Thursday, July 17, 2008

writing about nothing

These days I am working on a fairly boring paper that I intend to publish in one of our professional astronomy journals. The reason it is boring is because it is about nothing. Or, at least, it is about us looking for something and not finding it.

A couple months ago, my colleagues and I announced that we has seen variations in the light of a special type of white dwarf star. This represented the discovery of a new type of variable star, and we continue to work on understanding that star.

That new variable star was discovered as part of a targeted search. We were looking for variations in the light coming from stars such as the one we observed. And we found those variations in one star. But we didn't see them in other stars, and now we need to write a paper describing these non-detections.

There may be many reasons why we didn't see variations in the other stars we targeted:

  1. These other target stars aren't varying.
  2. The other targets are varying, but at a very low level that we can't detect.
  3. We messed up in our data analysis.
And there may be other reasons. Number 3 is fairly straightforward to check; I'm doing that now. I make sure that we were pointed at the right star, and that the other stars around it are acting pretty normal. I make sure that the weather wasn't too bad, and that the clouds weren't too thick. These checks are time consuming, but important. The hard part is choosing between possibilities one and two, and this is crucial for the science! If we claim (and we do) that these other target stars should not be varying, then we have to rule out variations to pretty low levels, better than one percent. And even that may not be good enough; one colleague of mine states that, "if you don't see a star varying, you haven't looked hard enough." And we have other reasons to thinking that variations smaller than the percent level would be something altogether different. But there is a big difference between saying, "it doesn't vary" and "it doesn't vary at a level larger than 1 percent."

A lot of astronomers don't bother publishing non-detections (or "null results," as we often call them). The papers are boring to write, because we usually aren't sure if we should have seen something or not, and it's not as fun as claiming to find something new. But, when it comes to testing theories, a null result can be just as important as finding something. If a theory were to predict that all of our stars should vary in brightness, but only one of a dozen does, than that theory can be ruled out.

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