The Faber Book of Science (55 page)

Men have lost their lives because the Sun has disrupted radio; nations are equally vulnerable, in this age of the ICBM.

You will recall that though the Sun shines with remarkable steadiness in the visible spectrum, it flares and sparkles furiously on
the long (radio) waves. Exactly the same thing happens with its X-ray emission, even though these waves are a billion times shorter. Moreover, both the Sun’s radio waves and its X-rays appear to come from the same localized areas of the solar surface – disturbed regions in the neighbourhood of sunspots, where clouds of incandescent gas larger than the Earth erupt into space at hundreds of miles a second.

For reasons not yet understood (there is not much about the Sun that we do thoroughly understand) solar activity rises and falls in an eleven-year cycle. The Sun was most active around 1957, which is why that date was chosen for the International Geophysical Year. In the 1960s it headed for a minimum but unfortunatley threatened to come back to the boil at around the time the first major space expeditions were being planned. The astronauts might have run into some heavy weather, since the Sun by then was shooting out not only vast quantities of ultraviolet, X-rays and radio waves, but other radiations which cannot be so easily blocked. (As it turned out, however, the risks were far less than had at one time been feared.)

We see, then, how complicated and how variable sunlight is, if we use that word in the widest sense to describe all the waves emitted by the Sun. Nevertheless, when we accept the evidence of our unaided eyes and describe the Sun as a yellow star, we have summed up the most important single fact about it – at
this
moment in time. It appears probable, however, that sunlight will be the colour we know for only a negligibly small part of the Sun’s history.

For stars, like individuals, age and change. As we look out into space, we see around us stars at all stages of evolution. There are faint blood-red dwarfs so cool that their surface temperature is a mere 4,000 degrees Fahrenheit; there are searing ghosts blazing at 100,000 degrees, and almost too hot to be seen, for the greater part of their radiation is in the invisible ultraviolet. Obviously, the ‘daylight’ produced by any star depends upon its temperature; today (and for ages past, as for ages to come) our Sun is at about 10,000 degrees Fahrenheit, and this means that most of its light is concentrated in the yellow band of the spectrum, falling slowly in intensity towards both the longer and the shorter waves.

That yellow ‘bump’ will shift as the Sun evolves, and the light of day will change accordingly. It is natural to assume that as the Sun grows older and uses up its hydrogen fuel – which it is now doing at the
spanking rate of half a billion tons
a
second
– it will become steadily colder and redder.

But the evolution of a star is a highly complex matter, involving chains of interlocking nuclear reactions. According to one theory, the Sun is still growing hotter and will continue to do so for several billion years. Probably life will be able to adapt itself to these changes, unless they occur catastrophically, as would be the case if the Sun exploded into a nova. In any event, whatever the vicissitudes of the next five or ten billion years, at long last the Sun will settle down to the white dwarf stage.

It will be a tiny thing, not much bigger than the Earth, and therefore too small to show a disc to the naked eye. At first, it will be hotter than it is today, but because of its minute size it will radiate very little heat to its surviving planets. The daylight of that distant age will be as cold as moonlight, but much bluer, and the temperature of Earth will have fallen to 300 degrees below zero. If you think of mercury lamps on a freezing winter night, you have a faint mental picture of high noon in the year ad 7,000 million.

Yet that does not mean that life – even life as we know it today – will be impossible in the Solar System; it will simply have to move in towards the shrunken Sun. The construction of artificial planets would be child’s play to the intelligences we can expect at this date; indeed, it will be child’s play to us in a few hundred years’ time.

Around the year 10,000 million the dwarf Sun will have cooled back to its present temperature, and hence to the yellow colour that we know today. From a body that was sufficiently close to it – say only a million miles away – it would look exactly like our present Sun, and would give just as much heat. There would be no way of telling, by eye alone, that it was actually a hundred times smaller, and a hundred times closer.

So matters may continue for another five billion years; but at last the inevitable will happen. Very slowly, the Sun will begin to cool, dropping from yellow down to red. Perhaps by the year 15,000 million it will become a red dwarf, with a surface temperature of a mere 4,000 degrees. It will be nearing the end of the evolutionary track, but reports of its death will be greatly exaggerated. For now comes one of the most remarkable, and certainly least appreciated, results of modern astrophysical theories.

When the Sun shrinks to a dull red dwarf, it will not be dying. It will
just be starting to live – and
everything
that
has
gone
before
will
be
merely
a
fleeting
prelude
to
its
real
history.

For a red dwarf, because it is so small and so cool, loses energy at such an incredibly slow rate that it can stay in business for
thousands
of times longer than a normal-sized white or yellow star. We must no longer talk in billions but of trillions of years if we are to measure its life span. Such figures are, of course, inconceivable. (For that matter, who can think of a thousand years?) But we can nevertheless put them into their right perspective if we relate the life of a star to the life of a man.

On this scale, the Sun is but a week old. Its flaming youth will continue for another month; then it will settle down to a sedate adult existence which may last at least eighty years.

Life has existed on this planet for two or three days of the week that has passed; the whole of human history lies within the last second, and there are eighty years to come.

In the wonderful closing pages of
The
Time
Machine,
the young H. G. Wells described the world of the far future, with a blood-red Sun hanging over a freezing sea. It is a sombre picture that chills the blood, but our reaction to it is wholly irrelevant and misleading. For we are creatures of the dawn, with eyes and senses adapted to the hot light of today’s primeval Sun. Though we should miss beyond measure the blues and greens and violets which are the fading afterglow of Creation, they are all doomed to pass with the brief billion-year infancy of the stars.

But the eyes that will look upon that all-but-eternal crimson twilight will respond to the colours that we cannot see, because evolution will have moved their sensitivity away from the yellow, somewhere out beyond the visible red. The world of rainbow-hued heat they see will be as rich and colourful as ours – and as beautiful; for a melody is not lost if it is merely transposed an octave down into the bass.

So now we know that Shelley, who was right in so many things, was wrong when he wrote:

For the radiance of eternity is not white: it is infra-red.

But to what extent can we
really
know the universe around us? Sometimes this question is posed by people who hope the answer will be in the negative, who are fearful of a universe in which everything might one day be known. And sometimes we hear pronouncements from scientists who confidently state that everything worth knowing will soon be known – or even is already known – and who paint pictures of a Dionysian or Polynesian age in which the zest for intellectual discovery has withered, to be replaced by a kind of subdued languor, the lotus eaters drinking fermented coconut milk or some other mild hallucinogen. In addition to maligning both the Polynesians, who were intrepid explorers (and whose brief respite in paradise is now sadly ending), as well as the inducements to intellectual discovery provided by some hallucinogens, this contention turns out to be trivially mistaken.

Let us approach a much more modest question: not whether we can know the universe or the Milky Way Galaxy or a star or a world. Can we know, ultimately and in detail, a grain of salt? Consider one microgram of table salt, a speck just barely large enough for someone with keen eyesight to make out without a microscope. In that grain of salt there are about 10
¹⁶
sodium and chlorine atoms. This is a 1 followed by 16 zeros, 10 million billion atoms. If we wish to know a grain of salt, we must know at least the three-dimensional positions of
each of these atoms. (In fact, there is much more to be known – for example, the nature of the forces between the atoms – but we are making only a modest calculation.) Now, is this number more or less than the number of things which the brain can know?

How much
can
the brain know? There are perhaps 10
¹¹
neurons in the brain, the circuit elements and switches that are responsible in their electrical and chemical activity for the functioning of our minds. A typical brain neuron has perhaps a thousand little wires, called dendrites, which connect it with its fellows. If, as seems likely, every bit of information in the brain corresponds to one of these
connections
, the total number of things knowable by the brain is no more than 10
¹⁴
, one hundred trillion. But this number is only one percent of the number of atoms in our speck of salt.

So in this sense the universe is intractable, astonishingly immune to any human attempt at full knowledge. We cannot on this level understand a grain of salt, much less the universe.

But let us look a little more deeply at our microgram of salt. Salt happens to be a crystal in which, except for defects in the structure of the crystal lattice, the position of every sodium and chlorine atom is predetermined. If we could shrink ourselves into this crystalline world, we would see rank upon rank of atoms in an ordered array, a regularly alternating structure – sodium, chlorine, sodium, chlorine, specifying the sheet of atoms we are standing on and all the sheets above us and below us. An absolutely pure crystal of salt could have the position of every atom specified by something like 10 bits of information.
*
This would not strain the information-carrying capacity of the brain.

If the universe had natural laws that governed its behavior to the same degree of regularity that determines a crystal of salt, then, of course, the universe would be knowable. Even if there were many such laws, each of considerable complexity, human beings might have the capability to understand them all. Even if such knowledge exceeded the information-carrying capacity of the brain, we might store the additional information outside our bodies – in books, for example, or in computer memories – and still, in some sense, know the universe.

Human beings are, understandably, highly motivated to find regularities, natural laws. The search for rules, the only possible way to understand such a vast and complex universe, is called science. The universe forces those who live in it to understand it. Those creatures who find everyday experience a muddled jumble of events with no predictability, no regularity, are in grave peril. The universe belongs to those who, at least to some degree, have figured it out.

It is an astonishing fact that there
are
laws of nature, rules that summarize conveniently – not just qualitatively but quantitatively – how the world works. We might imagine a universe in which there are no such laws, in which the 10
⁸⁰
elementary particles that make up a universe like our own behave with utter and uncompromising abandon. To understand such a universe we would need a brain at least as massive as the universe. It seems unlikely that such a universe could have life and intelligence, because beings and brains require some degree of internal stability and order. But even if in a much more random universe there were such beings with an intelligence much greater than our own, there could not be much knowledge, passion or joy.

Fortunately for us, we live in a universe that has at least important parts that are knowable. Our common-sense experience and our evolutionary history have prepared us to understand something of the workaday world. When we go into other realms, however, common sense and ordinary intuition turn out to be highly unreliable guides. It is stunning that as we go close to the speed of light our mass increases indefinitely, we shrink toward zero thickness in the direction of motion, and time for us comes as near to stopping as we would like. Many people think that this is silly, and every week or two I get a letter from someone who complains to me about it. But it is a virtually certain consequence not just of experiment but also of Albert Einstein's brilliant analysis of space and time called the Special Theory of Relativity. It does not matter that these effects seem unreasonable to us. We are not in the habit of traveling close to the speed of light. The testimony of our common sense is suspect at high velocities.

Or consider an isolated molecule composed of two atoms shaped something like a dumbbell – a molecule of salt, it might be. Such a molecule rotates about an axis through the line connecting the two atoms. But in the world of quantum mechanics, the realm of the very small, not all orientations of our dumbbell molecule are possible. It
might be that the molecule could be oriented in a horizontal position, say, or in a vertical position, but not at many angles in between. Some rotational positions are forbidden. Forbidden by what? By the laws of nature. The universe is built in such a way as to limit, or quantize, rotation. We do not experience this directly in everyday life; we would find it startling as well as awkward in sitting-up exercises, to find arms outstretched from the sides or pointed up to the skies permitted but many intermediate positions forbidden. We do not live in the world of the small, on the scale of 10–13 centimeters, in the realm where there are twelve zeros between the decimal place and the one. Our common-sense intuitions do not count. What does count is experiment – in this case observations from the far infrared spectra of molecules. They show molecular rotation to be quantized.

The idea that the world places restrictions on what humans might do is frustrating. Why
shouldn't
we be able to have intermediate rotational positions? Why
can't
we travel faster than the speed of light? But so far as we can tell, this is the way the universe is constructed. Such prohibitions not only press us toward a little humility; they also make the world more knowable. Every restriction corresponds to a law of nature, a regularization of the universe. The more restrictions there are on what matter and energy can do, the more knowledge human beings can attain. Whether in some sense the universe is ultimately knowable depends not only on how many natural laws there are that encompass widely divergent phenomena, but also on whether we have the openness and the intellectual capacity to understand such laws. Our formulations of the regularities of nature are surely dependent on how the brain is built, but also, and to a significant degree, on how the universe is built.

For myself, I like a universe that includes much that is unknown and, at the same time, much that is knowable. A universe in which everything is known would be static and dull, as boring as the heaven of some weak-minded theologians. A universe that is unknowable is no fit place for a thinking being. The ideal universe for us is one very much like the universe we inhabit. And I would guess that this is not really much of a coincidence.