Universe a grand tour of modern science Phần 8 pot

particles, but also the various forces. All matter particles feel the weak force
carried by W particles, made of an electron and antineutrino or vice versa, and
by Z particles with a neutrino and antineutrino, or some other composition.
Photons made of an electron and anti-electron carry the electric force, to which
all charged particles respond. The strong nuclear force, felt only by proton-like
matter particles and mesons, is carried by gluons (colour and anticolour) within
the particles and by mesons (quark and antiquark) operating between the
particles.
Mathematically, all of these forces are described by so-called gauge theories,
which give the same results wherever you start from. The electric force provides
a simple example of indifference to the starting point, in a pigeon perching
safely on a power line while being repeatedly charged to 100,000 volts. Signals
of a fraction of a volt continue to pass in a normal manner through the bird’s
nervous system.
Indifference to circumstances is a requirement if the various forces are to
operate in exactly the same way on and within a proton, whether it is anchored
in a mountain or whizzing through the Galaxy close to the speed of light, as a
cosmic-ray particle. In other words, gauge theories are compatible with high-
speed travel and Albert Einstein’s special theory of relativity. The obligation that
the force theories must be of this type strengthens the physicists’ confidence in
them.
Those four par agraphs sum up the Standard Model, a well-rounded theory and
one of the grandest outcomes of 20th-century science. It was created and largely
confir med in an era of unremitting excitement. Almost as fast as theorists
plucked ideas from their heads, experimenters manufactured the corresponding
particles literally out of thin air, in the vacuum of their big machines. It was as if
Mother Nature was in a mood to gossip with the physicists, about her arcane
ways of running the Universe.
The frenzy lasted for about 20 years, bracketed by the materializations of the
triply strange omega particle in 1964 and the Z carrier of the weak force in 1984.
But after that climax came a period of hush on the subject of the fundamental

particles and the forces operating between them. Most particle physicists had to
content themselves with confirming the predictions of the existing theories to
more and more decimal places.
If the Standard Model were complete in itself, and arguably the end of the story,
Mother Nature’s near-muteness at the end of the 20th century would have been
unsurprising. Yet neither criterion was satisfied. As Chris Llewellyn Smith of
CERN commented in 1998, ‘While the Standard Model is economical in
concepts, their realization in practice is baroque, and the model contains many
arbitrary and ugly features.’
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particle families
I Hoping for flaws
Two decades earlier, in the midst of all the excitement, Richard Feynman of
Caltech played the party pooper. He put his finger on one of the gravest
shortcomings of the-then emergent Standard Model. ‘The problem of the masses
has been swept into a corner,’ he complained.
Theorists have rules of thumb that work well in estimating the masses of
expected new particles, by reference to those of known particles. Yet no one can
say why quarks are heavier than electrons, or why the top quark is 44,000 times
more massive than the up quark. According to the pristine versions of the
theories all particles should have zero intrinsic mass, yet only photons and
neutrinos were thought to conform. The real masses of other particles are an
arbitrary add-on, supposedly achieved by introducing extraneous particles.
By the start of the 21st century physicists were beefing up their accelerators to
address the mass problem by looking for a particle called the Higgs, which
might solve it. They were also very keen to find flaws in the Standard Model.
Only then would the way be open to a superworld rich in other particles and
forces.
The physicists dreaded the thought of entering a desert with nothing for their
machines to find, by way of particle discoveries, to match the great

achievements of the previous 100 years. The first hint that they might not be so
unlucky came in 1998, with results from an underground experiment in Japan.
These indicated that neutrinos do not have zero mass, as required by the
Standard Model. Hooray!
E For more about the evolution of the Standard Model, see
Electroweak force, Quark
soup
, and Higgs bosons. For theories looking beyond it, see Sparticles and
Superstrings. Other related entries are Cosmic rays and Neutrino oscillations.
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particle families
‘G
reen plants spread the enormous surface of their leaves and, in a still unknown
way, force the energy of the Sun to carry out chemical syntheses, before it cools
down to the temperature levels of the Earth’s surface.’ Thus, in 1866, the Austrian
physicist Ludwig Boltzmann related the growth of plants to recently discovered
laws of heat. By stressing the large leaf area he anticipated the 21st-century view of
greenswards and the planktonic grass of the sea as two-dimensional
photochemical factories equipped with natural light guides and photocells.
Botanists had been strangely slow even to acknowledge that plants need light. In
1688 Edmond Halley told the Royal Society of London that he had heard from
a keeper of the Chelsea Physic Garden that a plant screened from light became
white, withered and died. Halley was emboldened to suggest ‘that it was
necessary to the maintenance of vegetable life that light should be admitted to
the plant’. But why heed such tittle-tattle from an astronomer?
The satirist Jonathan Swift came unwittingly close to the heart of the matter in
1726, in Voyage to Laputa, where ‘projectors’ were trying to extract sunbeams
from cucumbers. Half a century later Jan Ingenhousz, a Dutch-born court
physician in Vienna, carried out his Experiments on Vegetables, published in
London in 1779. He not only established the importance of light, but showed

that in sunshine plants inhale an ‘injurious’ gas and exhale a ‘purifying’ gas. At
night this process is partially reversed.
The medic Ingenhousz is therefore considered the discoverer of the most
impor tant chemical reactions on Earth. In modern terms, plants take in carbon
dioxide and water and use the radiant energy of sunlight to make sugars and
other materials needed for life, releasing oxygen in the process. At night the
plants consume some of the daytime growth for their own housekeeping.
Animal life could not exist without the oxygen and the nutrition provided by
plants. The fact that small communities on the ocean floor subsist on volcanic
rather than solar energy does not alter the big picture of a planet where the
chemistry of life on its surface depends primarily on combining atoms into
molecules with the aid of light—in a word, on photosynthesis. Thereby more
than 100 billion tonnes of carbon is drawn from the carbon dioxide of the air
every year and incorporated into living tissue.
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I Chlorophyll, photons and electrons
The machinery of photosynthesis gradually became clearer, in the microscopic
and molecular contents of commonplace leaves. During the 19th and early 20th
centuries scientists found that the natural green pigment chlorophyll is essential.
It concentrates in small bodies within the leaf cells, called chloroplasts. The key
chemical reaction of photosynthesis splits water into hydrogen and oxygen, and
complex series of other reactions ensue.
Another preamble to further progress was the origin of photochemistry. It
star ted with photography but was worked up by Giacomo Ciamician of Bologna
into a broad study of the interactions of chemical substances and light. The
physicists’ discovery that light consists of particles, photons, opened the way to
understanding one-on-one reactions between a photon and an individual atom
or molecule. Electrons came into the story too, as detachable parts of atoms.
Chlorophyll paints the land and sea green. Its molecule is shaped like a kite,
with a flat, roughly square head made mainly of carbon and nitrogen atoms, and

a long wiggly tail of carbon atoms attached by an acetic acid molecule. In the
centre of the head is a charged magnesium atom that puts out four struts—
chemical bonds—to a ring of rings, each made of four atoms of carbon and one
of nitrogen. Different kinds of chlorophyll are decorated with various
attachments to the head and tail.
From the white light of the Sun, chlorophyll absorbs mainly blue and red
photons, letting green light escape as the pigment’s colour. Because the
chlorophyll is concentrated in minute chloroplasts, leaves would appear white or
transparent, did they not possess an optical design that forces light entering a
leaf to ricochet about inside it many times before escaping again. This
maximizes the chance that a photon will encounter a chloroplast and be
absorbed. It also ensures that surviving green light eventually escapes from all
over the leaf.
The pace of discovery about photosynthesis quickened in the latter half of the
20th century. Using radioactive carbon-14 to label molecules, the chemist Melvin
Calvin of UC Berkeley and others were able to trace the course of chemical
reactions involving carbon. Contrary to expectation, the system does not act
directly on the assimilated carbon dioxide but f irst creates energy-rich molecules,
called NADPH and ATP. These are portable chemical coins representing free
energy that the living cell can spend on all kinds of constructive tasks.
Conceptually they link photosynthesis to the laws of heat, as Boltzmann wanted.
Teams in Europe and the USA gradually revealed that two different molecular
systems are involved. Somewhat confusingly they are called Photosystem II and
Photosystem I, with II coming first in the chemical logic of the process,
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photosynthesis
although it was the second to be discovered. II is where incoming light has its
greatest effect, in splitting molecules of water to make oxygen molecules and
dismembering the hydrogen atoms into positively charged protons and
lightweight, negatively charged electrons.

Water, H
2
O, is a stable compound, and splitting it needs the combined energy
of two photons of sunlight. But as you’ll not want highly reactive oxygen atoms
rampaging among your delicate molecules, you’d better liberate two and pair
them right away in a less harmful oxygen molecule. That doubles the energy
required for the transaction.
To accumulate the means to buy one oxygen molecule, by splitting two water
molecules at once, you need a piggy bank. In Photosystem II, this is a cluster of
four charged atoms of a metallic element, manganese. Each dose of incoming
energy extracts another electron from one manganese atom. When all four
manganeses are thus fully charged, bingo, the system converts two water
molecules into one oxygen molecule and four free hydrogen nuclei, protons.
The four electrons have already left the scene.
The other unit in the operation, Photosystem I, then uses the electrons supplied
by II, and others liberated by light within I itself, to set in motion a series of
other chemical reactions. They convert carbon dioxide into energy-rich carbon
compounds. Human beings are hard put to make sense of the jargon, never
mind to understand all the details. Yet humble spinach operates its two systems
without a moment’s thought, merrily splitting water in one and fixing carbon
from the air in the other.
I Pigments as a transport system
Like other plants, spinach also runs molecular r ailways for photons and
electrons. These are built of carefully positioned chains of pigment molecules,
mainly chlorophyll. For light, they can act first like antennas to gather the
photons, and then like glass fibres to guide their energy to the point of action.
It is mildly surprising to have pigment chains relaying light, but much more
remarkable that they also transport free electrons at an astonishing rate. The
possibility was unknown to scientists until the 1960s. Then the Canadian-born
theorist Rudolph Marcus, working in the USA, showed how electrons can leap

from molecule to molecule. In photosynthesis, this trick whisk s the liberated
electrons away along the molecular railway, before they can rejoin the wrong
atoms. It delivers them ver y precisely to the distant molecules where their
chemical action is required.
The separation of electric charges achieved by this means is the most crucial of
all the steps in the photosynthetic process. It takes place in a few million-
millionths of a second. Ultrafast laser systems became indispensable tools in
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photosynthesis
studying photosynthesis, to capture events that are quicker than any ordinary
flash. The production of oxygen within milliseconds seems relatively leisurely,
while the reactions converting carbon dioxide into other materials can take
several seconds.
The layout of the high-speed pigment railways became apparent in the first
complete molecular structure of a natural photocell, converting light energy into
electrical energy. Its elucidation was a landmark in photosynthesis research. In
1981, at the Max-Planck-Institut fu
¨
r Biochemie, Martinsried, Hartmut Michel
succeeded in making crystals of photosynthetic reaction centres from a purple
bacterium, Rhodopseudomonas viridis. This opened the way to X-ray analysis, and
by 1985 Johann Deisenhofer, Michel and others at Martinsried had revealed
the most complex molecular 3-D assembly ever seen at an atomic level, up
to that time.
This photocell passes in rivet fashion through a membrane in the bacterium.
When light falls on it, it creates a voltage across the membrane, sending a
negative charge to the far side. The molecular analysis revealed how it works.
Four protein molecules encase carefully positioned pigments, bacterial analogues
of chlorophyll, which create a railway that guides the light energy to a place
where two pigment molecules meet in a so-called special pair. There the light

energy liberates an electron, which then travels via a branch line of the pigment
railway to the dark side of the membrane. It settles with its negative charge on a
ring-shaped quinone molecule that has a useful appetite for electrons.
‘Although it is a purple bacterium that has first yielded the secrets of the
photosynthetic reaction centre,’ commented Robert Huber, who coordinated the
work at Martinsried, ‘there is no need to doubt its relevance to the higher green
plants on which human beings depend for their nourishment.’
I The gift of the blue-greens
Whilst it was certainly encouraging that so complicated a molecule could be
analysed, the photosystems of the higher plants, with two different kinds of
reaction centres, were a tougher proposition. They would keep scientists busy
into the 21st century.
There are evolutionary reasons for the greater complexity. Purple bacteria live
by scavenging pre-existing organic material, using light energy as an aid. This
would be a dead end, if other organisms did not make fresh food from scratch,
by reacting carbon dioxide with hydrogen. Some photosynthetic bacteria obtain
their hydrogen by splitting volcanic hydrogen sulphide, but others took the big
step to splitting water.
‘Think about it,’ said James Barber, a chemist at Imperial College London.
‘Water is the solvent of life. It was very odd that bacteria should start attacking
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photosynthesis
their solvent. That’s like burning your house to keep warm. Only the abundance
of water on the Earth made it a sustainable strategy. And of course the first
thing that plants do in a drought is to stop photosynthesizing.’
The key players in this evolutionary switch were blue-green algae, or
cyanobacteria, first appearing perhaps 2.4 billion years ago. Their direct
descendants are still among us. Blue-greens are commonplace in ponds and
oceans, and on the shore of Western Australia they build mounds called
stromatolites, with new layers growing on top of dead predecessors. Fossils

of similar stromatolites are known in rocks 2 billion years old.
Those remote ancestors of the present-day blue-greens possessed such an
excellent kit for photosynthesis that other, larger cells, welcomed them aboard
to make the first true algae. Whenever the cells reproduced themselves, they
passed on stocks of blue-green guests to their daughters. Much later, some of
the algae evolved into land plants. The green chloroplasts within the leaf cells
of plants, where the photosynthesis is done, are direct descendants of the former
blue-g reens.
What was so special about them? Until the ancestral blue-greens appeared on
the Earth, some photosynthetic bacteria, like the purples studied at Martinsried,
had used quinones as the end-stations to receive electrons released by light.
Others employed iron–sulphur clusters (Fe
4
S
4
) for that purpose. The blue-greens
beefed up photosynthesis by putting both systems together. As a result, their
descendent chloroplasts possess Photosystems II (using quinones) and I (using
iron–sulphur).
Although there are many variants of photosynthesis, they are all related.
Photosynthesis using chlorophyll seems to be a trick that Nature orig inated only
once. Investigators of molecular evolution at Indiana and Kanagawa traced the
whole story back in time, from the similarities and differences between proteins
involved in photosynthesis, in plants, blue-greens and other photosynthetic
bacteria alive today. Chlorophyll, the badge of sun-powered life, first appeared in
an ancient form in a remote common ancestor of purple and green
photosynthetic bacteria. Among the variants appearing later is chlorophyll a,
which is exclusive to blue-greens and plants.
I Engineering the photosystems
Although it is a quinone user like the purple bacterium, Photosystem II

generates a higher voltage. For its key job, it also has a special water-splitting
enzyme—a protein molecule whose modus operandi remains elusive. Like the
purple bacterium’s photocell, Photosystem II consists of a complex of protein
molecules supporting pigment antennas and railways, but it is bigger, with about
45,000 atoms in all.
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photosynthesis
By 1995, at Imperial College London, James Barber’s team had isolated the
Photosystem II complex from a plant—spinach. The material resisted attempts
to crystallize it for full X-ray examination. Nevertheless, powerful electron
microscopes operating at very low temperatures gave a first impression of its
molecular organization.
In Berlin, Wolfram Saenger and colleagues from the Freie Universita
¨
t and Horst
Tobias Witt and colleagues from the Technische Universita
¨
t had better fortune
with Photosystem II from a blue-green, Synechococcus elongatus, which lives in
hot springs. Athina Zouni managed to grow small crystals. They were not good
enough for very detailed analysis, but by 2001 the team had a broad-brush X-ray
view of the complex.
The blue-green’s Photosystem II was similar to what Barber was seeing in
spinach, and reminiscent of the purple bacterium’s photosynthetic machine too.
The team positioned about ten per cent of the 45,000 atoms, including key
metal atoms and chlorophyll molecules. They pinpointed the piggy bank—the
manganese cluster that accumulates electric charges for the break-up of water.
The Berliners were also working on the blue-green’s Photosystem I, and strong
similarities convinced them that I and II shared a common ancestry. The picture
grew clearer, of a treasured reaction centre originating long ago, spreading

throughout the living world, adapting to different modes of existence, but always
preserving essential structures and mechanisms in its core.
The Berlin group had better crystals of Photosystem I than they had of II. Ingrid
Witt first managed to crystallize groups of three robust Photosystem I units
from the blue-green S. elongatus, in 1988. That opened the possibility of X-ray
analysis down to an atomic level.
With so formidable a complex as Photosystem I, containing 12 different proteins
and about 100 chlorophyll molecules, this was no small matter. Very powerful
X-rays, available at the European Synchrotron Radiation Facility in Grenoble,
were essential. The crystals had to be frozen at the temperature of liquid
nitrogen to reduce damage to the delicate structures by the X-rays themselves.
By 2001 the Berliners’ analysis of Photosystem I was triumphantly thorough.
It showed the detailed arrangement of the proteins, of which nine are riveted
through the supporting membrane. Six carefully placed chlorophyll molecules
provide central transport links for light and electrons and make a special pair as
in the Martinsried structure. Most impressively, a great light-harvesting antenna
using another 90 chlorophylls surrounds the active centre of Photosystem I.
Orange carotene pigments also contribute to the antenna.
For outsiders who might wonder what value there might be in this strenuous
pursuit of so much detail, down to the atomic level, Wolfram Saenger had an
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photosynthesis
answer. ‘We don’t just satisfy our curiosity about the mechanisms and evolution
of this life-giving chemistry,’ he commented. ‘We have already gained a new and
surprising appreciation of how pigments, proteins, light and electrons work
together in living systems. And the physics, chemistry, biochemistry and
molecular biology, successfully marshalled in the study of photosynthesis, can
now investigate these and many other related molecular machines in living cells,
and find out how they really work.’
I Can we improve on the natural systems?

Practical benefits can be expected too. Growing knowledge of the genetics and
molecular biology of the photosynthetic apparatus, and of its natural control
mechanisms, may help plant breeders to enhance g rowth rates in crop plants.
Other scientists use biomolecules to build artificial photosynthetic systems for
generating electrical energy or for releasing hydrogen as fuel. In competition
with them are photochemists who prefer metal oxides or compound metals,
which are also capable of splitting water into hydrogen and oxygen when
exposed to light, without any need for living things.
In 1912 Ciamician of Bologna looked forward to a time when the secrets of
plants ‘will have been mastered by human industry which will know how to
make them bear even more abundant fruit than Nature, for Nature is not in
a hurry and mankind is’. In that sense, two centuries spent grasping the
fundamentals of photosynthesis may be just the precursor to a new relationship
between human beings and the all-nourishing energy of the Sun.
E For the geological impact of photosynthesis, see
Global enzymes and Tree of life.
For its present influences, see
Carbon cycle and Biosphere from space. For more
about proteins and their structures, see
Protein shapes. The molecular biology of
plants is dealt with more generally under
Arabidopsis. For alternative sources of
energy for life, see
Extremophiles.
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photosynthesis
P
otatoes are easy to grow, and when introduced into Ireland they meant
that you could keep your family alive while spending most of your time labouring
for the big landowners. This feudal social system worked tolerably until 1845,

when an enemy of the potato arrived on the wind from the European continent. It
was the potato blight Phytophthora infestans. Black spots and white mould on the
leaves foretold that the potatoes would become a rotten pulp.
The Great Irish Famine, which k illed and exiled millions, was neither the first
nor the last case of a crop being largely wiped out by disease. The potato blight
itself caused widespread hardship across Europe. Its effects reached historic
dimensions in Ireland partly because landowners continued to export grain while
the inhabitants starved. As Jane Francesca Wilde (Oscar’s mother) put it:
T here’s a proud array of soldiers—what do they round your door?
T hey guard our masters’ granaries from the thin hands of the poor.
At least 20 per cent of the world’s crop production is still lost to pests, parasites
and pathogens, and the figure rises to 40 per cent in Africa and Asia. Plant
diseases can also devastate species in the wild, as when the bark-ravaging fungus
Cryphonectria parasitica crippled every last stand of native American chestnut
trees between 1904 and 1926. But cultivated crops are usually much more
vulnerable to annihilating epidemics than wild plants are, because they are
grown from varieties with a narrow genetic base.
In the wars between living species that have raged since life began, human
beings often think that their natural enemies are big cats, bears, sharks,
crocodiles and snakes. In fact, the depredations of those large animals are
insignificant compared with disease-causing microbes. They either afflict people
directly or starve them by attacking their food supplies.
There is no difference in principle between the conflicts of organisms of any
size. All involve weapons of attack and defence, whether sharper canines versus
tougher hides, or novel viruses versus molecular antibodies. Given the
opportunities for improvements on both sides, biologists have called the
interspecies war an evolutionary arms race.
536
Hereditary systems provide much natural resistance to diseases in plants, as well
as animals. Many herbal medicines are borrowed from the plant k ingdom’s

arsenal of chemical weapons. An overview of the genetic system involved in
fighting disease became available in 2000, when a European–US–Japanese
consortium of labs in the Arabidopsis Genome Initiative read every gene in
arabidopsis, which is a small weed.
Very variable genes called R for resistance, of which arabidopsis possesses 150,
provide the means of identifying various kinds of parasites and pathogens
attacking the plant. Recognition of a foe triggers defence mechanisms in which
signalling molecules activate various defender genes and sometimes command
infected cells to die. Dozens of genes involved in these actions were tentatively
pinpointed, including eight thought to be responsible for a burst of respiration
that zaps the intruder with oxygen in a highly reactive for m.
Plants devote a lot of energy, in the literal sense, to protection against disease.
To keep up a guard against every possible enemy would, though, be far too
much work for any individual creature. Instead populations share the task
between individuals, by their genetic variability, especially in respect of the
R genes. How this arrangement comes about is a matter of intense interest to
plant breeders, and also to theorists of evolution.
I For a fleeter cheetah
The contest between diseases and their victims is a case of co-evolution, which
means an interaction on time-scales long enough for species to evolve together.
The prettiest example concerns flowers, nectar, fruits and nuts, which evolved as
lures and bribes for animals to help the plants in pollination and seed dispersal.
Insects, birds and many other animals including our primate ancestors took
advantage of the floral offerings in evolving in novel ways on their own account.
The co-evolution of flowers and bees seems be a case where both sides have
gained.
More antagonistic, and therefore perhaps more typical, is the contest between
grasses and grass-eating animals. Leaves of grass toughened by minerals can ruin
a casual muncher’s teeth, but grazing animals have acquired teeth that keep
growing throughout life, to compensate for the wear. Like the contest between

plants and diseases, this is reminiscent of military engineers trying to outdo one
another, with their missiles and their antimissile shields. But as with the ever-
rising prices of modern armaments, the capacity for attack or defence imposes a
tax on each creature’s resources.
When trees compete for sunlight in a dense forest, they may grow ever taller to
avoid being overshadowed. The upshot is that all of the species of trees involved
in a height contest tend to become less efficient. The leaves exposed to sunlight
537
plant diseases
in the canopy do not increase in total area, but they have to power the building
and maintenance of elongated tree trunks that are useless to the trees except for
giving them height.
Reflecting on the non-stop wars and competitions between species, Leigh van
Valen at Chicago propounded a new evolutionary law in 1973. Even if physical
conditions such as the climate don’t change, he reasoned, every creature is being
continually disadvantaged by changes in other species with which it is
co-evolving. It is therefore obliged to evolve itself, if it is to maintain its relative
position in the ecosystem.
Van Valen called his idea the Red Queen principle, citing Lewis Carroll’s Through
the Looking-Glass. ‘Now, here, you see, it takes all the running you can do, to keep
in the same place,’ the breathless chess-piece explains to Alice. ‘If you want to
get somewhere else, you must run at least twice as fast as that!’
Another perspective came from William Hamilton at Oxford, in 1982, with
special reference to diseases. The chief role of sexual reproduction, he argued, is
to shuffle and share out genes for disease resistance, among the individuals in a
species. Even if a disease breaks through on a broad front, there will still be well-
armed individuals in strongpoints that can’t be winkled out. So the defending
species will survive to fight another day. The name of the game is remembering
all the different kinds of adversaries encountered in the past, which might
reappear in future.

Among mammalian foes, the cheetah becomes fleeter, and the gazelle sharper in
its reactions and better at blending into the long grass. For Richard Dawkins,
also at Oxford, the evolutionary arms race was the chief way of driving
evolution onwards and upwards. His explanation in The Blind Watchmaker (1986)
of how the world’s magnificent, intricate organisms could have been created by
the blind forces of physics, relied at its core on the arms-race hypothesis.
‘Each new genetic improvement selected on one side of the arms race—say
predators—changes the environment for the selection of genes on the other side
of the arms race—prey,’ Dawkins wrote. ‘It is arms races of this kind that have
been mainly responsible for the apparently progressive quality of evolution, for
the evolution of ever-improved running speed, flying skill, acuity of eyesight,
keenness of hearing, and so on.’
I Antique weapons
The arms-race theories remained largely speculative until very late in the 20th
century, when progress in molecular biology at last began to expose them to
observational tests. These began with Joy Bergelson at Chicago setting her
graduate students to look closely at a gene conferring resistance in plants to
infection by the small bacterium Pseudomonas syringae, which causes a blight on
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plant diseases
young leaves. They were able to show that the anti-pseudomonas Rpm1 gene in
a small weed, arabidopsis, was nearly 10 million years old.
The molecular cunning that led them to this conclusion, in 1999, was a matter
of examining the anchors that hold the gene in place in the long chain of the
nucleic acid, DNA. The anchors consist of short lengths of DNA, but unlike the
gene itself they carry no important messages in the genetic code. As a result
they are free to accumulate random mutations in the DNA subunits as time
passes. High variability in the Rpm1 anchors enabled the Chicago team to
estimate the gene’s age.
In the essentially progressive view of the evolutionary arms race, as proposed by

Dawkins, you might expect each species to be armed to the teeth with the most
modern weapons. You’d not expect to see a paratrooper carrying a bow and
arrow. Yet the Rpm1 gene against the leaf-blight bacterium is, from this point
of view, just such an antique-collector’s piece.
‘The arms race theory has been a generally accepted model for the evolution of
disease-resistance genes because it is intuitive, but it’s never been scientifically
tested,’ Bergelson commented. ‘Our results were surprising in demonstrating
that an arms race is not occurring for the resistance gene we studied.’
She offered instead the metaphor of trench warfare. Disease epidemics alternate
with periods of high resistance in the plants, leading to ceaseless advances and
retreats for both plants and pathogens. Genes that have proved their worth in
the past may be retained indefinitely, while others from recent skirmishes may
die out.
By 2000, when the Arabidopsis Genome Initiative had its 150 R genes for disease
resistance, Bergelson and her group were able to check the evolutionary picture
quite thoroughly. It confirmed the trench-warfare idea. The scientists found
plenty of evidence of rapid adaptation of defences to meet new threats in the
past, but also many antiques. Although the R genes show a very wide range of
ages, they are far from being typically young, as would be expected in a reliably
progressive kind of arms race as described by Dawkins.
After the initial molecular verdicts concerning the plant’s armoury for resistance
against disease, what survives of the 20th-century ideas about the evolutionary
arms race? Co-evolution is strongly reconfirmed as a factor in evolution. There
are one-to-one correspondences between proteins manufactured by command of
the R genes and other proteins carried by disease-causing organisms, whereby
the plant recognizes the enemy.
The Red Queen principle of non-stop evolution, which implies the possibility of
retreat as well as advance, remains a valid basis for thinking generally about
co-evolution and its weaponr y. Hamilton’s idea of sex as a means of sharing out
539

plant diseases
responsibility for disease resistance between individuals, with wide variations in
their complements of R genes, was strongly supported by the early molecular
results from arabidopsis. It needs confirmation in other species, and perhaps
with other molecular techniques.
A new mathematical theory will be needed to explain in detail the range of ages
of R genes and how they relate to past battles, in which attacking diseases
sometimes triumphed, sometimes retreated, during trench warfare lasting many
millions of years. The development and testing of such a theor y will probably go
hand in hand with fresh research in molecular ecology. The search will be for
patterns of resistance in wild plants alive today, which can be related to their
recent experiences of disease.
E For the background to the weed’s genome, see
Arabidopsis. For Hamilton’s theory, see
Cloning. For diseases in wheat and rice, see Cereals. For the use of crown gall in
genetic engineering, see
Transgenic crops. For the arms race with human diseases,
see
Immune system. For other molecular insights into co-evolution, see Alcohol and
Global enzymes. For general perspectives on evolution, see Evolution, and cross-
references therein.
540
plant diseases
B
otany was a poor relation of zoology until recently. Animals are more
fun to watch than plants are, and their relevance to human biology and
medicine is plainer. Norman Borlaug’s failure to win a Nobel science prize, after
helping to save the world from mass starvation with his hybrid wheat, was a
symptom of the academic pecking order, although he got the Nobel Peace Prize
instead. For the Green Revolution that he started, see

Cereals. Other biologists
modified plants by introducing new genes, with controversial consequences—see
Transgenic crops.
The leap forward associated with the reading of entire complements of genes,
the genomes, puts plants on an equal footing with animals at the frontiers of
discovery. First off the production lines were the genomes of a humble weed and
of rice—see
Arabidopsis and Cereals. At once, many aspects of plant life were
illuminated—see
Flowering, Plant diseases and Genomes in general.
Plants grow using carbon dioxide and water, and the energy of sunlight. Their
molecular machinery for this purpose has been largely elucidated—see
Photosynthesis. The links between plants and other living things, and with the
physical and chemical environment of the Earth, are ancient and far-reaching—
see
Tree of life, Global enzymes and Alcohol. Maize and arabidopsis have
been used to demonstrate molecular mechanisms of evolution—see
Hopeful
monsters
.
Seasonal growth has large effects on the concentration of carbon dioxide in the
air, and on the surface life of the planet—see
Carbon cycle and Biosphere from
space
. Because the leaves of land plants adapt to changing carbon dioxide levels,
fossil leaves can help to monitor past changes—see
Carbon cycle.
The ecology of plant life looms large among the anxieties about the state of the
planet, but fundamental issues remain in dispute—see
Biodiversity. Ecologists

have become acutely aware of the importance of controlling the numbers of
plant eaters—see
Predators. For relationships between plants and people in
ancient and traditional settings, see
Human ecology. For a tidbit on the use of
medicinal plants by chimpanzees, see
Primate behaviour.
541
‘S
o your chimneys i sweep, and in soot I sleep,’ lamented William Blake’s
child of the Industrial Revolution. Two centuries later, one of the world’s dirtiest
jobs was to clean out machines used for experiments in controlled nuclear
fusion. Supposedly pointing the way to abundant energy supplies in the 21st
century, the machines called tokamaks became filthy with black dust. It was
manufactured, sometimes by the shovelful, as straying high-energy particles
quarried atoms from the internal walls of the reaction chamber. A full-scale
fusion reactor of that kind would make dust in radioactive tonnes.
Ever since they first ignited firewood, human beings have regretted the
efficiency with which their fuels made soot, but no one thought that any
explanation was needed. Not until late in the 20th century did physicists fully
wake up to the tricks of soot and other kinds of dust. Makers of microchips
created clean rooms with care and expense only to find that manufacturing
processes using beams of atomic particles made silicon sawdust that ruined
many chips. Like the begrimed cleaners of the fusion machines, they bore
witness that something very odd was going on.
A common factor in the tokamak s and the microchip factories was the
co-existence of dust grains and electrified gas, in what physicists call dusty
plasmas. Astronomers and space scientists encountered dusty plasmas too. They
occur in the vicinity of dying stars that puff off newly made chemical elements,
and in interstellar clouds where such material accumulates. Around newborn

stars, dusty plasmas provide material from which planets can form.
In our own Solar System, comets throw out dusty tails into the electrified solar
wind. Dust accumulates in the plane in which the planets orbit, and it is
sometimes visible after sunset as the zodiacal light. And inspections of the dusty
rings of Saturn, by NASA’s two Voyager spacecraft in 1980, showed very fast
variations in the structure of the rings that defied explanation at that time.
In 1986 a fusion physicist at General Atomics in California, Hiroyuki Ikezi,
considered what could happen when many charged dust particles were confined
within an electrified gas, or plasma. He speculated that the dust grains might
arrange themselves in neat rows, sheets and 3-D lattices, like atoms in an
ordinary crystal, although on a much larger scale. But he did not explain why
542
they should remain like that. Indeed, you’d expect the dust grains to accumulate
electric charges and simply repel one another.
Only if the dusty plasma somehow generated a special confining force of its
own, to hold the grains together, would the regular lattices proposed by Ikezi be
stable. But if there were such a force, you could have a previously unknown
state of matter, with liquid-like or crystal-like gatherings of dust grains. They
came to be called plasma crystals.
I The shadow force
The idea of a special force at work in dusty plasmas rang a loud bell with
astronomers of the Max-Planck-Institut fu
¨
r extraterrestrische Physik at Garching
near Munich. They were puzzled by a fantastically rapid production of dust near
dying stars. That atoms of newly created elements puffed into space from the
stars should combine to make microscopic grains of carbon, miner als and ice
was only to be expected. But according to traditional ideas, the grains would
grow very slowly, atom by atom, over millions of years. Although dust
formation must start very slowly, something else was accelerating the later

growth of grains, to cut the time required.
Of the main constituents in the dirty plasma around a dying star—electrons,
positively charged atoms and the dust grains—the electrons are the most mobile.
The dust therefore gathers a disproportionate number of electrons on its
surfaces and acquires a mostly negative electric charge. Alternatively, strong
ultraviolet light from the star, or its neighbours, might knock electrons out of
the dust grains, and so generate mostly positive charges.
Either way, you would then expect the dust grains to repel one another in
accordance with schoolroom laws of electrostatics. Yet mounting astronomical
evidence showed that this idea was completely wrong. Dust grains near dying
stars could grow as large as a millimetre in just a few decades. So far from
delaying the agglomeration of dust, the plasma somehow accelerates it,
circumventing the electrostatic repulsion.
The first revision of the theory is to visualize each negatively charged dust grain
attracting a cloud of positively charged atoms around it, which neutralizes the
charge and removes an obstacle to the grains getting together. Secondly comes
the more remarkable idea that, as the dust grains approach one another in a
plasma, they feel a mutual attraction. Experts now call it the shadow force.
A racing yacht creates a shadow in the wind, which can thwart a rival trying
to overtake it on the leeward side. In a plasma, the equivalent of the wind
blows from every direction, in the form of the randomly moving atoms that
generate pressure. Each dust grain shadows its neighbours, reducing the
pressure on the facing sides, so that the remaining pressure pushes the grains
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plasma crystals
together. The cloud of charged atoms around each dust grain makes its sail area
larger.
The strength of the shadow force depends on the sizes of the grains and the
distances between them. If you double the size, the electric repulsion increases
fourfold, but the shadow force driving the grains together is multiplied by 16.

Halving the distance between the grains quadruples the force. It’s the same
mathematical law as for Newton’s force of gravity. Indeed the 18th-century Swiss
physicist Georges-Louis Le Sage tried to explain gravity by a shadow force. He
imagined space filled with corpuscles moving rapidly in all directions but being
blocked by massive bodies, so that the bodies would be pushed towards one
another.
As the grains in the dusty plasma come closer, their clouds of positively changed
atoms merge and the grains eventually repel one another. Then they occupy
space like atoms in a crystal, but on a vastly larger scale—typically a fraction of a
millimetre, or a million times the width of an atom. That’s when plasma crystals
can form.
I ‘An exhilarating experience’
Gregor Morfill at the Max-Planck-Institut in Garching wanted to make plasma
crystals experimentally, but he foresaw difficulties. The dust grains fall under
gravity. So in 1991 he proposed a Plasmakristallexperiment to be done in
weightless conditions on the International Space Station, which was then being
planned. Ten years later, thanks to collaborative Russians, Morfill’s apparatus
became the very first experiment in physical science to operate on the station.
Meanwhile a graduate student in Morfill’s group, Hubertus Thomas, succeeded
in making plasma crystals on the ground. He used electric levitation to oppose
gravity and keep his microscopic plastic grains afloat in a plasma of electrified
argon gas, in a box ten centimetres wide. The grains spontaneously arranged
themselves in a neat honeycomb pattern, just like atoms in a crystal but spaced
a fraction of a millimetre apart. When lit by a laser beam, the astonishing
objects could be seen with the naked eye.
The Garching scientists were not alone in producing plasma crystals on the
ground. Independently, other teams in Taiwan, Japan and Germany had similar
success. By 1996 a team at the Russian Institute for High Energy Densities was
making plasma crystals with a different technique. In place of a high-frequency
generator for electrifying the gas, the Russians used a direct-current discharge.

In those pioneering experiments, the plasma crystals were flat, because of the
levitation required. The Garching team then undertook preliminary trials under
weightless conditions, in short-lived rocket flights and aircraft dives. These
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plasma crystals
confir med that 3-D plasma crystals could be made in space, just as Morfill had
predicted in proposing the space-station experiments.
‘Plasmas are the most disorganized form of matter—that was the common
wisdom,’ Morfill commented. ‘To discover that they can also exist in crystallized
form was, therefore, a major surprise. In a relatively young research field like
plasma physics you have to expect surprises, of course, but somehow you always
think that the major discoveries will be made by others. To actually see the
plasma crystallization happen, for the first time, was an exhilarating
experience.’
The co-leader of the experiment on the International Space Station, Anatoli
Nefedov of Russia’s Institute for High Energy Densities, died just a few weeks
before space operations began in March 2001. So the project was renamed
Plasmakristallexperiment-Nefedov. Tended by cosmonauts, dozens of experiments
provided researchers on the ground with movies and measurements of the
behaviour of plasma crystals in weightless conditions. They watched matter
performing in ways seen only sketchily before, or not at all.
In space, plasma crystals usually form with holes in the middle, like doughnuts,
and the holes are very sharp-edged. If a disturbance fills a hole, it quickly
re-forms. The grains make 3-D assemblies, arranged in various symmetric
patterns, similar to those shown by atoms in different crystals.
When mixed grains of two different sizes are injected into the plasma, they sort
themselves out to make two plasma crystals, each with only one size of grain.
Where they meet, the cr ystals weld together, with the boundary between them
strangely bent. Hit the crystals with a puff of neutral gas, and a very sharply
defined shock wave will travel through them.

If one side of the chamber is warmer than the other, a stronger wind of
molecules comes from that side and pushes the plasma crystal towards the
cooler side. Used on the ground, this thermal effect provides an alternative to an
electric field, for countering gravity and levitating the plasma crystals. A warm
floor and a cold ceiling in the experimental chamber will keep the plasma
crystals floating comfortably for hours on end.
The strange phenomena made sense, thanks to theories that developed in
parallel with the preparation of the experiments. Vadim Tsytovich of Russia’s
Gener al Physics Institute had predicted the sharp boundaries of the plasma
crystals and the separations of particles of different sizes. He and Morfill
together developed a more refined theory, called the collective shadow effect,
which operates through the whole plasma crystal and not just between
neighbouring grains. This plays its part within a more general scenario by
Morfill and Tsytovich concerning the instabilities in dusty plasmas that drive
them to make structures.
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plasma crystals
I Helping to make the Earth
An early surprise in the experiments on the International Space Station was
that dust grains acquire electric charges even when injected into a neutral
gas. Some grains gather an excess of positive charges (ions) from the gas, and
others more negative charges (electrons). In effect, the grains make their own
plasma.
As a result the grains attract one another by the ordinary electric force, with a
clump accumulating 100,000 grains in a second. That is a million times faster
than you’d expect just by the collision of uncharged grains. If this phenomenon
had been noticed sooner, as an alternative explanation for the rapid growth of
dust grains, then plasma crystals and the shadow force might have remained
unknown. Which would have been a pity, because large areas of science and
engineering will feel the effects of this discovery.

In the story of the shadow force and dust power, plasma crystals are a half-way
house towards large dust grains. When the plasma pressure overwhelms the
repulsion between small grains, a plasma crystal cannot survive. Instead the
dust grains coalesce and grow rapidly, in the space around stars. And as a new
force in the cosmos, dust power has consequences going beyond mere dust
itself.
To make stars and planets, a cloud of dusty gas collapses under the pull of its
own gravity. The cloud must be large and massive enough for gr avity to grasp,
and a theory dating from 1928, by the British astronomer James Jeans, defined
the critical size. But by 2000 Robert Bingham of the UK’s Rutherford Appleton
Laboratory, in collaboration with Tsytovich, was pointing out that, in interstellar
space, the force drawing dust grains together is initially far stronger than gravity.
It operates over smaller volumes, and marshals the dust much more rapidly than
gravity grabs the gas.
‘We suspect that the shadow force operates in relatively dense interstellar clouds
to build comets and all the small bodies used in planet-making,’ Bingham said.
‘If we’re right, the Earth was being prefabricated when the Sun was still only a
gassy possibility.’
Effects may continue today in the Earth’s atmosphere. Dusty plasma—electrified
gas containing small solid particles—is produced by meteors bur ning up in the
atmosphere, and also by dust from the surface mixing with air electrified by
lightning strokes, ultraviolet rays from the Sun, and cosmic rays from the
Galaxy. Dust grains play a daily role in providing the nuclei on which water
condenses or freezes, to make rain and snow. Whether the shadow force speeds
their growth to an effective size, for cloud formation, is a now a matter for
investigation.
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plasma crystals
I Smart dust
Plasma crystals give scientists the chance to study analogues of atomic

latticework on a vastly enlarged scale. Fundamental processes of crystallization
and melting in ordinary materials will be clarified. Perhaps research on the
shadow force will help the fusion engineers and microchip manufacturers to
escape from their dusty difficulties.
The plasma crystals may suggest how to make materials of new kinds. But they
are already a fascinating novelty in their own right. In their ability to interact
with electricity, magnetism, light, radio waves or sound waves, plasma crystals
could create novel sensors or tools.
So scientists speculate about smart dust. In sizing up the consequences and
opportunities of plasma crystals and dust power, the imagination is strongly
challenged. In Fred Hoyle’s science-fiction tale of The Black Cloud (1957) an
intelligent interstellar medium appeared. As a leading theorist of plasma crystals,
Tsytovich, too, toyed with the notion of living dust.
‘Imagine a large, self-organizing structure of plasma crystals floating in an
interstellar cloud,’ he said. ‘Feeding on the dusty plasma, it can grow and make
copies of itself. The complex structure has a memory, but it can mutate and
evolve very rapidly—for example in competition with similar structures. Should
we not say it is alive?’
E See also
Molecules in space. For other discoveries provoked by dusty stars and electric
devices, see
Buckyballs and nanotubes.
547
plasma crystals
P
rofits from danish lager paid for a round-the-world expedition by the
research ship Dana, from 1928 to 1930. So when, in the Indian Ocean, the
onboard scientists detected a chain of underwater mountains running south-east
from the Gulf of Aden at the exit from the Red Sea, they gratefully named it the
Carlsberg Ridge. For investigators of the solid Earth, that obscure basaltic hump

became the equivalent of Charles Darwin’s Galapagos Islands, sparking a
revolution in knowledge.
The Carlsberg Ridge was only the second feature of its kind to be discovered. A few
years previously the German research ship Meteor had explored a similar ridge in
the middle of the Atlantic, first encountered by ships laying telegraph cables across
the ocean in the 19th century. Using ultrasound generators developed for hunting
submarines, but adapted into hydrographical echo sounders, ocean scientists
gradually revealed more and more mid-ocean ridges, each of them a long
mountain chain. By the 1960s they were known in all the world’s oceans, with a
total length of 65,000 kilometres, and geography was transformed.
The Carlsberg Ridge showed more subtle features. First, in 1933–34, a British
expedition in John Murray found that the ridge was also a rift. It was curiously
like the Great Rift Valley in nearby East Africa, with bulging sides and a deep
gully running down the middle.
In 1962 Drummond Matthews, a scientist from Cambridge, was in the Indian
Ocean aboard a borrowed naval ship, Owen. It towed a magnetic instr ument
behind it as it passed to and fro over the Carlsberg Ridge. Sensitive
magnetometers, devised initially to detect the steel hulls of submerged
submarines, were at that time an innovation in geophysics.
Elsewhere the magnetometers had revealed strange patterns of stripes on the
ocean floor, such that the magnetism was sometimes stronger, sometimes
weaker. US expeditions surveyed large areas of the Pacific, without making sense
of the patterns. Matthews decided to undertake a close inspection of part of the
Carlsberg Ridge. Again he found a mixture of strong and weak fields, and when
he returned to Cambridge he gave the data to a graduate student, Fred Vine, to
try to interpret.
548
Not long before, Vine had heard a visitor from Princeton, Harry Hess, speaking
at a students’ geology congress in Cambridge about sea-floor spreading. The
suggestion was that rock welling up at the mid-ocean ridges would spread

outwards on either side, making the ocean gradually wider. It was an idea with
no evidence to support it, and very few adherents, least of all in Hess’s
homeland. But while studying the magnetic data from the Carlsberg Ridge, Vine
had a brainwave.
If molten rock emerges at a mid-ocean ridge and then cools down, it becomes
magnetized by the prevailing magnetic field, thereby intensifying the magnetism
measured by a ship cruising over it. Earlier in the century Bernard Brunhes in
Fr ance and Motonori Matuyama in Japan had discovered that the Earth reverses
its magnetism every so often, swapping around its north and south magnetic
poles. Rock that cooled during a period of reversed magnetism will be magnetized
in the opposite direction, so altering the Earth’s field as measured by
a passing ship.
To the sea floor spreading outwards from the mid-ocean ridge, as Hess
proposed, Vine added this notion of magnetization on freezing. His inspiration
was to see that the sea floor acts like a tape recorder, with the sectors of strong
and weak magnetism telling of their formation at different times. ‘If spreading of
the ocean floor occurs,’ Vine wrote in a landmark paper, ‘blocks of alternately
normal and reversely magnetized material would drift away from the centre of
the ridge and lie par allel to the crest of it.’
Vine recalled later that when he showed his draft to a leading marine
geophysicist at the Cambridge lab, ‘he just looked at me and went on to talk
about something else.’ The head of the lab, Edward Bullard, was less
discouraging but demurred from putting his name to the paper. If so crazy a
notion from a graduate student was to have any hope of publication, respectable
support was needed. Matthews, who supplied the magnetic data from the
Carlsberg Ridge, agreed to be co-author and thereby earned his own place in
scientific history.
I The world turned upside down
On its publication in 1963, the Vine–Matthews paper was greeted at first with
a stony silence from the world’s experts. How could the sea floor spread unless

continents moved to make room for it? Everyone except a few mavericks, mostly
in Europe, knew perfectly well that the continents had been rooted to their
spots since the creation of the world.
There followed the most comprehensive overthrow of previous beliefs to occur
in science during the 20th century. The next few years brought vindication of
Vine’s idea of the magnetic tape recorder, confirmation of continental drift, and
549
plate motions
a brand-new theory called plate tectonics. A key contribution came in 1965 from
Tuzo Wilson at Toronto. He realized that some of the long fault-lines seen in
the ocean floor are made by large pieces of the Earth’s outer shell sliding past
one another.
Together with the ridges where the sea floor is manufactured, and with deep
ocean trenches where old sea floor dives underground for recycling, Wilson’s
transform faults help to define the outlines of great moving plates into which
the outer shell of the Earth is divided. The plate boundaries are the scenes of
most of the world’s earthquakes and active volcanoes. Dan McKenzie of
Cambridge and Jason Morgan of Princeton developed the formal mathematical
theory of plate tectonics in 1967–68.
The story that began in Her Majesty’s Ship Owen over the Carlsberg Ridge in
1962 climaxed in 1969 when another vessel, the US drilling ship Glomar
Challenger, began penetrating deep into the sediments of the ocean floor. The
eminent scientists who still believed that the oceans were primordial features
of the planet expected billions of years of Earth history to be recorded in thick
deposits. Instead, Glomar Challenger’s drill-bit hit the basaltic bedrock quite
quickly.
Near the ridges, the sediments go back only a few million years. They are
progressively older towards the ocean margins, just as you’d expect in basins
growing from the middle outwards, by sea-floor spreading. Everywhere they
date from less than 200 million years ago. The Earth’s surface refurbishes itself

continuously, directly in the oceans that cover most of the planet, and by what
are literally knock-on effects in the continents.
Seven large plates account for 94 per cent of the Earth’s surface. In descending
order of size they are the Pacific, African, Eurasian, Indo-Australian, North
American, Antarctic and South American Plates. Small plates make up the rest,
the main ones being the Philippine, Arabian and Caribbean Plates, lying roughly
where their names indicate, plus the Cocos and Nazca Plates, which are oceanic
plates located west of Central and South America.
The plates shuffle about at a few centimetres per year, roughly the speed at
which your fingernails grow. Even at that rate the momentum is terrific and the
plates jostle one another very forcibly. They also transport the mighty continents
in all directions, like so many lunches on cafeteria trays.
Plate motions and continental drift are directly measurable, by fixing the relative
positions of stations on different continents and seeing how they change as the
years pass. Ordinary navigational satellites, used with special care, do the job
surprisingly well. So does laser ranging to NASA’s Lageos satellite, fitted with
cat’s-eye reflectors. The fanciest method of gauging the plate motions compares
the exact arrival times, at radio telescopes scattered around the world, of radio
550
plate motions
waves coming across billions of light-years of space from the quasars, which are
giant black holes.
I Looking for the rocky motor
After the revolution, while geologists and geophysicists were hurriedly rewriting
their textbooks in terms of plate tectonics, two fundamental questions remained.
The first, still totally obscure, is why the Earth’s magnetism reverses, to produce
the patterns spotted by Vine. Explanation is the more difficult because of great
variations in the rate of reversals, from 0 to 6 per million years. If we lived in a
magnetically tranquil phase, such as prevailed 100 million years ago, the tape
recorder of the Carlsberg Ridge would be blank.

The second basic question is why the plates move. The idea of rocks flowing is
not at all unacceptable, even though they are very viscous of course. Given time,
they yield to pressure, as you can see in the crumpled strata of mountain ranges
and the squashed fossils they contain. What’s more, a slushy, semi-molten layer
of rocks beneath the plates, called the asthenosphere, eases their motions, and
lubrication at plate boundaries comes from water.
Nor is there any problem in principle about a power supply for moving the
plates about on the face of the Earth. Internal heat first provided during the
formation of the planet, by the amalgamation and settling of material under
gravity, is sustained by energy released by radioactive materials present in the
rocks of the Earth’s interior. You can think of all activity at the surface as a
direct or indirect result of heat trying to escape. Eventually it will succeed to
such an extent that the planet will freeze, as its neighbour Mars has done
already, and geological action will cease.
A pan of water carries heat from the stove to the air, by the hottest water rising
and cooled water sinking back again. Convection of a similar kind must operate,
however sluggishly, inside the Earth. Even so, the mechanism that translates
heat flow and convection into plate motions has been a matter of much
conjecture and argument ever since the dawn of plate tectonics. It’s not easy
to tell what’s going on, deep below the ground we stand on.
The distance from the Earth’s surface to the centre is 6400 kilometres. The
record-breaking Kola Superdeep Borehole, near Zapolyarny in northern Russia,
goes down just 12 kilometres. So everything scientists know about the interior
has to be inferred from data collectable very near the surface. That includes
figuring out what rocky motor propels the plates.
Earthquake waves have for long been the chief illuminators of the Earth’s
interior. By timing their arrivals at various stations around the world,
seismologists can deduce how fast they have tr avelled, and by what routes.
Beneath the crust, typically 40 kilometres thick, the main body of the Earth
551

plate motions