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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. -
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The history of gravity
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Planet Vulcan
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. -
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. -
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 -
General relativity
Albert Einstein publishes a 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. -
Einstein theories stimulated emission
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. -
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. -
First measurement of gravitational redshift
Walter Sydney Adams examined light emitted from the surface of massive stars and detected a redshift, as Einstein predicted. -
Prediction of a galactic gravitational lensing
Swiss astronomer Fritz Zwicky proposed that an entire galaxy could act as a gravitational lens. -
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 -
Laser invented using stimulated emission
Theodore H. Maiman, a physicist at Hughes Research Laboratories in California, builds the first laser. -
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 centers. 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. -
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 -
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. -
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. -
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. -
LIGO receives funding
US National Science Foundation funds construction of the Laser Interferometer Gravitational-Wave Observatory (LIGO) -
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. -
LIGO construction begins
It took a long time, but the construction of LIGO finally began in Hanford, Washington, and Livingston, Louisiana. -
LIGO starts first search
In August 2002, LIGO starts searching for evidence of gravitational waves. -
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. -
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. -
Enhanced LIGO
An upgraded version called Enhanced LIGO starts new hunt for gravitational waves. -
Enhanced LIGO hunt ends
Enhanced LIGO fails to detect and gravitational waves. A major upgrade, called Advanced LIGO begins. -
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. -
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. -
Gravitational wave detection confirmed
After rigorous checks, the Advanced LIGO team announce the detection of gravitational waves.
