A persistent echo in spacetime
When a cataclysmic event like the merger of two black holes happens, it sends ripples across the universe. These are gravitational waves, and at facilities like the LIGO Hanford Observatory, scientists detect them as spacetime stretches and squeezes. But general relativity predicts another, stranger effect: after the wave passes, spacetime does not fully relax. A permanent distortion, a "memory" of the event, is left behind. This gravitational wave memory results from the non-linear nature of Einstein's equations, the idea that gravity itself can be a source of more gravity.
The effect is incredibly small. While the main gravitational wave might change the 4-kilometer length of LIGO's arms by one-thousandth the width of a proton, the memory effect is predicted to be about 20 times smaller still. This permanent change in the distance between two points is far too subtle for current detectors like LIGO and Virgo to isolate from a single event. They are designed to sense the oscillating (AC) part of the wave, not a permanent (DC) shift.
Varieties of cosmic memory
Physicists have identified two main types of gravitational memory. The first, called linear memory, was proposed in the 1970s. It is caused by sources that eject mass anisotropically, such as an asymmetric supernova explosion or particles flung away in a close stellar flyby.
The second, more powerful type is non-linear memory, often called the Christodoulou memory. Predicted by Demetrios Christodoulou in 1991, this effect arises from the energy carried by the gravitational waves themselves. According to E=mc², this energy has a gravitational influence, altering the background spacetime and leaving a permanent change long after the primary wave has passed. This is the dominant type of memory expected from the violent merger of black holes and neutron stars.
The search for this faint, permanent echo is an important target for the next generation of gravitational wave observatories. The space-based Laser Interferometer Space Antenna (LISA), with its 2.5-million-kilometer-long arms, will be sensitive to the low frequencies where the memory effect from supermassive black hole mergers should be apparent. Another method involves Pulsar Timing Arrays (PTAs). By monitoring the hyper-regular pulses from dozens of distant pulsars, scientists can look for a simultaneous, permanent shift in the arrival times of their signals, indicating a memory event has passed through our galaxy. While a single detection remains elusive, researchers believe that by stacking the data from hundreds or thousands of merger events, the faint memory signal could be statistically teased out of the noise in the coming years.