Heisenberg's Microscope
In his draft paper explaining his
Uncertainty Principle,
Werner Heisenberg proposed a way to visualize the inability to simultaneously
measure the position and momentum of an electron.
Heisenberg's example was a simple microscope that would allow us to see the electron. The problem is that if we use visible light to observe the particle, the wavelength of the photon is very much longer than the dimensions of the particle. Our image is at best fuzzy.
If on the other hand we use light with a wavelength comparable to the dimensions of the particle (Heisenberg noted that these would be gamma rays), such photons are extremely energetic, and the collision with the particle would cause the position and momentum of the particle to be changed dramatically.
Heisenberg said (erroneously as it turns out) that the act of observing the electron "disturbs" it and causes the resulting uncertainty. Niels Bohr embarrassed Heisenberg with his frank criticism and public discussion of Heisenberg's error in Bohr's 1927
Como Lecture
In the first years of interpreting quantum mechanics,
Neils Bohr and Heisenberg thought the uncertainty was epistemic, the result of our inability to measure accurately, a consequence of human ignorance, rather than something intrinsic to nature.
After Einstein in 1935 formalized his criticisms of quantum mechanics in the thought experiment known as the
Einstein-Podolsky-Rosen experiment, Bohr changed his mind and announced that the "uncertainty" was not our uncertainty, but an intrinsic "
indeterminacy" in the nature of the particles themselves, a consequence of their dual wavelike nature. Indeterminacy (
Unbestimmtheit) was the word Heisenberg had originally used for his principle.
The correct view is that the indeterminacy is intrinsic to the electron and has nothing to do with our attempts to measure it.
Many quantum processes in nature are equivalent to "measurements" and they need no conscious observers to make them happen.
In recent years, Gerhard Rempe has shown that simply identifying a particle by putting it into an excited state is enough to destroy the interference pattern in a
two-slit experiment. The photon can be a tiny fraction of the energy and momentum of the particle (Rempe used heavy Rubidium atoms), yet the interference pattern is destroyed completely.
For an extensive description of the Heisenberg Microscope and its significance see this explanation of
Heisenberg's Microscope at the American Institute of Physics.
Note that the AIP agrees that Heisenberg's Microscope is no longer considered the correct explanation for the Heisenberg-Bohr indeterminacy-uncertainty principle.
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