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Lise Meitner (1878-1968) was Otto Hahn's collaborator
for over thirty years until she had to leave Nazi Germany just before
World War II and was replaced by Strassmann. A share of Hahn's Nobel prize
eluded her, as well. She was, however, the first to publish a report of
nuclear fission from her refuge in Denmark, alerting the scientific community
to this momentous development.(Detail)
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Hideki Yukawa's (1907-1981) 1934 theoretical calculations
on the forces binding the atomic nucleus predicted the existence of new
subatomic particles, now called mesons. These particles were expected
to be 200 to 300 times as heavy as the electron and because of their mass
act at very short intranuclear distances. Many different types of these
highly unstable particles which may be charged or neutral were later observed
experimentally, including resulting from cosmic rays, and their study
has proved to be of fundamental importance for the understanding of the
forces acting in the atomic nucleus. Yukawa won the 1949 Nobel prize in
physics. |
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The Cerenkov effect manifests itself by the emission
of light when a charged particle, the result of radioactive decay, traverses
an inert medium, such as water. It is possible that such particles may
travel through the material with a speed faster than light, whose speed
is limited by the refractive index of the substance. The interaction of
the particle's electric field with the molecules of the material causes
the emission of light, most commonly a bluish glow, as in the case of
water. Any college student who has toured Brookhaven will not forget the
mysterious blue glow of the reactor pool. (See the center stamp under
Otto Hahn above). This effect was studied in various fluids by the Russian
physicist P. A. Cerenkov, who showed that it was not the result of fluorescence,
and was later explained by the theoretists I. Y. Tamm and I. M. Frank.
It is analogous to the phenomenon of the sonic boom, where an aircraft
travels at a speed faster than the speed of sound in air. The 1958 Nobel
prize in physics was awarded jointly to these three scientists. |
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Enrico Fermi (1901-1954) (Detail)
produced thermal neutrons by slowing them down in water, and then used
them to start the first self- sustainig chain reaction in uranium, at
the University of Chicago under the football stadium in 1942; it was the
beginning of the nuclear age. Very few stamps show the powerful image
of a nuclear explosion. The accompanying mushroom cloud is generally stylized
in deference to the real events in Hiroshima and Nagasaki in 1945. The
Mexican stamp at left commemorates the tenth anniversary of the nuclear
nonproliferation treaty in the Americas. The green living tree's shape
is mirrored by what might result if nuclear weapons are unchecked -- a
doomed earth in the lurid glow of destruction. |
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 Richard Feynman (1918-1988) was
an American physicist and Nobel prize winner who worked in QED, quantum
electrodynamics. Feynman diagrams were his way of representing qed events
and processes to make them more easily understood. However, the American public will remember him as the physicist
who, before a Congressional investigative committee, cut through red tape
and obfuscation to pinpoint the cause of the Challenger disaster - the
blowup of the shuttle right after lift-off - with a simple demonstration
involving an O-ring and a glass of ice water. The defective stamp at left showing
the doomed space vehicle, or one just like it, may serve as a reminder that great theoretical minds can also have keen commonsensical insights.
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Physicist Werner Heisenberg (1901-1976) derived
the uncertainty principle named after him, which states that the product
of the changes in momentum and position of a particle must be greater
than Planck's constant/2 pi, meaning that both its position and momentum
cannot be known simultaneously with complete accuracy. He invented a matrix
mechanics that was a non-commutative algebra of probability amplitudes.
For this work he received the Nobel prize in physics in 1932. 
