Why Is The Universe Fundamentally Left-Handed?
“It’s eminently true that we can describe the Universe as being perfectly symmetric between mirror-reflections, replacing particles with antiparticles, and interactions going forwards or backwards in time, for every force and interaction that we know of, except one. In the weak interactions and the weak interactions alone, however, none of these symmetries are conserved. As far as the weak interactions are concerned, every measurement we’ve ever made shows that Pauli would still be in disbelief today: more than 60 years after parity violation was first discovered, the weak interaction has still been shown to couple exclusively to left-handed particles.
Because neutrinos have mass, one of the most remarkable experiments to perform would be to travel extremely close to the speed of light: overtaking a left-handed neutrino so that its spin would appear to reverse from your perspective. Would it suddenly display the properties of a right-handed antineutrino? Would it be right-handed, but still behave as a neutrino? Whatever its characteristics, it might reveal new information about the fundamental nature of our Universe. Until that day arrives, indirect measurements — such as the ones happening at CERN and the searches for neutrinoless double beta decay — will be our best opportunity for discovering whether our Universe isn’t as left-handed as we presently think.”
In 1957, Wolfgang Pauli wrote to Victor Weisskopf, indignant that parity violation had been observed. “I cannot believe that God is a weak left-hander,” he wrote. More than 60 years later, every experiment we’ve performed still agrees: while all the other interactions are completely symmetric between left-and-right interactions, the weak interactions still only couple to left-handed particles. Pauli would still be displeased.
What does it all mean, and what are our best chances for discovering a right-handed weak interaction? Here’s where we are today.
