Human sight depends on response to subatomic photons traveling with the energies of visible light,
yet paradoxically the atom's interior cannot be seen. Exploration into the domain of the ultrasmall
is by theoretical physics, elegant mathematics, and giant machines. Within this strange
territory where
the raw materials for existence are processed matter
is bound energy, particles and waves are one, and
reality itself is a kind of uncertainty.

Through the Ages,
The discovery of the Atom

"UNCUTTABLE" is the literal meaning of the word atom in ancient Greek.
Democritus advanced the idea of atoms more than 2,000 years ago, rivaling the
view that the world was composed of air, earth, fire, and water.

Isaac Newton's work on light, which he conceived as being made of "corpuscles," or particles although later theory held that light was made up of waves. Chemist John Dalton argued that for each chemical element there is a corresponding atom, and that all else is made from combinations of those atoms. The discovery of radioactivity by Henri Becquerel, elaborated by Marie and Pierre Curie, indicated that atoms had internal structure. Work on cathode rays by J. J. Thomson, which identified electrons, charged particles much smaller than the hydrogen atom.

The uncuttable was cut; there was a subatomic world.


Scientists had difficulty explaining how a heated object like a black chunk of
iron could glow in the colors observed. Max Planck's assumption Energy
was not exchanged in a continuous flow but by individual packets,
or quanta energy moved not like a river but like raindrops



Albert Einstein was developing what would become his special relativity theory equating energy
and mass. That same year he proposed that light was itself quantized, or particle like, to explain
how electrons were emitted when light hit certain metals. His work was verified by
Robert Millikan, who also measured the negative charge of the electron.

Ernest Rutherford beamed charged particles at gold foil. He observed that while most particles went through the foil, some bounced back. He theorized that atoms must be mainly empty space with a small nucleus in the center. That being so, electrons must orbit the nucleus; yet wouldn't charged electrons lose all their energy and fall into the nucleus? Niels Bohr proposed that electrons behaved in quantum fashion. They remained in fixed orbits and moved from one orbit to another in quantum leaps when they emitted or absorbed energy.

THE NEUTRON, the atom's last part until accelerators revealed more.

As quantum theory was developed, the very nature of reality seemed elusive. Werner Heisenberg formulated the uncertainty principle: It is impossible (not just technically difficult) to measure simultaneously both the precise momentum and position of a subatomic particle.

Wolfgang Pauli proposed in the exclusion principle that no two electrons could occupy the same orbit, a theory necessary to understand the chemical bonds between atoms. Were electrons actually waves? Erwin Schrodinger's work suggested they were, but Max Born theorized that the wave idea was useful for describing the probability for an electron's location. Paul Dirac's mathematics predicted antimatter.

Taken together, these and other aspects of quantum theory postulate that observation not only affects reality but in a way creates it. We can choose to measure light as particles or as waves. Physicists now restrict subatomic particles to three families: gauge particles, quarks, and leptons. The desire for ultimate simplicity remains elusive. Niels Bohr is quoted as saying:

"It is wrong to think that the task of physics is to find out how Nature is.
Physics concerns what we can say about Nature"

A Unification of Forces



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