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Many Physicists Believe That Entanglement Is the Essence Of NEWS FEATURE SPACE. TIME. ENTANGLEMENT. n early 2009, determined to make the most annual essay contest run by the Gravity Many physicists believe of his first sabbatical from teaching, Mark Research Foundation in Wellesley, Massachu- Van Raamsdonk decided to tackle one of setts. Not only did he win first prize, but he also that entanglement is Ithe deepest mysteries in physics: the relation- got to savour a particularly satisfying irony: the the essence of quantum ship between quantum mechanics and gravity. honour included guaranteed publication in After a year of work and consultation with col- General Relativity and Gravitation. The journal PICTURES PARAMOUNT weirdness — and some now leagues, he submitted a paper on the topic to published the shorter essay1 in June 2010. suspect that it may also be the Journal of High Energy Physics. Still, the editors had good reason to be BROS. ENTERTAINMENT/ WARNER In April 2010, the journal sent him a rejec- cautious. A successful unification of quantum the essence of space-time. tion — with a referee’s report implying that mechanics and gravity has eluded physicists Van Raamsdonk, a physicist at the University of for nearly a century. Quantum mechanics gov- British Columbia in Vancouver, was a crackpot. erns the world of the small — the weird realm His next submission, to General Relativity in which an atom or particle can be in many BY RON COWEN and Gravitation, fared little better: the referee’s places at the same time, and can simultaneously report was scathing, and the journal’s editor spin both clockwise and anticlockwise. Gravity asked for a complete rewrite. governs the Universe at large — from the fall But by then, Van Raamsdonk had entered a of an apple to the motion of planets, stars and shorter version of the paper into a prestigious galaxies — and is described by Albert Einstein’s 290 | NATURE | VOL 527 | 19 NOVEMBER 2015 © 2016 Macmillan Publishers Limited. All rights reserved FEATURE NEWS Black holes such as the one depicted in Interstellar And in the years since, so many have rushed to (2014) can be connected by wormholes. explore this idea that Maldacena’s paper is now one of the most highly cited articles in physics. Einstein loathed the idea of entanglement, Among the enthusiasts was Van Raams- and famously derided it as “spooky action at a donk, who started his sabbatical by pondering distance”. But it is central to quantum theory. one of the central unsolved questions posed And Van Raamsdonk, drawing on work by by Maldacena’s discovery: exactly how does a like-minded physicists going back more than quantum field on the boundary produce grav- a decade, argued for the ultimate irony — that, ity in the bulk? There had already been hints3 despite Einstein’s objections, entanglement that the answer might involve some sort of might be the basis of geometry, and thus of relation between geometry and entanglement. Einstein’s geometric theory of gravity. “Space- But it was unclear how significant these hints time,” he says, “is just a geometrical picture of were: all the earlier work on this idea had dealt how stuff in the quantum system is entangled.” This idea is a long way from being proved, and is hardly a complete theory of quantum gravity. But independent studies have reached “I HAD UNDERSTOOD much the same conclusion, drawing intense interest from major theorists. A small indus- try of physicists is now working to expand the geometry–entanglement relationship, using all SOMETHING THAT the modern tools developed for quantum com- puting and quantum information theory. “I would not hesitate for a minute,” says NO ONE HAD physicist Bartłomiej Czech of Stanford Uni- versity in California, “to call the connections between quantum theory and gravity that have emerged in the last ten years revolutionary.” UNDERSTOOD GRAVITY WITHOUT GRAVITY Much of this work rests on a discovery2 BEFORE.” announced in 1997 by physicist Juan Maldacena, now at the Institute for Advanced with special cases, such as a bulk universe that Study in Princeton, New Jersey. Maldacena’s contained a black hole. So Van Raamsdonk research had led him to consider the relation- decided to settle the matter, and work out ship between two seemingly different model whether the relationship was true in general, universes. One is a cosmos similar to our own. or was just a mathematical oddity. Although it neither expands nor contracts, it He first considered an empty bulk universe, has three dimensions, is filled with quantum which corresponded to a single quantum field particles and obeys Einstein’s equations of grav- on the boundary. This field, and the quan- ity. Known as anti-de Sitter space (AdS), it is tum relationships that tied various parts of it commonly referred to as the bulk. The other together, contained the only entanglement in model is also filled with elementary particles, the system. But now, Van Raamsdonk won- but it has one dimension fewer and doesn’t dered, what would happen to the bulk universe recognize gravity. Commonly known as the if that boundary entanglement were removed? boundary, it is a mathematically defined mem- He was able to answer that question using brane that lies an infinite distance from any mathematical tools4 introduced in 2006 by given point in the bulk, yet completely encloses Shinsei Ryu, now at the University of Illinois general theory of relativity, announced 100 years it, much like the 2D surface of a balloon enclos- at Urbana–Champaign, and Tadashi Takanagi, ago this month. The theory holds that gravity is ing a 3D volume of air. The boundary particles now at the Yukawa Institute for Theoretical geometry: particles are deflected when they pass obey the equations of a quantum system known Physics at Kyoto University in Japan. Their near a massive object not because they feel a as conformal field theory (CFT). equations allowed him to model a slow and force, said Einstein, but because space and time Maldacena discovered that the boundary methodical reduction in the boundary field’s around the object are curved. and the bulk are completely equivalent. Like entanglement, and to watch the response in Both theories have been abundantly verified the 2D circuitry of a computer chip that the bulk, where he saw space-time steadily through experiment, yet the realities they encodes the 3D imagery of a computer game, elongating and pulling apart (see ‘The entan- describe seem utterly incompatible. And from the relatively simple, gravity-free equations glement connection’). Ultimately, he found, the editors’ standpoint, Van Raamsdonk’s that prevail on the boundary contain the same reducing the entanglement to zero would approach to resolving this incompatibility was information and describe the same physics as break the space-time into disjointed chunks, strange. All that’s needed, he asserted, is ‘entan- the more complex equations that rule the bulk. like chewing gum stretched too far. glement’: the phenomenon that many physicists “It’s kind of a miraculous thing,” says Van The geometry–entanglement relationship believe to be the ultimate in quantum weirdness. Raamsdonk. Suddenly, he says, Maldacena’s was general, Van Raamsdonk realized. Entan- Entanglement lets the measurement of one duality gave physicists a way to think about glement is the essential ingredient that knits particle instantaneously determine the state of quantum gravity in the bulk without thinking space-time together into a smooth whole — not a partner particle, no matter how far away it may about gravity at all: they just had to look at the just in exotic cases with black holes, but always. be — even on the other side of the Milky Way. equivalent quantum state on the boundary. “I felt that I had understood something 19 NOVEMBER 2015 | VOL 527 | NATURE | 291 © 2016 Macmillan Publishers Limited. All rights reserved THE ENTANGLEMENT CONNECTION The ghostly quantum phenomenon of entanglement may be what knits space-time into a smooth whole. In an innite model universe known as about a fundamental question that perhaps anti -de Sitter space, the eects of gravity at ry nobody had understood before,” he recalls: NATURE a any point x in the interior are mathematically d n equivalent to a quantum eld theory on its u x “Essentially, what is space-time?” o B boundary. This universe can be visuali z ed in 2D by lling it with imaginary triangles. ENTANGLEMENT AND EINSTEIN Anti-de Sitter Although the triangles are identical, they look NIK SPENCER/ space increasingly distorted as they approach the Quantum entanglement as geometric glue — boundary. this was the essence of Van Raamsdonk’s rejected paper and winning essay, and an idea Physicists noticed that this pattern that has increasingly resonated among physi- resembled diagrams called tensor networks, which were invented to show cists. No one has yet found a rigorous proof, so Tensor connections between quantum particles s the idea still ranks as a conjecture. But many le on a massive scale. These connections network ic t r independent lines of reasoning support it. are known as quantum entanglement . a p In 2013, for example Maldacena and Leonard ed gl 5 an Susskind of Stanford published a related con- nt What is quantum entanglement? E jecture that they dubbed ER = EPR, in honour In 1935, Albert Einstein, Boris Podolsky and Nathan Rosen (EPR) pointed out that a connection can exist of two landmark papers from 1935. ER, by Ein- between widely separated quantum systems: a measurement of one will determine the state of the other. stein and American-Israeli physicist Nathan 6 EXAMPLE Entangled spins: The Observation of Rosen, introduced what is now called a worm- if one particle is particles are one particle hole: a tunnel through space-time connecting spinning up, the separated.
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