# A Storm in Space and Time: Gravitational Waves Detected

In September, there was a storm in space and time. The scientists at the Laser Interferometric Gravitational wave Observatory (LIGO) **announced today** that on September 14th, 2015, they detected the unmistakable signal of two black holes merging together.

I cannot stress enough that this is a tremendous scientific breakthrough made by an instrument with unprecedented sensitivity. Many many scientists from diverse backgrounds invested years and years of time and effort into laying the groundwork of theory and technology for this detection to be made possible. They built an instrument that can detect distortions in space that are comparable to 1/1000th the size of the nucleus of an atom for crying out loud!

However, I'd like to allow myself to geek out a tiny bit. So today I'll pay homage to Einstein and explain his **theory of General Relativity** and what this discovery means for the past and future of our understanding of the universe.

Note that I'll be talking a lot about scientific theories and I'd like to make one thing exceptionally clear. Although colloquially we often use the word **theory** to describe something that is unfounded or not well proven, when I'm talking about scientific theories, this type of theory is an idea that is well understood, characterized, and repeatedly tested. The scientific community accepts it as what we would colloquially refer to as a fact.

**The Theory of General Relativity**

Meet Dr. Albert Einstein, technical assistant at the patent office, social activist, violinist, hater of socks, oh yeah, and famous theoretical astrophysicist.

Meet Newtonian physics, also known as classical mechanics, which is a theory about how objects with mass interact. Recall Newton's F=MxA (force = mass x acceleration). In classical mechanics, a force acting on an object with mass can completely describe its acceleration. You should feel comfortable with classical mechanics, since it uses common-sense approaches to describe how matter and forces work together in the universe; in classical mechanics we know that objects have a definite place in space and a knowable speed.

The setting: At the end of the 19th century, many scientists believed that all the important laws of physics had been discovered and that research should be concerned mainly with fixing tiny inconsistencies they saw in measurement errors of the universe.

But here's something unsettling. Around that time, astronomers made careful observations of the orbit of Mercury around the Sun and observed that its orbit precesses around the sun rather quicker than Newton's laws of gravity predict. (Precession means that Mercury's closest point to the sun shifts forward with each pass.) Something weird was going on with gravity.

Enter Einstein. After finishing his theory of Special Relativity,—a fascinating topic for another time—Einstein spent a decade mulling over gravity itself. He eventually developed a complicated mathematical formula to describe the curvature of spacetime itself (If you're curious, see Wikipedia for the math). He published his theory of General Relativity in 1915, explaining that spacetime itself can be warped by massive objects like the sun.

But what is spacetime? We're used to thinking of the universe as a three dimensional space. Spacetime incorporates time as an additional dimension of space, which is important because massive objects can slow down time itself (see Special Relativity).

Note that without General Relativity your cell phone's GPS would be off by as much as 10 km accumulated error per day! A satellite's clock runs faster by 38 microseconds a day due to the warping of spacetime around Earth. Lucky for us, scientists can make this GR correction for us behind the scenes.

Einstein published his theory along with some testable predictions. General relativity has predicted the observed precession rate of Mercury, something called gravitational lensing (light itself can be warped around massive objects like the sun), and frame dragging (the Earth's gravity actually causes the axes of gyroscopes aboard satellites to drift over time).

Another prediction of general relativity that until now has been untested observationally is gravitational waves. Which leads us to our second discussion point...

**Gravitational Waves**

Gravitational waves are different from gravity waves, which you can think of as waves in the ocean. Gravitational waves are disruptions in the very fabric of spacetime caused by massive objects undergoing very energetic gravitational interactions.

Scientists have run theoretical simulations and have been able to predict the frequency and type of gravitational waves that would theoretically arise from some of the most powerful and exotic gravitational interactions in the universe. For instance, gravitational waves could be produced from the interactions of neutron stars, which are the extremely dense remnants of massive stars. They could also result from a supernova, which is the result of a star much larger than the sun collapsing in on itself. They could also occur when black holes merging together.

The theoretical signal in gravitational waves of two black holes merging matches the signal detected back on September 14th of last year at LIGO.

The detection itself is exciting because it validates one of the main predictions of General Relativity exactly a century after the theoretical groundwork was laid by Einstein in his 1915 paper. However, this detection also lays the groundwork for some really exciting work. Scientists can now begin to learn about the energy released in this erstwhile undocumented event.

I'm an astrophysicist who studies the supermassive black holes at the centers of galaxies and how they interact with one another and their host galaxy. We have been able to predict that smaller black holes should merge together (such as the LIGO discovery) and this is how they grow to their morbidly-obese supermassive states that we observe today. HOWEVER, extragalactic astronomers have entirely relied upon either theoretical simulations, experiments on Earth, or looking at light itself to understand the universe outside our galaxy. Now we can learn about the tremendous energy released from events in our universe through the very warping of space and time itself.

In fact, based upon the gravitational wave signal, the LIGO scientists have announced that the signature was produced from the merging of black holes 1.3 billion light years away, one 36 times the mass of the Sun, and one 29 times the mass of the Sun. They merged to form a single black hole 62 times the mass of the Sun. Wait, 29+36 does not equal 62. This merging event was so powerful, three times the mass energy of the Sun was released into the universe. For comparison, when we talk about mass energy released in the largest nuclear bombs, we talk about masses of a couple of kg at the most. So even with this one detection, we have made huge advances in our understanding of merging black holes.

And so I leave you with this thought: Today, nearly a century after the theoretical groundwork of GR was laid in 1915, we are able to verify and observe one of the major predictions of general relativity. While my excitement cannot be contained about this exciting new discovery, I look forward to how our understanding of spacetime and the universe itself is about to change!