PHYSICS 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 solids – 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 recent inventions such as the Scanning Tunneling Microscope that
images atoms directly.
A world made of atoms
Demokritos in ancient Greece was the first to propose that all matter is fundamentally made of indivisible units he called atoms, literally meaning uncuttable.
The rest is empty space. Aristotle, however, ridiculed the idea, believing that matter 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 abbreviations 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
One of the key experimental remade up of more than one species of
sults
obtained by early chemists
atoms -- they are compound subwas
that
substances combine in
stances.
definite proportions to make up
The elements are usually arranged other substances, e.g. two parts hyin 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 ultisimilar 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.
9
PHYSICS FOR EVERYONE by S. T. Pantelides -- ? 2000
For example, NaCl, KCl, KBr and so on are very similar “salts”. NaCl is the familiar 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 Russians. They have gone all the way to atomic number 118 by now.
The early chemists thought the elements were Demokritos’s “uncuttable” atoms. They thought of them as solid-like objects that somehow hooked up together 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 structure. 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
10
PHYSICS FOR EVERYONE by S. T. Pantelides -- ? 2000
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 extremely 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 sentence. 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 colIt 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 technology 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 suspected (correctly) that the critter is responsible for electricity (electron is the
Greek word for amber; which was the first material known to produce electrical phenomena; see next chapter).
Fourteen years later, Ernest Rutherford, an Australian working in Cambridge, 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 physicists extract the deepest 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 maximum 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 amphitheater (the rules pop out of an equation that governs the behavior of electrons 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 column 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 penetrate 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 electrons 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 occupying 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 molecules are larger. Water molecules have three atoms (H2O), methane has five atoms (CH4), aspirin has 21 (C 9O4H8). 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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PHYSICS FOR EVERYONE by S. T. Pantelides -- ? 2000
Living tissue is made up of huge molecules 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 nitrogen atoms here and there. These huge
molecules are made up of identical segments that keep repeating. They are, therefore, called polymers, from the Greek poly
(meaning many) and meros (meaning part).
DNA, the main chemical in living cells, is a
huge helical molecule, shown in the accompanying figure.
Chemists learned how to make artificial
polymers with no signs of life. They are the
synthetic fibers that nylon stockings and dacron 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!
Image of a protein molecule
showing the ribbon-like structure
Solids
Solids are networks of atoms that are either 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 difthat their surfaces are faceted. As in mole- ferent colors to denote different
elements.
cules, the electron clouds in solids overlap
and act as the glue that holds everything together, 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 substantial 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 impurities 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.
14
PHYSICS FOR EVERYONE by S. T. Pantelides -- ? 2000
The scientific study of crystals is called crystallography. 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 surface very slowly so that they “feel” individual atoms and record their shape through
sophisticated electronics. The shapes are
then plotted by computers as threedimensional structures as shown in the accompanying pictures. Variations in color
are used for three-dimensional visualization
and/or to distinguish different species of
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, Switzerland, received the 199X Nobel Prize for Physics.
In addition to imaging individual atoms,
the tip of an STM can be used to push individual atoms around. This capability was
first demonstrated by IBM scientists led by
Don Eigler from IBM’s research laboratories in San Jose, California, who pushed
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 atoms 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
15
An “abacus” made by arranging
atoms on a solid surface.
STM image of the surface of Si crystal
STM image of the surface of a GaAs
crystal
PHYSICS FOR EVERYONE by S. T. Pantelides -- ? 2000
period at the end of this sentence. When we said that the tip
In 1886, Beckerel in France discovered
of the STM literally feels the atthat some rocks that he kept in a dark drawer
oms of the solid surface, we
exposed some photographic film that he kept
meant that the electron clouds
in there. He carried out experiments that esof the tip atom and the surface
tablished that some kind of radiation was
atom overlap in their tail ends.
emitted by these rocks. Soon after, Marie Curie, a Polish expatriate living in France, estabThe 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 overlapthe rocks contained a special few elements,
ping tails of the electron clouds
radium (Marie and her husband Pierre disenable electrons from the surcovered it and named it), throrium, and a few
face atoms to gently flow to the
others. It did not seem to matter what the
tip atom or the other way
chemical compounds were, as long as they
around. It’s done by connecting
contained one of these elements (this was
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 batThe phenomenon was called radioactivity and
tery to run? We’ll learn how batit 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
same kind that lights up light
chance to get him). Once the atom was unbulbs – hang in there and we’ll
derstood to have a nucleus surrounded by
learn all about it in due time) is
electrons around 1911, it became obvious that
large when the electron cloud
the pesky radiation is coming from the nucleus. 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 excontrast, the current is smaller
periment (see box, p. 9) using a beam of parwhen the overlap is smaller.
ticles that were accelerated to much higher
The STM “picture” is simply an
energies so that they could penetrate the nuimage of these tiny current
cleus .
variations. You actually take different pictures if you reverse the
current flow by reversing the
connections to the battery. The two pictures are “complementary”, a kind of positive 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 instrument 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. .
16
PHYSICS FOR EVERYONE by S. T. Pantelides -- ? 2000
Inside
the
nucleus
“In March
1995
scientists gathered at a hastily called meeting at Fermilab -- the
Fermi
National
Accelerator
Batavia,
Chicago
tocenter
witnessand
a a
So far we have learnedLaboratory
that atomsinare
madeIll.,
of near
a nucleus
at -the
historic
event.
In
back-to-back
seminars,
physicists
from
rival
experiments
within
the
cloud of electrons around it. Each element has a different number of electrons,
lab announced the discovery of a new particle, the top quark. A decades-long
equal
to its atomic number: 1, 2, 3, … all the way to more than 100. Now, you
search for one of the last missing pieces in the Standard Model of particle physics
would
not really
expect all elements to have the same nucleus. They don’t. Nuhad come
to an end.
clei are made up of protons (from the Greek word for “first”) and neutrons (well,
from The
the top
English
quarkword
is theneutral!
sixth, and
We’ll
quitesee
possibly
in the
thenext
last, quark.
chapterAlong
where
withthe
leptons
name
-the
electron
and
its
relatives
-quarks
are
the
building
blocks
of
matter.
The
lightcomes from).
est quarks, designated "up" and "down," make up the familiar protons and neutrons.
In each atom, the number of protons is exactly equal to the number of elecAlong with the electrons, these make up the entire periodic table. Heavier quarks
trons,
the charm,
atomicstrange,
number.top
There
is a good
reason
for that,though
but wait
until the
(suchi.e.
as the
and bottom
quarks)
and leptons,
abundant
next
chapter
to
learn
all
about
that.
The
number
of
neutrons
is
typically
close
in the early moments after the big bang, are now commonly produced only in accel-but
not
equal to the number of protons. Hydrogen is the one element that has no
erators.
neutrons; its nucleus is just a single proton.
Physicists had known that the top quark must exist since 1977, when its partthink about
it for a minute.
Wetop
started
with
well over ahard
hundred
distinct
ner,Now,
the bottom,
was discovered.
But the
proved
exasperatingly
to find.
Alatoms
are the elements
of the
table.
Eachthe
atom
is made
ofout
a nuthoughthat
a fundamental
particle with
no periodic
discernible
structure,
top quark
turns
cleus
containing
protons
and
surrounded
by athan
cloud
of theorists
electrons.
Just
to have
a mass as
large as
an neutrons,
atom of gold
and far greater
most
had
three
kinds
of
particles
make
up
everything!
That’s
simplicity
at
its
–
almost
anticipated. The proton, made of two ups and one down, has a mass that is about –
175 Itimes
Creating
top quark
thus be
required
concentrating
best.
guesssmaller.
just protons
andaelectrons
would
too simple.
Even so,immense
just about
amounts ofinenergy
into a minute
regionup
of space.
do this by accelerating
everything
the universe
is made
of onlyPhysicists
three ingredients:
protons, neutwo particles
and having
them smash
intohave
each been
other.jealous.
Out of aWhen
few trillion
trons,
and electrons.
Demokritos
would
this collisions
simple fact
at
least
a
handful,
experimenters
hoped,
would
cause
a
top
quark
to
be
created
out“unwas nailed down in the 1930’s, physicists were gloating. They had found
the
of
energy
from
the
impact.
What
we
did
not
know
was
how
much
energy
it
would
cuttable” ones and there were only three of them!
take.”
But
the euphoria did not last long. Nobody really expected it to last. The writExcerpt from Scientific American, The Discovery of the Top Quark” by T. M.
ing
was
wall already.
Radioactivity
(see box, this page) was one big worry.
Liss andon
P.the
L. Tipton,
September
1997
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 actually got worse before it got better. By the 1930’s, physicists had designed big machines 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 invisible 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, neutrons, and most of the exotic particles are made up from some even more fundamental 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. (remember, 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!
17
PHYSICS FOR EVERYONE by S. T. Pantelides -- ? 2000
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 exotic particles. The “atom smashers” were busy at work smashing highly accelerated 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 particles 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 created 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 critters later in the book (sign up for next semester!).
Light
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PHYSICS FOR EVERYONE by S. T. Pantelides -- ? 2000
We said earlier that just about everyAntimatter
thing in the universe is made up of protons, 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 oplight 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 “annihiory 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
Richard Feynman, German expatriate there was an almost equal number
Eugene Wigner 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 figured out what caused the asymThe bottom line is very simple: Light
is made up of particles called photons. metry that, mercifully, left an
abundance of particles.
So, just chalk up another particle on the
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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PHYSICS FOR EVERYONE by S. T. Pantelides -- ? 2000
In this Chapter we talked
about the structure of matter and
In 1913, Niels Bohr 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 subphotons or matter particles (pro- atomic world is governed by somewhat diftons, neutrons, electrons or ferent physical laws. Before Bohr, Max
other composite particles, in- Planck (1901) and Albert Einstein (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
physics”. Hold your breath until the next
dots that make up the letters on
this page. You don’t see them, chapter to really appreciate the word quantum. (You will also hear the term quantum
you don’t even care about them,
mechanics. Physicists use the word meyou simply ignore the fact. The
same way if you look at a chanics like nobody else. It’s just a subdibridge, you will see the pylons vision of physics that deals mostly with moand 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 entire 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 reflected 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
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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.
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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 belong 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 repeating 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 elements 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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