Scientists have proposed a rare phenomenon called "ball lightning" as a possible explanation. Of course, the other side counters with the fact that the pilots said that the lights seemed "intelligent"... Here's a link: http://www.reuters.com/newsArticle.jhtml?type=scienceNews&storyID=5152278
If it was indeed ball lightning, then why would it not be visible in the normal spectrum of light and only visible in the infrared?
Good point. I hadn't even thought about that issue. In spite of the approach I take a lot of times on et, I do think it's good to investigate the both sides and look for a natural explanation first. They'll have a tough time with this one though as, if I remember right, the pilots felt the lights played cat and mouse with them...
Ball lightning isn't nearly as rare as suggested in the Reuters article. In addition, depending on the energy spectra of the specific occurance, it's possible that most/all of the energy dissipation was in the infrared range. Also, ball lightning is well known to chase aircraft and people - e.g., there have been cases of ball lightning coming down a person's chimney and chasing them around the house bouncing off walls and furniture.