The particle which breaks time symmetry
The majority of the processes in our universe follow time symmetry. This theoretical idea in physics says that objects and processes in our universe are time-reversible (the mechanics work the same way forwards or backwards).
The Exception
This is where entropy comes in. Generally, it is pointed out that entropy is the exception to this; the second law of thermodynamics states that entropy increases as time moves forward, in a closed system. Because of this, a direction in time is created, with systems evolving from ordered (lower entropy) states to disordered (higher entropy) states.
At the microscopic level, the laws of physics are time-reversible, meaning particle interactions can occur forwards or backwards in time. However, at the macroscopic level, involving many particles, systems statistically tend to move towards higher entropy. This creates a clear directionality in time.
The increase in entropy defines the arrow of time. Entropy breaking time symmetry means that while fundamental physical laws are time-reversible, the behaviour of large systems is not. This is due to the many more possible higher entropy states than lower entropy. Thus, we perceive time as moving from lower to higher entropy states.
Now this exception raises the question of whether fundamental particles can tell the direction of time; this will be the question we will answer.
In particle physics, the behaviour of particles is governed by certain fundamental symmetries. These symmetries are foundational to our understanding of the laws of physics and help to determine how particles interact and behave. The three major symmetries are time, parity, and charge.
In particle physics, the discovery of parity violation in weak force interactions challenged the assumption that all fundamental particles obeyed symmetry laws without exception. The experiment by Chien-Shiung Wu on cobalt-60 decay in 1956 revealed that the weak force does indeed distinguish between left- and right-handedness; this is in contrast to previously held beliefs.
Following this discovery, physicists sought to restore symmetry through the concept of combined charge-parity (CP) symmetry, which put forth that while parity alone might be violated, CP symmetry could still hold. However, subsequent findings in 1964 demonstrated that CP symmetry can also be violated in certain particle interactions, prompting a retreat to the CPT symmetry.
CPT symmetry combines charge (C), parity (P), and time (T) symmetries, suggesting that the laws of physics remain unchanged under the combined operations of charge conjugation, parity inversion, and time reversal. Yet, experiments have shown that certain particles, such as those involving quark pairs, break time symmetry directly.
This raises questions about the nature of time and how it compares to other symmetries. While fundamental physical laws are theoretically time-reversible, the tendency of macroscopic systems to increase in entropy gives rise to the arrow of time— moving from lower to higher entropy states. The question of CPT symmetry thus continues to intrigue physicists, resulting in further exploration of the fundamental forces and the nature of our universe’s existence.
