The detection of gravitational waves by the LIGO and Virgo groups announced in 2016 was one of the biggest experimental breakthroughs in recent physics. Not only was it direct evidence for a prediction made all the way back in 1916 by Einstein, but also was the first observation of a binary black hole merger. But first, what are gravitational waves and why do we care so much about them?
When massive objects that exert tremendous gravitational fields orbit each other, they send faint ripples through the fabric of spacetime. Gravitational waves are a way of exerting gravity on other objects. According to Newton’s laws of motion, the information that a gravitational force is being exerted on an object is sent instantaneously. However, from Einstein’s special relativity we know that no information can travel at the speed of light. Therefore he predicted that gravity acts on other object through waves, same as light from the Sun travels to Earth in the form of light waves. Gravity is a very weak force, on the relative scale of 10-39 compared to the strong force, therefore these undulations are faint and difficult to detect.
These waves exist on a spectrum, and their frequencies are determined by the mass and acceleration of the body. They are emitted when an object experiences acceleration, provided it is not symmetrical such as the expansion and contraction of a sphere.
How does LIGO detect such faint signals? The basic principle is quite simple. Two light beams are split down 4km arms and reflected off mirrors. The presence of a gravitational wave alters the length of the arms, which would change the time taken for one beam to travel back and forward, desynchronising them. The beams travel in a vacuum to ensure that no gas particles disturb them, and there is painstaking maintenance and measurement work done to ensure perfection, as the detectors run months at a time collecting data points. The merger of the black holes referred to as GW150914 happened a billion light years away, so it changed the length of a LIGO arm that spans 4km by a thousand of a proton. Therefore, any noise due to weather, seismic events or equipment malfunctions is detrimental to the result.
During the detection of the event, the experimental data was compared with the theoretical prediction to see if there was any correlation and if this was not simply noise. As you can see, the data fit very well, but despite that debate raged over this result. Nonetheless, several more detections of binary neutron star gravitational waves in 2017 and 2018 put an end to the debate.
Why should we care? Not only is this another piece of evidence for general relativity, but it also showed that smaller black holes can merge to form bigger ones. The LIGO experiment pushed the envelope for what is possible in large scientific collaborations. Similar to the discovery of the Higgs Boson, the detection of gravitational waves shows that sky is the limit in terms of building very precise instruments. Perhaps, this will pave the wave to detecting gravitons and developing experimental evidence for string theory. For now, we should be proud of what these scientists have accomplished.
References
Daw, E. (2016, February 11). Gravitational waves discovered: how did the experiment at LIGO actually work? The Conversation. https://theconversation.com/gravitational-waves-discovered-how-did-the-experiment-at-ligo-actually-work-54510
Stierwalt, S., PhD. (2016, February 15). 5 Reasons You Should Care About the Discovery of Gravitational Waves. Quick and Dirty Tips. https://www.quickanddirtytips.com/education/science/5-reasons-you-should-care-about-the-discovery-of-gravitational-waves
Tate, K. (2014, April 11). Hunting Gravitational Waves with Lasers: How Project LIGO Works (Infographic). Space.Com. https://www.space.com/25445-how-ligo-lasers-hunt-gravitational-waves-infographic.html
What are Gravitational Waves? (n.d.). LIGO Lab | Caltech. https://www.ligo.caltech.edu/page/what-are-gw