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This question is basically a spin off from my friend's question - "What are we basically seeing? Is it the atom or the nucleus?" (He had referred to the huge amount of empty space inside the atoms). I replied that it must be the electronic transitions between different orbitals. It gets energy from outside, it then goes in some orbit inside, but it is unstable there, so it releases energy and comes back to its original orbit. If the corresponding wavelength matches a particular colour, we detect it. (Typical explanation based on Bohr's model).

Now another question arises that what we must be touching (suppose if we touch the keyboard), because most of the space inside the atom is empty. The size of the nucleus is almost 100,000 times less than the atom. So are we touching the atoms or the nucleus? Or the lattice arrangement of atoms? What about liquids then?

Another thing I know is that frictional & other contact forces have electromagnetic origins but I don't know exactly how. Can someone please explain?

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    $\begingroup$ Related: physics.stackexchange.com/q/23797/2451 and links therein. $\endgroup$ – Qmechanic Oct 20 '15 at 18:19
  • $\begingroup$ Don't think of atoms as "empty space". Think of them as electron orbitals with a teeny-tiny nucleus at the center. Nearly all of the mass of an atom is contained in the teeny-tiny nucleus, but virtually all of the other physical properties of ordinary matter are due to the electron orbitals. When you touch a surface, the force that you feel is due entirely to the interaction of electrons belonging to the atoms of your finger, interacting with electrons belonging to the atoms of the surface; and when you see a surface, it's photons interacting with electrons. $\endgroup$ – Solomon Slow Oct 20 '15 at 23:02
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When it comes to atomic scales the traditional separation into what is sensed and what senses it essentially loses its meaning, and to some extent so does the question of what is sensed. What is detected depends on the mode of interaction with the matter, technically we do not "touch" or "see" quantum objects, but participate in processes in which they also participate. If the mode of interaction is to hit atom with a photon then indeed an electron gets excited, jumps to a higher energy level and then drops releasing another photon, which is what we "see". Normally we do not hit them ourselves but rather sunlight does, and our eyes detect the "reflected" photons.

But Bohr's atom is only a semi-classical approximation, and so is the picture where atoms and electrons are "objects" in the void that we "hit" with other "objects". Already in full blown quantum mechanics they are distributed clouds more concentrated in some regions than others interacting with other such clouds all over the space. And in quantum field theoretic picture interaction of light and matter is even more complicated, and involves superpositions that are neither fully light nor matter. In a way the question of "seeing" is dissolved.

As for touching, what we feel derives from resistance to pressure, which may be a side effect of intermolecular forces, not atoms or nuclei, they counter our attempt to push atoms apart. Or it may be spin interaction of electron clouds if atoms get close enough together, the semi-classical description is given by the Pauli exclusion principle. How intermolecular forces are understood again depends on the level of description. Classically they are fields permeating space, in perturbative quantum field theory they are artifacts of particle exchange between constituents of atoms or molecules, with which our touching interferes, and in the most general description there is no separation into a toucher and a touchee, we are parts of a single interacting system, mostly localized in space.

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