“Suppose the TV-news suddenly reported one evening that visitors from outer space were planning to land on Earth; that the space travellers have radioed a demand for immediate information about the composition of the Earth. Does it consist of Matter or Antimatter? The answer to this question is one of life and death. The two kinds of matter are known to annihilate each other atom by atom. The space travelers claim, furthermore, that the nature of their own kind of matter was determined before leaving. What they now want to know is, whether the same tests have been made on Earth. ”
from The Nobel Prize in Physics 1980 Award Ceremony Speech
In a nutshell, the answer is “Yes”. A difference in behavior between matter and antimatter has been observed in various experiments. However, it’s rare and very subtle. It cannot be observed in everyday life, and not even in physics labs.
The laws of classical physics are symmetric in space and time – they do not change when all directions are reflected (parity symmetry), nor distinguish between forward and backward movements (time symmetry). There is also the particle-antiparticle symmetry: the sign of charge is flipped between a particle and its antiparticle. Parity (P) and charge (C) symmetries are conserved in electromagnetic interactions, but not in weak interactions (e.g. interactions responsible for radioactive decay). However, even in weak interactions, the combination of charge and parity symmetries (the CP symmetry) is conserved for the building blocks of atoms (electrons, neutrons and protons). Since there was no reason to believe that Nature has preference for either matter or antimatter, it was assumed that CP symmetry is always conserved.
The first evidence that CP-symmetry could be broken came in 1964 from an experiment that studied the decays of neutral kaons. The Nobel Prize in Physics 1980 was awarded to James Cronin and Val Fitch “for the discovery of violations of fundamental symmetry principles in the decay of neutral K-mesons.”
From 1990s and on, other CP violations have been observed. Still, nowadays more experiments are conducted and planned. Why do physicists put so much effort in this quest?
Despite the quote in the beginning of the post, physicists aren’t concerned that visitors from outer space might be composed of antimatter. As John Ellis wrote in 1999 article “Why does CP violation matter to the universe?”:
“The visible universe is composed of matter particles – protons, neutrons and electrons – rather than their antimatter partners – antiprotons, antineutrons and positrons. If the Moon were composed of antimatter, then lunar probes and astronauts would have vanished in a fireball of energy as soon as they touched the lunar surface. The solar wind and cosmic rays do not destroy us, implying that the Sun and the Milky Way are also made of matter.”
So, why does the disparity between matter and antimatter pose a problem?
In the prelude to The Mystery of The Missing Antimatter, Helen Quinn and Yossi Nir explain:
“Our theories suggest that, at very early times in the development of this Universe, matter and antimatter, all possible types of particles and antiparticles, existed equally in a hot, dense, and very uniform plasma. If equal amounts of matter and antimatter had persisted, then today the Universe would be a very dull place. At the early high temperatures, creation and annihilation of energetic matter and antimatter particles would have served not only to keep their numbers equal, but also to keep their numbers large. However, as the universe expanded and cooled, it reached a stage where annihilation could still occur when a particle met an antiparticle, but the reverse process, creation of a particle and an antiparticle, became more and more rare. There was essentially no radiation remaining with sufficiently high energy to cause it. Gradually all the particles and antiparticles would have disappeared. The Universe would have no visible objects in it.
Today, however, we do see a universe with huge structures made of matter: earth, solar system, galaxies, clusters of galaxies; all matter, with very little antimatter…All these…would not exist today if somehow matter had not won out over antimatter at some very early time in the evolution of the Universe.
How and when did the histories of matter and antimatter take such different courses? This is one of the great mysteries of science today.”