Most of us have already known that gravity is one of the fundamental forces that rule the whole Universe without any restriction in the space and time continuum except the speed of light. However, gravitational waves predicted by the general relativity theory are not a coherent fact yet for most of us as though it is a common knowledge thanks to the mainstream media. Let’s get started to comprehend this phenomenon step-by-step.
To better comprehend gravitational waves, we need to understand the structure of waves travelling around us everywhere. According to the cycle of motion, there are two types of waves ⚊ transverse waves and longitudinal or compression waves. It is about the angle of the direction the wave is travelling. For transverse waves, the cyclic motion is at right angles to the direction the wave is travelling, such as water waves(1). If you have ever watched water waves striking at a beach, you should know that they are always travelling straightforward. Conversely, for longitudinal waves, the cyclic motion is toward the direction of the wave is travelling, hence the back and forth movement. Sound waves are the most known longitudinal or compressive waves. According to the frequency of the sound wave, the section of the molecules is called a compression or a rarefaction. And, the distance between the crest(the highest point or compression) and the next one, or between the trough(the lowest point or rarefaction) and the next one is the wavelength of the wave(sound wave)(2). But, as we know instinctively, waves need an environment ⚊ a medium ⚊ to travel through in it. Which is why the ether theory ⚊ a matter compound fills the Universe ⚊ had been accepted by physicists, for many years, to explain how the electromagnetic waves(sunlight) travel through space without not having a medium. That is well-explained by Maxwell equations, in accord with the subatomic particles and the quantum theory.
So, what are gravitational waves and how do they behave? Most of the time, gravitational waves are described as sound waves travelling without a medium(3), but it is a false statement. The reason is as follows. Gravitational waves are not longitudinal waves as sound waves travelling towards a given point through the matter of its medium ⚊ a gravitational wave is a side-to-side wave as light. Gravitational waves travel, not like any other waves, through space itself, and therefore warp and alter the fabric of space and time indeed. In that regard, gravitational waves alter the matter itself by merely travelling across space. In other words, a gravitational wave squeezes and expands the matter(molecules, atoms and so on) whilst it pervades space. This type of pressure instigates a small electrical current by freeing up electrons(4), and it is known as the piezoelectric effect. And, gravitational waves have been detected because of their effects on the matter itself, such as the piezoelectric effect as explained above. Nevertheless, it is not a simple process to detect a gravitational wave because not only it is too hard to observe its effects on the matter, but also it has to be truly massive objects, like black holes, to expose the palpable effects of gravity on space. Which is why gravitational waves from a black hole merger observed by LIGO and Virgo are the First observation on gravitational waves on 14 September 2015(5).
Gravitational waves alter space itself by stretching and squeezing it in two-dimensions and therefore change all the matter while passing by through space. They change the trajectory of light and make objects look different, as predicted by the general relativity theory, by distorting space itself. This phenomenon would be known as the key to understanding the very core of the Universe and the fundamental forces.
(1) Clegg, Brain. "WHAT IS A WAVE?." Gravitational Waves: How Einstein's Spacetime Ripples Reveal the Secrets of the Universe. London: Icon Books, 2018. 12-14. Print.
(2) Knauss, Harold P. "Sound." The New Book Of Popular Science Vol-3. Grolier Incorporated, 1988. 258-259. Print.
(3) Clegg, Brain. "Einstein's Baby." Gravitational Waves: How Einstein's Spacetime Ripples Reveal the Secrets of the Universe. London: Icon Books, 2018. 30. Print.
(4) Clegg, Brain. "THE GRAVITATIONAL WAVE CHALLENGE." Gravitational Waves: How Einstein's Spacetime Ripples Reveal the Secrets of the Universe. London: Icon Books, 2018. 45. Print.
(5) “Gravitational waves from a binary black hole merger observed by LIGO and Virgo ” LIGO Caltech, 27 September 2017, https://www.ligo.caltech.edu/news/ligo20170927
Figure - 17.1 By NASA - http://www.nasa.gov/mission_pages/gpb/gpb_012.html, Public Domain, https://commons.wikimedia.org/w/index.php?curid=4072432
Figure - 17.2 http://thegophysics.com/wp-content/uploads/2017/12/042617_EC_quantum-equivalence_main_FREE.jpg
Figure - 17.3 https://slideplayer.com/slide/4178697/
Figure - 17.4 http://www.qsstudy.com/physics/transverse-wave
Figure - 17.5 http://www.physics-and-radio-electronics.com/physics/transverseandlongitudinalwaves.html
Figure - 17.6 http://time.com/4217820/gravitational-waves-history/
Figure - 17.7 https://kaiserscience.wordpress.com/physics/modern-physics/general-relativity/
Figure - 17.8 https://www.ligo.caltech.edu/news/ligo20170927
Figure - 17.9 https://phys.org/news/2016-12-strong-gravity-earth.html