Commentary

The Nature of the Unknown

While reacquainting myself with the research on gravitational waves, I ran across a comment that brought me back to my graduate student days. I was reading one of the conference papers on the LIGO website when I ran across the assertion that the cosmic string may be among the objects producing gravitational waves. I hadn't thought of this mythical beast in literally a decade; I hadn't realized that this idea was still actively active in the astronomical communities consciousness. When I was a Harvard graduate student in the early 1980s, some in the theoretical group were playing with them. At that time my interest was in gamma-ray bursts. One day when I was sitting at my desk, another graduate student burst into my office and proclaimed that he, too, was now working on gamma-ray bursts: he was looking into a theory in which a cosmic string whips through space and smashes into a star, causing the star to supernova. In his defense, the theory I was working on at that time turned out to be just as wrong as this theory was. But neutron stars, the source of gamma-ray bursts in my work, exist; the cosmic string is nothing more than a mathematician's (or perhaps more accurately, a field theorist's) dream. I have about as much faith in the existence of these things as I do in the existence of dragons—the Beowulf type, not the Komodo type.

When I read the reference to cosmic strings in the LIGO literature, it made me wonder, if Christopher Columbus were alive today, trying to get funding for his trip to America, would he be citing the possible existence of sea monsters, or perhaps one of the imaginary lands of St. Brendan's voyage, as a reason for the trip? But Columbus's voyage had very solid commercial benefits, so perhaps these things would have been detrimental to his funding.

The modern research project is a totally different beast. It is government funded at levels ranging from the millions to the billions. The NSF yearly spents $33 million on LIGO, and NASA spent billions of dollars on its Great Observatories. With no commercial benefit, these projects have to be sold as transformative of our understanding of the universe. With speculative theories so commonplace, citing one as an example of the unknown is irresistible.

There is no doubt that any instrument that looks for the first time into space through a new window will find the unexpected. X-ray and gamma-ray detectors have discovered a wide range of unanticipated sources that were invisible with optical telescopes. The gamma-ray burst is one example. When neutrino detectors started to measure the Sun's neutrino flux, they unexpectedly found that the Sun's neutrino luminosity is much smaller than anticipated. So if an instrument can be made sufficiently sensitive to detect gravitational waves, then it is likely that some unanticipated sources of gravitational waves will be found.

The difficulty is in anticipating the unanticipated. Humans have a hunger to fill in the unknown with imagination. Like a cartographer filling in unknown parts of his map with imaginary land masses, astrophysicists fill in his unknown spaces with mathematics. So we have the inventions of cosmic strings, magnetic monopoles, worm holes, the inflationary early universe, and a multitude of other objects that play central roles now in science fiction. To my mind, the ultimate is the assertion of superstring theorists (different from cosmic string theorists) that black holes, which are not yet proven experimentally to exist, are filled with a mass of tiny worm holes, which are objects that are pure conjecture; now when I think black hole, I see an image of a giant bait bucket filled with night crawlers. Usually theorists engaged in speculation are guided by rather subjective mathematical principals, but usually the universe does not follow our prejudices. More to the point, we like to fill the universe with new but simple objects, while the universe likes to fill itself with mundane but complex objects. The supernova creates the gamma-ray burst on its own; it doesn't need help from a cosmic string.

My own sense is that the most unexpected result that can come out of the gravitational wave experiments is to observe merging neutron star binaries and find that their behavior contradicts general relativity in the strong field limit. This is a plausible outcome, it would be a transforming outcome for physics, and given the wide acceptance of general relativity, it would be totally stunning to the community.