### Heisenberg uncertainty principle

**(uncertainty principle)**
(limit on observable position and momentum of an object)

The **Heisenberg uncertainty principle** is a limit on how precise
the position and momentum of an object is defined, specifically a
limit on the product of their precise values such that the more
precise the momentum is known to be, the less determined the location
is. The large mass of everyday items makes this lower limit
unnoticeably small for them, but the limit is significant at
atomic scale and smaller.
A concise statement of the principle:

σ_{x}σ_{p} ≥ ℏ/2

Since the object's momentum is directly related to its mass,
another way of stating the principle is that the definition of the object's
speed and position are limited by an amount inversely proportional to the
object's mass.
The principle is derived from quantum mechanics (QM), essentially
a theorem that can be used in QM calculations, and which can be thought
of as one of QM's weird aspects. The question of
whether the object really has no precise position and momentum
versus the notion that we are merely unable to determine both (hit
the object with a photon to attempt to locate it and you've
pushed it a bit) is under continual discussion, and depends upon
interpretation of QM, but whatever the answer, it must be consistent
with the established fact that putting a particle near a barrier
such that this principle suggests it might be on the other side of
the barrier does sometimes result in the particle being there
(i.e., resulting in quantum tunneling), which is not merely observed
in the lab, but used in everyday technology such as **tunnel diodes**.

(*physics,quantum mechanics*)
**Further reading:**

http://en.wikipedia.org/wiki/Uncertainty_principle

**Referenced by pages:**

electron degeneracy

ground state

line broadening

quantum fluctuations

quantum mechanics (QM)

quantum tunneling

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