One of the more exotic explanations for UFOs is that they are a space-time phenomenon.
Some researchers point out cases where a UFO is said to have simply contracted in size and winked out of existence, in front of a witness.
The 7 August 2010 issue of the magazine "New Scientist" pages 28-31 has an article by Anil Ananthaswamy, of special interest.
The article reports on the work of physicist Petr Horava, University of California. (Click here.) In January 2009 Horava published an article which galvanised physics. It revolved around reconciling "Einstein's general theory of relativity...and quantum mechanics." (p28.)
"...Horava began looking for ways to tweak Einstein's equations..." (p30.)
">>>So Hovra did the unthinkable and amended Einstein's equations in a way that removed Lorentz's symmetry..."
In short,Hovra's idea was to break the symmetry between space and time. (p31.)
If you have a chance, read the article.