Читать книгу The Davey Dialogues - An Exploration of the Scientific Foundations of Human Culture - John C. Madden - Страница 16

The highest endowments do not create – they only discover. All transcendent genius has the power to make us know this as utter truth. Shakespeare, Beethoven – it is inconceivable that they have fashioned the works of their lives; they only saw and heard the universe that is opaque and dumb to us. When we are most profoundly moved by them, we say, not “O superb creator” – but “O how did you know! Yes it is so.”

RUTH BENEDICT In Margaret Mead’s Ruth Benedict, 1974

– Well, I hope you have a good story to tell me today. I have just been listening to an astronomer give me his version of what I hoped you were going to say. He was very nice, but he fed me a lot of equations, and showed me a lot of pictures, which as you know I cannot see. All astronomers seem star-struck by the beauty of the heavens. I gather from him that what they see through their telescopes far surpasses the beauty that often causes wonder to non-astronomers when viewing the sky at night while far removed from city lights.

Davey and I had not spoken for a week, during which I had a wonderful time reminding myself about the historical peaks of astronomical research.

– I have to agree that for most of us humans, the night sky can be staggeringly beautiful, and for astronomers, who commonly see relative close-ups of a great variety of stellar events, the sights are even more moving and beautiful. Personally, I find it hard to improve on the photographs of our own planet taken from outer space or from the moon.

I have worked hard in the past week to assemble an overview of advances in human understanding of the heavens since those first discoveries of a thousand or more years ago. However, it appears that you may have heard about this already, and I don’t want to bore you.

Figure 4.1 – Earthrise over the lunar surface as photographed from the Apollo 8 spacecraft in 1968.

I was feeling just a little bit piqued that despite my hard work over the past week, he was evidently not relying overly much on what I might tell him!

– On the contrary, Peter. As I told you, the astronomers I spoke with did not quite give me the perspective on their work that I was seeking. I am expecting that you will do better. Perhaps we should get started.

I was partially mollified but once again nervous that I, too, would fail to provide Davey with information he could understand.

– Very well, but be sure to stop me if I am not telling you what you want to know.

At the dawn of civilization there was a lot about the world around us –as well as about ourselves – that seemed unknowable. Gradually, and with accelerating success, we have pushed back the barriers to our understanding. But barriers still remain. One classic barrier has been our understanding of the origins and evolution of the universe. In my lifetime an amazing amount has been learned, and we have reached some totally unexpected conclusions. Not surprisingly, the knowledge we have gained about the origins of our universe has only led to speculation as to whether or not there may be other universes! Today I shall summarize what I think were the key milestones in expanding our knowledge of our universe, and close with some very speculative conclusions that bear on how we might put our own species in an appropriate perspective.

With the possible exception of the Greeks, the major driver of our forebears’ interest in astronomy was their belief that by their studies they could learn more about the gods whose capricious will could bring disaster upon them. Nonetheless, their accomplishments were very impressive, and a good lesson in how much enquiring minds can learn without the benefit of much in the way of special equipment.

However, there is another lesson to be learnt from examining the more recent history of astronomy: with a combination of enquiring minds and ingenious equipment one can learn a lot more. What we have learnt about the heavens in the past fifty to one hundred years would have knocked the socks off the ancients!

– I’m surprised you think the ancients wore socks. When I last asked what the ancients wore, I was told either that they ran around barefoot wearing hairy skins, or were clad in simple cloth and wore sandals on their bare feet.

Was Davey attempting to be funny, or was he serious? I wondered. Perhaps I should avoid phrases that required a deeper knowledge of the English language than Davey might have.

– Well , um, yes.

– Don’t be so serious. Of course I know what you mean.

He still sounded strangely serious. I couldn’t think of a good comeback. He must have been joking, but if so, he had not yet mastered a joking tone of voice or even a muffled giggle.

After a brief pause to collect my thoughts, I carried on.

– Aristarchus of Samos, who was born in 310 BC, twelve years after the death of Aristotle, is generally credited as having been the first to conclude that Earth orbits the sun, rather than vice versa (thus beating out Indian mathematician-astronomer Aryabhata by about eight hundred years). Aristotle (384–322 BC), who was for twenty years a student of Plato and who later tutored Alexander the Great, concluded, as did most others at the time, that Earth was at the centre of the universe, with the sun and the moon orbiting us in circular paths, and the observable planets also orbiting Earth, but in orbits that were peculiarly hard to fathom. Aristotle believed that Earth was spherical, based in part on observing that the shadow of Earth on the moon, as seen during a lunar eclipse, is curved. He also believed that a sphere is the most perfect shape.[7] The stars, he concluded, were a static display stuck to an outer sphere that also rotated about Earth. Many of Aristotle’s writings, including some related to astronomy, were conserved in Athens up to the time of the Roman conquest of Athens by Sulla in 86 BC, when they were taken to Rome, where they attracted considerable scholarly attention.

In the late medieval period, Roman Catholic Church scholars “rediscovered” Aristotle, who became the bedrock philosopher for the re-awakening Church. Unfortunately, in the process, Aristotle’s views on astronomy and the universe were adopted as a part of the package, and became integrated into Church dogma. It is more than a little ironic that the Church should have adopted as its paramount philosopher a man who believed that “the true spirit of science and philosophy is born when problems are studied for their intrinsic interest, detached from practical interests.”[8] The Church at that time was still firmly of the view that it was the ultimate arbiter of the “creation story” at least as far as the Western world was concerned. The task of scientists was to confirm the words of God, as interpreted by senior Church officials.

For almost no one was this attitude more unfortunate than Galileo Galilei (1564–1642), who in 1609 was the first to use a telescope for astronomical purposes. Galileo’s cardinal sin was that his observations seemed to confirm that Earth orbited the sun, consistent with Copernicus’s rediscovery of what Aristarchus had observed about seventeen centuries before!

By the early 1900s, with lots of assistance from other early pioneers such as Tycho Brahe, Johannes Kepler, Isaac Newton and Giordano Bruno a lot was known about our solar system and rather less about the stars and galaxies beyond it. The time was ripe for some breathtaking changes in mankind’s perception of time and distance.[9]

Starting in 1863, some very powerful new tools and new theories fuelled remarkable advances in astronomy. The new tools, and the accompanying observations and theories, have enabled us to gain both a qualitative and a quantitative understanding of our universe.

In 1863 Sir William Huggins, an amateur English astronomer, was the first to examine the spectra of visible stars. Only four years previously, two Germans, Gustav Kirchoff and Robert Bunsen, had discovered that each gas has its own spectrum of lines of different colours, which are emitted when the gas is heated (in their case, in a Bunsen burner). Huggins found the spectral lines characteristic of hydrogen and helium in many stars, as well as in nebulas and comets. For the first time, humans knew that the stars were predominantly composed of these two gasses. Thenceforth astronomers focussed a lot of attention on examinations of the spectral lines present in the light coming from the various sources of light in the sky. This was the start of an accelerating series of discoveries, not the least of which was a vital helping hand that spectral analysis provided in calculating the distance to many visible stars.

There is an old joke about a physicist, an engineer and a businessman who, just after the First World War, somehow got involved in a bet about who could first determine the height of the steeple of a distant church. They all three raced over to the church. The engineer quickly got his transit out of the trunk of his Model T Ford car, measured off an appropriate distance over level ground from the steeple (d), and then measured the angle (A) from where he stood up to the steeple top. He then used high-school trigonometry to determine the steeple height (h), i.e. h=d.tanA. The physicist, on the other hand, stopped only to pick up a few stones, before scrambling to the steeple top. He threw the stones off the top and timed their descent. Since he knew the force of gravity (g), and he now knew how long the rocks took to fall (t), he was able to calculate the height (h) from Newton’s famous equation h=gt2/2. The businessman, too, went straight to work. He went into the church and asked the verger how high the steeple was. Of course, he won the bet hands down!

If the same threesome had instead bet on who would first discover the distance from Earth to a certain star, they would have been at a loss. The businessman would have found no one to give him an authoritative answer. The physicist, too, would obviously have been out of luck. If the engineer was really lucky, and one of the sun’s closest stars had been chosen for measurement, he would have found that with great care it was possible to measure the distance to such a star by observing the apparent change in position of the star as Earth moves in orbit around the sun. The radius of Earth’s orbit is fairly constant, varying between 149 and 152 million kilometres (or about eight light minutes[10]). The principle would have been the same as the one he used to measure the height of the church steeple, but even if he took his measurements at the most appropriate six-month interval, the difference in angle that he would measure for the closest star (Proxima Centauri) would be only 1.534 seconds of arc, or about four hundredths of a degree! No wonder most observers through history have assumed that stars hold their same relative position in the sky indefinitely.

At the current time, the limit of accuracy for the angular measurement of celestial objects is about 0.05 seconds of arc, a situation that allows the distance to less than two thousand of our closest stars to be measured using geometry.

I knew I could not show Davey any sort of image that would help him understand the method of measurement I described, so I had to hope that I’d explained it adequately. However I have shown a very rough diagram of the geometry involved in Figure 4.2 below. The representation is clearly not to scale. The distance from the sun to Earth is shown as “r”. If the distance to Proxima Centauri (d) were to scale, it would be about 2.7 kilometres off to the left of the page! The angle “a” shown on the diagram is half the difference in apparent position of the star from the two different viewing points (technically called the parallax angle) of 1.534 arc seconds referred to above.

Figure 4.2 – Measurement of the distance to Proxima Centauri by the parallax method.

– Davey, I wonder if you ever studied trigonometry in your equivalent of our high schools? If so, you may recall that the sine of the angle a is r/d. Hence d, the distance to Proxima Centauri is r/sin a, which works out to 4.2 light years.

– Peter, you continue to underestimate my intelligence. I suppose that humankind has been the most intelligent species on Earth for so long that it cannot admit to itself that there may be other beings that are more intelligent than humans. I’ll let you know if you ever tell me something I cannot understand!

– Thanks a lot! Perhaps I should call you Mr. Smartypants! I suppose I should have known that you would think you are a lot cleverer than I am, especially after all you have told me about yourself!

Davey noticed my sarcasm. He at once apologised and asked me to continue. His apology did not at all address his claim to be much smarter than me, a claim I was not yet ready to concede.

– Distance measurements are obviously fundamental to gaining an understanding of our universe. Some method of measuring our separation from more distant objects was badly needed. Solutions to the problem came from unexpected directions.

In 1895 Henrietta Leavitt was given a job at the Harvard University observatory. In accordance with the practices of the day, because she was a woman, she was not given full status as an astronomer but was instead given leadership of the department that studied the photographic images of stars to determine their magnitude. During her career there, she discovered over 2400 stars whose brightness varied periodically in magnitude, usually with a period ranging from several hours to several tens of days. These variable stars are called cepheids after the name of the first variable star discovered (in 1784), Delta cephei. Leavitt was able to determine that there is a direct correlation between the brightness of these stars and the period of magnitude variations. This was a very significant discovery, since it permitted observers to determine both the actual brightness of a star and to measure its apparent brightness when seen from Earth. Since brightness is known to decrease as the square of the distance from the light source, it became possible to determine the distance to the star, having first calibrated cepheids, which were close enough to measure using the previously described geometric (or parallax) method to provide the baseline distances.[11]

However, it is the third link in the chain of astronomical distance measuring techniques that is the most spectacular.

In the 1920s, the vast majority of astronomers believed that the universe and the Milky Way galaxy were one and the same thing. Nebulas were thought to be stars (in the Milky Way) in the process of formation. The universe was thought to be a static entity, though the theory included the idea that the birth and death of stars was part of a continual regeneration process.

The first real clue that this might not be so came from an American astronomer, Vesto Melvin Slipher who started in 1909 to study nebulas in the hopes of learning more about how our sun came about.[12] Slipher looked first at the Andromeda nebula and observed that the spectral lines of light from the nebula, including the lines from excited hydrogen gas, were displaced from their proper position on the frequency scale.

As you may already be aware, Davey, both light and sound from a source that is moving relative to the observer will undergo an apparent frequency shift audible or visible to the observer, a shift that is referred to as the Doppler shift (after the effect’s discoverer, Christian Doppler [1803–53], an Austrian mathematician and physicist). As it happens, the Andromeda nebula is moving toward us, so the frequency of the spectral lines were shifted toward the blue, or high-frequency, end of the visible spectrum. However, by 1914 Slipher had discovered that some other nebulas were apparently receding from Earth at very high velocities, since the spectral lines were shifted down in frequency, i.e. toward the colour red. Calculations showed that one nebula was apparently receding from us at an astounding 1100 km/sec, or about 4 million km/hr! How could something moving this fast be in our galaxy?

It was Edwin P. Hubble who provided the evidence to resolve this dilemma. Hubble was an American Rhodes Scholar who studied law at Oxford (to please his father), was briefly a school teacher, served in the US Army during the First World War, and, finally, in 1919 returned to his first and enduring passion, astronomy, when he accepted an offer of employment at Mount Wilson Observatory in California. In 1923 Hubble launched into a study of very bright stars (novae) in spiral nebulas. To his surprise, he came across a cepheid in the Andromeda nebula, and, by applying the by then standard calculations, discovered that this nebula was a million light years from us, putting it well outside the Milky Way. In rapid order, he found other cepheids, and found that many of them, too, were well removed from our own galaxy. The implication was clear. The Milky Way was not the only galaxy in town! There were many others. In fact the current estimate is that there are over one billion galaxies in our universe, each with, on average, about 100 billion stars! There are thus about 10²⁰ (i.e. 100,000,000,000,000,000,000) stars in the universe.[13]

But there was yet another major surprise still in store.

Slipher’s data had shown that some nebulas were moving away from us at high speeds. There were two physicists who dared to wonder aloud whether this phenomenon might be due to the expansion of the universe. The first was Alexander Friedmann, a Russian World War One fighter pilot and professor in St. Petersburg, and the second was Georges Lemaître, a young Belgian abbé and professor. To begin with, the evidence was spotty. Hubble changed all that. By 1929 he had collected enough evidence to show that the rate at which a galaxy was apparently receding from Earth was directly proportional to its distance from us. The constant of proportionality came to be called “the Hubble constant”[14] Alexander Friedmann and Georges Lemaître were right.

Humanity thus took another great leap toward insignificance, or at least to lack of centrality. Not only was our planet not at the centre of the universe, neither was the sun, nor even our galaxy. Our sun is only one of a hundred billion billion stars, and who knows how many habitable planets there are in the universe? Furthermore, like all the other planets and stars, we resulted from a cosmic Big Bang since calculated to have taken place about 13.8[15] billion years ago!

The idea of the Big Bang rose directly from Hubble’s observation that the universe seemed to be exploding outward. By running the events in reverse, everything seemed to come back together to a time of origin of one, huge, “universal” explosion over 13 billion years ago.

All this took some time to digest, even amongst the scientists. Arthur Eddington, the leading astronomer of the day, wrote about these findings.

I paused to find my quote from Eddington.

– “. . . it seems to require a sudden and peculiar beginning of things. . . . As a scientist I simply do not believe that the present order of things started out with a bang; unscientifically I feel equally unwilling to accept the implied discontinuity of the divine nature.”[16] Others have since found the concept that everything started with one Big Bang equally uncomfortable. Was there really nothing at all prior to the Big Bang? We all have some freedom to speculate here, since, as of now, there is really not much hope of making any observations outside of our universe.

At this point I waited deferentially for a comment from Davey, but none was forthcoming. Perhaps he was sleeping? In the circumstances, I would bet money that while the outer boundaries of our powers of observation and comprehension seem to be the boundaries of the universe we inhabit, it is likely that there are now, have been and will be other universes. But let’s be clear that this is not a scientific observation. In my case, it is based in part at least, on my unprovable, but for me very stimulating and real, conversations with Davey.

– In spite of Arthur Eddington’s unease, there was at least one very significant member of the cheering section for Hubble and his findings. Albert Einstein had discovered that his General Theory of Relativity predicted that the universe was not in a stable state but was either expanding or contracting. This was directly counter to the generally held belief in a stable universe prior to Hubble, a fact that had persuaded Einstein that his theory must be wrong. He therefore introduced a cosmological constant whose magnitude was calculated to ensure stability of the universe. Six years later when he first heard of Hubble’s discoveries, he revised his equations to remove the constant and referred to his earlier introduction of the constant as the biggest blunder of his life. He even made a special trip up Mount Wilson to thank Hubble for his work. By that time he had already taken pains to give special recognition and thanks to Friedmann and Lemaître.[17]

It is perhaps ironic that the discovery in 1998 that there is a small but continuous acceleration in the rate at which the universe is expanding has caused a revival of Einstein’s cosmological constant. The leading explanation for this surprising discovery is that the universe is suffused with a constant (but so far undetected) energy filling all space homogeneously. This energy has been labelled “dark energy”, and can be conveniently thought of as a cosmological constant. If you find this confusing, don’t be discouraged. There will likely be more changes coming soon. These are exciting times in the field of cosmology.

Apparently Davey, too, had noticed the anomaly. Finally, a response!

– I find it very interesting that even very distinguished scientists such as Einstein seem to incorporate new variables into their theories just to make them fit the facts, without, it seems, any real notion of what in the physical world that particular variable represents. There seems to be a lot your physicists don’t know about your surroundings.

– On the contrary, I think you should be amazed at how much they do know. It has taken us centuries to accumulate the knowledge we have of our environment, but in most cases we are now able to understand and to forecast what is going on in our universe to an astounding degree of accuracy. For instance, the Standard Model theory of particle physics was able to forecast not just the existence of new particles as yet undiscovered, but also predicted their mass correctly to better than four decimal places, as measured after experimental physicists had searched for and found the particles. When a theory is able to make predictions such as these, one has real confidence that the theory is on the right track.

– I suppose you may be right. But what exactly is the Standard Model of which you speak so highly. I don’t recall that you have mentioned it before.

– It is an intellectual triumph – a masterpiece of co-operative scientific endeavour stretching over sixty years or so. The model encompasses a theory of almost everything in that it describes all known forces except the force of gravity, and successfully predicts the properties of all known sub-atomic particles.

– What you seem to be telling me is that this great model of yours describes and predicts a lot of things, but it is of little, if any, help if we want to understand your universe, where the force of gravity reigns supreme.

– There is an element of truth to what you say, I conceded, but you should know that the nuclear reactions taking place in the billions and billions of stars in the universe are very accurately predicted and described by the Standard Model. That said, the Standard Model, while useful in providing an understanding of the life and death of stars, is currently of little help when it comes to understanding the overall structure and history of our universe.

Davey seemed ambivalent. It was somewhat annoying to realize that he might know far more than I was giving him credit for. Perhaps he knew the solutions already to some of the tough questions the cosmologists were tackling. I planned to talk a bit about these questions in a future dialogue, and I wondered if I could trick him into showing his hand at that time. In the meantime, I returned to the subject at hand.

– As you can see, the numbers associated with the study of astronomy are, well, astronomical! It takes our sun 250 million years to complete an orbit about the centre of our galaxy (by comparison, dinosaurs are thought to have appeared on Earth only about 230 million years ago). As previously mentioned, the average galaxy is estimated to contain one hundred thousand million (~10¹¹, or 1 followed by 11 zeros) stars, and astronomers believe that there are well over a billion galaxies in the universe! Over the past decade, it has been possible to observe the effects of large planets orbiting nearby stars. While not all stars have planets, clearly some of those that do, like our sun, have several planets.

At this time I know of no reliable estimate of the likely number of planets in the universe. It may be of the same order as the estimated number of stars (i.e. ~10²⁰), but even a number one thousand times smaller is still unimaginably large. Similarly, it is hard to find an estimate of how many, out of the total number of planets, could support life, but it would be exceedingly rash to imagine that our Earth is the only one amongst this huge number of candidates. A similar line of argument makes it seem extremely likely that more than one of these remote planets is home to life forms that are at least as intelligent as we are.

Most of us humans are still inclined to consider ourselves the wisest and most intelligent species in the universe. If there is a superior species one might reasonably expect that it would have been in touch with us by now, and left us with firm evidence of its (superior) existence. But is this really true?

Davey, you could really help me here. You say you don’t even come from our universe, but rather from another universe altogether. Assuming that this is true, and that you really have wandered about our universe in ways we are not yet capable of, perhaps you could let me know what you have found in the way of other life forms in our universe and perhaps also, by the way, what you know about life forms in your own universe.

– Sorry, Peter. I thought I had already explained to you that I cannot possibly answer such questions without first understanding whether what I say could unduly harm life in your universe. I understand your impatience, but you will just have to be patient for the time being.

– It’s all very well for you to ask for patience. You need to understand that I have to work hard to pull together the information you have asked for. Thus far I have received precious little from you for all my effort.

– I think you know that you have my sincere thanks as well as my appreciation. I was given to understand that most humans appreciate such gifts.

I rolled my eyes. As I did so I realized that he could not see the gesture. This realization led me to wonder how often he was misled by what he heard, as it would have been much more difficult for him to detect sarcasm than for the rest of us in the circumstances.

– Do please continue, Peter. What you are describing is a rather intricate method for determining the extent and nature of your universe. I admit that it seems rather awkward and unnecessarily involved to me, but given that you lack the tools that I have to understand my universe, I have to take my hat off to your scientists for what they have accomplished – and I am very interested to see how your story will end!

– All right. I suppose I should continue, at least for the time being.

In an attempt to introduce some order into what can at best be described as a highly speculative subject, Frank Drake, a physicist at the University of California in Santa Cruz, introduced in 1961 what has become known as the Drake Equation. (Carl Sagan, a well-known physicist and science writer of my time, gave the Drake Equation considerable prominence, to such an extent that it is also known as the Sagan Equation.)

Figure 4.3 – The Butterfly Nebula. This spectacular photo (NGC 6302) was taken in 2009 using the upgraded Hubble Space Telescope. It is representative of the great beauty frequently encountered by astronomers as they observe our universe.

The formula itself is simply the product of a set of fractions and average numbers multiplied by the rate of star formation in our galaxy; the equation addresses the likelihood of communication with intelligent life elsewhere in our galaxy only. If we wanted a likelihood for the whole universe, we might choose to multiply our result by the number of galaxies in the universe reduced by a factor that reflects the considerable difficulty of communication over distances greater than the diameter of our galaxy. If we sent a message to an intelligent being one million light years away, it would be at least two million years before we could receive a reply. And if we wanted to compute the likelihood of communication with intelligent life outside our universe, well, we probably would have to ask you, Davey, for an estimate!

Davey responded immediately.

– I’ve already given you an answer! Not today!

– That’s disappointing. Perhaps you have a headache?

I paused to emphasize my displeasure, then I continued.

– Drake’s formula looks like this:

N (the number of civilizations in our galaxy with which we might be able to communicate at any given time) equals:

R (the rate of star formation in our galaxy (about 6 per year), times

f_p (the fraction of those stars that have planets [current estimates are 20–50 per cent]), times

n_e (the average number of planets per star with planets that can potentially support life – a figure between 1 and 5 has been suggested), times

f_l (the fraction of planets in n_e where life actually evolves – fractions ranging from 0 all the way to 1 have been suggested), times

f_i (the fraction of f_l where intelligent life evolves – some believe that the fraction should be close to 1, because the survival advantage of intelligence is very large, hence intelligence is almost certain to develop; others see the development of brains in life forms as being highly improbable, perhaps as small as 1 chance in 10 million), times

f_c (the fraction of intelligent life (f_i ), which develops both the means and the desire to communicate externally – most guesses are in the 10–20 per cent range), times

L (the average expected duration of the intelligent and communicating life forms calculated above – now there is a challenging number to estimate! We have only our own experience to go on. Some pessimists have estimated an average duration as low as 68 years. Drake himself suggested 10,000 years. Perhaps he was an optimist, but humankind in our present form appears to have been around for about 160,000 years, of which we have been attempting to communicate with extraterrestrial beings for only 50 or 60 years. The pessimistic estimate has us almost overdue for disappearance! Even the optimistic suggestion of 10,000 years is not very long except when viewed against the average human lifetime.)

At this point, to my delight, Davey intervened with a question.

– Why would you necessarily assume that if one intelligent life form ceased trying to communicate, another on the same planet would not immediately supplant it?

– I imagine it is because we on Earth find it difficult to believe that another life form, the chimpanzees for example, even assuming they survived whatever holocaust resulted in the extinction of Homo sapiens, would gain intelligence rapidly enough to engage in extraterrestrial communication in a reasonable span of time.

– I see. But by now you must have realized that in my universe this assumption is not valid. I am the living proof that one intelligent life form can be superseded by another. In due course I will explain to you how that came about.

After a pause to encourage further elaboration, I again continued.

– As you are likely already suspecting, estimates of the value of N vary very widely. If you are a relative optimist, and expect that the average communicating intelligent civilizations will last about ten thousand years, and that about 20 per cent of planets that evolve life will also evolve intelligent life, then you would expect that there are about a thousand civilizations in our galaxy trying to communicate with us. On the other hand, it seems that you don’t have to be much of a pessimist to conclude that the likelihood of hearing from other civilizations in our galaxy is very small indeed, perhaps one chance in a million. Of course the likelihood of a two-way conversation is smaller still, since any communication we receive may easily have been transmitted halfway across the diameter of the galaxy, or from about 150,000 light years away. At that distance, any reply would be received at least three hundred thousand years after the original message was sent!

Davey, here is a place where you can be a big help to us! Let me ask you one more time. Have you run into intelligent life elsewhere in our universe? Does that life resemble us in any way? How many instances of intelligent life have you found? Are you even able to visit more than one planet in the universe at one time? And what about . . .

– Stop all those questions, please. Before I can say anything, I have to be aware of your understanding of your situation. Otherwise, what I know could be profoundly destructive. Think about it for a minute, and you will understand what I am saying.

What could he have meant? I wondered. Does he have bad news for us, such that life would lose all its meaning? Are we destined to be destroyed by extra-terrestrial life forms? Perhaps there is a big chunk of rock headed our way that will wipe us out? Or perhaps we are destined to be the instrument of our own destruction? I at once yearned for and dreaded the answer that my continued efforts might prompt him to provide.

After a long pause it was Davey who broke into my thoughts.

– So, what you are trying to tell me is that once your forebears understood that the sun and the moon were not living beings, but rather balls of matter, a ball of gas in the case of the sun and a ball of rock in the case of the moon, they decided not to worship the sun and the moon any more, and opened a search for gods elsewhere?

– Well, not quite. Religion in the developed world had already changed a lot before the truth about the sun and moon were known. The major factor influencing religious changes seems to have been the changes in organization of human societies, which went from being many small bands of hunter-gatherers operating independently to much larger societies with one person acting as the supreme leader. I suppose that in the circumstances it seemed natural that the heavens might be organized in the same way, so that the belief in a single omnipotent God became increasingly widespread. The Jewish faith is generally reckoned to have been the first successful monotheistic religion. It got its start in about 1000 BC, well ahead of the discoveries I have just reported.

– So, it looks as though science really does not have much influence on the direction your religions take after all. I thought you were trying to tell me that science is important!

– It is important. But the influence of science did not really become key until the discipline of science had itself evolved to the point where its influence and authority became widespread. This did not really happen until some societies became wealthy enough that some of its members could afford to devote themselves to scientific enquiry. But this did not happen until long after the founding of the major monotheistic religions.

– I see. But you must admit that this whole religious business is very strange. As far as I know, my friends in my universe were never religious.

– On the contrary. It is not at all strange. For us religion was and still is a logical activity closely connected with our desire to survive. Perhaps your people had little desire to survive. I would not be too surprised as you tell me they have died out already! But I shall be very interested to learn how they could have come to exist if they had no instincts for survival.

– You must be sure not to jump to conclusions too quickly. In any case, I assume you want to stop here and meet again in about a week’s time. I wonder what you plan to talk about then. Perhaps you will tell me how humans finally came to understand their place in your universe?

Davey was getting under my skin! I responded in kind.

– You must be sure not to jump to conclusions too quickly.

I smiled grimly. How likely was it, I wondered, that Davey’s people were really as intelligent and all-knowing as Davey seemed to think them to have been?