Einstein’s General Relativity theory, 1915-2015

Albert Einstein

By Don Barrett in the USA:

100 years of General Relativity

“Thus, the general theory of relativity as a logical edifice has finally been completed”—Albert Einstein, November 25, 1915

These words were spoken by Albert Einstein one hundred years ago, concluding a series of four lectures at the Prussian Academy of Sciences in Berlin on a new theory of universal gravitation, extending and amending the work Isaac Newton published 228 years earlier. In the accompanying paper, Die Feldgleichungen der Gravitation (The Field Equations of Gravitation), Einstein published for the first time the final and correct equations for what would come to be known as the general theory of relativity. This work, an elaboration on the special theory of relativity worked out ten years previously, remains one of the two central pillars of modern physics.

While the scientific community pursued and studied this work, the Great War raged in its second year. Rationing, hunger and international isolation were features of everyday life. Physicists such as Karl Schwarzschild were deployed on the various fronts. Two years later would see the conquest of political power by the masses under the leadership of the Bolshevik Party in Russia, redefining the political landscape for the rest of the century.

Produced in a revolutionary epoch, general relativity continues to be regarded as one of the momentous achievements of physics, yet its implications are far from exhausted. Even after the passage of a century, new insights connecting the theory and observed phenomena appear each year in the publications of thousands of physicists across the globe. It was the capstone to a century of intellectual struggle and the catalyst to a deeper understanding of the material world.

Einstein’s work did not emerge in isolation. The growth of 19th and early 20th century science was inseparable from the technical innovation driven by the engine of capitalism during the period of its rise as an economic system. The first high accuracy refracting telescope, built by Joseph Fraunhofer and installed at Tartu in 1824, was a derivative of the surveying theodolite, the essential tool for dividing up land and marking national boundaries. High accuracy clocks and sextants, necessary for quantitative astronomical measurements, were initially treated as strategic secrets of state because of their critical role in navigation. The invention of the telegraph allowed for closer and more immediate collaboration among astronomers both within their respective countries and across national borders. As these new more precise technologies emerged, questions regarding humanity’s understanding of matter could be revisited.

Joseph von Fraunhofer (1787-1826) together with his surveying theodolite and his Tartu refractor of 1824, the first modern telescope

This led to a renewed interest in theories of planetary motion, which are the origins of the study of gravity. Ptolemy begin this work in the second century BCE when he explained the positions of the known planets in the sky by a sort of clockwork motion of planets in concentric and slightly offset circles; this theory survived for 15 centuries. It was overturned by Johannes Kepler on the basis of a slight yet critical discrepancy of its predictions, which led Kepler to a “road to a complete reformation of astronomy.” His new theory not only correctly described the motion of the planets through the sky, but concluded that their paths in space must be ellipses around the Sun. Galileo’s observations of Jupiter’s four largest moons showed that Kepler’s theory was correct not just for planets orbiting the Sun, but for moons orbiting planets.

Newton provided a deeper understanding of the elliptical motion by reducing it to a simple and universal law of force between the Sun, the planets, in fact everything in nature, varying with the masses and distances of the attracting bodies. To elaborate the implications of his theory, Newton, alongside the German mathematician Gottfried Wilhelm Leibniz, developed an entire new branch of mathematics, differential and integral calculus. Newton’s theory, combined with the new precision of 19th century technique, culminated in the discovery in 1846 of Neptune, a planet whose existence was deduced from its slight effect on the motion of Uranus by mathematical analysis. The French scientist and republican François Arago famously remarked that Neptune had been discovered “with the point of [a] pen.”

But contradictions to Newton’s theory began to develop through the 19th century. Urbain Le Verrier, one of the astronomers who predicted Neptune’s orbit, noted that the motion of the Mercury, the innermost and most rapidly orbiting planet, was deviating very slightly from its predicted path. Most of the deviation could be specifically calculated and modeled away as due to tugs from the other planets; the remainder, an effect of only seven percent of the whole, remained unexplained. Searches were made for an undiscovered inner planet adding its own perturbations. None has ever been found.

Other areas of 19th century physics were challenged by discovery after discovery tied to improvements in technique and materials. Hitherto unknown types of radiation were detected, not directly by the human senses, but by their physical effects. Thus 1799 brought the discovery of infrared rays; 1800, ultraviolet rays; 1886, radio waves; 1895, x-rays. The apparent stability of matter itself was shattered with the discovery of radioactivity by Henri Becquerel in 1896.

The high point of this scientific turmoil was the discovery of two new types of universal forces: electricity and magnetism. Alessandro Volta’s pile, the first battery, was assembled in 1800. Flowing electricity was found to create magnetism by Hans Ørsted in 1820. Michael Faraday showed that changing magnetic fields created electricity in 1831, and thus created the basis for modern electrical power generation. Faraday also first described magnetism using the concept of a field of magnetic lines existing in definite physical relations with other forms of matter. Contemporaries viewed this field as merely a mathematical abstraction, but the later work of Maxwell and much of 20th century physics showed that fields do in fact exist as an independent material reality.

The full theory of these phenomena was worked out by James Clerk Maxwell, whose equations of 1861 and their refinement in 1865 would unify electricity and magnetism into a new theory of electromagnetism. This theoretical unity for a natural force was matched only by Newton’s work on gravitation, two centuries previously. The great physicist Ludwig Boltzmann gave some voice to the impact Maxwell’s contribution made when he quoted Goethe’s Faust: “Who was the god who wrote these lines?”

Maxwell’s equations, tested by Heinrich Hertz’s deliberate construction of equipment to produce and detect the predicted waves we now call radio, suggested the various other “rays” already discovered composed a common form as electromagnetic waves. What distinguished them was simply the wavelength between crests in an electromagnetic spectrum of radiation.

Into this ferment was born Albert Einstein in 1879, just months before Maxwell’s death. The young Einstein was captivated by Maxwell’s work. In autobiographical notes, he stated, “The most fascinating subject at the time I was a student was Maxwell’s theory.” Inspired by Maxwell’s study of light, a 16-year-old Einstein conducted his first significant gedankenexperiment (thought experiment). He imagined what it would be like to ride along such an electromagnetic wave at the same speed. Would it appear frozen, since the motion would be along its crest, like surfing along an ocean wave? The equations did not admit this possibility. More intriguingly, the theory of electromagnetism posited a fixed velocity—the speed of light—while Newton’s equations implied no such limit, instead describing the force of gravity operating with instantaneous effect.

Why should the form of the physical laws governing two different “fundamental forces” treat velocity so differently? By 1893 Oliver Heaviside would demonstrate that a modified theory of gravity incorporating a Maxwell-like speed limit must exhibit new behavior, including wave-like behavior. And what should the speed of Maxwell’s waves be measured against? These were questions posed to physicists as the end of the 19th century approached.

The speed of light had been measured crudely by Ole Rømer in 1676 by measuring the sixteen-minute difference in the clockwork motions of Jupiter’s moons from when Earth was on the near and far side of its orbit around the Sun. In 1879, Albert Michelson measured the speed of light not by using the natural motions of the solar system, but rather by reflecting pulses of light off a spinning mirror. By tuning the spinning rate, one could measure the speed. The accuracy of this technique was far above previous measurements: it was good to one part in six thousand.

Ten years later, in 1889, Michelson, together with Edward Morley, used a variation of this device to compare the speed of light in two separate directions at the same time. It was assumed there was some stationary luminiferous aether through which the electromagnetic vibrations of Maxwell’s equations (i.e., light) propagated. The refined Michelson-Morley device could compare the speed of light going in different directions, this time to one part in sixty thousand. Since the Earth travels about the Sun at 30 kilometers per second, an enormous effect should have been seen in the speed of light measured along and perpendicular to the presumed aether. But, as Einstein would write in 1916, “the experiment gave a negative result—a fact very perplexing to physicists.”

Several physicists lent their efforts to construct ad-hoc “corrections” to explain the null result of the Michelson-Morley experiment. These would apply to moving bodies and cancel the expected measurements of aether drift. Hendrik Lorentz and George FitzGerald proposed a contraction in the scale of length with speed, such that an exact cancellation would occur against the expected difference in the speed of light as measured from a platform in motion. Such were the efforts to save the laws of electromagnetism from the realities of experimental results at the close of the century.

Einstein’s solution to the problem was both brilliant and profoundly radical. He recognized that there was a fundamental incompatibility between the way Newton and Maxwell expressed time and space in their theories, and he settled this conflict in favor of Maxwell rather than Newton. It would take a decade to fully work out the consequences. Here he was influenced by a physicist who became more famous for his philosophical speculations than his (significant) discoveries in the field of mechanics, Ernst Mach (1838-1916).

Mach explored through thought experiments the contradictions of Newton’s assumption of an absolute frame of reference, an absolute space and time against whose background the motion of objects could be measured. He contended that such an absolute space and time had no real physical meaning: motion was relative to an observer, not absolute. In considering Mach’s arguments, Einstein developed the understanding that observers who were not accelerating in relation to each other should see the same physical laws, and that this must hold for any development of new physics.

Mach took his denial of absolute space and time to an idealist conclusion philosophically, claiming that matter itself could not be considered an absolute, i.e., existing independently of the observer. The world, he claimed, consisted entirely of sensations and complexes of sensations. Lenin made Mach, and idealist philosopher Avenarius, the principal targets of his polemic in defense of dialectical materialism, Materialism and Empirio-Criticism.

Einstein did not follow Mach on his erroneous philosophical path. On the hundredth anniversary of Maxwell’s birth in 1931, Einstein opened his comments with following: “The belief in an external world independent of the perceiving subject is the basis of all natural science.”

Even as the philosophical debates carried on, the paradox of the Michelson-Morley “null result,” showing the same speed of light in all directions, and the deeper contradiction was resolved by Einstein’s special theory of relativity, published in 1905. (See: One hundred years since Albert Einstein’s annus mirabilis) Einstein’s boldest stroke was to assume as a basic postulate that observers in uniform motion would all see the same laws of physics from their individual perspectives, including the same speed of light. The germ of this project was found in Maxwell’s equations, with the speed of light serving as a critical natural constant.

One of the consequences of Einstein’s theory is that mass and energy, rather than subject to separate and independent conservation laws, are conserved together and are therefore at some fundamental level equivalent. This famous equivalence of mass and energy is expressed in the equation known across the world: E=mc2. The speed of light links small amounts of mass to enormous amounts of potential energy. Originally thought to be only of academic interest, this conversion was later carried out through the mechanism of the nuclear fission chain reaction. The energy released by the two atomic bombs dropped by the United States at the end of World War II, which each incinerated a city, was the equivalent of less than one gram of mass.

There was, however, a more daunting task. Einstein and his contemporaries had managed to recast the equations of electromagnetic physics so that different uniformly moving observers saw the same laws. The same approach would not work for gravitational physics. Newton’s equations still differed from Maxwell’s equations in that they implied instantaneous action, that is, infinite velocities. Special relativity had decisively shown that the fastest velocity was that of light, thus limited the speed at which even gravity could influence matter. Integrating these concepts into Newton’s work would occupy Einstein and the broader physics community from 1907-1915, even as the world political situation tobogganed towards catastrophe.

To be continued

Part II of this is here. Part III is here.

Bird migration, new research

This video says about itself:

Physics of Bird Migration

29 April 2013

It is spring and we went to check out the migratory birds returning from their winter grounds. It is pretty incredible to think that some of them have crossed deserts and oceans on their journeys, and they still manage to find their way back to the same locations every year.

For example, did you know that the Arctic Tern is the World Record holder when it comes to migration amongst birds? It spends Northern Hemisphere summers in the Arctic and then for winter it flies all the way to the Antarctic! Absolutely crazy to think that in one year it has seen more of the world than most of us will in a lifetime. In this week’s video we take a look at the physics behind a few of the adaptations that the birds have evolved to be able to perform these annual migrations. Enjoy!

Produced by: Jonas Stenstrom

Filming help by: Louise Fornander & John-Mehdi Ghaddas

From the Annual Review of Physiology (2015):

The Neural Basis of Long-Distance Navigation in Birds


Migratory birds can navigate over tens of thousands of kilometers with an accuracy unobtainable for human navigators. To do so, they use their brains. In this review, we address how birds sense navigation- and orientation-relevant cues and where in their brains each individual cue is processed. When little is currently known, we make educated predictions as to which brain regions could be involved.

We ask where and how multisensory navigational information is integrated and suggest that the hippocampus could interact with structures that represent maps and compass information to compute and constantly control navigational goals and directions. We also suggest that the caudolateral nidopallium could be involved in weighing conflicting pieces of information against each other, making decisions, and helping the animal respond to unexpected situations. Considering the gaps in current knowledge, some of our suggestions may be wrong. However, our main aim is to stimulate further research in this fascinating field. Expected final online publication date for the Annual Review of Physiology Volume 78 is February 10, 2016. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.

Einstein and light, new film

This video says about itself:

22 January 2015

Official trailer for the International Year of Light 2015 (IYL2015), by Nickolas Barris.

This trailer kicked off the official United Nations/UNESCO opening ceremony of the IYL2015 in Paris on January 19, 2015.

The concept of this video ‘propagated light from the cosmos activating life on earth’, is based on Barris’ documentary film Einstein’s Light, which is in production and will be released in September 2015.

‘Einstein was wrong’, new research claims

This video says about itself:

Dutch Physicists Teleport Quantum Information

3 June 2014

For the first time, physicists from the Kavli Institute of Nanoscience at the Delft University of Technology in the Netherlands have reportedly been able to teleport information between a pair of quantum bits that are about ten feet apart. The ability to do this pokes a hole in Einstein’s theory about entanglement or connection of particles that are light years apart, and how the state of a particle instantly affects the state of another particle.

For the first time with 100 percent accuracy, physicists from the Kavli Institute of Nanoscience Delft, at the Delft University of Technology in the Netherlands have reportedly been able to teleport information between a pair of quantum bits that are about ten feet apart.

The ability to do this pokes a hole in Einstein’s disbelief in entanglement where particles remain connected with the state of one particle instantly affecting the state of another despite being light years apart.

There are several groups of scientists working on similar projects, and the Dutch researchers have been able to reliably teleport the information by trapping electrons in diamonds at extremely low temperatures, and observing their electron spins.

Ronald Hanson, a physicist who leads the group at Delft University of Technology is quoted as saying: “There is a big race going on between five or six groups to prove Einstein wrong. There is one very big fish.”

Now that they have been able to repeatedly teleport the information from ten feet away, the researchers want to see if they can send the information between even further distances.

This development might make it possible to have a faster generation of computer systems that also operate on completely secure communication networks.

From The Economist in Britain:

Hidden no more

One of the weirdest bits of physics is proved beyond doubt (almost)

Oct 24th 2015

IN THE 1930s Albert Einstein was greatly troubled by a phenomenon that emerged from quantum theory. Entanglement, as it is called, forever intertwines the fates of objects such as subatomic particles, regardless of their separation. If you measure, say, “up” for the spin of one photon from an entangled pair, the theory suggests that the spin of the other, measured an instant later, will surely be “down”—even if the two are on opposite sides of the galaxy. This was anathema to Einstein and others: it looked as if information was travelling faster than light, a no-no in the special theory of relativity. Einstein was quotably derisive, calling the idea “spooky action at a distance”. But after 80 years of physicists’ fretting, a cunning experiment reported this week proves that such action is in fact how the world works.

To save physics from the spooky, Einstein invoked what he called hidden variables (though others might describe them as fiddle factors) that would convey information without breaking the universal speed limit. It took until 1964, though, to tame this woolly idea into testable equations. John Bell, a British physicist, worked out the maximum effect hidden variables could have on a given test. Any influence beyond that, his equations suggested, must be down to spooky action. The Bell inequality, as it became known, sparked decades of clever experiments—sending entangled photons or atoms hither and thither with detectors triggered by this or that—each designed to catch nature out, to banish hidden variables once and for all.

Yet a number of loopholes remained—ways that hidden variables might exert some influence, though the purported mechanisms became increasingly contrived as years and experimental finesse advanced. One was the detection loophole. Reliably catching a single photon, for example, is tricky; lots of them go amiss in a given experiment. But if an experiment does not capture all of its participants, the loophole idea goes, perhaps hidden variables convey information through the missing ones. Another was the communication loophole. If the two measurements happen near enough to one another, some invisible hidden-variable signal might be passing between them (as long as that signal does not go faster than light).

Plenty of experiments have closed one or the other of these loopholes, for example by detecting particles that are more reliably caught than photons, or by sending photons so far apart that no slower-than-light signal could flit between them in time to have an effect. By now, most physicists reckon the hidden-variable idea is flawed. But no test had closed both loopholes simultaneously—until this week, that is.

Ronald Hanson of the University of Delft and his colleagues, writing in Nature, describe an experiment that starts with two electrons in laboratories separated by more than a kilometre. Each emits a photon that travels down a fibre to a third lab, where the two photons are entangled. That, in turn, entangles the electrons that generated the photons. The consequence is easily measured particles (the electrons) separated by a distance that precludes any shifty hidden-variable signalling.

Over 18 days, the team measured how correlated the electron measurements were. Perhaps expectedly, yet also oddly, they were far more so than chance would allow—proving quantum mechanics is as weird as Einstein had feared.

Though this experiment marks an end to hidden variables, Dr Hanson says it is also a beginning: that of unassailably secure, quantum-enabled cryptography. It was shown in 1991 that the very Bell tests used to probe hidden variables could also serve as a check on quantum cryptography. A loophole-free Bell test, then, could unfailingly reveal if a hacker had interfered with the fundamentally random, quantum business of generating a cryptographic key. So-called device-independent quantum ciphers would, Dr Hanson says, be secure from hackers “even if you don’t trust your own equipment—even if it’s been given to you by the NSA”.

There remains, alas, one hitch that could explain all these counterintuitive findings. Just maybe, every single event that will ever be, from experimenters’ choices of the means of measurement to the choice of article you will read next, were all predetermined at the universe’s birth, and all these experiments are playing out just as predetermined. That, however, is one for the metaphysicists.

See also here.

Ferguson, USA and Albert Einstein

This video from the USA says about itself:

Arise America: Einstein’s Stance on Racism

16 December 2014

Nobel Prize-winning physicist Albert Einstein is well known for his contributions to science, but not so publicized are his efforts to speak out against racism in the United States, as evidenced by papers written by Einstein recently made public. Ze’ev Rosenkranz, assistant director and senior editor of the Einstein Papers Project at the California Institute of Technology, joins Arise America to discuss this little known side of Einstein.

From the classical music blog Slipped Disc in Britain, by Norman Lebrecht:

Albert Einstein’s message to Ferguson, Missouri

August 11, 2015

The immortal physicist, moral philosopher and fervent violinist was so disturbed by the state of racial relations in his American homeland that in 1946 he published an agonised denunciation of ‘this deeply entrenched evil.’

The [American] sense of equality and human dignity is mainly limited to men of white skins. Even among these there are prejudices of which I as a Jew am clearly conscious; but they are unimportant in comparison with the attitude of the “Whites” toward their fellow-citizens of darker complexion, particularly toward Negroes. The more I feel an American, the more this situation pains me. I can escape the feeling of complicity in it only by speaking out.

His message went, and still goes, unheard.

What, however, can the man of good will do to combat this deeply rooted prejudice? He must have the courage to set an example by word and deed, and must watch lest his children become influenced by this racial bias.

Read the full article here.

Armed white men from Oath Keepers arrive in Ferguson, stoking tension: here.

Black Lives Matter protesters commemorate Michael Brown in New York City: here.

15-year-old schoolboy discovers new planet

This video from England is called Schoolboy finds new planet… while on work experience: Teenager spotted Jupiter-sized globe 1,000 light-years away.

From daily The Guardian in Britain:

Schoolboy on work experience discovers planet

Newcastle-under-Lyme pupil Tom Wagg spotted dip in light which revealed existence of a planet while on placement at Keele University two years ago

Jessica Elgot

Thursday 11 June 2015 17.12 BST

A schoolboy doing work experience with an astrophysics professor has discovered a new planet 1,000 light years from Earth.

Newcastle-under-Lyme school pupil Tom Wagg was 15 when he went for his work placement at Keele University, where he spotted a minuscule dip in the light from a faraway star that he knew could be caused by a planet passing in front of it.

Wagg kept in touch with the university’s Prof Coel Hellier while the potential planet was analysed by scientists from the universities of Geneva and Liege.

Two years later, the 17-year-old got the call confirming his discovery was indeed a new planet – a large gas planet with similar properties to Jupiter in the southern constellation of Hydra. Its characteristics mean it is very unlikely to support any form of life.

Although credited with the discovery, Wagg has not been allowed to name the planet he discovered, which will be decided by competition entries co-ordinated by the International Astronomical Union.

The new planet has been temporarily termed WASP-142b, because it is 142nd discovery by the Wide Angle Search for Planets (WASP) project, whose data Wagg had been searching through.

“I had no idea what kind of work I’d be doing on the placement, let alone what I’d discover,” Wagg said. “When I realised what it could be I was astonished, it’s been a real boost to me to carry on with science.”

Hellier said he had been impressed by his “bright” work experience pupil and said that good observation skills had been key to spotting the small dip which revealed the planet’s existence.

“Humans are far better at doing this than a computer algorithm,” he said. “It’s not that rare to discover a planet – we’ve probably discovered 1000 in the last 10 years – but I’m not aware of any others being discovered on work experience.”

Wagg admitted he was a little sad he would not necessarily have the planet named after him. “In a way I am sad, but I definitely didn’t expect it to be, I understand why it’s a competition,” he said. “I do hope it encourages other people to know that anyone can find a planet, if they get access to the data and they know what to look for.”

Wagg, who is studying physics, maths, further maths and latin for A-level next year, plans to continue with physics at university. But he has not quite decided whether he will be pursing planets.

“I’m torn between particle physics and astrophysics, which seem on the face of it pretty different because one deals with the smallest things in the universe, and the other with the biggest,” the young scientist said.

“But actually, there are real similarities because if you study one, you can understand both, because the laws of physics apply to everything. That’s the beauty of science.”

Albert Einstein visual arts exhibition

This is a video about a 2008 exhibition in the Lakenhal and Boerhaave museums in Leiden, the Netherlands about the Kamerlingh Onnes family. Some people in that family were physicists (with a special interest in cold temperatures), some were visual artists.

Albert Einstein, 1920 drawing by Harm Kamerlingh Onnes

Translated from NOS TV in the Netherlands:

Albert Einstein in Leiden museum

Today, 19:23

Leiden artist Harm Kamerlingh Onnes (1893-1985) has portrayed twenty renowned scholars in the years when they visited his uncle, the Nobel Prize winner Heike Kamerlingh Onnes. Among them was Albert Einstein. Boerhaave Museum in Leiden has now acquired these sketches and drawings. The majority was not known until now.

Harm was in 1920 and 1921 also regularly found in the laboratory of his uncle Heike, who was doing research on absolute zero temperature (-273 ° C). He made portraits and recorded how his uncle and staff were busy with their experiments.


The physicist Heike Kamerlingh Onnes received the Nobel Prize in 1913. In his Leiden home, Huize ter Wetering at the Galgewater, at that time many foreign guests visited.

The house was a meeting place for scholars and artists, including Marie Curie, Albert Einstein and Niels Bohr. They met there Dutch artists like Jan Toorop, Albert Verwey and Carel Lion Cachet.


The collection of drawings is from the estate of a son of Harm Kamerlingh Onnes. A selection will be on show from 21 February until 26 April at the Museum Boerhaave in Leiden. About the family of scientists and artists an accompanying booklet has been published with the title Koude, kunst, Kamerlingh Onnes [Cold, art, Kamerlingh Onnes], written by Dirk van Delft.

See also here. And here.