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The Great Alien Debate (Part 1)

The views expressed are those of the author and are not necessarily those of Scientific American.

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This post is one in a series covering, and expanding on, topics in the book The Copernicus Complex (Scientific American/FSG).






The conversation usually goes like this:

Do you think we’re alone in the universe?

Answer A) :

No, absolutely not. It’s a huge universe, we’re not at the center or central in any way and it would be the height of vanity to think humans or Earth are in any way special or significant.

Answer B):

We might be. There’s never been any firm evidence of extraterrestrial life, and our galaxy is old enough that intelligent civilizations should have spread everywhere by now.

I’ve lost track of how many times I’ve had this exchange, with scientists, with family, with random people at pubs and (once) on a heavily forested hillside in Norway with a pair of slightly suspicious looking hikers who had probably just hidden a body somewhere. But the fascinating thing is how we tend to fall into either camp A or camp B, and how strongly we feel about our answers.

There are some caveats. The ‘we’ in the original question is a loaded word. It can be taken to mean ‘technologically intelligent life’ (i.e. as modern humans like to think they are), or it can be taken to mean ‘slime’ – the single-celled microbial life that represents, and has always represented, the bulk of living matter on Earth. If the definition of ‘we’ gets refined using these categories the answers can change a bit. In fact both A and B responses tend to converge to a tepid middle-ground, along the lines of saying that there could be lots of microbial-type life in the cosmos, where it sits slime-like under a favorite rock rather than building pan-galactic empires, while more complex life is either very rare (as in the so-called Rare Earth hypothesis) or never makes it very far into interstellar space (part of an idea called the Great Filter).

This is an awfully unsatisfactory state of affairs, a set of answers that are enormously influenced by our interpretation of events here on Earth. The impasse would be broken if we could detect life with an independent origin elsewhere – either in the solar system or farther beyond – yet that’s a challenge that remains unmet.

It may not stay that way for much longer, between our exploration of Mars and our ambitions for exploring places like comets and icy moons, we really do seem to be getting closer to examining local possibilities for life. And with a stunning array of exoplanet detections, and near-term cataloging of all the best neighboring targets with upcoming missions like TESS, we should be able to apply the next generation of space and ground based ‘super observatories‘ to make crude measurements of the properties of a few potential Earth-analogs. But this is an optimistic overview. In all of these examples, the non-detection of life (whether as fossils or as chemical signatures) is unlikely to eliminate the possibility of life in these places – we simply won’t be able to be that thorough.

So, unfortunately, in ten years time we’re quite likely to still be having the A vs B conversation. On the face of it the compromise solution – that microbial-type life may be common, but life like us isn’t – seems like a decent answer. However, there’s a catch.

Most of this argument hinges on the idea that Earth’s complex-celled, multi-cellular life (everything from nematodes to sheep) only exists because of a sequence of very specific, but low probability, events – including the way and location in which the Earth formed (with water, with plate tectonics), to the presence of a large moon (keeping our spin axis from varying too much), to a chance merger of two equally simple, single-celled, organisms giving rise to complex, eukaryotic life 2 billion years ago. Thus the odds of a planet making creatures like us is vanishingly unlikely, and so this simply can’t have happened in many other places, even in a universe of hundreds of billions of galaxies and nearly 14 billion years old.

Except this is a very specific interpretation of the facts, after the events (a post-hoc analysis). To use an analogy, imagine that you’re woken up one morning by the telephone ringing. You answer and it’s a distant cousin just calling to let you know their new number. Later, as you walk to work, a bus honks at traffic and you glance up to see some of the digits of that phone number on its side. At lunchtime a torn piece of newspaper gets stuck on your foot, with the headline that the national lottery has a record prize draw. Back at work a colleague insists that you participate in a meeting where the word ‘prize’ gets used extensively. On your way home you stop at a newspaper stand and decide to buy a lottery ticket. The next morning you discover that you’ve won the huge jackpot!

What do you make of this? Your natural instinct is to look back at the events of the previous day, marveling at how, step-by-step, you were led to this point by a series of unlikely events. Taken altogether, you reason, this was incredibly improbable, it’s as if the cosmos has singled you out to win!

Yet this isn’t true at all. Someone, somewhere, was going to win the lottery draw. And whoever they were, in whatever circumstances, they would be having the same thoughts. The events of the previous day, week, or year would all take on new meaning in light of the outcome. There would be numbers that they’d seen, choices made, steps taken, random occurrences that appeared to lead up to this point.

Of course some of these events were necessary, but they would also be entirely different if another person had won. The point being that it would be hugely irrational to claim that the chain of events that led to you winning was the only way this could have happened.

All of which circles back to the idea of the ‘rarity’ of life’s trajectory here on Earth. The fact is that our perspective is not unlike the lottery winner’s. It’s easy to look back at 4 billion years of bio-geo-chemical evolution and say that organisms like us were a highly improbable outcome. But in truth we don’t know whether or not that’s actually true, and we can’t know because we have no information about how things have played out across billions of other worlds across the galaxy. In a sense, we have no idea how many other winning lottery tickets are out there in the cosmos, or how they were picked.

And that raises a new question (one that I will tackle in a later post, Part 2), about why we have no idea, and how complex, even technological, life elsewhere could have remained unseen to this point…

Caleb A. Scharf About the Author: Caleb Scharf is the director of Columbia University's multidisciplinary Astrobiology Center. He has worked in the fields of observational cosmology, X-ray astronomy, and more recently exoplanetary science. His books include Gravity's Engines (2012) and The Copernicus Complex (2014) (both from Scientific American / Farrar, Straus and Giroux.) Follow on Twitter @caleb_scharf.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. jtdwyer 3:27 pm 08/26/2014

    IMO, a more pertinent question is: are we alone in the universe – at this time? We’re looking through a very small window…

    Link to this
  2. 2. M Tucker 4:25 pm 08/26/2014

    Where and how did life begin? We have no idea.
    Did it begin on a planet? We don’t know.
    Did it begin in space? We don’t know.
    Did it begin on Earth or was it seeded here? We don’t know.
    How rare is life, microbial life, in the galaxy? We have no idea?
    Once established does life always tend toward intelligent creatures that can explore the fundamental forces of nature and the solar system they live in? We have no idea.
    All we can do is ask questions and have fun guessing at the answers.

    Link to this
  3. 3. GuillaumePascal 4:48 pm 08/26/2014

    No it’s oke !
    I’m GuillaumePascal .

    Link to this
  4. 4. GuillaumePascal 5:40 pm 08/26/2014

    We need to go back to the basics .
    I believe though, that the human mind is a universal quantity !
    What tangibly makes live possible on earth, excist in fact, all over in the universe !
    Light is the main factor in the excistence of live !
    It comes in all colors, and in every different way, on earth !
    In all fauna and in flora, light is the blessing to live and much moreover !
    What ever is alive out there will feed off ‘light’ probably in a different way like it happens here on earth for sure !
    Surely there are other similar earths in our universe similar to ours !
    But I bet those beings can’t travel through space just like we phisically can’t !
    The mind though is a universal quantity !
    All abilities we have in our mind and dream of having !
    Psychic abilities, going back and forth into the past and future !
    Being invisible, chance appearance radically like an octopus !
    If one can visualize it in one’s mind, it’s out there !
    I believe there is life in a different way !
    They could have all those abillities, in a very very supurb and sophisticated developed way !
    A matter of fact is, knowing time and space as we know it now;
    If there is live out there ?
    It had the time and space to develop itself, through trillion and trillion of light years of time !
    I cannot visualize what it looks like, except for how it shows itself !
    I recognize in all I see about alien life and extraterrestial life that there’s a simple mind behind it all !

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  5. 5. SciPhil 7:35 pm 08/26/2014

    When we discover life on another planet, it will be very interesting as to the form it takes and how it has evolved.

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  6. 6. RussAbbott 11:18 pm 08/26/2014

    Your example of the lottery winner makes the opposite point to the one you want to make. No matter who wins (or where life exists) if winning the lottery is analogous to life existing, it’s still a rare event. As you point out, the argument for it being rare does not hold. But if the analogy holds the failure of the argument doesn’t make it less rare — in either case.

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  7. 7. roj2003 4:10 am 08/27/2014

    Our own ‘window’ for either hearing from or listening to anyone has lasted for just 130 years in our ~200,000 years existence. The current direction/extent of our own expectancy MIGHT be no more than another 130 years before it collapses, for which the probability is quite high.
    If there is anyone else out there, they too have probably had the same or similar windows & expectancies. For their and our windows to coincide, probabilities are quite low.
    The more we search, the better the probability becomes.

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  8. 8. Caleb A. Scharf in reply to Caleb A. Scharf 6:18 am 08/27/2014

    In response to the point about the lottery analogy still making life rare – I don’t think this is the case. As I say in the post, for life in the cosmos we don’t know how many lottery tickets have won (or what the odds of winning are) – it could be lots of ‘winners’, but each winner (if they have our level of knowledge) will still interpret the chain of events leading to them as being potentially very low probability.

    Link to this
  9. 9. Abas2045 7:19 am 08/27/2014

    We don’t know
    we are dying to know!
    inst it fun

    Link to this
  10. 10. TorbjornLarssonOM 3:16 pm 08/27/2014

    Re the time horizon, Sara Seager is on record predicting that we will have a statistical answer, if life is common, within 1-2 decades given the next 2 generation space telescopes. That is, we can’t say for certain on any specific planet because the data is too uncertain, but we can have sufficiently low uncertainty on an aggregated set that we can say on a group of systems that they will contains some percentage of inhabited planets.

    I take a very dim view re the “Rare Earth” hypothesis. It is an open-ended (goalpost) moving bayesian model, where you can arrange to get any result you want.

    The Great Filter is mapping the static RE hypothesis to a dynamic, time-dependent, model by way of the Fermi Question. But apart from the same problem of non-constraint, the Fermi Question (not to confuse with his “Fermi questions” approach) is not a paradox, hence the original name. The existence of unconstrained false negatives makes a vast space of positives impossible to test.

    [ E.g. the galaxy can be densely colonized, but the easiest long-range colonization, of comets in the Oort clouds of stars, is likely silent. Such biospheres diverge fast culturally and evolutionary, so contact are lost quickly. And they have no interest of costly and risky descents into huge gravity wells (planets), even if they remember what they were and can reinvent (doubtful) descent/ascent technologies.]

    Chemical and biological evolution are both processes that aren’t finetuned, as few natural processes are. These processes produce results on a massive scale. Hence life should be common.

    On the other hand, biological evolution displays vast contingency and hence diversity. Some traits are convergent (the ovoid form of many free swimming water animals, say), most are not. Like the Elephantid trunk, language capable intelligence evolved once in 0.5 billion years of land life. Hence they are rare.

    Both of these hypotheses are standard biology. Why they are not standard astrobiology hypotheses is anybody’s guess. Too little biology studies?

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  11. 11. TorbjornLarssonOM 3:32 pm 08/27/2014

    @M Tucker: We have many ideas where and how life began. The problem is to pare them down with testing, but it is possible and is ongoing.

    Trait phylogeny say that alkaline hydrothermal emergence of life is much more likely than temperate ocean emergence, which in turn is more likely than the rest of the theories. E.g. they both have the same element ratios as modern cells and the same non-equilibrium chemistry (from redox and heat vs light and heat), but Hadean/Archean AHE had the same cellular compartments, membranes, pH differential, pH size, redox potentials and electron bifurcating atom “metabolic engines” as modern cells.

    I find it amazing that traditionally biological methods seem to be able to pinpoint the emergence locale of Earth. If it stands up to testing, that is…

    We do know that evolution has no “trend” and that most life is still simple as the article notes, biology 101 if you care to study it. Apart from the “diffusion” away from a simple ancestor (e.g. expect some more complicated forms as well).

    @RussAbbott: Winners are not rare, there is at least one every lottery. (Or the lottery is usually criminal.) =D

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  12. 12. Heteromeles 4:36 pm 08/27/2014

    The funny thing is that Earth apparently is already getting affected by a form of the Great Filter

    Our radio broadcasts are getting quieter, and pretty soon, even without catastrophes, it’s likely that no one listening on even the nearest stars would hear our radio broadcasts at all.

    Here’s the thing: any non-directional radiation attenuates as the square of the distance traveled, since it has to fill the surface of an expanding sphere. Now, when you take a radio station that normally reaches perhaps 10 or 100 miles, and attenuate it across a sphere, say, 10 light-years across, that’s a *very* weak broadcast.

    A dew decades ago, there were loud stations, like the bandit stations in Tijuana broadcasting into the US. Problem is, that kind of loud broadcasting takes a lot of energy, and not only is energy getting more expensive, engineers are getting a more efficient at getting the equivalent broadcast (say, via the internet) with less energy. The side effect of this totally rational move towards efficiency is that we’re going silent to the stars around us.

    While this isn’t the same as, say, searching for a world with an oxygen atmosphere, it’s worth noting that, in the 4.5 billion year history of the Earth to date, there will be perhaps a 100 year window when someone doing a SETI-type radio search for us would hear any of our radio broadcasts. We have to remember that there’s a big difference between being searchable (especially with radio telescopes) and being alive, and our visibility in the radio spectrum depends in large part on our own inefficiencies.

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  13. 13. jayjacobus 5:24 pm 08/27/2014

    One cannot do a statistical analysis on one observation. So, the problem only has non-analytical solutions.

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  14. 14. Jerzy v. 3.0. 6:45 am 08/28/2014

    Humans are searching for Aliens in the wrong way.

    Detecting efforts up to now were based on simplistic assumptions, for example that that Aliens communicate using radio like humans in 20. century (commenters before pointed that it is unlikely), or that exist enormous Star Trek style civilizations producing gigantic bursts of energy.

    Meanwhile, many observations were ignored. For example, chemical signature of upper atmosphere of Venus shows abnormalities consistent with presence of microbes. It is not investigated, because these wouldn’t fit precisely into biochemistry of microbes on Earth (Venus atmosphere lacks H2O and has excess of CO2, while opposite is true on Earth).

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  15. 15. SteveO 12:56 pm 08/28/2014


    Depends on what you are testing for. We can certainly rule out certain things analytically, or find things analytically that support the existence of life.

    Even if some day we have some number of technological civilizations that we learn about, I am not sure that would help statistically either. I think each one is likely to be like the lottery analogy – some crazy unlikely history for each one that got it to where it ended up.

    Just like each human! ;-)

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  16. 16. jayjacobus 5:26 pm 08/28/2014

    It sounds like we can start with a premise and follow that premise to some conclusions. But if we don’t know if the premise is true, can we say that we are being analytical.

    Isn’t that somewhat like religious thinking?

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  17. 17. Dr. Strangelove 5:02 am 08/29/2014


    We have one sample – Earth. We can infer the probability from this sample. Probability of planets in Goldilock’s zone = we have 8 planets, 1 in Goldilock’s zone = 1/8

    Bacteria evolved after a billion years. Probability of simple life = 1/1 billion years

    Probability of intelligent life after simple emerged = 1 species/total no. of species in earth’s history

    We can debate whether other planets will follow earth’s history but this is the best and only information we have. This is real data. Other histories are hypothetical.

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  18. 18. Postman1 11:16 am 08/29/2014

    Dr. Strangelove, I like your reasoning, but doesn’t Venus also fall within the Goldilocks zone? Mars might too, if it were larger, with a thicker atmosphere. I like the odds of 3/8 much better.

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  19. 19. And Then What? 9:21 pm 08/29/2014

    Our one-armed bespectacled Dr.S. correctly points out we have only one known example from which to draw conclusions, but beware ye all of Statisticians bearing conclusions based on statistical analysis alone. From my time playing in the field of Statistical analysis I learned one important lesson. Statistics like Humans can be forced to lie by clever tricks of formulation and/or assumption.
    The conclusions drawn from such lies then crumble under the weight of even minor scrutiny.
    The one example I remember went something like this: 33% of all women attending a certain college married their professors. The fact was that only 3 women were in the sample and one married a professor. so on the surface the results were at the same time both correct and useless for predictive purposes.

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  20. 20. largo1 3:16 pm 08/30/2014

    The basis for this question is spurious to those of us who have seen UFOs. When you have a self-validated (do not expect support from many ‘responsible’ parties) exposure to this phenomena your paradigm shifts.
    Quite literally, all of what you have known becomes as dross. One portion is that the ‘learned’ are as wise as their fellow traveler credentialers portray them as.
    Trash those thoughts. They have opinions based on cleverly engineered theories which repel intrusion of reality.
    I firmly suggest that you take a truly scientific viewpoint when anyone (even me) states a ‘truth’.
    We had already received a wholly anomalous encoded transmission/message from an extraterrestrial source at a fully sanctioned and vetted observatory. It was decided to NOT record this as genuine because it was not repeated.
    As the American-Indians (Sioux?) said, “All you need is one white crow to prove that all crows are not black.”
    Also the Universe does not normally care what you think as to what it shall do. You can alter that by conceding to it’s mastery and pressing along the channel of it’s movement. To press on with non-reality based conclusions is at best harmful. Things must go wrong if you keep citing self reinforced stupidity as your basis of reasoning.

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  21. 21. runner01 9:35 pm 08/30/2014

    Jupiter’s moon,Europa has sparked the interest of astrobiologists and and other scientists. They think that an ocean of water exist under Europa’s icy surface. And living organisms may live there. This type of life could be similar to ones that live at the bottom of oceans on earth, These life forms live without sunlight. They are warmed by hydrothermal vents.

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  22. 22. Dr. Strangelove 9:43 pm 08/31/2014

    Goldilock’s zone must include favorable atmosphere, not full of greenhouse gases. Mars and Venus will not qualify. Mars has virtually no atmosphere. Venus atmosphere is 96% CO2.

    @then what?
    3 samples may be representative of the population if the standard deviation is low. At SD close to zero, 1 sample is enough. There are many critics of Bayesian inference. They argue a few samples are not enough because they assume SD is high. Actually SD is unknown.

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  23. 23. And Then What? 7:50 pm 09/1/2014

    @ Dr.S.
    Inherent in the acceptance of SI as a useful tool is the assumption of uniformity of random external influence also I would argue that any apriori assumptions drawn may be incorrect as the sample size, when dealing with something as large as the entire Universe, may be so insignificantly minute that any conclusions drawn would be useless for predictive purposes. SI may be useful for “local” predictions but stretching out to some unknown extent is tricky at best. As my son is so fond of saying “it is what it is”. I always keep in mind that tools change as conditions requiring their use change. Although I was always top of the class in SI I still wouldn’t bet my life on some of my predictions based on it, but it was nice talking to you anyway even if we don’t always agree.

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  24. 24. Dr. Strangelove 9:58 pm 09/1/2014

    I’m not arguing Bayesian inference is certain. I’m arguing the position that it is useless is false. If you say you played a game of chance twice and won once. Without additional information, I will infer you probably played a toss coin rather than a lottery. I’m not certain my inference is correct but chance is on my side.

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  25. 25. And Then What? 9:52 am 09/2/2014

    @ Dr.S.
    Forgive me if I gave the impression that I believed Bayesian inference to be useless. I just think that it may not be the correct tool to to use to provide conclusions that I would have a high degree of confidence in when dealing with something as large and unknown as the Universe itself. Your conclusion that I probably played a coin toss versus the lottery would be valid if those were my only 2 choices.

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  26. 26. SciChallenger 3:27 pm 09/3/2014

    Recently published the article “Dyson Spheres” as an Alternative to the Dark Matter Explanation of Hidden Masses in Galaxies” that states that effects of hidden mass in galaxies can easily be explained if “Dyson Spheres” are real.

    The “Dyson Spheres” produce gravity but do not produce radiation. In this case the dark matter is not required at all. It is also interesting push in favor of existence of extra-terrestrial civilizations. Even if this hypothesis appears weird, in accordance to Occam razor it is more preferable than introduction of new type of matter. Quoting that article:
    “It also explain the puzzle “why we do not see the activity of hypothetic extra-terrestrial civilizations in our galaxy?” The answer may be very simple – This is because they live inside the Dyson Spheres and are invisible to external observers.”

    I cannot put all details here. If you wish you can read it on , #1321 in Astronomy. (It was also published in “IntellectualArchive” journal).

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  27. 27. QuantumTimeGuy 3:35 pm 09/9/2014

    From the article “Taken altogether, you reason, this was incredibly improbable, it’s as if the cosmos has singled you out to win!
    Yet this isn’t true at all.”
    There is not sufficient evidence to rule out “the Cosmos has singled you out to win!” hypothesis. Saying “this isn’t true all” is inaccurate. In fact if these same “good luck omens” were to occur the day of the next lottery draw, I would recommend that the lottery winner buy another ticket, since the reward is some millions of times larger than the cost, if the experiment works – bonus, if the ticket is not a winner perhaps it will diminish your confidence in “the Cosmos singled me out” hypothesis.
    That an idea is ridiculous does not make it wrong or not true.

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  28. 28. hkraznodar 4:24 pm 09/9/2014

    @largo1; If I throw a Frisbee and you see it but don’t know what it is, that is an UFO. If you mean “spaceship from another planet” then say “spaceship from another planet”.

    I strongly doubt that you observed anything even remotely like an alien spacecraft. You most likely observed something that you were unable to identify. The human mind makes things up all the time and not always in ways that our conscious minds would like. I don’t know what you saw and you don’t either or you wouldn’t have used the term UFO.

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  29. 29. hkraznodar 4:35 pm 09/9/2014

    My answer is; I neither know or care but I assure you that eventually we won’t be if we survive long enough to engage in interstellar travel. While most stars are M class and thus can’t have truly Earth like planets, there are a few like our star that might.

    We have an innate desire to expand and explore and if civilization survives long enough we will move off world to some degree. From local orbit to interplanetary and then interstellar we will expand.

    When we reach other stars I doubt that we will find ready made worlds. More likely we will have to build them by adjusting planetoid orbits to change planetary orbits or add mass. Then we will likely have to adjust water and atmosphere be relocating comets to the new planet’s surface. After that the simple life must be added and then higher forms. Each world may take thousands of years to prepare.

    If the human race survives to expand off of the Earth this is inevitable. Manifest destiny or something similar.

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  30. 30. LimitedUbiquity 5:36 pm 09/18/2014

    I woke up this morning realizing that I am an average sentient being on a life supporting planet in an otherwise unremarkable solar system located in a vast galaxy that is surrounded by an incomprehensibly large universe. Good luck to any other alien beings out there. BTW… I just purchased The Copernicus Complex. Can’t wait to read it.

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