I have a lovely photograph of him as an alien, in a green felt suit. He had a special voice programmed into his speech machine and went around the party saying "Take me to your leader". And, just as we described in the book, there was a scientist at that party who was dressed in red and who stood next to people and then moved away before asking them to guess what he was. He had come as red shift - the effect whereby electromagnetic radiation from distant objects is shifted towards the red end of the spectrum as a result of their movement away from Earth.
Did things like black holes get discussed at the family dinner table?
LH: Yes, very much. I grew up in Cambridge and there were always physicists coming to our house for dinner and discussing their work. Even at Dad's birthday party this year, there was a scientist trying to construct a black hole from a balloon and a pair of tights.
What kind of books did your father read to you when you were small?
LH: The other day, Dad suddenly quoted from the Bible and we were all taken aback because it was so unexpected. We were discussing the slogan on a tin of golden syrup, asking why it said "out of the strong came forth sweetness". Dad said it came from the Bible, and gave us the correct chapter and verse. He said that his father used to read him stories from the Bible when he was young. I said "I'm glad you read us Paddington Bear instead."
One thought I had on religion as I read George's Cosmic Treasure Hunt is that the big questions of physics seem to be supplanting the big religious questions.
SH: Science is increasingly answering questions that used to be the province of religion. The one remaining area that religion can still lay a claim to is the origin of the universe, but even here science is making progress, and should soon provide a definitive answer to how the universe began.
Whose idea was it to branch out into children's books about physics?
LH: Mine. I'd noticed that children were coming up to Dad and saying, "So what would happen if I fell into a black hole?". He gave them good, clear, funny answers, and I also noticed that all the adults gathered round to hear the replies too. I put it to him that it would be fun to work on an adventure story about physics. It was meant to be quite small-scale at first, something for his grandchildren, but it has grown from there.
It sounds as if collaborating has changed your relationship.
LH: Yes. I could never have foreseen that we would end up working together because our paths are so radically different, with him a scientist and me a creative writer. But the books gave us a lot of time together and it was a project that we both enjoyed. It was interesting how much my dad got into the creative side of it. Often I'd go to his house at mealtimes - because that's a good time to chat with him - and would read sections of the text while we had dinner. One time when I read him some of the opening chapters of the book he laughed so much that two people had to jump up and catch him because he fell out of his chair. I don't think I've ever seen him laugh that much.
He was a joy to work with because of his clarity of thought, and his ability to express things very simply. Of course, he does have to write everything via a labour-intensive process, using a communication machine.
Have you ever heard your father's real voice rather than his computer-generated one?