Greenhouse gases absorb and then re-emit the heat energy of infrared rays which have been partially absorbed and then reflected off the Earth from the sun, which is how the greenhouse effect works. I have heard, however, that in order for a gas in the atmosphere to be considered a greenhouse gas in the first place, it must have at least a permanent dipole or something that can induce a temporary dipole. Nobody in my chemistry class knows why and neither does my professor. We have all been looking for explanations as to why, but so far none of them are complete or satisfactory. For example, one explanation I found was that the changing electric charge in a dipole was the necessary condition for a molecule to interact with the electric field, which is what electromagnetic waves travel in. But this isn't consistent because, for example, oxygen gas (which, although irrelevant, isn't a greenhouse gas) doesn't have any dipoles whatsoever yet it can interact with UV light to break down into oxygen radicals and produce ozone. This makes the theory I found, therefore, inconsistent. Can anybody provide a consistent and complete explanation as to why greenhouse gases require a permanent dipole or something that can induce a temporary dipole? Thank you.