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<title>UFOs: The Extraterrestrial and Other Hypotheses</title>
<meta content="Carl Sagan" name="author"/>
<meta content="UFO's - A Scientific Debate" name="copyright"/>.
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<p><font class="TextBlack">There are many ideas which are charming if true, which would be fun to believe in, which are
  a delight to think about: reincarnation; the philosopher's stone to turn base metals into gold; the search for long or
  possibly indefinitely extended lifetimes; psychokinesis, the ability to foresse the future; telepathy, the ability to
  read somebody's else's mind; time travel; leaving one's body (the literal meaning of ecstasy); becoming one with the
  universe. There is a wide range of concepts which would be fascinating especially if only they were true. But
  precisely because these ideas have charm, exactly because they are of deep emotional significance to us, they are the
  ideas we must examine the most critically. We must consider them with the greatest skepticism, and examine in the
  greatest detail the evidence relevant to them. Where we have an emotional stake in an idea, we are most likely to
  deceive oursleves.</font></p>
<p>The idea of benign or hostile superbeings from other planets visiting the earth clearly belongs in such a list of
  emotion-rich ideas. There are two sorts of possible self-deceptions here: either accepting the idea of
  extraterrestrial visitation in the face of very meager evidence because we want it to be true; or rejecting such an
  idea out of hand, in the absence of sufficient evidence, because we don't want it to be true. Each of these extremes
  is a serious impediment to the study of UFO's; they affect different categories of people. A symposium such as this
  one must spend some time worrying about emotional predisposition.</p>
<p>I want to discuss first the <a href="/science/crypto/ufo/analyse/hypotheses/intelligence/HET">extraterrestrial
  hypothesis</a> of UFO origin, bearing in mind that its assessment depends upon a large number of factors about which
  we know little and a few about which we know literally nothing. What I want to lead up to is some crude numerical
  estimate of the probability that we are frequently visited by extraterrestrial beings.</p>
<p><font class="TextBlack">There is a range of hypotheses which can be examined in such a way. Let me give a simple
  example: consider the Santa Claus hypothesis. This hypothesis maintains that, in a periode of eight hours or so on
  December 24-25 of each year, an outsized elf visits fifty million homes in the United States. This is an interesting
  and widely discussed hypothesis. Some strong emotions ride on it, and it is argued that at least it does no harm. We
  can do some calculations. For example, suppose that the elf in question spends one second per house. This isn't quite
  the usual picture — "Ho Ho Ho" and so on — but I imagine he is terribly efficient, and very speedy; that
  would explain why nobody ever sees him very much. With 10<sup>8</sup> houses he has to spend three years just filling
  stockings. I've assumed he spends no time at all in going from house to house. Even with hyper-relativistic reindeer,
  the time spent in 10<sup>8</sup> houses is three years and not eight hours. This is an example of hypothesis testing
  independent of reindeer propulsion mechanisms or debates on the origins of elves. We examine the hypothesis itself,
  making very straightforward assumptions, and derive a result inconsistent with the hypothesis by many orders of
  magnitude. We would then suggest that the hypothesis is untenable.</font></p>
<p>We can make a similar examination, but with greater uncertainty, of the <a
  href="/science/crypto/ufo/analyse/hypotheses/intelligence/HET">extraterrestrial hypothesis</a> which holds
  that a wide range of unidentified flying objects viewed on the planet Earth are space vehicles from planets to other
  stars. The report rates, at least in recent years, have been several per day at the very least, but I will make the
  much more conservative assumption that one such report per year corresponds to a true interstellar visitation. Let's
  see what this implies. To pursue this subject we have to have some feeling for the number, <em>N</em>, of extant
  technical civilizations in the galaxy <font class="TextBlack">—</font> that is, civilizations vastly in advance
  of our own, civilizations which are able by whatever means to perform interstellar space flight (I will say a word
  about the means later, but the means don't enter into this discussion just as reindeer propulsion mechanisms don't
  affect our discussion of the Santa Claus hypothesis).</p>
<p>An attempt has been made to specify explicitly the factors which enter into a determination of the number of such
  technical civilizations in the galaxy. I will not here run through what numbers have been assigned to the various
  quantities involved <font class="TextBlack">—</font> it's a multiplication of many creases as we proceed down
  this list. <em>N</em> depends, first, on the mean rate at which stars are formed in the galaxy, a number which is
  known reasonably well. It depends on the number of stars which have planets, which is less well known but there are
  some data on that. It depends on the fraction of such <a href="/place/systeme/solaire/planete/index.html">planets</a>
  which are so suitably located with respect to their star that the environment is feasible for the origin of life
  occurs in which, after life has arisen, an intelligent form comes into being. It depends on the fraction of those
  planets in which intelligent forms have arisen which evolve a technical civilization substantially in advance of our
  own. And it depends on the lifetime of the technical civilization. It's clear that we are rapidly running out of
  examples as we go further and further along. That is, we have many stars, but only one instance of the origin of life,
  and only a very limited number <font class="TextBlack">—</font> some would only say one <font class="TextBlack">—</font>
  of instances of the evolution of intelligent beings and technical civilizations on this planet. And we have no cases
  whatever to make a judgment on the mean lifetime of a technical civilization. Nevertheless there is an entertainment
  (which is the way I put it) which some of us have been engaged in, making our best estimates about these numbers, and
  coming out with a value of <em>N</em>. The equation which comes out <span class="note">[I. S. Shklovskii and C. Sagan, Intelligent Life in the Universe (San Francisco: Holden-Day, 1966; New York: Delta Books, 1967]</span>
  is that <em>N</em> roughly equals 1/10 the average lifetime of a technical civilization in years. If we put in a
  number like 10<sup>7</sup> years for the average lifetime of advanced technical civilizations, we come out with a
  number for such technical civilizations in the galaxy of about a million: that is, a million other stars with planets
  on which today there are such advanced civilizations. Now I think you will recognize that this is quite a difficult
  calculation to do accurately and moreover that the choice of 10<sup>7</sup> years for the lifetime of a technical
  civilization is rather optimistic. But let's take these optimistic numbers and see where they lead us.</p>
<p>Let's assume that each of these million technical civilizations launch <em>Q</em> interstellar space vehicles a year;
  thus, 10<sup>6</sup><em>Q</em> insterstellar space vehicles are launched per year. Let's assume that there's only one
  contact made per journey. In the steady-state situation there are something like 10<sup>6</sup><em>Q</em> arrivals
  somewhere or other per year. Now there surely are something like 10<sup>10</sup> interesting places in the galaxy to
  visit (we have several times 10<sup>11</sup> stars) and therefore at least 10<sup>-4</sup><em>Q </em>arrivals at a
  given interesting place, let's say a planet, per year. So if only one UFO is to visit the earth each year, we can
  calculate what mean launch rate is required at each of these million worlds. The number turns out to be 10,000
  launches per year per civilization ; and 10<sup>10</sup> launches in the galaxy per year. This seems excessive. Even
  if we imagine a civilization very much further advanced that ourselves (I'll mention in a minute that it's a
  considerable undertaking to travel effortlessly between the stars), to launch 10,000 such vehicles for only one to
  appear here is probably asking too much. And if we were more pessimistic on the lifetime of advanced civilizations we
  would require a proportionately larger launch rate. But as the lifetime decreases, the probability that a civilization
  would develop interstellar flight very likely decreases as well.</p>
<p>There is a related point made by Hong-Yee Chiu <span class="source">[H.-Y. Chiu, book review, "The Condon Report, Scientific Study of Unidentified Flying Objects", <em>Icarus</em> 11 (1970): 447]</span>;
  he begins with more than one UFO arriving at Earth per year, but his argument follows the same lines as the one I have
  just presented. He calculates the total mass of metals involved in all of these space vehicles during the history of
  the galaxy. The vehicle has to be of some size <font class="TextBlack">—</font> it should be bigger than the
  Apollo capsule, let's say <font class="TextBlack">—</font> and you can calculate how much metal is required. It
  turns out that the total mass of half a million stars has to be processed and all their metals extracted. Or if we
  extend the argument and assume that only the outer few hundred miles or so of stars like the Sun can be mined by
  advanced technologies (further in it's too hot) we find that 2 x 10<sup>9</sup> such stars must be processed, or about
  1 per cent of the stars in the galaxy. This also sounds unlikely. Now you may say, <q>Well, that's a very parochial
    approach; maybe they have plastic spaceships</q>. Yes, I suppose that's possible. But the plastic has to come from
  somewhere, and calculating plastics instead of metals changes the conclusions very little. This calculation gives some
  feeling for the magnitude of the task when we are asked to believe that there are routine and frequent interstellar
  visits to our planet.</p>
<p>Let me say a few word about possible counterarguments. For example, it might be argued that we are the object of
  special attention: we have just developed all sorts of signs of civilization and high intelligence like nuclear
  weapons, and maybe, therefore, we are of particular interest to interstellar anthropologists. Perhaps. But we have
  only signaled the presence of our technical civilization in the last few decades. The news can be only some tens of
  light years from us. Also, all the anthropologists in the world do not converge on the Andaman Islands because the
  fishnet has just been invented there. There are a few fishnet specialists and a few Andaman specialists ; and these
  guys say, <q>Well, there's something terrific going on in the Andaman Islands; I've got to spend a year there right
    away because if I don't go now, I'll miss out</q>. But the pottery experts and the specialists in Australian
  aborigines don't pack up their bags for the Indian Ocean.</p>
<p>To imagine that there is something abolutely fantastic, you see, about what is happening right here goes exactly
  against the idea that there are lots of civilizations around. Because if there are lots of them around, then the
  development of our sort of civilization msut be pretty common. And if we're not pretty common then there aren't going
  to be many civilizations advanced enough to send visitors.</p>
<p>There is another argument: namely, that the space vehicles that are allegedly being seen are in fact just the local
  craft <font class="TextBlack">—</font> the shuttles that come from some large mother ship which is the real
  interstellar space vehicle (Drs. Grinspoon and Persly may be interested to hear that the vehicles in the UFO
  literature described as "mother ships" are the ones that are <a
      href="../../../../../../observation/pan/aspect/forme/cigare">cigar</a>-shaped, and I shudder to think what that
  means for their interpretation). But again the mother-ship idea changes things by factors of 10 or 100 at the very
  most; it doesn't resolve our problems. </p>
<p>So I deduce from these arguments that the extraterrestrial hypothesis is in some trouble if we're to imagine that
  even a smallish fraction of the ten or twenty thousand UFO cases reported in the last twenty to twenty-five years are
  interstellar in origin.</p>
<p>So far, I've said not a word about the methods of interstellar transport. There are serious problems in interstellar
  flight, principally because the space between the stars in enormous. There are a large number of stars <font
      class="TextBlack">—</font> about two hundred billion stars in our galaxy alone. There are at least a million
  other such galaxies. But the average distances between stars in our galaxy is a few light years; light, faster than
  which nothing that can slow down can travel, takes years to traverse the distances between the nearest stars. Space
  vehicles take that long at the very least. In order for a space vehicle to get from one star to another in a
  convenient period of time it has to go very fast. It has to go very close to the speed of light so that relativistic
  time dilation can enter into the problem, and so the shipboard clock can run more slowly compared to a clock left on
  the launch planet. To travel very close to the speed of light is difficult. There is a literature on the subject of
  relativistic interstellar flight, maybe thirty or forty papers in various scientific journals <span class="note">[C. Sagan, <em>Planetary and Space Science</em> 11 (1963): 485, reprinted in part in <em>The Coming of the Space Age</em>, ed. Arthur C. Clarke (New York: Meredith Press, 1967)]</span>.
  It is easy to see that carrying sufficient fuel for an interstellar flight is really out of the question, even if the
  fuel is half matter and half antimatter (never mind what's holding the antimatter). The ratio of mass of fuel to mass
  of usable payload that is required in such ventures is prohibitively large <span class="note">[E. Purcell, <em>Brookhaven Lecture Series</em> 1 (1960): 1; S. von Hoerner, "The General Limits of Space Travel", <em>Science</em> 137 (1962): 18]</span>.
  An alternative has been suggested by Bussard <span class="note">[R. W. Bussard, "Galactic Matter and Interstellar Flight", <em>Astronautica Acta</em> 6 (1960): 179]</span>
  : an interstellar ramjet with enormous frontal loading area which collects interstellar material on the way,
  accelerates it out the back, and therefore does not have to carry its own fuel. It doesn't run into the mass-ratio
  problem, but it does run into some other problems. The point of the Bussard ramjet is not that it is practical, for it
  surely isn't that: building a spacecraft several hundred kilometers across is only an engineering problem but it's not
  an engineering problem that's going to be solved tomorrow. But the Bussard ramjet does overcome this mass-ratio
  difficulty, which involves fundamental physics. There have been some recent discussions of Bussard's idea, for example
  one by Fishback <span class="source">[J. Fishback, "Relativistic Insterstellar Spaceflight", <span class="note"><em>Astronautica Acta</em> 15 (n° 1) (1969): 25]</span>]</span>
  which critically asseses the ramjet concept and judges that there are various mechanims that comes out the ramjet that
  makes stability at high velocities very difficult. But this is a second-order criticism. What I've learned from the
  Bussard idea is that it is possible even at the present time to think of methods of running between the stars. The
  fact that none of them may work well is, I think, not critical. What is critical is that there are conceivable ways of
  doing it without bumping into fundamental physical constraints. And this suggests that it is premature to say that
  interstelalr space flight it out of the question.</p>
<p>And so now I turn in the other direction. I believe the numbers work out in such a way that UFO's as interstellar
  vehicles is extremely unlikely, but I think it is an equally bad mistake to say that interstellar space flight is
  impossible. One may argue that space flight is not the most cost-effective way to communicate between civilizations
  and that <a href="../../../../../../observation/projet/SETI-RMW.html">interstellar radio contact</a> is a better way. Or
  one can imagine a wide range of other possibilities <font class="TextBlack">—</font> neutrino transmission,
  modulated gravity waves, tachyons; next year we'ell think of some more. But such considerations do not exclude
  interstellar flight. We do not know enough to exclure such visitations, but the probability of such visitations seems
  very small.</p>
<p>If all this is true (and even if we were to admit the possible existence of very strange and very reliably reported
  cases), why is it that extraterrestrial hypothesis of UFO's is so popular? Why is it even around? There is a wide
  range of other perhaps equally plausible hypotheses that we don't often hear about. Why is there no faction that urges
  that an unidentified flying object is a projection of mankind's collective unconscious? Psychiatrists have written on
  the collective unconscious; why not that? Or time travelers? Or visitors from another dimension? Or the halos of
  angels? Or apparitions from the spirit world, of from Middle Earth, or Witchland, or Perelandra? There are a wide
  range of possibilities that could be thought of. How about harbingers of divine wrath? If only we could interpret them
  properly! Or fulfillments of prophecies from the Bhagavad Gita? My question is: How can <em>these</em> possibilities
  be disproved? What is the critical tests for disproving the hypothesis that UFO's are angels' halos? It's difficult to
  think of a really critical test.</p>
<p>It seems to me that one runs into precisely the same problem with the extraterrestrial hypothesis. There is no good
  empirical test which could, for all cases, exclude this hypothesis. I would like therefore to ask: it is possible that
  we hear so much of this hypothesis because the idea of extraterrestrial visitation somehow resonates with the spirit
  of the times in which we live?</p>
<p>There are four such resonances that I can think of: religious aspects, the relief of boredom by believable novelty,
  military classification, and intolerance of ambiguity. I think it is pretty clear that over the last few centuries
  science has systematically expropriated areas which are the traditional concern of religion. It used to be that the
  opening of the flowers was due to the direct intervention of the Deity. That is not a view now often heard. As
  Darwinian evolutionary views became popular and mechanistic interpretations of the origin of the solar system and of
  cosmology became widely disseminated, part of the traditional domain of religion contracted, whether for good or for
  ill. At the same time, traditional forms of religion have been a very firm portion of nearly every culture or mankind
  ; it is unlikely that the needs for belief in the gods, whether valid or not, can be destroyed do easily. In a
  scientitic age what is a more reasonable and acceptable disguise for the classic religious mythos that the idea that
  we are being visited by messengers of a powerful, wise, and benign advanced civilization?</p>
<p>I have some direct experience with a few UFO cases in which this sort of thing is clearly part of the motivation for
  both exaggerating and denigrating the sightings. I certainly don't maintain that unfilled religious needs are
  responsible for all typical UFO sightings, but such needs a a likely resonance between the extraterrestrial hypothesis
  and quite unscientific aspects of the problem. Incidentally, I believe this view is politically dangerous: The
  expectation that we are going to be saved from ourselves by some miraculous interstellar intervention works against
  the necessity for us to solve our own problems.</p>
<p>The second point is the question of boredom and novelty, which I can best illustrate with a brief personal
  experience. Once when I was on the faculty at Harvard I gave a popular experience. Once when I was on the faculty at
  Harvard I gave a popular lecture on something or other, and in the question period at the end there were some
  questions about UFO's. I said that I felt at least a great fraction of them were misapprehended natural phenomena. For
  some reason that I don't understand, policemen are present at all such public gatherings, and as I walked out after
  the last question, two policemen outside the lecture hall were pointing up at the sky. I looked up and observed a
  strange brilliant light moving slowly overhead. Of course, I got out of there fast, before the crowd came out to ask
  me what it was. I joined some friends at a restaurant and said, <q>There's something terrific outside</q>. Everyone
  went outside. They really liked it <font class="TextBlack">—</font> it was a great fun. There is was. It wasn't
  going away. It was clearly visible, slowly moving, fading and brightening, no sound attached to it. Well, I went home,
  got my binoculars, and returned. Through the binoculars I was able to resolve the lights; the bright light was really
  two closely spaced lights, and there were two lights on either side, blinking. When the thing got brighter we could
  hear a mild drone; when the thing got dimmer, we couldn't hear a thing. In fact it turned to be a <a
      href="/org/us/nasa">NASA</a> weather airplane. When I showed my friends at the restaurant that
  what we were seeing was in fact an airplane, the uniform response was disapointment. I mean, it's no fun to go home
  and say, <q>You'll never guess what happened. I was in this restaurant, there was a bright light outside, it was an
    airplane</q>. That's not a memorable story. But suppose no one had a pair of binoculars. Then the story goes. <q>There
    was this great light out there and it was circling the city and we don't know anything about it. Maybe it's visitors
    from somewhere else</q>. That's a story worth talking about. Despite all the novelties of our times, there is a kind
  of drudgery to everyday life that cries out for profound novelties; and the idea of extraterrestrial visitation is a
  culturally acceptable novelty.</p>
<p>The third point is classification. There is in our society, I think everyone will recognize, a certain paranoid
  aspect to some of the UFO cases (The paranoia in our society is not, of course, restricted to UFO cases). For example,
  there was a feeling in some circles a few years ago that <a href="/org/us/nasa/">NASA</a> was keeping to itself
  photographic evidence which showed that the earth was not round. There were buttons one could buy in Berkeley which
  said something like <q>Where are the photos of the whole earth?</q> There were many space vehicles that went up and
  took pictures of the arth but they were always close-up pictures <font class="TextBlack">—</font> continents, oceans,
  but never the entire spherical earth. Well, when space vehicles flew far enough from the earth to take such pictures,
  they were taken <font class="TextBlack">—</font> and sure enough, the earth was round.</p>
<p>In such a climate of opinion Air Force classification of UFO reports resonates exactly. The armed forces have a
  tendency to classify everything in sight, including, or maybe even especially, bizarre cases which are inadequately
  examined and which involve military personel. Then the fact that such cases are classified starts rumors. Somebody who
  is in a position to know realizes the Air Force does have relevant data; and it is just a short step to the idea of
  official conspiracy to suppress the truth. Had the data not been classified, then independent scientific judgment
  would have been possible. In many cases, such independent scientific analysis would show that the cases have a natural
  explanation. The culprit is classification. I have a friend who says that in America today if you're not a little
  paranoid you're out of your mind. The military has a responsibility not to add further to the paranoia.</p>
<p>The fourth point is a widespread intolerance for ambiguity. It's more difficult to keep two ideas in my mind than it
  is to keep just one idea in my mind. This point comes out very clearly if you've ever written a popular book in
  science. I did that once for a major publishing enterprise. I would write, <q>Here's the observation; some people
    think this is the explanation, some people think this is the explanation</q>. The story I would get from their
  editors would be <q>Dont bother me with the alternatives; just tell me what's true</q>. I think it's a fact of life
  that many people are uncomfortable with ambiguity, with a judgment withheld. But, it seems to me, this is precisely
  where we ought to be on the UFO problem: to say that there aren't enough data, that good judgment isn't possible yet,
  and that an open mind should be kept. <a href="/people/scientifiques.html">Scientists</a> are particularly bound to
  keep open minds; this is the lifeblood of <a href="/science">science</a>. </p>
<p>As a concluding word let me say that I believe the search for extraterrestrial intelligence to be an exceedingly
  important one both for science and for society. It is difficult to think of a more important scientific question. But
  I do not believe that the most efficient method of examining this topic is via the UFO problem. The best hope for such
  investigations is NASA's unmanned planetary program and attemps at interstellar radio communication. </p>
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