Many of the profound ideas in nature manifest themselves as symmetries. In a scientific experiment, a symmetry suggests that something is conserved or remains constant during it – in fact, conservation laws and symmetries seem to have some sort of inseparable link with one another.

The "CPT symmetry" is a fundamental symmetry which all particles in our universe seem to obey. It combines three separate symmetries: charge conjugation (C), parity (P), and time reversal (T). Charge conjugation refers to the operation that changes all particles to their corresponding antiparticle. Parity and time transformations involve reversing the spatial and temporal dimensions of all particles in the system. The symmetry itself states that the laws of physics should be unchanged if one applies all three operations (C, P, and T) simultaneously. In other words, if we switch all particles with their corresponding antiparticle (C), invert the spatial coordinates of the system (P), and reverse the arrow of time (T), then the physical laws which govern that system would remain unchanged.

Some implications of CPT

• CPT symmetry places constraints on violations of other symmetries, such as CP (charge-parity) symmetry. CP violation is important in attempting to explain the observed imbalance between matter and antimatter in the universe. CPT symmetry implies that if there is a CP violation, then there must also be a T violation.

• CPT symmetry tells us that if some particle exists, then so must its antiparticle, which by necessity has the same mass and lifespan.

• Conservation: certain physical quantities – such as energy, momentum, and angular momentum – must be conserved in particle interactions. Antiparticles have the same mass and spin as their corresponding particle, but an opposite charge.

If we take our entire universe and apply the CPT transformation to it, we'd expect to obtain another universe which is identical to ours in every respect. This hypothetical universe is referred to as the "antiuniverse" as it is comprised entirely of antimatter, and would obey the exact same laws of physics in our universe! It’d be a backwards-running reverse universe that, in essence, exploded in the opposite direction from ours when the big bang took place 14.7 billion years ago.

The existence of such a universe is a fascinating consequence of CPT symmetry as it implies that matter and antimatter are fundamentally symmetric. However, the observed imbalance between matter and antimatter in our own universe suggests that this symmetry must have been broken at some point in the past. Otherwise, we would expect to see a universe with equal amounts of matter and antimatter (or we technically wouldn't, as this would cause annihilation between the particle-antiparticle pairs and thus a universe filled with only radiation and no humans or matter!)

The origin of this matter-antimatter asymmetry is still a mystery in cosmology. We know that we should be thankful for it, though, because without it we would all cease to exist! There are many many theories which try to explain this apparent asymmetry – I'll explore some of these ideas in a future post :)

published: 09/05/23 by kaan evcimen