This Particle Breaks Time Symmetry

Most physical processes work the same forwards or backwards in time, making it impossible to determine if a video is playing normally or in reverse

Entropy and the second law of thermodynamics are typically cited as the only exception, but this is an emergent property of many particles, not a fundamental one

Can individual particles themselves detect the direction of time, or does there exist a physical process at the particle scale that looks different forwards versus backwards?

Yes, certain fundamental particles can break time symmetry and distinguish between forward and backward time direction

• Charge (C): Interactions remain unchanged when all charges are swapped, showing nature treats positive and negative charge equally • Parity (P): Physics laws are indifferent to left- or right-handedness, working the same in a mirrored universe • Time (T): Interactions work identically forwards or backwards in time

Imagine a giant mirror reversing the z-axis so right hands become left hands; the laws of physics should work identically in this mirror world with no preference for handedness

In the 1950s, all three symmetries were believed to be unbreakable fundamental laws of physics

In 1956, Li and Yang published a paper pointing out that parity symmetry had never been tested in experiments involving the weak force

• Physicist Chien Shang Wu abandoned her Christmas vacation to test parity symmetry in the weak force • Cooled cobalt-60 atoms to three-thousandths of a degree above absolute zero • Applied a strong magnetic field to align all nuclear spins in the same direction • Measured the direction electrons were emitted during radioactive cobalt-60 decay via the weak nuclear force

Under parity symmetry, electrons should be equally likely to emit in any direction, making normal and mirror experiments give identical results

Electrons were predominantly emitted in one direction, predominantly opposite to the nuclear spin direction, not just by a little but as much as physically possible

• Destroyed a fundamental assumption in theoretical physics that had existed for decades • Shocked the physics world; Wolfgang Pauli famously said 'That's total nonsense' • The discovery of parity violation won the Nobel Prize in 1957, the very same year the results were published

Physicists proposed that while the weak force violates parity, it might respect the combined charge-parity (CP) symmetry, where the mirror also swaps particles for antiparticles with reversed charges

In 1964, experiments found that some particles also violate the combined charge-parity symmetry, earning another Nobel Prize and breaking another fundamental rule

Physicists retreated to the combined symmetry of CPT, combining charge, parity, and time; CPT violation would require rewriting special relativity and quantum field theory

• If CP can be violated but CPT cannot be violated, then time symmetry must also be broken • This is the only way to maintain CPT as a true symmetry while CP is broken • Physicists have confirmed through experiments that certain particles directly break time symmetry

When a pair of quarks are held together by the strong force, they can exist in two different possible arrangements and switch between them via the weak force

Switching from one arrangement to the other takes longer than switching back, creating an asymmetry in time

If recorded, this event would look different when played forwards versus backwards, which is the definition of breaking time symmetry

• Fundamental particles can distinguish between forward and backward time direction • The second law of thermodynamics is not the only physical process that prefers one direction in time • This may relate to why humans perceive time flowing in only one direction, though the ultimate origin remains unknown

Physicists once thought parity, charge, and time symmetries were unbreakable, but over time each has been demonstrably violated by experiments

Is the ultimate CPT symmetry also unbreakable, or will it eventually fall, taking quantum field theory and special relativity with it?

Scientists still have no idea why time only goes in one direction, despite understanding the symmetry violations at the particle level

Perhaps another physicist will give up their vacation to resolve these big basic unresolved mysteries in understanding the universe