My first Magazine Sky & Telescope - 01.2019 | Page 6

SPECTRUM by Peter Tyson The How and the Why The Essential Guide to Astronomy Founded in 1941 by Charles A. Federer, Jr. and Helen Spence Federer WHEN IT COMES TO ASTROPHYSICAL studies these days, it’s often hard to decide what’s more amazing: how scientists do them, or why scientists do them. Both can boggle the mind. Take the search for low-frequency gravitational waves. LIGO detected high-frequency gravitational waves from the merger of small black holes (S&T: Jan. 2018, p. 10). But LIGO can’t observe the longer waves emanating from the most massive black holes of all: supermassive black hole binaries. These are pairs of behemoths each millions to billions of times the mass of our Sun that, after the galaxies that host them merge, wind up orbiting each other and eventually combining. As they circle and when they col- lide, they generate these low-frequency gravitational waves, which radiate out- ward at the speed of light and ripple the fabric of spacetime across the universe. These waves have extremely long wavelengths, with a billion seconds (>30 years) between peaks. And their influence when they pass objects like Earth is exceedingly subtle: Even a “strong” gravitational wave will cause an object to stretch or shrink by just one part in a quadrillion. Facing such challenges, researchers have devised an inge- nious solution for how to detect these ripples, as Bob Naeye Artist’s concept of a describes beginning on p. 22. They rely on the exquisitely pulsar pulling in mat- stable timing of radio flashes from pulsars, a variety of ter from a nearby star neutron star that whirls like a lighthouse beacon, dispatch- ing its beams into the ocean of space. So predictable are these celestial clocks that astronomers can time, often decades into the future, when radio signals from the most stable pulsars will reach Earth to within a few hundred nanoseconds. By monitoring across the galaxy a scattering of such cosmic clocks — which together form a pulsar timing array — astronomers can use the Milky Way itself as a detector. Consider it a kind of Galaxy Positioning System. The idea is to watch for pulses that arrive at our radio telescopes ever so slightly earlier or ever so slightly later than forecast. In this way, radio astronomers are striving to use pulsars as stellar buoys to reveal the swells of passing gravitational waves. Why go to all this trouble? Spacetime is full of ripples, warps, and holes that we can’t see but we can “hear.” Successfully doing so will open up a whole new area of astronomy, allowing us to address questions we can’t address any other way. How do galaxies merge and grow over cosmic time? What characterizes the ultra-exotic environments around supermassive black hole binaries? In such environments, we’ll be operating at the limits of our understanding. What we find will doubtless challenge us with entirely new, unfore- seen discoveries that will lead to even more remarkable hows and whys. Editorial Correspondence (including permissions, partnerships, and content licensing): Sky & Telescope, 90 Sherman St., Cambridge, MA 02140-3264, USA. Phone: [phone]. E-mail: [email protected]. Website: skyandtelescope.com. Unsolicited proposals, manuscripts, photographs, and electronic images are welcome, but a stamped, self-addressed envelope must be provided to guarantee their return; see our guidelines for contributors at skyandtelescope.com. 4 Advertising Information: Tim Allen 773-551-0397, Fax: [phone]. E-mail: [email protected] Web: skyandtelescope.com/advertising Customer Service: Magazine customer service and change-of-address notices: [email protected] Phone toll-free U.S. and Canada: 800-253-0245. Outside the U.S. and Canada: 386-597-4387. JA N UA RY 2 019 • SK Y & TELESCOPE Senior Contributing Editors J. 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