Toss a pebble in a pond and watch the tiny ripples radiate outwards. The waves you’ve created bear striking resemblance to another phenomenon, one taking place trillions of miles away at the centre of a distant galaxy. The difference is, when two black holes collide, their impact surges across the fabric of spacetime itself.
Black hole collisions are among the most powerful events in the Universe. But the aftershocks of these cataclysmic mergers — gravitational waves — have proven nearly impossible to detect. That’s because the energy imparted by gravitational waves causes atoms to shake by about one part in 1,000,000,000,000,000,000,000.
And yet we’d very much like to catch these elusive spacetime ripples in action. Unlike electromagnetic radiation, which bounces off atoms as it traverses space, gravitational waves cut through all matter like an invisible knife. In theory, their detection could allow physicists to “see” back to the very beginning of time.
Scientists are currently building ground-based detectors with the sensitivity to see high-frequency gravitational waves — those produced by black holes a few times the mass of our Sun. But outside Earth’s gravity well, we can cast a wider net, and in theory detect gravitational ripples over a broader range of wavelengths. That’s why this fall, the European Space Agency is launching the LISA pathfinder, a miniature astrophysics laboratory that’ll sit a million miles from Earth at the L1 Lagrange point. In orbit, LISA Pathfinder will start test-driving technologies that could root out low-frequency gravitational waves emanating from distant galaxies.
In the meanwhile, physicists are using models to imagine what spacetime ripples might look like. The image above, created by a NASA computer simulation, may be the closest we’ll ever get to witnessing one of the most mysterious cosmic phenomena of all.
Image Credit: NASA/C. Henze