Here are two more open questions about physics. I have a question of my own at the end!
Why are the laws of physics not symmetrical when we switch left and right, or future and past, or matter and antimatter? Why do the laws of nature even violate “CP symmetry”? That is: why are the laws not symmetrical under the operation where we simultaneously switch matter and antimatter and switch left and right?
Violation of P symmetry, meaning the symmetry between left and right, is strongly visible in the Standard Model: for example, all directly observed neutrinos are “left-handed”. But violation of CP symmetry is subtler: in the Standard Model it appears solely in interactions between the Higgs boson and quarks or leptons. Technically, it occurs because the numbers in the Cabibbo–Kobayashi–Maskawa matrix and Pontecorvo–Maki–Nakagawa–Sakata matrix (discussed in the previous question) are not all real numbers. Interestingly, this is only possible when there are 3 or more generations of quarks and/or leptons: with 2 or fewer generations the matrix can always be made real.
Does the strong force violate CP symmetry? In the Standard Model it would be very natural to add a CP-violating term to the equations describing the strong force, proportional to a constant called the “θ angle”. But experiments say the magnitude of the θ angle is less than 2 × 10-10. Is this angle zero or not? Nobody knows. Why is it so small? This is called the “strong CP problem”. One possible solution, called the Peccei–Quinn mechanism, involves positing a new very light particle called the axion, which might also be a form of dark matter. But despite searches, nobody has found any axions.
• Wikipedia, CP Violation.
• Wikpedia, Strong CP Problem.
• Michael Beyer, editor, CP Violation in Particle, Nuclear, and Astrophysics, Springer, Berlin, 2008.
It’s a theorem that quantum field theories are symmetrical under CPT: the combination of switching matter and antimatter, left and right, and future and past. Thus, a violation of CP implies a violation of time reversal symmetry. For more on this, see:
• R. G. Sachs, The Physics of Time Reversal, University of Chicago Press, Chicago, 1987.
What are the electric dipole moments of the electron and the neutron?
As of 2020, experiments show the electric dipole moment of the electron is less than 1.1 × 10-29 electron charge centimeters. According to the Standard Model it should have a very small nonzero value due to CP violation by virtual quarks, but various extensions of the Standard Model predict a larger dipole moment.
Also as of 2020, experiments show the neutron’s electric dipole is less than 1.8 × 10-26 e·cm. The Standard Model predicts a moment of about 10-31 e·cm, again due to CP violation by
virtual quarks, and again various other theories predict a larger moment.
Measuring these moments could give new information on physics beyond the Standard Model.
• Wikipedia, Electron Electric Dipole Moment.
• Wikipedia, Neutron Electric Dipole Moment.
• Maxim Pospelov and Adam Ritz, Electric Dipole Moments as Probes of New Physics.
Here’s my question. Do you know papers that actually calculate what the Standard Model predicts for the electric dipole moments of the electron and neutron?