No man is an island, entire of itself, said poet John Donne.
And no atom neither. Even in the middle of intergalactic space, atoms feel the
electromagnetic field---also known as the cosmic microwave background---left
over by the Big Bang. The cosmos is filled with interactions that remind atoms
they are not alone. Stray electric fields, say from a nearby electronic device,
will also slightly adjust the internal energy levels of atoms, a process called
the Stark effect. Even the universal vacuum, presumably empty of any energy or
particles, can very briefly muster virtual particles that buffet electrons
inside atoms, further shifting their energies; this form of self-interaction is
known as the Lamb shift.