UFO's : The E.T. and other hypotheses by Carl Sagan

There are many ideas which are charming if tnie, which would be fün to believe in, which are a delight to think about: reincarnation; the philosopher's stone to tum base metals into gold; thé search for long or possibly indefinitely extended lifetimes; psychokinesis, thé ability to move inanimate objects by thinking at them; precognition, thé ability to foresee thé future; telepathy, the ability to read somebody eise's mind; time travel; leaving one's body (thé literal meaning ofecstasy); becoming one with thé universe. There is a wide range of concepts which would be fascinating especially if oniy they were true.

But precisely because thèse ideas have charm, exactiy because they are of deep emotional significance to us, they are thé ideas we must examine most critically. We must consider them with thé greatest skepticism, and examine in thé greatest détail thé évidence relevant to them. Where we hâve an emotional stake in an idea, we are most likely to deceive ourselves.

Thé idea of benign or hostile superbeings from other planets visiting thé earth clearly belongs in such a list of emotion-rich ideas. There are two sorts of possible self-déceptions hère: either accepting the idea of extraterrestrial Visitation in thé face of very meager évidence because we want it to be true; or rejecting such an idea out of hand, in thé absence of sufficient évidence, because we don't want it to be true. Each of thèse extrêmes is a serions impediment to thé study of UFO's; they affect différent catégories of peuple. A symposium such as this one must spend some time worrying about emotional prédisposition.

Rétrospective and Perspective

l want to discuss first thé extraterrestrial hypothesis of UFO orip' bearing in mind that its assessment dépends upon a large number f factors about which we know little and a few about which we know l'r erally nothing. What l want to lead up to is some crude numerical estimate of the probability that we are frequently visited by extraterrestrial beings.

There is a range of hypotheses which can be examined in such a wav Let me give a simple example: consider thé Santa Claus hypothesis This hypothesis maintains that, in a period of eight hours or so on December 24-25 of each year, an outsized elf visits fifty million homes in thé 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 thé elf in question spends one second per house.

This isn't quite the usual picture—"Ho Ho Ho" and so on—but l imagine he is terribly efficient, and very speedy; that would explain why nobody ever sees him very much. With 108 houses he bas to spend three years just filling stockings. l've assumed he spends no time at ail in going from house to house. Even with hyper-relativistic reindeer, the time spent in 10" houses is three years and not eight hours. This is an example of hypothesis testing independent of reindeer propulsion mechanisms or debates on thé origins of elves.

We examine the hypothesis itself, making very straightforward assumptions, and dérive a resuit inconsistent with thé hypothesis by many orders of magnitude. We would then suggest that thé hypothesis is untenable.

We can make a similar examination, but with greater uncertainty, of thé extraterrestrial hypothesis which hoids that a wide range of unidentified flying objects viewed on thé planet Earth are space vehicles from planets of other stars. The report rates, at least in récent years, have been several per day at thé very least, but l will make thé 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 hâve some feeling for the number, N, of extant technical civilizations in thé galaxy—that is, civilizations vastly in advance of our own, civilizations which are able by whatever means to perform interstellar space flight (l will say a word about thé means later, but the and Other Hypotheses means don't enter into this discussion just as reindeer propulsion mechanisms don't affect our discussion of the Santa Claus hypothesis).

An attempt has been made to specify explicitly the factors which enter into a détermination of thé number of such technical civilizations in thé galaxy. l will not hère run through what numbers hâve been assigned to thé various quantities involved—it's a multiplication of many probabilities, and thé likelihood that we can make a good judgment decreases as we proceed down this list. N dépends, first, on the mean rate at which stars are formed in thé galaxy, a number which is known reasonably well.

lt dépends on thé number of stars which have planets, which is less well known but there are some data on that. lt dépends on thé fraction of such planets which are so suitably located with respect to their star that the environment is feasible for thé origin of life.

lt depends on thé fraction of such otherwise feasible planets on which thé origin of life in fact occurs. lt dépends on thé fraction of those planets on which thé origin of life occurs in which, after îife has arisen, an intelligent form cornes into being.

lt depends on thé fraction of those planets in which intelligent forms have arisen which evolve a technical civilization substantially in advance of our own. And it dépends on thé lifetime of thé technical civilization. It's clear that we are rapidiy running oui of examples as we go further and further along. That is, we hâve many stars, but oniy one instance of the origin of life, and oniy a very limited number—some would oniy say one—of instances of thé évolution of intelligent beings and technical civilizations on this planet.

And we have no cases whatever to make a judgment on thé mean lifetime of a technical civilization. Nevertheless there is an entertainment (which is the way l put it) which some of us have been engaged in, making our best estimâtes about thèse numbers, and coming out with a value of N. The equation which cornes out ' is that N roughiy equals l/10 the average life time of a technical civilization in years.

If we put in a nurnber like 10" years for thé average lifetime of advanced technical civilizations, we corne out with a number for such technical civilizations in thé galaxy of about a million: that is, a million other stars with planets on which today there are such advanced civilizations. Now l think you will recognize that this is quite a difficult calculation to do accurately and moreover that thé choice of 107 years for the lifetime of a technical civilization is rather optirnistic. But let's take these optim' tic numbers and see where they lead us.


Let's assume that each of these million technical civilizations launch Q interstellar space vehicles a year; thus, 10 6 Q interstellar space vehicles are launched per year. Let's assume that there's oniy one contact made per journey. In thé steady-state situation there are something like 10<'Q arrivais somewhere or other per year. Now there surely are somethine like 10'" interesting places in thé galaxy to visit (we hâve several times 10" stars) and therefore at least 10-4 Q arrivais at a given interesting place, let's say a planet, per year. So if oniy one UFO is to visit thé earth each year, we can calculate what mean launch rate is required at each of thèse million worids. Thé number turns out to be 10,000 launches per year per civilization; and 10L" launches in thé galaxy per year.

This seems excessive. Even if we imagine a civilization very much further advanced than ourselves (111 mention in a minute that it's a considérable undertaking to travel effortiessiy between thé stars), to launch 10,000 such vehicles for oniy one to appear hère is probably asking too much. And if we were more pessimistic on thé lifetime of advanced civilizations we would require a proportionately larger launch rate. But as thé lifetime decreases, thé probability that a civilization would develop interstellar flight very likely decreases as well.

There is a related point made by Hong-Yee Chiu; r he begins with more than one UFO arriving at Earth per year, but his argument foilows thé same lines as thé one l hâve just presented. He calculâtes thé total mass of metals involved in ail of thèse space vehicles during thé history of thé galaxy. Thé vehicle has to be of some size—it should be bigger than thé Apollo capsule, let's say—and you can calculate how much metal is required.

It turns out that thé total mass of haif a million stars has to be processed and ail their metals extracted. Or if we extend thé argument and assume that oniy thé outer few hundred miles or so of stars like thé Sun can be mined by advanced technologies (further in it's too hot) we find that 2 x 109 such stars must be processed, or about l per cent of thé stars in thé galaxy. This aiso sounds uniikely. Now you may say, "Well, that's a very parochial approach; maybe they hâve plastic spaceships." Yes, l suppose that's possible. But thé plastic has to corne from somewhere, and calculating plastics instead of metals changes the conclusions very little. This calculation gives some feeling for thé magnitude of thé task when we are asked to believe that there are routine and fréquent interstellar visits to our planet.

Let me say a few words about possible counterarguments. For exarnple, it might be argued that we arc thé object of spécial attention: we hâve just developed ail 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 oniy signaled thé présence of our technical civilization in thé last few décades. Thé news can be oniy some tens of light years from us. Aiso, ail thé anthropologists in thé worid do not converge on the Andaman Islands because thé fishnet has just been invented there.

There are a few fishnet spccialists and a few Andaman specialists; and thèse guys say, "Well, there's something terrifie going on in thé Andaman Islands; l've got to spend a year there right away because if l don't go now, 111 miss out." But thé pottery experts and thé specialists in Australian aborigines don't pack up their bags for thé Indian Ocean.

To imagine that there is something absolutely fantastic, you see, about what is happening right hère goes exactiy against thé idea that there arc lots of civilizations around. Because if there are lots of them around, then thé development of our sort of civilization must be pretty common. And if we're not pretty common then there aren't going to be many civilizations advanced enough to send visitors.

There is another argument: namely, that the space vehicles that are allegediy being seen are in tact just thé local craft—thé shuttles that corne from some large mother ship which is thé real interstellar space vehicle. (Drs. Grinspoon and Persky may be interested to hear that thé vehicles in thé UFO literature described as "mother ships" are thé ones that are cigar-shaped, and l shudder to think what that means for their interprétation.) But again thé mother-ship idea changes things by factors of 10 or 100 at thé very most; it doesn't résolve our problems.

So l deduce from thèse arguments that thé extraterrestrial hypothesis is in some trouble if we're to imagine that even a smallish fraction of thé ten or twenty thousand UFO cases reported in thé last twenty to twenty-five years are interstellar in origin.

So far, l've said not a word about thé methods of interstellar transport. There are serions problems in interstellar flight, principallv h cause thé space between thé stars is enormous. There are a large num ber of stars—about two hundred billion stars in our galaxy alon There are at least a million other such galaxies.

But the average dm tances between stars in our galaxy is a few light years; light, faster tha which nothing that can slow down can travel, îakes years to traverse thé distances between thé nearest stars. Space vehicles take that long at thé very least.

In order for a space vehicle to get from one star to another in a convenient period of time it bas to go very fast. lt bas to go verv close to thé speed of light so that relativistic time dilation can enter into thé problem, and so thé shipboard dock can run more slowly compared to a dock left on the launch planet. To travel very close to thé speed of light is difficult. There is a literature on thé subject of relativistic interstellar flight, maybe thirty or forty papers m varions scientific journals.3

It is easy to see that carrying sufficient fuel for an interstellar fiight is really out of thé question, even if thé fuel is haif matter and haif antimatter (never mind what's holding thé antimatter). Thé ratio of mass of fuel to mass of usable payload that is required in such ventures is prohibitively large.* An alternative bas been suggested by Bussard: 5 an interstellar ramjet with enormous frontal loading area which collects interstellar material on thé way, accélérâtes it out thé back, and therefore does not hâve to carry ils own fuel.

It doesn't run into thé mass-ratio problem, but it does run into some other problems. Thé point of thé Bussard ramjet is not that it is practical, for it surely isn't that: building a spacecraft several hundred kilometers across is oniy an engineering problem but it's not an engineering problem that's going to be solved tomorrow. But thé Bussard ramjet does overcome this mass-ratio difficulty, which involves fundamental physics.

There have been some récent discussions of Bussard's idea, for example one by Fishback « which critically assesses thé ramjet concept and judges that there are varions mechanisms including magnetobrehmstrahlung and problems in thé turbulence of thé stream that cornes out thé ramjet that makes stability at high velocities very difficult.

But this is a second-order criticism. What l've leamed from thé Bussard idea is that it is possible even at the present time to think of methods of running between thé stars. Thé fact that none of them may work well is, l think, not critical. What is critical is that there are conceivable ways of doing it without bumping into fondamental physical constraints. And this suggests that it is prémature to say that interstellar space flight is out ofthe question.

And so now l tum in the other direction, l believe thé numbers work out in such a way that UFO's as interstellar vehicles is extremely unlikely, but l think it is an equally bad mistake to say that interstellar space flight is impossible. One may argue that space flight is not thé most cost-effective way to communicate between civilizations and that interstellar radio contact is a better way.

Or one can imagine a wide range of other possibUities—neutrino transmission, modulated gravity waves, tachyons; next year we'll think of some more. But such considérations do not exclude interstellar flight. We do not know enough to exclude such visitations, but thé probability of such visitations seems very small.


If ail this is true (and even if we were to admit thé possible existence of very strange and very reliably reported cases), why is it that thé extraterrestrial hypothesis of UFO's is so popular? Why is it even around? There is a wide range of other perhaps equally plausible hypothèses 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 thé collective unconscious; why not that? Or time travelers? Or visitors from another dimension? Or thé halos of angels? Or apparitions from thé spirit worid, or from Middie Earth, or Witchiand, or Perelandra?

There are a wide range of possibilities that could be thought of. How about harbingers of divine wrath? If oniy we could interpret them properly! Or fulfillments of prophecies from thé Bhagavad Gita? My question is: How can thèse possibilities be disproved? What is thé critical test for disproving thé hypothesis that UFO's are angels' halos? Ifs difficult to think of a really critical test.

It seems to me that one runs into precisely thé sarne problem with thé extraterrestrial hypothesis. There is no good empirical test which could, for ail cases, exclude this hypothesis. l would like therefore to ask: is it possible that we hear so much of this hypothesis because thé idea of extraterrestrial Visitation somehow resonates with thé spirit of thé times in which we live?

There are four such resonances that l can think of: religious aspects thé relief of boredorn by believable novelty, military classification, and intolérance of ambiguity. l think it is pretty clear that over thé last few centuries science has systematically expropriated areas which are thé traditional concern of religion.

It used to be that thé opening of the flowers was due to thé direct invervention of thé Deity. That is not a view now often heard. As Darwinian evolutionary views becarne popular and mechanistic interprétations of thé origin of thé solar system and of cosmology became widely disseminated, part of thé traditional domain of religion contracted, whether for good or for ill.

At the same time, traditional forms of religion hâve been a very firm portion of nearly every culture of mankind; it is uniikely that thé needs for belief in thé gods, whether valid or not, can be destroyed so easily. In a scientific âge what is a more reasonable and acceptable disguise for thé classic religions mythos than the idea that we are being visited by messengers of a powerfui, wise, and benign advanced civilization?

l have some direct expérience with a few UFO cases in which this sort of thing is clearly part of thé motivation for both exaggerating and denigrating thé sightings. l certainly don't maintain that unfilled religious needs are responsible for ail typical UFO sightings, but such needs are a likely résonance between thé extraterrestrial hypothesis and quite unscientific aspects of thé problem. Incidentally, l believe this view is politically dangerous: Thé expectation that we are going to be saved from ourselves by some miraculous interstellar intervention works against thé necessity for us to solve our own problems.

The second point is thé question of boredorn and novelty, which l can best illustrate with a brief personal expérience. Once when l was on thé faculty at Harvard l gave a popular lecture on something or other, and in thé question period at thé end there were some questions about UFO's. l said that l feit at least a gréât fraction of them were misapprehended natural phenomena. For some reason that l don't understand, policemen are présent at ail such public gatherings, and as l walked out after thé last question, two policemen outside thé lecture hall were pointing up at thé sky. l looked up and observed a strange brilliant light moving slowly overhead. Of course, l got out of there fast, before thé crowd came out to ask me what it was. l joined some friends at a reslaurant and said, "There's something terrifie outside." Everyone went outside. They really liked it—it was gréât fun. There it was. lt wasn't going away.

It was clearly visible, slowly moving, fading and brightening, no sound attached to it. Well, l went home, got my binoculars, and retumed. Through thé binoculars l was able to résolve thé lights; thé bright white 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 thé thing got dimmer, we couidn't hear a thing. In fact it tumed out to be a NASA weather airplane. When l showed my friends at thé restaurant that what we were seeing was in fact an airplane, thé uniform response was disappointment. l mean, it's no fun to go home and say, "You'11 never guess what happened. l was in this restaurant, there was a bright light outside, it was an airplane."

Thats not a memorable story. But suppose no one had a pair of binoculars. Then thé story goes, "There was this great light out there and it was circling thé city and we don't know anything about it. Maybe it's visitors from somewhere eise." That's a story worth taiking about. Despite ail thé novelties of our times, there is a kind of drudgery to everyday life that cries out for profound novelties; and thé idea of extraterrestrial Visitation is a culturally acceptable novelty.

The third point is classification. There is in our society, l think everyone will recognize, a certain paranoid aspect to some of thé UFO cases. (Thé paranoia in our society is not, of course, restricted to LIFO cases.) For example, there was a feeling in some circles a few years ago that NASA was keeping to itself photographie évidence which showed that thé earth was not round. There were buttons one could buy in Berkeley which said something like "Where are thé photos of thé whole earth?" There were many space vehicles that went up and took pictures of thé earth but they were aiways close-up pictures—continents, océans, but never thé entire spherical earth. Well, when space vehicles fiew far enough from thé earth to take such pictures, they were taken—and sure enough, thé earth was round.

In such a climate of opinion Air Force classification of UFO reports resonates exactiy. Thé armed forces hâve a tendency to classify everything in sight, including, or maybe even cspecially, bizarre cases which are inadequately examined and which involve military personnel. Then the fact that such cases are classified starts rumors. Somebody who is in a position to know realizes thé Air Force does hâve relevant data; and it is just a short step to thé idea of officiai conspiracy to suppress thé truth. Had thé data not been classified, then independent scientificjudg. ments would hâve been possible. In many cases, such independent scientific analysis would show that thé cases hâve a natural explanation. Thé culprit is classification, l hâve a friend who says that in America today if you're not a little paranoid you're ont of your mind. Thé military bas a responsibility not to add further to thé paranoia.

The fourth point is a widespread intolérance for ambiguity. lt's more difficult to keep two ideas in my mind than it is to keep just one idea in my mind'. This point cornes out very clearly if you've ever written a popular book on science, l did that once for a major publishing enterprise.l would write, "Here's thé observation; some people think this is the explanation, some people think that is the explanation." The story l would get from their editors would be "Don't bother me wtth thé alternatives; just tell me what's true." l think it's a fact of life that many people are uncomfortable with ambiguity, with a judgment withheld. But, it seerns to me, this is precisely where we ought to be on thé 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. Scientists are particularly bound to keep open minds; this is thé lifeblood of science.

As a concluding word let me say that l believe thé 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 l do not believe that thé most efficient method of examining this topic is via thé UFO problem. Thé best hope for such investigations is NASA's unmanned planetary program and attempts at interstellar radio communication.



3. C. Sagan. Planetary and Space Science ]l (1963): 485, reprinted in part in Thé Coming of thé Space Age, éd. Arthur C. Clarke (New York: Meredith Press, 1967).
4. E. Purcell, Bnokhaven Lecture Série! l (1960); l; S. von Hoerner, "The General Limitsof Space Travel.''Xciencc 137 (1962); 18.
5. R. W. Bussard, "Galactic Matter and Interstellar Flight," Aittonautica ACM 6 (1960); 179.
6. J. Fishbacll, "Relativistic Interstellar Spaceflight," Astronautica Acta 15 (No. l) (1969): 25.

NOTES

l. l. S. Shklovskii and C. Sagan, Intelligent Life in thé Universe (San Francisco: Holden-Day, 1966; New York: Delta Books, 1967).
2. H.-Y. Chiu, book review, "Thé Condon Report, Scientific Study of Un' identified FlyingObjects," Icarui II (1970): 447.

Reference : UFO's A scientific debate; Editedby Carl Sagan and Thornton Page. The Norton Library and Cornell University 1972.