In the context of nuclear physics, a physicist wrote in a non large audience document (not available on the web nor in books) that the free nucleons don't interact with each other while they do in matter (nucleus, nuclear matter, neutron stars, etc.)
Is it true ?
If it is true, is the difference of behaviour understood ?
Some comments ?
In the limit of low energy interactions, the nucleon-nucleon force is well-described by a series of Yukawa potentials
$$ V(r) \propto \frac{e^{-r/r_0}}{r} $$
where the constant of proportionality depends on the quantum numbers that are exchanged during the interaction. These potentials are associated with mesons, and the range parameter in the exponential shorter for mesons with larger mass, $r_0 = \hbar c / m c^2$. The least-massive strongly-interacting meson is the pion, and the Yukawa potential for pions has a range parameter of a little more than one femtometer. All of the other strongly-interacting mesons have higher masses and correspond to shorter-range forces.
What that means is that two nucleons which are separated by more than a few femtometers basically don't feel any strong interaction between them. This is part of the reason that the periodic table ends. A uranium nucleus ($A\sim 240$) has a diameter of about $\rm7\,fm$; two protons on opposite ends of a U nucleus are repelled from each other because they have the same electric charge, but they don't feel very much strong attraction because $e^{-7}$ is small.
Free protons still interact with each other via electromagnetism. Free neutrons, with no net charge and small magnetic moment, basically don't interact with their environment unless they happen to overlap with a nucleus. However, neutrons at room temperature have de Broglie wavelengths $\lambda = h/p$ comparable to usual interatomic spacings, so cold neutrons in solid matter may be coherently overlapping with (and therefore interacting with) many nuclei at the same time.
You suggest in a comment:
But academicallly speaking they still interact with an almost null interaction because of the exponential term of the Yukawa potential. Thus, is it meaning that there is not an intrinsic need of matter for nucleons to interact with nuclear interactions by exchange of pions. It is just that "concretely", the interactions of free nucleons is small because there are distant.
First, the long-distance nuclear interaction is pion exchange, just like electromagnetism is photon exchange. The Yukawa potential describes pion exchange from an energy perspective. (For that matter, electromagnetism is described by a Yukawa potential in the limit $r_0\to\infty$, because the photon is massless.)
But more importantly, by your logic, no interaction is ever zero. That's technically true, but it's not a productive way to look at the world. In an experiment we have to choose the few most important interactions, and things we haven't thought of yet present at "noise." Consider a universe that contains only two neutrons, separated by some distance $r$. The longest-range parts of the strong, electromagnetic, and gravitational interactions are
\begin{align} V_\text{strong} &\sim r^{-1} e^{-r/r_0} \\ V_\text{dipole} &\sim r^{-3} \\ V_\text{grav} &\sim r^{-1} \end{align}
At internuclear distances, the most important of these is the strong interaction. At interatomic distances, the winner is the magnetic dipole-dipole interaction. (Magnetic scattering of neutrons in matter is associated with milli-eV transitions, but that's not necessarily a purely electromagnetic interaction; the neutron's magnetic moment is about fifty nano-eV per tesla.) For any set of coupling constants, there will be some distance where the $1/r^3$ magnetic interaction becomes energetically less important than the $1/r$ gravitational attraction between them. There is basically zero circumstance where any experimental outcome is affected by the gravitational attraction between two subatomic particles (though there are some which are affected by the attraction between one subatomic particle and the entire Earth). If you are comfortable with sometimes ignoring the gravitational attraction between low-mass objects, you should be comfortable with sometimes ignoring the nuclear force.
