Gravitational Waves

Gravitational waves produced by two black holes orbiting each other
Credit: NASA (retrieved from www.space.com)

Gravitational waves are undulations in space-time that propagate as waves from a given source. Although mass curves space-time, the curve stays centered on that specific mass. With gravitational waves, these curves propagate throughout the universe, far away from the center of mass from which they are originating.

Credit: Malter
Retrieved from Wikimedia Commons

Credit: University of Minnesota

 

Gravitational waves are sinusoidal, meaning they oscillate smoothly according to the y-coordinate of a rhythmic trace of a circle. These waves, called “sine” waves, are everywhere. Light waves, sound waves, and even water waves are sinusoidal. For example, the music note “A” above is a sine wave.

And here’s a side view. You can really see the sinusoidal nature of the wave.

There are many different types of gravitational waves. The one above is a “linearly polarized” gravity wave. There are other polarizations, such as circular and elliptical polarization, and the polarization has to do with the electromagnetic nature of the wave. Additionally, these waves occur at a vast variety of frequencies corresponding to the objects that produced the waves and how far the waves have traveled.

Credit: LIGO Scientific Collaboration

So, gravitational waves move temporarily distort time and space for certain regions before they continue their journey onward throughout the rest of the universe. But how the heck do we measure these things?

I hope nobody was in that lookout!
Credit: Defense Research and Development Canada

As we saw, the gravitational waves moving through something distort its shape. However, there are lots of waves that can move through things and distort their shapes. Seismic waves distort the shape of the ground. Water waves distort the shape of the water. Shock waves, such as the one you see emanating from the explosion above, can set off car alarms, bring down buildings, and scare the living crap out of your dog. Therefore, how did these scientists deduce that the waves they observed were of the gravitational variety?

The LIGO Control Room in Hanford, Washington
Credit: Tobin Fricke

Enter LIGO, which stands for the “Laser Interferometer Gravitational-Wave Observatory.” LIGO is primarily funded by the National Science Foundation and draws scientists from all around the world. It was originally founded in 1992 by a group of scientists from MIT and Caltech, and began operations in 2002. Between 2002 and 2010, it failed to discover any gravitational waves, and was subsequently shut down for enhancements, finally coming back online in February of 2015.

Ligo Hanford (left) and Ligo Livingston (right)
Credit: LIGO Scientific Collaboration

LIGO consists of two giant L-shaped observatories, one in Hanford, Washington, and one in Livingston, Louisiana, with each “arm” being 4 kilometers long. As the LIGO acronym suggests, these observatories are “Laser Interferometers,” meaning they split a laser beam and see how the two laser beams interfere with each other. The beams are calibrated so that, under normal conditions, there will be perfect “destructive interference,” which is where the light beams (which are sine waves) are perfectly opposite each other so that adding them together creates a straight line and thus a complete absence of light.

LIGO interferometer design.
Credit: LIGO Scientific Collaboration

When a gravitational wave comes, space-time is distorted, and as such, the arms of the observatory lengthen and contract rhythmically, with one arm lengthening as the other contracts and vice versa. Due to this changing length, the laser beams no longer perfectly cancel each other out, and light shines through.

The change in arm length is on the small side – typical changes are on the order of 1/10,000th the width of a proton! Incredibly, the interferometers are sensitive enough to measure this change. I don’t know how they do it. To determine whether the readings were due to gravitational waves and not – say – a 9.0 Cascadia Subduction Zone earthquake, scientists are on the lookout for specific patterns that suggest the passing of a gravitational wave. Here are their exact measurements from the gravitational wave everybody is talking about.

Credit: Abbott et al. (2016)

Gravitational waves are made by extremely violent events in the universe, and these gravitational waves were theorized to have been created by the collision of two black holes, one 36 times the mass of the sun and the other 29 times as massive. They circled and approached each other at half the speed of light and eventually collided to make a single black hole 62 times as massive as the sun. Three solar masses were transformed into energy (remember E=mc²?), and 50 times as much power as the output of all stars in the universe was sent radiating into the space at the speed of light.

1.3 billion years later, on September 14, 2015, scientists found the specific pattern they were looking for, and the fact that it occurred at both the Hanford and Livingston observatories at almost exactly the same time proved that it was not simply a local disturbance. The signal only lasted for 20 milliseconds and only moved the LIGO mirrors four thousandths of the diameter of a proton, but they had finally captured direct evidence of both black holes and gravitational waves. Although gravitational waves are not sound waves, the frequency of these waves is a frequency that our ear can detect, so by converting the gravitational waves to sound waves, we can hear the sounds of these two black holes colliding. Listen to them here.

The discovery of these waves is one of the most important astronomical discoveries of all-time. According to Abhay Ashtekar, a physics theorist at Penn State, “it’s really comparable only to Galileo taking up the telescope and looking at the planets. Our understanding of the heavens changed dramatically.” I think a good comparison is to the discovery of radio waves 130 years ago by Heinrich Hertz. Since then, radio waves have completely changed our way of life.

So far, the vast majority of our observations of the universe have come from electromagnetic radiation. Radio waves, infrared waves, visible light, x-rays, gamma rays… you get the idea. However, not everything in the universe emits electromagnetic radiation. Black holes don’t… that’s why they are called black holes! We would be able to see dark matter and dark energy. But that’s not what I’m most excited about.

Credit: NASA

According to the big bang theory, the very early universe was opaque to electromagnetic radiation, and we cannot see further back than 380,000 years after the big bang. However, it was not opaque to gravitational waves, meaning that if we get a clear enough view, we should actually be able to see the big bang itself. We’ll go all the way back to the very beginning of time: the “singularity,” where all of the matter in the universe was contained in an infinitely small, infinitely dense, and unimaginably hot point. Or we’ll discover that the Big Bang didn’t happen at all. However, the existence of gravitational waves adds credence to the big bang theory.

“Imagination is more important than knowledge,” Einstein once said. For knowledge is limited to all we now know and understand, while imagination embraces the entire world, and all there ever will be to know and understand.” I can only imagine what we’ll discover next!

Written by Charlie Phillips – charlie.weathertogether.net. Last updated 12/1/2017

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