Photo Credit: Wikipedia

Black Holes, general relativity and gravitational waves – My Interview With Andy Bohn, Astrophysicist

Key Vocabulary: General relativity, medium, spacetime, black hole, event horizon, gravity, nuclear fusion, gravitational waves, frequency, pendulum

Next Generation Science Standards: 

  • HS-PS4-1. Use mathematical representations to support a claim regarding relationships among the frequency, wavelength, and speed of waves traveling in various media.
  • HS-PS4-5. Communicate technical information about how some technological devices use the principles of wave behavior and wave interactions with matter to transmit and capture information and energy.*

Article Guide: Gravity_Waves_Article Guide

In February, scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced that they had directly observed–for the first time–gravitational waves or ripples in the spacetime continuum. Perhaps the biggest physics discovery of the year, it confirmed a prediction made by Albert Einstein over 100 years ago when he published his theory of general relativity.   

But the idea of a wave that travels through the medium of spacetime is a challenging one to wrap your head around. You must have an understanding of general relativity, that the force of gravity is due to a curvature in the geometry of both space and time. It’s heavy duty, doc. Which is why I sat down with Andy Bohn to help me have a better grasp of why this discovery is so significant.

Dr. Bohn, who recently finished defending his Ph.D in physics from Cornell University, worked at LIGO for several months. Describing his chance to studying merging black holes using supercomputers as an opportunity he couldn’t pass up, I knew he would be the perfect person to explain what was discovered and to put it in perspective. We spoke a few months ago on his involvement on the project and its significance as a scientific discovery. Our conversation, which has been lightly edited for length and clarity, follows.


Chris Anderson: What inspired you to be a scientist? Was there any one thing or person that guided you to your current work?

Andy Bohn: Indirectly, my dad was the biggest reason I became interested in science and physics. He wasn’t trained in math or science, but he approached everything he did in a very methodical and logical way. Because of this, math and science classes were always intuitive for me.

I got into physics, however, because my brother and I were very competitive with each other. In high school, he told me that AP physics was the hardest class our school had to offer. I decided that I had to take the class and prove to him that I would have no trouble. It turned out that it was a perfect fit for me, because physics tries to describe how nature works, and nature is always logical.

CA: We hear a lot about black holes and event horizons through popular culture and science fiction and you’ve been studying these phenomena for years. Can you explain to someone who isn’t familiar with those terms what exactly they are?

AB: First you’ve got to understand a little about General Relativity. 100 years ago, Einstein developed his theory of General Relativity (GR) which describes space and time as one continuum, replacing Newton’s theory of gravity. The idea is based on geometry: matter tells spacetime how to curve, and spacetime tells matter how to move. Instead of thinking of Earth’s orbit being due to a force from the sun, GR says that the sun bends spacetime around it, and the Earth is just moving in a straight line through this curved spacetime. (Editor’s note: For more on general relativity, check out this excellent video)

Earth’s gravity curves the spacetime around it. Photo credit: Wikipedia


Now consider a star for a moment. Due to the enormous amount of mass, gravitational forces are constantly trying to compact the star. At the same time, most stars are very hot due to the nuclear fusion inside their cores. This heat provides an outward pressure that balances the inward force of gravity. However, if nuclear fusion were to stop, the star would begin to would cool and gravity would force the star to collapse, which is what happens as stars die.

As these star collapse on themselves, their gravitational field becomes stronger. If it’s massive enough, the speed required to escape the compressing star’s gravity becomes greater and greater until it is faster than the speed of light. This is when a black hole forms. An event horizon essentially defines the size of a black hole, it’s the boundary beyond which not even light can escape, so anything that passes the event horizon surface, cannot come back out.

CA: So what are gravitational waves and how do you detect such a thing?

AB: Another prediction of GR is that things such as merging black holes will release an enormous amount of energy as they orbit closer to each other and eventually merge into one. This emission is called gravitational waves, which are essentially ripples in spacetime, similar to waves from dropping a rock in a pond.

 Simulation of two black holes merging. Video credit: Wikipedia

LIGO recently detected these gravitational waves produced from the merger of two black holes, one 36 times the mass of our sun, and another 29 times the mass of our sun. When the two black holes collided, they formed a black hole 62 times the mass of our sun.  We started with 65 solar masses and ended with 62, meaning we lost energy equal to 3 times the mass of our sun to the emitted gravitational waves. This is a lot of energy, however, it happened about 1.3 billion light-years away, so the waves were extremely weak.

What LIGO does is detect those ripples by shooting a laser at mirrors in two perpendicular tubes, and measuring the time it takes for the light to return. If a gravitational wave passes through, spacetime would be distorted and the distance between the detector and the end mirrors will be altered, changing how long the light takes to travel back to the detector. To give you an idea how hard this measurement is to take, LIGO is trying to measure the distance between a set of mirrors that are 4 kilometers apart to an accuracy of 1/1000th the diameter of a proton. Meanwhile, things like earthquakes, people walking around, and even the wind blowing nearby trees can alter the distance between the mirrors, making the readings less accurate.


LIGO uses many tools and tricks to try to reduce these noise sources.  For example, imagine you are holding a string with a ball attached, like a pendulum. If you move your hand back and forth slowly, the ball will follow the motion of your hand. However, if you shake your hand back and forth very quickly, the ball will hardly move at all. This principle is used in LIGO to reduce meaningless data (statisticians call this noise) in the detector above a certain frequency. The mirrors in LIGO are suspended by pendulum attached to a pendulum repeatedly for a total of 4 pendula to get an accurate measurement. I did some work on the fibers on the pendula when I first worked at LIGO.

CA: But more recently you have worked on the data side of the project. Can you explain more about the work you’ve done at LIGO?

AB: Our group codes the equations of General Relativity into a computer program to simulate many hundreds of different mergers to see what the gravitational waves would look like in different situations. If LIGO knows what the signal is supposed to look like, it can then use the information to find the signal through the noise. Otherwise, you’re essentially trying to find a needle in a haystack without knowing what a needle looks like. The simulation of mergers such as this is called numerical relativity.

Once we know that we have a detection, the next step is trying to determine what the properties of the system were. If someone makes a prediction for that system, such as the size of the black holes merged, we can simulate the system to see if it agrees with the data. We can also use our simulations to improve what we know about the system.

I was also asked by the LIGO collaboration to produce a video for their February press conference. You can see a high-resolution version of the video I made here:

I also made a high resolution still of what our simulated stars look like without any black holes:

And also a high resolution still of a simulation with the same parameters as the one LIGO detected:

CA: What is so important about this discovery? Why should people care about it?

AB: This discovery is almost certainly worth a Nobel Prize for some members of the collaboration. In addition to being an incredibly difficult measurement, it matches exactly with the predictions of general relativity, proving Einstein correct about GR and giving us the first direct evidence for the existence of black holes.

But beyond that, we’ve been staring at the sky forever with our eyes, observing visible light to understand the universe. We’ve made technological advances to be able to see “invisible” light, such as radio waves or x-rays in the sky, giving us even more information about our universe and how everything started. But these are all just different forms of light. If we continue to make gravitational wave detections, it will be a completely new way to look at the universe and make discoveries. It’s similar to only being able to see in black and white, then suddenly being able to see in color, having access to a new dimension of information.

Dr. Bohn has Ph.D in physics from Cornell University. He currently works for SpaceX in Seattle, Washington.

3 Comments Posted

    • really? 13 billion light years to move 1/1000th the diameter of a proton, what does that even freaking prove? These colliding black holes are extremely massive, if in fact they do exist in reality, so why would they only move some particles 1/1000th THE DISTANCE OF A PROTON. Black holes have never been directly observed anyway…only mathematically predicted. Thats like me saying “hey look over there it’s a 1/2*pi*8.932/16.3344332^6.9867.” The simplest answer is normally the correct one…and this by no means is simple.

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