In this blog post, we’ll explore the concept of gravitational waves, the process of their discovery, and what their detection means for modern physics and cosmology.
What are gravitational waves, and why is this such an important discovery?
On February 11, 2016, an international research team officially announced that they had directly detected gravitational waves for the first time. This news captured the attention of the global media and was widely reported as major news in South Korea as well. The research team, which played a pivotal role in the observation of gravitational waves, went on to win the 2017 Nobel Prize in Physics, and the direct detection of gravitational waves is regarded as one of the most significant achievements in 21st-century physics, alongside the “discovery of the Higgs boson.” However, for most people who aren’t particularly interested in physics, the very concept of gravitational waves would have been unfamiliar, and it would have been difficult to understand why this was such an important discovery. What exactly are gravitational waves, and why did humanity have to wait 100 years to discover them? And why is the discovery of gravitational waves considered such a great achievement?
First, let’s try a simple quiz. The Earth orbits the Sun along an elliptical path due to the Sun’s gravitational pull. So, if the Sun were to suddenly disappear, how would the Earth move? Many people think that the moment the Sun disappears, the Earth would immediately be flung off in the tangential direction of its orbit. In fact, scientists during Newton’s time thought similarly. However, that is not the correct answer. The Earth would continue moving along its existing orbit for about 8 minutes before finally heading off in the tangential direction. This is because, just as it takes about 8 minutes for light to travel from the Sun to Earth, changes in gravity are also transmitted at the speed of light. In other words, the existing gravitational force continues to act until the information that the Sun has disappeared reaches Earth. This phenomenon, in which distortions in spacetime are transmitted through space in the form of waves, is called a gravitational wave.
How were gravitational waves predicted?
The first person to predict the existence of gravitational waves was Einstein himself. In his General Theory of Relativity, he explained that no information can travel faster than light. Since a change in gravity is also a form of information, a change in spacetime occurring at one point cannot be transmitted instantly to another point. Therefore, changes in gravity must propagate through space like waves, and this is the very concept of gravitational waves.
Although countless physicists attempted to verify this prediction, gravitational waves remained undetected for a full 100 years. The main reason is that gravitational waves themselves are an extremely faint signal. Since gravitational waves cause space-time itself to expand and contract ever so slightly, the equipment used to measure them is also affected. In other words, because not only the object being observed but also the observation apparatus itself is simultaneously distorted, gravitational waves cannot be detected using simple methods.
To solve this problem, scientists employed a method that compares the minute changes in distance occurring at two separate points. When a gravitational wave passes by, the two points experience different degrees of distortion in spacetime; by measuring this difference with extreme precision, the existence of gravitational waves can be confirmed.
The device developed for this purpose is LIGO (Laser Interferometer Gravitational-Wave Observatory). LIGO detects the extremely small changes in distance caused by gravitational waves by sending lasers into two perpendicular tunnels, each approximately 4 km long, and measuring the resulting interference. However, in addition to gravitational waves, numerous factors—such as earthquakes, vehicle vibrations, wind, and thermal expansion—can affect the laser measurements. Therefore, to eliminate such errors, identical observatories were installed at two locations in the United States, approximately 3,000 km apart, to compare whether the same signal was observed simultaneously at both sites. As such, detecting gravitational waves required both massive and extremely precise observational equipment and analytical techniques.
How was the existence of gravitational waves first confirmed?
If so, why did scientists go to such great lengths and expense to search for gravitational waves that had not even been directly observed? The reason was that there was already strong indirect evidence supporting the existence of gravitational waves.
In the 1970s, Russell Hulse and Joseph Taylor conducted long-term observations of a binary system of neutron stars orbiting each other. As a result, they discovered that the orbital period of the two celestial bodies was gradually shortening. This meant that the two bodies were continuously losing energy as they orbited each other.
Scientists calculated the amount of energy lost and confirmed that it matched almost perfectly with the energy emitted by gravitational waves, as predicted by the general theory of relativity. In other words, the energy lost by the neutron stars was being emitted into space in the form of gravitational waves. This discovery was a major achievement that indirectly confirmed the existence of gravitational waves, and the two scientists were awarded the 1993 Nobel Prize in Physics for this work.
Where did humanity’s first gravitational waves come from?
The first gravitational waves ever directly observed by humanity were generated by a collision of black holes.
On September 14, 2015, LIGO detected a gravitational wave signal for the first time in history. After undergoing a very rigorous verification process over several months, the research team officially announced this finding in February 2016.
Analysis revealed that, in a region of space approximately 1.3 billion light-years away, two black holes—with masses of about 36 and 29 times that of the Sun, respectively—collided and merged into a single, larger black hole. During this process, a mass equivalent to about three Suns was converted into energy and emitted in the form of gravitational waves. This massive ripple in spacetime traveled through the universe for about 1.3 billion years before passing by Earth, and LIGO detected that faint signal.
Why is the discovery of gravitational waves so significant?
Ultimately, humanity has directly proven the existence of gravitational waves—which Einstein predicted in 1916—after about 100 years. So why is this discovery so significant?
The first reason is that it has provided a new observational tool for directly studying black holes and neutron stars. The LIGO research team explained the significance of this discovery by comparing it to the discovery of X-rays. Just as X-rays allowed us to observe the inside of the human body without making an incision, gravitational waves have opened a new window onto the universe—one that was previously invisible to conventional electromagnetic waves.
Since the invention of the telescope, humanity has observed the universe using not only visible light but also various electromagnetic waves, such as radio waves, infrared, ultraviolet, X-rays, and gamma rays. However, these methods had a fundamental limitation: they could only observe celestial objects that emit or reflect light. Celestial objects like black holes, from which not even light can escape, were extremely difficult to observe directly.
However, by using gravitational waves, we can study black holes directly and obtain various pieces of information—such as their mass, rotation speed, and collision processes—that were difficult to determine using only conventional electromagnetic waves.
The second reason is that gravitational waves allow us to study the history of the universe. The first gravitational waves mentioned earlier originated about 1.3 billion light-years away. In other words, this means we are now observing an event that occurred approximately 1.3 billion years ago. Since gravitational waves carry the exact traces of massive collisions and explosions that occurred in the universe all the way to Earth, analyzing them allows us to gain a deeper understanding of the history and evolution of the universe.
The third reason is the potential for future technology. Because gravitational waves interact very little with matter, there is theoretical potential for them to overcome some of the limitations of existing electromagnetic wave communications. However, at the current level of technology, communication using gravitational waves has not yet been realized as a practical technology and is considered a very long-term research project.
Gravitational waves have opened a new window for humanity to view the universe, much like when the telescope was first invented. Currently, not only LIGO but also the Virgo Collaboration, KAGRA, and other international observatories are working together to build an observation network, and space-based gravitational wave observation projects are also being pursued. The era of full-fledged gravitational-wave astronomy has now begun. No one yet knows what new phenomena and secrets of the universe will be revealed through gravitational waves in the future. But one thing is certain: humanity now has another window through which to observe new aspects of the universe that were previously invisible.
