by a very fast quark of another impacting proton during a particle accelerator experiment. However, quark–gluon plasmas have been observed. Residual strong force Contrary to the description above of distance independence, in the post-Big Bang universe it is not the case that every quark in the universe attracts every other quark. Color confinement implies that the strong force acts without distance-diminishment only between pairs of quarks, and that in compact collections of bound quarks (hadrons), the net color-charge of the quarks essentially cancels out, resulting in a limit of the action of the color-forces: From distances approaching or greater than the radius of a proton, compact collections of color-interacting quarks (hadrons) collectively appear to have effectively no color-charge, or "colorless", and the strong force is therefore nearly absent between those hadrons. However, the cancellation is not quite perfect, and a residual force (described below) remains. This residual force does diminish rapidly with distance, and is thus very short-range (effectively a few femtometres). It manifests as a force between the "colorless" hadrons, and is known as the nuclear force or residual strong force (and historically as the strong nuclear force). The nuclear force acts between hadrons, known as mesons and baryons. This "residual strong force", acting indirectly, transmits gluons that form part of the virtual π and ρ mesons, which, in turn, transmit the force between nucleons that holds the nucleus (beyond hydrogen-1 nucleus) together. The residual strong force is thus a minor residuum of the strong force that binds quarks together into protons and neutrons. This same force is much weaker between neutrons and protons, because it is mostly neutralized within them, in the same way that electromagnetic forces between neutral atoms (van der Waals forces) are much weaker than the electromagnetic forces that hold electrons in association with the nucleus, forming the atoms. Unlike the strong force, the residual strong force diminishes with distance, and does so rapidly. The decrease is approximately as a negative exponential power of distance, though there is no simple expression known for this; see Yukawa potential. The rapid decrease with distance of the attractive residual force and the less-rapid decrease of the repulsive electromagnetic force acting between protons within a nucleus, causes the instability of larger atomic nuclei, such as all those with atomic numbers larger than 82 (the element lead). Although the nuclear force is weaker than the strong interaction itself, it is