This article on LIGO and gravitational waves is a guest article by Ariana Vlad, senior at the International Computers High School of Bucharest, Romania, where she focuses on studying Physics and Mathematics.
What Are Gravitational Waves?
Gravitational waves are, as predicted by Einstein’s theory of general relativity, ripples in space-time that propagate with the speed of light when energy is liberated in a process. The first proof of this prediction came more than 50 years after the theorem was developed. In 1974 two scientists from a Puerto Rico Observatory discovered a binary pulsar and started recording the rate of energy loss of this system. After gathering eight years worth of data and solving extremely complicated equations, the astronomers could safely conclude that their observations matched the theory. This was a great step towards proving the initial prediction, but the first direct proof came from a group of Caltech scientists and their ingenious project, LIGO.
Why Detect Them?
Since the beginning of astronomical studies, scientists rely mostly on detecting electromagnetic radiation to discover what happens in space. The discovery of gravitational waves has now opened a new window that allows us to complete our understanding of these phenomena. Since waves carry out energy as they move, they also carry information about the nature of the original phenomenon. This can help scientists “view” cosmic events that were invisible to devices that recorded electromagnetic radiation, such as two black holes colliding. Another advantage of gravitational wave detection is their weak interaction with matter, which means initial information will suffer little to no distortions or alterations.
Laser Interferometer Gravitational Waves Observatory – The LIGO
LIGO is one of the greatest observatories ever built both in dimension and in sensitivity. Each such device consists of two 4 kilometers long arms, comprising 1.2 meters wide steel vacuum tubes and protected by a 3 meters wide and almost 4 meters tall concrete shelter. This protective structure is necessary to isolate the observatory from the exterior noise and keep a high sensitivity. Because of this, LIGO can detect variations of even 1/10000 the size of a proton, which is roughly the amplitudes of the greatest gravitational waves recorded.
LIGO is built similarly to a Michelson interferometer. A Michelson interferometer is composed of two coherent laser sources, perfect mirrors that lengthen the light’s path, and a screen or detector that receives light beams from both sources. All these are placed in a way that ensures the light’s paths are perpendicular to one another. The screen serves as a recorder of the interference pattern of the two beams, a measure of the phase difference of light for the two distinct paths. Let’s say that, initially, the two paths are identical.
This means the phase difference between the laser beams on the screen in zero (constructive interference). What will happen if the paths are slightly different? The new phase difference will be very small, but certainly not zero, therefore the interference pattern can’t be the same as in the first case. Measuring the shift in the pattern or the intensity of the light in the central point of the screen, scientists can find the path difference that determined this change. At LIGO, the same path difference is due to gravitational waves passing through the system at that moment.
Detection Of Gravitational Waves
The first direct observation of gravitational waves was recorded on 14 September 2015 at LIGO and the partner observatory Virgo. The data matched the theoretical predictions of general relativity, which helped scientists understand what determined the recorded gravitational waves: the merger of two black holes, one around 36 solar masses and the other around 29 solar masses. The energy radiated from this process in the form of gravitational waves corresponded to the rest energy of three solar masses.
In August 2017, LIGO took a step further when it detected a new source of gravitational waves with unknown position and origin. Combining data from all gravitational-wave observatories, scientists were able to notify the astronomers from electromagnetic radiation-based telescopes, who observed the visible aftermath of what was supposedly the collision of two neutron stars.
This represented only the first collaboration between astronomers at these two types of observatories, and many more are now expected. Now that a new window towards a better understanding of the universe has opened, we can only hold our breaths in anticipation of what will follow.