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Colonization and Panspermia If we assume, as discussed, that there there have been no visits of extraterrestrials to earth then the following argument has been proposed: 1) An advanced civilization would migrate from star to star, colonizing as it went, and would quickly colonize the galaxy 2) We observe no visits from extraterrestrial civilizations. 3) Therefore advanced extraterrestrial civilizations do not exist. Here I will analyse this argument. 1) How easy is travel between stars? Current human technology: The Voyager space craft, launched 1977 are traveling at about 16,000 m/s ≈ 0.5 x 10-4 c 3x108m/s The nearest star is Proxima Centauri, about 4 light years away. Therefore it would take the Voyagers about 4/ 0.5 x 10-4 = 80,000 years to reach it What is physically possible for interstellar travel? There is a universal ‘speed limit’ of c=3 x 108 m/s. Near c, the energy cost of acceleration grows as mc2(1-v2 /c2)-1/2 so travel at exactly c is impossible (requires infinite energy). Engineering estimates suggest a practical upper limit of about c/10 at enormous expense. How would a civilization migrate? Not known, but one possibility is that is would spread the way species spread through habitats on the surface of the earth. Biologists have studied the species migration problem mathematically. Models are complex but they are elaborations of a simple diffusion model which I will use to illustrate the possibilities. Diffusion: Walk on a square lattice: Choose a direction at random each hop time τhop and take a step of length l. On average, the distance from the origin R increases as (<R2>)1/2 = (t/ τhop )1/2l Estimate filling time: l = R/Ngal1/3 τfill = τhop(R/l)2=τhopNgal2/3 What’s τhop ? Distance between stars is of order a few light years. At a maximum imaginable speed of c/10, this gives a minimum time of order 10 years. Human technology now gives times of about 105 years so 10< τhop < 105 years With Ngal = 1011 this gives 2x108< τfill < 2x1011 years The upper limit exceeds the age of the universe. How should we interpret this? The likelihood of seeing another civilization nearby is proportional to the local density of civilizations The likelihood of seeing another civilization nearby is proportional to the local density of civilizations nciv . This is (determined by a balance of birth and death rates: ngal/τb – nciv/τd- nciv/τfill=0 if τd << τfill then nciv =(τd/τb)ngal (Drake equation) and if τd >> τfill then the diffusion (colonization) rate determines the local density. Forms of the Drake Equation (Appendix 1.1) From SETI: N = R* • fp • ne • fl • fi • fc • L Where, N = The number of civilizations in The Milky Way Galaxy whose electromagnetic emissions are detectable. R* =The rate of formation of stars suitable for the development of intelligent life. fp = The fraction of those stars with planetary systems. ne = The number of planets, per solar system, with an environment suitable for life. fl = The fraction of suitable planets on which life actually appears. fi = The fraction of life bearing planets on which intelligent life emerges. fc = The fraction of civilizations that develop a technology that releases detectable signs of their existence into space. L = The length of time such civilizations release detectable signals into space. From Hart: Nciv=Ngalfstarfplanetflife Here: Nciv =(τd/ τb)Ngal The f’s in the last part of the SETI form have a slightly different meaning In Drake’s form: flfifc is the probability that life will ever appear on the planet during the stars lifetime Correspondences are flife= flfifc(τd/τstar) , L=τd Ngalfstar=R*τstar fplanet=fpne fstarfplanetflife=τd/τb Conclusion: (within a diffusion model) For civilizations with lifetimes more than 100 million years, we may be more likely to see them because they drift in (colonization) than because they appeared and grew in their original environment. Thus the argument that they haven’t colonized us therefore they don’t exist could only apply to very long lived civilizations (> 100 million years). Results of a more detailed model (Appendix 6.2) kR α (τhop /τd )1/2 Ngal1/3 Other models for colonization dynamics: Nonlinear diffusion models. For example article by Jones in the book 'Extraterrestrials Where are They? (reference in the book). Gives similar filling times. Self avoiding random walk. This is like the diffusion model but the walker never steps where he has already been. The result is that the distance from the starting point is, on average <R2>=(t/τhop)6/5 l2 (6/5 replaces 1 in the diffusion model.) This makes quite a difference in the filling times. As you might expect, the filling occurs faster With the self avoiding diffusion model. Nevertheless, the filling times are at least 10 million years, and by the same argument as before, only the local density of civilizations living longer than this will be affected by colonization and migration. The hop times only take account of travel times. In fact it is likely that the need to settle around a new star will greatly increase the minimum hop time and therefore the minimum fill time. Correspondingly, the minimum lifetime of civilizations excluded by the argument will be increased by this effect. So how likely is it that civilizations would last 1 million years or more? Well of course we don’t know but there is an argument which suggests that the human example may not be very encouraging. Consider two events: Human civilization continues for sometime τd longer than it has lived so far and then dies. Call this event H. Human civilization survives sometime between its present age, call it t, and its beginning. Call this event E. This event has occurred. Now consider these probabilities: P(E), the probability of E P(H) the probability of H P(H|E) the probability of H given E P(E|H) the probability of E given H P(E,H) the probability of E AND H For any pair of events these are related by P(E,H)= P(E|H) P(H)= P(H|E)P(E) H: Human civilization continues for sometime τd longer than it has lived so far and then dies. E: Human civilization survives sometime between its present age, call it t, and its beginning. P(E,H)= P(E|H)P(H)= P(H|E)P(E) What we want to know is P(H|E). Solve for it P(H|E)= P(E|H)(P(H)/P(E)) Clearly P(H)<P(E). If there is NOTHING SPECIAL ABOUT THE PRESENT P(E|H)=t/τd So P(H|E)< t/ τd P(H|E)< t/ τd Now the number we assign to t depends on what we mean by ‘civilization’. The SETI people, who are looking for Electromagnetic signals from a civilization, Would say that our ‘electromagnetic lifetime’ Is about 100 years. (We’ve been sending Out radio signals that long.) If we put t=100years then we get P(H|E)< 100years/ τd So the probability of surviving 10,000 years is less than 1% and the probability of surviving 1 million years is less than 1 part in 10,000 (.01%) A few comments about this result: It refers to human civilization and makes no specific statement about other civilizations. However, since human civilization is the only one we know, assumptions about others which are inconsistent with what we know about humans are questionable. There is one other perspective which strengthens the plausibility of the result: If our civilization is to live 1 million years with probability 10-4 or greater then it is easy to show that the probability per year of civilization destruction must be about 10-5 or less. Ln(1-ε)N ≈ -Nε = ln 10-4 =-4ln 10; ε = 4 ln 10/N Can we assume that annihilation probabilities are as small as that? This is hard to quantify but one can make a start. Some annihilation risks that have been discussed include Impact by a near earth object (asteroid) see http://neo.jpl.nasa.gov/risk/ . For example object 2008 AF4 is given a probability of about 4.3 x 10-5 of impacting the earth during the next century (most probably in 2089). That would be about 4.3 x 10-7/yr from this object. Another object, 2007 VK184 is given a probability of 3.4e-04 (that would be in 2048) or 3.4 x 10-6/yr . These are the objects currently listed as most hazardous. But asteroid impact probability alone seems to bring us to within the range suggested by the Carter argument. The asteroid 2007 VK184 which is given about one chance in 3000 of impacting the earth probably in 2048. Its diameter is about 0.13km and its mass is 3.3x$10^9$ kg. If it hits the earth, the estimated impact velocity would be 19.19 km/s. There are numerous other ways in which human civilization could die. For an example of a discussion see Existential Risks: Analyzing Human Extinction Scenarios and Related Hazards Nick Bostrom, Journal of Evolution and Technology, Vol. 9, March 2002. Assessing quantitative probabilities of most of these is very difficult, but they can only increase the likelihood of extinction. Conclusion: The human example gives no grounds for optimism concerning the likelihood that extraterrestrial civilizations will live 1 million years or more. Panspermia: Here we are concerned with the transport of presumably ‘unintelligent’ or ‘uncivilized’ life. A similar Hart-like argument could be made in this case. It would have the form 1) Simple forms of life can easily and quickly (on the relevant time scales) move from star to star 2) Simple forms of life have not arrived on earth in this way. 3) Therefore simple forms of life do not exist elsewhere in the galaxy. Actually this argument is not made very often in this way. One reason is historical: Arrenhius first proposed panspermia as a way to understand the prebiotic evolution problem in the late 19th century. There are at least two problems with the proposal that life started on earth by transport of microorganisms to earth on meteorites. It does not solve the prebiotic evolution problem: life still has to get started somewhere within less than 14 billion years and It is not clear if microorgansims can survive the trip on a meteorite. This also bears on point 1) of the Hart like argument in the preceding slide. Another reason that the Hart-like argument is not usually made for panspermia is because it is less clear in the case of microorganisms that point 2 is correct. In fact scientists continue to suggest that it has occurred or will occur. Now we consider these issues for panspermia one at a time. Can microorganisms easily move from planet to planet or from star to star? When meteorites hit the earth they can knock bits of the earth’s surface off and accelerate them to escape velocity. It is conceivable that such bits will carry microobes with them. So a plausible mechanism exists for launching microobes on meteorites into space from earth by natural (nonhuman) processes. The same processes could occur on other planets on which microorganisms existed. There is strong evidence that meteorites get here from Mars in that way. (They compare the isotopes found in the meteorites with the distribution of isotopes found on Mars.) In 1996 there was a report that structures found in those meteorites appeared to be fossils of organisms. (picture on next slide) However subsequent work has shown that these structures are probably the result of nonorganic crystallization processes. We conclude that transport of meteorites between planets (and possibly also between stars) is possible. If these meteorites were to carry microbes, would the microbes survive the trip? Not if they were ordinary organisms. The radiation levels in space are about 6 times higher than they are on the earth’s surface and most organisms would be killed by them on a trip of more than a few years. However, recently microbes have been discovered on earth which can survive much higher radiation levels. A few things about radioactivity. It arises from changes in the structure of nuclei of atoms and the resulting radiation is of three basic types all carrying a lot of energy and capable of damaging living organisms. The biological harm depends on the total energy deposited in the organism (not on the rate at which it is deposited) . The units of radiation dose include the rad = (1/100)joules/kilogram And the rem = Quality factor x rad The quality factor takes account of the different effects of different types of radiation. For relevant types, QF is always larger than 1. On earth the radiation levels are around 1/2rem/yr whereas in the solar system they average around 3 rem/yr. Any life dose above 30rem is judged to be a serious risk for humans, but some extremophile bacteria can tolerate more than 3 million rem. Though no bacteria have yet been found which simultaneously tolerate the extremes of temperature, dessication and temperature which they would encounter in an interstellar journey on an asteroid, it seems possible that some may exist. We conclude that undirected panspermia may be possible and could have occurred at least once on earth. It is unlikely to have occurred often because if if had, we would see more biochemical diversity in the biosphere than we do. How much would undirected panspermia affect the likelihood of finding life on any particular star or planet? We can consider a diffusion model, as before. It is more likely to be a good model here than in the case of civilizations. What hopping times should we use? Relativistic speeds are unlikely. Thus the filling times are likely to exceed the age of the universe. Recall that diffusion is irrelevant if filling times exceed lifetimes. Actually bacteria are ‘immortal' under favorable conditions. We don’t know if their lifetimes can exceed 100 billion years but that is irrelevant because the maximum time available for diffusion is 14 billion years. It is possible that lifetimes of some bacteria might exceed 14 billion years. However that time is less than the estimated filling time, so in any case diffusion is irrelevant to estimation of the likelihood of finding bacteria on a habitable planet. Thus local evolution (that is the Drake equation) will determine the probability. ‘Directed’ panspermia Here one imagines that a civilization, about to die (say because its star is about to supernova or become a red giant) , sends a packet of essential biochemical material (‘seeds’) to a nearby hospitable star. Speeds could be relativistic depending on postulates about the nature of the civilization. Transport would not need to be diffusive because the civilization could ‘aim’ the probe toward one or a few selected targets. Material might be radiation protected. Conclusions concerning directed panspermia: Most of the previous arguments fail (diffusion model). The only evidence against it is the failure to observe biochemical diversity. This puts a limit on the frequency with which it could have occurred here. This appears to be a practically unfalsifiable hypothesis Summary: Colonization: The failure to observe colonization is evidence that civilizations that live more than about 10 million years do not exist. The exact age depends on the model used to describe their transport but relativistic limits in this argument are unlikely to change. Nothing is learned about more short lived civilizations. Other arguments that civilizations will not live that long were given. Panspermia: Undirected panspermia is probably possible and may have occurred once on earth. It is unlikely to be the dominant factor determining the probability of observing life on planets. Directed panspermia is conceivable but appears to be an unfalsifiable hypothesis.