Increasing the wavelength decreases the disturbance, because photons of longer wavelength have less momentum. However, increasing the wavelength reduces the precision of the position measurement. Decreasing the wavelength allows better position measurement, but increases the disturbance to the momentum. 

   The principle that measurement disturbs that which is being measured was already well established in science. What was so radical about Heisenberg's discovery was that this quantum disturbance could not be compensated for. Heisenberg calculated that the product of the uncertainty in position and the uncertainty in momentum is never less than an amount involving h, which is Planck's constant, named after the German physicist Max Planck. If Dp is the uncertainty in a particle's momentum and Dx the uncertainty in its position then:
Dp × Dx ³ h / 4p
implying that, as one quantity is measured more precisely, the other must get more uncertain. It is impossible to specify both exactly, as h has a non-zero, although very small, value, equal to 6.62 × 10^-34 joule-seconds.

   Reaction to the announcement of the uncertainty principle was initially mixed. The Danish physicist Niels Bohr, one of the founders of quantum theory, immediately grasped its importance, but differed with Heisenberg as to the interpretation of the idea. To Bohr the uncertainty principle did not describe an inability of physicists to measure quantities, but a limitation of the way in which momentum and position could be defined. Bohr felt that physical quantities were defined by the experiments that measured them-they were not properties possessed independently by particles in themselves. In other words, when an electron's position is measured with light that disturbs its momentum, it is not that the electron has a well-defined momentum of which we are ignorant-rather, the electron does not have a well-defined momentum under those conditions.

   This idea was unacceptable to Albert Einstein, who deeply believed in the independent reality of the laws of physics. To him it was nonsense to say that such a property was defined by the way in which scientists measure it. A vigorous debate between Bohr, Heisenberg, and Einstein followed. In the end Einstein was forced to admit that quantum theory was logically consistent, but he always hoped that it would be replaced by a new theory. To date quantum theory has been tested in a variety of experiments of increasing precision and has always been shown to provide a full account of the experimental results.

   The uncertainty principle extends to other "complementary" quantities as well, such as energy and time. If the energy of a particle is measured over a time period (Dt), the uncertainty in energy (DE) is related to the duration of the measurement:
DE × Dt ³ h / 4p
Bohr extended the idea of complementarity to include concepts outside physics. For example, he suggested that a description of a living creature in biological terms was complementary to a description in physical terms: to know all about the inner workings of the organism would require taking it to pieces, which would kill it. Many other thinkers have commented on parallels between quantum mechanics and ideas of Eastern mysticism.

Wave-Particle Duality
   Possession of both wave-like and particle-like properties by subatomic objects. The fundamental principle of quantum theory is that an entity that we are used to thinking of as a particle (such as an electron) can behave like a wave, while entities that we are used to thinking of as waves, such as light waves, can also be described in terms of particles (in this case, photons).

   This wave-particle duality is most clearly seen in "double-slit" experiments, in which either electrons or photons are fired, one at a time, through a pair of holes in a barrier, and detected on a screen (like a TV screen) on the other side. In both cases, particles leave the gun on one side of the barrier and arrive at the detector screen, each making an individual spot on the screen. However, the overall pattern that builds up on the screen as more and more particles are fired through the two holes is an interference pattern, made up of light and dark stripes, which can only be explained in terms of waves passing through both holes in the barrier and interfering with each other. This gives rise to the aphorism that quantum entities "travel as waves but arrive as particles".

   Wave-particle duality is also related to the uncertainty principle. This says that the exact position of a particle and its exact momentum (essentially, its speed and direction of movement) can never be known simultaneously. Position is a particle property-particles exist at a point. Waves are extended entities by nature, which do not have a position, although they do have momentum. Entities that are both wave and particle are never quite sure either where they are or where they are going.
The wavelength l and momentum p of a quantum entity are related by the equation pl = h, where h is a constant known as Planck's constant.

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