Zero dipole moment in stationary state?

[paulzhen]

P.W. Anderson in his essay "more is different" said that:

"no stationary state of a system has an electric dipole moment". He used an example of NH₃ to illustrate that. I then checked online and found that, Chemists said there is dipole moment in NH₃ molecule, but (nuclear) physicists claim it is zero. Any one could help me to understand this?

Here is a link to Anderson's essay:
http://robotics.cs.tamu.edu/dshell/cs689/papers/anderson72more_is_different.pdf

Thanks!

 

[Bill_K]

P.W. Anderson in his essay "more is different" said that:

"no stationary state of a system has an electric dipole moment". He used an example of NH₃ to illustrate that. I then checked online and found that, Chemists said there is dipole moment in NH₃ molecule, but (nuclear) physicists claim it is zero. Any one could help me to understand this?

Here is a link to Anderson's essay:
http://robotics.cs.tamu.edu/dshell/cs689/papers/anderson72more_is_different.pdf

The essay doesn't seem to be publically accessible, but the correct statement is

"no stationary state of a system" with a well-defined parity "has an electric dipole moment"

That is, the system must be symmetrical under reflection. The NH3 molecule clearly is not.

The statement needs to be qualified even for fundamental particles. CP-violating effects will produce electric dipole moments. Experiments are under way to detect and measure the electric dipole moment of the neutron. The Standard Model predicts an extremely small but nonzero electric dipole moment for the neutron, while larger values are predicted by other theories such as supersymmetry.