OBJECTIVES

After you have finished this lesson you should be able to

        *       define elementary particles and energy scales and relate them to states
of technology.
        *       understand how we measure time and distance and thus define time
and size scales.
        *       describe the structure of atoms.
        *       describe how we recognize stability and change.
        *       describe the relationship between the existence of properties and their
measurement.
        *       describe the fundamental forces in the universe and the relationship
between them.


Traditionally physics has followed a reductionist theory of nature in trying to
explain complex phenomena through the properties of their smallest parts. We
shall follow that pathway in this course. In order to carry out this program of
analysis, however, we must first produce parts. How do we do this? The answer
lies in a most primitive behavior of humans-we break things; we destroy! Of
course, we do this in an orderly manner, breaking larger objects into smaller
ones, carefully noting and observing what we obtain. And what do we need to
break objects? The answer is force: When one hits a rock with a hammer, one is
applying a force, and if chips of rock splinter off, one is applying enough force to
disrupt the forces holding the rock together. How small are the pieces we can
break materials into? That depends on how much force we can exert on the
material. If there is no force available that can disintegrate objects beyond a
certain size or form, then these smallest objects are elementary particles. The
strength of a force is strongly related to energy associated with that force.
Therefore, we could just as well discuss the amount of energy as the strength of a
force and reformulate the concept of "elementary particle" in terms of energy, as
the smallest particles that can be produced with the amount of energy available.
The amount of energy available is clearly dependent on the state of technology,
which is a function of time.

        The ancient Greeks Leucippus and Democritus introduced the concept of
atoms, particles that were indivisible and postulated to constitute all matter. The
ancient Greeks did not have much energy available to test such theories, but since
their world view did not rely much on empirical evidence to substantiate their
hypotheses, this was not a problem. The term "elementary particle" discussed
above actually has the properties put forward by Leucippus and Democritus and
could rightly be called instead an "atom." However, in the early nineteenth
century, Dalton used the term "atom" to describe the smallest constituent of
matter that could be obtained through the action of chemical energies. He based
his theory on earlier work carried out by Proust and Boyle in the late eighteenth
century. The term "atom" has thus since been reserved for such particles.
Chemical energies are found in everyday processes like the burning of wood, coal,
and gas, or, for instance, when you add ammonia solution to sulfuric acid. These
energies were the greatest that could be generated by the technology of those times
and could transform structures formed of atoms, called molecules, into other
molecules and sometimes break down molecules into atoms.

        In the early twentieth century controlled energies were developed that were
enough to smash atoms into protons, neutrons, and electrons. Protons carry one
unit of positive charge and electrons one unit of negative charge, while neutrons
carry no charge (i.e., they are neutral). Protons and neutrons are a thousand
times heavier than electrons. Rutherford experimentally found that atoms are
constructed with a heavy central core about 10-15m in diameter made up of
neutrons and protons and surrounded by electrons that fill a region of about 10-9m
in diameter. He considered electrons to be like satellites orbiting a heavy central
body, just as the earth is orbiting the sun or the moon is orbiting the earth. The
electrons are very light, hence the density of the electron region of an atom is very
low, almost like nothing. And since all normal states of matter are formed from
atoms, everything around us (including ourselves) is mostly empty space!

        Atoms themselves are electrically neutral and the number of protons must
always equal the number of electrons. The name of the atom is determined by this
number. The number of neutrons, however, can vary and these numbers
correspond to different isotopes of the given atom. The isotope has the same name
as the atom  (e.g., carbon twelve, which has 12 neutrons, and carbon fourteen
which has 14 neutrons).  If an atom, such as carbon, loses or gains electrons so
that it becomes positively or negatively charged, it is then called an ion. The ion
has the same name as the atom, but is qualified with the amount of charge it has
on it (e.g., sodium plus or chlorine minus ions). Ions are important in the
conduction of electricity and in the formation of certain solids like sodium
chloride, which is table salt.

        Energies needed to produce reactions among neutrons and protons are very
large and are in general only found in the center of stars or in hydrogen bombs.
At these energies neutrons and prtons are the analogs of the atoms and the
different nuclei, which are combinations of neutrons and protons, are the analogs
of molecules. In these reactions the dreams of the alchemists can become true
and lead can be transmuted to gold! Some nuclei found on the earth are unstable
and spontaneously disintegrate to form lighter nuclei, releasing energy and sub
atomic particles. This process is called radioactivity, and is the basis of nuclear
power stations and the atomic bomb.

        In the middle and later part of the twentieth century we developed machines
called accelerators that can accelerate objects to immense speed and then let them
crash into each other or into a stationary solid object. The products of these
collisions are many strange sub atomic particles that usually have very short
lifetimes, less than 10-6 of a second (a microsecond). When these particles crash
into each they often do not form composite particles or molecule analogs, but
rather they transform into mixtures of each other. (In a mixture of objects the
individual objects maintain their identity, while in compounds, which are
molecules or molecule analogs, the properties of the constituent particles are
lost.) The question then arises of whether there are any more basic particles than
these found in such accelerators and whether they might be induced to show
themselves in collisions at still higher energies or whether we have reached the
ultimate elementary particles.

        The problem with this last hypothesis is that by the late 1950s there was a vast
array of these elementary particles already. But in 1963 Murray Gell-Mann
introduced the quark model of these particles. He suggested that all the
elementary particles were made up of molecule analogs of combinations of three
different quarks. These quarks were pretty strange objects having charges +2e/3,
-e/3, and +e/3, where e was the charge on the proton. The force between these
quarks increased when the quarks got further apart, a property that explains
quark confinement and hence the fact that quarks have never been observed! Such
a force is not really too bizarre though; a common spring acts in this fashion. The
quark theory is pretty much accepted these days with a few additions, some more
quarks, and more combinations. But if quarks are never observed, why is the
theory accepted? It satisfies the basic requirements that it clarifies and explains
the relationship between observed phenomena in the most eloquent way so far,
and it can be used to predict new observations. However, these new observations
are hard to come by, usually requiring huge amounts of energy that are not available yet.

        Protons and neutrons belong to a family of sub atomic particles called
hadrons, which are composed of interacting quarks; electrons belong to another
family of sub atomic particles called leptons; while the particle forming light, the
photon, belongs to a third family called gauge particles. The leptons and the
gauge particles are so far considered to be elementary, but are the quarks? Today
some people think they might be composite particles. Perhaps even the leptons
and the gauge particles could be broken down with enough energy, though there
are no hints at present that this would be the case. But with enough energy who
can tell? There is, of course, an upper bound to the energy that will ever be
available-which is the total amount of energy contained in the universe! Thus
there has to be an ultimate particle or family of particles.

        How do we see or observe these very small objects that we have been
discussing? In fact, what is "seeing" in the first place? We see when light enters
and interacts with our eyes, causing nerve cells to send messages to certain areas
of our brain. It is this chain of events that we call "seeing." But where does this
light come from? It must be associated with the object we see in one of three main
ways:

        1.      The object is the source of the light and emits the light directly to our
                eyes.
        2.      The object reflects light coming from another source towards our eyes.
        3.      Light coming from another source is transmitted through an object on
                the way to our eyes.

How does light give us information about the objects? It must interact (i.e., be
disturbed by) the object in some way.

        Light is composed of photons, which have properties of both particles and
waves (a strange phenomena that we shall discuss in depth later); the waves are
composed of fluctuations of electricity and magnetism and are called
electromagnetic radiation. A good pictorial model for waves of any kind is a water
wave, which has the form





If an object fits into a trough (i.e., if its size is less than one wavelength), the wave
will not notice its presence and the object will not cause any disturbance to the
wave. On the other hand if the object has a size bigger than one wavelength, it will
disturb the wave and secondary waves will propagate1 from the object. These
secondary waves then travel to the observer and carry information about the
object.






        What is the wavelength of visible light? Actually visible light is a mixture of
electromagnetic waves that have wavelengths in the range 4.0x10-7 meters to
6.5x10-7 meters. However the size of atoms is approximately 10-10 to 10-9 meters in
diameter! Thus atoms fit easily into the troughs of visible light. This means it is
impossible to see atoms with visible light no matter what the sophistication of the
microscope at your disposal. In order to "see" atoms we must thus use
electromagnetic radiation of smaller wavelengths, such as X-rays. Electrons, like
photons, also have wave-like properties and they form electron waves. These
waves are totally distinct from electromagnetic waves, being made up of electrons
not photons. The wavelength of electron waves are usually 10-11 meters, about the
same as X-rays, so they can also be used to "see" atoms. Electron waves are used
in electron microscopes, which are thus about 1000 times more powerful than the
best light microscopes.

        In order to make the atom's disturbing effect on X-rays or electron waves
perceptible to human beings, these disturbances must be echoed with
manifestations directly experienced by our senses, such as visible ones. This can
be accomplished, for example, with film sensitive to X-rays, which produces
visible effects when exposed to such radiation. This example represents a general
procedure for observing microscopic objects: the objects must disturb an
intermediary, which either interacts in a macroscopic way with our senses or
interacts with another intermediary or a chain of intermediaries so that the last
one in the chain interacts in a macroscopic fashion. In diagrammatic form






In the case of "elementary" particles such as neutrons and protons, whose size is
about 10-15m3, the intermediary can be a Cloud Chamber where the particles can
be seen by the pathway of water droplets condensed from the super saturated
water vapor (in other words, the cloud) by the passage of the particle.

        So far we have discussed how to observe objects either directly or by their
interaction with an intermediary, but how do we compare the properties of
objects? To do this we must measure their properties and in order to measure we
must have standard units. These standards are defined as the length, duration,
mass, charge, and so forth, of standard objects and processes, and are kept in the
archives of national physics laboratories around the world. All measured lengths,
durations, masses, charges, and so on, are then fractions and multiples of these
standard objects and processes.

        One of the most important units is that of time duration. The progress of time
is only evident with respect to processes that repeat themselves, which are called
cyclic processes, each cycle of such a process being called an oscillation. An
example of such a process is the rising and setting of the sun, where the time
duration between two successive settings or risings roughly defines the unit of
time, "the day." This, of course, is pretty inaccurate unless one takes into account
the season, although even then this unit is only accurate to a degree. Further
refinements taking into account the earth's rotation about its axis and around the
sun and various other astronomical phenomena allow one to define "the second,"
a very small part of a day. However, the second defined in such a fashion has also
been found to be inaccurate when compared to atomic processes, and nowadays
the second is defined as the duration of time taken by a certain number of
oscillations of electromagnetic radiation when leaving a hydrogen atom during a
certain prescribed standard process. The definition of the meter in terms of the
length of a standard object was also found not to be accurate enough and is now
defined in terms of the distance travelled by such radiation in a vacuum in a
certain fraction of a second. Standard units are by necessity macroscopic.


        In order to measure objects and duration of processes in terms of these
standard units, there has to be interaction with the object. The interaction usually
involves light or electron beams or collision with atomic and subatomic particles.
With macroscopic objects this interaction does not essentially disturb the object;
that is, the magnitude of the disturbance is negligible compared to the magnitude
of the property that is being measured. For instance, in order to see how many
meter lengths (a piece of wood whose length is one meter) span a desk top in a
given direction, we compare the desk top to the length of the meter length by using
light reflected from the two. The light does not disturb the meter length or the
desk top within the accuracy we are measuring. This, however, is not the case
when we are measuring the size of an atom, using photons that have a
wavelength of about 10-10m. These photons are quite energetic and exert enough
force to disturb atoms. Thus the act of observing/measuring perceptibly disturbs
the object being measured; it can move or actually change its size! Hence some
uncertainty is introduced into the measurement of properties of microscopic
objects as the measuring process involves direct interaction with other
microscopic objects that have comparable energies and sizes. Measuring
microscopic objects with microscopic objects can be compared to measuring the
size and position of a TV set by bouncing other TV sets off of it. The stream of
incident TV sets are disturbed, thus registering size and position information
about the TV being measured, but at the same time the size and position of this
TV set is definitely being altered!

        Is there a limit to the smallness of the sizes we can measure? In order to
disturb a wave, the object must be at least one wavelength in size. Thus to
measure smaller and smaller objects one needs smaller and smaller
wavelengths. However, as the wavelength decreases, the energy of the wave
increases; thus very small objects require higher and higher energy waves for
their measurement, which of course also increases the uncertainty of the
measurement. If the energy in the universe is finite, then this energy limits the
size of the smallest particle that can be measured. What about the smallest
duration of time? In order to measure a given duration, one must have a "clock"
that oscillates with a period (the time taken for one oscillation) shorter than that
duration. It also takes more and more energy to produce processes with shorter
and shorter periods of oscillation; thus again in a finite universe there has to be a
smallest time duration that can be measured. Present day theory predicts that the
smallest length should be at most 10-35m and that the smallest time duration
should be at most 10-44s.

        The question can then be posed that if there is no measuring process that
could determine durations of time and distances less than these values, can we
say that such small quantities exist? If we are consistent with our prescribed
scientific philosophy that only accepts as physical facts phenomena that can be
measured or have the possibility of being measured4, we must say NO! Thus at
this level time and space have a discrete grainy structure, though of course this

will only be experienced by objects of comparable size and frequency5. Larger,
less energetic objects will experience space and time as a smooth continuum.

        Let us emphasize the conclusion of the preceding paragraph and formalize it
as the Principle of Reality:

In order to make measurements and characterize the properties of events, objects, and processes, they must exist long enough and in great enough quantity for us to carry out the procedures necessary. The amounts and times needed vary from procedure to procedure. For instance, to carry out a full chemical analysis takes a relatively long time, while a spectra can be recorded in a fleeting moment. The longer the existence and the greater the quantity of events, objects, and processes, the more properties we can determine. These considerations are particularly pertinent for unstable elementary particles and nuclei. When, however, do we use the terms stable and unstable? The term "stable" is relative to the time period one considers. If the events, objects, and processes do not change during the time period under consideration, then for all intents and purposes they are stable. Sometimes this time period may be short, other times the age of the universe, so context is all important in the usage of the terms stable/unstable. In this commentary we have discussed the importance of forces and thus energies in the concept of elementary particles. But what are the forces in the universe and how can they be classified? By the 1960s observed physical phenomena could be essentially6 explained by four forces: Electromagnetic, Strong, Weak, and Gravity. The electromagnetic force was the unification of two forces that had been considered in the 19th century to be distinct-the electric force and the magnetic force. However, Einstein's theory of special relativity showed that they were just two different ways of looking at the same phenomena. The electromagnetic force is felt between electrically charged particles such as electrons and protons and is a long-range force decreasing slowly with distance between the interacting objects. The strong force is a short-range force felt between hadrons, its range being less than 10-15m, and is used to describe the interaction between nucleons (i.e., protons and neutrons). It thus controls the structure of nuclei. The weak force is an interaction that can be found between leptons and leptons, leptons and hadrons, and hadrons and hadrons. It is also extremely short-range, less than 10-15m, and is weaker than the strong force. The weak force is used in explaining radioactivity (the spontaneous decay of some nuclei). Gravity is a force between objects that have mass; unlike the other forces, however, it is only attractive, and it is enormously less powerful than the other forces. In the late 1970s the electromagnetic and weak forces were "unified" into the Electro-weak force. A consistent theory was developed based on the hypothesis that at very high energies there is no distinction between the weak force and the electromagnetic force, just different ways of looking at the same thing, exactly analogous to the situation with electricity and magnetism. Inspired by this and Einstein's dream of a single unified force, physicists have produced a theory that unifies this force with the strong force at even higher energies. This theory is called a Grand Unification Theory (GUT). However, it is not clear if some of the predictions of this theory are actually valid, and the GUT is a topic of current research. Theoretical physicists have gone even further in unifying gravity with this single electro-weak-strong force in a theory called The Theory Of Everything (TOE). This unification is supposed to occur at even higher energies, which probably could only exist in the first few moments of existence of the universe-at the instant of the "Big Bang." These unified forces at high energy are very abstract and far from everyday experience, but not so the forces of gravity and electromagnetism. These we feel directly every moment of every day. The effects of gravity are clear: It causes objects on the surface of the earth to drop towards the center of the earth and it keeps objects pressed to the surface of the earth. In the preceding pages we have discussed the facts that material objects are composed of atoms in various combinations and arrangements and that atoms are mostly a thin mist of negative electricity surrounding an extremely small, dense, heavy nucleus. Thus matter is mostly made up of this ephemeral mist of insignificant mass, almost nothingness. Why then do we not crash through the surface of the earth under the force of gravity or pass through closed doors or even each other? It is because of electromagnetic repulsion. When objects come close to each other, their outer mists of negative electricity become close, negative charge repels negative charge, and this force of repulsion increases dramatically as the distance between the charges becomes smaller and smaller. The charges can never actually meet if the objects maintain their identity as the force increases to infinity at zero distance. Thus the force of gravity doesn't actually keep us on the ground, as it cannot overcome the force of electromagnetic repulsion, but keeps us floating a small distance above where the force of gravity on us is equal and opposite to the electromagnetic force between us and the ground. When we come close to objects and other people, we feel that we touch when this electromagnetic force reaches a certain threshold; the contact we feel is actually the experience of this force! This understanding of why we don't fall through floors or pass through solid objects, even though they are not really substantial in the sense described above, allows one's imagination to construct fantasy beings that could have the property of ghosts. These beings would have to be constructed of atoms that were constituted of electrically neutral material with dense nuclei surrounded by thin mists and held together by a long range "strong" or "weak" force. Such beings would just pass through material that felt solid to us. But we all know there are no such things as ghosts-don't we?