Timeline: the history of gravity

Geraint Lewis, University of Sydney

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

---

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.

Wes Mountain/The Conversation, CC BY-ND

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!

Wes Mountain/The Conversation, CC BY-ND

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.

Wes Mountain/The Conversation, CC BY-ND

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.

Wes Mountain/The Conversation, CC BY-ND

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.

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.

Wes Mountain/The Conversation, CC BY-ND

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.

Wes Mountain/The Conversation, CC BY-ND

1925: 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.

Wes Mountain/The Conversation, CC BY-ND

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.

Wes Mountain/The Conversation, CC BY-ND

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.

Wes Mountain/The Conversation, CC BY-ND

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.

Wes Mountain/The Conversation, CC BY-ND

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.

 

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.

Wes Mountain/The Conversation, CC BY-ND

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

Wes Mountain/The Conversation, CC BY-ND

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.

2016: Gravitational wave detection confirmed

After rigorous checks, the Advanced LIGO team announce the detection of gravitational waves.

Wes Mountain/The Conversation, CC BY-ND

Geraint Lewis, Professor of Astrophysics, University of Sydney

This article was originally published on The Conversation. (Reblogged by permission). Read the original article.

The art and beauty of general relativity

Margaret Wertheim, University of Melbourne

One hundred years ago this month, an obscure German physicist named Albert Einstein presented to the Prussian Academy of Science his General Theory of Relativity. Nothing prior had prepared scientists for such a radical re-envisioning of the foundations of reality.

Encoded in a set of neat compact equations was the idea that our universe is constructed from a sort of magical mesh, now known as “spacetime”. According to the theory, the structure of this mesh would be revealed in the bending of light around distant stars.

To everyone at the time, this seemed implausible, for physicists had long known that light travels in straight lines. Yet in 1919 observations of a solar eclipse revealed that on a cosmic scale light does bend, and overnight Einstein became a superstar.

Einstein is said to have reacted nonchalantly to the news that his theory had been verified. When asked how he’d have reacted if it hadn’t been, he replied: “I would have felt sorry for the dear Lord. The theory is correct.”

What made him so secure in this judgement was the extreme elegance of his equations: how could something so beautiful not be right?

The quantum theorist Paul Dirac would latter sum up this attitude to physics when he borrowed from poet John Keats, declaring that, vis-à-vis our mathematical descriptions of nature, “beauty is truth, and truth beauty”.

Art of science

A quest for beauty has been a part of the tradition of physics throughout its history. And in this sense, general relativity is the culmination of a specific set of aesthetic concerns. Symmetry, harmony, a sense of unity and wholeness, these are some of the ideals general relativity formalises. Where quantum theory is a jumpy jazzy mash-up, general relativity is a stately waltz.

As we celebrate its centenary, we can applaud the theory not only as a visionary piece of science but also as an artistic triumph.

What do we mean by the word “art”?

Lots of answers have been proposed to this question and many more will be given. A provocative response comes from the poet-painter Merrily Harpur, who has noted that “the duty of artists everywhere is to enchant the conceptual landscape”. Rather than identifying art with any material methods or practices, Harpur allies it with a sociological outcome. Artists, she says, contribute something bewitching to our mental experience.

It may not be the duty of scientists to enchant our conceptual landscape, yet that is one of the goals science can achieve; and no scientific idea has been more enrapturing than Einstein’s. Though he advised there’d never be more than 12 people who’d understand his theory, as with many conceptual artworks, you don’t have to understand all of relativity to be moved by it.

There is a beauty in spacetime. NASA, CC BY-NC

In essence the theory gives us a new understanding of gravity, one that is preternaturally strange. According to general relativity, planets and stars sit within, or withon, a kind of cosmic fabric – spacetime – which is often illustrated by an analogy to a trampoline.

Imagine a bowling ball sitting on a trampoline; it makes a depression on the surface. Relativity says this is what a planet or star does to the web of spacetime. Only you have to think of the surface as having four dimensions rather than two.

Now applying the concept of spacetime to the whole cosmos, and taking into account the gravitational affect of all the stars and galaxies within it, physicists can use Einstein’s equations to determine the structure of the universe itself. It gives us a blueprint of our cosmic architecture.

Synthesis

Einstein began his contemplations with what he called gedunken (or thought) experiments; “what if?” scenarios that opened out his thinking in wildly new directions. He praised the value of such intellective play in his famous comment that “imagination is more important than knowledge”.

The quote continues with an adage many artists might endorse: “Knowledge is finite, imagination encircles the world.”

But imagination alone wouldn’t have produced a set of equations whose accuracy has now been verified to many orders of magnitude, and which today keeps GPS satellites accurate. Thus Einstein also drew upon another wellspring of creative power: mathematics.

As it happened, mathematicians had been developing formidable techniques for describing non-Euclidean surfaces, and Einstein realised he could apply these tools to physical space. Using Riemannian geometry, he developed a description of the world in which spacetime becomes a dynamic membrane, bending, curving and flexing like a vast organism.

Where the Newtonian cosmos was a static featureless void, the Einsteinian universe is a landscape, constantly in flux, riven by titanic forces and populated by monsters. Among them: pulsars shooting out giant jets of x-rays and light-eating black holes, where inside the maw of an “event horizon”, the fabric of spacetime is ripped apart.

One mark of an important artist is the degree to which he or she stimulates other creative thinkers. General relativity has been woven into the DNA of science fiction, giving us the warp drives of Star Trek, the wormhole in Carl Sagan’s Contact, and countless other narrative marvels. Novels, plays, and a Philip Glass symphony have riffed on its themes.

At a time when there is increasing desire to bridge the worlds of art and science, general relativity reminds us there is artistry in science.

Creative leaps here are driven both by playful speculation and by the ludic powers of logic. As the 19th century mathematician John Playfair remarked in response to the bizzarities of non-Euclidean geometry, “we become aware how much further reason may sometimes go than imagination may dare to follow”.

In general relativity, reason and imagination combine to synthesise a whole that neither alone could achieve.

Margaret Wertheim, Vice-Chancellor’s Fellow in Science Communication, University of Melbourne

This article was originally published on The Conversation. (Reblogged by permission). Read the original article.

Isaac Newton on seeing further

Sir Isaac Newton PRS MP (25 December 1642 – 20 March 1726/7) was an English physicist and mathematician (described in his own day as a “natural philosopher“) who is widely recognised as one of the most influential scientists of all time and as a key figure in the scientific revolution. His book Philosophiæ Naturalis Principia Mathematica (“Mathematical Principles of Natural Philosophy”), first published in 1687, laid the foundations for classical mechanics. Newton made seminal contributions to optics, and he shares credit with Gottfried Leibniz for the development of calculus.

Brian Cox on gravity