The Pauli exclusion principle
http://en.wikipedia.org/wiki/Pauli_exclusion_principle
> The Pauli exclusion principle is the quantum mechanical principle
that no two identical fermions (particles with half-integer spin) may
occupy the same quantum state simultaneously.
> The Pauli exclusion principle helps explain a wide variety of
physical phenomena. One particularly important consequence of the
principle is the elaborate electron shell structure of atoms and the way
atoms share electrons, explaining the variety of chemical elements and
their chemical combinations. An electrically neutral atom contains bound
electrons equal in number to the protons in the nucleus. Electrons,
being fermions, cannot occupy the same quantum state, so electrons have
to "stack" within an atom, i.e. have different spins while at the same
place.
> Astronomy provides a spectacular demonstration of the effect of the
Pauli principle, in the form of white dwarf and neutron stars. In both
types of body, atomic structure is disrupted by large gravitational
forces, leaving the constituents supported by "degeneracy pressure"
alone. This exotic form of matter is known as degenerate matter. In
white dwarfs atoms are held apart by electron degeneracy pressure. In
neutron stars, subject to even stronger gravitational forces, electrons
have merged with protons to form neutrons. Neutrons are capable of
producing an even higher degeneracy pressure, albeit over a shorter
range. This can stabilize neutron stars from further collapse, but at a
smaller size and higher density than a white dwarf. Neutrons are the
most "rigid" objects known; their Young modulus (or more accurately,
bulk modulus) is 20 orders of magnitude larger than that of diamond.
See:
http://en.wikipedia.org/wiki/Pauli_exclusion_principle
The Question for you is does the quantum state of an electron in
an atom "over there" effect the quantum state of an electron in
an atom over here?