The universe is filled with spectacular events that we don't understand. Right now, a star is going supernova, but we don't know what the inside looks like; two neutron stars are spiraling toward each other, but we don't know what the collision will look like. And a black hole is swallowing another black hole, but we don't know how they're shaped.
What makes these phenomena so hard to fathom is that the electromagnetic messages they send out are limited. But those messages aren't their only signals: According to Einstein's general theory of relativity, such events should create powerful waves of gravity—ripples in the curvature of space-time that alternately stretch and compress everything in their path.
Although scientists have been trying for decades, they have yet to directly observe these waves. Scientists have already built the first generation of large-scale gravity-wave detectors, and a second generation, expected to make the very first observation later this decade, is under construction. But a team of European scientists and engineers is already thinking beyond the second-generation instruments: On 19 May, they presented a design for an €800 million device 10 times as sensitive and capable of seeing 1000 times the volume of space. They call it the Einstein Telescope.
"We're hoping to see new things, things that no one knows of," says Harald Lück, deputy scientific coordinator of the Einstein Telescope design study. "As astronomers in the field like to say, with gravity waves, we could see the dark side of the universe."
The instrument, which won't start making observations until 2025 at the earliest, isn't a telescope in the Galilean sense; it and its predecessors are laser interferometers that measure tiny distortions—much smaller than the diameter of a proton—in the length of tubes several kilometers long. The telescope design includes three detectors, each consisting of two 10-kilometer-long arms—more than twice the size of the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), the largest second-generation gravitational interferometer. The arms will lie parallel to Earth's surface and connect at a 60-degree angle, with a laser, a beam splitter, and a photodetector situated at the junction. A beam will shoot into the splitter, and each resulting half will do laps up and down tubes in one of the arms, bouncing back and forth off mirrors.
In the absence of a detectable gravitational wave, the split beams will recombine and cancel each other out, shining no light on the photodetector. But when a strong wave—from, say, a swirling binary star—hits the instrument, one of the arms will contract minutely and the other will expand, then vice versa, shifting the phases of the split beams and allowing a signal to pass through.
In making the Einstein Telescope supremely sensitive, the challenge lies in canceling out noise. To detect a broader range of cosmic events, the telescope is designed to pick up gravitational waves of 3 to 10 hertz, lower in frequency than those picked up by second-generation models. As a result, earthquakes, tides, and even human activity could really throw the device off. To avoid some of this interference, the detectors will be built away from the coast and 150 meters underground, according to Lück. But even in a ditch, the instrument would be so sensitive that the random motion of the mirrors' molecules would get in the way. The solution is to chill the mirrors to an icy 10 kelvins.
Over the next few years, Lück's team will research and develop the telescope's high-tech subsystems, look for potential sites, and flesh out the design further. But before the project can move forward, second-generation systems need to show that they can detect gravitational waves. "Without that last experimental proof, such a big investment will not be made," Lück says.
According to Joseph Giaime, head of LIGO's Louisiana detector, that could still be five or six years away.