The Varieties of Scientific Experience: A Personal View of the Search for God

by Carl Sagan

  The outline of a general solution of this problem was provided, independently as far as we can tell, by both Immanuel Kant and by Pierre-Simon, the marquis de Laplace.

  Newton, Laplace, and Kant all lived after the invention of the telescope and therefore after the discovery that Saturn has an exquisite ring system that goes around it, a portion of which you see here in this far-encounter photograph. It is a flat plane of clearly fine particles. The first clear demonstration that it's made of many particles, that it isn't a solid sheet, was made by a Scottish physicist, James Clerk Maxwell.

  Here's a closer view of the rings of Saturn. And you can see an enormous sequence of such rings and a gap-the so-called Cassini Division in the rings.

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  If you take a close-up look at this portion, you can see a succession of rings. We now know that there are many hundreds of these rings, all in a flat plane, and we now know, as both Kant and Laplace guessed, that they're made of tumbling boulders and dust particles. The rings of Saturn, by the way, are thinner compared to their lateral extent than is a piece of paper.

  Kant also knew about objects that were then called nebulae. It was not clear whether they 'were within our Milky Way or beyond-we now know, of course, most of them are beyond. Some of the nebulae were again flattened systems made, we now know, of stars.

  So Kant and Laplace, both of them explicitly mentioning the rings of Saturn, and Kant explicitly mentioning the elliptical nebulae, proposed that the solar system came from such a flattened disk and that somehow the planets condensed out of the disk. But if that's the case, the disk, after all, has some rotation. Everything that condenses out of it will be going around in the same direction. And if you think about it for a moment, you will see that as the particles come together and make larger objects, they will have a common sense of rotation as well.

  What Kant and Laplace proposed is what we now call a solar nebula, or accretion disk, whose flattened form was the ancestor of the planets, and that it is perfectly easy to understand how it is that the planets are in the same plane with the same direction of revolution and the same sense of rotation.

  What is more, we now know that the random orientation of the comets is not primordial and that very likely the comets began in the solar nebula, all going around the Sun in the same sense, were ejected by gravitational interactions with the major planets, and then, by the gravitational perturbations of passing stars, had their orbits randomized.

  So Newton was wrong in both senses: (a) in the sense of believing that the chaotic distribution of cometary orbits is what you would expect in a primordial system and (b) in assuming that there was no natural way in which the regularities of planetary motion could be understood without divine intervention, from which he deduced the existence of a Creator.

  Well, if Newton could be fooled, this is something worth paying attention to. It suggests that we, of doubtless inferior intellectual accomplishment, might be vulnerable to the same sort of error.

  I would just like to lock in what I've been saying about the solar nebula with three more images.

  Here is an attempt to illustrate what I've just been saying. An originally irregular interstellar cloud is rotating. It gravita-tionally contracts; that is, the self-gravity pulls it in. Because of the conservation of angular momentum, it flattens into a disk. A way to think of it is that centrifugal force does not oppose the contraction along the axis of rotation but does in the plane of rotation. So you can see that the net result will be a disk. Through processes that need not detain us here (although remarkable progress has been made in our understanding during the last twenty years), there are gravitational instabilities that produce a large number of objects, which then fall together by collision and produce a smaller number of objects. It's clear that

  fig. 17

  if there were a huge number of objects with crossing orbits, they would eventually collide, and you would wind up with fewer and fewer objects. So the idea here is that there is a kind of collisional natural selection-the evolutionary idea as applied to astronomy-in which you must eventually wind up with a small number of objects in orbits that do not cross each other. And that is certainly the present configuration of the planetary system shown up here.

  This is just another artist's conception of an early stage in the origin of our solar system, showing some of the multitude of small objects a few kilometers across, from which the planets were formed. And that this is not solely a theoretical construct has been made clear in recent years by the discovery of a number of flattened disks around nearby stars.

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  This one is around the star Beta Pictoris. It's in a Southern Hemisphere constellation. But Vega, one of the brightest stars in the Northern sky, also has such a flattened disk of dust and maybe a little gas around it. And many people think that it is in the final stages of sweeping up a solar nebula, that planets have already formed there, and that if you come back in only a few tens of millions of years you will find the disk entirely dissipated and a fully formed planetary system.

  So I would like now to come to what is called the anthropic principle. If you study history, it's almost irresistible to ask the question, what if something had gone in a different direction? What if George III had been a nice guy? There are many questions; that's not the deepest, but you understand what I'm saying. There are many such apparently random events that could just as easily have gone another way, and the history of the world would be significantly different. Maybe-I don't know that this is the case-but maybe Napoleon's mother sneezed and Napoleon's father said, "Gesundheit," and that's how they met. And so a single particle of dust was responsible for that deviation in human history. And you can think of still more significant possibilities. It's a natural thing to think about.

  Now, here we are. We're alive; we have some modest degree of intelligence; there is a universe around us that clearly permits the evolution of life and intelligence. That's an unremarkable and, I think, as secure a remark as can be made in this subject: that the universe is consistent with the evolution of life, at least here. But what is interesting is that in a number of respects the universe is very fine-tuned, so that if things were a little different, if the laws of nature were a little different, if the constants that determine the action of these laws of nature were a little different, then the universe might be so different as to be incompatible with life.

  For example, we know that the galaxies are all running away from each other (the so-called expanding universe). We can measure the rate of expansion (it is not strictly constant with time). We can even extrapolate back and ask how long ago were all the galaxies so close that they were in effect touching. And that will surely be, if not the origin of the universe, at least an anomalous or singular circumstance from which we can begin dating. And that number varies according to a number of estimates, but it's roughly 14,000 million years.

  Now, the period of time that was required for the evolution of intelligent life in the universe-if we are unique and we define ourselves immodestly as the carriers of intelligent life (a case could be made, you know, for other primates and dolphins, whales, and so on)-but for any of those cases it took something like 14,000 million years for intelligence to arrive. Well, how come? Why are those two numbers the same? Put another way: If we were at a much earlier stage or a much later stage in the expansion of the universe, would things be very different? If we were at a much earlier stage, then there would not be, according to this view, enough time for the random aspects of the evolutionary process to proceed, and so intelligent life would not be here, and so there would be nobody to make this argument or debate about it. Therefore the very fact that we can talk about this demonstrates, it is argued, that the universe must be a certain number of years old. So if only we had been wise enough to have thought of this argument before Edwin Hubble, we could have made this spectacular discovery about the expansion of the universe just by contemplating our nave
ls.

  There is to my mind a very curious ex post facto aspect of this argument. Let's take another example. Newtonian gravitation is an inverse square law. Take two self-gravitating objects, move them twice as far apart, the gravitational attraction is one-quarter; move them ten times farther apart, the gravitational attraction is one-hundredth, and so on. It turns out that virtually any deviation from an exact inverse square law produces planetary orbits that are, in one way or another, unstable. An inverse cube law, for example, and higher powers of the negative exponent mean that the planets would rapidly spiral into the Sun and be destroyed.

  Imagine a device with a dial for changing the law of gravity (I wish there were such a device, but there isn't). We could dial in any exponent, including the number 2 for the universe we live in. And when we do this, we find that a large subset of possible exponents leads to a universe in which stable planetary orbits are impossible. And even a tiny deviation from 2-2.0001, for example-might, over the period of time of the history of the universe, be enough to make our existence today impossible.

  So, one may ask, how is it that it's exactly an inverse square law? How did it come about? Here is a law that applies to the entire cosmos that we can see. Distant binary galaxies going around each other follow exactly an inverse square law. Why not some other sort of law? Is it just an accident, or is there an inverse square law so that we could be here?

  In the same Newtonian equation, there is the gravitational coupling constant called "big G." It turns out that if big G were ten times larger (its value in the centimeter-gram-second system is about 6.67 x 10-8), if it were 10 times larger (6.67 x 10-7), then it turns out the only kind of stars we would have in the sky would be blue giant stars, which expend their nuclear fuel so rapidly that they would not persist long enough for life to evolve on any of their planets (that is, if the timescales for the evolution of life on our planet are typical).

  Or if the Newtonian gravitational constant were ten times less, then we would have only red dwarf stars. What's wrong with a universe made with red dwarf stars? Well, it is argued, they're around for a long time because they burn their nuclear fuel slowly, but they are such feeble sources of light that to be warmed to the temperatures of liquid water, let's say, [2] then the planets would have to be very close to the star in order to be at this temperature. But if you put the planets very close to the star, there is a tidal pull that the star exerts on the planet so that the planet always keeps the same face to the star, and therefore, it is said, the near side will be too hot and the far side will be too cold and it's inconsistent with life. So isn't it remarkable that big G has the value it does? I'll come back to this.

  Or consider the stability of atoms. An electron with something like one eighteen-hundredth the mass of a proton has precisely the same electrical charge. Precisely. If it were even a little different, the atoms would not be stable. How come the electrical charges are exactly the same? Is it so that 14 billion years later we, who are made of atoms, could be around?

  Or if the strong nuclear force coupling constant were only a little weaker than it is, you can show that only hydrogen would be stable in the universe and all the other atoms, which surely are required for life, we would say, would never have been made.

  Or if certain specific nuclear resonances in the nuclear physics of carbon and oxygen were a little different, then you could not build up in the interiors of red giant stars the heavier elements and again you would have only hydrogen and helium in the universe and life would be impossible. How is it that everything works out so well to permit life when it's possible to imagine quite different universes?

  (What I'm about to say now is not an answer to the question I've just posed.) It is not difficult to see teleology hiding in this sequence of arguments. And, in fact, the very phrase "anthropic principle" is a giveaway as to at least the emotional if not the logical underpinnings of the argument. It says something central about us; we're the anthropos. And that's the reason I am saying that this is another ground, somewhat covert, on which the Copernican conflict is being worked out in our time. J. D. Barrow, one of the authors and promoters of the anthropic principle, is quite straightforward about it. He says that the universe is "designed with the goal of generating and sustaining observers"-namely, us.

  Now, what can we say about this? Let me make, in conclusion, a few critical remarks. First of all, in at least parts of this argument there is a failure of the imagination. Let's take that red dwarf argument, in which if the gravitational constant were an order of magnitude less, then we would only have those red giants. Is it true that you could not have life in that situation for the reasons I mentioned? It turns out it isn't, for two different reasons. Let's look again at that tidal locking argument. Yes, for a close-in planet and the star, it seems possible that the net result would be the same kind of situation as for the Moon and the Earth, namely, that the secondary body makes one rotation per revolution, therefore always keeping the same face to the primary. That's why we always just see one Man in the Moon and not some Woman in the Moon on the back that we see as well. But if you look at Mercury and the Sun, you find a close-in planet not in a one-to-one resonance, but it's a three-to-two resonance. There are many more than just this one kind of resonance that are possible. What is more, if we're talking about planets that have life, we're talking about planets with atmospheres. A planet with an atmosphere carries the heat from the illuminated to the unilluminated hemisphere and redistributes the temperature. So it's not just the hot side and the cold side. It is much more moderate than that.

  And then let's take a look at the more distant planets that you might imagine were too cold to support life. This neglects what is called the greenhouse effect, the keeping in of infrared emission by the atmospheres of the planet. Let's take Neptune, at thirty astronomical units from the Sun, so you would figure that it has almost a thousand times less sunlight. And yet there is a place we can see with radio waves in the atmosphere of Neptune that is as warm as it is in the cozy room I'm in. So what has happened here is that an argument has been put forward, but in insufficient detail. It has not been looked at closely enough. And I bet that will turn out to be the case in some of the other examples I present.

  The second possibility is that there is some new principle hitherto undiscovered, which connects various apparently unconnected aspects of the universe in the same way that natural selection provided a wholly unexpected solution to a problem that seemed to have no conceivable solution whatever.

  And thirdly, there is the so-called many worlds or, better, many universes idea. And this is what I had in mind when I was talking about history at the beginning. Namely, that if at every microinstant of time the universe splits into alternate universes in which things go differently, and that if there is at the same moment an enormously, tremendously large, perhaps infinitely large array of other universes with other laws of nature and other constants, then our existence is not really that remarkable. There are all those other universes in which there isn't any life. We just, by accident, happen to be in the one that has life. It's a little bit like a winning hand at bridge. The chance of, let's say, being dealt twelve spades is an absurdly low probability. But it is as likely as getting any other hand, and therefore, eventually, if you play long enough, some universe has to have our laws of nature.

  Well, I believe that we are seeing a still largely unexplored area of physics being projected upon by the same sorts of human hopes and fears that have characterized the entire history of the Copernican debate.

  I wanted to say just two final things. One is, if the very strong version of the anthropic principle is true, that is, that God-we might as well call a spade a spade-created the universe so that humans would eventually come about, then we have to ask the question, what happens if humans destroy themselves? That would make the whole exercise sort of pointless. So if only we could believe the strong version, we would have to conclude either (a) that an omnipotent and omniscient God did not create th
e universe, that is, that He was an inexpert cosmic engineer, or (b) that human beings will not self-destruct. Either alternative, it seems to me, is a matter of some interest, would be worth knowing. But there is a dangerous fatalism lurking here in the second branch of that fork in this road.

  Well, I would like to conclude, then, by just a few lines of poetry, this one from Rupert Brooke, called "Heaven."

  FISH (fly-replete, in depth of June, Dawdling away their wat'ry noon) Ponder deep wisdom, dark or clear, Each secret fishy hope or fear.

  Fish say, they have their Stream and Pond; But is there anything Beyond? This life cannot be All, they swear, For how unpleasant, if it were!

  One may not doubt that, somehow, Good Shall come of Water and of Mud; And, sure, the reverent eye must see A Purpose in Liquidity.

  We darkly know, by Faith we cry, The future is not Wholly Dry. Mud unto mud!-Death eddies near- Not here the appointed End, not here!

  But somewhere, beyond Space and Time, Is wetter water, slimier slime! And there (they trust) there swimmeth One, Who swam ere rivers were begun,

  Immense, of fishy form and mind, Squamous, omnipotent, and kind; And under that Almighty Fin, The littlest fish may enter in.

  Oh! never fly conceals a hook,

  Fish say, in the Eternal Brook,

  But more than mundane weeds are there,

  And mud, celestially fair;

  Fat caterpillars drift around,

  And Paradisal grubs are found;

  Unfading moths, immortal flies,

  And the worm that never dies.

  And in that Heaven of all their wish,

  There shall be no more land, say fish.

  Three

  THE ORGANIC UNIVERSE