I posted a few days ago about pulsars, and used the clunky analogy of an object on a trampoline to describe how matter sits in the fabric of space-time. Essentially, the mass of the object will bend space-time and cause it to curve towards the object. The higher the density, the more massive the object, the greater the curve and the greater the gravity.
There is another essential part of objects that exist in space and time. They are constantly in motion. Everything moves, on the macro scale right down to the quantum.
For the purposes of this post, I want to focus on angular momentum; spin. Specifically, how massive objects effect space-time when they spin at a constant rate, and when their motion is acted upon by an outside force or object.
I’m going to change analogies now, from one clunky metaphor to another.
Think of anything falling into a body of water. A drop of rain in a puddle, a stone in a pond, a spoon into a cup of tea. When these object hit the liquid they send out ripples. These ripples are waves that move through the water at certain frequencies and wavelengths. Like sound waves and light.
This effect, of ripples moving out through water, is similar to a phenomenon known as gravitational waves.
First predicted by Einstein when he postulated his idea of general relativity, it was indirectly proven in 1974 by two astronomers studying a pair of pulsars. These pulsars rotated around each other and, using general relativity as a basis, the astronomers predicted the movement of the binary system with the assumption they would be emitting gravitational waves. Eight years of data collecting later and they were proven to be right. The two pulsars moved together at exactly that rate.
But this was indirect evidence, humans didn’t have the ability to detect gravitational waves directly until 2015.
The Laser Interferometer Gravitational-Wave Observatory (LIGO) recorded gravitational waves rippling through space from the collision of two black holes over a billion years away. Gravitational waves travel at the speed of light and, like a stone in a pond, move outward from the center of the event that causes them. It took over a billion years for these waves to reach Earth and LIGO and by that time even the cataclysmic event of two black holes colliding has been reduced to tiny ripples, smaller than atoms.
But LIGO was designed for just this type of event and on September 14th 2015, Earth recorded gravitational waves for the first time.
This detection was one type of gravitational waves, those from a binary source. Meaning two things had to interact for the waves produced to be big enough to reach us whilst they were still detectable. There is another kind, one that every moving body in the Universe creates, but usually they’re just to small to notice.
Continuous gravitational waves are thought to be detectable from the spin of neutron stars. These incredibly dense objects spin at incredible rates, and if their surfaces aren’t perfectly smooth, any bump or irregularity would throw gravitational waves into space.
As of yet, these types of gravitational waves have yet to be detected and there is a simple reason why. They’re being drowned out by the cavalcade of noise produced by thunderous collisions in space. Trying to detect the constant hum that is predicted to be produced by not-perfectly-formed spinning neutron stars is the equivalent of hearing a whisper across a crowded room while a heavy metal band played next to your face and a plane flew overhead.
It’s incredibly difficult to do but in May three papers published by researchers of these continuous gravitational waves postulated three different ways to try and detect them. One of these is to look in areas of recent supernovae where young neutron stars may have just been formed. These young stars will most likely be deformed and have larger protrusions, therefore creating a louder whisper that may be heard of the racket of colliding objects in space.
But why search for these at all and why focus on nuetron stars?
Well, we really don’t know much about them. We don’t know what they’re made of, if they are layered or if they are a single type of matter. There are theories but the detection of gravitational waves could enable us to learn the secrets of the strange stars. They are the last step of matter before a black hole and the more we know about them, the more we can figure out the laws of the Universe in the most extreme of conditions.
For now, the hunt for continuous gravitational waves continues, but with LIGO having now detected over 56 candidates, we are well on our way to finding them and taking one more step to a more complete understanding of the Universe.
Further Reading/References
GraceDB | Public Alerts (ligo.org)
Sources and Types of Gravitational Waves | LIGO Lab | Caltech
Deep space listening: 6000 hours of research to hear continuous gravitational waves (phys.org)
Physics and Fiction 
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