4 ms·

https://www.reddit.com/r/Physics/comments/4pt3g6/barium144_nucleus_is_pearshaped_octupole/d4nnwtq?context=3 https://www.reddit.com/r/Physics/comments/4pt3g6/bar

https://www.reddit.com/r/Physics/comments/4pt3g6/barium144_nucleus_is_pearshaped_octupole/d4nnwtq?context=3 https://www.reddit.com/r/Physics/comments/4pt3g6/barium144_n...

personjerry 10y ago
My gosh, that was a great answer. Now if only I could understand any of it...
- personjerry 10y ago
  To clarify, I was saying that I as a layman cannot properly understand that answer on Reddit. My attempt at saying it with a touch humor has failed.
  - jacobolus 10y ago
    Θ⁻¹HΘ is a way to write a new Hamiltonian which is the “reflection” with respect to time of the original Hamiltonian H, see https://en.wikipedia.org/wiki/Hamiltonian_mechanics https://en.wikipedia.org/wiki/Hamiltonian_mechanics for an explanation of what the Hamiltonian is. Θ⁻¹HΘ = H means that when you reverse time, your physics looks the same as it did before. Such a thing (time-reversal invariance) is being called “time-even”. In the same way a function f(x) would be “even” with respect to its argument if f(x) = f(–x). The author of that comment is pointing out that if we assume the Hamiltonian is time-even, then we can’t have a particle with an electric dipole moment. This is because the contribution to the Hamiltonian from the dipole interacting with an external electric field would be time-even. That interaction can be expressed as a kind of dot product of two terms, an electric dipole operator part and an electric field part, and the action of our time-reversal operator Θ can be applied separately to each part. Since the electric field is time-even, that means the dipole itself must also be time-even. But this can’t really work, because a solitary particle has spin but no other symmetries which could generate the electric dipole moment. The spin gets reversed if we run time backwards, a “time-odd” symmetry. So if we find a particle with a static electric dipole moment in nature, then there must be some contradiction in our assumption that the Hamiltonian was time-even. The rest of his comment is a somewhat analogous kind of argument related to a slightly more complicated situation and slightly more complicated kind of symmetry. See https://en.wikipedia.org/wiki/Electric_dipole_moment#Dipole_moments_of_fundamental_particles https://en.wikipedia.org/wiki/Electric_dipole_moment#Dipole_...
andrewflnr 10y ago
Ok, but how does that affect, say, closed timelike curves? No need to run physics backwards for those, which it seems is really what's affected here.
- mirimir 10y ago
  Right. It starts with: > True time-reversal symmetry would prevent certain static multipole moments from existing in nuclei. Not "Time travel ..."
  - imglorp 10y ago
    Why doesn't the entropy portion of thermodynamics have anything to say about time reversal symmetry? Or does it say that but we're not smart enough to see it yet?
deleted 10y ago
[deleted]

https://www.reddit.com/r/Physics/comments/4pt3g6/barium144_nucleus_is_pearshaped_octupole/d4nnwtq?context=3 https://www.reddit.com/r/Physics/comments/4pt3g6/barium144_n...

personjerry 10y ago

My gosh, that was a great answer. Now if only I could understand any of it...

personjerry 10y ago
To clarify, I was saying that I as a layman cannot properly understand that answer on Reddit. My attempt at saying it with a touch humor has failed.
- jacobolus 10y ago
  Θ⁻¹HΘ is a way to write a new Hamiltonian which is the “reflection” with respect to time of the original Hamiltonian H, see https://en.wikipedia.org/wiki/Hamiltonian_mechanics https://en.wikipedia.org/wiki/Hamiltonian_mechanics for an explanation of what the Hamiltonian is. Θ⁻¹HΘ = H means that when you reverse time, your physics looks the same as it did before. Such a thing (time-reversal invariance) is being called “time-even”. In the same way a function f(x) would be “even” with respect to its argument if f(x) = f(–x). The author of that comment is pointing out that if we assume the Hamiltonian is time-even, then we can’t have a particle with an electric dipole moment. This is because the contribution to the Hamiltonian from the dipole interacting with an external electric field would be time-even. That interaction can be expressed as a kind of dot product of two terms, an electric dipole operator part and an electric field part, and the action of our time-reversal operator Θ can be applied separately to each part. Since the electric field is time-even, that means the dipole itself must also be time-even. But this can’t really work, because a solitary particle has spin but no other symmetries which could generate the electric dipole moment. The spin gets reversed if we run time backwards, a “time-odd” symmetry. So if we find a particle with a static electric dipole moment in nature, then there must be some contradiction in our assumption that the Hamiltonian was time-even. The rest of his comment is a somewhat analogous kind of argument related to a slightly more complicated situation and slightly more complicated kind of symmetry. See https://en.wikipedia.org/wiki/Electric_dipole_moment#Dipole_moments_of_fundamental_particles https://en.wikipedia.org/wiki/Electric_dipole_moment#Dipole_...

andrewflnr 10y ago

Ok, but how does that affect, say, closed timelike curves? No need to run physics backwards for those, which it seems is really what's affected here.

mirimir 10y ago
Right. It starts with: > True time-reversal symmetry would prevent certain static multipole moments from existing in nuclei. Not "Time travel ..."
- imglorp 10y ago
  Why doesn't the entropy portion of thermodynamics have anything to say about time reversal symmetry? Or does it say that but we're not smart enough to see it yet?

deleted 10y ago

[deleted]