by Alan J. Slavin,
Department of Physics, Trent University,
This brief history and philosophy of physics has been written
to give physics students some appreciation of where their discipline has
come from, and of the philosophical principles underpinning it. It is hoped
that this will provide students with a sense of physics as a living, evolving
discipline, and of their place in its evolution. Physics, indeed all of
science, is not a static agglomeration of proven facts and inviolable theories.
While there are many theories which are so well tried that they are generally
accepted as being correct, all scientific theories are still open to attack
from some new, reproducible experiment which disagrees with them. The history
below bears this out.
Furthermore, while science per se may be value-less, neither
good or bad, the teaching or application of science always has values attached.
If it is taught by a scientist without any mention of the need to use the
learning responsibly, then students may assume that scientists need not
be concerned about the application of science. If this happens, then the
scientist ultimately abdicates to the politician or manufacturer the decision
on the use of her or his own work, even though it is unlikely that either
the politician or manufacturer will understand as well as the scientist
the effects of its use. By this, I am not suggesting that scientists are
the only ones qualified to decide on the application of science; scientists
can also be blind to the potential in what they do. It is hoped that this
paper will contribute to the ability of students to ask the necessary questions
regarding the science they and others participate in, both now and throughout
This summary is designed to outline the general development of the main branches of physics as we know them today. It is presented here as occurring in a fairly linear fashion, and discusses only the principal figures in each area. However, it must always be remembered that there were a great many more people working on these problems than mentioned here, with many of them being unaware of the work of the others. As a result, many of these areas progressed in a more-or-less "random walk" between theory and experiment until about the last two hundred years, when improved communications made it much easier to keep up to date with developments world-wide.
Given the fact that half the world's population is female, there is
a notable absence of women in this history. This is largely because women
have been systematically excluded from science over the centuries until
very recently, with few exceptions. Even when women did make major contributions
as part of a larger team in relatively recent times, as was the case of
the women "computers" in astronomy at Harvard College Observatory in the
late 1800s, usually only the male team leader gained recognition [Rossiter].
One can only mourn the loss to the discipline from the exclusion of other
Marie Curies, and work towards encouraging the participation of many more
women in the future.
Earliest Beginnings, and the Greeks
People have always been acutely aware of the regularities in nature:
the sun rises every day; the moon appears at the same place in the sky
roughly every twenty-seven days, about the same as a woman's menstrual
cycle; the seasons always follow in the same order; the pattern of the
"fixed" stars (all the heavenly bodies except for the planets, sun, moon
and comets) repeats itself at the same time every year; snowflakes all
have six points; a dropped stone always falls. In fact, the very well-being
of a family depended until recent times on knowing when to plant, or when
to move camp for the next season's game.
This obvious order begged for explanation, and the earliest people attributed
it to a range of gods and goddesses who controlled the world. With the
Greeks, for example, Gaea was the earth goddess, Zeus threw lightning bolts,
and Apollo drove the fiery chariot of the sun once per day across the heavens.
"Science" is the attempt to give a rational, rather than religious or magical, explanation for the order in nature. People in different parts of the world began to develop science at different times, with different emphases. As one example, as early as 36 B.C. [Cole, p.46] the Mayan people of what is now Mexico and Central America used a calendar with an accuracy equivalent to knowing the length of the year to within six seconds, and plotted the movement of the sun, moon and planets. They also used a "place system" for numbers (like our decimal place system) at the time when the Romans were still using a new symbol for every new power of ten they encountered, and the Mayans employed the zero centuries before Europe. (The zero was used in India from about 850 A.D.) Although the Mayans had recorded much of their customs and learning on hundreds of books made of beaten-bark paper, very little remains today. Their Spanish conquerors systematically destroyed almost all of this "heathen" literature.
The first European attempts to provide a rational explanation for the
workings of nature began with the Greeks, about 600 B.C. For example, Pythagoras
(582-500 B.C.) and his followers belonged to a religious fraternity dedicated
to the study of numbers. They believed that the world, like the whole number
system, was divided into finite elements, an early precursor to the idea
of atoms ("atom" means "indivisible"). Their discovery of irrational numbers
such as Ö2, which could not be expressed
as a ratio of whole numbers, was a serious threat to this system, and history
tells us that they killed the Pythagorean who released this secret to the
The Greeks Leucippus (~440 B.C.), Democritus (~420 B.C.) and Epicurus
(342-270 B.C.) put forward the hypothesis that matter was composed of extremely
small atoms, with different materials being composed of different combinations
of these atoms. Aristarchus of Samos (310-230 B.C.) is the first person
known to have proposed that the earth rotates once per year around the
sun, rather than the intuitive explanation that the sun rotates around
the earth. He also attempted to calculate relative sizes for the earth,
moon and sun. However, it was not considered necessary by the Greeks to
test such hypotheses experimentally; all that most of them were looking
for was a self-consistent explanation of the world based on a small number
of philosophical principles.
Aristotle is generally credited with providing the most comprehensive
of such explanations. He believed that there were four earthly elements:
earth, water, air and fire. Each had its natural place determined by its
weight. Earth, being the heaviest, "wanted" to be at the centre of the
universe. Water was above the earth, with air above water, and then fire.
This order makes intuitive sense. Solid ("earthy") bodies sink in water;
if you release air under water the air bubbles to the surface; and flames
leap upward during burning. (Wood could float even though it was a solid
body, because it contained both earth and fire; the fire was released on
burning.) The farther a body was from the earth, the more perfect it became.
Hence the moon was the least perfect of the heavenly bodies, as could be
seen by its uneven appearance, while the fixed stars were the most perfect
of all, and were composed of a fifth element (the "quintessence") which
had no weight at all.
In Aristotle's physics, a moving body of any mass had to be in contact with a "mover", something which caused its motion, or it would stop. This mover could either be internal as for animals, or external as in the case of a bowstring pushing on an arrow. The arrow was kept in flight by air displaced from the front rushing to the back to fill the vacuum left by the arrow. Since Aristotle said that a vacuum was impossible ("nature abhors a vacuum"), this explanation of an arrow's motion was again internally consistent. However, because the stars were without mass, once they were put in motion by a "prime mover" they could continue to move by themselves.
The Greeks spent much effort trying to explain the motion of the sun,
moon, planets and stars. Since this motion also played a major role in
the development of modern science, it is worth discussing in some detail.
The stars are so far from us that their relative motions cannot be observed
except over timescales of a few centuries. Therefore, to someone standing
on the earth the stars appear to be fixed in a vast sphere, concentric
with the earth. This sphere rotates at constant speed about the earth at
a rate of just more than once in twenty-four hours, returning to almost
the same position at a given time of day once every year. Similarly, the
sun and moon appear to lie on spheres, which rotate about the earth once
per day and once every 27 days, respectively. The motions of the planets
appear much more complicated to an earthly observer. We now know that the
planets are all on orbits with different average distances from the sun,
and orbital periods that increase the farther the planet is from the sun.
For example Venus, Earth's nearest and brightest planetary neighbour, has
a period of 225 days, compared to Earth's 365. This means that as Venus
makes its annual pilgrimage through the night sky as viewed from Earth,
it occasionally moves backwards relative to the fixed stars, in
"retrograde motion", as its orbit carries it opposite to the direction
the earth is moving. (Hence the name "planet", meaning "wanderer".)
The Greeks usually described this motion using a device invented by
Eudoxus of Cnidus (409-356 B.C.), who was apparently the first Greek to
use quantitative observation to develop a mathematical description. Noting
that the motion of the planets was periodic, he developed a system of spheres
each of which carried a planet, with each sphere centred on the earth but
with its axis of rotation fixed in a larger sphere. This explanation fitted
with the Greek belief that the circle was the most perfect geometrical
form. However, this system was approximate at best. Apollonius of Perga
(~220 B.C.) suggested, instead, that each planet was attached to a small
sphere which, in turn, rolled on a large sphere centred on the earth, with
the larger one rotating roughly once per day. The large sphere accounted
for the daily motion of the planet, while the small one (the "epicycle")
explained the retrograde motion. A later addition was the use of the "eccentric",
which allowed the centre of rotation of the large sphere for each planet
to lie away from the centre of Earth.
As the accuracy of the mathematical description increased, so did the
need for reliable observations. This was recognized by Hipparchus of Nicea
(190-120 B.C.) who had studied the observational records of the earlier
Greeks and Babylonians, with the latter dating back to the seventh century
B.C. In this process, Hipparchus discovered the "precession of the equinoxes";
that is, that it takes the sun about 20 seconds more to return to its position
at the equinox every year than it does to return to its position among
the fixed stars. To satisfy the need for accurate data, Hipparchus catalogued
the position and brightness of 1080 stars. By the time of Ptolemy (85-165
A.D.), who observed at Alexandria in Egypt, the system of system of epicycles
and eccentrics required eighty circles to describe the known periodicities
of the heavens.
Of course, the Greeks did not restrict their science to physics. For
example, the Hippocratic oath sworn by doctors today takes its name from
Hippocrates of Cos (~460-377 B.C.). Aristotle's most lasting contribution
to science was in biology, where he classified about 540 animal species,
and carried out careful dissections of at least 50 different animals. Archimedes
(287-212 B.C.), scientist-engineer, has been described as one of the three
greatest geniuses of all time [Kramer]. He invented the Archimedean screw
for raising water, discovered the principle of buoyancy of a body in a
liquid, and calculated an accurate value for p,
among other accomplishments.
In light of his future influence on the course of European science,
it is of interest to look at Aristotle's attitude towards the role of women.
In his "Generation of Animals" he says, "Wherever possible and so far as
possible the male is separate from the female, since [he] is something
better and more divine in that [he] is the principle of movement for generated
things, while the female serves as their matter ... We should look upon
the female state as it were a deformity, though one which occurs in the
ordinary course of nature." [French p.130]. This attitude was not shared
by all Greeks. For example, Pythagoras admitted women to his school equally
with men. [French, p.144.]
The Dark Ages, and the Translations
With the fall of the Roman empire about 400 A.D., most of the
Greek learning was lost to Europe as it entered the Dark Ages. Even the
knowledge that the Earth was round, known to the Greeks who had a good
estimate for its diameter, was replaced by the conception of a flat Earth.
(This does not mean that all learning stopped during the Dark Ages; important
technological discoveries were made during this period, such as the invention
of the plough and the water wheel.) The Greek knowledge itself, however,
was not lost. It had migrated into the Middle East and Egypt under the
Greek and Roman empires, and was translated into Arabic by the people who
lived in these regions. The Arabs not only kept Greek science alive, they
added to it considerably. For example, the Arabs had important medical
schools and first discovered the law of refraction, now known as Snell's
law. They also translated major Indian scientific works into Arabic, and
began to use the numerals and algebra developed in India. Al-Battani (~858-929
A.D.) measured a value for the precession of the equinoxes that was more
accurate than Ptolemy's. The Arabs also transported the art of paper-making
from China to the west. Their contribution remains enshrined in Arabic
words which we still use today, including algebra and algorithm.
When Christians recaptured Spain in the eleventh century, the bridge
was formed to carry this learning back into Europe. A major translation
centre was set up in Toledo after it was captured in 1085, with a lesser
centre in Sicily after it fell to the Christians in 1091. Translation was
done primarily into Latin, the language of learning in Europe at this time.
However, most of the translators focused on the Greek works, and some Arabic
and Persian works remain untranslated today.
The Middle Ages
The scholarly work in Europe during the Dark Ages (roughly from the
fall of Rome to the beginning of the Middle Ages, or Medieval period, about
1100) had been primarily concerned with the copying of church manuscripts.
As a result, it was natural that as ancient learning began to reach Europe
it should be studied first in the cathedral schools. These schools evolved
into the first universities, with colleges in Cambridge and Oxford, for
example, being founded in the 1200s. These were followed by universities
set up by both city (e.g. Bologna, Padua) or state (e.g. Naples) governments.
The scholars in these early universities laid much of the groundwork for
later scientific developments.
One of the most important schools for the development of physics was
in Oxford, where the impetus theorists, beginning with William of Ockham
(~1295-1349), investigated the cause of motion. They believed that a body
in motion did not need to be in contact with a "mover" to stay in motion
as Aristotle had claimed, but did so out of its own "impetus". This was
a precursor to our modern concept of momentum. Another major contribution
has become known as "Ockham's Razor". This principle states that the best
scientific theory, other things being equal, is the one which requires
the fewest new starting assumptions. It is still accepted today. It was
important historically because it provided an objective means for choosing
between two theories and did not attempt to answer the question of which
The flood of ancient, "pagan" knowledge into Europe through the translations
from Arabic produced a crisis for Christian theologians: How could one
accept a world philosophy which was not rooted in the Christian faith?
This problem was largely overcome, at least for the time being, by St.
Thomas Aquinas (1225-74) who integrated Aristotelian philosophy and Greek
logic with Catholic theology. For example, his first proof of the existence
of God was that the fixed stars needed a source of motion, which he identified
with Aristotle's "Prime Mover".
One must ask why, when so many of the early scientific discoveries were
made in the east, the development of modern science was primarily in the
west. Alfred North Whitehead, in Science and the Modern World, suggests
that this was due to the integration of Greek rationality with Christian
monotheism under Thomas Aquinas. The all-seeing God of Christianity created
the world in an ordered, logical fashion as related in the biblical book
of Genesis. Therefore it was only natural to look for a rational explanation
of the phenomena of nature.
The Renaissance (1300-1700)
The rebirth ("Renaissance") of knowledge and learning in Europe, which
followed the rediscovery of Greek and Arab learning, affected all of society.
Awakened to the fact that there was so much "new" knowledge to be explored,
people became free to invent their own. The arts flourished, with Durer
inventing perspective drawing in Germany, Michelangelo studying anatomy
to give life to his sculpture in Italy, and orchestral music being born.
It saw the beginning of the Protestant Reformation in 1517, with Martin
Luther nailing his 95 theses to the door of Wittenburg Cathedral. This
was the period of the great European voyages of discovery, with Columbus
arriving in America in 1492 and Magellan sailing around the tip of South
America. Unfortunately, this period also saw the destruction of much of
the learning of the peoples "discovered" by the Europeans, who still believed
that non-Christian/European culture was valueless. This Eurocentrism is
still active today, as witnessed by the almost complete omission of the
great Central American civilizations from today's school curriculum in
However, during the Renaissance Aquinas' integration of Greek, and particularly
Aristotelian, philosophy with Catholic theology eventually led to as many
problems for the church as it had solved. Copernicus' suggestion (about
1530) that the Earth and the other planets moved around the sun, rather
than the reverse, was seen as heresy by the Church. Not only did it contradict
Aristotle's teaching and several Biblical assertions that the Earth was
stationary, it also challenged the authority of the Church by questioning
the hierarchical structure on which its entire existence was based. If
the Earth was not stationary at the centre of the universe, perhaps Heaven
was not outside the sphere of the stars, and where did this leave God,
not to mention all of His ecclesiastical delegates? The idea of a moving
Earth was so revolutionary that Copernicus did not agree to have it published
until he was on his death bed (1543). It is no surprise that the two people
most responsible for the publishing of Copernicus' book were followers
of Martin Luther, who had dared to question the authority of the Catholic
church on scriptural matters.
The Renaissance also saw the beginnings of modern science under Galileo
Galilei (1564-1642). One of Galileo's greatest contributions was to recognize
that the role of the scientist was not to explain "why" things happened
as they do in nature, but only to describe them. In one of his "Dialogues"
he asks a colleague why objects fall when released. When the colleague
replies that everyone knows that gravity makes them fall, Galileo replies
that he has not explained anything, just given it a name. This new role
greatly simplified the work of the scientist, who no longer had to wonder
why God would have caused a particular phenomenon to occur. It sufficed
to recognize that it did occur, and allowed one to get on with the job
of deciding how best to describe it.
This leads us to Galileo's second major contribution, the description
of natural phenomena using mathematics and the appeal to nature through
experimentation to see if the description is correct. This was a major
deviation from the qualitative science of Aristotle in which, for the most
part, all that was required of an explanation was that it agreed qualitatively
with reality: solid objects fell because they were composed of earthy material
whose natural place was at the centre of the universe. In Galileo's science,
on the other hand, one had to describe mathematically how far an object
fell in a given time, and then verify experimentally that this description
was correct. Moreover, he recognized that the experimenter had to devise
the experiment so as to isolate the phenomenon being studied; for example,
to minimize the effect of friction in the study of falling bodies.
Galileo's most important applications of these ideas was in the mechanics
of falling bodies, building on the early ideas of the impetus theorists.
He showed that all compact bodies fell at the same rate, such that the
distance covered was proportional to the square of the elapsed time of
fall. Because objects in free fall drop too fast for easy measurement,
Galileo did his measurements by rolling balls down an inclined plane. Even
so, there were no clocks at the time accurate enough to make the measurements
Galileo has recorded. (Galileo is, in fact, credited with the suggestion
of using a pendulum as clock.) Stillman Drake, a Canadian who was one of
the world's foremost scholar of Galileo, has noted that a person can keep
time while singing with a precision of about 0.01 seconds. Drake shows
that Galileo could have made his measurements by noting where the rolling
ball was at each beat in a song [Drake, 1975].
Galileo is probably best known for his conflict with the Catholic church
over his support for Copernicus' description of the solar system. When
Galileo heard of the invention of the telescope, he designed and built
one for himself. This, the first telescope usable for astronomical observations,
quickly led Galileo to realize that Copernicus' theory was more than just
an alternative to the Ptolemaic approach for calculating the positions
of the planets. He saw that Jupiter had moons, and so was a miniature model
of the solar system in itself; that Venus showed phases similar to those
of the moon, as it must under the Copernican system; and that the moon
had mountains and so was similar to the Earth. No wonder the church saw
him as a threat! Galileo, aged sixty-eight, was tried by the Inquisition
and sentenced to house arrest for the remainder of his life for daring
to support Copernicus' theory, even though he recanted when faced with
the death penalty. Ironically, he used this time to develop mechanics to
the point at which it could explain why the planets would not fall into
the sun if they were not held up by their "natural place".
Development of The Scientific Method
Francis Bacon (1561-1626) takes credit for providing much of the philosophical
basis for our modern scientific method. His major works, published in 1605
and 1620, were very influential in directing the approach to science over
the next two hundred years and remain relevant today. Bacon had a vision
that science could greatly improve the lot of humanity, and set out how
he thought this could best be accomplished. This belief in human "progress",
that humanity is moving towards some ultimate state of happiness in which
war, illness and poverty will be abolished, was unique to the west. Part
of this vision was his belief, founded in the Genesis story of creation,
in the right of man to dominate nature, "to bind her to your service and
make her your slave" [French, p.117]. This right of domination over the
rest of nature has been a guiding principle of science and technology for
most of the time since Bacon. It is only now beginning to be challenged
by the developing ecological awareness that people, too, are part of nature,
and that they ignore the inter-relationship at their peril. Marilyn French
goes on to argue that, since nature has generally been seen as "female",
Bacon's claim for the right of men to dominate nature has helped perpetrate
the domination of women by men.
Bacon's approach was basically experimental, qualitative and inductive.
He rejected a priori assumptions such as the idea of the perfection
of spherical motion used by the Greeks. Rather, Bacon believed that if
enough observations could be made which involved a particular phenomenon,
an observer could use these to induce the fundamental principles involved.
The first step of this process, then, was the gathering of as many unbiased
facts as possible, drawing heavily on information already available in
craft and industrial processes. The next was to correlate these so as to
discern the fundamental truths within them.
René Descartes (1596-1650), from France, proposed a different
approach to the development of science. Instead of starting with raw facts,
as Bacon had suggested, Descartes believed that the basic principles ruling
nature could be obtained by a combination of pure reason and mathematical
logic (e.g., "I think, therefore I exist.") His approach was analytic.
It involved breaking down a problem into its parts and arranging them logically,
a technique which is still used constantly in science today. It is termed
"reductionism", because its basic assumption is that we can reduce a phenomenon
to a collection of independent components; if we can understand each of
them taken independently, then we can understand the entire phenomenon,
in a way similar to our understanding of the operation of a machine. This
approach has dominated scientific investigation over the last three hundred
years, and has proven very successful in areas in which in which the parts
really are largely independent. "Holism", the opposite of reductionism,
assumes that some phenomena, at least, can only be understood as integrated
wholes, and so cannot be broken down into independent parts. An excellent
discussion of the need for more holistic thinking in modern science can
be found in Fritjof Capra's The Turning Point. Capra argues that
the need for a holistic approach has a theoretical basis in the quantum
nature of matter, as discussed below.
Descartes' "mathematical-deductive" approach was diametrically opposed
to Bacon's "qualitative-inductive" method, whereas modern science uses
a combination of the two. Given Bacon's emphasis on experimentation, and
Descartes' emphasis on deductive reasoning, it is not too surprising that
in the next hundred years English scientists stressed experimentation while
French scientists stressed mathematical theory. In developing his approach,
Descartes made several important mathematical contributions of his own.
Principal among these was the invention of cartesian geometry, which describes
geometrical figures in the form of algebraic equations.
Descartes really believed that the world and most of what was in it were essentially machines. God had created and wound up the system at the beginning, and it had been running ever since under the laws of nature without further intervention. The one exception to a machine was the soul (or mind) of a human, which was divine and separate from the mechanical body. Since animals did not possess a mind, they were pure machines which could not feel pain. For a period there were Cartesian followers who would vivisect animals to show how well a machine made by nature could mimic suffering. This concept of the world as a machine persisted for many years, and was strengthened by Newton's mechanics. In fact, in 1812 Laplace, a great mathematical physicist, made the following statement, [Schneer, p.129] "If an intelligence, for a given instant, recognizes all the forces which animate Nature, and the respective positions of all things which compose it, and if that intelligence is sufficiently vast to subject these data to analysis, it will comprehend in one formula the movements of the largest bodies of the universe as well as those of the minutest atom; nothing will be uncertain to it, and the future as well as the past will be present to its vision. The human mind offers in the perfection which it has been able to give to astronomy, a modest example of such an intelligence.
The Development of Classical Physics: Mechanics, Heat, Optics, Electromagnetism, Atoms
Sir Isaac Newton (1642-1727), born the year Galileo died, is the most
important figure in the development of mechanics. His three "laws" form
the base on which all of mechanics prior to 1900 was constructed. This
model of building an edifice of theory on the foundation of a few fundamental
definitions and laws is essentially that used by Euclid in his geometry.
It became the ideal for all future physical theories, including thermodynamics
with three basic laws (zeroth, first and second), optics (laws of reflection
and refraction) and electromagnetism (Maxwell's laws). Much of the physics
of the hundred years after the death of Newton was spent in applying his
three laws to different phenomena.
Newton's crowning accomplishment was the application of his mechanics
to show that the entire universe obeyed the same laws of nature, as published
in his Mathematical Principles of Natural Philosophy (the Principia)
in 1687. By assuming that two masses attracted each other with a force
inversely proportional to the square of the distance between them, Newton
proved that the mechanics which determined how bodies fall on Earth also
explained the periodic motions of the planets. However, Newton did not
restrict his work to mechanics; he also did extensive studies on light
and shares the credit for the invention of calculus with the German, Gottfried
Wilhelm Leibnitz (1646-1716), with whom he fought a long battle over who
was first. Newton also wrote on theology, and was Master of the Royal Mint.
The invention of a practical steam engine by Thomas Newcomen (1663-1729)
prompted great scientific interest in the study of heat, and was a major
contribution to the industrial revolution which began in England in the
mid 18th century. (It is ironic that the industrial revolution, which began
to apply scientific principles to the production of goods as predicted
by Bacon one hundred years earlier, also led to the virtual slave labour
of children and the poor in mines and factories.) Sadi Carnot (1796-1832),
a French engineer, laid the basis for our understanding of heat engines
(any engine which uses heat to produce power, such as the automobile engine,
or a coal or nuclear electrical power station). He compared the operation
of a heat engine with that of a waterwheel, with heat "falling" from a
higher to a lower temperature. Joseph Black (1728-99), the professor of
medicine at Glasgow University, began to quantify heat by the measurement
of the specific heat capacities (the amount of heat required to raise the
temperature of a given mass by one degree) of different substances, compared
to that of water. Motivated by the heat generated in the boring of cannons,
Count Rumford (1753-1814), first showed that heat could be produced in
limitless quantities by friction, and so was not a material substance (caloric)
as had been believed previously.
James Prescott Joule (1818-89), by rotating a "paddle wheel" under water
and measuring the increase of temperature, established a numerical equivalence
between work and heat. He also showed that the heat produced by an electrical
current I in a wire of resistance R was given by I2R, a relationship
now known as Joule's law. Joule's quantitative work on the interconversion
of energy laid the basis for the first law of thermodynamics, which says
that the change in the energy of a system is equal to the heat input to
it plus the mechanical work done on it. This law was first stated explicitly
by the German Rudolph Clausius and Englishman William Thomas Kelvin in
1851. Clausius also realized that a heat engine could utilize only some
of the available heat to do work, and from this developed the concept of
entropy, the quantity of heat transferred divided by the temperature. Clausius
showed that the entropy always increased in any spontaneous natural process,
and so established the second law of thermodynamics. As with Newton's three
laws, the laws of thermodynamics form the foundation for the understanding
of thermal physics.
Light and Optics
The Greeks had applied the methods of geometry to the study of optics,
and Ptolemy had a crude approximation to the law of refraction. This work
was extended by the Arab Al-Hazen (965-1038), who showed that Ptolemy's
law was just an approximation, valid at small angles. Al-Hazen also carried
out experiments which brought him close to the thin lens formula for convex
lenses. The telescope and compound microscope were invented in Holland
near the beginning of the seventeenth century, with the telescope used
to advantage by the early astronomers including Galileo. In 1621 Willebrod
Snell rediscovered the correct formula for the refraction of light, which
now bears his name.
From the time of Descartes there was considerable debate as to whether light consisted of small particles which were localized and travelled in straight lines, or of waves which spread out in space. Descartes adhered to the former explanation whereas in the late 1600s Christian Huygens argued for a wave theory, with the waves travelling through an ether which permeated all space and all objects. Newton used a combination of the two approaches: while light itself consisted of "corpuscles", he believed that these particles could induce vibrations in the ether through which they travelled, which in turn could affect the transport of the particles. For example, he used this theory to explain "Newton's rings", alternating light and dark bands which appear when a slightly curved lens is placed in contact with a flat mirror. For a century after Newton, the majority of scientists adhered to the corpuscular theory.
Thomas Young (1773-1829) revived the wave theory for light. It was generally
accepted that sound was transported by waves carried through the air, and
Young argued that light travelled in a similar way. He used the interference
pattern produced in his famous "two-slit experiment", still studied in
introductory physics courses today, as proof of this wave nature. (A similar
pattern, in the form of a cross, can be seen with the naked eye by looking
at a distant street light through a window screen, although using binoculars
improves the image.) From these patterns he was able to measure the wavelength
of light which he proved to be very small. He went on to show that this
led to light travelling in approximately straight lines for the vast majority
of common cases, although it did bend slightly around objects to produce
patterns in their shadows, patterns which could be explained by his wave
theory. Then, in 1817, the Frenchman Augustin Fresnel showed that all known
optical phenomena could be explained by the wave theory provided that,
following a suggestion of Young's, the vibrations were transverse (perpendicular
to the direction of light propagation) rather than parallel to it as for
sound waves. This firmly established the wave theory as dominant, although
it did raise the question of how a fluid such as the ether could support
a transverse vibration, since fluids usually have only longitudinal vibrations.
This problem was a harbinger of an upcoming debate over the very existence
of the ether.
The study of electromagnetism began in experimental studies of such
effects as static electricity and magnetism. People had known from ancient
times that rubbing certain materials on dry hair would make the two attract
each other, and the naturally occurring, magnetic lodestone was used as
a navigating compass by the Chinese from about 100 B.C. Systematic studies
of electricity began in earnest once apparatus had been invented for generating
and storing electrical charge. The first electrostatic generator, a machine
which rubbed a cloth against a rotating ball of sulphur, was invented by
Otto von Guerike (1602-86), while Pieter van Muschenbroek (1692-1761) made
the first Leiden jar to store electrical charge. In contrast to the spark
discharges of an electrostatic generator, the voltaic cell (battery), invented
by Volta in Italy in 1799, could provide a continuous flow of current.
In a famous (and dangerous!) experiment in 1752, Benjamin Franklin used
a kite to collect charge from a thunder cloud and store it in a Leiden
jar. He then showed that this charge had identical properties to that produced
by an electrostatic generator, proving that lightning was just one manifestation
of electricity. However, Franklin's main contribution to the theory of
electricity was his suggestion that charge came in two types, which he
called positive and negative, with like charges repelling each other and
unlike charges attracting. By these simple assumptions he could explain
all known experimental facts about electricity, whereas previous theories
had required about 20 different assumptions, including different shapes
for particles of electricity in different media. This is one example of
the use of Ockham's Razor in deciding between rival theories. Franklin
also showed that there was a connection between electricity and magnetism,
because iron needles could be magnetized by placing them near a wire carrying
an electrical current.
In 1750 John Mitchell, at Cambridge, had discovered the inverse-square
repulsion of magnetic poles, by using a "torsion balance" to measure the
twisting of a thread supporting one magnet when another was brought close.
In a period beginning in 1785, the Frenchman Charles Augustin Coulomb reinvented
the torsion balance and showed that both magnetic and electric forces experienced
an inverse-square dependence on distance, now called "Coulomb's law" in
the case of electrostatics.
In Germany there developed a separate school of thought, that of the
"nature philosophers". They believed that matter was not inert, as claimed
by the mechanist school, but alive, with a universal world spirit that
interconnected all forces. One member of this movement was the philosopher
Immanuel Kant (1724-1804), who asserted that it was the interplay of innate
repulsive and attractive forces that governed matter. If only repulsive
forces existed, all matter would disperse; if only attractive forces were
present, all matter would coalesce into a point. This balance between attractive
and repulsive forces is today the starting point for the theoretical analysis
of the structure of solids and liquids, although the forces are no longer
believed to reflect a life force.
The study of both electricity and magnetism was popular with German scientists, because the presence of opposite polarities in these phenomena fitted with their philosophy. These ideas also led to the conviction that every effect in nature had its inverse effect, since the vital forces were all connected. This idea that every effect has its inverse is fundamental to modern physics. For example, if you connect two wires made of different materials, and heat the junction, a voltage develops between the free ends of the wires. This effect, discovered by Thomas Seebeck, another German Nature-Philosopher, is the principle behind the use of a "thermocouple" for measuring temperatures. Conversely, a voltage applied with the correct polarity across the free ends of the two wires causes the junction to decrease in temperature. This is the principle behind the "thermoelectric cooler", often used to cool devices in electronic circuits.
The belief in the interconnectedness of all forces in nature led Hans
Christian Oersted, in Copenhagen, to announce in 1807 that he was looking
for a connection between magnetism and electricity. He found that a magnet
would move in a circle around a wire carrying a current, and that a wire
carrying a current would move around a magnet. This is the principle required
for the construction of an electric motor. The magnetic forces near current-carrying
wires were the first forces which had been discovered which did not operate
radially from the two interacting bodies. The next major contributions
in electricity and magnetism came from the theoretician André Marie
Ampère in France, and the experimentalist Michael Faraday in England.
Ampère (1775-1836) developed a theory for the calculation of magnetic
forces caused by a given electrical current, and suggested that the magnetic
effects of some solids were caused by small circulating currents in the
particles making up these materials.
Faraday (1791-1867), on the other hand, had very little mathematics
but was a superb experimentalist. His most important experimental observation
in electromagnetism was that of induced currents, made in 1831: a wire
loop would have an electric current developed in it, if either the loop
was moved near a magnet, or the magnet was moved. This is the principle
behind the generation of electricity by mechanical means, as occurs in
every hydro- or thermo-electric power generating station, or in every car
Even though mathematically unlearned, Faraday made a very important contribution to the development of the theory of electromagnetism by constructing a qualitative model of how electrical and magnetic forces acted. He supposed that each "particle" of electricity or magnetism produced a "line of force" which emanated from a positive pole of a particle and returned to a negative pole. These lines tended to contract along their length, and to expand perpendicular to their length. The lines could not cross. The number of such lines passing through a given area (i.e. the areal density) was a measure of the strength of the force provided by them. These assumptions explained the repulsion and attraction of magnetic and charged bodies: the tendency to contract lengthwise would pull bodies of opposite polarity together, whereas the tendency for them to expand laterally would push bodies of opposite polarity apart. Since the area of a sphere increases with the square of the radius, the inverse-square decrease in intensity of the forces was a natural consequence of the decrease in the areal density of the lines of force with distance from the charge or magnetic poles. The visual appeal of these lines of force still plays an important role in our understanding of electromagnetic phenomena. Moreover, Faraday believed that the lines of force would be present even if only a single charged or magnetic object existed; that is, even if there were no other body on which the first one could exert a force. Thus he invented the concept of the "field", as a physical presence which had the ability to produce a force -- magnetic, electric or gravitational -- if a second body happened to come into its vicinity. The concept of the field has served as one of the most powerful of all theoretical tools of modern physics.
James Clerk Maxwell (1831-79) set out to make Faraday's ideas quantitative.
He described the lines of force using Newtonian mechanics, envisioning
them as rotating tubes of fluid (the ether) which had the properties required
by Faraday: the rotation would cause the tubes to expand laterally and
contract longitudinally. The resulting set of only four equations ("Maxwell's
equations") described all known electric and magnetic phenomena exactly.
Maxwell, however, realized that the enormous machinery with which he had
filled all space was not an essential part of his theory, and eventually
just used his equations as though the machinery did not exist. This is
how we use his equations today. The relationship between the original machinery
and the final equations was not without its detractors, however. One French
reader stated that when he started to read Maxwell's work he expected to
find himself in the midst of the quiet groves of electromagnetic theory,
and instead found himself inside a factory! [Williams, p.122].
One of the unexpected results of Maxwell's work was that it predicted
that electromagnetic waves could be produced which would propagate at the
speed of light. This showed that light was an electromagnetic phenomenon,
and not a separate subject.
Discoveries in electromagnetism were applied quite rapidly to the development
of useful devices. For example, the telegraph was invented in 1837 by Charles
Wheatstone only one year after the development of the first reliable battery,
and the first practical electrical generator was invented by Werner Siemens
in Germany in 1866, 35 years after Faraday's discovery of induced currents.
Until the twentieth century, the development of the atomic theory of
matter was pursued by scientists who are often more closely identified
with chemistry than with physics. In 1789 Antoine Lavoisier published his
of Chemistry. In this work, he emphasized the need for quantitative
methods in chemistry. By carefully devised experiments, he was able to
isolate 23 elements, fundamental substances that could not be broken down
into simpler forms. In England in the late 1700s, the experimentalists
Joseph Black, Henry Cavendish and Joseph Priestley isolated several different
gases and showed how they could be produced. Schneer makes the interesting
point that a large number of the most successful scientists of this era,
including Priestley, Dalton, Faraday, James Watt (who greatly improved
the steam engine), Thomas Young, and Franklin, were all Quakers, a non-conforming
religious group who dared to challenge the established beliefs of the day.
Then in 1802 John Dalton, an English schoolmaster, revived the theory
of atoms. It was known by this time that gases always combine in fixed
ratios by mass. For example one gram of hydrogen burns with eight grams
of oxygen to produce nine grams of water. Dalton proposed that these ratios
of whole numbers could be explained if the gases were formed of atoms whose
masses were, themselves, in the ratio of simple integers. The formation
of water discussed above could then be explained by the combination of
two hydrogen atoms with one oxygen atom. At this time, Dalton was unaware
that both hydrogen and oxygen gas consisted of "molecules" which were each
composed of two atoms, but his theory was correct in essence.
In 1869 Dimitri Mendeleev of Russia, combining Dalton's atomic description
with the fact that certain groups of elements had similar chemical properties,
constructed the first periodic table. He pointed out that the gaps in this
table should correspond to as-yet-undiscovered elements, and was able to
predict their properties and atomic masses. Armed with this knowledge,
scientists very quickly discovered most of the missing elements.
Darwin's Theory of Evolution
A brief mention must be made here to the theory of biological evolution,
because of its philosophical relevance to the physical idea of an evolving
universe. A basic tenet of the theory of evolution is that the world as
we know it today has evolved from an earlier form of the world under the
pressures of natural forces which were in existence at the time, such as
erosion and sedimentation, and not by divine intervention in this process.
This idea of "uniformitarianism" was first put forward by James Hutton
of Edinburgh in 1785, as an explanation for the formation of the geological
structures of the earth. He found part of his justification for this theory
in the motion of the planets, which required only the forces of nature
to keep them moving in their orbits forever. In analogy to the timeless
motion of the planets, Hutton assumed that the formation of the earth had
occurred over extremely long periods of time. Hutton's ideas were unpopular
in his time because they were perceived to be in conflict with the teaching
of the Bible. They were received little better by scientists when revived
by Charles Lyell in The Principles of Geology published in 1830-33,
but were accepted much more readily by the populace. Mason suggests that
one of the reasons for this change in reception was that the idea of the
progress of humanity, championed by such writers as Francis Bacon and the
economist Adam Smith who published An Enquiry into the Nature and Causes
of the Wealth of Nations in 1776, was now generally accepted by society.
Charles Darwin acknowledges that it was the concept of uniformitarianism
that led him to his theory of evolution, the idea that biological species
might evolve in the same way that the earth's geology did, under the natural
forces continually in existence. The part that needed to be added was the
answer to what determined the direction of this evolution. Offspring are
born with characteristics which are slightly different from those of the
parents. Darwin claimed that when these new characteristics better prepared
the organism to live to reproductive age, then it would be able to pass
these characteristics on to its children: thus, nature selected those offspring
for survival much as a cattle owner selected for breeding those animals
born with desirable characteristics. His theory did not require a reason
for the variation readily observed in offspring, although he speculated
that it might be due to changes in food or climate. However, he believed
that these changes were exceedingly slight, and could result in a new species
(a class of life that is only fertile within that class) over very long
periods of time. Knowing that his theory was in contradiction with a literal
interpretation of the Bible, Darwin spent twenty years amassing data before
the publication of On The Origin of the Species in 1859.
Although this book raised a furore when first published, the logic of its arguments and its philosophical consistency with other scientific theories gradually won the day. Indeed, evolution turned out to be a useful, though fallacious, argument for justifying both colonialism and racism. Herbert Spencer coined the phrase "survival of the fittest" to replace Darwin's "natural selection", and applied it to the evolution of society. With the idea of human progress fully ensconced in society's thinking, it was a short step to assume that the race or nationality in power deserved to be there, because it was the one most fit to rule. "Survival of the fittest" soon became "might is right", a belief which is still at work in the world today.
Modern Physics: Relativity and Quantum Physics
By the end of the nineteenth century, most physicists were feeling quite
smug. They seemed to have theories in place that would explain all physical
phenomena. There was clearly a lot of cleaning up to do, but it looked
like a fairly mechanical job: turn the crank on the calculator until the
results come out. Apart from a few niggling problems like those lines in
the light emitted by gas discharges, and the apparent dependence of the
mass of high-speed electrons on their velocity ....
Twenty-five years later, this complacency had been completely destroyed
by the invention of three entirely new theories: special relativity, general
relativity, and quantum mechanics. The outstanding figure of this period
was Albert Einstein. His name became a household word for his development,
virtually single-handedly, of the theory of relativity, and he made a major
contribution to the development of quantum mechanics in his explanation
of the photoelectric effect.
Einstein was a clerk in a Swiss patent office when he published his
special theory of relativity in 1905. He claimed in later life that the
need for this theory emerged out of Maxwell's equations. Those equations
changed their form when one rewrote them from the conventional perspective
of a person moving at constant velocity. On the other hand, our experience
tells us that we cannot tell if we are moving as long as our velocity is
constant: you can throw a ball back and forth in a rapidly moving train
car just as you can when the train is still. It is only when it accelerates
-- slows down or speeds up -- that one experiences a change. Moreover,
Maxwell's equations indicated that the speed of light did not depend on
the speed of the person measuring this speed, whereas if one throws a stone
while running, the speed of the runner contributes to the speed of the
stone. To overcome these apparent difficulties with Maxwell's theory, which
Einstein believed to describe reality correctly, he considered the effect
of two postulates. The first was that all physical phenomena must obey
the same equations for people moving at different constant velocities (the
principle of relativity), and the second was that the speed, c, measured
for light does not depend on the speed of the "observer" (the person carrying
out the measurement).
These two postulates led directly to almost unbelievable results. They
showed that the measurement of space and time depended on each other (that
the time you measured for an occurrence depended on your position), and
also depended on the speed of the observer. One immediate result is that
"simultaneity " is relative to the observer. Two "events" that occur at
the same time for one observer occur at different times as seen by an observer
in motion relative to the first, provided that the events occur at different
spatial locations; the concept of absolute time and space which had underpinned
mechanics for two centuries lay in shatters. Einstein's theory also showed
that the measured mass of an object depended on its velocity, and that
mass (m) could be converted to energy (E) according to E=mc2,
the principle behind the atomic bomb and nuclear power plants.
One of the beauties of Einstein's theory was that, as you let a body's
speed become small compared to the speed of light, the equations would
reduce to those of Newtonian mechanics. This requirement of physics, that
a more general theory must reduce in some limit to more restrictive theories,
is called the "correspondence principle". Thus we see that the development
of the special theory of relativity in no way diminishes the stature of
Newton. Although his concept of absolute space and time were incorrect,
his genius remains: Newton's mechanics is still correct except for bodies
whose speeds approach that of light.
It is important to discuss the fact that the results of the special
theory contradict "common sense": we know that we do not have to correct
our watches after we have been in a car, and that people who are running
do not appear thinner than when at rest. The problem here is that our common
sense is, by definition, the sense of how the common world works. However,
the effects predicted by the special theory are significant only at a speed
approaching that of light, and none of us has ever moved at such a speed
relative to another object with which we can interact. Therefore, we must
not assume that our low-speed common sense also applies at very high speeds.
Similarly, we will see that the mechanics governing sub-microscopic bodies
such as atoms is quite different to the mechanics describing 60-kg human
In 1887 the Americans Albert Michelson and Edward Morley had attempted
to measure the speed of the Earth through the ether by measuring the difference
in the speed of light travelling in two perpendicular directions. A difference
was expected, for the same reason that the speed of a water wave relative
to you depends on whether you are travelling in the same direction as the
wave or otherwise. They found no dependence on the direction of motion
of the light, and interpreted this null result by claiming that the Earth
dragged the ether with it. But if the ether interacted with matter in this
way, why could it not be detected directly? Moreover, the observation by
James Bradley in 1725 of stellar aberation rules out the hypothesis of
ether drag. (Stellar aberation is the apparent movement of the stars in
a small ellipse over the course of a year, because the Earth is moving
and it takes some time for the light of the stars to reach Earth.) In 1892,
Hendrik Lorentz and G.F. Fitzgerald independently hypothesized that the
size of Michelson and Morley's measuring device must depend on its velocity
so as to contract in the direction of motion exactly enough to give the
Einstein's second postulate presented yet another possibility: the measured
speed of light was intrinsically independent of the speed of the observer.
However, it went much beyond interpreting the Michelson -Morley result
and explained, for example, the experimental observation that an electron's
mass depended on its velocity. In fact, Henri Poincaré, a renowned
physicist, had suggested a year before Einstein's publication that a whole
new mechanics might be required, in which mass depended on velocity. Einstein's
theory cleared up so many outstanding problems that it was quite quickly
accepted by most physicists.
Before leaving special relativity it is important to discuss briefly
Einstein's role in the development of nuclear weapons. Nuclear fission
had been discovered in Germany in 1938, just after the invasion of Austria
by Hitler's forces. In 1939, faced with the threat that Germany would develop
a nuclear bomb, Einstein was convinced by physicist Leo Szilard to write
to President Roosevelt, pointing out the possibility and encouraging American
research in this direction. In spite of this, Einstein actively opposed
further development of nuclear weapons following the Second World War.
In fact, he and British philosopher/mathematician Bertrand Russell founded
the Pugwash organization, named after its first meeting in Pugwash, Nova
Scotia, in 1954. This organization of leading scientists throughout the
world, and its student wing, still meet regularly to discuss issues concerning
the impact of science on society, and to prepare position papers for presentation
to governments and the United Nations.
The General Theory of Relativity extended Einstein's ideas to bodies
which are accelerating, rather than moving at constant velocity. Einstein
showed that spacetime near masses could not be described by Euclidean geometry,
but rather that a geometry invented by Riemann must be used. In this way,
gravitation was shown to be a result of the curvature of spacetime in the
vicinity of mass. The general theory allowed Einstein to predict the amount
of the deflection of light in the eclipses of 1919 and 1921, a value which
agreed with that measured. However, Einstein's theory of general relativity
was not the last word on the subject. General relativity is still an active
area of research today, partly because it provides us with much evidence
on the evolution of the universe including such questions as, "Will the
universe someday begin to collapse back upon itself under its gravitational
Einstein's theories of relativity were developed in a way close to Descartes' mathematical-deductive method. The special theory came from an attempt to harmonize electromagnetic theory with the principle of relativity. The general theory evolved from trying to reconcile the fact that inertial mass, the "resistance" to the force in the equation F=ma, has the same value as gravitational mass, even though the two are totally unrelated in Newtonian mechanics. Quantum physics, on the other hand, emerged from attempts to explain experimental observations. In the late 1800s a major area of research centred on the explanation of "blackbody" radiation: a black object such as a fireplace poker, when heated until it begins to glow, emits light whose intensity depends on wavelength in a way which depends largely on the temperature of the body and little on its material of construction. Because of the universal nature of this phenomenon, it was apparent that it must depend on fundamental physical principles. In 1900 Max Planck used a "lucky guess" [Jammer p.19] to obtain a mathematical equation which fitted the experimental data accurately. Three months later he derived the expression theoretically. To do this he assumed that a blackbody contained many small oscillators which emitted the light, much the way the oscillations of electrons along a transmission antenna emit radio waves. However, he had to allow these oscillators to emit energy only at certain frequencies rather than with a continuous range of frequencies, as would be expected from classical electromagnetism. Planck had no physical basis for this assumption; it was just the only way that he could fit the data.
Einstein used Planck's idea in his explanation of the photoelectric
effect, in which electrons are ejected from a metal when it is exposed
to light whose frequency exceeds a certain value. Einstein extended Planck's
ideas on the emission of light from a blackbody to the general statement
that light, itself, came in packets of energy, or quanta (called "photons"
from the Greek "photos" meaning "light"). Each quantum has an energy E=hf,
where f is the frequency of light and h is "Planck's constant". This was
a bold move, since the work of Young and Fresnel had seemed to establish
beyond all doubt that light acted as a wave, and Maxwell's theory did not
include any mention of a particle nature to light. However, Einstein's
assumption explained the fact that even an intense light below a certain
frequency could not cause the emission of electrons: if each incoming light
quantum gave all its energy to an electron in the metal, the electron could
not escape if this energy was less than the binding energy of the electron.
This explanation dismayed Planck, who never expected his suggestion to
be applied so broadly.
In 1911 Ernest Rutherford fired very small particles, emitted in radioactive
decay, at a thin film of gold. From the scattering pattern of the particles,
he determined that the atom consisted of a small, heavy, positively charged
nucleus surrounded by very light electrons. Niels Bohr used this model
and the quantum ideas of Planck and Einstein in 1913 to explain why the
light from gas discharges was emitted at only a few, discrete frequencies;
this light formed emission "lines" of different colours when the light
was passed through a slit and dispersed by a prism. Bohr suggested that
the electrons in an atom were only allowed to occupy certain orbits of
definite radius r around the nucleus, namely orbits whose angular momentum
was given by mvr=nh/2p where m and v are the
mass and velocity of the electron, and n is an integer. When an electron
gained energy and was "excited" to a higher orbit during the gas discharge,
it could lose this energy only by falling back to one of the lower allowed
orbits, with its energy loss DE being carried
off by the emission of a quantum of light of energy f=DE/h.
The predicted frequencies for hydrogen matched the experimental values.
Beginning with the claim that mechanical models such as Bohr's were inappropriate because they tried to use the mechanics which had been developed for macroscopic bodies in situations where it might not apply, Werner Heisenberg in 1925 derived a purely mathematical theory that incorporated directly the empirical data, such as the wavelengths of spectral lines. The same year, Louis de Broglie argued that if light could act both as a wave and as a particle (photon) with definite energy, then perhaps material particles such as electrons could as well. He suggested that such a particle should have a wavelength given by l=h/mv, where m is the particle's mass and v is its velocity.
By the next year, de Broglie's hypothesis had been used by Erwin Schrödinger
to explain the quantization of Bohr's orbits. Moreover, Schrödinger
showed that his wave mechanics was equivalent to Heisenberg's theory. By
1927, C.J. Davisson and L.H. Germer had confirmed de Broglie's hypothesis
directly by producing a diffraction pattern by scattering electrons from
the ordered atoms on the surface of a nickel sample, much like the two-slit
interference pattern used by Thomas Young to prove that light behaved as
a wave. This result is impossible if we consider the electron as a classical
particle: it means that the electron must scatter off more than one nickel
atom simultaneously or, in the two-slit analogy, go through both slits
at the same time!
Rather than placing the electrons in the atom in definite orbits as
envisioned by Bohr, Schrödinger's wave mechanics, as interpreted by
Born, treated the square of the particle's wave amplitude y
as giving the probability that the electron was at a particular
place in space, with the most probable positions corresponding to Bohr's
orbits. From this discussion it is clear that we are treating the electron
as a particle and a wave. Consider Young's two-slit experiment again, but
using electrons instead of light as the incident radiation. Suppose we
position a fluorescent screen behind the two holes, and decrease the intensity
of the electron beam until only one electron hits the screen at a time.
Experimentally we see that each electron produces a tiny flash on the screen,
as though it were struck by a particle rather than a wave. However, the
number of particles arriving in a given region of the screen is greater
where the diffraction pattern has its maxima. The electron acts like a
particle when we demand a particle-like response, but like a wave when
we demand a wave-like response. This is the conclusion come to by Bohr,
in establishing his "principle of complementarity": the wave and particle
descriptions of matter (or electromagnetic radiation) are complementary,
in the sense that our experiments can test for one or the other, but never
for both properties at the same time.
In 1927 Heisenberg proved that it was impossible to determine both a particle's position and momentum with arbitrary precision; if one is known very accurately, then the uncertainty in the other becomes large. This "Uncertainty Principle" showed that there are theoretical limits on a person's ability to describe the world. The limits are not a serious consideration for large bodies, but become very important for bodies the size of an atom or smaller. The uncertainty principle also makes it clear that the presence of the experimenter always affects the results of an experiment at some level. For example, if we try to determine the position of a small particle very accurately we must, in principle, change its momentum by the very act of observing it.
Quantum mechanics has now been extended to explain a wide range of phenomena
at the sub-microscopic level, including the structure of the atomic nucleus.
Experimentally, this structure has been determined in a manner similar
in principle to Rutherford's scattering experiment, using accelerators
which produce incident particles of very high energy.
Philosophically, the developments of quantum mechanics were far-reaching.
Like relativity, they again showed that humans could not assume that the
physical laws which seem to govern a 60-kg person moving at speeds up to
several hundred kilometres per hour also applied to bodies far from this
regime. They also brought into question the assumption of the perfectly
deterministic world proposed by Laplace. Clearly it was impossible to predict
the position and velocity of every body for all future times if you could
not even know these coordinates accurately at a single instant in time.
This conclusion has even been used as the basis of the claim that humans
have free will, that all is not predetermined as would seem to be the case
in a purely mechanistic, deterministic world governed by the laws of physics.
These ideas are still heavily debated today, as in a recent article by
Roger Penrose in the book Quantum Implications.
Indeed, Einstein himself was never able to accept fully the uncertainty
implied in quantum mechanics, declaring that he did not believe that God
played dice (Clark, pp.414,415). In an attempt to show that quantum theory
was at variance with the real world, he helped develop the Einstein-Podolsky-Rosen
(EPR) paradox, a "thought experiment" which shows that quantum mechanical
theory must lead to what seems like an impossible situation: what you do
to one particle can affect a second, even if they are sufficiently separated
in space that a light signal could not pass from the first to the second
fast enough to cause the observed effect. That is, either the knowledge
of the event can travel between the particles faster than the speed of
light, or the two particles really are not separate but remain interconnected
in some fundamental sense. It was the latter option which was under debate.
An experiment designed to test this hypothesis was carried out by D.
Aspect and coworkers in 1981 [Physical Review Letters 47,460
(1981) and 49, 91 (1982)] and was shown to confirm what was predicted:
the two particles really were connected over large distances by "non-local"
forces acting instantaneously. That is, the EPR paradox, rather than showing
a basic inconsistency in quantum theory, actually points to one more aspect
of nature that contravenes common sense.
The Unification of Physical Phenomena
The work of Maxwell represents the first great theoretical unification
of physical phenomena, in this case the integration of magnetic, electrical
and optical theory into one all-encompassing framework. Again, this must
be seen as desirable under Ockham's Razor, which argues for economy of
understanding. Such economy is the strength of modern analytical science,
which emphasizes the logical description of a vast range of physical phenomena
from a few basic principles, rather than the memorization of a large number
of isolated facts or formulae. The former approach enables the user to
predict effects not seen previously, to invent, whereas the latter restricts
one to what already is known.
Other great unifications that have taken place in physics include the
integration of classical mechanics, quantum physics and heat in the development
of statistical mechanics. This subject assumes that the properties of large
systems, such as gases or solids, can be calculated by working out the
average of the properties of all their constituent particles. For example,
the relationship between the temperature and pressure of a gas can be calculated
by treating the gas as being made up of a very large number of independent
molecules, and calculating the average force they produce as they collide
with the container walls, using Newtonian mechanics for the particles.
This approach was followed for gases by Maxwell and Ludwig Boltzmann (1844-1906).
Boltzmann also showed that Clausius' entropy could be interpreted as a
measure of the disorder of a system. In particular, he proved that the
value for entropy can be obtained from a knowledge of the total number
of different states in which a system can be found. That, in turn, depends
on the number of different potential configurations of all the particles
which comprised the system. This statistical approach has led to the development
of "quantum statistics", the application of statistical mechanics to quantum
Perhaps the greatest such unification that has taken place in this century
is the integration of electromagnetism and quantum mechanics, in quantum
electrodynamics (QED). This feat earned Richard Feynman, Julian Schwinger,
and Sin-itiro Tomonaga the Nobel Prize for physics in 1965. It is capable
of predicting the spin g-factor of the electron with a numerical accuracy
of 1 part in 1010!
In 1979, Sheldon Glashow, Abdus Salam, and Stephen Weinberg were given
the Nobel Prize for their "electroweak theory" that unified the electromagnetic
and weak nuclear forces. Attempts have also been made to form a quantum
theory of the strong nuclear force. Because of its similarity to QED, it
has been called quantum chromodynamics (QCD). "Chromo" comes from the Greek
word for colour, and refers to the fact that the quarks that make up neutrons
and protons come in several varieties that have been given the names red,
blue and green, and their antiparticles. (These names have been chosen
in analogy to light. These three colours can be combined to give white
light; the three quarks combine to give a "colourless" particle.) The combination
of electroweak theory and QCD comprises what is called the "Standard Model".
Attempts are still under way to integrate QCD and electroweak theory into
a single "Grand Unified Theory" (GUT).
Much effort has also gone into trying to unify electromagnetism and
gravitation. In fact, Einstein spent most of the latter part of his life
trying to create a quantum form of the general theory of relativity. As
can be seen from these few examples, the nineteenth-century belief that
the main theoretical work of physicists was over could not have been further
from the truth!
Dissemination of the Results of Scientific Research
Written exchange of information among scientists in different countries
was common from before the time of Galileo, and books on science were published
from shortly after the development of the printing press in Europe by 1450.
Starting in 1644 in England, John Wilkins, a Puritan clergyman, organized
weekly meetings of several scientists in London, who called themselves
the "Philosophical College". They met to discuss scientific theory and
carry out experiments, first at a pub and then at Gresham College. When
the Puritans under Cromwell came to power, Wilkins was appointed the head
of Wadham College in Oxford. There he established the Philosophical Society
for the discussion of science. Under the Commonwealth, interest in science
had increased substantially, and shortly after the restoration of Charles
II to the throne in 1660 a group of forty-one persons founded a college
for scientific learning which became the "Royal Society for the Improvement
of Natural Knowledge" two years later, with about one hundred members;
John Wilkins was one of its two secretaries. This organization eventually
became the Royal Society of London, which persists to today. Similar societies
emerged on the continent. These organizations published regular journals
of the findings of their members.
Today, there are hundreds of scientific societies world-wide, some discipline-based
and national in focus such as the Canadian Association of Physicists, and
some research-area-based and very international in membership, such as
the American Vacuum Society. Most hold meetings annually or more often.
There are more than 100,000 articles published per year in physics alone.
With this enormous amount of information, it has been necessary to develop
bibliographic search tools just to enable researchers to find papers of
interest. In physics these include three major journals. Physics Abstracts,
published monthly, catalogues by subject and author almost all the articles
published in physics in the previous period. Current Contents, published
weekly, lists by journal, author and subject all papers in the main journals.
Citation Index, published monthly, lists articles covering all the
sciences, which have been published or cited (referred to) in the previous
period. This last journal enables researchers to use their knowledge of
a seminal article in a given field to find the most current related work.
These search tools have become immensely more powerful recently, with the
application of computer programs which provide rapid searching, cross-referencing
and automatic print-outs. Searching can even be done on-line using remote
Bacon's vision of the application of science for human use has been
realized this century, with tens of thousands of scientists and engineers
working world-wide to develop usable products. However, the deal has been
Faustian. We have our jumbo jets, cellular telephones, catscans, personal
computers and CD-players, all direct applications of physics which we enjoy.
We have also developed the fission bomb which killed 110,000 in Hiroshima
and similar numbers in Nagasaki, with some 2500 people continuing to die
per year for decades from radiation-related illness (the fusion bombs currently
deployed are typically 50 times more powerful); modern conventional weapons
and communications keep millions of the world's people in economic slavery;
the world's ecosystem, of which we are a part, is endangered by the pollution
resulting from our technological successes; the technologically developed
world consumes some ten times that of the lesser developed world per capita,
so limiting the economic viability of the rest of the world.
As suggested by Capra in The Turning Point, it is time to take
a lesson from the EPR paradox and consider the world more holistically.
Physics still has a powerful role to play in the evolution of our society,
and it is our individual and collective responsibility to choose its direction
The motivation for writing this paper arose from long discussions with my partner, Linda, on the need for physics students to question their role in the world. The material presented above has been chosen as that which the author has found most useful in doing this for himself. It has come from a wide variety of secondary sources, many of which are given in the attached bibliography. However, the dates and other details have been confirmed for this writing using primarily the excellent book, A History of the Sciences by Stephen F. Mason, with some assistance from Schneer's The Evolution of Physical Science. Many useful comments from Peter Dawson have been incorporated into the text.
A Partial Bibliography
Butterfield, H., The Origins of Modern Science, 1300-1800
(Clarke-Irwin, Toronto) 1977. A good discussion of the interplay between science and society.
Capra, F., The Turning Point (Simon and Schuster, New York) 1982. Reductionist vs. holistic science, from a physicist's perspective.
Clark, R.W., Einstein, The Life and Times (Avon, New York) 1971.
Cline, B.L., Men who Made a New Physics (previously entitled The Questioners) (Signet, New York) 1965. A very readable account of the origins of quantum physics and relativity.
Cole, M.D., The Maya, 3rd ed. (Thames and Hudson, London) 1984.
Dijksterhuis, E.J., The Mechanization of the World Picture (Oxford University) 1961.
Drake, S., Telescopes, Tides and Tactics: A Galilean Dialogue about the Starry Messenger and Systems of the World (University of Chicago Press, Chicago) 1983. This book includes a translation of Galileo's description of his first astronomical observations, and MUST be read. It contains copies of Galileo's original sketches of the appearance of the Moon and of the moons of Jupiter.
Drake, S., The Role of Music in Galileo's Experiments Scientific American, p. 98, June 1975.
Finocchiaro, M.A., The Galileo Affair, A Documentary History (University of California Press, Berkeley) 1989. Gives the context for Galileo's trial, and a translation of a number of the original documents.
French, M., Beyond Power (Ballantine, New York) 1985. A feminist perspective on patriarchal society.
Hawking, S.W., A Brief History of Time (Bantam, 1988). A discussion of modern cosmology for the layperson, from one of the world's experts.
Horgan, J., Quantum Philosophy, Scientific American, July 1992, p.94. A discussion of recent investigations of the EPR paradox.
Hiley, B.J. and Peat, F.D. (editors), Quantum Implications - Essays in Honour of David Bohm (Routledge, New York) 1987. An excellent but fairly mathematical consideration of the implications of quantum theory.
Kramer, E., Nature and Growth of Modern Mathematics, (Princeton University Press, New York) 1982.
Jammer, M., The Conceptual Development of Quantum Mechanics, (McGraw-Hill, New York) 1966. This book is quite mathematical.
Mason, S.F., A History of the Sciences (Collier, New York), 1962. An excellent general history, very complete.
Rossiter, M.W., Women Scientists in America: Struggles and Strategies to 1940, (John Hopkins University Press, Baltimore) 1982.
Schneer, C.J., The Evolution of Physical Science (Grove Press, New York) 1960. Greeks to modern physical science.
Tuana, N. (editor), Feminism and Science (Indiana University Press, Bloomington) 1989. Addresses gender bias in science.
Whitehead, A.N., Science and the Modern World, (Cambridge University Press) 1933.
Williams, L.P., The Origins of Field Theory (Random House, Toronto) 1966. (Not in Trent Library).