Gravitational Waves Detected: The Next Era of Astronomy Begins

"Ladies and gentlemen, we have detected gravitational waves. We did it!" With these words, David Reitze, Executive Director of LIGO (Laser Interferometer Gravitational-Wave Observatory), confirmed what the astronomy community has been guessing for weeks. Gravitational waves, a concept proposed by Einstein in his general theory of relativity, have finally been detected and proven to exist. This discovery is the equivalent to Galileo pointing his telescope at the planets for the first time. A whole new era of astronomy has begun. So what's the big deal? And what is a gravitational wave anyway? 

Check out this excellent video by PHD Comics explaining what gravitational waves are, and how they are detected.

As the video above explains, we can imagine that space is a giant sheet. Items with mass bend the fabric of spacetime. The Sun is massive compared to the planets, and so bends spacetime around it, creating a gravity well, a dip in spacetime. The planets can't move in a straight line past the Sun because of that curve, and so are trapped in orbits around the Sun.

Image Source

Items that have even more mass, like black holes, neutron stars, and entire galaxies bend the fabric of spacetime even more.  Einstein theorized that when a very massive object accelerates, or when two massive objects interact with one another, the movement causes a ripple in spacetime, a gravitational wave. The wave compresses and stretches spacetime as it passes by. If a gravitational wave passed though Earth, our planet would ever so slightly bend and stretch. This change would be so infinitesimal that Einstein thought it could never be detected. (He also had no hope of detecting gravitational lensing, and that has been measured very well.) The big news of course is that a gravitational wave has been detected, after years and years of listening for it! 

In a press conference today (you can watch the whole thing here), scientists from LIGO announced that they measured a gravitational wave caused by two black holes merging. The detail they were able to tease out from the signal is incredible. The measurement took place on September 15, 2015, and was detected at the LIGO facility in Louisiana, and then seven milliseconds later, the same wave was detected at the facility in Washington. The wave measured very closely matches the wave predicted by supercomputing models of what a gravitational wave caused by two colliding black holes would look like. 

The first part of the wave, with short peaks, is the normal "noise" in the signal. The large peaks are the gravitational wave, bending spacetime. Image Credit 

By analyzing the data, scientists were able to tell that the two black holes collided about 1.3 billion years ago. They were 29 and 36 solar masses (meaning they were 29 and 36 times more massive than the Sun), and were orbiting one another until they got too close, and merged. When they merged, they formed a new black hole that was 62 solar masses. That is less than the combined total of the two - 3 solar masses were converted into energy, and that energy is what rippled through space in the form of a gravitational wave. The event itself only took 20 milliseconds to happen! LIGO scientist Gabby Gonzalez described these findings, and noted that the wave length is in the same range as sounds we can hear. So if we convert the gravitational wave into a sound wave, this is what it sounds like: 

This video released by LIGO has nice visualizations to help explain what happened, and how it was detected. 

So, we have detected something very difficult to find, proving that another piece of Einstein's theory was correct. What's the big deal? Why are astronomers so excited about this? Kip Thorne, a theoretical physicist who helped design and found LIGO (and also wrote the book that inspired large parts of the movie Interstellar), said this detection is a "scientific moonshot". Now that we have the capability to detect gravitational waves, we have a whole new way to observe the Universe. When we started using telescopes, we only observed the optical light in the Universe - light we can see with our eyes. This is just a tiny fraction of all the STUFF in the cosmos however.Then we developed the ability to observe in the infrared and x-rays, and eventually, the entire electromagnetic spectrum. And now, we can observe in an entirely different spectrum made of gravitational waves. We will be able to detect gravitational waves from black holes colliding and pulling apart neutron stars, neutron stars spinning and colliding, and possibly from cosmic strings. Every time we have found a new way to observe the Universe, the discoveries have astounded us. We can only imagine the discoveries that will follow this first detection. Brian Greene summarizes this discovery and what it means in this excellent video below: 

Further Reading:

Official Press Release 

Finding Beauty in the Darkness 


Gravitational Waves Exist: The Inside Story of How Scientists Finally Found Them
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