Detecting Orphan Memory from Gravitational Waves

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Nikolaus Prusinski1, 2, Nancy Aggarwal1, Christopher Berry1

1 Northwestern University, CIERA/Dept. of Physics and Astronomy
2 University of Wisconsin - Milwaukee, CGCA/Dept. of Physics

Background on Gravitational Waves

Gravtiational Waves (GWs) are distortions in spacetime caused by accelerating masses. We have measured GWs from both neutron star and black hole mergers using the LIGO and Virgo detectors. Similar to sound and light, GWs occur in a spectrum of frequencies. Our research looks at common sources between low and high frequency detectors, specifically those from which we may be able to observe memory.

GW Spectrum
Gravitational wave spectrum infographic created by CIERA/Northwestern. Original image is here.

Today's GW Detectors

The LIGO and Virgo detectors are large L-shaped detectors with arms extending over two miles. When a GW passes the LIGO detector, each arm is stretched, and that strain is measured. For binary black holes, the oscillatory strain, \(h\), is approximately \(\frac{\Delta L}{L}\sim 10^{-21}\).

Future GW Detectors

A future detector, called the Levitated Sensor Detector (LSD) will be a fraction of LIGO’s size, and observe high frequency GWs. This high frequency detector is a small cavity with a nanosphere or microdisk optically trapped in an antinode (green circle). When a GW passes, the trap-equilibrium position shifts with respect to the particle (orange circle). This applies an effective force on the particle proportional to the strength of the GW. When the GW frequency is in resonance with the trap frequency, the particle undergoes a large displacement due to this force [1].

LSD Cavity Cartoon
Cartoon representation of LSD cavity with nanospheres represented by green/orange circles.


In this work, we aim to study memory from gravitational waves: a permanent strain left in the spacetime after a GW has passed by. This displacement is small, leading to a memory strain which is ~ 20x smaller than the oscillatory strain.

GW Burst and Memory Time Series Plot
Gravitational wave burst with memory. Note how \(h(t)\) for the Total GW does not return to zero after the wave passes.

Orphan Memory

McNeil et al. show that if a high freq. detector like LSD were to observe the oscillatory part of the GW signal, LIGO would most likely observe the memory part of that signal. In the plot below, we see an example of this with a high frequency burst and memory trailing off to lower frequencies. The name “orphan” comes from the lack of a detectable parent GW signal [2].

Gravitational Wave Signal and Memory Plot
Gravitational wave burst in frequency domain with memory in the LIGO band.

SNR Comparison

Above, we see a signal-to-noise ratio (SNR) comparison for a sample high frequency GW burst along with its memory counterpart. This shows that for such a high frequency burst detectable with an SNR of ~ 15 in LSD, the memory from it might be detectable in LIGO with an SNR of ~ 100.

SNR Comparison Bar Chart
SNR comparison of a high frequency GW burst. SNR of memory in LIGO in magenta, and SNR of LSD detector in blue.


[1] Arvanitaki, A., & Geraci, A. A. (2013). Detecting High-Frequency Gravitational Waves with Optically Levitated Sensors. Physical Review Letters, 110(7), 071105.

[2] McNeill, L. O., Thrane, E., & Lasky, P. D. (2017). Gravitational Waves from Orphan Memory. Physical Review Letters, 118(18), 181103.


Special thanks to Aaron Geller, Eve Chase, and the entire 2019 CIERA REU cohort for discussions, peer review, and an all around welcoming research environment throughout the summer. It’s been a blast!

This material is based upon work supported by the National Science Foundation under Grant No. AST-1757792, a Research Experiences for Undergraduates (REU) grant awarded to CIERA at Northwestern University. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Pertinent Links

Nik's Background Page
CIERA REU Homepage
Aaron Geller's Homepage

Copyright © 2019 Nikolaus Zen Prusinski