The Theory That Took 100 Years to Prove

by Rahul Jayaraman '19

In 1916, Einstein formulated his general theory of relativity, which posits that spacetime can be thought of as an infinite fabric that is disrupted by the presence of massive objects. When two of these massive objects interact through the gravitational force, Einstein predicted that they send out little ripples in the fabric of spacetime (similar to waves on the ocean). These ripples are known as “gravitational waves, and they can be felt across the universe, up to billions of years after they were generated -- helping us study conditions in the infant cosmos.

While numerous experiments have been conducted over the years to verify other aspects of general relativity, such as the slower passage of time in a gravitational field, the direct detection of gravitational waves eluded scientists until recently. In the 1970's, a binary star system was identified as a candidate system to produce gravitational waves. Over the past 40 or so years, this system was carefully monitored, and changes in the stars’ orbits were found to agree with Einstein’s model of general relativity to high precision.

However, this was not considered a direct detection. To directly detect these waves and their effects, scientists from the National Science Foundation (NSF), Caltech, and MIT built the LIGO interferometers -- one in Washington state and the other in Louisiana. If these waves were to exist, when they passed through each observatory, they would cause a small compression and contraction of the 4 km long L-shaped structure. This contraction was predicted to be 10,000 times smaller than the diameter of a proton; for scale, this is akin to measuring the distance to the nearest star, Proxima Centauri, to an accuracy within the width of a human hair. (That’s pretty precise, if you couldn’t tell already.)

On September 14, 2015, early in the morning, a signal from the collision of two massive black holes 1 billion light years away was detected by LIGO during its “enhanced” phase, in which its sensitivity was increased fourfold from prior runs. The signal matched all the predictions made by general relativity about the effects of gravitational waves and, after confirmation and careful analysis of the results, was unveiled to the public in February 2016. This discovery earned Kip Thorne, Rainer Weiss, and Barry C. Baris the 2017 Nobel Prize in Physics.

Gravitational waves were undoubtedly one of the most sought-after discoveries in physics. Now that we have direct proof of their existence, we can learn more about gravitational interaction in the context of space-time, trace signals from the evolution of our universe (since many collisions/interactions producing gravitational waves occurred billions of years ago), and further refine the predictions of general relativity based on new observations.

Read the original paper here.

Bibliography:

1. Abbott B, Abbott R, Abbott T, Abernathy M, Acernese F, Ackley K et al. Observation of Gravitational Waves from a Binary Black Hole Merger. Physical Review Letters. 2016;116(6).

2. LIGO's Interferometer [Internet]. LIGO Lab | Caltech. 2017 [cited 28 October 2017]. Available from: https://www.ligo.caltech.edu/page/ligos-ifo

3. What are Gravitational Waves? [Internet]. LIGO Lab | Caltech. 2017 [cited 28 October 2017]. Available from: https://www.ligo.caltech.edu/page/what-are-gw