In this blog post, we explore the scientific significance and new possibilities brought about by humanity’s first direct detection of gravitational waves, examining how it transformed modern physics and the way we observe the universe.
The 2017 Nobel Prize in Physics was awarded to three American physicists—Kip Thorne, Rainer Weiss, and Barry Barish—who played a decisive role in discovering gravitational waves. They were recognized for their achievement in successfully detecting gravitational waves directly for the first time in February 2016 at LIGO (Laser Interferometer Gravitational-Wave Observatory). So what exactly are gravitational waves, and why is confirming their existence significant enough to warrant a Nobel Prize in Physics? To understand this, let’s first examine the concept of gravitational waves.
The concept of ‘gravitational waves’ has been around for over a century. Albert Einstein first predicted them in 1916 through his General Theory of Relativity. According to this theory, objects with mass warp spacetime, and gravity is the phenomenon arising from this warping. Furthermore, accelerating objects cause this curved spacetime to ripple, and these ripples propagate outward as waves at the speed of light. These are gravitational waves. When a gravitational wave passes through, space itself becomes distorted, stretching in one direction and contracting in another.
However, the distortion of space caused by gravitational waves is extremely small, making them undetectable under normal circumstances. They are only produced at significant levels during massive cosmic events like the collision of two black holes or a supernova explosion, yet even then, the signal is extremely faint. Current gravitational wave detectors measure the change in how much space has stretched. The disturbance created by the gravitational wave detected by LIGO this time stretched and contracted space by only about 10⁻²¹ times. To measure this minute change, the length must be measured with a precision smaller than one-thousandth of a neutron’s radius within a detector approximately 5 km long. This was practically impossible. Therefore, before LIGO, it was impossible to directly detect gravitational waves; their existence could only be inferred indirectly.
So how did LIGO manage to directly detect these minute length changes? In other words, how could gravitational waves be observed? LIGO fundamentally uses the principle of an interferometer. An interferometer is an instrument that utilizes the phenomenon of light interference to measure distance changes at an ultra-precise level. To understand this, let’s first look at wave interference.
Waves are akin to ripples. When two waves of the same form meet, their amplitude either increases (constructive interference) or decreases (destructive interference) depending on how they overlap. If both waves arrive with the same phase, constructive interference occurs. However, if one wave arrives later, causing the combined waves to be out of phase, destructive interference happens. Thus, the time difference in arrival between two waves causes a change in the amplitude of the combined wave, and this is the interference phenomenon.
Since light is also a wave, interference occurs when two light waves combine. Therefore, analyzing the amplitude of the combined wave allows us to calculate the time difference in arrival between the two light waves, which is equivalent to calculating the distance difference. This is because if two light beams depart simultaneously, the greater the distance, the larger the difference in arrival time.
LIGO is based on the ‘Michelson interferometer’ among such interferometers. The Michelson interferometer is also a historically significant experimental apparatus, notably featured in the Michelson-Morley experiment. This experiment revealed that the speed of light is independent of direction and that light requires no separate medium to propagate.
The Michelson interferometer operates on the following principle. Light from a single source is split into two beams by a centrally located beam splitter (a device that transmits half the light and reflects the other half). The two beams are reflected by mirrors placed at fixed distances and then recombine to form an interference pattern. If the speed of light varied with direction, the time taken for the two split beams to recombine would differ, resulting in a change in the interference pattern. The Michelson-Morley experiment predicted such a change in the interference pattern, but no change was observed, leading to the conclusion that the speed of light is constant. This fact later provided crucial clues for Einstein to formulate his theory of relativity.
LIGO is essentially a massive-scale extension of this Michelson interferometer. The distance between LIGO’s beam splitter and its reflecting mirror reaches approximately 4 km. However, this distance alone was insufficient for reliably detecting gravitational waves, so LIGO incorporated a ‘Fabry-Pérot tube’. This technique reflects light approximately 400 times within a 4-kilometer-long tube, effectively creating a path length of 1,600 kilometers. This allows for more precise measurement of minute distance changes. Thanks to this sophisticated technological combination, LIGO succeeded in directly observing gravitational waves generated by the collision of two black holes for the first time on September 14, 2015.
So why is the discovery of gravitational waves such a monumental event? First, its significance lies in directly confirming Einstein’s theory of relativity once again. The existence of gravitational waves, predicted by relativity, has been experimentally verified. But its greater value lies elsewhere. It means humanity has gained a completely new tool for observing the universe. This change is comparable to the moment humanity first created the telescope. Until now, astronomy has relied solely on light—that is, electromagnetic waves—to observe celestial objects. But with gravitational waves establishing themselves as a new observational tool, a realm previously inaccessible to electromagnetic waves has finally opened.
For instance, in the case of supernova explosions, we have never been able to directly observe what occurs within their cores. This is because the immense layer of material enveloping the supernova core blocks light from passing through. Gravitational waves, however, pass through matter with minimal interference, allowing us to capture phenomena unfolding deep within celestial objects.
Humanity now stands at the threshold of a new era called ‘gravitational-wave astronomy’. Gravitational waves will provide us with a completely new perspective on the universe, enabling us to uncover the secrets of various celestial bodies and take a step closer to understanding the origin of the universe. We eagerly anticipate what discoveries these observations will lead to and hope that gravitational-wave research will further expand humanity’s understanding of the cosmos.
