Reinventing Discovery - Michael Nielsen [51]
As another example of an SDSS-enabled discovery, in 2009 the astronomers Todd Boroson and Tod Lauer used the SDSS to discover two black holes orbiting around one another. The way Boroson and Lauer found the paired black holes was—you won’t be surprised!—by using a computer to search galaxy images from the SDSS. Now, black holes have no color, and don’t show up directly in photos. But black holes are surrounded by huge amounts of glowing matter that’s falling in, and so in a sense it’s possible to “see” black holes in the galaxy images, to talk about them having a color, and so on. The key to Boroson and Lauer’s work was a clever guess they made, which was that if two black holes were orbiting one another, they would appear to have slightly different colors. The reason they made this guess is interesting. When objects are moving at a high enough speed—it needs to be a considerable fraction of the speed of light—their apparent color changes appreciably. Why this happens is a long story, which we won’t get into, but as an example, a red object that’s moving very quickly toward the Earth actually looks a little bit bluer. Boroson and Lauer reasoned that two black holes orbiting one another would have different velocities relative to the Earth, and so one would be ever so slightly bluer than the other. Armed with this double coloring idea, Boroson and Lauer used their computer to go hunting in the SDSS data. Their hope paid off when they found a galaxy four billion light-years away with exactly the double coloring signature they were looking for. They followed up with a more detailed examination of the galaxy, confirming the presence of the orbiting black holes, and revealing that they are both staggeringly large, 20 million and 800 million times the mass of the sun, respectively, and a third of a light-year apart, orbiting one another roughly once every 100 years. The discovery has excited great interest, and also set off a debate, with other astronomers wondering if there might be some other explanation for what Boroson and Lauer are seeing. At the moment, the orbiting black hole theory remains the leading candidate among several possible explanations. But no matter what the truth turns out to be, no one doubts that Boroson and Lauer have discovered something remarkable.
All these discoveries are striking, but they don’t fully convey the enormous impact the SDSS has had on astronomy. One way to grasp that impact is to look at how many times the results of the SDSS have been cited (i.e., referred to) in other scientific papers. Most papers in astronomy are cited just a few times, if they’re cited at all. A paper that’s cited tens of times is quite successful, while a paper that’s cited hundreds of times is either famous or well on its way. The original SDSS paper has been cited in other papers more than 3,000 times. That’s more citations than many highly successful scientists receive over their entire career. To give you some feeling for what an achievement this is, Stephen Hawking, probably the world’s most famous scientist, has just a single paper with more than 3,000 citations. Hawking’s paper, which he published in 1975, in fact has just over 4,000 citations as of 2011. By contrast, the SDSS paper was published in 2000, and already has more than 3,000 citations. It will soon catch up to and surpass Hawking’s paper. Several follow-up s describing other aspects of the SDSS have also received