By Geraint Lewis - Professor of Astrophysics, University of Sydney

Our understanding of gravity has gone through a few permutations, from Newton’s equations through to Einstein’s general relativity. With the discovery of gravitational waves, we look back on how our grasp of gravity has evolved over the centuries.

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1687: Newtonian gravity

Isaac Newton publishes Philosophiae Naturalis Principia Mathematica, giving a comprehensive account of gravity. This gave astronomers an accurate toolbox for predicting the motions of planets. But it was not without its problems, such as calculating the precise orbit of the planet Mercury.

All planets' orbits precess – with the closest point of their orbit moving slightly with each revolution – due to the gravitational tugs from other planets.

The issue with Mercury’s orbit was that the amount of precession did not match what Newton’s theory predicted. It was only a small discrepancy, but big enough for astronomers to know it was there!

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1859: Planet Vulcan

To explain Mercury’s odd behaviour, Urbain Le Verrier proposed the existence of an unseen planet called Vulcan, which orbited closer to the sun. He suggested that the gravity from Vulcan was influencing Mercury’s orbit. But repeated observations revealed no signs of Vulcan.

1905: Special relativity

Albert Einstein shakes up physics with his special theory of relativity. He then started incorporating gravity into his equations, which led to his next breakthrough.

1907: Einstein predicts gravitational redshift

What we now call gravitational redshift was first proposed by Einstein from his thoughts in the development of general relativity.

Einstein predicted that the wavelength of light coming from atoms in a strong gravitational field will lengthen as it escapes the gravitational force. The longer wavelength shifts the photon to the red end of the electromagnetic spectrum.

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1915: General relativity

Albert Einstein publishes general theory of relativity. The first great success was its accurate prediction of Mercury’s orbit, including its previously inscrutable precession.

The theory also predicts the existence of black holes and gravitational waves, although Einstein himself often struggled to understand them.

1917: Einstein theorises stimulated emission

In 1917, Einstein publishes a paper on the quantum theory of radiation indicating stimulated emission was possible.

Einstein proposed that an excited atom could return to a lower energy state by releasing energy in the form of photons in a process called spontaneous emission.

In stimulated emission, an incoming photon interacts with the excited atom, causing it to move to a lower energy state, releasing photons that are in phase and have the same frequency and direction of travel as the incoming photon. This process allowed for the development of the laser (light amplification by stimulated emission of radiation).

1918: Prediction of frame dragging

Josef Lense and Hans Thirring theorise that the rotation of a massive object in space would “drag” spacetime around with it.

1919: First observation of gravitational lensing

Gravitational lensing is the bending of light around massive objects, such as a black hole, allowing us to view objects that lie behind it. During a total solar eclipse in May 1919, stars near the sun were observed slightly out of position. This indicated that light was bending due to the sun’s mass.

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925: First measurement of gravitational redshift

Walter Sydney Adams examined light emitted from the surface of massive stars and detected a redshift, as Einstein predicted.

1937: Prediction of a galactic gravitational lensing

Swiss astronomer Fritz Zwicky proposed that an entire galaxy could act as a gravitational lens.

1959: Gravitational redshift verified

The theory was conclusively tested by Robert Pound and Glen Rebka by measuring the relative redshift of two sources at the top and bottom of Harvard University’s Jefferson Laboratory tower. The experiment accurately measured the tiny change in energies as photons travelled between the top and the bottom.

1960: Laser invented using stimulated emission

Theodore H. Maiman, a physicist at Hughes Research Laboratories in California, builds the first laser.

1960s: First evidence for black holes

The 1960s was the beginning of the renaissance of general relativity, and saw the discovery of galaxies that were powered by the immense pull of black holes in their centres.

There is now evidence of massive black holes in the hearts of all large galaxies, as well as there being smaller black holes roaming between the stars.

1966: First observation of gravitational time delays

American astrophysicist Irwin Shapiro proposed that if general relativity is valid, then radio waves will be slowed down by the sun’s gravity as they bounce around the solar system.

The effect was observed between 1966-7 by bouncing radar beams off the surface of Venus and measuring the time taken for the signals to return to Earth. The delay measured agreed with Einstein’s theory.

We now use time-delays on cosmological scales, looking at the time differences in flashes and flares between gravitationally lensed images to measure the expansion of the universe.

1969: False detection of gravitational waves

American physicist Joseph Weber (a bit of a rebel) claimed the first experimental detection of gravitational waves. His experimental results were never reproduced.

1974: Indirect evidence for gravitational waves

Joseph Taylor and Russell Hulse discover a new type of pulsar: a binary pulsar. Measurements of the orbital decay of the pulsars showed they lost energy matching the amounts predicted by general relativity. They receive the 1993 Nobel Prize for Physics for this discovery.

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1979: First observation of a galactic gravitational lens

The first extragalactic gravitational lens was discovered, when observers Dennis Walsh, Bob Carswell and Ray Weymann saw two identical quasi-stellar objects, or “quasars”. It turned out to be one quasar that appears as two separate images.

Since the 1980s, gravitational lensing has become a powerful probe of the distribution of mass in the universe.

1979: LIGO receives funding

US National Science Foundation funds construction of the Laser Interferometer Gravitational-Wave Observatory (LIGO).

1987: Another false alarm for gravitational waves

A false alarm on direct detection from Joseph Weber (again) with claimed signal from the supernova SN 1987A using his torsion bar experiments, which consisted of large aluminium bars designed to vibrate when a large gravitational wave passed through it.

1994: LIGO construction begins

It took a long time, but the construction of LIGO finally began in Hanford, Washington, and Livingston, Louisiana.

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2002: LIGO starts first search

In August 2002, LIGO starts searching for evidence of gravitational waves.

2004: Frame dragging probe

NASA launches Gravity Probe B to measure the spacetime curvature near the Earth. The probe contained gyroscopes that rotated slightly over time due to the underlying spacetime. The effect is stronger around a rotating object which “drags” spacetime around with it.

The gyroscopes in Gravity Probe B rotated by an amount consistent with Einstein’s theory of general relativity.

2005: LIGO hunt ends

After five searches, the first phase of LIGO ends with no detection of gravitational waves. The sensors then undergo an interim refit to improve sensitivity, called Enhanced LIGO.

2009: Enhanced LIGO

An upgraded version called Enhanced LIGO starts new hunt for gravitational waves.

2010: Enhanced LIGO hunt ends

Enhanced LIGO fails to detect and gravitational waves. A major upgrade, called Advanced LIGO begins.

2014: Advanced LIGO upgrade completed

The new Advanced LIGO has finished installation and testing and is nearly ready to begin a new search.

2015: False alarm #3 for gravitational waves

The indirect signature of gravitational waves in the early universe was claimed by the BICEP2 experiment, looking at the cosmic microwave background. But it looks like this was dust in our own galaxy spoofing the signal.

2015: LIGO upgraded again

Advanced LIGO starts a new hunt for gravitational waves with four times the sensitivity of the original LIGO. In September, it detects a signal that looks likely to be from the collision between two black holes.