Well, I don't know that much about it myself. I did a little googling and didn't come up with much. Apparently there has been some theoretical work on the matter. The only thing I have come up with is that they may or may not be possible. If they are they create some strange effects. I do recall that Scientific American had an article about them and I managed to find out what issue it was in. That was the Jan. 21, 2009 issue. I don't have it, it went to my daughter years ago. You can read it on line if you are a subscriber, or you might be able to find it at a library.

Bill Gill

C is not the speed of light in a vacuum.
C is the universal speed limit.