When two objects are near each other, we say that they exchange heat energy by radiation and absorption. When they come into physical contact with each other, we say they exchange energy by conduction (not to imply that this stops them radiating or absorbing).
But given that physical contact between two objects is not really as intimate, at an atomic scale, as we think of it in an everyday sense, is the heat still moving between the objects by radiation and absorption (of photons)? If not, what’s actually happening? (direct interaction between electromagnetic fields? - if so, is this still mediated by photons?)
(to keep things simple, let’s say we’re talking about two objects in a vacuum, in the dark, completely isolated from external influences - even if that setup is impossible in the real world)
Pretty much everything in everyday life that doesn’t just serve to keep your feet on the floor, including physical contact (which keeps your feet from going through the floor), is due to electromagnetic interaction, which is indeed mediated by photons; however, heat is really a macroscopic notion, depending on the macroscopic notion of a ‘system’ where energy can be dissipated into unobservable microscopic degrees of freedom, which you don’t really have in the microscopic picture – everything there pretty much simplifies to a transfer of kinetic energy.
I’m pretty sure that most of the energy transfer for objects in contact is via direct electrostatic energy transfer. Heat energy is a measure of motion within a body, usually by vibration. When two bodies are in contact, the electrostatic forces of the surface atoms exert a force on each other, ultimately preventing the two bodies from passing through each other. If one of those bodies is hotter than the other, then the pattern of electrostatic forces will be vibrating, exerting a changing set of forces on the surface of the other body. This will transfer energy from the hotter to the cooler body. Of course (as you note), at the atomic level the physical contact area is very small, so surface smoothness and plasticity comes into play (this is why we use heat sink compounds, which contain rigid heat conductive particles that fill the gaps between surfaces). In a vacuum, there are no trapped gasses to mediate transfer. Energetic electrons can directly transfer energy, too, but is the gap is too big for a direct jump, then the electron drops back to a lower energy level by emitting photon radiation.
If you want to be quantum about it, the electromagnetic forces between the vibrating atoms in two objects in contact are mediated by virtual photons, but it doesn’t make sense to call this radiation and absorption. For radiation and absorption you need real photons.
All the forces involved in heat and heat transfer are mediated by photons, but this doesn’t mean that they are all accounted for by black body radiation of photons.
In a solid heat transfer can be via vibrations carried by electrostatic forces - which vibrates the whole atom - which can be usefully modelled by a virtual quantum particle - the phonon, and by direct movement of electrons acting on one another - much as current flows, and which is a mechanism confined to electrical conductors (i.e metals.)
The radiant heat transfer is mediated by photons, but these photons are emitted and absorbed as a black body.
A thought experiment. In the near field - where the distance between two extended objects is much less than the size of the object, the fall off in radiant energy with distance is unity. That is, if you double the distance, the energy density remins the same, and if you halve it, it remains the same too. (It is only for point sources that the inverse sqaure law holds.) Thus if you have two large objects that have perfectly flat surfaces, parallel to one another and reasonably close, they are in one another’s near field, and if you bring them closer and closer together the coupling via radiant energy will remain the same. If thermal conduction is mediated by an equivalent of radiant energy in a solid, we would expect that the thermal conductivity of the two objects separated by a small gap (and in a vacuum if we want) would be the same as if they were touching. Since it isn’t, we can assume that the hypothesis is disproven.
You could also contemplate the question about what happens if you polish the surfaces to a high lustre reflectivity.
I don’t think it’s right to say that physical contact forces all involve electromagnetic force. Atoms and molecules push against one another according to the Pauli exclusion principle that prevents fermions from occupying identical quantum states, and not just due to any of the “fundamental” forces. If you look at the Lennard-Jones potential function, the softly tapering forces at larger distances are electromagnetic, but the steep crowding force is Pauli exclusion. That’s why atoms take up space in the first place.