FOR EVERYONE by S. T. Pantelides -- ? 2000

Chapter 2 The structure of matter and radiation

Everything in the universe – matter and radiation -- is made of a few fundamental ingredients we call particles. In this Chapter we take a grand tour of these elementary particles and the composite objects that build up to atoms, molecules, and – all the states of matter – and radiation. We will learn about big accelerators that probe the depths of the fundamental particles and create exotic forms of matter, including antimatter. We will also learn about re- cent inventions such as the Scanning Tunneling that images atoms directly.

A world made of atoms Demokritos in ancient Greece was the first to propose that all matter is fun- damentally made of indivisible units he called atoms, literally meaning uncuttable. The rest is empty space. Aristotle, however, ridiculed the idea, believing that mat- ter is continuous. Aristotelian physics prevailed for about two thousand years until the 17th century when alchemists and the early chemists demonstrated the existence of atoms by experimentation and measurement. Compelling evidence in favor of atoms accumulated rapidly and today we know that 92 such distinct species of atoms, called “elements”, are found in our physical world. Most of them have familiar names, hydrogen, oxygen, iron, sulfur, and so on, but others have names you probably never heard before, such as hafnium, scandium, and praseodemium. They are denoted by shorthand abbre- viations consisting of one or two letters, usually related to their Latin name that often coincides with the English name. For example, H is used for hydrogen, O for oxygen, I for iodine, Ca for calcium. However, Fe is used for iron (from the Latin ferrum), Na for sodium (from the Latin natrium). Most substances are made up of more than one species of One of the key experimental re- atoms -- they are compound sub- sults obtained by early chemists stances. was that substances combine in definite proportions to make up The elements are usually arranged other substances, e.g. two parts hy- in a Periodic Table that reflects the fact drogen and one part of oxygen to that atoms in the same column have make water. Such discoveries ulti- similar chemical properties, i.e., they mately led to convincing evidence combine with other elements in similar about the existence of atoms as the ways to form similar compound sub- building blocks of all matter. stances.

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For example, NaCl, KCl, KBr and so on are very similar “salts”. NaCl is the famil- iar table salt whereas KCl is used on streets in the winter to melt ice. Similarly you have NH3 (ammonia) and also PH3 (phosphine) AsH3 (arsine) and so on. The designation KCl simply means that the substance is made of equal numbers of K atoms and Cl atoms. Similarly, NH3 means that the substance is made up of three H atoms for every one N atom. Note that these “chemical formulas” don’t tell you anything else about the substance, e.g. if it is gaseous or liquid at room temperature or how the atoms are arranged relative to each other. We’ll have a good deal to say about such issues later in the chapter. The Periodic Table of the elements shown on the previous page contains 113 elements. From number 93 and above, the elements are not found in nature. They are only made in the laboratory. Making the next element -- and getting to name it -- has been a battlefield of national pride between Americans and Rus- sians. They have gone all the way to atomic number 118 by now. The early chemists thought the elements were Demokritos’s “uncuttable” at- oms. They thought of them as -like objects that somehow hooked up to- gether in different ways, but did not have any good ideas how they did that. It turned out in the end that the elements are not the ultimate “atoms”. For this reason, before we look at how atoms combine to make up everything around us, it is useful to first take a quick peak inside atoms. We’ll return later and take a closer look. For the time being we just need some bare essentials.

Inside the atom The Periodic Table of the elements was an early telltale sign that atoms must have some internal structure that gives rise to the similarities and differences in the ways various elements combine to form compound substances. In the 19th century several other experimental observations pointed toward internal struc- ture. Around the turn of the century, it all became clear. Each species of atom is made up of a nucleus (the word means pit in Latin, as in olive pit, one of those

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erudite Latin choices of yore) surrounded by a “cloud” of electrons. The idea of “cloud” is that the electron is very tiny and is moving ex- tremely fast so it is literally everywhere at the same time. Atoms, therefore, are not solid-like objects but mostly empty space. They are very tiny, generally spherical and only about 10-8 cm across. There are about 1018 (a trillion trillion) of them in the dot of ink at the end of this sen- tence. If you were to blow up an atom to the The electron cloud around the size of a football stadium, the nucleus would nucleus of a hydrogen atom be the size of a grain of sand! Different species of atoms simply have a different number of electrons. In fact, the serial number of each element in the Periodic Table, called the atomic number, is simply the number of electrons in that species of atom. Now you are probably wondering what sets apart the col-

It was just over 100 years ago, in 1997 that J. J. Thompson in England first pried electrons out of atoms and made a beam with them in a glass tube from which air had been removed completely. He used a relatively new tech- nology for creating such a “vacuum” in a glass tube. At each end of the tube was a metal plate and the two plates were hooked to a battery (Batteries were invented in the 18th century by the Italian Volta; we will learn how they work later in the course). The beam glowed and Thompson did some clever experiments with which he established that the glowing beam was made up of tiny particles that are about 2000 times lighter than hydrogen atoms, the lightest atoms of them all! He coined the name electron because he sus- pected (correctly) that the critter is responsible for electricity (electron is the Greek word for amber; which was the first material known to produce electri- cal phenomena; see next chapter). Fourteen years later, Ernest Rutherford, an Australian working in Cam- bridge, England, did some other clever experiments and established that an atom is made of a hard tiny nucleus at the center and electrons buzzing around it. He and his students made the discovery by bombarding very thin gold foils with beams of particles (by that time scientists learned how to make such beams) and charting the directions in which they scattered. Most went through the foil virtually undeflected and very few bounced directly back as if they had struck something very hard. Indeed they had! Since Rutherford, accelerating beams of particles to high energies and smashing them into targets has been a big business, known as high-energy physics. The big machines that produce and accelerate the particles are popularly known as atom smashers. Fancy detectors controlled by computers now record the products of collisions from which extract the deep- est secrets of nature about the structure of matter.

11 PHYSICS FOR EVERYONE by S. T. Pantelides -- ? 2000 umns of the periodic table, i.e., what makes the elements in the same column exhibit similar chemical behavior. It’s quite simple: the electron cloud is actually made up of shells, like the shells of an onion. There are rules about the maxi- mum number of electrons that can go in each consecutive shell, very much like there is a maximum number of people that can sit in each row of seats in an am- phitheater (the rules pop out of an equation that governs the behavior of elec- trons in an atom). It turns out that for elements of the first column of the periodic table, the outermost shell has only one electron, for elements of the second col- umn the outermost shell has two electrons, and, you guessed it right, so it goes all the way to the eighth column. Neat! The outermost shell, called the valence shell, never has more than eight electrons. Mendeleev would have been very proud!

Molecules – Gases and Liquids Atoms make up molecules. A molecule is simply a bunch of atoms hooked up together. When atoms come close to each other, their electron clouds pene- trate and overlap. Since all electrons are the same, indistinguishable from each other, it is not possible to label which electrons belong to which atom. The elec- trons are in effect shared by the atoms. It is this overlap of electron clouds or sharing of electrons that is the effective “glue” that holds atoms together so they can form macroscopic matter as we know it. All the action of penetrating and overlapping in fact takes place among the valence electrons, namely those occu- pying the outermost or valence shells around the nucleus. That’s why the number of electrons in this shell determines the overall chemical behavior of elements. Remember? Elements in the same column of the Periodic Table have the same number of valence electrons. Air and gases are made up of dilute concentrations of molecules that are running around bouncing off each other. The air we breathe is mostly nitrogen and oxygen molecules. A nitrogen molecule is made up of two N atoms and is denoted as N2. Similarly, oxygen molecules are O2. These are small molecules in the sense that each molecule has only a small number of atoms. Other mole- cules are larger. Water molecules have three atoms (H2O), methane has five at- oms (CH4), aspirin has 21 (C9O4H8). Gasoline molecules in gasoline fumes are fairly large, with lots of carbon and hydrogen atoms. Those stinky gases that come off when you burn rubber have even larger molecules, again made up mostly of carbon and hydrogen atoms. In gases, the average distance between molecules is quite large compared with the molecule's dimensions. Thus molecules are running around pretty much freely, occasionally bumping into each other and changing direction. Liquids are also made up of molecules, but they are a lot closer together and they kind of keep track of each other as they move about. Think of them as couples dancing the fox trot in a crowded ballroom. They do move about, but they keep track of the other couples. You can see molecules moving about in a liquid if you put a drop of food coloring in a cup of water.

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Living tissue is made up of huge mole- cules that are tangled up and don’t move very much (e.g. molecules that make up skin and flesh), but are all bathed in water and other liquids. They are mostly strands of carbon atoms with hydrogen atoms attached all over the place, plus some oxygen and ni- trogen atoms here and there. These huge molecules are made up of identical seg- ments that keep repeating. They are, there- fore, called , from the Greek poly (meaning many) and meros (meaning part). DNA, the main chemical in living cells, is a Image of a protein molecule huge helical molecule, shown in the accom- showing the ribbon-like struc- panying figure. ture Chemists learned how to make artificial polymers with no signs of life. They are the synthetic fibers that nylon stockings and da- cron sweaters and all manner of plastic stuff are made of. The raw materials come from petroleum (“oil”) that we find buried deep in the ground (only in some lucky countries). The big molecules that make up oil, in a somewhat different form, were once living tissue that was fossilized and decayed. It’s a cosmic recycling process!

Solids Solids are networks of atoms that are ei- ther ordered in symmetric patterns or are relatively random. We call the ordered ones crystals and the random ones glasses. In crystals, the atoms form rows of planes so A model of DNA using balls of dif- that their surfaces are faceted. As in mole- ferent colors to denote different cules, the electron clouds in solids overlap elements. and act as the glue that holds everything to- gether, while the nuclei are just vibrating about a fixed place. Think of them as people sitting in a theater. They are not holding still, but they are pretty much stuck in their seats during the show. If there are some vacant seats, people can move about, and so do atoms in solids occasionally. Thus, atoms in solids can get mixed up but rather slowly, compared with liquids. You can get the atoms in solids to move about a bit faster at higher temperatures. Materials processing aims to rearrange atoms and mix different impurities. That is why most materials processing is done at high temperatures. There are ways to do it without cooking

13 PHYSICS FOR EVERYONE by S. T. Pantelides -- ? 2000 the stuff in ovens, but it’s tricky. In fact, non-thermal (meaning without heat) processing is one of the frontiers in materials science. We designate solids by the species of atoms they contain. For example pure solid aluminum is simply designated as Al. Pure silicon is Si. NaCl is sodium chloride (table salt), SiO2 is silicon dioxide (sand). Do not confuse this notation with molecules. The distinction is usually obvious from the context., Most things around us are solids. Aluminum atoms make up aluminum foil and the aluminum bars from which patio doors are made. We call them aluminum doors, but they are not really made of just Al atoms. It’s mostly Al, with all kinds of impurities and additives that improve the properties of the material. Another example is steel that is mostly iron (remember, Fe stands for iron), with substan- tial doses of other elements. Many structural materials are actually alloys, namely blended mixtures of two or more elements. For example, nickel-aluminum (Ni-Al) alloys are used in jet engines. Even gold and silver are not that pure. You can buy gold that is 999 (three nines) pure or 9999 (four nines) pure, meaning that impurities are only one part in 1000 or one part in 10000 (three nines means that one atom out of a thousand is an impurity, the other 999 being gold atoms).

Sand is made up of solid grains of silicon dioxide (SiO2). Most glass panes are also mostly SiO2 with all kinds of additives. By melting sand (yes, you can melt sand at high temperatures), you can pull pure silicon into fat salami-like rods, as much as a foot in diameter! This salami is then sliced into very thin disks called wafers which form the substrate on which electronics is fabricated. A big disk like that is carved into individual “chips” about a square inch. Those are the chips that drive your computer and your cell phone. Silicon can be made purer than any other material. Remember the three nines and four nines of gold and silver. May be you can do five nines. Silicon can be made at a purity of a dozen nines and more! Still, it is not the purity that makes Si useful for electronics. It’s the fact that you can “dope” it with special im- purities in special ways. We’ll see all about that later in the course. Just hang in here and you’ll get to understand the miracles wrought out of Si.

Natural crystals showing facets

Chunks of crystals abound in nature and have beautiful facets and colors. Sand grains are in fact crystals but the most beautiful crystals are found in exotic forests. Ironically, fine crystalware is just glass that has been cut into facets. They are not real crystals. Well, so much for consistency in our language.

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The scientific study of crystals is called . Crystallographers determine the atomic arrangements and symmetries of different crystals.

“Seeing” atoms Today we can literally “see” individual atoms with special instruments that can probe surfaces of solid materials with very fine metal tips that end at single atoms. These tips can be scanned over the sur- face very slowly so that they “feel” individ- ual atoms and record their shape through sophisticated electronics. The shapes are then plotted by computers as three- dimensional structures as shown in the ac- companying pictures. Variations in color are used for three-dimensional visualization An “abacus” made by arranging and/or to distinguish different species of atoms on a solid surface. atoms. The choices of colors are arbitrary. The instrument that takes these “pictures” is called Scanning Tunneling Microscope (STM for short) and was invented in the early 1980’s. Its inventors, Gerhard Binnig and Heinrich Rohrer of the IBM Research Laboratory near Zurich, , re- ceived the 199X for Physics. In addition to imaging individual atoms, the tip of an STM can be used to push indi- vidual atoms around. This capability was first demonstrated by IBM scientists led by from IBM’s research laborato- ries in San Jose, California, who pushed STM image of the surface of Si crystal krypton atoms around on a solid surface and spelled the letters I-B-M. Since then, scientists have created and taken pictures of all kinds of fascinating arrangements of atoms on otherwise flat solid surfaces. There are also movies of atoms being prodded about by the STM tip or simply at- oms that are moving about on their own (yes atoms are not sitting still! We’ll talk more about that in the next chapter). The “pictures” of atoms made by the STM are in effect pictures of the electron clouds of the atoms. On the scale of the figure, the nucleus is a dot smaller than the STM image of the surface of a GaAs crystal

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period at the end of this sen- tence. When we said that the tip In 1886, Beckerel in France discovered that some rocks that he kept in a dark drawer of the STM literally feels the at- exposed some photographic film that he kept oms of the solid surface, we in there. He carried out experiments that es- meant that the electron clouds tablished that some kind of radiation was of the tip atom and the surface emitted by these rocks. Soon after, Marie Cu- atom overlap in their tail ends. rie, a Polish expatriate living in France, estab- The picture is taken by lished that there are three distinct types of scanning the tip at an absolutely such radiation and they are emitted only when constant height. The overlap- the rocks contained a special few elements, radium (Marie and her husband Pierre dis- ping tails of the electron clouds covered it and named it), throrium, and a few enable electrons from the sur- others. It did not seem to matter what the face atoms to gently flow to the chemical compounds were, as long as they tip atom or the other way contained one of these elements (this was around. It’s done by connecting one of those many indications that atoms the sample and the instrument’s must have internal structure). Beckerel and tip to the two poles of a battery Curie got Nobel prizes for their discoveries. (doesn’t everything need a bat- The phenomenon was called radioactivity and tery to run? We’ll learn how bat- it proved to be very dangerous to one’s teries make electrons flow later health. Marie’s husband Pierre was the first to in the book). The electron flow get cancer from it (he died when he was run (it’s an electrical current, the over by a horse cart before cancer had a chance to get him). Once the atom was un- same kind that lights up light derstood to have a nucleus surrounded by bulbs – hang in there and we’ll electrons around 1911, it became obvious that learn all about it in due time) is the pesky radiation is coming from the nu- large when the electron cloud cleus. The radiation was also a telltale sign tails of the tip atoms and the that the nucleus has internal structure. Then, surface atoms overlap a lot; in it was essentially a repeat of Rutherford’s ex- contrast, the current is smaller periment (see box, p. 9) using a beam of par- when the overlap is smaller. ticles that were accelerated to much higher The STM “picture” is simply an energies so that they could penetrate the nu- image of these tiny current cleus. variations. You actually take dif- ferent pictures if you reverse the current flow by reversing the connections to the battery. The two pictures are “complementary”, a kind of posi- tive and negative. Theorists (remember, these are the physicists that don’t do experiments but work with the mathematics to figure out what goes on) also create pictures of the electron clouds of atoms that make up macroscopic matter. Though a real in- strument can only take pictures of the surface atoms, theorist’s tools have no such limitation. By solving the right equations, theorists can map out the electron cloud distribution of interior atoms as well. .

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Inside“In March the 1995 nucleus scientists gathered at a hastily called meeting at Fermilab -- the FermiSo Nationalfar we have Accelerator learned Laboratorythat atoms in are Batavia, made Ill., of neara nucleus Chicago at --the to centerwitness and a a cloudhistoric of event.electrons In back around-to-back it. Eachseminars, element physicists has froma different rival experiments number of within electrons, the equallab announced to its atomic the number:discovery 1, of2, a3, new … allparticle the ,way the totop more quark. than A decades100. Now,-long you search for one of the last missing pieces in the Standard Model of particle physics wouldhad come not reallyto an end.expect all elements to have the same nucleus. They don’t. Nu- clei are made up of protons (from the Greek word for “first”) and neutrons (well, from Thethe topEnglish quark word is the neutral!sixth, and We’ll quite see possibly in the the next last, quark.chapter Along where with the leptons name comes-- the from).electron and its relatives -- quarks are the building blocks of matter. The light- estIn quarks, each designatedatom, the "up"number and of"down," protons make is upexactly the familiar equal protonsto the numberand neutrons. of ele c- Along with the electrons, these make up the entire periodic table. Heavier quarks trons,(such i.e.as thethe charm,atomic strange,number. top There and bois ttoma good quarks) reason and leptons,for that, though but wait abundant until the nextin the chapter early momentsto learn afterall about the big that. bang, The are number now commonly of neutrons produced is typically only in close acce l-but noterators. equal to the number of protons. Hydrogen is the one element that has no neutrons; its nucleus is just a single proton. Physicists had known that the top quark must exist since 1977, when its part- ner,Now, the bottom,think about was discovered.it for a minute. But theWe top started proved with exasperatingly well over a hard hundred to find. distinct Al- atomsthough that a fundamental are the elements particle withof the no periodicdiscernible table. structure, Each the atom top quarkis made turns of outa nu- cleusto have containing a mass asprotons large asand an neutrons,atom of gold surrounded and far greater by a thancloud most of theoristselectrons. had Jus t threeanticipated. kinds ofThe particles proton, mademake ofup two everything! ups and one That’s down simplicity, has a mass at itsthat –is almost about – best.175 Itimes guess smaller. just protons Creating and a electrons top quark would thus berequired too simple. concentrating Even so, immense just about everythingamounts ofin energy the universe into a minute is made region up of space.of only Physicists three ingredients: do this by acceleratingprotons, ne u- trons,two particles and ele andctrons. having Demokritos them smash would into have each beenother. jealous. Out of a When few tri llionthis collisionssimple fact wasat least nailed a handful, down in experimenters the 1930’s, physicistshoped, would were cause gloating. a top quarkThey tohad be foundcreated the out “u n- cuttable”of energy ones from and the there impact. were What only we three did ofnot them! know was how much energy it would take.” But the euphoria did not last long. Nobody really expected it to last. The writ- Excerpt from Scientific American, The Discovery of the Top Quark” by T. M. ingLiss was and on P. theL. Tipton, wall already. September Radioactivity 1997 (see box, this page) was one big worry. The road to unravel the question whether protons and neutrons are composite particles was long to hoe with twists and turns. We’ll spare you most of them. It ac- tually got worse before it got better. By the 1930’s, physicists had designed big ma- chines that made up beams of charged particles, accelerated them to high speeds and smashed them into fixed targets or into each other. They detected the particles flying out during the collisions with big sensitive detectors and found all kinds of new and exotic species. When they used the same kind of detectors to check if any in- visible radiation hits the earth from outer space, they found protons and electrons but also some of these other strange particles. To make sense out of this zoo of strange particles Murray Gell-Mann of the California Institute of Technology in 1964 made a bold suggestion: Protons, neu- trons, and most of the exotic particles are made up from some even more funda- mental building blocks he whimsically called quarks, a word that appears in the novel Finnegan’s Wake by James Joyce (physicists read novels too). Obviously running out of words, he called the two most important quarks up and down. (re- member, positive and negative were already taken!). A proton is made up of two ups and a down and a neutron is made up of one up and two downs. There are a few more quarks that are needed to make up all the exotic particles. The existence of quarks was verified years later, garnering Nobel Prizes for Gell-Mann and the new discoverers. The experiments were again very similar to those of Rutherford, but were carried out at much higher energies to penetrate the protons and neutrons and find that there is something hard inside!

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How on earth did those certain nuclei, made up of just protons and neutrons, emit radiation? In the meantime other mysteries were piling up in the form of ex- otic particles. The “atom smashers” were busy at work smashing highly acceler- ated beams of protons and electrons into targets and detecting all kinds of strange particles. Some of these exotic particles were also found in the so-called “cosmic rays”, a kind of radiation that arrives on earth from outer space. The unraveling of the mystery was long and arduous but the answer turned out to be simple and elegant. Protons and neutrons are made of even tinier parti- cles called quarks. Just two quarks, called "up" and "down", make up protons and neutrons. Two "up"’s and a "down " make a proton, two "down "’s and an "up" make up a neutron. The "up" and "down " quarks, plus four more (called whimsically strange, charm, top and bottom) also make up a zoo of exotic particles that are only cre- ated in big accelerators. Some of them arrive on earth in “cosmic rays”, radiation that comes from outer space and whose precise origin is still unknown. The discovery of quarks led to the resolution of the radioactivity conundrum, namely how do nuclei emit that pesky radiation. It turns out that quarks are not forever. They do not have a rigid identity. They can, and do, change their identity (say from up to down or vice versa) by emitting that hallmark radiation! That means protons can turn into neutrons and vice versa! You are probably tired. It seems like an endless game. What’s inside the quarks? As far as we know today, quarks are the end of the road. All matter as we know it is made up of up and down quarks and electrons. Three tiny critters that are the ultimate uncuttable “atoms” of Demokritos. Yes, he would have been proud. And Aristotle would have been livid. We are going to drop the ball for a while and let radioactivity and the exotic particles rest while we focus on “normal” matter whose nuclei are stable. That’s most of matter as we know it. If we whetted your appetite about these things, that’s good. We’ll actually touch upon radioactivity again in the next chapter – to whet our appetite a bit more. But then you’ll have to wait in deference to normal matter. Hang in there, however, and we’ll get to radioactivity and the exotic crit- ters later in the book (sign up for next semester!).

Light

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We said earlier that just about every- thing in the universe is made up of pro- Antimatter tons, neutrons, and electrons. The exotic Physicists have discovered particles are one exception. Another big that for every particle, there exists exception is light. Beautiful light beams an “antiparticle” that has virtually that come from the sun and the stars, all the same properties except op- light from flames or light bulbs – what is posite charge. As far as anybody it made of? can tell they do not exist in nature Getting to understand light has a like particles do, but they can be long and arduous history, fraught with created by particle collisions or more pitfalls than matter. We won’t drag when particles change identity as you through that. Today, we actually un- in radioactivity. Thus, there exists derstand light better than matter. The an antielectron (the antiparticle theory of light and the way it interacts that has its own name, positron), with matter – ultimately with electrons an antineutrino and antiquarks. A and nuclei – is the most successful the- particle-antiparticle pair “annihi- ory in the history of mankind. Formulated late” into photons. It is believed in the 1940’s independently by American that in the very early universe , German expatriate there was an almost equal number and Japanese Sin-Itiro of particles and antiparticles. They Tomonaga (they shared a Nobel Prize), annihilated except for the excess it has made predictions and checked particles that now make up the against experimental measurements galaxies and animals and humans with incredible numerical accuracy. on earth. Physicists have not fig- The bottom line is very simple: Light ured out what caused the asym- is made up of particles called photons. metry that, mercifully, left an So, just chalk up another particle on the abundance of particles. list of two quarks and an electron and we are done. That’s the universe. The most important difference between photons and matter particles is that photons do not bind up to form composite particles and, when in beams, the familiar light rays, they always travel with one and only one average speed, the speed of light. Beams made up of matter particles can approach but never equal or exceed the speed of light. What makes up colors? Patience. We’ll get to that in the next chapter. There are also forms of light that our eyes cannot detect. We already talked about that in the first chapter. X-rays is one such form of invisible light. Microwave radiation is another. We’ll get back to that later on too. We close with yet another form of radiation. Particles called neutrinos. I hate to call them exotic because they are actually everywhere, even more than light, but we don’t see them. Stars emit neutrinos too, not just photons, and the pesky critters move just about as fast as light – maybe exactly as fast as light, but we are not sure yet.

From the microscopic to the macroscopic

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In this Chapter we talked about the structure of matter and In 1913, of Denmark boldly radiation. Both are made up of proposed a set of rules for the electrons elementary bits we called parti- that did not follow from the known laws of cles. Matter is ultimately made physics. The rules accounted for a great up of quarks and electrons and deal of observations, however, and were radiation is made up of either taken seriously as a hint that the sub- photons or matter particles (pro- atomic world is governed by somewhat dif- tons, neutrons, electrons or ferent physical laws. Before Bohr, Max other composite particles, in- Planck (1901) and (1903), cluding some of the exotic parti- both of Germany, proposed similarly bold cles). Yet, material objects and rules about the nature of light (see later in radiation in the macroscopic this chapter). All three got Nobel Prizes for world appear continuous be- their bold insights that, by 1925, led to the cause what matters is the collec- formulation of the general physical laws tive behavior of the particles. that govern the subatomic world. They are Remember the zillions of ink known by the strange moniker “quantum dots that make up the letters on physics”. Hold your breath until the next this page. You don’t see them, chapter to really appreciate the word quan- you don’t even care about them, tum. (You will also hear the term quantum you simply ignore the fact. The mechanics. Physicists use the word me- same way if you look at a chanics like nobody else. It’s just a subdi- bridge, you will see the pylons vision of physics that deals mostly with mo- and the I-beams and all the rest, tion and forces. It is not really a sensible but you don’t think about the at- term, but history never dies. oms that make up the I-beams and so on. The reality, however, is that the atomic arrangements in the -Ibeam ultimately determine its strength, its resistance to cracking or warping or sagging. When you wish to describe the sagging or warping, it makes no sense to give the positions of all the atoms, you only need to know the outline of the external edges. The same way when you toss a football in the air or watch the Indy-500 race, it would be senseless to describe the motion of the football or the speeding car by describing the trajectories of all the atoms that make them up. Instead, you describe the motion of the football and the speeding car in a satisfactory way in terms of a few macroscopic quantities, such as the speed and position of the en- tire car, the speed, position and spin of the football, and so on. The same is true of light and other forms of radiation. Though light is in effect a stream of particles, we perceive it as “rays” that travel in a straight line, get re- flected by mirrors, get transmitted through glass and get focused by lenses. There is no need to worry about the individual photons when we are talking about rays. The laws of nature describing the behavior of the tiny particles that make up macroscopic objects and light are different from those that describe the motion of the macroscopic objects themselves and the behavior of light rays. The laws of the macroscopic world were discovered first, starting with Galileo and Newton in

20 PHYSICS FOR EVERYONE by S. T. Pantelides -- ? 2000 the 17th century and were virtually complete by the end of the 19th century. In fact, toward the end of the 19th century, many physicists thought that their quest to describe the laws of nature was essentially complete. Around the turn of the century the microscopic world of atoms and the subatomic particles burst on the scene. The 20th century let to incredible new discoveries about the laws that govern this microscopic world and to technologies that exploit those laws, the biggest of which is the "transistor" that is the heart and soul of computers and the laser. The two sets of laws are consistent with each other. In fact, the equations of the macroscopic laws can be derived from those of the microscopic laws by taking averages over large numbers of atoms. The macroscopic laws of physics formulated prior to the 20th century are known as “classical physics”. The new physics of the 20th century is known as “modern physics”. Most physics textbooks cover classical physics first and then introduce physics by tracing the history of its development. This book blends the two and emphasizes the unity of physics and the major concepts that underlie all of physics.

21 PHYSICS FOR EVERYONE by S. T. Pantelides -- ? 2000

STUDY QUESTIONS – HOMEWORK

1. Write down the chemical symbols for the following elements: oxygen, hydrogen, helium, iron, sulfur, calcium, carbon and silicon. 2. If it takes 4 parts hydrogen to one part carbon to make methane, what is its chemical formula? 3. Look at the periodic table of the elements and decide which other compounds be- long in the same family as a) GaAs , and b) CaF2 . 4. When Mendeleev composed the first periodic table of the elements he left some spots empty. Why? 5. What can you say about the elements with atomic numbers larger than 92? 6. What is the number of electrons in a calcium atom whose atomic number is 20? How about gold with an atomic number of 79? 7. If you blow up an atom to the size of a football stadium, what would be the size of the nucleus? 8. What happens to the electron beam in a vacuum glass tube (as in Thompson’s experiment) when you hold a magnet against the tube? 9. What was the key observation that led Rutherford and his students to conclude that atoms must have a tiny hard nucleus at the center? 10. Look at the periodic table. How many electrons are in the outer shell of a) K , b) Ge , c) Kr ? 11. What function is unique to the valence electrons of atoms? 12. What is the generic name for long molecules that are made up of identical re- peating segments? 13. What is the primary element in steel? 14. Describe how silicon wafers are made. 15. Why are crystal surfaces faceted? 16. What do crystallographers do? 17. Who are Gerhardt Binnig and Heinrich Rohrer? 18. How do scientists push atoms around on the surface of a crystal to make letters of the alphabet? 19. What does an STM picture record? 20.How was it established that radioactivity is a property of individual chemical ele- ments and not of particular compound substances? 21. Name one major experimental indication the protons and neutrons had internal structure. 22. What are cosmic rays?

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