Phil Evans, an X-ray astronomer in England and frequent guest blogger for Geeked On Goddard, sends us this report on the fascinating nature of coincidence in science.
I have the power to make stars explode!
No, seriously. True, I can’t draw my sword and turn miraculously into a muscle-bound hero, like He-Man, nor can I turn my pet cat (Tinkabell) into Battle Cat, He-Man’s ferocious feline familiar.
But I really can make explosions at the other end of the universe. Skeptical? Here is the proof:
Last year, NASA’s Swift satellite (data from which I use in my work) was going through a bit of a lean observing period, with no gamma ray bursts (GRBs) detected for some time. GRBs are vast releases of energy from collapsing or collidign stars.
So, just as my duty week began at the University of Leicester, I tweeted, “Wake up universe!”
In the next 24 hours, Swift snagged four GRBs. Coincidence?
The only other time that we have had that many bursts in one day was the day celebrated science fiction author Arthur C. Clarke died? Coincidence?
Well, actually — yes. The thing is that coincidences happen all of the time.
A couple of years ago on her BBC Radio show, Sarah Kennedy asked people to send in their “coincidence” stories. Countless people mailed in about times they’d gone around the world on holiday, and met someone from three streets away. The response was continually, “Wow! Isn’t that amazing?” when what the was program actually demonstrating was that these “unlikely” events actually happen regularly.
In fact, when people respond to these stories by saying, “Small world,” they’ve got it totally wrong! It’s because it’s a big world that these things happen. Imagine something that only affects 1 in a million people. Pretty unlikely? Well, it will affect something like 300 Americans, and 60 Brits!
Coincidences happen. And this can be a real pain for astronomers. I’ve got some data, there’s a cluster of pixels close together. Is it a faint source, or just a coincidence that some background light has clustered? (See image at right.) This spectrum shows a blip. Is it a real feature, or just noise?
Fortunately, using statistics we can at least quantify how likely things are. Typically in astronomy we would only claim we’d found a source, for example, if there was less than a 0.3% chance that it was just a “lucky” fluctuation in the background. Even this happens, well, 0.3% of the time!
For Swift, we have to be even more conservative. When the Burst Alert Telescope (BAT) thinks it’s found a GRB, there has to be only a 0.0000000000008% chance that it’s just a fluctuation in the background [for us to interpret the observation as "probably real." This threshold was carefully determined to minimize the number of false alarms, without losing real (possible) GRBs.]
Despite this, we do get a few false alarms every year, because of the number of times and ways the BAT looks for GRBs. We tried a “subthreshold” test a couple of years ago, where we triggered on things which were more likely to be spurious, that is, there was a 0.00000000006% chance of them being a random change in the background. We expected, and got, about 2 false alarms a day.
Overall, I’d say we get maybe 5 false alarms a year — but about 100 real GRBs. And the false alarms we usually identify within 20 minutes or so, so they take very little of our time.
So, next time someone tells you something unusual that’s happened, and asks if it could be coincidence, the best answer is probably, “Yes!”
Check out Phil’s twitter feed: @swift_phil
OH AND DID I MENTION? All opinions and opinionlike objects in this blog are mine alone and NOT those of NASA or Goddard Space Flight Center. And while we’re at it, links to websites posted on this blog do not imply endorsement of those websites by NASA.