Gravity and Gravitational Waves: What Are They?

Two black holes that are merging produce many gravitational waves

 

We all know what gravity is. It is the force that affects our daily lives in many ways by pulling us towards the earth and by making sure that we don’t float away to the vacuum of space.  Yet, have you ever wondered what it looks like? Most people believe gravity is a hidden force that pulls us in a straight line towards the center of earth. This was what was proposed by Newton and was the commonly accepted theory by scientists, but in the last century, all of this has been made obsolete. World renowned scientist Albert Einstein proposed that gravity is not, in fact, just straight lines, but ripples and distortion in the space-time fabric.

 

Gravity and Space-time

This is a theoretical example of Einstein’s Theory on General Relativity. Earth here is bending the fabric of space-time.

 

Imagine space-time as one big sheet of fabric. If anything with mass is placed on that sheet, it will cause the sheet to distort. The greater the object’s mass, the greater the distortion of the fabric. This distortion in gravity causes all objects to produce a gravitational field, which will attract nearby objects. An object will always try to go in a straight line through space-time. If it gets caught in an object’s distortion, it will begin to follow the objects distortion and get attracted to it. Now imagine a ball is placed in the center of the fabric, and a second, smaller ball is rolled in the larger balls distortion area. Instead of going in a straight line towards the ball, it will orbit the ball before coming together. The same happens with us and the earth. Both we and the earth cause a distortion, but since the earth has greater mass, we are attracted to the earth with a greater force. There is no orbit as our distortion of space time is miniscule compared to the earth. That is what happens with all objects in our universe. Yet, Albert Einstein suggested that while the objects are changing position, something else is produced: gravitational waves.

 

Gravitational Waves

 

This is a theoretical example of Einstein’s Theory on General Relativity. Earth here is bending the fabric of space-time.

 

Like ripples in a pond, gravitational waves are produced when any object changes its position in the space-time fabric. When you shake a stick in water, you produce ripples in the water surface. These are waves. Imagine that instead of a stick, it’s a celestial object. When celestial objects change their position, usually due to attraction to each other’s distortion, they produce gravitational waves. Like the stick, it will produce ripples, but instead of rippling the water, it will ripple the space-time fabric of the universe. These ripples distort the area around them, stretching the vertical area and squeezing the horizontal area in the wave. Effectively, they are stretching the matter in the universe. These changes, however, are so small; we cannot feel them, so how were they discovered?

 

In 1905, French physicist and mathematician Henri Poincare hypothesized that if electric charges produce electromagnetic waves when displaced, then large objects like planets that have a gravitational field should produce a gravitational wave. This was a theory, and could not be proven at that time. Albert Einstein took note of his theory. In 1916, Albert Einstein published the famous theory of General Relativity, which explained, in the easiest way possible, that objects flow through space-time, and that if they stumble into another object’s distortion, it will follow that object’s field.  Many controversies followed the publication of Einstein’s theory. Many tried to disprove his theory, calling it fake. Yet, his theory couldn’t be proven or disproven until years later by a team of scientists in charge of the LIGO project.

 

The LIGO Project

This is a theoretical example of Einstein’s Theory on General Relativity. Earth here is bending the fabric of space-time.

LIGO (Laser Interferometer Gravitational-Wave Observatory) is a research lab, funded by the NSF (National Science Foundation) that was built by countless scientists. Joseph Weber, Mikhail Gertsenshtein, and Vladislav Pustovoit constructed the basic design for the LIGO in the 1960s. Many scientists were tasked with building the high tech experiment. The LIGO is made up of two, 4km long vacuum tubes. They are placed perpendicular to each other in an “L” shape with both tubes attaching at one end. At both ends there is a mirror that is so refined that it almost reflects everything perfectly. The mirrors are suspended and isolated to reduce motion. This makes sure everything is stable. Also along the tubes are input mirrors, which help focus the laser. At the intersection is a refined Beam-Splitter lens placed at a 45 degree angle.

 

Well, how is this going to prove anything? Gravitational waves distort matter to the point we can’t measure or feel it, as any physical measurements would also get distorted, but unlike physical matter, the speed of light doesn’t change. Therefore, if light is shown between two objects that are being affected by a gravitational wave, we might not notice a change with our eyes, but the light beam will be longer. In the LIGO, a one kilowatt laser is shown on the Beam-Splitter lens, splitting the wave in two. The laser is extremely precise so that an accurate wavelength is produced every time. Each laser goes through an input mirror, travels down the 4 kilometer path, and gets reflected back without any interference, because there is no air. When they meet back at the lens, they should cancel each other and nothing should happen. If there are gravitational waves, one of the tubes should be longer than the other, as one compresses and the other expands. The wave lengths will be different thus not canceling each other out. This will produce a faint laser at the lens. The most powerful sensor in the world is tasked with detecting the faint laser. This sensor is so precise to the extent that it can detect a change in the position of one ten thousandth of a proton. To put that in perspective, imagine being tasked to sense a change of 1 centimeter in a 10,000,000 meter rod.  This inconceivable experiment has proved that gravitational waves exist, as the faint laser was detected.

This is a theoretical example of Einstein’s Theory on General Relativity. Earth here is bending the fabric of space-time.

 

Why are waves that we can’t feel important?  Our greater understanding of gravitational waves can assist us in finding new and interesting discoveries. This can greater develop our understanding of the space-time around us, and possibly find ways of harnessing these waves for the good of humanity. If we detect waves produced by the original Big Bang, we can learn more about our universe. This is not something hidden in the eyes of the science communities, as many new LIGO sites are popping up around the world. We are uncertain of what this will bring us, but we are certain that these studies will help further our knowledge of the enigma which is our universe.

 

Sources:

http://www.pbs.org/wgbh/nova/next/physics/advanced-ligo/

https://www.nasa.gov/feature/goddard/2016/nsf-s-ligo-has-detected-gravitational-waves/

https://www.sciencenews.org/article/einsteins-genius-changed-sciences-perception-gravity

https://www.sciencenews.org/article/gravitational-waves-explained

https://cosmosmagazine.com/physics/probing-creation-with-waves

https://en.wikipedia.org/wiki/Gravitational_wave

https://www.youtube.com/watch?v=4GbWfNHtHRg

https://www.youtube.com/watch?v=B4XzLDM3Py8

https://www.youtube.com/watch?v=xj6vV3T4ok8

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