Gravitational wave astronomy will soon be in full swing, and permit studying subtle gravitational wave physics and intriguing “spacetime memory” effects.
In February the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced the first detection of gravitational waves – ripples in the fabric of spacetime predicted by the Einstein’s field equations of General Relativity. The breakthrough, achieved by Advanced LIGO detectors able to measure relative displacements smaller than one-ten-thousandth the diameter of a proton, opened the field of gravitational wave astronomy.
The first gravitational waves were detected from a catastrophic cosmic event very far away. The fusion of two black holes 1.3 billion years ago created a spinning black hole, and released gravitational waves that reached the Earth from a distance of 1.3 billion light years.
In June LIGO announced a new detection of gravitational waves from colliding black holes 1.4 billion light years away. The two confirmed events (the first is indicated as GW150914 and the second as GW151226) in the first few months of operations of LIGO’s advanced detectors indicate that LIGO should be able to detect many black hole collision events, and gravitational wave astronomy could soon be in full swing.
Gravitational Waves Permanently Stretch or Squeeze the Fabric of Spacetime
Once gravitational wave astronomy reaches maturity, it could permit detecting subtler effects of gravitational waves in the fabric of spacetime. A research paper titled “Detecting Gravitational-Wave Memory with LIGO: Implications of GW150914” shows that Advanced LIGO will allow for the detection of gravitational wave memory: a permanent displacement of spacetime that comes from strong-field, general relativistic effects.
High-energy astrophysical events such as black hole collisions produce gravitational waves that cause small oscillations in the distance between two test masses here on Earth, which is the basis of gravitational wave detection. The test masses come back to relative rest after the passage of the gravitational waves, but their rest distance will change permanently. The gravitational waves will have permanently stretched or squeezed the fabric of spacetime, explains Monash University astrophysicist Paul Lasky, lead author of the paper, in a PBS Nova Next story.
In other words, spacetime has a permanent memory of the collision of these two black holes.
The effect, a consequence of general relativity often indicated as “Christodoulou memory,” was described by physicist Demetrios Christodoulou in 1991. In a 2009 book titled “The Formation of Black Holes in General Relativity,” Christodoulou described the “memory effect” of gravitational
waves as “a manifestation of the nonlinear nature of the asymptotic gravitational laws at future null infinity.”
“As the gravitational waves from a binary’s coalescence depart from their source, the waves’ energy creates (via the nonlinearity of Einstein’s field equations) a secondary wave called the ‘Christodoulou memory’,” explained LIGO co-founder Kip Thorne in 1998. “Unfortunately, the memory is so weak that in LIGO only advanced interferometers have much chance of detecting and studying it.”
Lasky explains that effect is so weak that “people did not believe we would be able to measure it with LIGO.” But the new paper proposes a way to detect the memory effect using the results of many observations.
“We turn our attention to Christodoulou [gravitational wave] memory, a purely strong-field gravitational effect,” note the researchers.
We show memory can be probed in the near future using an ensemble of observations of binary black hole systems.
The memory component a gravitational wave signal from a black hole merger measured by Advanced LIGO is expected to account for only a small fraction of the total signal, making it improbable that Advanced LIGO will detect memory from an individual event. But LIGO is expected to detect tens to hundreds of events over the next few years, and analyzing many gravitational wave signals from different events together is expected to permit studying the gravitational wave memory of spacetime. In the paper, the scientist note that the strategy for detecting memory relies on the coherent summation of an ensemble of subthreshold signals.
“Our work has shown that the combination of all these mergers will enable us to measure the memory effect over time,” says Lasky. “The key is being able to stack the signals from all of the events in a clever way.”
“This is a very clever way of measuring gravitational-wave memory and exploring it observationally,” says Thorne. “I never thought it’d be possible with LIGO.” Thorne adds that gravitational-wave memory could shed light on fundamental information physics.
Images from LIGO.