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Last week marked the historic announcement of the first detection of gravitational waves. A big press conference was held, and physicists around the world celebrated. The discovery was even compared to Galileo looking through a telescope for the first time. So why all the fanfare? Why are gravitational waves such a huge deal?
1. Gravitational Waves Are an Entirely New Way of Observing the Universe
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Astronomers observe the universe across the electromagnetic spectrum, from X-ray and ultraviolet through optical and down to radio frequencies. Emission in each of these frequency ranges provides different information and thus a different perspective on our astronomical points of interest.
For example, we know that there are millions of stars clustered toward the center of our Galaxy which emit mostly at optical wavelengths, but there is also a lot of dust near the Galactic center as well. So to study those dust-enshrouded stars, astronomers must observe them at either infrared wavelengths (where the dust emits) or radio wavelengths (which can penetrate through the dust more effectively than shorter optical wavelengths). All of these wavelengths offer a unique perspective on the universe, but they are all the same kind of light, electromagnetic radiation, and so behave in similar, understood ways.
Gravitational waves are an entirely new phenomenon different from anything on the electromagnetic spectrum. In 1915, Albert Einstein proposed a radically different way of looking at gravity with his theory of general relativity. Rather than thinking of gravity as a force pushing and pulling massive objects in different directions, he described gravity as being manifested in a curvature of spacetime. In other words, the space (and time) around a massive object is curved, which then dictates how passing objects can move through that space.
This may sound crazy, but we can actually observe many of the effects predicted by Einstein’s theory. For example, general relativity informs us that time passes more slowly by an ever so small margin down here on Earth than it does for GPS satellites in orbit, an effect known as time dilation, a result of the curvature of spacetime. Without adjusting for this small time difference in our satellite communications, we would never get to where we are trying to go.
A consequence of the general relativity framework is that when objects accelerate through this warping of spacetime, they produce ripples known as gravitational waves. These waves propagate through space, compressing it in one direction and stretching it in another.
The frequencies predicted for these fluctuations are within the human hearing range. We can hear gravitational waves and already scientists and artists have teamed up to explore other artistic interpretations of their sound.
So why did it take 100 years to detect them? These ripples are tiny, on order of a thousandth of the size of a proton nucleus, so we need a pretty violent event to occur to produce enough of them for us to detect. We also need, of course, a very sensitive detector.