By Frederick Yeung
A droplet falling into water, the sun in our solar system, speakers at a loud festival–these create what we know in classical physics as waves. A wave is a wiggling of a thing which moves energy through space. For example, our everyday sound waves are just very quick patterns of compressions and stretching of air molecules, which is energy propagating through the air, pushing and pulling particles to carry itself along. What on earth (or any extraterrestrial mass or object for that matter (I could do another pun, on matter instead, but I felt it wasn’t solid and wouldn’t flow…(sorry))) is a gravitational wave, then? Can gravity wiggle??
For any mass in our universe to undergo some acceleration, Einstein’s General Theory of Relativity proposes that gravitational radiation is emitted. Think light (electromagnetic radiation) specifically, which is caused by charges undergoing periodic motion; constantly accelerating back and forth. Acceleration creates outward-traveling-waves. Well, we don’t see this acceleration in the form of a planet bouncing up and down attached to some spring, but rather in a binary (two things) system of large masses. Black holes we’re talking about. When two moving black holes pass near each other, where they are in the effects of the other’s gravity, they begin to orbit one another, forming a circular path which both black holes are moving along but are always across each other. They are pulling each other closer and closer, accelerating faster and faster, until they finally collide and merge into one larger black hole (gravitational radiation exists in times where the final parsec problem occurs too, just no merging of said black holes).
This process is in fact similar to the up and down oscillations which create ripples and waves. If we push a beach ball on the surface of water up and down many times, it sends large ripples outwards in all directions, along the surface of the water. Imagine we are watching the star on the same plane as its circular path, in such a way that they only appear to be moving in a horizontal line: we’d also see a kind of bobbing up and down motion as a result of this circular motion. Thus, the emitted gravitational radiation of the two masses travels outwards into space at the speed of light (3108 ms-1), as ripples in the fabric of space–spacetime itself stretching and contracting–are sent outwards along the plane of their circular path. This stretching and contraction is invisible to us, occurring at a scale of 1000 times smaller than the diameter of a proton, so it makes sense that we have yet to observe any major day to day distortions of the things around us.
One may ask: why so special? These waves can help to satiate our metaphysical curiosities, helping astrophysicists to better uncover the nature of our universe. Researchers currently have a few methods of detecting these waves, one of which is using the Laser Interferometer Gravitational wave Observatory (LIGO). LIGO uses two massive multi-kilometer spanning detectors, one in Washington and the other in Louisiana, which utilize the wave phenomenon of interference to detect these ripples in space. One may now ask: how so? Interference is what happens when waves meet. Essentially, when the trajectories of two waves intersect, how much the wiggles of both waves are aligned with each other determine how strong or weak the combined resultant wave becomes: perfectly aligned, double the maximum wiggle, known as constructive interference; perfectly misaligned (where a wiggle up meets a wiggle down), 0 resultant final wiggle, known as destructive interference.
This phenomenon has been cleverly used to detect the presence of gravitational waves. LIGO splits a powerful laser, sending them off in perpendicular directions, remerging them towards a detector and seeing variations in their interference patterns. When a gravitational wave passes through earth, it stretches and compresses the spacetime which earth exists in. This creates a marginal misalignment in extremely precise and long (4km) lasers, causing them to change from perfect misalignment–and perfect destructive interference–to align and constructively interfere, which is detected as a presence of light on the detector!
Let’s summarize: energy in the form of waves can travel through air as well as empty space, and spacetime itself can propagate energy, known as gravitational waves. These waves are measured using the Laser Interferometer Gravitational wave Observatory, utilizing a property of waves known as Interference to determine their presence. I would like to leave the evaluation and thinking in your hands now. It was built in the 90’s and detected its first gravitational wave in 2015, and has been making more observations since then. Why do you think astronomers and astrophysicists are so interested in understanding space in the first place? Does the pursuit of knowledge outside the scope of our daily lives really matter in the grand scheme of things––as in, what does this really mean for us? Does anything have to mean something in order for it to be a pursuit?
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