Let's say you have a magnet levitating off a
metallic surface. Classically, there is no work
done so there is no change in the energy state of
the magnet. We can expect the magnet to float
forever. Quantum mechanically, the electrical
repulsion is produced by the exchange of virtual
photons. A naive picture of this imagines the
virtual photons to be like bullets: the magnet
stays aloft by shooting these photons like bullets
off the surface. We can't expect the magnet to
have an infinite supply of bullets. However,
unless somehow the magnet never runs out of
bullets,(which makes you wonder about energy
conservation), the magnet would eventually settle
to the ground. So, what is the correct take on
this situation? Thank you. 
Answer 1:
First of all, it turns out a stationary magnet
can't levitate over an ordinary metallic surface
(and by "metallic," I assume you mean
"ferromagnetic" or something like that); it turns
out getting stable magnetic levitation requires
either superconductors, some kind of feedback
mechanism (like a computer controlling an
electromagnet), a diamagnetic material (that's a
material which is repelled by magnetic fields,
instead of attracted to them), or motion (like a
spinning, magnetized top).
Each of these mechanisms of levitation works on
a different principle, but I don't think any of
them get at the crux of your question. In short,
it seems like you're asking for a fundamental,
quantum mechanical picture of how the
electromagnetic interaction works. It's true that
we (physicists) often say that photons mediate the
electromagnetic force via the exchange of virtual
photons between electrical charges, but this isn't
"fundamentally" true. The picture of virtual
photon exchange comes from quantum field theory,
where we use pictures called Feynman diagrams to
describe the interactions between particles.
These Feynman diagrams are where the notion of
virtual particles comes from, but the important
thing to bear in mind is that the "real" physical
phenomena correspond to adding up an infinite
number of these Feynman diagrams. While the
picture that the Feynman diagrams give us is very
useful, we should really think of them as an
approximation to the "true" physics, and not as a
description of what's fundamentally going on.
So, if the virtual particles in Feynman
diagrams are only an approximation, what's the
"true" physical picture? Well...we don't know!
To understand what's "really" going on in the
physics, we'd need to be able to solve quantum
field theory exactly. But the whole reason we use
Feynman diagrams is that solving problems in
quantum field theory exactly is notoriously
difficult (and often impossible), so we make do
with the approximate approach that the Feynman
diagrams provide us.
So unfortunately I can't give you an exact
answer. I can tell you that for macroscopic
objects like a magnet, there's absolutely nothing
wrong with ignoring quantum mechanics and just
thinking of classical (i.e. nonquantum)
electromagnetic fields and ignoring the issue of
virtual photons entirely. If you do want to think
of virtual photons, I can tell you that these
virtual photons do not need to have positive mass
or energy, so you don't need to worry about energy
conservation (that's what it means for a particle
to be "virtual").
Anyway, I'm sorry I couldn't give a better
answer, but you're really pushing at the limits of
our current knowledge! Good question!
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