What is analog gravity, and where did the idea come from? I

think it was 1970.

I was a postdoc.

No, 71.

I was a postdoc with Roger Penrose.

and then a shama i met dennis shama and he said you know we should have you over he was at oxford at the time and he invited me over to oxford to give a colloquium on black holes they were the brand new fresh new thing and i was desperately trying to think of some analogy that i could convey that would give people the feel of what it was.

And I came up with the idea of a waterfall where the velocity of the water was greater than the velocity of sound in the water.

So how do you know that what you're seeing can really be ported onto black hole physics?