----------------------------------------
What are 'bare' and 'dressed' particles?
----------------------------------------

A bare electron is the formal entity discussed in textbooks
when they do perturbative quantum electrodynamics. The intuitive
picture generally given is that a bare electron is surrounded
by a cloud of virtual photons and virtual electron-positron pairs
to make up a physical, 'dressed' electron. Only the latter is real
and observable. The former is a formal caricature of the latter,
with paradoxical properties (infinite mass, etc.).

On a more substantial level, the observable electrons are produced
from the bare electrons by a process called renormalization,
which modifies the propagators by self-energy terms
and the currents by form factors. As the name says, the latter define
the 'form' of a particle. (In the above picture, it would correspond
to the shape of the virtual cloud, though it is better to avoid
giving the virtual particles too much of meaning.)

The dressed object is the renormalized, physical object,
described perturbatively as the bare object 'clothed' by the
cloud of virtual particles. The dressed interaction is the 'screened'
physical interaction between these dress objects.

To draw an analogy in nonrelativistic quantum mechanics
think of nuclei as bare atoms, electrons as virtual particles,
atoms as dressed nuclei and the residual interaction between atoms,
computed in the Born-Oppenheimer approximation, as the dressed
interaction. These were the situations physicists had in mind when 
they invented the notions of bare and dressed particles.
Of course, it is only an analogy, and should not be taken very 
seriously. It just explains the intuition about the terminology used.
The modern word for ''dressed'' is ''renormalized'', which is silent
about assigning some (often unjustified) sort of virtual physical 
reality to the bare part of the model under consideration.
For the serious version of renormalization, see the chapter on 
renormalization in this FAQ and my tutorial
    A. Neumaier,
    Renormalization: An elementary tutorial
    http://www.mat.univie.ac.at/~neum/papers/physpapers.html#ren

To give a specific example in which the bare level still makes sense, 
we take Argon atoms, whose measurable renormalized interaction is 
something close to a Lennard-Jones potential, while the underlying 
model consists of bare atoms and bare electrons interacting to a good 
approximation by Coulomb repulsion terms. As always when the bare level
has a physical interpretation, no divergences appear in the 
derivation of the Lennard-Jones residual force from the Coulomb forces. 
But divergences appear when trying to derive it directly from QED 
in a naive way, since naive QED starts from bare electrons with very 
unphysical properties.


The electrons in QM are real, physical electrons that can be isolated. 
The reason is that they are good eigenstates of the Hamiltonian. 

On the other hand, virtual particles don't have this nice attribute 
since the relativistic Hamiltonian H from field theory contains 
creation and annihilation operators which mess things up. 

The bare particles correspond to 1-particle states in the Hilbert 
space (though that is not quite true since there is no good Hilbert 
space picture in conventional interacting QFT). Multiplying them 
with H introduces terms with other particle numbers, hence a bare
particle can never be an eigenstate of H, and thus never be
observable in the way a nonrelativistic particle is.

The eigenstates of the relativistic Hamiltonian are, instead, 
complicated multibody states consisting 
of a superposition of states with any number of particles and 
antiparticles, just subject to the restriction that the total quantum 
numbers come out right. These are the dressed particles.

For the computational side of dressing, see, e.g., nucl-th/0102037,
or http://www.geocities.com/meopemuk/