3 - Littler And Littler: The Structure Of Matter
One of the words that fascinates scientists in the 1960s is "quark."
No one has ever seen a quark or come across one in any way. It is far too small to see and no one is even sure it exists. Yet scientists are anxious to build enormous machines costing hundreds of millions of dollars to try to find quarks, if they exist.
This is not the first time scientists have looked for objects they weren't sure existed, and were too small to see even if they did exist. They were doing it as early as the very beginning of the nineteenth century.
In 1803, an English chemist, John Dalton, suggested that a great many chemical facts could be explained if one would only suppose that everything was made up of tiny particles, too small to be seen under any microscope. These particles would be so small that there couldn't be anything smaller. Dalton called these particles "atoms" from Greek words meaning "not capable of being divided further." Dalton's suggestion came to be called the "atomic theory."
No one was sure that atoms really existed, to begin with, but they did turn out to be very convenient. Judging by what went on in test tubes, chemists decided that there were a number of different kinds of atoms.
When a particular substance is made up of one kind of atom only, it is an "element." Iron is an element, for instance, and is made up only of iron atoms. Gold is an element; so is the oxygen in the air we breathe.
Atoms can join together into groups and these groups are called "molecules." Oxygen atoms get together in groups of two and these two-atom oxygen groups are called oxygen molecules. The oxygen in the air is made up of oxygen molecules, not of separate oxygen atoms.
Atoms of different elements can come together to form molecules of "compounds." Water is a compound with molecules made up of two hydrogen atoms and one oxygen atom.
Dalton and the nineteenth century chemists who followed him felt that every atom was just a round little ball. There was no reason to think there was anything more to it than that. They imagined that if an atom could be seen under a very powerful microscope, it would turn out to be absolutely smooth and shiny, without a mark.
They were also able to tell that the atom was extremely small. They weren't quite certain exactly how small it was but nowadays we know that it would take about 250 million atoms laid side by side to stretch across a distance of only one inch.
The chief difference between one kind of atom and another kind, in the nineteenth century view, lay in their mass, or weight. Each atom had its own particular mass, or "atomic weight." The hydrogen atom was the lightest of all, and was considered to have an atomic weight of l. An oxygen atom was about sixteen times as massive as a hydrogen atom, so it had an atomic weight of 16. A mercury atom had an atomic weight of 200, and so on.
As the nineteenth century wore on, the atomic theory was found to explain more and more things. Chemists learned how atoms were arranged inside molecules and how to design new molecules so as to form substances that didn't exist in nature.
By the end of the century, the atomic theory seemed firmly established. There seemed no room for surprises.
And then, in 1896, there came a huge surprise that blew the old picture into smithereens. The chemists of the new twentieth century were forced into a new series of investigations that led them deep into the tiny atom.
What happened in 1896 was that a French physicist, Antoine Henri Becquerel, discovered quite by accident that a certain substance had properties no one had ever dreamed of before.
Becquerel had been interested in x rays, which had only been discovered the year before. He had samples of a substance containing atoms of the heavy metal uranium in its molecules. This substance gave off light of its own after being exposed to sunlight and Becquerel wondered if this light might include x rays.
It didn't, but Becquerel found it gave off mysterious radiations of some kind; radiations that went right through black paper and fogged a photographic film. It eventually turned out that it was the uranium atoms that were doing it. The uranium atoms were exploding and hurling small fragments of themselves in every direction.
Scientists had never expected atoms could explode, but here some of them were doing it. A new word was invented. Uranium was "radioactive."
Other examples of radioactivity were found and physicists began to study the new phenomenon with great interest as the twentieth century opened.
One thing was clear at once. The atom was not just a hard, shiny ball that could not be divided into smaller objects. Small as it was, it had a complicated structure and was made up of many objects still smaller than atoms. This had to be, for the uranium atom, in exploding, hurled outward some of these still smaller "subatomic particles."
One of the most skillful of the new experimenters was a New Zealander, Ernest Rutherford. He used the subatomic particles that came flying out of radioactive elements and made them serve as bullets. He aimed them at thin films of metal and found they passed right through the metal without trouble. Atoms weren't hard shiny balls at all. Indeed, they seemed to be mostly empty space.
But then, every once in a while, one of the subatomic bullets would bounce off sharply. It had hit something hard and heavy somewhere in the atom.
By 1911, Rutherford was able to announce that the atom was not entirely empty space. In the very centre of the atom was a tiny "atomic nucleus" that contained almost all the mass of the atom. This nucleus was so small that it would take about 100,000 of them, placed side by side, to stretch across the width of a single atom.
Outside the nucleus, filling up the rest of the atom, were a number of very light particles called "electrons." Each different kind of atom had its own particular number of electrons. The hydrogen atom had only a single electron; the oxygen atom had eight; the iron atom had twenty-six; the uranium atom had ninety-two, and so on.
All electrons, no matter what atom they are found in, are alike in every way. All of them, for instance, carry an electric charge. There are two kinds of electric charges-positive and negative. All electrons carry a negative electric charge and the charge is always of exactly the same size. We can say that every electron has a charge of just -1.
The atomic nucleus has an electric charge, too, but a positive one. The charge on the nucleus just balances the charge on the electrons. A hydrogen atom has a single electron with a charge of -l. Therefore, the charge on the hydrogen nucleus is +1.
An oxygen atom has eight electrons with a total charge of -8. The oxygen nucleus has a charge of +8, therefore. You can see, then, that the iron nucleus would have to have a charge of +26, the uranium nucleus one of +92, and so on.
Both parts of the atom-the tiny nucleus at the centre and the whirling electrons outside-have been involved in unusual discoveries since Rutherford made his announcement in 1911. In this chapter, however, we are going to be concerned only with the nucleus.
Naturally, physicists were interested in knowing whether the atomic nucleus was a single particle. It was so much smaller than the atom that it would seem reasonable to suppose that here at last was something as small as it could be. The atom had proved a surprise, however, and scientists were not going to be too sure of the nucleus either.
Rutherford bombarded atoms with subatomic particles, hoping to discover something about the nucleus if he hit them enough times.
He did. Every once in a while, when one of his subatomic bullets hit a nucleus squarely, that nucleus changed its nature. It became the nucleus of a different variety of atom. Rutherford first discovered this in 1919.
This change of one nucleus into another made it seem as though the nucleus had to be a collection of still smaller particles. Changes would come about because the collection of still smaller particles was broken apart and rearranged.
The smallest nucleus was that of the hydrogen atom. That had a charge of +1 and it did indeed seem to be composed of a single particle. Nothing Rutherford did could break it up (nor have we found a way to do so even today). Rutherford therefore considered it to be composed of a single particle which he called a "proton."
The proton's charge, +1, was exactly the size of the electron's, but it was of the opposite kind. It was a positive electric charge, rather than a negative one.
The big difference between the proton and electron, however, was in mass. The proton is 1,836 times as massive as the electron though to this day physicists don't know why that should be so.
It soon seemed clear that the nuclei of different atoms had different electric charges because they were made up of different numbers of protons. Since an oxygen nucleus had a charge of +8, it must contain eight protons. In the same way, an iron nucleus contained twenty-six protons and a uranium nucleus ninety-two protons.
This is why the nucleus contains just about all the mass of the atom, by the way. The nucleus is made up of protons which are so much heavier than the electrons that circle about outside the nucleus.
But a problem arose at this point that plagued physicists all through the 1920s. The protons could account for the electric charge of the nucleus, but not for all its mass. Because the oxygen nucleus had a charge of +8, it therefore had to contain eight protons, but it also had a mass that was sixteen times as great as a single proton and therefore twice as great as all eight protons put together. Where did the extra mass come from?
The uranium nucleus had a charge of +92 and therefore had to contain ninety-two protons. However, the mass of the uranium nucleus was two and a half times as great as all those ninety-two protons put together. Where did that come from?
Physicists tried to explain this in several ways that proved to be unsatisfactory. A few, however, speculated that there might be particles in the nucleus that were as heavy as protons but that didn't carry an electric charge.
Such uncharged particles, if they existed, would add to the mass of the nuclei without adding to the electric charge. They would solve a great many puzzles concerning the nucleus, but there was one catch.
There seemed no way of detecting such uncharged particles, if they existed. To see why this is so, let's see how physicists were detecting ordinary charged particles in the 1920s.
Physicists used a number of techniques for the purpose, actually, but the most convenient had been invented in 1911 by a Scottish physicist, Charles Thomson Rees Wilson.
He had begun his career studying weather and he grew interested in how clouds came to form. Clouds consist of very tiny droplets of water (or particles of ice) but these don't form easily in pure air. Instead, each one forms about a tiny piece of dust or grit that happens to be floating about in the upper air. In the absence of such dust, clouds would not form even though the air was filled with water vapour to the very limit it would hold, and more.
It turned out also that a water droplet formed with particular ease, if it formed about a piece of dust that carried an electric charge.
With this in mind, Wilson went about constructing a small chamber into which moist air could be introduced. If the chamber were expanded, the air inside would expand and cool. Cold air cannot hold much water vapour, so as the air cooled the vapour would come out as a tiny fog.
But suppose the moist air introduced into the chamber were completely without dust. Then even if the chamber were expanded and the air cooled, a fog would not form.
Next suppose that a subatomic particle comes smashing through the glass and streaks into the moist air in the chamber. Suppose also that the particle is electrically charged.
Electric charges have an effect on one another. Similar charges (two negatives or two positives) repel each other; push each other away. Opposite charges (a negative and a positive) attract each other.
If a negatively charged particle, like an electron, rushes through the air, it repels other electrons it comes near. It pushes electrons out of the atoms with which it collides. A positively charged particle, like a proton, attracts electrons and pulls them out of the atom. In either case, atoms in the path of electrically charged particles lose electrons.
What is left of the atom then has a positive electrical charge, because the positive charge on the nucleus is now greater than the negative charge on the remaining electrons. Such an electrically charged atom is called an "ion."
Water droplets, which form with particular ease about electrically charged dust particles, also form with particular ease about ions. If a subatomic particle passes through the moist air in the cloud chamber just as that air is cooled, droplets of water will form about the ions that the subatomic particle leaves in its track. The path of the subatomic particle can be photographed and the particle can be detected by the trail it leaves.
Suppose a cloud chamber is placed near a magnet. The magnet causes the moving subatomic particle to curve in its path. It therefore leaves a curved trail of dewdrops.
The curve tells volumes. If the particle carries a positive electric charge, it curves in one direction and if it carries a negative electric charge it curves in the other. The more massive it is, the more gently it curves. The larger its charge, the more sharply it curves.
Physicists took many thousands of photographs of cloud chambers and studied the trails of dewdrops. They grew familiar with the kind of tracks each particular kind of particle left. They learned to tell from those tracks what was happening when a particle struck an atom, or when two particles struck each other.
Yet all of this worked well only for charged particles. Suppose a particle didn't carry an electric charge. It would have no tendency to pull or push electrons out of an atom. The atoms would remain intact and uncharged. No ions would be formed and no water droplets would appear. In other words, an uncharged particle would pass through a cloud chamber without leaving any sign.
Still, might it not be possible to detect an uncharged particle indirectly? Suppose you faced three men, one of whom was invisible. You would only see two men and if none of them moved you would have no reason to suspect that the third man existed. If, however, the invisible man were suddenly to push one of his neighbours, you would see one of the men stagger. You might then decide that a third man was present but invisible.
Something of the sort happened to physicists in 1930. When a certain metal called beryllium was exposed to a spray of subatomic particles, a radiation was produced by it which could not be detected by cloud chamber.
How did anyone know there was that radiation present then? Well, if paraffin were placed some distance away from the beryllium, protons were knocked out of it. Something had to be knocking out those protons.
In 1932, an English physicist, James Chadwick, argued that the radiation from beryllium consisted of uncharged particles. These particles were electrically neutral and they were therefore called "neutrons."
Neutrons were quickly studied, not by cloud chamber, but by the manner in which they knocked atoms about, and much was learned. It was found that the neutron was a massive particle, just a trifle more massive than the proton. Where the proton was 1,836 times as massive as the electron, the neutron was 1,839 times as massive as the electron.
Physicists now found that they had a description of the structure of the nucleus that was better than anything that had gone before. The nucleus consisted of both protons and neutrons. It was the neutrons that accounted for the extra mass of the nucleus.
Thus, the oxygen nucleus had a charge of +8 but a mass of 16. That was because it was made up of 8 protons and 8 neutrons. The uranium nucleus had a charge of +92 and a mass of 238; it was made up of 92 protons and 146 neutrons. The atomic nucleus, small as it was, did indeed consist of still smaller particles (except in the case of hydrogen). Indeed, the nuclei of the more complicated atoms were made up of a couple of hundred smaller particles.
This does not mean that there weren't some serious questions raised by this proton-neutron theory of nucleus structure. For instance, protons are all positively charged and positively charged particles repel each other. The closer they are, the more strongly they repel each other. Inside the atomic nucleus, dozens of protons are pushed together so closely they are practically touching. The strength of the repulsion must be enormous and yet the nucleus doesn't fly apart.
Physicists began to wonder if there was a special pull, or force, that held the protons together. This force had to be extremely strong to overcome the "electromagnetic force" that pushed protons apart. Furthermore, the new force had to operate only at very small distances, for when protons were outside nuclei, they repelled each other with no sign of any attraction.
Such a strong attraction that could be felt only within nuclei is called a "nuclear force."
Could such a nuclear force exist? A Japanese physicist, Hideki Yukawa, tackled the problem shortly after the neutron was discovered. He carefully worked out the sort of thing that would account for such an extremely strong and extremely short-range force.
In 1935, he announced that if such a force existed, then it might be built up by the constant exchange of particles by the protons and neutrons in the nucleus. It would be as though the protons and neutrons were tossing particles back and forth and held firmly together as long as they were close enough to toss and catch. As soon as the neutrons and protons were far enough apart so that the particles couldn't reach, then the nuclear force would be no longer effective.
According to Yukawa, the exchange particle should have a mass intermediate between that of the proton and the electron. It was therefore eventually named a "meson" from a Greek work meaning "intermediate."
But did the meson really exist?
The best way of deciding whether it existed and if Yukawa's theory was actually correct was to detect the meson inside the nucleus, while it was being tossed back and forth between protons and neutrons. Unfortunately, that seemed impossible. The exchange took place so quickly and it was so difficult to find out what was going on deep inside the nucleus, that there seemed no hope.
But perhaps the meson could be somehow knocked out of the nucleus and detected in the open. To do that the nucleus would really have to be made to undergo a hard collision.
According to a theory worked out by the German-Swiss physicist, Albert Einstein, in 1905, matter and energy are two different forms of the same thing. Matter is, however, a very concentrated form of energy. It would take the energy produced by burning twenty million gallons of petrol to make one ounce of matter.
To knock a meson out of the nucleus of an atom would be very much like creating the amount of matter in a meson. To produce that quantity of matter doesn't really take much energy, but that energy has to be concentrated into a single tiny atomic nucleus and doing that turns out to be very difficult.
All through the 1930s and 1940s, physicists devised machines for pushing subatomic particles by electromagnetic forces and making them go faster and faster, piling up more and more energy, until finally, crash-they were sent barreling into a nucleus.
Gradually, more and more energy was concentrated into these speeding particles. Such energy was measured in "electron volts" and by the 1940s particles with energies of ten million electron volts (10 Mev) were produced. This sounds like a great deal, and it is, but it still wasn't enough to form mesons.
Fortunately, physicists weren't entirely stopped. There is a natural radiation ("cosmic rays") striking the Earth all the time. This is made up of subatomic particles of a wide range of energies; some of them are enormously energetic.
They originate somewhere deep in outer space. Even today, physicists are not entirely certain as to the origin of cosmic rays or what makes them possess so much energy. Still, the energy is there to be used.
Cosmic rays aren't the perfect answer. When physicists produce energetic particles, they can aim them at the desired spot. When cosmic rays bombard Earth, they do so without aiming. Physicists must wait for a lucky hit; when a cosmic ray particle with sufficient energy just happens to hit a nucleus in the right way. And then he must hope that someone with a detecting device happens to be at the right place and at the right moment.
For a while, though, it seemed that the lucky break had taken place almost at once. Even while Yukawa was announcing his theory, an American physicist, Carl David Anderson, was high on Pike's Peak in Colorado, studying cosmic rays.
The cosmic ray particles hit atoms in the air and sent other particles smashing out of the atoms and into cloud chambers. When there was finally a chance to study the thousands of photographs that had been taken, tracks were found which curved in such a way as to show that the particle that caused them was heavier than an electron but lighter than the proton. In 1936, then, it was announced that the meson had been discovered.
Unfortunately, it quickly turned out that this meson was a little too light to be the particle called for by Yukawa's theory. It was wrong in several other ways, too.
Nothing further happened till 1947. In that year, an English physicist, Cecil Frank Powell, was studying cosmic rays far up in the Bolivian Andes. He wasn't using cloud chambers, but special photographic chemicals which darkened when a subatomic particle struck them.
When he studied the tracks in these chemicals, he found that he, too, had a meson, but a heavier one than had earlier been found. Once there was a chance to study the new meson it turned out to have just the properties predicted by Yukawa.
The first meson that had been discovered, the lighter one, was named the "mu-meson." The heavier one that Powell had discovered was the "pi-meson." ("Mu" and "pi" are letters of the Greek alphabet. Scientists often use Greek letters and Greek words in making up scientific names.)
It is becoming more and more common to abbreviate the names of these mesons. The light one is called the "muon" and the heavy one the "pion."
The new mesons are very unstable particles. They don't last long once they are formed. The pion only lasts about twenty-five billionths of a second and then it breaks down into the lighter muon. The only reason the pion can be detected at all is that when it is formed it is usually travelling at enormous speed, many thousands of miles a second. Even in a billionth of a second it has a chance to travel a few inches, leaving a trail as it does so. The change in the kind of trail it leaves towards the end shows that the pion has disappeared and a muon has taken its place.
The muon lasts much longer, a couple of millionths of a second, and then it breaks down, forming an electron. The electron is stable and, if left to itself, will remain unchanged forever.
By the end of the 1940s, then, the atomic nucleus seemed to be in pretty good shape. It contained protons and neutrons and these were held together by pions flashing back and forth. Chemists worked out the number of protons and neutrons in every different kind of atom and all seemed well.
But it did seem that there ought to be two kinds of nucleithe kind that exists all about us and a sort of mirror image that in the late 1940s, no one had yet seen.
That possibility had first been suggested in 1930 by an English physicist, Paul Adrien Maurice Dirac. He calculated what atomic structure ought to be like according to the latest theories and it seemed to him that every particle ought to have an opposite number. This opposite could be called an "antiparticle."
In addition to an electron, for instance, there ought also to be an "antielectron" that would have a mass just like that of an electron but would be opposite in electric charge. Instead of having a charge of -1, it would have one of +1.
In 1932, C. D. Anderson (who was later to discover the muon) was studying cosmic rays. He noticed on one of his photographs a cloud-chamber track which he easily identified as that of an electron. There was only one thing wrong with it; it curved the wrong way. That meant it had a positive charge instead of a negative one.
Anderson had discovered the antielectron. Because of its positive charge, it is usually called a "positron." The existence of the antielectron was strong evidence in favor of Dirac's theory, and as time went on more and more antiparticles were uncovered.
The ordinary muon, for instance, has a negative charge of -l, like the electron, and it is usually called the "negative muon." There is an antimuon, exactly like the muon except that it has a positive charge of +1 like the positron. It is the "positive muon."
The ordinary pion is a "positive pion" with a charge of +1. The antipion is the "negative pion" with a charge of -1.
By the close of the 1940s, it seemed quite reasonable to suppose that there were ordinary nuclei made up of protons and neutrons with positive pions shifting back and forth among them; and that there were also "antinuclei" made up of "antiprotons" and "antineutrons" with antipions shifting back and forth.
Physicists didn't really feel they actually had to detect antiprotons and antineutrons to be sure of this but, of course, they would have liked to.
To detect antiprotons is even more difficult than to detect pions. An antiproton is as massive as a proton, which means it is seven times as massive as a pion. To form an antiproton requires a concentration of seven times as much energy as to form a pion.
To form a pion required several hundred million electron volts, but to form an antiproton would require several billion electron volts. (A billion electron volts is abbreviated "Bev.")
To be sure, there are cosmic ray particles that contain several Bev of energy, even several million Bev. The higher the energy level required, however, the smaller the percentage of cosmic ray particles possessing that energy. The chances that one would come along energetic enough to knock antiprotons out of atoms just when a physicist was waiting to take a picture of the results were very small indeed.
However, the machines for producing man-made energetic particles were becoming ever huger and more powerful. By the early 1950s, devices for producing subatomic particles with energies of several Bev were built. One of these was completed at the University of California in March 1954. Because of the energy of the particles it produced, it was called the "Bevatron."
Almost at once, the Bevatron was set to work in the hope that it might produce antiprotons. It was used to speed up protons until they possessed 6 Bev of energy and then those protons were smashed against a piece of copper. The men in charge of this project were an Italian-born physicist, Emilio Segré, and a young American, Owen Chamberlain.
In the process, mesons were formed; thousands of mesons for every possible antiproton. The mesons, however, were much lighter than antiprotons and moved more quickly. Segré's group set up detecting devices that would react in just the proper manner to pick up heavy, slow-moving, negatively charged particles. When the detecting devices reacted properly, only something with exactly the properties expected of an antiproton could have turned the trip.
By October 1955, the detection devices had been tripped sixty times. It could be no accident. The antiproton was there and its discovery was announced.
The antiproton was the twin of the proton. The great difference was that the proton had a charge of +l and the antiproton had a charge of -1.
Once enough antiprotons were produced for study, it was found that occasionally one would pass close by a proton and the opposite charges would cancel. The proton would become a neutron and the antiproton would become an antineutron.
You might wonder how you could tell an antineutron from a neutron since both are uncharged. The answer is that although the neutron and antineutron have no electric charge, they spin rapidly in a way that causes them to behave like tiny magnets. The neutron is like a magnet that points in one direction while the antineutron is like a magnet that points in the opposite direction.
By the mid-1950s, it was clear that antiprotons and antineutrons existed. But could they combine to form an antinucleus?
Physicists were sure they could but the final answer did not come till 1965. In that year, at Brookhaven National Laboratories in Long Island, New York, protons with energies of 7 Bev were smashed against a beryllium target. Several cases of an antiproton and antineutron in contact were produced and detected.
In the case of ordinary particles, there is an atomic nucleus that consists of one proton and one neutron. This is the nucleus of a rare variety of hydrogen atom that is called "deuterium." The proton-neutron combination is therefore called a "deuteron."
What had been formed at Brookhaven was an "antideuteron." It is the very simplest antinucleus that could be formed of more than one particle, but that is enough. It proved that it could be done. It was proof enough that matter could be built up out of antiparticles just as it could be built of ordinary particles. Matter built up of antiparticles is "antimatter."
When the existence of antiparticles was first proposed, it was natural to wonder why if they could exist, they weren't anywhere around us. When they were detected at last, they were found only in tiny quantities and even those quantities didn't last long.
Consider the positron, or antielectron. All around us, in every atom of all the matter we can see and touch on Earth, are ordinary electrons. Nowhere are there any antielectrons to speak of. Occasionally, cosmic ray particles produce a few or physicists form a few in the laboratory. When they do, those antielectrons disappear quickly.
As an antielectron speeds along, it is bound to come up against one of the trillions of ordinary electrons in its immediate neighbourhood. It will do that in perhaps a millionth of a second.
When an electron meets an antielectron, both particles vanish. They are opposites and cancel out. It is like a peg falling into a hole which it fits exactly. Peg and hole both disappear and nothing is left but a flat surface.
In the case of the electron and antielectron, however, not everything disappears. Both electron and antielectron have mass, exactly the same amount of the same kind of mass. (We only know of one kind of mass so far.) When the electron and antielectron cancel out, the mass is left over and that turns into energy.
This happens with all other particles and antiparticles. A positive muon will cancel a negative muon; a negative pion will cancel a positive pion; an antiproton will cancel a proton, and so on. In each case both particles disappear and energy takes their place. Naturally, the more massive the particles, the greater the amount of energy that appears.
It is possible to reverse the process, too. When enough energy is concentrated into a small space, particles may be formed out of it. A particle is never formed out of energy by itself, however. If an electron is formed, an antielectron must be formed at the same time. If a proton is formed, an antiproton must be formed at the same time.
When Segré and Chamberlain set about forming antiprotons, they had to allow for twice as much energy as would be sufficient just for an antiproton. After all, they had to form a proton at the same time.
Since this is so, astronomers are faced with a pretty problem. They have worked up many theories of how the universe came to be, but in all the theories, it would seem that antiparticles ought to be formed along with the particles. There should be just as much antimatter as there is matter.
Where is all this antimatter? It doesn't seem to be around. Perhaps it has combined with matter and turned into energy. In that case, why is there all the ordinary matter about us left over. There should be equal amounts of each, and each set should cancel out the other completely.
Some astronomers suggest that there are two separate universes, one made out of matter (our own) and another made out of antimatter. Other astronomers think there is only one universe but that some parts of it (like the parts near ourselves) are matter, while other parts are antimatter.
What made the matter and antimatter separate into different parts of the universe, or even into different universes, no one can yet say. It may even be possible that for some reason we don't understand, only matter, and no antimatter, was formed to begin with.
The problem of the universe was something for astronomers to worry about, however. Physicists in 1947 were quite satisfied to concentrate on particles and antiparticles and leave the universe alone.
And physicists in that year seemed to have much ground for satisfaction. If they ignored the problem of how the universe began and just concentrated on how it was now, they felt they could explain the whole thing in terms of a little over a dozen particles altogether. Some of these particles they had actually detected. Some they had not, but were sure of anyway.
Of course, not everything was absolutely clear, but what mysteries existed ought to be cleared up, they hoped, without too much trouble.
The particles they knew, or strongly suspected they were soon going to know, fell into three groups, depending on their mass. There were the light particles, the middle-sized particles, and the heavy particles. These were eventually given Greek names from words meaning light, middle-sized, and heavy: "leptons," "mesons," and "baryons."
The leptons, or light particles, include the electron and the antielectron, of course. In order to explain some of the observed facts about electrons, the Austrian physicist Wolfgang Pauli suggested, in 1931, that another kind of particle also existed. This was a very small one, possibly with no mass at all, and certainly with no charge. It was called a "neutrino." This tiny particle was finally detected in 1956. There was not only a neutrino but also an "antineutrino."
Although the muon was considered a meson, to begin with, it was soon recognized as a kind of heavy electron. All its properties but mass were identical with those of the electron. Along with the muon, a neutrino or antineutrino is also formed as in the case of the electron. In 1962, this muonneutrino was found to be different from the electron-neutrino.
Two other particles might be mentioned. Light, together with other radiation similar to it (like x rays, for instance) behaves in some ways as though it were composed of particles. These particles are called "photons."
There is no antiparticle for a photon; no antiphoton. The photon acts as its own opposite. If you were to fold a sheet of paper down the middle and put the particles on one side and the antiparticles on the other, you would have to put the photon right on the crease.
Then, too, physicists speculate that the reason different objects pull at each other gravitationally is because there are tiny particles called "gravitons" flying between them. Some of the properties of the graviton have been worked out in theory; for instance, it is its own antiparticle. The graviton is so tiny, however, and so hard to pin down, that it has not yet been detected.
This is the total list of leptons so far, then:
The leptons pose physicists some problems. Does the graviton really exist? Why does the muon exist; what is the purpose of something that is just a heavy electron? Why and how are the muon-neutrinos different from the electron-neutrinos? These puzzles are intriguing but they don't drive physicists to despair.
In 1947, only three particles were coming to be known which would now be considered mesons. Two of them were the positive pion and the negative antipion. The third was a neutral pion which, like the photon and the graviton, was its own antiparticle.
Only four particles were known in 1947 that would now be classified as baryons. These are the proton, antiproton, neutron, and antineutron. Both antiproton and antineutron had not yet actually been detected, but physicists were quite sure they existed.
The situation with regard to the nucleus seemed particularly well settled. There was the nucleus made up of protons and neutrons held together by pions, and the antinucleus made up of antiprotons and antineutrons held together by antipions. All seemed well.
But in 1947, the very year which saw the discovery of the pion and the apparent solution of the problem of the nucleus, there began a new series of discoveries that upset the applecart again.
Two English physicists, George Dixon Rochester and Clifford Charles Butler, studying cosmic rays with cloud chambers in 1947, came across an odd V-shaped track. It was as though some neutral particle, which left no track, had suddenly broken into two particles, which each had a charge and left a track, and which hastened away in different directions.
The particle that moved off in one direction and formed one branch of the V seemed to be a pion, but the other was something new. From the nature of the track it left, it seemed to be as massive as a thousand electrons, or as three and a half pions. It was half as massive as a proton.
Nothing like such a particle had ever been suspected of existing. It caught the world of physicists by surprise, and at first all that could be done was to give it a name. It was called a "V-particle," and the collision that produced it was a "V-event."
Once physicists became aware of V-events, they began to watch for them and, of course, soon discovered additional ones. By 1950, V-particles were found which seemed to be actually more massive than protons or neutrons. This was another shock. Somehow physicists had taken it for granted that protons and neutrons were the most massive particles there were.
The astonished physicists began to study the new particles carefully. The first V-particle to be discovered, the one that was only half as massive as a proton, was found to have certain properties much like those of the pion. The new particle was therefore classified as a meson. It was called a "K-meson" and the name was quickly abbreviated to "kaon." There were four of these: a positive kaon, a negative antikaon, a neutral kaon, and a neutral antikaon.
The other V-particles discovered in the early 1950s were all more massive than the proton and were grouped together as "hyperons." There were three kinds of these and each kind was given the name of a Greek letter. The lightest were the "lambda particles," which were about 20 percent heavier than protons. These came in two varieties, a lambda and an antilambda, both of them uncharged.
Next lightest were the "sigma particles," which were nearly 30 percent heavier than the proton. There was a positive sigma, a negative, and a neutral, and each had its antiparticle. That meant six sigma particles altogether.
Finally, there were the "xi particles," which were 40 percent heavier than the proton. There was a negative xi particle and a neutral one (no positive variety) and each had its antiparticle, making four altogether.
All these hyperons, an even dozen of them, had many properties that resembled those of the proton and neutron. They were therefore lumped with them as baryons. Whereas there had been four baryons known, or suspected, in 1947, there were sixteen in 1957.
But then things grew rapidly more complicated still. Partly, it was because physicists were building machines capable of producing particles with more and more energy. This meant that nuclei were being smashed into with greater and greater force and it was possible to turn the energy into all sorts of particles.
What's more, physicists were developing new and better means of detecting particles. In 1952, a young American physicist, Donald Arthur Glaser, got an idea for something that turned out to be better than the cloud chamber. It was, in fact, rather the reverse of the cloud chamber.
A cloud chamber contains gas that is on the point of turning partly liquid. Charged particles, racing through, help the liquid to form and leave trails of water droplets.
But suppose it were the reverse. Suppose there was a chamber which contained liquid that was on the point of boiling and turning into gas. Charged particles passing through the liquid would form ions. The liquid immediately around the ion would boil with particular ease and form small bubbles of gas. The tracks would be gas bubbles in liquid, instead of liquid drops in gas.
This new kind of detecting device was called a "bubble chamber."
The advantage of a bubble chamber is that the liquid it contains is much denser than the air in a cloud chamber. There are more atoms and molecules in the liquid for a flying particle to collide with. More ions are formed and a clearer trail is left behind. Particles that could scarcely be seen in a cloud chamber are seen very clearly in a bubble chamber.
By using bubble chambers and finding many more kinds of tracks, physicists began to suspect, by 1960, that there were certain particles that came into existence very briefly. They were never detected but unless they existed there was no way of explaining the tracks that were detected.
These new particles were very short-lived indeed. Until now the most unstable particles that had been detected lasted for a billionth of a second or so. That was a long enough time for them to make visible tracks in a bubble chamber.
The new particles, however, broke down in something like a hundred thousandth of a billionth of a billionth of a second. In that time, the particle has only a chance to travel about the width of a nucleus before breaking down.
These new particles were called "resonance particles" and different varieties have been deduced in great numbers since 1960. By now there are over a hundred baryons known that are heavier than protons. The heaviest are over twice as massive as protons.
Some of the new particles are mesons, all of them heavier than the pion. There are about sixty of these.
In the 1960s then, physicists were faced with the problem of finding some way of accounting for a large number of massive particles for which they could think of no uses and whose existence they couldn't predict.
At first all that physicists could do was to study the way in which one particle broke down into another; or the way in which one particle was built up into another when energy was added. Some changes could take place, while some changes could not. Particle A might change into particles B and C, but never into particles D and E.
Physicists tried to work out rules which would explain why some changes could take place and some could not. For instance, a neutron couldn't change into only a proton, because the proton has a positive electric charge and that can't be made out of nothing.
A neutron might, however, change into a proton plus an electron. In that case, a positive and a negative charge would be formed simultaneously. Together, they might be considered as balancing each other, so it would be as though no charge at all were formed.
But then to balance certain other qualities, such as the particle spin, more was required. In the end, it turned out that a neutron had to break down to three particles: a proton, an electron, and an antineutrino.
Matters such as electric charge and particle spin were enough to explain the events that were known in the old days when only a dozen or so different particles were known. In order to explain all the events that took place among nearly 200 particles, more rules had to be worked out. Quantities such as "isotopic spin," "hypercharge," "parity," and so on, had to be taken into account.
There is even something called "strangeness." Every particle is given a "strangeness number" and if this is done correctly, it turns out that whenever one group of particles changes into another group, the total strangeness number isn't altered.
The notion of strangeness made it plainer that there were actually two kinds of nuclear forces. The one that had first been proposed by Yukawa and that involved pions was an extremely strong one. In the course of the 1950s, however, it became clear that there was also a much weaker nuclear force, only about a hundred trillionths as strong as the strong one.
Changes that took place under the influence of the strong nuclear force took place extremely rapidly-just long enough to allow a resonance particle to break down. Changes that took place under the influence of the weak nuclear force took much longer-at least a billionth of a second or so.
Only the baryons and the mesons could take part in strong force changes. The leptons took part only in weak-force changes. The baryons and mesons are therefore lumped together sometimes as "hadrons."
Even when physicists gradually worked out the rules that showed what particle changes could take place and what couldn't take place, they were very unsatisfied. They didn't understand why there should be so many particles.
More and more physicists began to wonder if the actual number of particles was unimportant. Perhaps particles existed in families and they ought to concentrate on families of particles.
For instance, the first two baryons discovered were the proton and the neutron. They seemed two completely different particles because there was an important unlikeness about them. The proton had a positive electric charge and the neutron had no electric charge at all.
This seemed to be an enormous difference. Because of it, a proton could be detected easily in a cloud chamber and a neutron couldn't. Because of it a proton followed a curved path when brought near a magnet but a neutron didn't.
And yet when the strong nuclear force was discovered, it was found that it affected protons and neutrons exactly the same, as though there were no difference between them. If the proton and neutron are considered from the standpoint of the strong nuclear force only, they are twins.
Could it be, then, that we ought to consider the proton and neutron as two forms of a single particle which we might call the "nucleon" (because it is found in the nucleus)? Certainly, that might simplify matters.
You can see what this means if you consider people. Certainly, a husband and a wife are two different people, very different in important ways. To the income tax people, however, they are just one tax-paying combination when they file a joint return. It doesn't matter whether the husband makes the money, or the wife, or both make half; in the return it is all lumped together. For tax purposes we simply have a taxpayer in two different forms, husband and wife.
After 1960, when the resonance particles began to turn up, physicists began to think more and more seriously of particle families. In 1961, two physicists, Murray Gell-Mann in the United States and Yuval Ne'eman in Israel, working separately, came up with very much the same scheme for forming particle families.
To do this, one had to take all the various particle properties that physicists had worked out and arrange them in a very regular way. There were eight different kinds of properties that Gell-Mann worked with in order to set up a family pattern. Jokingly, he called his system the "Eightfold Way," after a phrase in the teachings of the Indian religious leader Buddha. The more formal name of his scheme is "SU (3) symmetry."
In what turned out to be the most famous example of SU (3) symmetry, Gell-Mann prepared a family of ten particles. This family of ten can be pictured as follows. Imagine a triangle made up of four objects at the bottom, three objects above them, two objects above them, and one object all by itself at the apex.
The four objects at the bottom are four related "delta particles" each about 30 percent heavier than a proton. The chief difference among them is the electric charge. The four delta particles have charges of -1, 0, +1, and +2.
Above these are three "sigma particles" more massive than the deltas and with charges of -1, 0, and +1. Above that are two "xi particles," which are still more massive and which have charges of -1, and 0. Finally, at the apex of the triangle is a single particle that is most massive of all and that has a charge of -1. Gell-Mann called this last particle the "omegaminus" particle, because "omega" is the last letter in the Greek alphabet and because the particle has a negative electric charge.
Notice that there is a regular way in which mass goes up and the number of separate particles goes down. Notice also that there is a regular pattern to the electric charges: -1, 0, +1, +2 for the first set; then -1, 0, +l; then -1, 0; finally -1.
Other properties also change in a regular way from place to place in the pattern. The whole thing is very neat indeed. There was just one problem. Of the ten particles in this family, only nine were known. The tenth particle, the omegaminus at the apex, had never been observed. If it did not exist the whole pattern was ruined. Gell-Mann suggested that it did exist; that if people looked for it and knew exactly what they were looking for, they would find it.
If Gell-Mann's pattern was correct, one ought to be able to work out all the properties of the omega-minus by taking those values that would fit into the pattern. When this was done, it was found that the omega-minus would have to be a most unusual particle for some of its properties were like nothing yet seen.
For one thing, if it were to fit into its position at the top of the triangle it would have to have an unusual strangeness number. The deltas at the bottom of the triangle had a strangeness number of 0, the sigmas above them a strangeness number of -1, and the xis above them one of -2. The omega-minus particle at the top would therefore have to have a strangeness number of -3. No strangeness number that large had ever been encountered and physicists could scarcely bring themselves to believe that one would be.
Nevertheless, they began to search for it.
The instrument for the purpose was at Brookhaven, where, as the 1960s opened, an enormous new device for speeding particles was put into operation. It could speed up particles to the point where they would possess energies as high as 33
Bev. This was more than five times the quantity of energy that was enough to produce antiprotons some years before.
In November 1963, this instrument was put to work in the search for the omega-minus particle. Along with it was a new bubble chamber that contained liquid hydrogen. Hydrogen was liquid only at very frigid temperatures, hundreds of degrees below the ordinary zero.
The advantage to the use of liquid hydrogen was that hydrogen nuclei were made up of single protons (except for the very rare deuterium form of the element). Nothing else could supply so many protons squeezed into so small a space without any neutrons present to confuse matters.
The liquid hydrogen bubble chamber was nearly seven feet across and contained over 900 quarts of liquid hydrogen. There would be very little that would escape it.
Physicists had to calculate what kind of particle collisions might possess sufficient energy plus all the necessary properties to form an omega-minus particle, if one could be formed at all. You would have to have a collision that would supply the necessary strangeness number of -3, for instance. It would also have to be a collision that would supply no quantity of something called "isotopic spin," for the isotopic spin of omega-minus would have to be 0 if it were to fit Gell-Mann's pattern.
It was finally decided that what was needed was to smash high-energy negative kaons into protons. If everything went right, an occasional collision should produce a proton, a positive kaon, a neutral kaon, and an omega-minus particle.
A beam of 5 Bev negative kaons was therefore shot into the liquid hydrogen bubble chamber and by January 30, 1964, fifty thousand photographs had been taken. Nothing unusual was found in any of them.
On January 31, however, a photograph appeared in which a series of tracks were produced which seemed to indicate that an omega-minus particle had been formed and had broken down to form other particles. If certain known and easily recognized particles were followed backward, and it were calculated what kind of particles they must have come from, and then those were followed backward, one reached the very brief existence of an omega-minus particle.
A few weeks later, another photograph showed a different combination of tracks which could be worked backward to an omega-minus particle.
In other words, a particle had been detected which had broken down in two different ways. Both breakdown routes were possible for the omega-minus particle if it had exactly the properties predicted by Gell-Mann. Since then, a number of other omega-minus particles have been detected, all with exactly the predicted properties.
There seemed no question about it. The omega-minus particle did exist. It had never been detected because it was formed so rarely and existed so briefly. Now that physicists had been told exactly what to look for and where to look for it, however, they had found it.
Physicists are now satisfied that they must deal with particle families. There are arguments as to exactly how to arrange these families, of course, but that will probably be straightened out.
But can matters become simpler still? It has often happened in the history of science that when matters seemed to grow very complicated, it could all be made simpler by some basic discovery.
For instance, there are uncounted millions of different kinds of materials on Earth, but chemists eventually found they were all formed out of a hundred or so different kinds of elements, and that all the elements were made up, in the main, of three kinds of particles: protons, neutrons, and electrons.
In the twentieth century, as physicists looked more and more closely at these subatomic particles and found that nearly two hundred of them existed altogether, naturally they began to think of going deeper still. What lies beyond the protons and neutrons?
It is a case of digging downward into the littler and littler and littler. First to atoms, then beyond that to the nucleus, then beyond that to the proton and neutron, and now beyond that to-what?
Gell-Mann, in working out his family patterns, found that he could arrange them by letting each particle consist of three different symbols in different combinations. He began to wonder if these different symbols were just mathematical conveniences or if they were real objects.
For instance, you can write one dollar as $1.00, which is the same as writing 100�. This would make it seem that there are one hundred cents in a dollar, and there certainly are. But does this mean that if you were to take a paper dollar bill and tear it carefully apart you would find a hundred one-cent pieces in it? Of course not!
The question was, then, if you tore a proton apart, would you find the three smaller objects that represented the three symbols used by Gell-Mann.
Gell-Mann decided to give the particles a name at least. He happened to think of a passage in Finnegan's Wake by James Joyce. This is a very difficult book in which words are deliberately twisted so as to give them more than one meaning. The passage he thought of was a sentence that went "Three quarks for Muster Mark."
Since three of these simple particles were needed for each of the different baryons, Gell-Mann decided, in 1963, to call them "quarks."
If the quarks were to fit the picture, they would have to have some very amazing properties. The most amazing was that they would have to have fractional electric charges.
When the electron was first discovered, its electric charge was set at -1 for simplicity's sake. Since then, all new particles discovered have either no electric charge at all or have one that is exactly equal to that of the electron or to an exact multiple of that charge. The same held for positive charges.
In other words, particles can have charges of 0, -1, +1, -2, +2, and so on. What has never been observed has been any fractional charge. No particle has ever yet been found to have a charge of +11/2 or -21/3.
Yet a fractional charge was exactly what the quarks would have to have. Charges of -1/3 and +2/3s would have to be found among them.
An immense search is now on for the quarks, for if they are found, they will simplify the physicist's picture of the structure of matter a great deal.
There is one important difficulty. Gell-Mann's theory makes it quite plain that when quarks come together to form ordinary subatomic particles, the process gives off a great deal of energy. In fact, almost all the mass of the quarks is given off as energy and only about one-thirtieth is left to form the particle. This means that quarks are thirty times as massive as the particles they produce.
(This sounds strange, but think about it. Suppose you see three balloons blown up almost to bursting. Would you suppose it were possible to squeeze them into a small box just an inch long in each direction? All you would have to do would be to let the air out of the balloons and what is left can easily be packed away in a small box. Similarly, when three quarks combine, you "let the mass out" and what is left can easily fit into a proton.)
If you want to form a quark by breaking apart a proton or some other particle, then you have to supply all the energy that the quarks gave up in the first place. You have to supply enough energy to form a group of particles thirty times as massive as a proton. You would need at least fifteen times as much energy as was enough to form a proton and antiproton in the 1950s, and probably even more.
There is no instrument on Earth, not even Brookhaven's 33-Bev colossus, that can supply the necessary energy. Physicists have two things they can do. First, they can turn to the astronomers and ask them to watch for any sign of quarks in outer space. There are cosmic ray particles with sufficient energy to form quarks. Most cosmic ray particles are protons and if two of them smash together hard enough they may chip themselves into quarks.
However, this would happen very rarely and so far astronomers have detected nothing they could identify as quarks. The second possibility is to build a device that will produce particles with sufficient energy to form quarks. In January 1967, the government of the United States announced plans to build such an instrument in Weston, Illinois.
This will be a huge device, nearly a mile across. It will take six or seven years to build and will cost 375 million dollars. Once it is completed, it will cost 60 million dollars a year to run.
But when it is done, physicists hope it will produce streams of particles with energies up to 200 Bev. This may be enough to produce quarks-or to show that they probably don't exist.
Physicists are awaiting the completion of the new instrument with considerable excitement and the rest of us should be excited, also. So far, every new advance in the study of the atom has meant important discoveries for the good of mankind.
By studying atoms in the first place, chemists learned to put together a variety of dyes and medicines, fertilizers and explosives, alloys and plastics that had never existed in nature.
By digging inside the atom and studying the electron, physicists made possible the production of such devices as radio and television.
The study of the atomic nucleus gave us the various nuclear bombs. These are not very pleasant things, to be sure, but the same knowledge also gave us nuclear power stations. It may make possible the production of so much cheap energy that our old planet may possibly reach towards a new era of comfort and ease.
Now physicists are trying to find the quarks that lie beyond the subatomic particle. We can't predict what this will result in, but it seems certain there will be results that may change the world even more than plastics, and television, and atomic power.
We will have to wait and see. Once. the new device is put into action at Weston, it is just possible we may not have to wait long.
From: Twentieth Century Discovery by Isaac Azimov