Reversibility on Microscopic Level

[StevieTNZ]

Hi there,

Quick question - I know the processes in the microscopic realm are reversible. However, in 'Quantum physics: illusion or reality', Rae says that this is true except for one or two subatomic particle processes. Which particles are these, and why aren't they reversible?

 

[JesseM]

Hi there,

Quick question - I know the processes in the microscopic realm are reversible. However, in 'Quantum physics: illusion or reality', Rae says that this is true except for one or two subatomic particle processes. Which particles are these, and why aren't they reversible?

All reactions show CPT symmetry (charge, parity and time symmetry) but not all show T-symmetry (time) alone. For the ones that violate T-symmetry, basically that means if you had a movie of such a reaction then if you played it backwards you'd get something that violated the laws of physics, but the CPT-symmetry means that if you played it backwards and reversed all the spatial axes (reversing left and right as in a mirror for example) and switched the labels of charges so that particles became antiparticles and vice versa, then the result would no longer violate the laws of physics.

In terms of CPT symmetry, violations of T-symmetry are equivalent to violations of CP-symmetry, see this wikipedia page for examples of reactions that violate CP symmetry.

 

[StevieTNZ]

When you speak of T-symmetry, is that equivalent to me walking forward -- then when reversibility comes into play, I'm walking backwards the same path...
much like breaking a cup, then all the fragments forming the cup again?
Would you also see this happen if the reaction showed CPT symmetry?

 

[JesseM]

When you speak of T-symmetry, is that equivalent to me walking forward -- then when reversibility comes into play, I'm walking backwards the same path...much like breaking a cup, then all the fragments forming the cup again?
Would you also see this happen if the reaction showed CPT symmetry?

Both you walking backwards and the cup-reforming should be physically possible according to both T-symmetry and CPT-symmetry, in the sense that neither would violate the fundamental laws of physics, however they might be very unlikely for thermodynamic reasons (certainly the cup scenario would be, and the walking backwards scenario would be too if you paid close attention to things like air dust movements around your feet). The fact that some phenomena are statistically more likely to happen in one direction of time than the other is thought to trace back to the low-entropy state of the universe around the time of the Big Bang, which introduced a thermodynamic asymmetry in the subsequent evolution of the universe (good discussions of this "arrow of time" problem can be found in Brian Greene's The Fabric of the Cosmos and Sean Carroll's From Eternity to Here)

 

[StevieTNZ]

So you have a process A + B -> C + D
Under the CPT theorem, where A-D are particles, suddendly those particles turn into their corresponding anti-particles A*-D*, so the process now is
C* + D* -> A* + B* ?

So the original process is simply not reversed: C + D -> A + B. Then why could their be that kind of reversibility in the example I stated:

When you speak of T-symmetry, is that equivalent to me walking forward -- then when reversibility comes into play, I'm walking backwards the same path...
much like breaking a cup, then all the fragments forming the cup again?
Would you also see this happen if the reaction showed CPT symmetry?

Isn't that just a process A -> B, being reversed to B -> A?

 

[JesseM]

So you have a process A + B -> C + D
Under the CPT theorem, where A-D are particles, suddendly those particles turn into their corresponding anti-particles A*-D*, so the process now is
C* + D* -> A* + B* ?

No, there is no question of the particles "suddenly" turning into antiparticles, the idea is simply that it would be allowed by the laws of physics to have a different set of particles somewhere else in the universe that behaved like the original particles but which charge, parity, and time-order reversed. So somewhere else in the universe you might see a sequence C* + D* -> A* + B*, with a movie of this sequence looking just like a backwards movie of the original sequence A + B -> C + D (and besides looking like a temporally reversed version of the first, the system would also be mirror-reflected along all three spatial axes, that's a reversal of "parity" or the "P" in CPT symmetry)

So the original process is simply not reversed: C + D -> A + B.

But nearly all particle reactions--certainly all involving the particles that make up ordinary atoms and light--are T-symmetric, so you can have simple reversals in this case.

 

[StevieTNZ]

Ah ok. Thanks for clearing that up. My confusion has gone away.

So particles that make up atoms, therefore atoms, show T-symmetry?

 

[JesseM]

Ah ok. Thanks for clearing that up. My confusion has gone away.

So particles that make up atoms, therefore atoms, show T-symmetry?

Yep, violations of T-symmetry are equivalent in quantum field theory to violations of CP-symmetry, and while reactions that violate CP-symmetry have been observed they only involve some particles like "neutral kaons" that don't make up ordinary matter, see the wiki article on CP violation.