Frank Tipler is an accomplished physicist;
his basic textbook was one I used as an undergrad, and his contributions to science have been welcome. But at some point, as some scientists do, he went, well, in a different direction. Talking about immortality, for one, and trying to use science (poorly) to support some religious beliefs.
I've been dithering over writing about this for years, but now that procrastination has paid off: Sean at Cosmic Variance already did it. I agree with what he wrote, so I'll let him do the talking.
One thing though: the question remains as to why so many scientists go off the deep end later in life! It's not a majority by any means, it's just some, but off they go nonetheless. I'd love to see some studies done on this.