The Faber Book of Science

by John Carey

  No, she’s our quiet mother, sensible.

  But therefore down-to-earth, not suffering

  Fools who play fast and loose among the mountains,

  Who fly in her face, or, drunken, clown on cornices.

  She taught our ways of walking. Her affection

  Adjusted the morning grass, the sands of summer

  Until our soles fit snug in each, walk easy.

  Holding her hand, we’re safe. Should that hand fail,

  The atmosphere we breathe would turn hysterical,

  Hiss with tornadoes, spinning us from earth

  Into the cold unbreathable desolations.

  Yet there – in fields of space – is where she shines,

  Ring-mistress of the circus of the stars,

  Their prancing carousels, their ferris wheels

  Lit brilliant in celebration. Thanks to her

  All’s gala in the galaxy.

  Down here she

  Walks us just right, not like the jokey moon

  Burlesquing our human stride to kangaroo hops;

  Not like vast planets, whose unbearable mass

  Would crush us in a bear hug to their surface

  And into the surface, flattened. No: deals fairly.

  Makes happy each with each: the willow bend

  Just so, the acrobat land true, the keystone

  Nestle in place for bridge and for cathedral.

  Lets us pick up – or mostly – what we need:

  Rake, bucket, stone to build with, logs for warmth,

  The fallen fruit, the fallen child … ourselves.

  Instructs us too in honesty: our jointed

  Limbs move awry and crisscross, gawky, thwart;

  She’s all directness and makes that a grace,

  All downright passion for the core of things,

  For rectitude, the very ground of being:

  Those eyes are leveled where the heart is set.

  See, on the tennis court this August day:

  How, beyond human error, she’s the one

  Whose will the bright balls cherish and obey

  – As if in love. She’s tireless in her courtesies

  To even the klutz (knees, elbows all a-tangle),

  Allowing his poky serve Euclidean whimsies,

  The looniest lob its joy: serene parabolas.

  Source: John Frederick Nims, The Six-Cornered Snowflake and Other Poems, New York, New Directions, 1990.

  Otto Frisch Explains Atomic Particles

  Otto Frisch (1904–79) was the physicist who, in collaboration with his aunt Lise Meitner, first realized that uranium atoms, bombarded with neutrons, split into atoms of lighter elements. Frisch and Meitner named the process ‘fission’. At the time (1939) Frisch was working in Copenhagen with Niels Bohr, who passed on the news to Einstein and others in the USA. During the Second World War Frisch was a member of the Los Alamos atomic research project. He wrote this popular introduction to atomic particles in 1960.

  For good reasons this has become known as the atomic age. Power from atomic nuclei is about to transform our world – and threatens to destroy it. Nearly every recent advance in engineering is based on what we know about the structure of atoms – high-strength alloys and plastics no less than fluorescent lamps, transistors and ferrites. Even in the study of the phenomena of life the stage has been reached when we examine effects of single atoms on living organisms. For these reasons alone it is clear that the study of atoms and their components is of great importance, but physicists have another strong incentive: curiosity. They just want to know what the world is made of, what are the smallest particles and how they behave. Therefore they are always ahead of practical applications; there always exists some knowledge about which people can ask ‘what is it good for?’ It is true that at the moment nobody can see any use for the ‘newer’ particles – mesons and hyperons – yet in the past any new discovery has invariably, within a few decades, found some practical use, or at least has become such an indispensable part of our knowledge that many practical advances would have been impossible without it. I do not foresee meson guns or hyperon boilers, but if applications for these particles are ever found, it is unlikely that even the most imaginative of present science-fiction writers will have envisaged them correctly.

  Let me first recapitulate what is known about the structure of atoms. Since 1911 we have known that each atom consists of a heavy core or nucleus, surrounded by a number of much lighter particles called electrons. Different atoms have different kinds of nuclei, but electrons are all alike. Electrons all weigh the same and have the same negative electric charge, but an atom is not electrically charged, the negative charge of its electron being offset by an equal positive charge of the nucleus. To put it the other way round: the nucleus has a positive charge which is Z times the charge of an electron; hence Z electrons become arranged around the nucleus to form an atom, which is electrically neutral (i.e., uncharged).

  The number (Z) of positive electronic charges on the nucleus, or of electrons around it, is called the atomic number. This determines the chemical properties of the atom, and many of its physical properties as well. Thus atoms with the same Z will stay together and not become separated when passed through chemical reactions; they belong to the same chemical element. Each element is characterized by its Z. Thus hydrogen has Z = 1; carbon, 6; copper, 29; and uranium, 92.

  Though all the nuclei in any one element carry the same positive charge, they have not necessarily the same mass. Nuclei with the same charge but different mass are said to belong to different isotopes of the same element. The mass of a nucleus is, however, always very nearly an exact multiple of a certain unit mass which, in turn, is very nearly equal to the mass of the lightest nucleus of all, that of the lightest (and most common) hydrogen isotope, the proton. For instance, a gold nucleus weighs about 197 of these units; we say its mass number A is 197. But it cannot consist simply of 197 protons, for then its atomic number Z would also be 197 whereas it is really only 79. We now know that it consists of 197 nuclear particles of which, however, only 79 are protons; the other 118 have practically the same mass but no electric charge and are called neutrons. Protons and neutrons are known collectively as nucleons.

  Free neutrons were first recognized in 1932 when it was found that they could be knocked out of certain light nuclei. For the knocking, nature had very kindly supplied fast-moving helium nuclei which are emitted from the nuclei of certain heavy elements such as uranium, radium, and others; when they were first observed their nature was not known and they were labelled ‘alpha particles,’ a name which has stuck. An alpha particle – like any other fast-moving charged particle – damages the atoms in the air through which it passes, leaving behind a trail of atoms or molecules which are electrically charged by having either lost an electron or gained an extra one, and which are called ‘ions.’ But a neutron slips through the air without making ions and is for that reason hard to detect; that is why it was discovered so late. A neutron can only be detected if it happens to strike a nucleus, which is then sent flying or is broken up; in that case, fast charged particles are formed which makes ions and can thereby be detected.

  C. T. R. Wilson’s cloud chamber provides a means of making trails of ions visible as fine tracks of water droplets, and so of detecting neutrons at second hand. It was a very convincing proof of the existence of neutrons to see the track of a nucleus, struck by a neutron somewhere in the gas of a cloud chamber, and sent flying by the collision. By placing the chamber between the poles of a strong electromagnet one can deflect the nuclei so that their tracks become curved; the degree of curvature indicates the speed of the nucleus, and hence the speed of the neutron that sent it flying. This kind of technique has been used again and again in the discovery and study of new particles.

  When the neutron was first found, its study amounted to research of the purest kind. Nobody could have foreseen any practical use for it. This changed dramatically with the discovery (1939) tha
t neutrons could cause the fission of uranium nuclei, with the liberation of more neutrons; in this way a chain reaction became possible, and this is now our chief source of neutrons and an increasingly important source of power. Today neutrons are an industrial commodity; several tons have been produced (and immediately consumed) in the brief history of atomic energy. Only ten years elapsed between the discovery of the neutron and the operation of the first atomic pile!

  A nucleus always weighs a little less than the sum of the neutrons and protons – or briefly the nucleons – of which it consists. The reason is that the nucleons are bound together by strong forces; it would require a considerable energy, the so-called binding energy, to take a nucleus completely to pieces. Now it is one of the consequences of Einstein’s theory of relativity that an energy E possesses a mass m, the exchange rate being given by the famous formula E = mc2 where c is the speed of light. The energy contained in 700 domestic units of electricity corresponds to only one millionth of an ounce; so in ordinary life the mass equivalent of energy can be neglected. But if you could assemble a nucleus out of its nucleons the energy liberated would amount to about one per cent of the total mass. Even minor rearrangements inside nuclei cause changes in mass which can be accurately measured with the so-called mass spectrometer. The energies, too, can be measured, and in this way Einstein’s equation has been checked and confirmed many times. Energies here are measured in MeV (the energy gained by an electron on being accelerated through one million volts), 1 MeV corresponds to one 940th of the mass of a proton, or to twice the mass of an electron.

  Almost at the same time as the neutron, the positive electron or positron was discovered, and this opened entirely new vistas. Physicists had often asked themselves why protons were always positively and electrons negatively charged when the fundamental laws of electricity were quite symmetrical in respect of charge. Indeed the quantum theory of the electron, as developed in England by P. A. M. Dirac (1928), showed that positively charged electrons must be possible, and that one could produce a positron and a (negative) electron ‘out of nothing,’ provided the necessary energy was supplied. Indeed when gamma rays pass through matter, positrons are produced, and when they pass through a cloud chamber the production of pairs of electrons, one deflected to the right, the other to the left, by the electromagnet, can be clearly seen. The opposite process also occurs: when a positron meets an electron both disappear in a flash of gamma radiation – they are said to annihilate each other. This is the reason why positrons are so rare: they disappear within less than a millionth of a second in contact with matter although they can exist indefinitely in a perfect vacuum. The positron is said to be the ‘antiparticle’ of the electron, and vice versa.

  As soon as the positron was discovered, it was realized that there ought to be an antiproton as well, a proton of negative charge, capable of annihilating itself with an ordinary proton. But since the proton is 1,836 times heavier than the electron, 1,836 times as much energy is needed to produce a proton-antiproton pair than to produce a positron-electron pair. For the latter process one needs about 1 MeV energy, an amount possessed by many ordinary gamma rays, but gamma rays of several thousands MeV are rare even in the cosmic radiation, the fine rain of very fast particles that comes to us from outer space. An accelerator was therefore built (in Berkeley, California) which could accelerate protons to about 6,000 MeV, and a determined and successful search was made for antiprotons. Particles were found which weighed as much as protons, but had the opposite (i.e., negative) charge, and which suffered annihilation in the expected manner on passing through matter.

  In the same experimental setup, antineutrons were also found. It may sound surprising that the neutron, which has no charge, should possess an ‘electric mirror image’ different from itself; but the neutron is a little magnet, which spins about its magnetic axis rather like the earth. In the antineutron, not only the electric but also the magnetic properties are reversed; so a neutron and an antineutron, spinning in the same direction, have their magnetic poles pointing in opposite ways, and this makes them different particles. Nobody has yet thought of a way of measuring the magnetic properties of antineutrons (though with neutrons it can be done); but there can be little doubt that the neutral particles, created under the same conditions as antiprotons and suffering annihilation in the same way, are indeed antineutrons.

  Let us take stock. We have mentioned six particles so far: the electron, the proton, and the neutron, each with its respective antiparticle – protons and neutrons are jointly called nucleons. The positive electron (the ‘antielectron’) is usually called the positron. The word electron is sometimes used for both kinds and sometimes just for the negative kind, the meaning being usually clear from the context.

  There is also the photon, the quantum of electromagnetic radiation, recognized by Einstein as early as 1905. Its existence, he realized, was a necessary consequence of the quantum theory of Planck (Germany) who had concluded – from a rather subtle argument about heat radiation – that radiation must be emitted and absorbed, not continuously as people had previously thought, but in packets whose energy-content was proportional to the frequency of oscillation in the radiation in question. For instance, the main part of the gamma radiation of radium has an oscillation frequency of 5.3 × 1020 per second, which corresponds with an energy content of 2.2 MeV for photons of this radiation. The frequency of visible light is a million times lower and hence its photons have a million times less energy; on the other hand, photons a million times more energetic are found in the cosmic radiation. But all these photons are basically the same thing, only endowed with different energies.

  Next on my list is the neutrino, which has an extremely interesting story. Almost as soon as radioactivity was discovered at the turn of the century the various types of radiation were roughly classified and labelled with Greek letters. I have already mentioned alpha particles, the least penetrating of all, which were later identified as fast helium nuclei, and gamma rays, which are energetic photons rather like X-rays. Those with intermediate penetrating power were called beta rays and were soon recognized as fast electrons. Some of them were just atomic electrons which had been set in motion by energy from the nucleus, but others came out of the nucleus itself, created in the transformation of a neutron into a proton. It was possible to calculate with what energy they ought to come out (from the mass of the nucleus before and after), and the disturbing fact emerged that they never came out with this full energy, but with a distribution of lower energies. In each of these ‘beta transformations’ a random amount of energy was missing and could not be traced.

  Where something is missing there must be a thief, so a young Austrian theoretician, W. Pauli (in Switzerland), suggested that the beta transformation consisted in the emission, not of one particle (the electron that is observed), but of two, which share the available energy, one of them escaping unobserved. Lead blocks had been set up by the experimental physicists in order to trap the missing energy, and Pauli had to assume that his unobserved particle was penetrating enough to go through all of them, and so had to be electrically neutral. It was therefore soon nicknamed il neutrino (the little neutral one) by the Italian physicist E. Fermi, who was the first to take it seriously. It also had to be very light in order to give good agreement with the observed distribution of the electrons. More and more indirect evidence accumulated that Pauli had been right, but the neutrino itself escaped all the traps set for it and proved to be millions of times more elusive than Pauli had assumed at first. Even so it could not be completely elusive: if atomic nuclei could send out neutrinos they must also be capable of stopping them. It was possible to calculate the minimum stopping-power and it turned out to be extremely small: the chance that out of a million neutrinos traversing the entire earth a single one could be stopped was about one in a million. Yet, in 1956, it was announced that neutrinos had been caught and their existence definitely confirmed. This was made possible by using an atomic pile as a source (s
ome 1018 neutrinos every second) and by means of special counters capable of detecting any neutrino that got stopped in a cubic yard or so of scintillating fluid. Even so only a few neutrinos per hour were recorded, but this was enough to identify them, and a proud day it was for Pauli, whose bold guess of some twenty-five years ago had at last been fully vindicated.

  There are also antineutrinos, and the difference between the two is that they spin in opposite directions about their direction of flight, like bullets fired respectively from barrels with right-handed and left-handed rifling. You may object that this is not a valid distinction, that to an observer overtaking a neutrino its direction of flight would appear the opposite, and hence it would look like an antineutrino to him. But a neutrino always travels with the speed of light and thus cannot be overtaken since no object can move faster. It looks like a neutrino to any observer, whichever way and however fast he moves.

  You may feel that I have spent too much time on an almost unobservable particle, but in the first place I think the physicists can be proud of having traced and pinned down something so elusive, and secondly, the effects of neutrinos in the large are not negligible. The energy of the sun and stars comes from nuclear transformations in their interiors. Some of the transformations are of the beta kind, and it has been computed that no less than about one-sixth of the energy is taken away by neutrinos and lost in the depths of space.

  How did the photon, the quantum of electromagnetic radiation, come to be discovered? Our eyes being sensitive to a particular kind of radiation which we call light, once the laws of the quantum theory were understood, the existence of light quanta or photons followed of necessity. But would a race of blind physicists have discovered the photon? I think they would. They could have discovered electric and magnetic phenomena and found out their laws, and from these laws they would have deduced the existence of electromagnetic waves (just as Maxwell did in 1864, though the fact that he knew light might have helped him!) and hence of photons. Once the electromagnetic forces were known the rest could be done by mathematics alone.