is also needed, requiring a longer-term program. First, the frequency with which Earth-size planets occur in zones around stars where liquids such as water are stable on planetary surfaces must be measured (see Box 2.1). Stars will then be targeted that are sufficiently close to Earth that the light of the companion planets can be separated from the glare of the parent star and studied in great detail; this will allow us to find signatures of molecules that indicate a potentially habitable environment. Here, the opportunities are suddenly bountiful, as we have understood over this past decade that, for example, stars much lower in mass than our Sun may have orbiting habitable planets that are much easier to spot. Thus, the plan for the coming decade is to perform the necessary target reconnaissance surveys to inform next-generation mission designs while simultaneously completing the technology development to bring the goals within reach. This decade of dedicated preparatory work is needed so that, one day, parents and children can gaze at the sky and know that a place somewhat like home exists around “THAT” star, where life might be gaining a toehold somewhere along the long and precarious evolutionary process that led, on Earth, to humankind. And perhaps it is staring back at us!

A Bold New Frontier: Gravitational Radiation

In the coming decade, a radically new window on the cosmos will open, with the potential to reveal signals of phenomena ranging from the processes that shaped the earliest era of the universe to the collisions and mergers of black holes in the more recent history of the universe. Einstein’s theory of relativity tells us that space and time are inextricably linked to form space-time (Figure 2.2). Space-time is malleable: its shape is determined by the distribution of mass and energy in the universe. Massive bodies ripple space-time as they move, creating gravitational waves that propagate through the cosmos at the speed of light, unimpeded by even the densest material. The direct detection of gravitational waves requires measurements at a level of exquisite precision and sensitivity that is just now within our reach.

The daunting challenges associated with building kilometer-size detectors whose distortion by passing gravitational waves can be measured to less than one-thousandth the radius of a single proton have been overcome. By mid-decade a worldwide array of ground-based detectors such as Advanced LIGO will be operating. Like electromagnetic waves, gravitational waves span a spectrum, with more massive objects typically radiating at longer wavelengths. These ground-based experiments will probe the short-wavelength part of the spectrum, enabling us to observe the mergers of neutron stars and possibly to see the collapse of a stellar core in the fiery furnace of a supernova explosion.

However, even more promising are signals in a completely different part of the gravitational wave spectrum, at longer wavelengths, predicted to result from mergers of massive black holes during the build-up of galaxies. Detecting these



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