In 1935 a Japanese physicist Hideki Yukawa suggested that exchange forces might also be used to describe the strong force between nucleons. However, virtual photons were not strong enough for this force, so a new virtual particle was needed. Yukawa used Heisenberg's uncertainty principle to explain that a virtual particle could exist for an extremely small fraction of a second. Since its time of existence is so exact, there would be a great uncertainty in the energy of the virtual particle. This uncertainty allowed the particle to be very strong only at certain times and the particles could slip in and out of existence. He also calculated that these particles should be about 250 times as heavy as an electron. Later, in 1947, the physicist Cecil F. Powell detected this particle and was called the pion.
Although the pions have been described as the transmitters of the strong force, they are not classed with the other force-transmitting particles, such as the photon or the W and Z particles. Pions are now known not to be elementary particles but composites made up of quarks. The strong force is transmitted by the pions only at the larger
