answer:Michael Kaku’s website has a Contact link, but it appears to be for media people only. Stephen Hawking discourages contact from the general public. I’ve done a little looking around, and can’t seem to find any recent papers by either which list their address (usually the “corresponding author” will publish their email on the title page), So, it might just be that both have withdrawn their contact information because simply too many non-scientists are trying to reach them. I find this disappointing, because one of my own science heroes, Richard Feynman, would field letters from civilians all the time. Some of these letters were collected here.