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The Uncertainty Principle
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In order to predict the future position of an object, be it a baseball or an
electron, you need to know its position, its momentum and its direction. A
baseball in flight can be checked for each of these factors and its ultimate
fate easily determined, as any outfielder can attest. Electrons and other
subatomic phenomena are more slippery, due to the fact that they have the
nature of a wave as well as the nature of a particle. For an object to be
seen, it must give off light. Physics tells us that for an electron to be
observed, a photon must be bounced off it. This is duly detected by a
microscope--but then the photon causes the electron to recoil in a direction
which cannot be known to us without making another observation. Thus, an
electron cannot be observed without changing its state.
The dual wave-particle nature we use to describe these micro-entities is to
blame for our frustration. If the microscope lens has a large diameter, the
image on the film will be small, but due to the wave nature of the photon it
will make an image somewhere within one wavelength-not an exact point that
can be replicated, but a general cloud of spots will appear if the
experiment is repeated. It is possible to know the speed of the electron's
recoil, but it is impossible to know the exact direction of the photon upon
entry into the wide lens. Thus momentum is fairly certain, but position is
not, so we can't know the direction of the electron's recoil. If the lens is
narrowed, the wave property of the photon causes it to diffract, making its
wavelength much greater, so while the momentum of both photon and electron
can be pinned down, the greater wavelength makes it impossible to accurately
assess where the electron actually was. In a nutshell, the wide lens allows
for an accurate measure of the electron's position, but makes it impossible
to know where it is going, while the narrow lens reduces momentum
uncertainty while making knowledge of the position less accurate.
There is an inverse proportional relationship between knowledge of position
and momentum which seems to preempt any attempt at prophesying the future of
a single atom. The mechanists' old dream of plotting the future vector of
each particle in the universe was destroyed, but this was not the most
disturbing feature of the new atomic model. What really astonished the
physicists of the 1920's, when quantum theory assumed the shape it still has
today, was the fact that the existence of the phenomena they were studying
was dependent on their consciousness cognizing the event.