 World Year of PHYSICS 2005
Einstein in the 21st Century
Ahead of his time
What would Albert Einstein think if he were alive today? As someone who disliked the limelight, he would probably be embarrassed by the celebrations that are planned as part of the International Year of Physics to mark the centenary of his remarkable achievements in 1905. As a theorist who was interested in experiments, in his early career at least, he would be pleased to know that a small band of 21stcentury physicists are still trying to find flaws in the special theory of relativity,while others are busy checking out the predictions of the general theory. And having spent the final years of his life trying to unify general relativity with electromagnetism, without success, he could be forgiven for thinking that criticisms of his relative non-productivity in those years were somewhat unfair. No-one else has succeeded where he failed. It is impossible to overstate the importance of what Einstein did in 1905. His work on Brownian motion provided the theoretical framework for experiments to prove that atoms were real.Hard as it might be to believe now, at the time the majority of physicists did not believe in atoms. The special theory of relativity completely changed our notions of space and time, while E=mc2 led to the remarkable  conclusion that mass and energy are one and the same. And his work on the photoelectric effect was the start of a love–hate relationship with quantum mechanics that still fascinates physicists today. And 1905 was just the beginning.The general theory of relativity – his truly outstanding achievement – followed 10 years later, with its predictions for the bending of light by mass being confirmed a few years after that during the solar eclipse of 1919. But even then Einstein did not abandon his interest in atoms, photons and quantum mechanics. The Einstein A and B coefficients for spontaneous and stimulated emission – without which we would not have lasers – made their debut in 1916, and the prediction of Bose–Einstein condensation – one of the hottest topics in experimental physics for the past decade – followed in the 1920s. This special issue of Physics World covers all this and more. On page 19 Mark Haw describes Einstein's theory of Brownian motion as a "slower, subtler revolution" than his work on relativity or quantum mechanics, but just as influential nonetheless. On page 27 Clifford M Will provides an update on the renaissance in experimental gravitational physics and reports how the general theory has so far survived all scrutiny, although it has not yet been tested in the strong-field limit. Most exciting, however, is the fact that theories that seek to unify gravity with the three other fundamental forces of nature predict departures from general relativity that will soon be within experimental reach. Of course, the outstanding prediction of general relativity that has yet to be confirmed is the existence of gravitational waves: on page 37 Jim Hough and Sheila Rowan describe the almost superhuman efforts that are being made to find out if Einstein was right on this occasion. And as if to show that the great physicist could also be wrong, on page 47 Harald Weinfurter reports on the state of the art in quantum entanglement – the phenomenon that Einstein once dismissed as "spooky action at a distance". Other topics covered range from Einstein's love of music to the way his image is protected by the Hebrew University of Jerusalem and a Hollywood agent. These articles are obviously preaching to the physics converted, but the organizers of the International Year of Physics – also known as World Year of Physics and Einstein Year – have much loftier ambitions. Through a world-wide programme of events, demonstrations and other activities they hope to inspire the next generation of physics students. Einstein would have approved.
In his own words
The supreme task of the physicist is to arrive at those universal elementary laws from which the cosmos can be built up by pure deduction.
A brief history of Albert Einstein
Born in Germany in 1879,
Einstein became the most famous physicist the world has ever seen
The early years
1879 Born 14 March at Bahnhofstra?e 135, Ulm, Germany
1880 Einstein's family moves to Munich, where his father founds a firm manufacturing electrical equipment
1888 Enters Luitpold Gymnasium in Munich
1894 Family moves to Italy; Albert stays in Munich, but gets depressed without his family and does not complete his schooling
1895 Albert joins family in Italy; fails entrance exam for the ETH Zurich; moves to Aarau, Switzerland
1896 Obtains diploma from cantonal school in Aarau,which allows him to enrol for the ETH Zurich; relinquishes German citizenship
1900 Receives diploma from Zurich, scoring 5 (out of a possible 6) for theoretical physics, experimental physics and astronomy, and 5.5 for theory of functions
Life after college
1901 Becomes a Swiss citizen, but declared unfit for military service due to flat feet and varicose veins; gets a few temporary schoolteaching jobs
1902 Appointed technical expert (third class) at the patent office in Bern with a salary of SwFr 3500; fiancée Mileva Maric′ – a fellow student from Zurich – gives birth to illegitimate daughter Lieserl
1903 Marries Mileva on 6 January
1904 First son, Hans Albert, born 14 May
1905 Einstein's annus mirabilis: submits PhD thesis on molecular dimensions to University of Zurich, as well as two papers on special relativity, one on quantum theory and another on Brownian motion to Annalen der Physik
1906 Promoted to technical expert (second class), salary raised to SwFr 4500
1907 Einstein has "the happiest thought of my life" – that a gravitational field is equivalent to acceleration
Turning professional
1909 Resigns from patent office and starts work as associate professor at University of Zurich on 15 October
1910 Second son, Eduard, born 28 July
1911 Appointed full professor at the German University of Prague, where he works out that the bending of light should be detectable during a solar eclipse; attends first Solvay Congress in Brussels
1912 Returns to Switzerland as professor at the ETH Zurich
1914 Becomes professor at the University of Berlin; moves into a bachelor apartment after separating from Mileva,who returns with sons to Zurich
1915 Completes theory of general relativity; co-signs an anti-war manifesto urging people to join a "League of Europeans"
1916 Writes 10 papers, including first paper on gravitational waves, and one on the spontaneous and stimulated emission of light; publishes The Origins of the General Theory of Relativity; succeeds Max Planck as president of the German Physical Society
1917 Becomes founding director of Kaiser-Wilhelm Institut, Berlin; writes paper on the twin paradox; introduces the cosmological constant; overwork triggers liver problem, stomach ulcer and jaundice that together confine him to bed for several months – looked after by his cousin Elsa Einstein L?wenthal
Public fame
1919 Marries Elsa on 2 June; divorce settlement with Mileva stipulates that she would receive any Nobel-prize money from Einstein; eclipse watchers confirm his prediction that the Sun bends distant starlight, leading to headlines around the world
1920 Toys with leaving Germany after attacks on relativity by anti-semites
1921 Visits the US for first time
1922 Awarded 1921 Nobel Prize for Physics for his "services to theoretical physics and in particular for his discovery of the law of the photoelectric effect" – prize money of about $32 000 given to Mileva; completes first paper on unified field theory
1924 Einstein Institute founded in Potsdam; predicts Bose–Einstein condensation
1927 Attends fifth Solvay Congress in Brussels and starts debate on quantum theory with Niels Bohr
Life in the US
1933 Leaves Germany after Nazis take power and joins the Institute for Advanced Study in Princeton – a "quaint and ceremonious village of demigods on stilts"; rejects cosmological constant
1935 Publishes strident attack on quantum theory with Boris Podolsky and Nathan Rosen
1936 Elsa dies
1939 Signs letter to President Roosevelt warning of dangers of atomic bomb
1940 Becomes US citizen,while retaining Swiss citizenship
1944 Retires from Princeton, aged 65; writes out by hand his original 1905 paper on special relativity for auction, raising $6m for US war effort
1946 Becomes chairman of the Emergency Committee of Atomic Scientists; calls for world government to be formed
1952 Turns down an offer to be President of Israel
1955 Signs "Russell–Einstein manifesto" on 11 April urging nations to renounce nuclear weapons; dies in Princeton at 1.10 a.m. on 18April from ruptured abdominal aorta; brain removed by pathologist Thomas Harvey; body cremated at the Ewing Crematorium
Five papers that shook the world
In 1905 an anonymous patent clerk in Bern rewrote the laws of physics in his spare time.
Matthew Chalmers describes Einstein's miraculous year
Most physicists would be happy to make one discovery that is important enough to be taught to future generations of physics students. Only a very small number manage this in their lifetime, and even fewer make two appearances in the textbooks. But Einstein was different.  Genius at work – Einstein was just 26 when he made three ground-breaking contributions to physics in a single year. Here he is pictured at the Swiss patent office in early 1906. | In little more than eight months in 1905 he completed five papers that would change the world for ever. Spanning three quite distinct topics – relativity, the photoelectric effect and Brownian motion – Einstein overturned our view of space and time, showed that it is insufficient to describe light purely as a wave, and laid the foundations for the discovery of atoms. Perhaps even more remarkably, Einstein's 1905 papers were based neither on hard experimental evidence nor sophisticated mathematics. Instead, he presented elegant arguments and conclusions based on physical intuition. "Einstein's work stands out not because it was difficult but because nobody at that time had been thinking the way he did," says Gerard 't Hooft of the University of Utrecht,who shared the 1999 Nobel Prize for Physics for his work in quantum theory. "Dirac, Fermi, Feynman and others also made multiple contributions to physics, but Einstein made the world realize, for the first time, that pure thought can change our understanding of nature." And just in case the enormity of Einstein's achievement is in any doubt,we have to remember that he did all of this in his "spare time".
Statistical revelations
In 1905 Einstein was married with a oneyear- old son and working as a patent examiner in Bern in Switzerland. His passion was physics, but he had been unable to find an academic position after graduating from the ETH in Zurich in 1900. Nevertheless, he had managed to publish five papers in the leading German journal Annalen der Physik between 1900 and 1904, and had also submitted an unsolicited thesis on molecular forces to the University of Zurich, which was rejected. Most of these early papers were concerned with the reality of atoms and molecules, something that was far from certain at the time. But on 17 March in 1905 – three days after his 26th birthday – Einstein submitted a paper titled "A heuristic point of view concerning the production and transformation of light" to Annalen der Physik. Einstein suggested that, from a thermodynamic perspective, light can be described as if it consists of independent quanta of energy (Ann. Phys., Lpz 17 132–148). This hypothesis, which had been tentatively proposed by Max Planck a few years earlier, directly challenged the deeply ingrained wave picture of light.However, Einstein was able to use the idea to explain certain puzzles about the way that light or other electromagnetic radiation ejected electrons from a metal via the photoelectric effect. Maxwell's electrodynamics could not, for example, explain why the energy of the ejected photoelectrons depended only on the frequency of the incident light and not on the intensity.However, this phenomenon was easy to understand if light of a certain frequency actually consisted of discrete packets or photons all with the same energy. Einstein would go on to receive the 1921 Nobel Prize for Physics for this work, although the official citation stated that the prize was also awarded "for his services to theoretical physics". "The arguments Einstein used in the photoelectric and subsequent radiation theory are staggering in their boldness and beauty," says Frank Wilczek, a theorist at the Massachusetts Institute of Technology who shared the 2004 Nobel Prize for Physics. "He put forward revolutionary ideas that both inspired decisive experimental work and helped launch quantum theory." Although not fully appreciated at the time, Einstein's work on the quantum nature of light was the first step towards establishing the wave– particle duality of quantum particles. On 30 April, one month before his paper on the photoelectric effect appeared in print, Einstein completed his second 1905 paper, in which he showed how to calculate Avogadro's number and the size of molecules by studying their motion in a solution. This article was accepted as a doctoral thesis by the University of Zurich in July, and published in a slightly altered form in Annalen der Physik in January 1906. Despite often being obscured by the fame of his papers on special relativity and the photoelectric effect, Einstein's thesis on molecular dimensions became one of his most quoted works. Indeed, it was his preoccupation with statistical mechanics that formed the basis of several of his breakthroughs, including the idea that light was quantized. After finishing a doctoral thesis, most physicists would be either celebrating or sleeping. But just 11 days later Einstein sent another paper to Annalen der Physik, this time on the subject of Brownian motion. In this paper, "On the movement of small particles suspended in stationary liquids required by the molecular-kinetic theory of heat", Einstein combined kinetic theory and classical hydrodynamics to derive an equation that showed that the displacement of Brownian particles varies as the square root of time (Ann. Phys., Lpz 17 549–560). This was confirmed experimentally by Jean Perrin three years later, proving once and for all that atoms do exist (see "Einstein's random walk" on page 19). In fact, Einstein extended his theory of Brownian motion in an additional paper that he sent to the journal on 19 December, although this was not published until February 1906.
---------------- "The arguments Einstein used were staggering in their boldness and beauty."
A special discovery
Shortly after finishing his paper on Brownian motion Einstein had an idea about synchronizing clocks that were spatially separated. This led him to write a paper that landed on the desks of Annalen der Physik on 30 June, and would go on to completely overhaul our understanding of space and time. Some 30 pages long and containing no references, his fourth 1905 paper was titled “On the electrodynamics of moving bodies” (Ann. Phys., Lpz 17 891–921). In the 200 or so years before 1905, physics had been built on Newton's laws of motion, which were known to hold equally well in stationary reference frames and in frames moving at a constant velocity in a straight line. Provided the correct “Galilean” rules were applied, one could therefore transform the laws of physics so that they did not depend on the frame of reference. However, the theory of electrodynamics developed by Maxwell in the late 19th century posed a fundamental problem to this “principle of relativity” because it suggested that electromagnetic waves always travel at the same speed. Either electrodynamics was wrong or there had to be some kind of stationary “ether” through which the waves could propagate. Alternatively, Newton was wrong. True to style, Einstein swept away the concept of the ether (which, in any case, had not been detected experimentally) in one audacious step. He postulated that no matter how fast you are moving, light will always appear to travel at the same velocity: the speed of light is a fundamental constant of nature that cannot be exceeded. Combined with the requirement that the laws of physics are the identical in all “inertial” (i.e. non-accelerating) frames, Einstein built a completely new theory of motion that revealed Newtonian mechanics to be an approximation that only holds at low, everyday speeds. The theory later became known as the special theory of relativity – special because it applies only to non-accelerating frames – and led to the realization that space and time are intimately linked to one another. In order that the two postulates of special relativity are respected, strange things have to happen to space and time, which, unbeknown to Einstein, had been predicted by Lorentz and others the previous year. For instance, the length of an object becomes shorter when it travels at a constant velocity, and a moving clock runs slower than a stationary clock. Effects like these have been verified in countless experiments over the last 100 years, but in 1905 the most famous prediction of Einstein's theory was still to come. After a short family holiday in Serbia, Einstein submitted his fifth and final paper of 1905 on 27 September. Just three pages long and titled “Does the inertia of a body depend on its energy content?”, this paper presented an “afterthought” on the consequences of special relativity, which culminated in a simple equation that is now known as E=mc2 (Ann. Phys., Lpz 18 639–641). This equation, which was to become the most famous in all of science, was the icing on the cake. “The special theory of relativity, culminating in the prediction that mass and energy can be converted into one another, is one of the greatest achievements in physics – or anything else for that matter,” says Wilczek. “Einstein's work on Brownian motion would have merited a sound Nobel prize, the photoelectric effect a strong Nobel prize, but special relativity and E=mc2 were worth a super-strong Nobel prize.” However, while not doubting the scale of Einstein's achievements,many physicists also think that his 1905 discoveries would have eventually been made by others. “If Einstein had not lived, people would have stumbled on for a number of years, maybe a decade or so, before getting a clear conception of special relativity,” says Ed Witten of the Institute for Advanced Study in Princeton. 't Hooft agrees. “The more natural course of events would have been that Einstein's 1905 discoveries were made by different people, not by one and the same person,” he says.However, most think that it would have taken much longer – perhaps a few decades – for Einstein's general theory of relativity to emerge. Indeed,Wilczek points out that one consequence of general relativity being so far ahead of its time was that the subject languished for many years afterwards.
The aftermath
By the end of 1905 Einstein was starting to make a name for himself in the physics community, with Planck and Philipp Lenard – who won the Nobel prize that year – among his most famous supporters. Indeed, Planck was a member of the editorial board of Annalen der Physik at the time. Einstein was finally given the title of Herr Doktor from the University of Zurich in January 1906, but he remained at the patent office for a further two and a half years before taking up his first academic position at Zurich. By this time his statistical interpretation of Brownian motion and his bold postulates of special relativity were becoming part of the fabric of physics, although it would take several more years for his paper on light quanta to gain wide acceptance. 1905 was undoubtedly a great year for physics, and for Einstein. “You have to go back to quasi-mythical figures like Galileo or especially Newton to find good analogues,” says Wilczek. “The closest in modern times might be Dirac, who, if magnetic monopoles had been discovered, would have given Einstein some real competition!” But we should not forget that 1905 was just the beginning of Einstein's legacy. His crowning achievement – the general theory of relativity – was still to come.
The 1919 eclipse: a celebrity is born
Einstein shot to fame in 1919 when a team of astronomers led by Arthur Eddington found that the light from a distant star can be bent by the Sun, as predicted by relativity. But as Matthew Stanley explains, Eddington's expedition was partly motivated by a desire to heal the wounds between Britain and Germany after the First World War
In the spring of 1919,while Europe was just beginning to recover from the effects of the First World War, teams of British astronomers thousands of miles from home laboured to measure a tiny effect predicted by an obscure German scientist. This scientist was Albert Einstein, and when those astronomers presented their results he would move from little-known physicist to global celebrity. How did this dramatic turn of events come to be? It was in 1907 that Einstein first began systematic work to include gravity and acceleration in his earlier special theory of relativity. One of his first insights toward this new “general” theory was the equivalence principle, which postulated that there  Peacemaker – Arthur Eddington (bottom left) used the 1919 eclipse to show that Einstein's general relativity, which Willem de Sitter (top right) had promoted during the war, was correct. Also shown here are Einstein, Paul Ehrenfest (back row, centre) and Hendrik Lorentz (bottom right). | can be no observable difference between a gravitational field and uniform acceleration (at least as measured over the distance scales typical of laboratories). An immediate consequence of this was a thought experiment in which it seemed that a beam of light would be bent slightly in a gravitational field. Early attempts to observe this effect were uniformly unsuccessful, which turned out to be fortunate for Einstein: he later changed the quantitative value of his prediction using a more refined version of his theory. When Einstein presented his full field equations for general relativity in 1915, there were tremendous obstacles preventing the dissemination of his ideas to the world scientific community. Einstein was working in Berlin, and Germany had been isolated from the basic channels of scientific communication soon after the beginning of the First World War. Einstein's technical achievement went almost completely unnoticed on the other side of the trenches. The astronomer Willem de Sitter, working from the neutral Netherlands, sent his own presentation of general relativity to Britain, where he hoped to find someone receptive to Einstein's ideas. In a fortunate turn of history, de Sitter's papers landed on the desk of Arthur Eddington, head of the Cambridge Observatory and an officer of the Royal Astronomical Society. Not only was Eddington one of just a handful of British scientists who were familiar enough with tensors and differential geometry to understand Einstein's theory, he was also one of an even smaller group of British scientists that was willing to pay attention to German science at all. Soon after the beginning of the war the British scientific community became outraged at the apparent complicity of German intellectuals with the Kaiser's treaty-breaking army. (In addition to waging what was seen as an aggressive and atrocity-laden conflict, the Germans had flagrantly broken their commitment to respect the neutrality of Belgium.) The lack of trustworthiness this implied led to calls for Germany to be exiled from international science. Just as the German violation of neutral Belgium had made the claims of its politicians unreliable, it was felt that its scientists' reports were now worthless. Scientific journals from allied countries would no longer be sent to Germany or Austria, and foreign members from those countries were expelled from the Royal Society and other organizations.
Quaker, pacifist and adventurer
Eddington was one of the few voices that continued to argue for scientific internationalism. As a Quaker, he was a pacifist and believed strongly that international co-operation was critical to good science, particularly astronomy. He worked furiously and unsuccessfully to push back the emerging jingoism of British science, and he seized on relativity as a tool to break down wartime barriers. This was groundbreaking science coming from a peaceful German, and Eddington set out to both gain support for Einstein and to use that support to help heal the wounds of war. The debate over relativity developed quickly, with Eddington becoming known as the theory's primary defender: he was Einstein's bulldog. However, nationalistic considerations, in addition to the technical  Solar power – measurements that were made during the 1919 eclipse agreed with the predictions of general relativity. | difficulty and metaphysical strangeness of general relativity, limited the number of Einstein's supporters in Britain. This was despite the fact that in 1915, when first presenting the theory, Einstein used it to explain the long-known anomalies in the orbit of Mercury. People were impressed, but wanted further proof. Much of the discussion therefore turned to the possibility of tests of the theory: a predicted redshift in the solar spectrum appeared to be too difficult to observe, which left only a phenomena known as gravitational deflection. The curvature of space–time near massive bodies described by Einstein, if correct, would result in an apparent shift in position of stars near the Sun's edge.This shift would be minuscule and could only be observed during a solar eclipse, when stars could be seen during the day. Frank Dyson, Britain's Astronomer Royal, pointed out that there would be a solar eclipse on 29 May 1919 directly in front of the Hyades, a dense field of stars perfect for trying to detect the Einstein deflection. Unfortunately for the British scientists, the path of the eclipse was across difficult-to-reach parts of the southern hemisphere.
On the trail of the eclipse
Two teams were organized – Eddington and colleagues went to the island of Principe, which lies off the west African coast, while Andrew (A C D) Crommelin led a team to Sobral, Brazil. Both used techniques that were very similar to those used for standard eclipse observations of the day: a telescope was laid horizontally and a clockworkdriven mirror placed at the front to track the Sun's motion across the sky, with large glass photographic plates placed at the back of the telescope to capture images of the solar corona and nearby stars. The plan was to compare photographs of the gravitationally deflected stellar images surrounding the eclipsed Sun with “check plates” of the same star fields taken when the Sun was absent. Einstein predicted that stars at the edge of the Sun would appear to be only 1.75 arcseconds from their normal position in the sky – a small difference that was equivalent to about one-sixtieth of a millimetre on the photographic plates. Many physicists were sceptical of making such a small measurement, but, in reality, contemporary astronomers were quite comfortable detecting such changes thanks to their long experience performing conventional stellar-parallax measurements. Eddington's observations in Principe were nearly ruined by the weather, but he managed to bring back several good photographs with an average deflection of 1.61±0.3 arcseconds. The observations in Brazil were somewhat more complicated. The team there had two telescopes, one of which performed splendidly and returned results of 1.98±0.12 arcseconds. The second telescope, however, suffered an optical defect (astigmatism) that corrupted the photographs. Crommelin, who was the chief observer in Brazil, declared on the scene that the results should not be trusted. For the sake of completeness, however, the plates were still measured, and a deflection of 0.93 arcseconds (or 1.52 arcseconds if the astigmatism was accounted for) was derived.
A legend is born
Back in London, at a joint meeting of the Royal Society and the Royal Astronomical Society, Eddington presented the results on 6 November 1919 with all the skill of a practised showman. He dramatically portrayed the expedition as a crucial test between two master scientists – Newton and Einstein. Repeatedly emphasizing the international character of the theory and its test, he announced that Einstein's esoteric prediction had been confirmed by the expedition's photographs, and that space was in fact warped and that light had weight. The mass media, with significant encouragement from Eddington, picked up the story and ran with it. The appeal of a German theory being proved by British scientists so soon after the war captured the imagination, and Einstein was catapulted from an obscure physicist to worldwide celebrity literally overnight. His mythical reputation as an inscrutable sage was born instantly when the New York Times declared that no more than “12 wise men” in all the world could understand relativity. In the resulting demand for information about relativity and Einstein, Eddington led the popularization of the theory and the man, using the opportunity to show that science could rise above wartime hatreds. Einstein, as the Newtonsupplanting genius trapped behind nationalistic barriers, was presented as a powerful argument for international science. It is sometimes suggested that Eddington's internationalism led him to “fudge” the data from the expedition to ensure a positive result for Einstein.There is no reason to think this was the case. Usually those proposing this myth claim that Eddington threw out results that were unfavourable (meaning the second telescope from Brazil). In fact, those results were declared unusable by observers in the field who did not include Eddington. Furthermore, copies of the photographic plates from all three telescopes were distributed to astronomers around the world for them to make their own measurements and analysis. No contemporary accused Eddington of altering the results – this is purely a modern myth based on poor understanding of the optical techniques in use at the time. The influence of Eddington's pacifism is to be found in his championing of the expedition as a scientific goal and his popularization of Einstein as a major scientific figure, not in manipulated data. Einstein was pleased with Eddington's efforts on his behalf, although he was not too concerned as he always said he knew what the result of the eclipse expedition was going to be. The pair later met on a couple of occasions and appeared to get on well together: Einstein said that he wanted to learn English so that he could talk to Eddington about relativity. But as both were in the main solitary investigators, they never collaborated formally. Thanks to Eddington, the expedition has entered our collective memory as a great victory for scientific internationalism, and its triumphant and dramatic confirmation of general relativity launched Einstein to worldwide fame. Our image of Einstein as the scientific rebel who overthrew Newton was thus a result of surprising contingencies of war, peace and nationalism.
Strange ways of light and atoms
Two of Einstein's less well-known discoveries . Bose.Einstein condensation and stimulated emission . have had a huge impact on the modern world, explains Charles W Clark
Einstein is best known for relativity and his other 1905 breakthroughs . explaining the photoelectric effect and his work on Brownian motion . but his ideas also underpinned the development of the laser and the creation of a new state of matter called the Bose.Einstein condensate. These discoveries, which were made in 1916 and 1924, respectively, were based on Einstein's investigations into “bosonic” particles such as photons. Moreover, Bose.Einstein condensation was predicted to occur in one of the simplest physical systems: the ideal gas. An ideal gas is a system of non-interacting particles that are in thermal equilibrium . hardly a promising vehicle for surprising discoveries. Indeed, it is the epitome of disorder, with atoms and molecules flying about randomly. But Einstein showed that for any temperature there is a density above which the particles in an ideal gas do not participate in the thermal agitation. In other words, if we take an ideal gas and compress it at a constant temperature by, say, squeezing the walls of its container, then the gas will eventually separate into two components. One component remains engaged in the familiar wild party of thermal motion, while the other is quiescent, effectively at zero temperature, even though it is surrounded by a mob of hot atoms. As the density is increased, more atoms fall into the zero-temperature component, which eventually dominates the gas. In practice, researchers cool a gas with a given density until atoms start to enter this zerotemperature component.
 Quantum insight . Einstein predicted that a gas of bosons will collapse into a single quantum state called a condensate, which was created for the first time in 1995. These images show how such a state emerges in a gas of rubidium atoms that has been cooled from its classical state (left) to close to absolute zero (right). The appearance of a second, asymmetric profile in the middle figure is the “smoking gun” of Bose. Einstein condensation. | Bose.Einstein condensates
This phase transition, which cannot be understood in classical physics, is called Bose. Einstein condensation and is one of the most active areas of research in physics today (see Physics World September 2003 pp37.40). But, as the name suggests, it was not all down to Einstein: the existence of this new state of matter was predicted when Einstein applied to material particles ideas about the statistical mechanics of photons that had been proposed by the Indian physicist Satyendra Nath Bose. In 1923 Bose sent Einstein a paper that described a new way to derive Planck's radiation law by treating photons as indistinguishable particles. At the time, Bose was a little-known lecturer in physics at Dacca University (now in Bangladesh), and his paper had been rejected by The Philosophical Magazine. Einstein, on the other hand, was the most famous physicist in the world, and was sufficiently impressed by Bose's paper to translate it from English into German and submit it to the Zeitschrift fur Physik, where it was published under Bose's name. Bose considered a system of photons, and proposed that any number of photons could occupy a given quantum state. This led to a system that was in thermal equilibrium in accordance with Planck's law of black-body radiation. Einstein's contribution was to extend Bose's idea to material particles, postulating that phase space could be divided into elementary cells of volume h3, where h is Planck's constant, and that any number of particles could occupy a given cell. An alternative prescription was proposed by Enrico Fermi in 1926, in which no more than one particle can occupy an elementary cell. Today, we recognize that all the elementary particles in nature are either bosons or fermions, and are described either by Bose. Einstein or Fermi.Dirac statistics.
The quantum viewpoint
From the standpoint of quantum mechanics, the transition from a gas of bosons to a condensate is straightforward. In a classical ideal gas, which is described entirely by its temperature and density, there is only one characteristic length scale of microscopic origin: the mean distance between the atoms or molecules. For example, in an ideal gas at room temperature and atmospheric pressure this distance is about 3 nm. Quantum mechanics, however, introduces another microscopic length scale: the de Broglie wavelength, ル=h/p, where p is the momentum of the particle. Bose.Einstein condensation occurs when the de Broglie wavelength becomes comparable to the average separation between particles. For the nitrogen molecules in the atmosphere at room temperature, the de Broglie wavelength is about 0.02 nm,which is much smaller than the classical molecular separation. We might therefore think that we could create a condensate by compressing ordinary air by a factor of about a million. However, this will not work because the mean distance between the air molecules would become about 10 times less than the length of a normal molecular bond, and so we would be left with a solid with an incredibly high density, rather than an ideal gas. Indeed, no familiar substance can approach the conditions required for Bose. Einstein condensation, which led many to regard the phenomenon as nothing more than a mathematical curiosity. In 1938, however, superfluidity was discovered in liquid helium, and Fritz London noted that the conditions for the onset of superfluidity were remarkably similar to those for Bose.Einstein condensation. London recognized that the helium-4 atoms . which, like photons, are bosons . in these conditions could hardly be considered an ideal gas because the interactions between the atoms were so strong. However, he felt that some relic effect of condensation might drive a quantum phase transition in such a strongly interacting system.
Laser cooling
The concept of Bose.Einstein condensation as the iconic quantum phase transition, combined with its possible links to superfluidity, made it a “holy grail” for experimentalists. But it took almost 70 years to realize. In 1995 Eric Cornell, Carl Wieman and coworkers at the JILA laboratory in Boulder, Colorado, created the first condensate in a gas of laser-cooled rubidium atoms. This work, which has since been followed by demonstrations in some 40 laboratories worldwide, has placed Bose.Einstein condensates . and their fermionic counterparts . at the forefront of modern research. In 2001 Cornell,Wieman and Wolfgang Ketterle of the Massachusetts Institute of Technology shared the Nobel prize for their work on Bose.Einstein condensation. The creation of the first condensates relied on the use of lasers to trap and cool atoms . work that was recognized with the award of the 1997 Nobel prize to Steven Chu, Claude Cohen-Tannoudji and Bill Phillips. Remarkably, the development of the laser can also be traced to the work of Einstein. In 1916 Einstein found that quantum mechanics meant that atoms were more likely to emit photons into electromagnetic modes that already contained photons than into modes that did not . a process called stimulated emission. In other words, a photon with a particular energy, and therefore frequency, can cause an atom to emit a photon with the exact same frequency. Einstein related the probability of stimulated emission to that of spontaneous emission using two expressions that are now called the Einstein A and B coefficients. At the time this discovery did not have immediate practical consequences because the stimulated light . which is said to be coherent because it consists of photons with a single frequency . had to be amplified in some way. This was first achieved by Charles Townes and Arthur Schawlow in the microwave region with the development of the “maser” in 1954, and implemented in the optical regime by Theodore Maiman in 1960. Einstein's work on stimulated emission thus presaged a device that is now found in households around the world, and which is an essential accessory in virtually every field of science and engineering.
Hating the inherent randomness of quantum mechanics, Einstein tried to show that the theory was incomplete by drawing attention to a phenomenon that we now call entanglement. As it turns out, entangled particles are the key to quantum computing
The power of entanglement
EINSTEIN is rightly famed for his revolutionary work on relativity. But he was also one of the founders of quantum physics and in 1905 became the first physicist to apply Max Planck's quantum hypothesis to light. Einstein realized that the quantum picture can be used to describe the photoelectric effect . that only light above a certain frequency can eject electrons from the surface of a metal. Indeed, it was mainly for deriving the law of the photoelectric effect that he was awarded the 1921 Nobel Prize for Physics. Despite the undeniable success of quantum theory, Einstein never liked all of its implications. In particular, he simply could not accept the idea that randomness should be an inherent principle of nature. He felt that the theory did not . and could not.  Does God play dice? . Einstein thought not. | explain why quantum effects should appear random to us. Einstein's hope was that quantum mechanics could be completed by adding various as-yet-undiscovered variables.These "hidden" variables, he thought, would let us regain a deterministic description of nature. He expressed his discomfort in his celebrated saying, "[God] does not play dice". Einstein spent many years debating the pros and cons of quantum theory with the leading physicists of his day, particularly the Danish theorist Niels Bohr. This culminated in a final attack in 1935 when Einstein, Boris Podolsky and Nathan Rosen (together known as EPR) published a famous paper in which they outlined their objections to quantum mechanics.The title alone . "Can quantum-mechanical description of physical reality be considered complete?" . hinted at their concerns. In their paper, EPR argued that any description of nature should obey the following two properties. First, anything that happens here and now can influence the result of a measurement elsewhere, but only if enough time has elapsed for a signal to get there without travelling faster than the speed of light. Second, the result of any measurement is predetermined, particularly if one can predict it with complete certainty; in other words a result is fixed even if we do not carry out the measurement itself. Einstein, Podolsky and Rosen then examined what impact these two conditions would have on observations of quantum particles that had previously interacted with one another. They concluded that such particles would have very peculiar properties. In particular, the particles would exhibit correlations that lead to contradictions with Heisenberg's uncertainty principle. Quantum mechanics, it seemed, was incomplete. Later in 1935 Erwin Schrodinger published a response to the EPR paper, in which he introduced the notion of "entanglement" to describe such quantum correlations.He said that entanglement was the essence of quantum mechanics and that it illustrated the difference between the quantum and classical worlds in the most pronounced way. Schrodinger realized that two entangled particles have to be seen as a whole, rather than as two separate entities. If, say, the polarization of two photons is entangled,we will find that the polarization of each photon, when measured separately, appears to be random. However, if we find that one photon is circularly polarized in a right-handed sense, then we know immediately that the other photon is polarized in a left-handed sense . even if we do not actually measure the second photon.
 Entanglement is not so spooky
The problem, as far as Einstein was concerned, was that measuring the spin of one photon should have an instantaneous effect on the other photon, even though the two photons might be physically far apart. Einstein did not like this "non-localism" . or what he later called "spooky action at a distance" . because nothing should be able to travel faster than the speed of light. He wanted nature to be local and deterministic. For the next 50 years entanglement was seen as a somewhat weird effect that was essential only for answering the rather philosophical questions that EPR had raised about nature itself. Only recently, however, have physicists begun to realize that entanglement is not just an abstract concept. It is also important for understanding a variety of effects, such as "decoherence" . the process by which quantum effects die away and the classical world takes over. Moreover, entanglement has real practical consequences and lies at the heart of the emerging field of quantum information, which includes quantum computing, quantum cryptography and quantum teleportation (see Physics World March 1998 pp33.57). In the case of quantum computing, entanglement enables certain computational tasks to be performed much faster than is possible using classical physics. A quantum computer could be built from any system that can store information in a two-level quantum state, such as an atomic nucleus with a spin that can point either up or down. Unlike the bits in a classical computer,which can only be "0" or "1", these quantum states, known as "qubits", can be in a superposition of both states and hold any value between 0 and 1. Moreover, a quantum computer with N qubits can exist in 2N different states. Each of these states can be processed at the same time. This "quantum parallelism" might lead to an entangled quantum state allowing a quantum computer to, for example, factorize large numbers exponentially faster than a conventional computer.
En route to entanglement
But how can we generate and observe entanglement between particles in the first place? There are basically two options. One method is to let a particle emit (or decay into) other particles. Conservation rules dictate that the properties of these daughter particles will be strongly correlated and possibly entangled.The other option is to "engineer" entanglement by allowing two particles to interact for a fixed length of time. If the interaction depends on the states of the two systems, they can become entangled once the period of interaction is over. Of course, all particles interact with each other in one way or another,which means that entanglement is not such a special feature of nature at all. In fact, the challenge for experimental physicists who want to observe entangled particles is to isolate them completely from anything else. If the particles do interact with any further particles, the initial entanglement between them is easily lost. But thanks to huge progress in laser physics, atom optics and superconducting technology, physicists can now generate and observe entanglement in quantum systems using any of these techniques. Although photons do not interact strongly enough to be entangled directly, they can be entangled through various emission processes, many of which are well known. Indeed, correlations between photons that are stronger than those allowed by classical physics were first observed by Chien- Shiung Wu and Irving Shaknov at Columbia University in New York back in 1950. They carried out experiments in which an electron collides with a positron to create positronium . a short-lived state in which the electron and positron are bound together.This state then rapidly decays to produce entangled gamma-ray photons. The two photons have spins pointing in opposite directions, so that if one photon is found to be spin-up, then the other will have to be spin-down. Two photons can also be entangled when they are emitted in quick succession from an excited atom.The only proviso is that the photons are emitted when an electron falls in two steps to lower energy levels, such that the initial and the final state both have zero orbital angular momentum. If the first photon is, say, left circularly polarized and has a quantum state │L1, then the second photon has to be right circularly polarized and will have a quantum state │R2. Similarly, if the first photon is right circularly polarized (│R1) then the second photon will be left circularly polarized (│L2). Provided that the final state of the atoms is the same in both cases, a "coherent" superposition of the two decay options is obtained and the overall wavefunction for the two entangled photons is │プ=(│R1│L2.│L1│R2)/☆2, with the minus sign reflecting the fact that the final state has zero spin. The wavefunction is no longer the product of the quantum states of the two photons separately and their quantum states are intimately interlinked. Atomic-cascade experiments,which were pioneered in the 1970s and 1980s, are not easy. They require lots of equipment, including a vacuum vessel for the atomic beam, strong lasers that are exactly tuned to excite the atoms, and large lenses to collect enough photons, which are emitted in all directions. Currently the best way of creating pairs of entangled photons is to use a technique called parametric down conversion, which involves shining blue or ultraviolet laser light onto a crystal with nonlinear optical properties (figure 1). The crystals are special in that they distort an incoming electromagnetic wave in such a way that, for example, its frequency is exactly doubled. Very occasionally this process is reversed and a blue photon is converted into two new photons that have exactly half the energy (and frequency) of the original photon.The directions in which the photons are emitted depends on the polarization and direction of the incoming beam, as well as on the orientation of the crystal axis. Using this technique we can arrange for the two photons to be either vertically or horizontally polarized and to be emitted in two different directions. Provided that these two options are in a coherent superposition the two photons are entangled. Depending on the type of light used and the nature and orientation of the crystal it is also possible to entangle other properties of the photons, such as their frequency or direction. Entangled photons that can be sent down two separate fibreoptic cables can also be created.
Inspired by Einstein
All of these experimental advances were largely inspired by the questions that Einstein originally raised. In the early 1960s, for example, the Irish physicist John Bell tried to find a way of showing that the notion of hidden variables could remove the randomness of quantum mechanics.These hidden variables might, for example, provide values for all components of the polarization of a photon at all times and dictate whether it is left or right circularly polarized. In 1964 Bell therefore proposed a famous experiment that would give one result if quantum mechanics is correct and another result if hidden variables are needed. As it turned out, hidden variables were pretty much ruled out first by experiments carried out by Stuart Freedman and John Clauser in 1972 at the University of California at Berkeley and later by a comprehensive series of high-precision tests using atomic-cascade emission by Alain Aspect and co-workers at Orsay near Paris in the early 1980s. Thanks to the high quality of the crystals used for parametric down conversion it is now possible to observe entangled particles that are separated by a distance of almost 10 km. None of these experiments supports the need for hidden variables, although we cannot be totally sure because they do not detect a big enough fraction of the total flux of photons.The ultimate experimental test would not only involve detecting a high proportion of entangled particles but also performing measurements so fast that any mutual faster-than-light influence can be ruled out. If and when this test is carried out, we will be able to say once and for all that nature is deterministic and local as Einstein believed . or whether he was wrong.
 Entangling more particles
In recent years physicists have sought to entangle more and more particles at the same time. One reason for this interest is that multiparticle entangled states will be useful for quantum information. Such states can also refute EPR's arguments more directly . a fact that was first pointed out by Daniel Greenberger, Michael Horne and Anton Zeilinger in 1989. In practical terms, complex,multiphoton entangled states can be created by firing high-power pump lasers at several parametric-down-conversion crystals, which simultaneously emit several pairs of photons. These photons can then be brought together using specially arranged semi-transparent mirrors and other optical devices. For example, Anton Zeilinger at the University of Vienna and colleagues have used this technique to entangle three, and later four, infrared photons (figure 2). And last year Jian-Wei Pan and colleagues at the University of Science and Technology of China in Hefei even managed to observe non-classical correlations from five photons. In addition, several groups of researchers are trying to increase the yield by entangling photons emitted by "quantum dots". These are nanometre-sized islands of conducting material that confine electrons in three dimensions and therefore exhibit discrete energy levels, very much like atoms.Although no-one has yet succeeded, mainly because of inhomogeneities and distortions in the dots, I fully expect this to change soon. The problem with these methods is that the probability of generating . and then observing . entangled photon pairs is very low. Indeed, the more photons you try to entangle, the less chance you have of creating them. However, novel crystals, better laser systems and improved optical resonators to tailor the emission will boost the number of entangled photons further and allow such systems be used for multiparty communication.
 Engineered entanglement
The experiments described so far generate entanglement using photons originating from an emission process. But we cannot deliberately engineer entanglement between photons because they interact so weakly.However, this process is possible with atoms . very much in the spirit of EPR's proposal. The first experiment to entangle three atoms was carried out in 2000 by Serge Haroche, Jean Michel Raymond, Michel Brune and colleagues at the Ecole Normale Superieure in Paris.They used the electromagnetic field of a microwave resonator to mediate the interaction between three highly excited rubidium atoms. As an atom passes through the resonator there is a 50% chance of it dropping to a lower energy state and depositing a photon in the resonator.The resonator then contains either no photons or one photon, with the atom either in the excited or the lower state. This means that the atom is entangled with the field of the resonator. The resonator is then detuned so that the next atom that passes through it only undergoes a phase shift if there is a photon already present.What this means is that if the second atom is prepared in a superposition of the two states, it is entangled both with the state of the resonator and with the first atom. The resonator is then tuned back to resonance so that when a third atom passes through it all three atoms are entangled with each other . but not with the resonator. The problem with this method is that the atoms come randomly out of an oven, which means that the chance of detecting a certain number of entangled atoms within a given time again falls rapidly with number. The solution to this problem is to first capture a controlled number of atoms and only then let them interact with each other. Ideas for performing such experiments have been developed over the last 10 years, mainly by Ignacio Cirac and colleagues at the Max Planck Institute for Quantum Optics in Garching, Germany, and by Peter Zoller and co-workers at the University of Innsbruck in Austria. Currently the most advanced way of entangling quantum particles is to use a linear chain of ions that have been trapped in the electric field between a pair of elongated electrodes. At room temperature the ions oscillate vigorously back and forth along the chain. However, using the technique of "laser cooling" it is possible to slow down the ions so that they end up near to absolute zero. Lasers can then be used to excite the atoms so that they move in tandem. This collective centre-of-mass oscillation has the energy of a single quantum of motion, known as a phonon. The key points about this experiment are that it is then possible to excite the phonon by letting any ion in the chain interact with a laser beam and that subsequent interactions depend on whether the phonon has been excited.The quantum state of an ion can therefore be transferred to the quantum state of motion. Since its excitation is simultaneously shared with all the other ions, another laser beam can then be used to entangle a second ion with the motional state of the chain. Finally, that state can be transferred back to the first ion, which leaves the two ions entangled. Manipulating the quantum states in this way can be viewed as the application of a quantum logic gate, which is the basic component of a quantum computer. In 2003 Ferdinand Schmidt-Kaler, Rainer Blatt and coworkers in Innsbruck entangled up to three ions by carrying out the controlled-NOT (CNOT) operation, which corresponds to the XOR gate operation of a classical computer. The Innsbruck team trapped calcium ions (figure 3) and used focused laser beams to manipulate two particularly long-lived electronic states of each ion. These two states . and any superposition of them . carry the quantum information of the ions.The advantage of the technique is that it could, in principle, be modified to include many more ions, provided that the total time to engineer the states is less than the decoherence time.This time is a measure of how fast entanglement is lost, which occurs, for example, when the ions scatter off any residual atoms in the ultra-high vacuum of the trap. Last year a group led by David Wineland at the National Institute of Standards and Technology (NIST) in Boulder, Colorado, used a slightly different approach to entanglement that does not require ground-state cooling and is less sensitive to experimental imperfections. In this experiment a pair of beryllium ions is exposed to two laser beams simultaneously. The beams apply an oscillating force to the ions . but only if they are in specific internal electronic states. This "statedependent" coupling is what is needed to achieve entanglement. The NIST group is now trying to use this approach to entangle more ions by developing a "multitrap" architecture where ions are physically moved between memory and processing segments of a large trap.
 Entanglement on a grand scale
But if you want a truly large number of entangled atoms, a group led by Immanuel Bloch at the University of Munich (now at the University of Mainz) has found the way forward. In an experiment reported last year, Bloch and co-workers began by creating a dense, ultra-cold gas of rubidium atoms in which all of the atoms were in the same quantum state . a Bose.Einstein condensate. They then transferred about 10 000 of these atoms to an "optical lattice" . a periodic 2D intensity pattern that is formed where two standing waves at 90' to each other interfere (figure 4). The atoms sit in a regular array at every position of maximum brightness, a bit like eggs in an egg-box. Since each atom can be put in two alternative quantum states, Bloch and colleagues therefore set up two different standing waves along one axis of the lattice. One wave trapped atoms that were in one quantum state, while the other trapped atoms that were in the other quantum state. The team then tweaked the polarization of one of the laser beams, which moved the position of one of the standing waves . and all the atoms trapped in it. Pairs of neighbouring atoms then approach each other and collide, but only if they are in one of the four possible distinguishable states. However, by preparing the atoms in a superposition of their two quantum states, Bloch's team ensured that all the atoms collided and became entangled in a common quantum state. The main experimental difficulty is to measure the properties of each of the atoms separately. Bloch's team used light with a wavelength of 780 nm, which led to a lattice spacing of only 390 nm . too close to resolve each of the atoms. But by exciting all the atoms simultaneously and then observing their fluorescence, it was shown that many of them had formed one huge entangled state . ideal for quantum computation.
What's next?
A long time has passed since Einstein, Podolsky and Rosen's seminal work of 1935. But in recent times . and the last 15 years in particular . physicists have made significant headway in understanding the fascinating non-classical features of entangled states and in creating entangled quantum systems experimentally. Physicists have learned about the variety and power of entangled states, and found ways of engineering these states very much in the spirit of EPR's original prescription. The work opens the door to new methods of quantum communication and quantum information processing, and to improved high-precision measurements. New ways of entangling particles are being reported almost every month. Entanglement has, for example, been observed between macroscopic systems, such as clouds of atoms or bright pulses of light. In the case of a cloud of atoms, the collective spin of the atoms become entangled with the spin of atoms in another cloud in a separate glass cell. Entanglement has also been observed in solid-state systems. In 2003, for example, Yuri Pashkin and co-workers at NEC and the RIKEN research lab in Japan entangled two micrometer-size superconducting qubits. Each qubit is based on a superconducting loop with a transistor formed from a single "Cooper pair" of electrons. This results in charge qubits in which the two states (0 and 1) are de- fined by a lack or excess of these pairs. Although Pashkin's team only indirectly observed the effects of two-qubit entanglement, the work moves us a step closer to solid-state quantum information processing. It is even possible to entangle two different types of particle, such as an atom and a photon.To pull off this trick you need an excited atom that can decay to two alternative ground states. Chris Monroe and colleagues at the University of Michigan in the US demonstrated this effect in 2004 by analysing the correlation between the polarization of a photon and a trapped cadmium ion. This research could lead to quantum processors that are connected to each other, just as conventional PCs are linked over the Internet. Another possibility is for the processors to be used as basic repeater stations or error-correction units for communicating quantum information over long distances. Although Einstein's objections to quantum mechanics were never confirmed during his lifetime, physicists are now reasonably sure that what he stood for . determinism and locality . are not properties of nature. But until we have definite experimental proof, it is too early to say that he was wrong. Still, it is ironic that entanglement, which Einstein first highlighted in objection to quantum theory, is a real phenomenon that researchers can not only understand but also put to practical use.
The king is dead. Long live the king!
There is the Einstein who grew up, worked and died, but there is also the Einstein who became the public face of science. Robert P Crease explains the difference
In his classic work The King’s Two Bodies: A Study in Medieval Political Theology the historian Ernst Kantorowicz examined the development of the political doctrine that distinguished between a monarch’s natural body and his or her political body. Whereas the monarch’s natural body is mortal – it lives, breathes, becomes ill and dies – the political body,which is the embodiment and representative of the state, is immortal. Yet somehow the two bodies comprise a single unit in making appointments, conducting wars and signing treaties. The paradox is encapsulated in the expression, “The king is dead. Long live the king!”. Einstein has such a great and enduring cultural  More than a physicist – this statue of Einstein sits at the National Academy of Sciences in Washington, DC. | visibility that it is tempting to try to understand him in similar terms. He had a natural body that emerged into the world one day in March 1879,matured, and passed out of the world in April 1955, his ashes dispersed by the currents of the Delaware river. But Einstein also has another kind of body – it is too dynamic and influential to be called an icon – that is as alive as ever half a century after his death. It features in magazines, movies, novels, the arts, advertisements, commercials, cartoons, and in just about every niche of popular culture, including “Baby Einstein” toys. It also features prominently in the minds of professional physicists. The king’s political body – symbol and agent of the realm – was officially defined, generally sought-after, and often a struggle to maintain. Einstein’s political body was thrust upon him, and he was ambivalent about it. As he once wrote to a friend, “Take pleasure that only a few care about you and, believe me, it has a good side. Better an understanding spectator than an electrically illuminated actor”. Einstein’s political body continues to represent science itself. Like that of the king, it is linked in some way with his private body.
Uniting the bodies
Scientists generally prefer to separate Einstein’s two bodies; after all, his scientific work is what is important. Indeed, anyone who tries to tether a scientist’s work and personality can get their fingers burned, as Robert Oppenheimer once found to his cost. Having previously worked with Einstein at the Institute for Advanced Study in Princeton, Oppenheimer was invited to give a talk at a UNESCO conference that was held in Paris in December 1965 to mark the 10th anniversary of Einstein’s death. The occasion called for polite words about Einstein’s political body. But Oppenheimer chose instead to speak about Einstein’s background and its limitations, pointing out that in his later years Einstein worked all by himself on what many considered to be a fruitless quest – a unified field theory. Although this was something that many scientists had said privately for years, they had never openly admitted it at a public event. Some colleagues were furious. Wounded, Oppenheimer declined an invitation to speak about Einstein a few weeks later. The public, however, is not content to separate the two bodies, and is endlessly fascinated by information about Einstein’s private body and its relation to his political one. Where did Einstein get his ideas? How did he treat women? Was he a good parent? What were his views on the Jewish people? Vegetarianism? World peace? The craving for answers to such questions can elicit what may seem to be excessive responses from those able to satisfy it. Consider, for instance, the tone of The Private Albert Einstein, a book written by Peter Bucky, the son of one of Einstein’s close friends. In the opening chapter Bucky claims to have known Einstein probably “as intimately as did any other man on Earth”. He goes on to provide us with reminiscences of Einstein’s early-morning “jolly whistling…echoing in the bathroom”, of the smells of “the not unpleasant aroma of his pipe tobacco“, of Einstein’s clothes, eating habits, picnics and other things that make for irresistible reading but seem to shed little light on his science.
Scientist or symbol?
But is Einstein’s political body really a scientist, or is it a mere symbol of science, like the flag of a country? Does it not clean up and oversimplify the complex and messy process of real science? As the French intellectual Roland Barthes once pointed out, photographs of Einstein – i.e. of his private body – generally show him next to a blackboard covered with equations, while popular images generally depict him next to a clean blackboard with only one equation, E=mc2, as if giving birth to it were that simple. This might be fine for science museums and children’s textbooks, but is it at the cost of abandoning real science? This distance between Einstein’s political body and Einstein the working scientist is cleverly parodied in a new musical called Einstein’s Dreams: A Musical Romance, a version of which will be performed at the Prince Music Theater in Philadelphia next month. Based on the best-selling novel by the physicist Alan Lightman, the musical includes a scene in which Einstein, the private body, explains E=mc2 to his friend Michele Besso, who was an engineer, and a later scene in which Einstein the political body appears at a news conference. Forced to speak about the equation, he stammers and cannot do it. “E equals…E equals something, I’m fairly sure,” Einstein blurts out, “and whatever it equals I’m sure it’s important.” It is therefore tempting to dismiss the significance of Einstein’s public body as having nothing to do with science. But that would be a mistake. For it plays an important role in the interaction between scientific and popular culture. When two cultures interact, they never engage each other simultaneously at all levels.Rather, they meet through what ethnographers call “congeners” – little lenses through which one culture looks at, tries to understand, and responds to the other, accompanied by deepening curiosity and interest. A congener is thus more than something that symbolizes or denotes another culture; it crystallizes an interaction with it. Einstein serves, in effect, as a congener. He is the means through which many nonscientists acquire more than a superficial understanding of science; he is the conduit through which they become acquainted with key theories, individuals and events in science history. The frontier between science and the public needs more such congeners. Albert Einstein is dead. Long live Einstein!
Digested from <Physics World>
(Edited by phyfan)
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