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By the late 19th cent. most of classical physics was complete, and
optimistic physicists turned their attention to what they considered minor
details in the complete elucidation of their subject. Several problems,
however, provided the cracks that eventually led to the shattering of this
optimism and the birth of modern physics. On the experimental side, the
discoveries of X rays by Wilhelm Roentgen (1895), radioactivity by A. H.
Becquerel (1896), the electron by J. J. Thomson (1897), and new
radioactive elements by Marie and Pierre Curie raised questions about the
supposedly indestructible atom and the nature of matter. Ernest Rutherford
identified and named two types of radioactivity and in 1911 interpreted
experimental evidence as showing that the atom consists of a dense,
positively charged nucleus surrounded by negatively charged electrons.
Classical theory, however, predicted that this structure should be
unstable. Classical theory had also failed to explain successfully two
other experimental results that appeared in the late 19th cent. One of
these was the demonstration by A. A. Michelson and E. W. Morley that there
did not seem to be a preferred frame of reference, at rest with respect to
the hypothetical luminiferous ether, for describing electromagnetic
phenomena.
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