A brief history of gravitational waves
Albert Einstein predicted the existence of Gravitational waves in his famous 1916 paper describing general relativity. A century after Einstein’s prediction of these mysterious gravitational waves was made, proof of their existence was detected in September 2015. Einstein, as it turns out was not the first scientist to describe or predict gravitational waves but he was the first to accurately describe the phenomena. Einstein wrestled with the idea of gravitational waves for many years after publishing his paper on general relativity which indicated that these waves could, in fact, be a consequence of his theory of general relativity.
A British physicist Oliver Heaviside first proposed gravitational waves in 1893. In 1905 Henri Poincare predicted the existence of gravitational waves in his paper On Electron Dynamics where he states that a consequence of space-time geometry gravitation must produce waves that travel at the speed of light in a fashion close to electromagnetism. While there is some argument as to who first described the concept of gravitational waves it seems clear that Einstein was the first to correctly describe gravitational waves through his theory of general relativity.
What is a gravitational wave and what causes it?
A gravitational, wave according to NASA’s space place website, is “an invisible (yet incredibly fast) ripple in space. These waves travel at the speed of light through space-time which is “incredibly fast” indeed. These ripples in space physically alter the fabric of space-time as they travel. These waves stretch space in one direction and squeezes space in a direction perpendicular to the direction of stretch. These waves travel at the speed of light, in all directions, through space-time away from the source of the gravitational wave.
Gravitational waves are caused by massive objects which are accelerating around each other and may cause this ripple in the fabric of space-time when they eventually collide or merge with each other. Neutron stars or black holes are examples of objects that are massive enough to cause gravitational waves. Events which may be described as cataclysmic, such as the merger of two neutron stars or black holes or a neutron star going supernova likely produce the strongest of these waves.
How are gravitational waves detected?
The first gravitational waves that were verified were detected by LIGO (Laser Inferferometer Gravitational-Wave Observatory) located in Livingston, Louisiana and its twin inferferomter in Hanford, Washington on September 14th, 2015. The event that caused these waves is believed to be the merger of two black holes that occurred 1.3 billion years ago. The black holes reportedly collide at nearly .5c or 1/2 the speed of light to form a single massive black hole. The result is the release of an enormous amount of energy, in this case the amount of energy that was converted was equal to 3 times the mass of the sun. This process occurs in accordance with Einstein’s equation E=mc^2 which states that mass can be converted to energy. The mass that is converted to energy is discharged in the form of gravitational waves. It is these gravitational waves that were detected by the twin LIGO detectors in September of 2015.
The LIGO equipment consists of two 4 kilometer detector arms in an “L” configuration which can detect the distortion of space by as little as 1/10,000th the diameter of a proton. These distortions are the result of extremely violent events such as the merger of black holes, neutron stars, or a neutron stat going supernova. According to the LIGO Caltech website the 4 kilometer arms were “long enough that the curvature of the Earth was a factor in their construction.”
The Virgo interferometer is located in Italy which has arms that are 3 kilometers in length and there are plans for two more detectors, one will be located in India and will be a joint operation between LIGO and three research facilities in India. Another detector will be an underground detector called KAGRA located in Japan. Here is a quick link describing how LIGO detects gravitational waves https://www.sciencemag.org/news/2016/02/gravitational-waves-einstein-s-ripples-spacetime-spotted-first-time
Earth based interferometer can detect waves with a frequency of 30-400 hertz (Hz). These ground based detectors have the ability to detect waves that are longer than the 3-4 kilometer arms of the detectors. Space based interferometers which are slated for deployment in the 2030s are projected to be able to detect waves with a frequency of .1-100 milliHz. LISA, or Laser Interferometer Space Antenna consists of three probes that have the ability to detect waves to much lower frequencies than their ground based counterparts. Scientists are attempting to develop methods of detecting subtle variations from pulsars located within the Milky Way using “pulsar arrays” which are located in Europe, Australia, North America, and one being developed in China. These variations may be caused by the propagation of gravitational waves through our home galaxy. The pulsar arrays can detect frequencies from 1-320 nanoHz
What can we learn from gravitational waves?
So we know Albert Einstein predicted gravitational waves in his paper on general relativity and we know that the technology used to detect these waves is amazing but why should we study these waves? Why should we care about waves that may have been generated millions or billions of years ago?
Gravitational waves are unrelated to electromagnetic radiation and this allows us a fuller picture of events in the universe. Black holes for example are invisible to electromagnetic radiation but can be studied by the gravitational waves they create as they merge or collide with one another. Scientists are hoping to answer some fundamental questions regarding black holes and how they end up pairing and circling one another prior to colliding.
The study of gravitational waves led scientists to the origin of heavy elements in the universe. In 2017 scientists were able to witness two neutron stars merging by detecting the gravitational waves associated with the merger. Scientists were able to detect the heavy element strontium in the aftermath of the neutron star merger and the resulting explosion and burst of gamma rays known as a kilonova. LIGO Caltech defines a kilonova as “a phenomenon by which the material that is left over from the neutron star collision, which glows with light, is blown out of the immediate region and far out into space.” It is from this event that the scientists were able to prove that the heavy element strontium was created in the explosion of a neutron star. The study of gravitational waves may reveal information about the rate of expansion of the universe, the origin of black holes, and the composition of neutron stars.
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