Although there are many similarities and differences within the Standard Model: between quarks and leptons, between fermions and bosons, between particles and antiparticles, etc., many conventional symmetries only hold under specific conditions. However, the combination of changing particles for antiparticles, objects for their mirror-image reflections, and a forwards-moving clock for a backwards-moving one, also known as CPT symmetry, must never be broken. (Credit: Symmetry Magazine)

The Universe’s most unbreakable symmetry

The combination of charge conjugation, parity, and time-reversal symmetry is known as CPT. And it must never be broken. Ever.

The ultimate goal of physics is to accurately describe, as precisely as possible, exactly how every physical system that can exist in our Universe will behave. The laws of physics need to apply universally: the same rules must work for all particles and fields in all locations at all times. They must be good enough so that, no matter what conditions exist or what experiments we perform, our theoretical predictions match the measured outcomes. And having predictive power, explicitly, means that if you know the initial conditions of your system and the laws that govern it, you can predict what the outcomes — or the relative probability of the set of possible outcomes — will always turn out to be.

The most successful physical theories of all are twofold:

• the quantum field theories that describe each of the fundamental interactions that occur between particles,
• as well as General Relativity, which describes spacetime and gravitation.

And yet, there’s one fundamental symmetry that applies to not just all of these physical laws, but for all physical phenomena: CPT symmetry…