It would seem to me the best physical proof for an afterlife is the devil himself. He has appeared to many of the saints who have experienced him first hand. Jesus himself had a number of conversations with the devil in the people who were possessed by the devil.
And then there is the latest flurry of possessions where the Vatican has increased the number of exorcists and a school to educate new exorcists. And in addition, right here on CAF, we have witnesses of those whose homes he has invaded and has done a number of things to the people living in them.
If we know the devil’s activities and see them first hand, then it seems to me that only the blind would deny he exists. And if he exists, then there can not be a denial that another world exists that we may know little about.
However that being said, anyone can deny anything because God gave them the ability to do this, even contrary to the evidence.