The Cosmic Connection

by Carl Sagan

  But such cultural homogenization of the Galaxy will take a long time. One round-trip communication by radio between us and the center of the Milky Way Galaxy requires sixty thousand years. Cultural homogenization of the Galaxy would require many such exchanges, even if each exchange involved very large amounts of information conveyed very efficiently. I find it difficult to believe that fewer than one hundred exchanges between the remotest parts of the Galaxy would be adequate for galactic cultural homogenization.

  The minimum lifetime for the homogenization of the Galaxy would thus be many millions of years. The constituent societies must, of course, be stable for comparable periods of time. Such homogenization need not be desirable, but there are still strong and obvious pressures for it to occur, as is also the case on the Earth. If there exists a galactic community of civilizations that truly embraces much of the Milky Way, and if we are right that no information can be transmitted at a velocity faster than light, then most of the members – and all of the founding members – of such a community must be at least millions of years more advanced than we are. For this reason, I think it a great conceit, the idea of the present Earth establishing radio contact and becoming a member of a galactic federation – something like a bluejay or an armadillo applying to the United Nations for member-nation status.

  These velocity-of-light limitations on the speed of communication can also be applied to the homogenization of the cultures of different galaxies, after a hypothetical period of millions of years in which the stellar civilizations of a given galaxy achieve a common culture. We can imagine attempts to make contact with such galactic federations in other galaxies.

  The nearest spiral galaxies are several million light-years away. This means that a single element of the dialogue – a message and its reply – would take periods of time of several millions to about ten million years. If a hundred such exchanges are required, the time scale for homogenization of a group of nearby galaxies is then of the order of a billion years. The galactic societies would have to be stable and preserve continuity for such periods of time. This would mean that an immensely old civilization within our Galaxy might have strong learned commonalities with similar galactic federations in other members of what astronomers modestly call the "local" group of galaxies.

  These homogenization time scales are beginning to reach a point that strains credulity. There are sufficient natural catastrophes and statistical fluctuations in the universe that a stable society – even residing on many different planets simultaneously for more than a billion years – begins to sound unlikely. Also, during these immense periods of time the communicating galactic societies will themselves be evolving; many contacts will be required to maintain homogenization. The galaxies are so distant one from another that they will always retain their cultural individuality.

  In any case, all bets are off beyond the local group. To have cultural homogenization with the next such cluster of galaxies like our own, and engage in a hundred message-exchange pairs, would require a time longer than the age of the universe. This is not to exclude long individual messages from one galaxy to another. It may be that enormous amounts of information – about the history of a given galactic federation, for example – may be well known to civilizations in other galaxies. But there will not be enough time for dialogues. At most, one exchange would be possible between the most distant galaxies in the universe. Two exchanges of information at the velocity of light would take more time than there is, according to modern cosmology.

  We conclude that there cannot be a strongly cohesive network of communicating, unifying intelligences through the whole universe if (1) such galactic civilizations evolve upward from individual planetary societies and if (2) the velocity of light is indeed a fixed limit on the speed of information transmission, as special relativity requires (i.e., if we ignore such possibilities as using black holes for fast transport: See Chapter 39). Such a universal intelligence is a kind of god that cannot exist.

  In a way, St. Augustine and many other thoughtful theologians have come to rather the same conclusion – God must not live from moment to moment, but during all times simultaneously. This is, in a way, the same as saying that special relativity does not apply to Him. But supercivilization gods, perhaps the only ones that this kind of scientific speculation admits, are fundamentally limited. There may be such gods of galaxies, but not of the universe as a whole.

  36. A Passage to Elsewhen

  One of the most pervasive and entrancing ideas of science fiction is time travel. In The Time Machine, the classic story by H. G. Wells, and in most subsequent renditions, there is a small machine, constructed usually by a solitary scientist in a remote laboratory. One dials the year of interest, steps into the machine, presses a button, and presto, here's the past or the future. Among the common devices in time-travel stories are the logical paradoxes that accompany meeting yourself several years ago; killing a lineal antecedent; interfering directly with a major historical event of the past few thousand years; or accidentally stepping on a Precambrian butterfly – you are always changing the entire subsequent history of life.

  Such logical paradoxes do not occur in stories about travel to the future. Except for the element of nostalgia – the wish we all have to relive or reclaim some elements of the past – a trip forward in time is surely at least as exciting as one backward in time. We know rather much about the past and almost nothing about the future. Travel forward in time has a greater degree of intellectual excitement than the reverse.

  There is no question that time travel into the future is possible: We do it all the time merely by aging at the usual rate. But there are other, more interesting possibilities. Everyone has heard about, and now even a fair number of people understand, Einstein's special theory of relativity. It was Einstein's genius to have subjected our usual views of space, time, and simultaneity to a penetrating logical analysis, which could have been performed two centuries earlier. But special relativity required for its discovery a mind divested of the conventional prejudices and the blind adherence to prevailing beliefs – a rare mind in any time.

  Some of the consequences of special relativity are counterintuitive, in the sense that they do not correspond to what everybody knows by observing his surroundings. For example, the special theory says that a measuring rod shrinks in the direction in which it is moving. When jogging, you are thinner in the direction in which you are jogging – and not because of any weight loss. The moment you come to a halt you immediately resume your usual paunch-to-backbone dimension. Similarly, we are more massive when running than when standing still. These statements appear silly only because the magnitude of the effect is too small to be measured at jog velocities. But were we able to jog at some close approximation to the speed of light (186,000 miles per second), these effects would become manifest. In fact, expensive synchrotrons – machines to accelerate charged particles close to the speed of light – take account of such effects, and work only because special relativity happens to be correct. The reason these consequences of special relativity seem counterintuitive is that we are not in the habit of traveling close to the speed of light. It is not that there is anything wrong with common sense; common sense is fine in its place.

  There is a third consequence of special relativity, a bizarre effect important only close to the speed of light: The phenomenon called time dilation. Were we to travel close to the speed of light, time, as measured by our wristwatch or by our heartbeat, would pass more slowly than a comparable but stationary clock. Again, this is not an experience of our everyday life, but it is an experience of nuclear particles, which have clocks built into them (their decay times) when they travel close to the speed of light. Time dilation is a measured and authenticated reality of the universe in which we live.

  Time dilation implies the possibility of time travel into the future. A space vehicle that could travel arbitrarily close to the speed of light arranges for time, as measured on the space vehicle, to move as slowly as d
esired. For example, our Galaxy is some sixty thousand light-years in diameter. At the velocity of light, it would take sixty thousand years to cross from one end of the Galaxy to the other. But this time is measured by a stationary observer. A space vehicle able to move close to the speed of light could traverse the Galaxy from one end to the other in less than a human lifetime. With the appropriate vehicle we could circumnavigate the Galaxy and return almost two hundred thousand years later, as measured on Earth. Naturally, our friends and relatives would have changed some in the interval – as would our society and probably even our planet.

  According to special relativity, it is even possible to circumnavigate the entire universe within a human lifetime, returning to our planet many billions of years in our future. According to special relativity, there is no prospect of traveling at the speed of light, merely very close to it. And there is no possibility in this way of traveling backward in time; we can merely make time slow down, we cannot make it stop or reverse.

  The engineering problems involved in the design of space vehicles capable of such velocities are immense. Pioneer 10, the fastest man-made object ever to leave the Solar System, is traveling about ten thousand times slower than the speed of light. Time travel into the future is thus not an immediate prospect, but it is a prospect conceivable for an advanced technology on planets of other stars.

  There is one further possibility that should be mentioned; it is a much more speculative prospect. At the end of their lifetimes, stars more than about 2.5 times as massive as our Sun undergo a collapse so powerful that no known forces can stop it. The stars develop a pucker in the fabric of space – a "black hole" – into which they disappear. The physics of black holes does not involve Einstein's special theory of relativity; it involves his much more difficult general theory of relativity. The physics of black holes – particularly, rotating black holes – is rather poorly understood at the present time. There is, however, one conjecture that has been made, which cannot be disproved and which is worthy of note: Black holes may be apertures to elsewhen. Were we to plunge down a black hole, we would re-emerge, it is conjectured, in a different part of the universe and in another epoch in time. We do not know whether it is possible to get to this other place in the universe faster down a black hole than by the more usual route. We do not know whether it is possible to travel into the past by plunging down a black hole. The paradoxes that this latter possibility imply could be used to argue against it, but we really do not know.

  For all we do know, black holes are the transportation conduits of advanced technological civilizations – conceivably, conduits in time as well as in space. A large number of stars are more than 2.5 times as massive as the Sun; as far as we can tell, they must all become black holes during their relatively rapid evolution.

  Black holes may be entrances to Wonderlands. But are there Alices or white rabbits?

  37. Starfolk

  I. A Fable

  Once upon a time, about ten or fifteen billion years ago, the universe was without form. There were no galaxies. There were no stars. There were no planets. And there was no life. Darkness was upon the face of the deep. The universe was hydrogen and helium. The explosion of the Big Bang had passed, and the fires of that titanic event – either the creation of the universe or the ashes of a previous incarnation of the universe – were rumbling feebly down the corridors of space.

  But the gas of hydrogen and helium was not smoothly distributed. Here and there in the great dark, by accident, somewhat more than the ordinary amount of gas was collected. Such clumps grew imperceptibly at the expense of their surroundings, gravitationally attracting larger and larger amounts of neighboring gas. As such clumps grew in mass, their denser parts, governed by the inexorable laws of gravitation and conservation of angular momentum, contracted and compacted, spinning faster and faster. Within these great rotating balls and pinwheels of gas, smaller fragments of greater density condensed out; these shattered into billions of smaller shrinking gas balls.

  Compaction led to violent collisions of the atoms at the centers of the gas balls. The temperatures became so great that electrons were stripped from protons in the constituent hydrogen atoms. Because protons have like positive charges, they ordinarily electrically repel one another. But after a while the temperatures at the centers of the gas balls became so great that the protons collided with extraordinary energy – an energy so great that the barrier of electrical repulsion that surrounds the proton was penetrated. Once penetration occurred, nuclear forces – the forces that hold the nuclei of atoms together – came into play. From the simple hydrogen gas the next atom in complexity, helium, was formed. In the synthesis of one helium atom from four hydrogen atoms there is a small amount of excess energy left over. This energy, trickling out through the gas ball, reached the surface and was radiated into space. The gas ball had turned on. The first star was formed. There was light on the face of the heavens.

  The stars evolved over billions of years, slowly turning hydrogen into helium in their deep interiors, converting the slight mass difference into energy, and flooding the skies with light. There were in these times no planets to receive the light, and no life forms to admire the radiance of the heavens.

  The conversion of hydrogen into helium could not continue indefinitely. Eventually, in the hot interiors of the stars, where the temperatures were high enough to overcome the forces of electrical repulsion, all the hydrogen was consumed. The fires of the stars were stoked. The pressures in the interiors could no longer support the immense weight of the overlying layers of star. The stars then continued their process of collapse, which had been interrupted by the nuclear fires of a billion years before.

  In contracting further, higher temperatures were reached, temperatures so high that helium atoms – the ash of the previous epoch of nuclear reaction – became usable as stellar fuel. More complex nuclear reactions occurred in the insides of the stars – now swollen, distended red giant stars. Helium was converted to carbon, carbon to oxygen and magnesium, oxygen to neon, magnesium to silicon, silicon to sulfur, and upward through the litany of the periodic table of the elements – a massive stellar alchemy. Vast and intricate mazes of nuclear reactions built up some nuclei. Others coalesced to form much more complex nuclei. Still others fragmented or combined with protons to build only slightly more complex nuclei.

  But the gravity on the surfaces of red giants is low, because the surfaces have expanded outward from the interiors. The outer layers of red giants are slowly dissipated into interstellar space, enriching the space between the stars in carbon and oxygen and magnesium and iron and all the elements heavier than hydrogen and helium. In some cases, the outer layers of the star were slowly stripped off, like the successive skins of an onion. In other cases, a colossal nuclear explosion rocked the star, propelling at immense velocity into interstellar space most of the outside of the star. Either by leakage or explosion, by dissipation slow or dissipation fast, star-stuff was spewed back to the dark, thin gas from which the stars had come.

  But here, later generations of stars were aborning. Again the condensations of gas spun their slow gravitational pirouettes, slowly transmogrifying gas cloud into star. But these new second- and third-generation stars were enriched in heavy elements, the patrimony of their stellar antecedents. Now, as stars were formed, smaller condensations formed near them, condensations far too small to produce nuclear fires and become stars. They were little dense, cold clots of matter, slowly forming out of the rotating cloud, later to be illuminated by the nuclear fires that they themselves could not generate. These unprepossessing clots became the planets: Some giant and gaseous, composed mostly of hydrogen and helium, cold and far from their parent star; others, smaller and warmer, losing the bulk of their hydrogen and helium by a slow trickling away to space, formed a different sort of planet – rocky, metallic, hard-surfaced.

  These smaller cosmic debris, congealing and warming, released small quantities of hydrogen-rich gases, trapped in t
heir interiors during the processes of formation. Some gases condensed on the surface, forming the first oceans; other gases remained above the surface, forming the first atmospheres – atmospheres different from the present atmosphere of Earth, atmospheres composed of methane, ammonia, hydrogen sulfide, water, and hydrogen – an unpleasant and un-breathable atmosphere for humans. But this is not yet a story about humans.

  Starlight fell on this atmosphere. Storms were driven by the Sun, producing thunder and lightning. Volcanoes erupted, hot lava heating the atmosphere near the surface. These processes broke apart molecules of the primitive atmosphere. But the fragments reassorted into more and more complex molecules, falling into the early oceans, there interacting with each other, falling by chance upon clays, a dizzying process of breakdown, resynthesis, transformation – slowly moving toward molecules of greater and greater complexity, driven by the laws of physics and chemistry. After a time, the oceans achieved the constituency of a warm dilute broth.

  Among the innumerable species of complex organic molecules forming and dissipating in this broth there one day arose a molecule able crudely to make copies of itself – a molecule which weakly guided the chemical processes in its vicinity to produce molecules like itself – a template molecule, a blueprint molecule, a self-replicating molecule. This molecule was not very efficient. Its copies were inexact. But soon it gained a significant advantage over the other molecules in the early waters. The molecules that could not copy themselves did not. Those that could, did. The number of copying molecules greatly increased.

  As time passed, the copying process became more exact. Other molecules in the waters were reprocessed to form the jigsaw puzzle pieces to fit the copying molecules. A minute and imperceptible statistical advantage of the molecules that could copy themselves was soon transformed by the arithmetic of geometrical progression into the dominant process in the oceans.