By Stephen Hughes
In 1915, ten years after the publication of the theory of special relativity, Albert Einstein published a new theoretical description of gravity, the theory of general relativity. Before general relativity, gravity was understood in terms of Newton’s law of gravitation, which gives excellent agreement between the predicted and observed positions for most of the planets in the Solar System. However, the predictions regarding the planet Mercury’s orbit around the Sun are slightly wrong. One of the first confirmations that general relativity was the correct description of gravity came from the theory accounting for this discrepancy. A theoretical model, such as general relativity, should lead to a better understanding of the phenomena it is trying to describe. There should also be predictions regarding the outcome of experiments when measurements are made. Ideally the predictions should not only explain what is already known but provide some insight into new effects. General relativity gives many predictions regarding previously unknown effects while accounting for everything that is already known from Newton’s work.
In 1916 Einstein found his theory predicted the existence of gravitational waves. Many of the other predictions of general relativity have been confirmed experimentally in the early decades that followed. Einstein’s gravitational waves have eluded direct experimental confirmation for a century, leaving some uncertainty if they actually exist. During the past century many experiments have been designed to detect and study gravitational waves. The instruments used to detect gravitational waves need to be extremely sensitive to small disturbances in space-time. Gravity is the result of mass and energy curving space-time. Gravitational waves are ripples in space-time, propagating at the speed of light. When propagating through a region of space-time, the gravitational waves change the curvature of space-time by a small amount in that region. This is what the experiments try to measure. Only large scale processes, such as the collision of two stars, are likely to produce significant gravitational waves to be detected based on present instrument sensitivity. Gravitational waves carry away some of the energy from the collision. Detecting and measuring the gravitational waves gives information about the colliding masses.
In September 2015 the gravitational wave detectors comprising LIGO (Laser Interferometer Gravitational-Wave Observatory), resumed after undergoing an upgrade designed to increase the sensitivity of the instruments. Shortly after this upgrade in February 2016 the paper ‘Observation of Gravitational Waves from a Binary Black Hole Merger’ was published in the journal Physical Review Letters. This work represents the collaborative effort of a large team of scientists, engineers and mathematicians from different countries, working together over many years. Their work documents the first direct confirmation of gravitational waves. While this is a triumphant confirmation of Einstein’s ideas about gravity it is also the beginning of a new era of astronomical observations. Traditionally objects in the Universe are observed by collecting visible light through the aperture of a telescope. Visible light represents only a small range of the electromagnetic spectrum. Observing the Universe using only visible light restricts the information that can be obtained. Extending the range to include other parts of the electromagnetic spectrum gives more information, leading to a better understanding. When viewing nebulae, the birth place of stars, while only detecting visible light, the features of these systems can be obscured by a large cloud of gas and dust surrounding the newly forming stars. However, observing the same systems with detectors sensitive to light from other regions of the electromagnetic spectrum reveals more structural detail. The gas and dust in this case are not preventing the light from leaving the system.
Astronomers now have a new technique for observing objects and events in the Universe. These first gravitational waves measured by LIGO originated when two black holes merged together, the first time this type of event has ever been observed. There are presently several gravitational wave detectors being constructed and others planned for construction in the near future. Perhaps the most promising of these are DECIGO (DECI-hertz Interferometer Gravitational wave Observatory), and eLISA (Evolved Laser Interferometer Space Antenna), which are anticipated to be launched in 2027 and 2038 respectively. These space based instruments will be more sensitive and capable of detecting gravitational waves from a greater range of astronomical events. Since gravitational waves also travel through space-time unaffected by other events, those produced in the early Universe are still propagating through space-time today. If measured these primordial gravitational waves could lead to a better understanding of the origin of the Universe.
To learn more about Einstein’s general relativity, space-time and gravitational waves why not enrol onto 101 Years of General Relativity with Stephen Hughes starting Monday 4th April 2016 you can enrol here