strange, temporary geocentric orbit. It is a discarded Saturn V stage from Apollo 12. 6Q0B44E orbits the Earth-Moon system every 80 days. From its brightness, it is a few meters across. Presumably artificial junk from Earth research. 1991VG has a heliocentric, near-Earth orbit, passing us every 16 years. It is ~9-19 m across. In 1991, it approached within km of the Earth. Its unstable orbit argues it is a recently captured (lunar?) meteoroid. Glinting brightness variations suggest it is metallic, but its orbit does not match with any space missions. Steel (1995) notes that: P natural + P space junk + P alien =100% If both P natural and P space junk are very small, then P alien is close to 100%!
26 3:4 Aliens choose to observe us quietly The problem with all these hypotheses (of aliens watching us quietly) is that galactic shear would move us to new stellar systems in a few million years; enforcement of a “DO-NOT-CONTACT-EARTHLINGS” rule would require cooperation at a galactic scale. In other words, if you think we are being quietly observed by aliens… …the fact that we have not seen them suggests there must be a galactic federation of allied planetary civilizations! It starts to sound like, “the fact that we have never seen leprechauns, proves that they exist and are magical
27 4:4 Deadly Probes scenario Suppose there were bad von Neumann probes in the galaxy… – –Created by a xenophobic race? – –Created to terraform planets, but disregarding the natives? – –Arising from imperfect probe replication, resulting in the evolution of a hostile strain? – –Arising from an unanticipated technological singularity? The new programming… To explore strange new worlds; To seek out new life and new civilizations; To boldly go where no probe has gone before… …then replicate massive attack forces, and sterilize it all. In response, civilizations would tend to be quiet (hide) and cautious about making contact with other civilizations. Contact beacons would be hidden or nonexistent.
28 The Deadly Probes scenario explains all the observations we have made so far. – –The galaxy is in equilibrium—it has been fully explored by previous aliens, and especially Deadly Probes. – –We are certainly not the first race to explore the galaxy; in compliance with the Copernican Principle, it has already been explored by aliens—including the Deadly Probes. – –The galaxy is filled with aliens—but they are not communicating with us because they are hiding from the Deadly Probes. Hence the Fermi Paradox and Great Silence. – –Problems with other hypotheses (such as unlikely 100% efficiencies for social changes) do not plague this scenario—it only had to happen once. The Deadly Probes scenario makes a prediction! The Deadly Probes Scenario will probably play out in our lifetime—we have contacted about 2500 star systems within 50 LY! These star systems need not be inhabited…they only need to have Deadly Probes, which are about to descend upon us, and stop us in our tracks! 4:4 Deadly Probes scenario
29 A semester in review Astrobiology – –The building blocks of life occur throughout the galaxy. – –Life evolved with great speed on Earth. – –Primitive life may occur elsewhere in our solar system. – –Primitive life seems likely at many sites in our galaxy. – –It is quite possible that we are not alone. – –It is quite possible that we are the only life in the galaxy. – –Might we become members of a Galactic Union? – –Are we destined to be explorers of the ruins of prior civilizations? – –Are we fated, by probability, to destroy ourselves? – –Against all odds, are we the first technological species to develop? – –Are, even now, alien von Neumann probes such as 6Q0B44E and 1991VG preparing to overwhelm us with superior numbers and technology? Any of those possibilities is profound.