The scientific history of gravity marks the perpetual quest to understand the beginning and structure of the universe. From the Newtonian equation in 1687 to the discovery of gravitational waves in 2016, the understanding of gravity has considerably evolved throughout the years.
From Aristotle to Einstein: Early scientific history of gravity
Around 330 BCE: The Aristotelian four elements—Greek philosopher and scientist Aristotle argued that the four elements—air, earth, fire, and water—have a natural position or place in which they travel. He further argued that objects heavier than others or those that contain more amount of earth would fall toward the ground faster and their speed would increase as they near their natural place.
1589: Leaning Tower of Pisa experiment by Galileo—A biography by Vincenzo Viviani claimed that his mentor, Italian scientist Galileo Galilei, performed an experiment that involved dropping two spheres of different masses from the Leaning Tower of Pisa to demonstrate that the time of decent of the two objects is independent of their mass.
This experiment supposedly contradicted the argument of Aristotle that heavy objects fall faster than lighter ones. However, it is important to take note that it remains unclear whether Galileo in fact performed the experiment.
1687: Newtonian apple and the Theory of Gravitation—In his Philosophiae Naturalis Principia Mathematica, Isaac Newton provided one of the most significant contributions to the theory of gravity. He argued that the orbit of the moon depended on the same type of force that causes an apple to fall on Earth.
Furthermore, this argument also proposed the inverse-square law of universal gravitation. This law claims that the magnitude of the force decreases in inverse proportion to the square of the distance from the centre of the Earth.
The Newtonian theory of universal gravitation gave astronomers a tool for predicting the motion of planets. This theory further received success when it successfully predicted the existence of Neptune through a series of calculations done in 1840s. Accordingly, the motion of Uranus could not be accounted for by the actions of other planets.
1859: Mercury and the search for the planet Vulcan—There was a problem with calculating the precise orbit of Mercury using the Newtonian theory of universal gravitation. By the end of the 19th century, researchers discovered that the orbit of Mercury showed slight perturbations. This discrepancy did not match what the theory predicted.
To explain the discrepancy with the orbit of Mercury, French mathematician who specialised in celestial mechanics Urbain Le Verrier proposed that there was another planet orbiting closer to the sun. He called this Vulcan. Verrier argued that the gravity from Vulcan was influencing the orbit of Mercury. However, repeated observations revealed no signs of this planet.
1905: Special relativity theory of Einstein—Albert Einstein introduced the theory of special relativity that modified the Newtonian theory of universal gravitation. He initially wanted to describe gravity in a way that was independent of the motions of observers and of the coordinates chosen to label events.
The special relativity theory argues that the laws of physics are the same for all non-accelerating observers. Furthermore, it also argues that the speed of light within a vacuum is the same regardless of the speed at which an observer travels. This theory also proposed that space and time are linked within a single continuum called space-time.
With the introduction of this theory, Einstein began incorporating gravity in the picture to describe it exclusively within the geometrical concept of space-time continuum.
1907: Einstein and the gravitational redshift—While developing another relativity theory that incorporates gravity in the equation, Einstein also proposed the wavelength of light coming from atoms trapped in a strong gravitational field stretches or lengthens as it tries to escape the force of the gravity. This lengthening of light wavelength results in the display of the electromagnetic spectrum. The longer wavelength shifts the photon to the red end of the spectrum.
The described lengthening of light wavelength paved the way for the understanding of so-called gravitational redshift. This concept is a process by electromagnetic radiation originating from a source that is in a gravitational field is reduced in frequency when observed in a region of a weaker gravitational field.
1915: General relativity theory of Einstein—In his attempt to describe gravitation exclusively within the geometrical concept of space-time continuum, Einstein came up with the general relativity theory. This theory argues that gravity affects all forms of matter and energy, all of which moves in spacetime. Massive objects cause a distortion in spacetime and such is felt as gravity.
Einstein successfully generalised the special relativity theory and the Newtonian theory of universal gravitation. The theory of general relativity provides a unified description of gravity as geometric property of space and time. A curvature in the spacetime occurs because it is directly related to the energy and momentum of whatever mass or radiation that is present.
The theory also predicted the existence of black holes and gravitational waves. Einstein struggled to understand these concepts further however. Nonetheless, general relativity theory opened a window opportunities that ushered in a new era for understanding the university. The scientific history of gravity took a considerable leap because of this theory.
From gravitational lensing to waves: Modern scientific history of gravity
1918: Predicting frame dragging—Austrian physicists Josef Lense and Hans Thirring used the general relativity theory as a framework for predicting the Lense-Thirring effect or frame dragging. They theorised that the rotation of a massive object in space would drag spacetime around it.
The National Aeronautics and Space Administration launched a project called Gravity Probe B to measure the spacetime curvature near the Earth. Using a probe that gyroscopes that rotated slightly over time due to the underlying spacetime, researchers found that frame dragging is stronger around a rotating object which “drags” spacetime around with it. The amount of rotation of the gyroscopes was consistent with the general relativity theory.
1919: Observing gravitational lensing—Observers witnesses during a total solar eclipse in May 1919 that stars near the sun seemed slightly out of position. This seemed to indicate that the light was bending due to the mass of the sun. Nonetheless, this was the first observation of gravitational lensing—the bending of light around a massive object allowing observers to view objects behind it.
Swiss astronomer Fritz Zwicky further predicted in 1937 that galaxy clusters could act as gravitational lenses. In other words, light coming from objects can bend around entire galaxy. This would allow observers to view objects behind this massive galaxy cluster.
In 1979, astronomers Dennis Walsh, Bob Carswell, and Ray Weymann observed two identical quasars or quasi-stellar objects. Further observation revealed that these objects were actually one quasar that appeared as two separate objects. This is the first observation of galactic gravitational lensing.
1952: Measuring and confirming gravitational redshift—American astronomer Walter Sydney Adams observed and analysed the light emitted from the surface of massive stars. His examination resulted in the detection of lengthening of light wavelength, particularly the detection of red light. This was similar to the prediction made by Einstein.
Robert Pound and Glen Rebka performed an experiment in 1959 that involved confirming the existence of gravitational redshift. Doing so required measuring the redshift in spectral lines using Iron-57 gamma source over a vertical height of 22.5 metres—or at the top and bottom of the Jefferson Laboratory tower at Harvard University. The experiment precisely measured the minute change in energies as photons journeyed between the top and the bottom.
1960: Observing the existence of black holes—The general relativity theory gained further momentum beginning the 1960s due to the discovery of galaxies maintained by an enormous pull of black holes in the centre. Gravity is responsible for this pull.
All massive galaxies are now found to have massive black holes. Smaller black holes also exist roaming between the stars.
1966: First proof of gravitational time delays—Using the general relativity as a framework, American astrophysicist Irwin Shapiro predicted that the gravity of the sun will slow down radio waves as they travel and bounce around the solar system.
Series of experiments were performed between 1966 and 1977s that involved firing radar beams on the surface Venus and measuring the time taken for it to return to Earth. The bouncing created delays that were consisted with the general relativity theory of Einstein.
Gravitational time delay or dilation is now understood as an actual difference of elapsed time between two events as measured by observers situated at varying distances from a gravitating mass.
1969: Search for gravitational waves began—Joseph Weber made a claim that his experiment led to the detection of gravitational waves. However, others were not able to replicate the results of his experiment thereby concluding that the claim was invalid.
Joseph Taylor and Russell Hulse discovered the binary pulsar in 1974. Further measurements of the orbital decay of the pulsars revealed that the two lost energy. This loss in energy matched the amounts predicted by the general relativity theory. Nonetheless, this discovery marked an indirect evidence for gravitational waves.
Joseph Weber made another claim in 1987 that he discovered gravitational waves. His so-called torsion bar experiments involved using large aluminium bars engineered to vibrate when a large gravitational wave passed through it.
1979 to 2005: Funding and conceiving the LIGO—The United States National Science Foundation provided support and funding for the construction of the Laser Interferometer Gravitational-Wave Observatory or LIGO—a large-scale physics experiment and observatory to detect gravitational waves—starting in 1979.
It is important to note that Einstein came up with a theory in 1917 that described stimulated emission. In his paper about the quantum theory of radiation, he theorised the process of spontaneous emission in which an excited atom returns to a lower energy state by releasing energy. This process paved the way for the development of light amplification by stimulated emission of radiation or LASER.
American engineer and physicist Theodore Harold Maiman invented the first laser in 1960. His invention led to the development many other types of and uses for lasers.
The concept behind LIGO centres on the use of massive laser interferometers located thousands of kilometres apart to exploit the physical properties of light and of space itself to detect and understand the origins of gravitational waves
Nonetheless, the construction of LIGO began in 1994 in Hanford, Washington and Livingston, Louisiana. In August 2002, it started searching for evidence of gravitational waves. The search however ended in 2005 after five attempts. Researchers gathered that the sensors needed upgrading to improve sensitivity.
2009 to 2016: Improved LIGO and the discovery of gravitational waves—The so-called Enhanced LIGO started the new hunt for gravitational waves in 2009. However, by 2010, the search yielded no results. A new major upgrade began and the resulting product was the Advanced LIGO.
The Advanced LIGO was completed in 2014 after installation and rounds of testing. A new search began in 2015. This iteration to the LIGO has four times the sensitivity of the original version.
In September 2015, the Advanced LIGO detected a signal that appeared to come from the collision of two black holes. A thorough analysis of the data revealed that gravitational waves were finally detected. The announcement was made in February 2016 and it marked another turning point in the scientific history of gravity.