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History of molecular theory wikipedia , lookup

By Nienke Adamse
A theory in science is a scientific statement about empirical information,
information that is gained by means of observation or experimentation. The
purpose of such a statement is to describe and explain the correlation between
various phenomena. Even though a theory needs to be supported by many ways of
experimentation, the real accuracy of a scientific theory can never be verified.
According to Karl Popper (1902-1994), a British philosopher of science, even
though there have been numerous confirmed observations that support a theory,
there is always the possibility that the next observation does not.
History, therefore, has proven that most theories eventually need to be revised or
even replaced by other theories. Very few theories, however, in the history of
science have been completely rejected. Most theories start with a basic
understanding of a phenomenon. These theories always lead to more questions and
these questions are the main motivators for new generations of scientists to do
more and deeper research, which leads to more insights and to increasing the
complexity of the theory. One such a theory is the Atomic Theory of Matter.
Atomic Theory of Matter
The word atom is derived from the Greek word ‘atomos’, which means indivisible or
infinitesimal. Greek philosophers, such as Plato (424 BC-348 BC), thought that
matter was infinitely divisible. Democritus (460 BC- 370 BC), however, disagreed
with that statement and argued that matter was made out of indivisible particles
with a defined size, shape and mass.
It took more than two millennia before the scientist John Dalton (1766-1844) shed
more light on the world of the atoms with his work: “A New System of Chemical
Philosophy”, that he published in 1808. In this work he postulated the principles of
atomism with the statement that the natural world consists of two fundamental
parts: indivisible particles and empty void. He called the indivisible particles atoms
and he stated that each element was made of only one kind of atom with its own
unique properties and mass. He stated that the atoms are indestructible and keep
their own identity after a chemical reaction. Dalton’s theory raised the inevitable
question: “What are atoms made of?”
Several scientists had suggested that atoms were built up from a more fundamental
unit and it was Joseph Thomson (1856-1940) who discovered in 1897 negatively
charged sub-atomic particles --now known as electrons-- with his cathode rays
experiments. The question whether atoms really existed was seen as irrelevant for
the scientific world before that time, but it became the key question of 19th century
science when society (the military and industry) needed the knowledge to use the
new technology of steam engines more efficiently. In order to understand and
predict the behavior of steam, scientists became occupied with finding the truth
about atoms. Much controversy about the existence of the atom began. Some
scientists believed that atoms were mere mathematical conveniences rather than
physical objects. Ludwig Boltzmann (1844-1906), an Austrian physicist, believed
that matter ultimately is made of basic building blocks. When he was accused of
sacrilegious ideas, turning the magic of God’s creation into a matter of just collisions
between tiny atoms, he ended his life in 1906. Had he known that a year before he
died, the existence of atoms was indisputably proven by a young German scientist
named Albert Einstein (1879-1955), and that his theory was totally vindicated, he
might have had a different view on the value of his life.
Einstein, who at the age of 26 worked a very undemanding job as a patent clerk in
Vienna, had plenty of time to think deep thoughts and he published many scientific
papers within a few months. In 1905 he published a paper about the Brownian
motion (the dancing movement of pollen in water that was described by Robert
Brown (1773-1858) in 1827) and presented it as a way to indirectly confirm the
existence of atoms and molecules. Einstein’s idea was that in order for the pollen to
move they need to be pushed by smaller particles of the water. He suggested that
the water was made out of atoms or atom like particles. He also proved
mathematically that the size of an atom must be one tenth of a millionth of a
millimeter across ( 10 m). To say it differently: there are more atoms in a glass of
water then there are glasses of water in all the oceans of the world. The Atomic
Theory of Matter now verified not only the existence but also the size of the atoms.
Still the question remained: “What do atoms look like?” This question led to a new
form of physics: atomic physics.
Atomic Physics
In 1910, the world center for atomic physics was in Manchester, England. Two
famous physicists worked there: Ernest Rutherford (1871-1937), a New Zealand
born British physicist and Niels Bohr (1185-1962) from Copenhagen, Denmark.
Both scientists were prepared to disregard three centuries of scientific convention if
that did not fit what they believed to be true. Rutherford was obsessed with
radioactivity and after years of painstakingly experimenting with gold foil and
radioactive alpha rays, he was able to visualize a picture of the shape and inside of
the atom. He found that the atom looked like a planetary system with a tiny nucleus,
ten thousand times smaller than the whole atom, and electrons surrounding the
nucleus like orbiting planets. Rutherford’s model of the atom was almost entirely
empty space, if you would be able to suck the empty space out of the atoms of a
human body, you would shrink it to the size of a grain of salt with the same weight
as the whole body.
This model, however, created the next problem: according to the known laws of
science, the electrons ought to crash into the nucleus within a split second. The
established scientists of that time, including Einstein, were puzzled by this
mysterious contradiction: how can this empty atom make up a solid world?
It required a new generation of scientists, bold and brilliant, fearless enough to
disregard common sense and human intuition to find an explanation.
Bohr was determined to solve the problem of why the atoms did not collapse. He
looked for clues by studying light and he realized that the spectra (the distinctive
colors with which different substances glow) were telling something about the inner
structure of the atom. He replaced Rutherford’s planetary model of the atom with
the Bohr model. He suggested that the electrons were confined into clearly defined,
quantized orbits, and could jump between these, but could not exist in between. An
electron must absorb or emit specific amounts of energy when it jumps (a quantum
jump) between these energy levels. The larger the jump, the more energy was
emitted or absorbed. This concept was so difficult to visualize and to understand
that Bohr himself once said: “Anyone who is not shocked by quantum theory has not
understood it”. His theory was the beginning of many Bohr-Einstein debates,
because Einstein and many other older generation scientists claimed the quantum
jumps as nonsense. They thought that not being able to visualize a scientific concept
seemed to be going against the whole purpose of science. Conflict between these
two generations of scientists was inevitable.
Einstein was supported by an Austrian scientist Erwin Schrödinger (1887-1961)
and Werner Heisenberg (1901-1976), a German theoretical physicist, joined the
camp of Bohr. Schrödinger believed that the electron surrounding the nucleus
behaved like a wave of energy and he came up with a new equation, which
completely described this wave and described the atom in traditional physics terms.
The older generation of scientists loved this explanation, it allowed them to
visualize the atom in simple terms and it allowed them to use their intuition. The
progressive scientific generation, however, could not reconcile with the new theory
because it still did not account for Bohr’s strange instantaneous quantum jumps.
Heisenberg thought that the strangeness of the quantum jumps was actually the key
to understanding the atom. He believed that trying to visualize the atom with
familiar images would always fail; he described the atom using pure mathematics
alone. He realized that the atom did not only defy visualization but also traditional
mathematics. With the help of the German mathematician Max Born (1882-1970),
he developed a whole new theory of the atom. This theory was called: Matrix
Mechanics, it required a new matrix mathematics in which the order of
multiplication of the position and the speed of an electron seemed to matter.
Heisenberg described the atom as inherently unknowable. The ambiguity of the
atom --we cannot know where it is and how fast it is moving at the same time-- is
the fundamental truth about the way nature behaves at the subatomic scale. This
became known as Heisenberg’s Uncertainty Principle.
At the Solvay Conference in Brussels in 1927, all of the world’s leading atomic
physicists gathered and the battle between Einstein and Bohr ended. After a week of
brushing away all of Einstein’s criticism with convincing arguments, Bohr was
considered victorious and his theory was accepted as the Copenhagen
This interpretation is still accepted today and it is at the heart of the new form of
atomic physics: Quantum Mechanics.
This monumental scientific achievement changed our view of the world; it explains
how everything in the universe fits together. However, with accepting this theory
we need to realize that atoms are unimaginable and self-contradictory; it behaves
like a particle when you look to see where it is and it behaves like a wave when you
are not looking at it.
From then on the development of new theories were taking place at breakneck
speed: the discovery of matter and anti-matter by Dirac, the discovery of the
positron by Anderson, the physics of quantum electro mechanics (QED), the
visualization of the QED theory (much to the disgust of Bohr) by Feynman (the
Feynman diagrams), the discovery of new particles, the organization of the ‘particle
zoo’ and the discovery of the quarks by Gell-Mann. The development of new
technologies such as particle accelerators made it possible to confirm the existence
of quarks. We now know that matter is made up of only two different particles:
leptons and quarks. The deeper science digs into the tiny world of the atom the
more we discover laws that tend to defy reason and contradiction. We understand
more about how an individual atom behaves but we are still in the dark about how
all these trillions of atoms behave together and create the world around us.
According to quantum physics, an empty space is full of activity, virtual particles
come and go and the matter we see are leftovers from this virtual activity. New
theories that combine Quantum Theory with gravity such as the String Theory, the
Brain Theory and the Quantum Loop Gravity Theory are all theories that still need
to be explored. We are all waiting for the next generation of Einsteins and Bohrs.
The analogy of the development of scientific theories can be the reversed life span of
a human being. Death represents the cautious first steps of a new theory. The old
age is the status quo and the stubbornness of the existing theories and (religious)
resistance. The younger years between adolescence and adulthood mark the battle
between the different generations of scientists. The infant years are the rapid
developments of the theory as a result of a growing technology, and eventually, the
birth of the baby represents the birth and the acceptance of a new scientific theory.
It is interesting to see how generational conflicts play out in the history of a
scientific theory. The young, fearless scientists have no attachments to the old
science convention nor to the older, wise but conservative scientists who believe in
their own theories and hold on to the conventional scientific ideas. Although I have
mentioned only a few scientists in this essay who are regarded as the main
contributors or developers of the Atomic Theory of Matter, the development of a
theory is not possible without the contribution of many more scientists. Some of
them came to the same discoveries or conclusions simultaneously; others of them
contributed the small steps in thinking that ultimately influenced the scientists that
have been credited throughout the history of science.
The atomic Theory of Matter started with the simple idea that matter is built with
fundamental building blocks: atoms. Now our language is no longer adequate to
describe the sub-atomic phenomena we observe. We, according to Bohr, can only
use language as poetry when it comes to atoms. We now wrestle with all that we
have discovered and we realize how nature is really beyond our wildest
imagination. We rethink what we mean by past and future, by cause and effect,
where the universe comes from and where it is going. And who knows, one day, if
we as science teachers have done our job well, one of our next generation science
students stands up in class and says: “What the heck, Mrs. Science teacher, how
come it took us three thousand years to realize that Plato was right all along, that
matter is infinite divisible and that what we see is just a shadow of reality: an
Al-Khalili, J. (2007)Movie: Clash of the Titans. BBC
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