After this appeared on BBC4 the other night I had a lot of emails asking me to explain my `black hole in a dustbin’ joke. The programme was Some Boffins with Jokes and I never actually got to see it as I was abroad at the time it was aired, but the basic format was some scientists standing up telling some of their favourite jokes. I told several but judging from emails the only one in the final cut was this:

What happens if you put a black hole into a indestructible rubbish bin  (garbage can if you are telling this in the USA)? ... Answer, It takes off!

OK so what is this about? I’m afraid its a bit of a physics joke that popped into my head circa 1992 and I’ve found it funny ever since. This would be a mini black hole of course, you can have black holes of any size. When you toss in the black hole it would start to suck in air from the top of the bin. Then by conservation of momentum, or if you like by Newton’s law of equal and opposites, there would be a tendency for the black hole + bin to rise up into the air and I have a vision of it  shooting off into space, which I find hilarious. 

I should say this is hypothetical (where would you actually get an indestructible bin anyhow? I mean one so tough that it can contain the black hole) and also makes assumptions such as ignoring the effects of the Earth’s gravity, for example. But hey, lets not overanalyse!

Click on the image to go through to the BBC page for as long as it stays there (although the programme  is not available at the time of writing).