From the July 2019 issue

What Creates Gravitational Waves?

Just as a boat creates waves on a lake as it slices forward through the water, stars and other bodies in the universe create ripples in the fabric of space-time.
By | Published: July 1, 2019 | Last updated on May 18, 2023
WAVY BANG. Supernova explosions, like the one that produced the Crab Nebula (M1) in the constellation Taurus in 1054, are significant producers of gravitational waves.
NASA/The Hubble Heritage Team
In 1916, Albert Einstein revolutionized our understanding of the universe when he published his general theory of relativity. In it, the German-born physicist described the complex relationship between the fabric of space-time and the mass of celestial bodies. Space-time is the combination of three spatial directions (height, width, and depth) with the time dimension.

The easiest way to interpret gravitational interactions, Einstein said, is to think of the space-time continuum as a stretchable material that bends as massive objects “sit” inside it. While this two-dimensional analogy does not represent what is happening in four-dimensional space-time, it serves as a capable model.

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CENTRAL ENGINE. Black holes are important creators of gravitational waves. This artwork shows the central supermassive one that powers the active galaxy PKS 0521–36.
Dana Berry/STScI

When you stretch a pliable plastic sheet tautly, and you place a softball on it, the gravity well around the ball pushes the sheet downward and curves the fabric. The same thing happens in the four-dimensional universe. Near actual massive objects that have large gravitational pulls, the “fabric” of space-time curves and stretches.

Massive objects also cause another effect in the fabric of space-time. Just as a boat creates waves on a lake as it slices forward through the water, stars and other bodies in the universe create ripples in the fabric of space-time as they move. Astronomers call these ripples gravitational waves.

Immense objects like black holes create larger gravitational waves than less massive objects. Likewise, objects moving rapidly through space create more sustained gravitational waves than slower moving ones. When these gravitational-wave signals finally reach Earth, however, they are extremely weak. Like waves in water, gravitational waves diminish as they move outward from their origin. So gravitational waves prove difficult to detect and interpret once they reach us from a variety of distant locations.

But after years of searching, in late 2015, researchers finally detected the first clear signal of a gravitational wave passing through Earth. This signal, dubbed GW150914, originated from the merger of two black holes with a combined mass of roughly 60 suns. Since then, scientists have confirmed gravitational-wave signals from four additional merging pairs of black holes, as well as a merging pair of neutron stars for good measure.

With these detections, astronomers now know the interaction of two compact and massive bodies usually produces gravitational waves. The interactions can be between binary black holes or neutron stars, but they also may be between merging galaxies or normal stars simply encountering each other.

SPINNING STAR. Gravitational waves originate from a variety of objects, including pulsars, rapidly rotating neutron stars. Using NASA’s Rossi X-ray Timing Explorer, astronomers found in 2003 the upper limit to a pulsar’s spin, based on an outburst on a pulsar, shown in this series of illustrations.
Dana Berry/STScI
To help detect these faint gravitational-wave signals, astronomers use a technique called interferometry. Two large test masses placed a great distance apart serve as detectors. The masses are free to move in all directions, and lasers continuously measure the exact distance between them. When a gravitational wave passes through them, the cosmic ripple causes their distance to fluctuate slightly. It’s an ingenious technique, and scientists have used such devices in several places around the world to hunt for gravitational waves.

The Advanced Laser Interferometer Gravitational-wave Observatory (LIGO), a joint project between MIT and Caltech, has two locations: one in Hanford, Washington, and the other in Livingston, Louisiana. Teaming up with LIGO is the French- and Italian-led Virgo collaboration, which operates the Advanced Virgo interferometer, a third gravitational-wave detector that enables researchers to better pinpoint the sources of gravitational-wave events like those found in recent years. 

Above all these Earth-based projects, the European Space Agency (ESA) plans to launch the Laser Interferometer Space Antenna (LISA) in 2034. To test the space-based technologies needed for such an enormous mission, ESA, with the help of NASA, launched the LISA Pathfinder satellite in 2016. Since Pathfinder far exceeded expectations, the LISA project will likely provide the best observatory for detecting gravitational waves and will give astronomers significant clues about the interaction of matter and space-time, and how the universe came to be.