Whereas electromagnetic astronomy is about seeing the universe, gravitational wave astronomy is about hearing it. Although sound waves don’t travel through space, the ripples in spacetime predicted by Einstein’s theory of general relativity do, forming the soundtrack of our universe in the process. Other than a poetic analogy, you may be wondering: what do gravitational waves have to offer us?
To answer that question, let’s start by addressing what gravitational waves are and why scientists care about detecting them. In Albert Einstein’s theory of general relativity, we live in a four-dimensional universe, with three spatial and one-time dimensions that we collectively refer to as spacetime. Within that framework, mass produces spacetime curvature. That curvature affects the motion of massive objects, producing the effect we call gravity. Mass in nonuniform, nonspherical motion generates ripples in spacetime curvature that propagate (move) at the speed of light, called gravitational waves. One of the main reasons that scientists are interested in detecting them is their weak coupling to matter: gravitational waves are barely absorbed by matter on the path from their source to the Earth. Consequently, gravitational waves allow us to receive information from much more distant events than electromagnetic radiation (Hartle, 2003). Gravitational wave observations from black holes, for example, provide opportunities to test Einstein’s general relativity against alternative theories of gravity. Since these theories only differ from general relativity in very strong gravitational fields, scientists need data from extreme objects like black holes to compare their predictions. Gravitational radiation is also a useful tool for modeling astronomical systems and can provide insight into our universe’s cosmography (structure and kinematics) and early structure formation (e.g. exploring the relationship between supermassive blackhole formation and galaxy formation) (Sathyaprakash & Schutz, 2009). Other questions that gravitational waves have the potential to address include: How does matter behave under extreme densities and pressures? What drives gamma-ray bursts? (LIGO's Impact on Science and Technology, n.d.) Does the universe have a gravitational-wave background (CWB), and if so, what causes it? The answer to the first part of this question was recently revealed to be an exciting yes, and scientists are now investigating the CWB’s potential origins (“The NANOGrav 15 Yr Data Set: Evidence for a Gravitational-Wave Background,” 2023). In summary, physicists and astronomers can use gravitational waves to fill in the gaps left by other astronomical messengers.
If you’re not in physics or astronomy, though, you might not be that fascinated by the prospect of modeling black hole mergers or galaxy formation. So, what’s in it for you? As with most research in astronomy-related fields, gravitational waves’ short-term societal impact will likely take place in the form of “spinoff technologies,” or technologies made/improved for research purposes that prove useful to the public. The Laser Interferometer Gravitational-Wave Observatory (LIGO), for example, is a ground-based detector that uses laser interferometry to detect the tiny changes in distances caused by gravitational waves passing the Earth. Because these length differences are so small (less than 1/10,000 the diameter of an atomic nucleus) and easily confused with noise, LIGO involves the development and improvement of advanced optics, quantum optics, and laser systems, as well as computational methods to extract signals from data (Technology Development and Migration, n.d.). These technologies are frequently transferred to other fields: the initial laser LIGO used is now sold for making computer chips and smartphone circuit boards, algorithms developed to find gravitational wave signals in LIGO data are now used in radar, sonar, and laser pulses (The Science of LSC Research, n.d.), and federated identity management tools developed for use in LIGO are now being incorporated in the development of COmanage, a tool for collaborative organizations to manage effective and secure access to applications and services (Technology Transfer Case Studies, n.d.). In the long run, we can also expect that scientific discoveries from gravitational waves will eventually result in new innovations for the public. When relativity was discovered, for example, it allowed for the development of GPS technology (Ashby, 2003). If gravitational wave observations allow us to find a better theory of gravity than Einstein’s general relativity, it is likely that this new and more precise theory will lead to technological breakthroughs we cannot imagine yet.
Gravitational wave observations can help answer fundamental questions in physics, astronomy, and cosmology, adding to the information scientists gather from other astronomical sources. To make these observations possible, technological innovations in optics and computation are developed, which are transferable to a variety of different fields. Whether you’re an astrophysicist, a scientist from a different field, or a member of the public, gravitational wave astronomy shows great promise in the years to come as new melodies from our universe are uncovered.
References
Ashby, N. (2003). Relativity in the Global Positioning System. Living reviews in relativity, 6(1), 1. https://doi.org/10.12942/lrr-2003-1
Hartle, J. B. (2003). Gravitational Waves. In Gravity : an introduction to Einstein's general relativity (pp. 331-346). Addison-Wesley.
LIGO's Impact on Science and Technology | LIGO Lab | Caltech. (n.d.). LIGO Caltech. Retrieved August 4, 2023, from https://www.ligo.caltech.edu/page/science-impact
The NANOGrav 15 yr Data Set: Evidence for a Gravitational-wave Background. (2023). The Astrophysical Journal Letters, 951(1).
Sathyaprakash, B. S., & Schutz, B. F. (2009). Physics, Astrophysics and Cosmology with Gravitational Waves. Living Review in Relativity, 12(1). https://link.springer.com/article/10.12942/lrr-2009-2
The science of LSC research. (n.d.). LIGO Scientific Collaboration - The science of LSC research. Retrieved July 25, 2023, from https://www.ligo.org/science/faq.php#spinoffs
Technology Development and Migration | LIGO Lab | Caltech. (n.d.). LIGO Caltech. Retrieved August 4, 2023, from https://www.ligo.caltech.edu/page/technology-transfer
Technology Transfer Case Studies | LIGO Lab | Caltech. (n.d.). LIGO Caltech. Retrieved August 4, 2023, from https://www.ligo.caltech.edu/page/technology-transfer-case-studies