One of the most common complaints deniers made about my global warming skeptic challenge was "you cannot prove a negative" as an excuse for why they were unable to prove manmade global warming wasn't real. This, I told them, was a false statement. In fact, all experiments are designed to prove the negative. It's called the null hypothesis. The most common example I gave of proving the negative consisted of my saying the door is locked. If you were to try the door and find it is unlocked, you have proven the negative. I could come up with countless other examples (the Sun is in the backyard, cats are actually dogs, there is a gold coin under a given rock, deniers accept science, etc.), but that one works.
I was reminded of this when I saw an article from the National Science Foundation: Techniques to prove or disprove existence of other planets. Surely, that is just a title. They don't really mean they are proving a negative. Do they?
To the dismay of deniers everywhere - yes, they do mean that. The article is about how habitable planets had been identified around a particular star, Gliese 581, but new technology and methods proved the planets did not exist - "it was disappointing to disprove the habitable zone planets in the Gliese 581 system."
So, if you ever have someone say to you, "You can't prove a negative," be sure to tell them they are wrong and you can prove it. Isn't that actually proving a negative in itself?