In the vast swath of time since then, intelligence might have died out, arisen again, and grown inconceivably rich. The beyond-all-reckoning wealthy beings near the center could afford to lavish a pittance on a luxury—blaring their presence out to all those crouched out in the galactic suburbs, just getting started in the interstellar game.

Whatever forms dwelled farther in toward the center, they knew the basic symmetry of the spiral. This suggested that the natural corridor for communication is along the spiral’s radius, a simple direction known to everyone. This maximized the number of stars within a telescope’s view. A radius is better than aiming along a spiral arm, since the arm curves away from any straight-line view. So a beacon should broadcast outward in both directions from near the center.

So, rather than look nearby, the twenty-first-century SETI searchers began to look inward. They pointed their antennas in a narrow angle toward the constellation Sagittarius. They listened for the big spenders to shout outward at the less prosperous, the younger, the unsophisticated.

But how often to cup an ear at the stars? If Earth was mediocre, near the midrange in planetary properties, then its day and year were roughly typical. These were the natural sequences any world would follow: a daily cycle atop an annual sway of climate.

If aliens were anything like us, they might then broadcast for a day, once a year. But which day? There was no way to tell—so the SETI searchers began to listen every day, for roughly a half hour, usually as the radio astronomers got all their instruments calibrated to make their usual maps of the microwave sky. They watched for narrow-band signals that stood out even against the bright hub’s glow.

Radio astronomers also had to know what frequency to listen to. The universe is full of electromagnetic noise at all wavelengths, from the size of atoms to those of planets. Quite a din.

There was an old argument that water-based life might pick the “watering hole”—a band near 1 billion cycles/second where both water and hydroxyl molecules radiate strongly. Maybe not right on top of those signals, but nearby, because that’s also in the minimum of all the galaxy’s background noise.

Conventional searches had spent a lot of effort looking for nearby sources, shifting to their rest frame, and then eavesdropping on certain frequencies in that frame. But a beacon strategy could plausibly presume that the rest frame of the galactic center was the obvious gathering spot, so anyone broadcasting would choose a frequency near the “watering hole” frequency of the galaxy’s exact center.

Piggybacking on existing observing agendas, astronomers could listen to a billion stars at once. Within two decades, the strategy worked. One of the first beacons found was from the Sagittarius Architecture.

Most of the signals found since the early days proved to have a common deep motivation. Their ancient societies, feeling their energies ebb, yet treasuring their trove of accumulated art, wisdom, and insight, wanted to pass this on. Not just by leaving it in a vast museum somewhere, hoping some younger species might come calling someday. Instead, many built a robotic funeral pyre fed by their star’s energies, blaring out tides of timeless greatness: