Gravitational Waves, Part 1: Ripples Across Spacetime (Introduction)

On September 14, 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) observed gravitational waves for the first time. These waves were the results of a collision of two black holes of 29 and 36 solar masses. The 2017 Nobel Prize in Physics was awarded to Rainer Weiss, Barry Barish, and Kip Thorne - the pioneers of gravitational wave observation. Let’s find out more about gravitational waves and why they bring excitement to any modern astrophysicist (or astrophysics student like me)!

From Newton to Einstein: The Evolution of Gravity Theory

You probably have heard the legendary myths of apples dropping onto Isaac Newton’s head, sparking ideas of gravity. In 1687, Newton was the first scientist to develop the laws of gravity. His theory is simple: two objects apply the same attractive force onto each other. The heavier the objects and the closer they are, the greater the force. However, Newton could not find a medium where the force occurs. The objects seem to apply force onto each other without any contact.

Over 2 centuries later, Albert Einstein published the Theory of General Relativity, which solves the problem of Newton’s ideas. Einstein proposed that objects warp spacetime, and this curvature of spacetime affects the motion of surrounding objects, which experience what we call “gravity”. Spacetime becomes the medium of gravity.

Gravitational Waves

General relativity leads to some interesting results. Accelerating objects produce distortions in spacetime propagating outwards like ripples spreading across the water when you throw a rock into the pond. The ripples in spacetime are called “gravitational waves”. 

We create gravitational waves ourselves every day when we move, but our gravitational waves are so tiny that they are non-existent. Even gravitational waves from massive objects like black holes and neutron stars are still very small. They only become barely great enough to be detectable when black holes and/or neutron stars collide with each other, but even that took scientists decades to figure out how to detect. On the bright side, we should be grateful that these gravitational waves do not shred us into spaghetti.

Just to give you some intuition, gravitational waves from typical black hole collisions would stretch a 1-meter region by 0.000000000000000000001 meters (yes, there are 21 zeros), or 10-21 meters, the size of an atomic nucleus. 

Multi-Messenger Astronomy

Throughout the majority of our history, astronomers had to rely on electromagnetic waves (the mediator of photons, or light) to study the universe. You can think of electromagnetic waves as extroverts - they love to interact with other matter. They can be absorbed and re-emitted by objects on their journey from the source material to us, so they can be unreliable at times. Sometimes, they cannot reach us at all when something is blocking their way or when the early universe was too dense in the first 380,000 years. 

Meanwhile, gravitational waves are introverts - they interact very little with other matter and pass through objects mostly unchanged. This helps them to preserve the information of their source material, a very useful feature for astronomers. They provide a new pair of eyes into objects that emit little light like black holes or into the early universe when light could not pass through. Astronomers now can combine both electromagnetic waves, gravitational waves, and detections of particles like neutrinos to have a more complete view of the universe in the new era called “multi-messenger astronomy”.

In the next articles of the Gravitational Waves series, we will explore the technology of gravitational wave detectors and future possibilities of this research field. Stay tuned!