What are Gravitational Waves and Why Do We Care About Them?
On September 14, 2015 gravitational waves were detected from the collision of two black holes 30 times the mass of the sun, a billion light years away. This detection was made by LIGO, the Laser Interferometer Gravitational-wave Observatory, and it was the first time gravitational waves were directly observed.
How We See the Unseeable in Space
Measuring these waves is a major accomplishment for science because nearly everything we know about the universe outside the Earth comes from looking into space with some form of light. When we look at the stars, we see the light they are giving off. When we look at the planets in our solar system, we see the light they are reflecting from our sun. Not all light astronomers use is ‘visible’. For example, you can’t see radio waves or X-rays with your eyes, but they are both a form light nonetheless. Looking into space using gravity gives us a whole new way to look at the universe, allowing us to see things in ways we never could before. Black holes, as the name suggests are black. They don’t give off light. However, their gravity is more intense than anything else in the universe. By observing their gravitational waves, we can learn a lot more about them, and about gravity itself. It lets us see things that would otherwise be unseeable.
What exactly are gravitational waves?
Einstein’s Theory of General Relativity predicts their existence. Everything in relativity is based on one basic principle; nothing can travel faster than the speed of light. This includes gravity. For example, the Earth stays in orbit around the sun because of the sun’s gravity. If you were to make the sun suddenly disappear, the Earth would fly off into empty space…but not until 8 minutes later. During that time, the Earth will stay in its orbit until it finally feels the change in gravity. Mars would then feel it sometime later, then Jupiter, and on out to Pluto and beyond. This change in gravity spreads out from the sun in all directions, at the speed of light. If instead you wiggled the sun back and forth, you would make waves, gravitational waves, spreading out in all directions.
Detecting the Waves
LIGO has built two observatories to try to detect these waves from big massive things wiggling around in space. One observatory is in Hanford, WA, the other in Livingston, LA. The picture below shows the Hanford Observatory. Only the most violent events create waves big enough to see, things like colliding black holes and exploding stars. The waves are hard to see because gravity is the weakest force in the universe. For example, if you rub a balloon on your hair, you can make it stick it to a wall. It sticks because the small amount of static charge you create on the balloon is strong enough to resist the gravity from the entire Earth.
To measure the waves, the LIGO observatories have two, 2.5-mile-long arms at a right angle to each other. When the waves pass through, they will make one arm slightly shorter and the other slightly longer. These length changes are measured very carefully using laser beams bouncing up and down the arms, using a technique called interferometry. A photo of one of the mirrors that steers these beams is shown below. The length changes of the arms are tiny. LIGO measures changes a trillion times smaller than the thickness of spider web strands to detect the waves!
Interestingly, the waves LIGO observed can be listened to. To hear what it sounds like when black holes collide, go to
https://www.ligo.caltech.edu/video/ligo20160211v2
For more explanation on how gravitational waves work, see
To help LIGO find gravitational waves, see the Einstein@Home project at
https://www.einsteinathome.org/
More information about LIGO can be found at http://ligo.org/.
Brett Shapiro is a postdoctoral scholar at Stanford University and has been contributing to the LIGO project in varying capacities for the past 11 years. He lives with his wife in Palo Alto, California and enjoys traveling around the world to various research facilities!