Other topics:
Emergence
Strings
Gravity and Cosmology
Condensed Matter Physics
Nanoscience
Quantum Mechanics
Elementary Particles
Quantum Fluids
ELEMENTARY PARTICLES
The beginning of the 20th century was heralded by the discovery of the idea of the quantum (by Planck in 1900) and by the theoretical proof that matter was made of atoms (by Einstein in 1905). Thus was vindicated the Ancient Greek idea of 'fundamental building blocks' (atomos), but with a twist- these had to be quantum mechanical. Quantum mechanical particles are seen as 'probability waves', governed by wave-particle duality instead of as hard little balls- but one feature of the old ideas of Democritus has survived. This is that their structure is governed essentially by geometrical considerations. In modern theory all particles/waves arise as possible oscillations of some 'field' (like waves on a string or on a water surface). Only certain waves are allowed, and these are the 'elementary particles'.
As physics advanced in the 20th century, bigger and
bigger machines were built to probe the structure of
atoms and of the nucleus. A bewildering zoo of
different 'elementary' particles was found- it was not
clear which of them really were elementary. Finally, in
the 1960's and 1970's, 
brilliant theoretical
work allowed a unified picture of all the particles
except those connected with gravity -- this 'standard model' of
particle physics has now survived all tests for 35
years. The fundamental particles are 'heavy' particles
(the quarks), from which matter is constructed, and
other particles like photons or gluons, which tie the
heavy particles together.
The big blot on this beautiful picture is that it does not include gravity -- the problem is that we cannot yet find a theory which unifies gravity with quantum mechanics. This is the task which string theory has set itself.
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