Early in the morning on September 14, 2015, scientists observed an extraordinary event: the collision of two black holes. Each of these black holes was about 30 times more massive than our Sun. They had been orbiting each other for millions of years, gradually spiraling closer together. As they neared, they orbited faster and faster until they finally collided and merged into a single, larger black hole. Just before they collided, they sent out a ripple through the universe at the speed of light. This ripple was detected on Earth by an instrument called the Laser Interferometer Gravitational-Wave Observatory, or LIGO. The signal lasted only a fifth of a second, marking the first time gravitational waves were observed.
So, what exactly are these ripples in space? To understand them, we need to start with gravity, the force that pulls objects toward each other. This force is universal, affecting everything from the smallest particles to the largest galaxies. The more massive an object, the stronger its gravitational pull. Conversely, the farther away an object is, the weaker its pull. Since every mass in the universe influences every other mass, changes in gravity can tell us about the activities of those objects. These changes are known as gravitational waves.
Gravitational waves spread out from their source like ripples on a pond, weakening as they travel farther away. But what do these ripples affect? When Einstein developed his Theory of Relativity, he described gravity as a bending of space-time, the fabric of the universe. A massive object creates a dip in space-time, and anything moving nearby will follow a curved path, as if drawn by the object’s gravity. When the object moves, it creates ripples in space-time, which are the gravitational waves.
If we could feel gravitational waves, it would be like being stretched sideways while being compressed vertically, and then stretched up and down while being compressed horizontally. This would happen repeatedly as the wave passed through us. However, these changes are so tiny that we can’t feel them. That’s why we have detectors like LIGO to sense these waves for us. LIGO is one of several gravitational wave detectors around the world.
These detectors are L-shaped instruments with long arms. Lasers measure the lengths of these arms, and if the lengths change, it might mean gravitational waves are stretching and compressing them. Once a detector senses a gravitational wave, scientists can learn about its source. In a way, detectors like LIGO are like giant radios for gravitational waves. Just as we can’t hear radio waves without a radio, we can’t detect gravitational waves without these instruments. LIGO picks up the signal, and scientists analyze it to learn about the object that created it, such as its mass and orbit.
We can even listen to gravitational waves by turning their signals into sound, much like how a radio turns radio waves into music. The sound from the collision of two black holes is like a chirp, a signature sound of two objects spiraling into each other. The black hole collision is just one example of what gravitational waves can reveal. Other cosmic events, like a star collapsing before a supernova or neutron stars colliding, also create gravitational echoes.
Every time we create a new tool to observe the universe, we make unexpected discoveries that can change our understanding of space. LIGO is no exception. In its short time of operation, LIGO has already revealed surprises, such as the fact that black holes collide more often than we thought. While we can’t predict what we’ll find next, it’s exciting to think about the new discoveries that might be heading our way, reshaping our view of the universe.
Using a large piece of fabric and some heavy objects like balls or weights, simulate the bending of space-time. Place the weights on the fabric to create dips, and roll smaller balls around them to observe how they move. This will help you visualize how massive objects like black holes affect space-time and generate gravitational waves.
Access real data from LIGO’s public database. Try to identify patterns or signals that indicate gravitational waves. Use software tools to analyze the data and understand how scientists determine the source and characteristics of these waves.
Find audio files of gravitational wave signals converted into sound. Listen to the “chirps” and discuss what these sounds represent. Consider how different cosmic events might produce different sounds and what information can be gathered from them.
Choose a specific gravitational wave event detected by LIGO or another observatory. Research the event’s details, such as the masses of the objects involved and the distance from Earth. Present your findings to the class, explaining the significance of the event.
Participate in a class debate about the future of gravitational wave research. Discuss the potential discoveries and technological advancements that could arise from this field. Consider the implications for our understanding of the universe and the challenges that scientists might face.
At about six o’clock in the morning on September 14, 2015, scientists witnessed something unprecedented: two black holes colliding. Both were approximately 30 times as massive as our Sun and had been orbiting each other for millions of years. As they approached one another, they circled each other increasingly faster until they finally collided and merged into a single, larger black hole. A fraction of a second before their collision, they emitted a vibration across the universe at the speed of light. On Earth, a detector known as the Laser Interferometer Gravitational-Wave Observatory, or LIGO for short, picked up this signal. The signal lasted only a fifth of a second and marked the detector’s first observation of gravitational waves.
What are these ripples in space? The answer begins with gravity, the force that attracts any two objects together. This applies to everything in the observable universe. You are pulling on the Earth, the Moon, the Sun, and every single star, and they are pulling on you. The more mass an object has, the stronger its gravitational pull. Conversely, the farther away an object is, the weaker its pull. If every mass influences every other mass in the universe, no matter how small, then changes in gravity can inform us about the activities of those objects. Fluctuations in gravity from the universe are referred to as gravitational waves.
Gravitational waves propagate outward from their source, similar to ripples on a pond, diminishing in strength as they travel farther from their origin. But what do these ripples affect? When Einstein formulated his Theory of Relativity, he envisioned gravity as a curvature in a framework known as space-time. A mass in space creates a depression in space-time, and a ball rolling across this depression will curve as if attracted to the other mass. The larger the mass, the deeper the depression and the stronger the gravitational force. When the mass creating the depression moves, it generates ripples in space-time, which are gravitational waves.
What would it feel like to experience a gravitational wave? If our bodies were sensitive enough to detect them, we would feel as though we were being stretched sideways while being compressed vertically, and then stretched up and down while being compressed horizontally, and so on. This oscillation would occur repeatedly as the gravitational wave passed through us. However, this happens on such a minuscule scale that we cannot perceive it. Therefore, we have constructed detectors that can sense these waves for us. This is the function of the LIGO detectors, and they are not alone; there are gravitational wave detectors located around the world.
These L-shaped instruments have long arms, the lengths of which are measured using lasers. If the lengths change, it may indicate that gravitational waves are stretching and compressing the arms. Once the detectors sense a gravitational wave, scientists can extract information about its source. In a sense, detectors like LIGO act as large gravitational wave radios. Radio waves are constantly traveling around us, but we cannot feel them or hear the music they carry without the appropriate detector. LIGO identifies a gravitational wave signal, which scientists then analyze for data about the object that generated it, such as its mass and the shape of its orbit.
We can also listen to gravitational waves by converting their signals into sound, similar to how a radio extracts music from radio waves. The sound produced by the collision of those two black holes resembles a chirp, which is the signature of any two objects spiraling into one another. The black hole collision was just one example of what gravitational waves can reveal. Other high-energy astronomical events, such as the collapse of a star before it explodes in a supernova or the collision of very dense neutron stars, will also leave gravitational echoes.
Every time we develop a new tool to observe space, we uncover unexpected discoveries that may transform our understanding of the universe. LIGO is no exception. In its brief operational period, LIGO has already unveiled surprises, such as the fact that black holes collide more frequently than previously anticipated. While it is impossible to predict, it is thrilling to consider what new revelations may be traveling through space toward our small blue planet and its evolving perception of the universe.
Gravitational – Relating to the force of attraction between any two masses. – The gravitational pull of the Earth keeps the Moon in orbit around it.
Waves – Disturbances that transfer energy through space or matter, often characterized by oscillations. – Gravitational waves were first predicted by Einstein and were detected by LIGO in 2015.
Black – Referring to black holes, regions of space where the gravitational pull is so strong that nothing, not even light, can escape. – The black hole at the center of our galaxy is known as Sagittarius A*.
Holes – In the context of black holes, these are regions in space with extremely strong gravitational effects. – Scientists study the event horizon of black holes to understand their properties.
Gravity – The natural force of attraction exerted by a celestial body, such as Earth, upon objects at or near its surface, drawing them toward the center of the body. – Gravity is responsible for keeping planets in orbit around the Sun.
Space-time – The four-dimensional continuum in which all events occur, integrating the three dimensions of space and the one dimension of time. – Einstein’s theory of relativity describes how mass and energy can warp space-time.
Universe – The totality of known or supposed objects and phenomena throughout space; the cosmos; macrocosm. – The universe is constantly expanding, as evidenced by the redshift of distant galaxies.
Detectors – Instruments or devices used to observe and measure phenomena such as gravitational waves or cosmic radiation. – LIGO and Virgo are detectors that have successfully observed gravitational waves.
Mass – A measure of the amount of matter in an object, typically in kilograms or grams, which is not affected by gravity. – The mass of an object determines the strength of its gravitational pull.
Light – Electromagnetic radiation that is visible to the human eye and is responsible for the sense of sight. – Light from distant stars takes millions of years to reach Earth, allowing us to look back in time.
