Even Stephen made some logical errors. ""The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future."
- We could be, but they are not making themselves known. (This is what I believe)
- Time travel devices could be tightly controlled and secured by the governments
Dr. Kaku stated that nothing in Physics prevents time travel. He didn't refer to forward or backward.
"Recently, I went on a binge of reading experiences of people going into time slips." A lot of us here have experienced them. They are sort of parallel universe crossovers.
I personally believe, without proof, that one can travel both ways. It's just math.