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!!!