pi

Hideki Yukawa predicted the pion in 1935. Inside the nucleus of an atom these particles should help the harons sticking together, he reasoned. The particle could not be detected by its mass or otherwise, since it travels a very short distance and and exists very briefly. This is is the principle of inaccuracy that Heisenberg brought to us. If we had a complete emptiness, some energy could still momentarily exist there, if we can not detect it. Physics is about things that can be measured. Hope you see the paradox! However this short-time-existence against the first principle of thermodynamics seems to hold. A particle like the one Yukawa had predicted was found by Cecil Powell in 1947: the pi-meson. It contains two quarks and is electrically charged or neutral. pion (image from the Internet)