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Dear Marshall,

That limerick is original with me (and copyrighted, so others should not reproduce it without my consent).

As I recall The Universe in a Nutshell -- it's been a while since I read it -- Hawking discusses that nature seems to protect causality. As I recall, he wasn't confident of the mechanism that would prevent a wormhole from being a conduit to another time. He may have even mentioned more than one.

Or it could be the erroneous statement to which you are referring (but still haven't explained to the inquiring minds of Science Blog)?

My limerick essentially says this: Hawking declares that because nature protects causality, any time travel hypothesis should be consigned to the black hole of failed ideas.

I think his notion of protecting causality is sensible, but that doesn't mean he didn't err in the details of The Universe in a Nutshell.

Fred Bortz -- Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)