5 Reasons You Should Care About the Discovery of Gravitational Waves

The world of physics was abuzz last week with the historic announcement of the first ever detection of gravitational waves. But why is it such a big deal?

Sabrina Stierwalt, PhD
5-minute read
Episode #181

2. The Instrument That Made the Gravitational Wave Detection Is the Most Precise Measuring System Ever Built

To detect such tiny distortions in spacetime, physicists use a technique called laser interferometry. A focused beam of light is sent in different directions to bounce back and forth between two sets of mirrors before being sent to a detector. If a gravitational wave passes by the interferometer during all of this bouncing, the distance between the mirrors will change ever so slightly and this change will translate to a difference in the two signals as measured by the detector.

Not only is the signal from gravitational waves incredibly weak, there is also a significant amount of competing noise attempting to drown it out. To increase the detectability of such a signal against the background noise, the path the laser travels must be a long one. The Laser Interferometer Gravitational-Wave Observatory (LIGO), the instrument that made the historic detection, is four kilometers long on each side. The detectors are further suspended in the air in hopes of isolating the slightly faster wiggles due to gravitational waves from terrestrial interference.

To further fight back against false detections, LIGO has not one but two detectors: one in Hanford, Washington and the other in Livingston, Louisiana. Detecting the same signal at both, widely separated locations would mean that signal was likely not a local one. And that is exactly what happened on September 14, 2015—a signal with the precise characteristics predicted for gravitational waves was observed at both detectors only milliseconds apart.

3. We Now Know That Massive Black Holes Can Merge to Create Even Bigger Black Holes

Famous theoretical physicist and author Kip Thorne described the event that produced the detected gravitational waves as a "violent storm in the fabric of space and time". Around 1.3 billion years ago, when multicellular life was just beginning here on Earth, two black holes orbiting each other began to close in on one another. As these dense objects got closer, they began to accelerate to nearly half the speed of light in the presence of their shared strong gravitational field - the perfect mix for producing gravitational waves.

From fitting the waveform of the gravitational wave detection and comparing it to simulations done with a supercomputer, astronomers can tell that the two black holes were originally 29 and 36 times the mass of our Sun. They merged to form a 62 solar mass black hole which means that an amount three times the mass of our Sun was emitted away as energy in the form of gravitational waves, all in the 20 milliseconds it took for the collision to happen! That’s a power output of 50 times greater than all of the power put out by all of the stars in the universe put together.

Before this first detection, astronomers were not even sure that mergers between black holes existed, and now the details of this particular event are known to a high degree of certainty.


Please note that archive episodes of this podcast may include references to Ask Science. Rights of Albert Einstein are used with permission of The Hebrew University of Jerusalem. Represented exclusively by Greenlight.

About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.