Now synonymous with “genius”, Albert Einstein has turned out to be the gift that keeps on giving. In the early 1900s, Einstein had the incredible intuition and scientific foresight to produce work that would guide scientific endeavor for the next century. His theories about mass-energy equivalence (E = mc2) and general relativity have changed the way we understand the world when it comes to large, high energy objects. The nuance of his ideas has inspired revolutionary scientific and technological developments to this day in the effort to verify some seemingly bizarre characteristics of the natural world like the bending of space, time, and light. Though Einstein’s hypotheses contributed profoundly to the understanding of space-time and the movement of celestial bodies, the consequences of the phenomena he described were very difficult to measure and remained theoretical. That is, until September 14, 2015, when the Laser Interferometer Gravitational-Wave Observatory (LIGO) became the first to measure gravitational waves.
In his theory of general relativity, Einstein presents theories about the effect of mass on space in a way that strays from our typical Newtonian intuition about mechanics. Isaac Newton’s idea of motion is perfectly valid for an apple falling from a tree or a truck driving on the road, but when Newton’s apple has the mass of a planet, the rules are slightly different. To begin, at extreme scales (supergalactic or subatomic), space and time can no longer be treated separately and are actually better understood as an intertwined continuum intuitively termed space-time. The most intuitive visualization of space-time is as a Euclidean space, or a typical three-dimensional coordinate plane.
These waves travel at the speed of light and, because they are literal bends in space, distort the masses they pass through periodically according to the frequency of the wave. This would be like having an elastic sheet with a ball drawn on it; if the sheet is bent or stretched, the drawing will also contort and then return to its original shape when the sheet is released. What actually happens is analogous, but in three dimensions; when a gravitational wave passes through an object — a planet, for instance — the planet will be compressed in one dimension and stretched in the other two dimensions.
To put this into perspective, gravitational waves share a lot of qualities with light and sound, including the inverse square law. That is, they are weaker further from the source (just as a light is dimmer and a sound is softer). Moreover, these waves are not isolated since lots of events that may cause waves of various sizes occur simultaneously and individual waves can be superimposed. Now, it begins to make sense why these waves are so difficult to measure since each wave in question needs to be distinct and measurable. Because the waves dissipate with distance according to the inverse square law, a measurable wave should be the result of a relatively large, nearby event. For this reason, LIGO, funded by the National Science Foundation (NSF), was a leap of faith.
Due to the precision of the measurements needed and the very specific conditions required for success, LIGO was a very expensive and risky undertaking. However, it was also immeasurably useful, as sensing a gravitational wave from an event reveals both that the event occurred and where and when it occurred based on the angle and strength of the signal. In other words, LIGO essentially provided a way to read the information documented by gravitational waves about events in the past. This is most likely why NSF took that leap, allowing two identical observatories to be built in Livingston, Louisiana and Hanford, Washington in order to confirm any observed waves.
Finally, in September 2015, the waves from a suitable event — the merging of two black holes — were detected. The signal, GW150914, was measured in both observatories with a small lag, corresponding to the amount of time it would have taken light to travel the distance between them. It was not only the first measurement of gravitational waves, but also of a binary black hole, confirming both that binary black holes exist and that they could happen within the age of the universe. Additionally, LIGO measured the masses of the two black holes and the merged result, finding a discrepancy between the sum of the two originals and the product. This difference matched the energy dissipated during the merger into the wave by the equivalence of mass and energy according to E = mc2. Furthermore, the lag in the measurement between the two observatories allowed researchers to determine the approximate angle and position of the original event. Because the frequency of the wave fell within human hearing range, the gravitational wave could be translated to a sound wave of the same frequency so that one can, quite literally, listen to the sound of the universe. Furthermore, the measurement of the wave has astronomical implications, not least among which is the confirmation of Einstein’s conception of space-time.
LIGO’s success marked the beginning of gravitational wave astronomy and contributed to inquiries regarding black holes, stars, and the expansion of the universe. It also confirms an unproven prediction of Einstein’s general relativity and affirms a structured, complete theory. And, finally, gravitational wave detection promises a way for us to learn about past celestial events and listen to the universe’s history.