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particle families
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 paragraphs sum up the Standard Model, a well-rounded theory and one of the grandest outcomes of 20th-century science. It was created and largely confirmed 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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reen plants spread the enormous surface of their leaves and, in a still unknown way, force the energyof the Sun to carryout 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 recentlydiscovered 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 important 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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photosynthesis
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 started 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 first 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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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, H2O, 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 railways 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 whisks the liberated electrons away along the molecular railway, before they can rejoin the wrong atoms. It delivers them very 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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