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Hundred years old, still a work of genius: Einstein’s Theory of General Relativity

Hundred years old, still a work of genius: Einstein’s Theory of General Relativity

On the occasion of the first public release of Albert Einstein’s Theory of General Relativity, J. BROOKS SPECTOR contemplates its importance and place in history.

There was a young lady named Bright,
Whose speed was much faster than light;
She set out one day in a relative way,
And returned home the previous night!

Limerick, author unknown, in honour of relativity

Back in the early 1950s, together with the author’s parents, the writer once went to Princeton, New Jersey for the day to have supper with a relative who was living in Princeton at the time. She was an artist, a lesser-known devotee of the school of abstract expressionists then thriving in nearby New York City. (In actual fact, while she made her living typing scientific and other academic manuscripts for the authors too busy – or unfamiliar with a keyboard – to produce clean copy for submission to academic journals, she lived the artistic life in that university town).

A necessary part of any trip to Princeton back in those days, of course, was to drive slowly past 112 Mercer Street, Albert Einstein’s modest home where he lived from the time he arrived in the United States to join Princeton’s Institute for Advanced Study (just ahead of the Nazi Holocaust) until his death in 1955. The hope of any such a drive-by was to catch at least a glimpse of the man who had changed the world – whatever it was that he had really discovered. That bit was a little unclear to most people, but the fact he was the ‘father’ of the atomic bomb, well everyone knew that. On that particular day we did not actually see him, but we did see the house of the man who had changed everything under, around, and inside the Sun. Especially, inside the Sun.

Photo: Albert Einstein’s house in 112 Mercer Street (Wikimedia Commons)

By 25 November 1915, Einstein had already become a famous theoretical physicist from his work on the photoelectric effect and what he had called special relativity. He would finally gain a Nobel Prize in 1921 for this earlier work on the Law for Photoelectric Effect, since his work on relativity remained, years later, still too controversial for too many in physics to be blessed with a Nobel award. In the years following this research and his increasing fame from four famous papers published in 1905, he moved from an obscure position as a patent examiner for the Swiss patent office, and on to a much more visible and honoured position in the Kaiser Wilhelm Institute in Berlin, with the illustrious Max Planck as mentor. It was from that perch that he launched his most extraordinary scientific thunderbolt.

While working at the patent office, Einstein had the time and solitude to advance ideas that had first taken hold during his studies at Polytechnic, and then brought together his thoughts on what would become known as the principle of relativity. In that year of 1905 — seen by many as a “miracle year” for the theorist – he published four separate papers in the Annalen der Physik, one of the best-known, most prestigious physics journals of that period. These four papers focused, respectively, on the photoelectric effect, Brownian motion, the special theory of relativity (the most widely circulated of the write-ups), and the matter/energy relationship. The final one took physics in an extraordinary, even revolutionary, new direction. In that fourth paper, Einstein used his famous equation E = mc2, suggesting the astounding idea that tiny particles of matter could be converted into huge amounts of energy, thereby foreshadowing the development of atomic power, and overthrowing a fundamental distinction between matter and energy. (According to some science historians, French physicist Henri Poincaré came up with his E/c2=m equation some three months before Einstein – Ed)

Then, if all this were not sufficient to carve out a place in physics’ pantheon, in 1915 Einstein came along with his work on general relativity as well. Of this, David Kaiser, professor of the history of science, technology and society at the Massachusetts Institute of Technology (MIT), said, “Einstein changed the way we think about the most basic things, which are space and time. And that opened our eyes to the universe, and how the most interesting things in it work, like black holes.” And this new understanding firmly launched the 20th century’s understanding of the space-time continuum – even as it added even more problems for reconciling the then newly evolving world of quantum physics with Einstein’s relativistic one.

In describing the impact of Einstein’s work, The Economist has written, “General relativity was presented to the Prussian Academy of Sciences over the course of four lectures in November 1915; it was published on December 2nd that year. The theory explained, to begin with, remarkably little, and unlike quantum theory, the only comparable revolution in 20th-century physics, it offered no insights into the issues that physicists of the time cared about most. Yet, it was quickly and widely accepted, not least thanks to the sheer beauty of its mathematical expression; a hundred years on, no discussion of the role of aesthetics in scientific theory seems complete without its inclusion.

Today its appeal goes beyond its elegance. It provides a theoretical underpinning to the wonders of modern cosmology, from black holes to the Big Bang itself. Its equations have recently turned out to be useful in describing the physics of earthly stuff too. And it may still have secrets to give up: enormous experiments are under way to see how the theory holds in the most extreme physical environments that the universe has to offer.

The theory built on the insights of Einstein’s first theory of relativity, the ‘special theory’, one of a trio of breakthroughs that made his reputation in 1905. That theory dramatically abandoned the time-honoured description of the world in terms of absolute space and time in favour of a four dimensional space-time (three spatial dimensions, one temporal one). In this new space-time observers moving at different speeds got different answers when measuring lengths and durations; for example, a clock moving quickly with respect to a stationary observer would tell the time more slowly than one sitting still. The only thing that remained fixed was the speed of light, c, which all observers had to agree on (and which also got a starring role in the signature equation with which the theory related matter to energy, E=mc2).”

Following his initial presentation of his paper in Berlin, this work then appeared in the March 1916 issue of Annalen der Physik. It was a major departure from the precise, mechanistic, clockwork universe of Isaac Newton’s, the one that had held sway for nearly 250 years and that had let to so many discoveries in astronomy. In general relativity, Einstein had argued that “space and time are not fixed, which was what others had thought, but are flexible, dynamic phenomena like other processes of the universe. So space bends and time warps, and it was a whole new way at looking at gravity,” explained Michael Turner, director of the Kavli Institute for Cosmological Physics.

This new exploration of Einstein’s ideas of general relativity had built upon the physicist’s earlier theory of special relativity in which he had left out gravity as a consideration, but where he had first advanced a consideration of the complex relationship between space and time. Now, in his theory of general relativity, with the addition of gravity as a key factor, Einstein explained that time would move more slowly in proximity to a powerful gravitational field, such as that of a planet in the void of space. This idea was eventually confirmed by comparisons of paired atomic clocks, in which one remained on Earth and the other was carried aloft in a high-altitude airplane where it showed a slight but distinct delay. In a thoroughly practical application of general relativity’s principles, today’s global positioning systems make use of this phenomenon, where satellites have clocks precisely adjusted to account for these small, but vital time differences. General relativity has also made the full panoply of modern science fiction possible as well, what with all those intricate ways around the ultimate limit by virtue of the speed of light, and the presumably inherent contradictions of time travel (and all those ingenious ways around them that have been the basis for so many great films and stories).

Another key element of general relativity was the point that light is also warped by powerful gravitational fields, something that British astronomer Arthur Eddington managed to confirm in 1919, with his observations of the slight, but real deflections of starlight by the Sun through his meticulous measurements from his temporary observing post on the island of Príncipe (of São Tomé and Príncipe). Eddington went there to observe the 29 May 1919 solar eclipse – and this became a crucial proof of general relativity’s predictions.

Watch: Einstein and Eddington, HBO trailer

Einstein’s general relativity led to the realisation that at the end of their lives, stars would collapse under their own gravity. Their external gaseous envelope would explode as a supernova, while their core would form a very dense object such as a neutron star, or a rapidly spinning pulsar. They could also transform into a black hole, which had such a huge gravitational field that space and time themselves could not escape. Per Einstein’s calculations, such celestial bodies, given their masses, would provoke waves in space-time much like a tossed stone caused ripples in water.

MIT’s Kaiser explains that gravitational waves are what astronomers are now hoping, finally, to observe and measure first-hand. Such observations, Kaiser says, would “confirm one of the last great, but as yet untested predictions from Einstein, equation that space and time are not really dynamical, but they can ripple, like the surface of a pond.” The Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States and VIRGO, a gravitational wave detector in Italy, are now poised to do this precise heavy lifting.

What Einstein’s general theory of relativity also did, however, was set up a vast challenge for physics – the need to reconcile, somehow, relativity with quantum physics. The latter, in contrast to Einstein’s approach effectively describes phenomena on the atomic and sub-atomic levels and has, as a consequence, led to applications crucial to the contemporary world such as transistors and computers.

As things stand now, one approach for this bringing of things together has been something really abstruse and hard to visualise, called string theory. This approach asserts that fundamental particles are not so fundamental, and aren’t even particles, but rather a kind of elastic “string” that vibrate at different frequencies and different dimensions to generate their physical presence. String theory, in turn, leads to the possibilities of numerous other extra dimensions beyond the usual three plus time.

A further implication is that the describable dimensions of time and space could also be different than even what relativity offers. Einstein, of course, had recognised this problem early on, and he had laboured for years – especially during his years in Princeton – on what he called his general field theory, arguing the case via his famous aphorism about god not playing with the universe as if he had celestial dice. This preoccupation of Einstein’s, to find a way out of the contradictions flowing from the clash of relativity and quantum physics, meant that for many physicists, Einstein was viewed as increasingly out of the mainstream of physics thinking and research.

Watch: Albert Einstein documentary

The paradox was that his personal star continued to rise in the popular imagination, despite the presumed fading of his research star. So much so was this true that the very image of Einstein – that famous head of unkempt, wild hair; a demeanour of absent-mindedness to the point of forgetting to wear socks; an uncaring dress style that eagerly embraced baggy pants and sweatshirts – all of these became a stand-in for the very image of any scientist, and most especially of a great one. It is a strange mark of his public fame and a sense of his presumed scientific uniqueness that, quite literally, the location and ownership of his actual, physical brain swimming in a glass jar became a kind of whodunit mystery – as some chose to study its structure for insights into Einstein’s special genius and how different his brain tissue was thought to be from that of the rest of humanity’s.

As Scientific American noted, “On April 18, 1955, Albert Einstein died at Princeton Hospital of a ruptured aortic aneurysm. Within hours the pathologist on call, Thomas Harvey, acting on his own initiative, removed the famed physicist’s brain without the family’s permission. He then preserved the organ, counter to Einstein’s stated wish to be cremated. Harvey managed to secure a retroactive blessing from Einstein’s son Hans Albert, with the stipulation that the brain would be used only for scientific purposes. But Harvey himself lacked the expertise needed to analyse the organ, so he began to seek out specialists to help him. It would take him 30 years to find one. The quest changed the course of Harvey’s life and consigned his precious specimen to a fate that is at once strange, sad and fraught with ethical complications.”

Four years after Einstein had come to the US and over twenty since his theoretical work had been at the very forefront of physics, a delegation of some of the brightest minds in physics – many of them also exiles from Nazi Germany’s shadow – had come to Einstein in 1939 in order to ask him to sign a letter addressed to President Franklin Roosevelt that sketched out the possibilities of a powerful weapon that converted uranium into nuclear energy, based on Einstein’s famous equation, E = mc2. The letter then urged the president to initiate a crash project to develop just such a weapon – before the Germans got there first.

In spite of their respective reputations in their collective field, none of the delegation – nor their colleagues and friends – thought anyone’s name besides Einstein’s would be sufficiently in the public mind (or that of the president) to get the chief executive’s attention on the problem – and they were probably right. He did, and the Manhattan Project and the nuclear age evolved from that communication, even though Einstein, a committed pacifist, later said he deeply regretted the eventual outcome of his intervention, even as he detested the Nazi menace.

In an appreciation of Einstein’s genius, The Economist wrote on this anniversary, “Most scientific findings are sedimentary, slowly building upon the edifice of understanding. Rare is the idea that marks a fundamental change to a system of thought, forcing the rest of science to bend to its own vision. However, on November 25th 1915, Albert Einstein published a theory that did just that. The ten equations of his general theory of relativity set out a new concept of gravity—not as its own, independent force, but as the warping of the fabric of space and time in the presence of mass.

In the intervening century, Einstein himself has become a byword for cartoonish genius. His theory, however, is cherished less than it should be. That is partly because of its complexity; general relativity survived some trying experimental tests early on, but few scientists focused on it—in large part because its equations were so damnably hard to solve. And when the theory did take firm hold, it swiftly became so ubiquitous in describing astronomical goings-on that it began to be taken for granted. As a result, relativity’s revelations are less widely appreciated than the ideas of Charles Darwin, or even Einstein’s predecessor in gravitational thought, Isaac Newton.

Yet the world has much to thank Einstein for. Because of him, scientists think of space as, well, relative: what you measure depends on your vantage point, and on what mass is around you. An understanding of gravity’s most subtle effects informs both the exalted and the everyday. Relativity permitted the New Horizons mission this year to steer a space probe through a 150km-wide ‘keyhole’ near Pluto, nearly 5 billion kilometres away, after a nine-and-a-half-year journey. A more quotidian example of the extraordinary precision of relativity comes from satellite-navigation systems. Einstein’s theory shows that satellites experience an ever-so-slightly different stretching of space-time in orbit than people do on the surface of the Earth—so the positional data streamed to smartphone users, and the time-stamps used for transactions in industries from banking to energy, must take in relativistic adjustments.

The theory has yielded odd surprises. It predicted, and then helped explain, the black holes that have captured public imagination. Efforts to join relativity with quantum mechanics, in a field called string theory, are shedding light on science that is wholly unconnected to the heavens, including materials that conduct electricity without resistance and new kinds of information processing. This concordance across phenomena that seem so disparate is a tantalising hint that scientists may yet come up with a grand theory that incorporates all the physical forces.

Relativity’s most overlooked triumph, though, has been to reframe the sorts of questions that stargazers ask. After the invention of telescopes in the early 17th century, astronomy concerned itself chiefly with discrete objects in the cosmos—peering at the planets and working out how they move, mapping how stars are distributed in the sky, and so on. General relativity got its start there, too, resolving a long-standing mystery of Mercury’s orbit. But the implications of stretchy space-time quickly raised bigger questions: by the 1970s, relativity had become integral to describing the Big Bang. Not since Johannes Kepler’s ‘Mysterium Cosmographicum’, a 16th-century attempt to reveal the structure of the cosmos, were thinkers so inspired to consider the universe as a whole: its organising principles, its ultimate origins and what makes it tick.”

Einstein came up with his ideas for his new relativistic universe through a careful series of what he called “thought experiments” – or precisely set out imaginary assessments of some finely scripted, highly stylised circumstances. One such thought experiment back in 1907 resulted in what he later called his “happiest thought”. This was the understanding that someone falling off a roof would not feel his own weight since an object in free-fall, at least until it hit the ground, would not experience gravity. However, the curved trajectories gravity makes seemed to imply some sort of pushing or pulling. But, if golf balls, planets, and the man falling from the roof felt neither push nor pull, why do they not fall in straight lines? The core of general relativity was in the author’s realisation they did. Objects falling free, like rays of light, follow straight lines through space-time but, crucially, it was space-time itself that was curved. Contrary to our quotidian, everyday sense of things, gravity is not a force. Rather, it is a distortion of space-time. Or as physicist, John Wheeler, said some years after Einstein’s paper, “Space-time tells matter how to move; matter tells space-time how to curve.”

It has been frequently noted that within just one half century, from 1859 to 1900, three major thinkers – Charles Darwin (along with Alfred Wallace), Karl Marx and Sigmund Freud – thoroughly recast humankind’s relationship with the universe. Darwin’s theory of evolution in On The Origin of Species (1859) severed the hallowed link between humans and a sense of their unique, divine creation, separate from all other living creatures. Freud’s The Meaning of Dreams in 1900 had opened up a window into the subconscious mind, pointing to an entire inner landscape that created or shaped human actions, interactions, emotions and thoughts. And Marx, in Das Kapital, in 1867, had pointed to an entire subterranean landscape of mechanisms – unknown to most people – that governed and structured the very nature of human undertakings and society. But with due recognition of the claims for that august list, it would seem churlish in the extreme not to include Einstein for his influence in overthrowing the perfect clockwork celestial machinery and, in its place, offering a new vision of an extraordinary universe where matter can become energy, where gravity bends light, and where stars quite literally disappear into themselves from the force of their own gravitation. And it was at the end of 1915, when he fully drew those lines that connected pretty much everything to pretty much everything else. DM

For more, read:

  • After 100 years, Einstein’s theory stands test of time at Physics.org;
  • Celebrating Einstein at Celebrating Einstein;
  • The general theory of relativity – Thanks, Albert at the Economist;
  • Celebrating Einstein events to mark 100th anniversary of the general theory of relativity at MIT News;
  • Einstein (special anniversary issue) of Scientific American;
  • The most beautiful theory – A century ago Albert Einstein changed the way humans saw the universe. His work is still offering new insights today at the Economist;
  • Albert Einstein Biography at Biography.com;
  • The strange afterlife of Einstein’s brain at the BBC;
  • The Quest for Genius in Einstein’s Brain in Scientific American;
  • Einstein and Eddington (an HBO made for TV movie);

Photo: Albert Einstein.

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