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 photon may be detected by a microscope, which gives us information on where the electron was - but then the photon causes the electron to recoil in a direction that 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. (21)
There is an inverse proportional relationship between knowledge of position and momentum that 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 implication that the existence of the phenomena they were studying was dependent on the event of their observation.
Copyright © 2005 Dan Haig