Gravity is mathematically relatable to dynamics of subatomic particles

Gravity, the force that brings baseballs back to Earth and governs the growth of black holes, is mathematically relatable to the peculiar antics of the subatomic particles that make up all the matter around us.

Albert Einstein’s desk can still be found on the second floor of Princeton’s physics department. Positioned in front of a floor-to-ceiling blackboard covered with equations, the desk seems to embody the spirit of the frizzy-haired genius as he asks the department’s current occupants, “So, have you solved it yet?”

Einstein never achieved his goal of a unified theory to explain the natural world in a single, coherent framework. Over the last century, researchers have pieced together links between three of the four known physical forces in a “,” but the fourth force, gravity, has always stood alone.

No longer. Thanks to insights made by Princeton faculty members and others who trained here, gravity is being brought in from the cold—although in a manner not remotely close to how Einstein had imagined it.

Though not yet a “theory of everything,” this framework, laid down over 20 years ago and still being filled in, reveals surprising ways in which Einstein’s theory of gravity relates to other areas of physics, giving researchers new tools with which to tackle elusive questions.

The key insight is that gravity, the force that brings baseballs back to Earth and governs the growth of , is mathematically relatable to the peculiar antics of the that make up all the matter around us.

This revelation allows scientists to use one branch of physics to understand other seemingly unrelated areas of physics. So far, this concept has been applied to topics ranging from why black holes run a temperature to how a butterfly’s beating wings can cause a storm on the other side of the world.

This relatability between gravity and subatomic provides a sort of Rosetta stone for physics. Ask a question about gravity, and you’ll get an explanation couched in the terms of subatomic particles. And vice versa.

“This has turned out to be an incredibly rich area,” said Igor Klebanov, Princeton’s Eugene Higgins Professor of Physics, who generated some of the initial inklings in this field in the 1990s. “It lies at the intersection of many fields of physics.”

From tiny bits of string

The seeds of this correspondence were sprinkled in the 1970s, when researchers were exploring tiny subatomic particles called quarks. These entities nest like Russian dolls inside protons, which in turn occupy the atoms that make up all matter. At the time, physicists found it odd that no matter how hard you smash two protons together, you cannot release the quarks—they stay confined inside the protons.

One person working on quark confinement was Alexander Polyakov, Princeton’s Joseph Henry Professor of Physics. It turns out that quarks are “glued together” by other particles, called gluons. For a while, researchers thought gluons could assemble into strings that tie quarks to each other. Polyakov glimpsed a link between the theory of particles and the theory of strings, but the work was, in Polyakov’s words, “hand-wavy” and he didn’t have precise examples.

Meanwhile, the idea that fundamental particles are actually tiny bits of vibrating string was taking off, and by the mid-1980s, “string theory” had lassoed the imaginations of many leading physicists. The idea is simple: just as a vibrating violin string gives rise to different notes, each string’s vibration foretells a particle’s mass and behavior. The mathematical beauty was irresistible and led to a swell of enthusiasm for string theory as a way to explain not only particles but the universe itself.

One of Polyakov’s colleagues was Klebanov, who in 1996 was an associate professor at Princeton, having earned his Ph.D. at Princeton a decade earlier. That year, Klebanov, with graduate student Steven Gubser and postdoctoral research associate Amanda Peet, used string theory to make calculations about gluons, and then compared their findings to a string-theory approach to understanding a black hole. They were surprised to find that both approaches yielded a very similar answer. A year later, Klebanov studied absorption rates by black holes and found that this time they agreed exactly.

That work was limited to the example of gluons and black holes. It took an insight by Juan Maldacena in 1997 to pull the pieces into a more general relationship. At that time, Maldacena, who had earned his Ph.D. at Princeton one year earlier, was an assistant professor at Harvard. He detected a correspondence between a special form of gravity and the theory that describes particles. Seeing the importance of Maldacena’s conjecture, a Princeton team consisting of Gubser, Klebanov and Polyakov followed up with a related paper formulating the idea in more precise terms.

Another physicist who was immediately taken with the idea was Edward Witten of the Institute for Advanced Study (IAS), an independent research center located about a mile from the University campus. He wrote a paper that further formulated the idea, and the combination of the three papers in late 1997 and early 1998 opened the floodgates.

“It was a fundamentally new kind of connection,” said Witten, a leader in the field of string theory who had earned his Ph.D. at Princeton in 1976 and is a visiting lecturer with the rank of professor in physics at Princeton. “Twenty years later, we haven’t fully come to grips with it.”

Two sides of the same coin

This relationship means that gravity and subatomic particle interactions are like two sides of the same coin. On one side is an extended version of gravity derived from Einstein’s 1915 theory of general relativity. On the other side is the theory that roughly describes the behavior of subatomic particles and their interactions.

The latter theory includes the catalogue of particles and forces in the “standard model” (see sidebar), a framework to explain matter and its interactions that has survived rigorous testing in numerous experiments, including at the Large Hadron Collider.

In the standard model, quantum behaviors are baked in. Our world, when we get down to the level of particles, is a quantum world.

Notably absent from the standard model is gravity. Yet quantum behavior is at the basis of the other three forces, so why should gravity be immune?

The new framework brings gravity into the discussion. It is not exactly the gravity we know, but a slightly warped version that includes an extra dimension. The universe we know has four dimensions, the three that pinpoint an object in space—the height, width and depth of Einstein’s desk, for example—plus the fourth dimension of time. The gravitational description adds a fifth dimension that causes spacetime to curve into a universe that includes copies of familiar four-dimensional flat space rescaled according to where they are found in the fifth dimension. This strange, curved spacetime is called anti-de Sitter (AdS) space after Einstein’s collaborator, Dutch
astronomer Willem de Sitter.

The breakthrough in the late 1990s was that mathematical calculations of the edge, or boundary, of this anti-de Sitter space can be applied to problems involving quantum behaviors of subatomic particles described by a mathematical relationship called conformal field theory (CFT). This relationship provides the link, which Polyakov had glimpsed earlier, between the theory of particles in four space-time dimensions and string theory in five dimensions. The relationship now goes by several names that relate gravity to particles, but most researchers call it the AdS/CFT (pronounced A-D-S-C-F-T) correspondence.

Tackling the big questionsThis correspondence, it turns out, has many practical uses. Take black holes, for example. The late physicist Stephen Hawking startled the physics community by discovering that black holes have a temperature that arises because each particle that falls into a black hole has an entangled particle that can escape as heat.

Using AdS/CFT, Tadashi Takayanagi and Shinsei Ryu, then at the University of California-Santa Barbara, discovered a new way to study

entanglement in terms of geometry, extending Hawking’s insights in a fashion that experts consider quite remarkable.

In another example, researchers are using AdS/CFT to pin down chaos theory, which says that a random and insignificant event such as the flapping of a butterfly’s wings could result in massive changes to a large-scale system such as a faraway hurricane. It is difficult to calculate chaos, but black holes—which are some of the most chaotic quantum systems possible—could help. Work by Stephen Shenker and Douglas Stanford at Stanford University, along with Maldacena, demonstrates how, through AdS/CFT, black holes can model quantum chaos.

One open question Maldacena hopes the AdS/CFT correspondence will answer is the question of what it is like inside a black hole, where an infinitely dense region called a singularity resides. So far, the relationship gives us a picture of the black hole as seen from the outside, said Maldacena, who is now the Carl P. Feinberg Professor at IAS.

“We hope to understand the singularity inside the black hole,” Maldacena said. “Understanding this would probably lead to interesting lessons for the Big Bang.”

The relationship between gravity and strings has also shed new light on quark confinement, initially through work by Polyakov and Witten, and later by Klebanov and Matt Strassler, who was then at IAS.

Those are just a few examples of how the relationship can be used. “It is a tremendously successful idea,” said Gubser, who today is a professor of physics at Princeton. “It compels one’s attention. It ropes you in, it ropes in other fields, and it gives you a vantage point on theoretical physics that is very compelling.”

The relationship may even unlock the quantum nature of gravity. “It is among our best clues to understand gravity from a quantum perspective,” said Witten. “Since we don’t know what is still missing, I cannot tell you how big a piece of the picture it ultimately will be.”

Still, the AdS/CFT correspondence, while powerful, relies on a simplified version of spacetime that is not exactly like the real universe. Researchers are working to find ways to make the theory more broadly applicable to the everyday world, including Gubser’s research on modeling the collisions of heavy ions, as well as high-temperature superconductors.

Also on the to-do list is developing a proof of this correspondence that draws on underlying physical principles. It is unlikely that Einstein would be satisfied without a proof, said Herman Verlinde, Princeton’s Class of 1909 Professor of Physics, the chair of the Department of Physics and an expert in string , who shares office space with Einstein’s desk.

“Sometimes I imagine he is still sitting there,” Verlinde said, “and I wonder what he would think of our progress.”


Is Our Understanding of Gravity Really That Wrong?

Albert Einstein was a brilliant man; there’s no denying it.  But, what he proposed about gravity, and what we have believed for a long time, may not be quite right. It was first proposed back in 2010, that gravity doesn’t work in the way that Einstein hypothesized, and there is now evidence to prove so. The study that was conducted was done on more than 30,0000 galaxies, and the new hypothesis that has arisen as a result is referred to as ‘Verlinde’s hypothesis of gravity’ after Eric Verlinde, creator of the hypothesis and theoretical physicist from the University of Amsterdam.

The problem with Einstein’s theory of gravity is that, even though it’s widely accepted throughout the physics community, it doesn’t account for everything within the universe. Verlinde, on the other hand, takes a different approach to tackling gravity. His idea is that gravity is a side effect of what is happening in the universe and not the cause. But, not much more had been done with the theory since 2010, until a team at Leiden University in the Netherlands decided to test the theory and came up with positive results. The testing involved analyzing the distribution of matter of 33,000+ galaxies and involved studying gravitational lensing, which is a tried and tested method of measuring dark matter.  However, the team discovered that is they just take Verlinde’s modified gravity theory into consideration; there was no need to factor in the idea of dark matter.

Upon comparing the results of the team’s testing and Einstein’s predictions, it was found that they both work. However, with Einstein’s theory and the presence of dark matter, four free parameters were needed to match the team’s observations, but with Verlinde’s theory, none were needed. Margot Brouwer, the leader of the research, says, “The dark matter model fits slightly better with the data than Verlinde’s prediction.  But then if you mathematically factor in the fact that Verlinde’s prediction doesn’t have any free parameters, whereas the dark matter prediction does, then you find Verlinde’s model is performing slightly better.” But, it will still be some time before Verlinde’s theory is accepted over Einstein’s (if ever), no matter how much proof there is. Brower goes on to say, “The question now is how the theory develops, and how it can be further tested.  But the results of this first test looks interesting.”

Will the LHC Prove the Existence of Higher Dimensions?

In Brief
  • To achieve an accurate description of the universe, physical theories are increasingly invoking extra dimensions to explain the mysteries of nature.
  • The problem is—how do you prove the existence of something so elusive? New experiments with the LHC may finally prove just how many extra dimensions, if any, our universe really has.

How many dimensions are there? Is time a dimension? Or is our 3-dimensional space-time just one element—and a minor one at that—of a greater hyperdimensional universe?

It’s a question that’s been asked many, many times, and the answers can be almost as varied as there are potential extra dimensions. From Paul Ehrenfest’s exploration of 3-dimensional physics in 1917 to the M-theory of the 1990s, experts throughout the years have proposed their own answers—some more forcefully than others.

But with advances in technology, and armed with new mathematical models and theories, we might be in a unique position today to begin to understand one of the natural world’s most baffling mysteries.

Dimensions, Gravity, and Light

At the heart of almost all theories that deal with the number of dimensions are the fundamental forces of gravity and light, both of which are possibly the most observed and easily the most studied phenomena in the physical universe. Among the four fundamental forces in nature—the others being the strong and weak nuclear forces—gravity and electromagnetism (which is responsible for generating light) are the trickiest to deal with. Individually, they’ve caused grief to countless scientists and theorists; and when put together, they wreak absolute havoc.

Models generally draw from these observable features of the universe to build theories and conjectures about how things work. The simpler ones proposed that the universe was made up of three dimensions: length, width, and depth. This is especially easy to grasp since it’s how we perceive the world; it’s intuitive and entirely logical.

Illustration of gravity leaking from space-time
Illustration of gravity leaking from space-time “branes” into the hyperdimensional “bulk.”

But this neat, trinary division of the universe doesn’t exactly sum up how we experience it. To build on this, some mathematicians—notably Hermann Minkowski—combined the three spatial dimensions with a fourth, temporal dimension, to construct a space-time description of reality.

This is where things start to become knotty. There are embarrassing discrepancies and inconsistencies that just don’t seem to tally. For instance, why does gravity operate on such a massive scale—planets, stars, galaxies—whereas the other forces act upon such a tiny scale? Or, put somewhat differently, why is gravity so much weaker than the other four fundamental forces?

In an interesting essay for PBS, Paul Halpern illustrates the problem using a simple example: “Pick up a steel thumbtack with a tiny kitchen magnet, and see how its attraction overwhelms the gravitational pull of the entire earth.”

So a number of theories were evolved to attempt to compensate for these discrepancies. Building on the work of Theodor Kaluza and Oskar Klein in the 1920s, superstring theories advanced the idea that the vibrations of minuscule energy strings were responsible for all that we observe in nature; these theories only worked, however, in a universe comprising ten or more dimensions, with the extra six or so all “compactified” into a tiny space beyond the limits of ready observation.

Another approach (M-theory) subsumed this 10-dimensional universe, composed of strings and energetic membranes, within a large, potentially observable extra dimension called the “bulk.” In this notion, matter and energy and most of the fundamental forces cling timidly to the energetic space-time membranes, or “branes;” gravity, however, is something of a free agent, operating alike on the branes and within the hyperdimensional bulk. For this reasons, gravitons—the carriers of gravitational energy—can bleed off in to the bulk, diminishing the small-scale strength of gravity but still allowing it to exert undue power over large distances.

A Large Hadron Collision of Ideas

Enter the Large Hadron Collider. The machine, based in Geneva, Switzerland, just might hold the answer to the dimensional puzzle. Capable of running extremely high-energy particle collisions, experts are able to construct specialized experiments which might, in turn, yield data that point to theories that actually hold water.

Right now, scientists are looking for three specific occurrences to prove that higher dimensions exist: the presence of massive particle traces, sort of like reverberating echoes; missing energy caused by gravitons migrating to higher dimensions; and microscopic black holes.

Ongoing experiments will explore these possibilities, just as scientists are hotly pursuing an elusive theory that unifies all the laws of the universe. If the volume of discoveries in recent years is an indicator, then we just might be closer than we ever thought.


Controversial New Theory That Says There’s No Gravity or Dark Matter Passes Its First Test.

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Dutch theoretical physicist Erik Verlinde has been shaking up the physics world with his controversial theory of “emergent gravity”, which sees gravity not as a fundamental force but rather as a force that comes into existence as a result of microscopic changes in the spacetime’s structure. Verlinde came out with this theory in 2010, taking on the laws of Newton and calling gravity “an illusion”. In 2016, his follow-up paper argued that there is also no mysterious “dark matter” in existence, which is supposed to (along with dark energy) make up to 95% of the universe but has so far not been detected.

Now a team of Dutch astronomers, led by Margot Brouwer from Leiden Observatory, has tested one aspect of Verlinde’s theory and found that it actually worked!

Brouwer’s team relied on the effect of “gravitational lensing” to test Verlinde’s prediction of gravity distribution around 33,000+ galaxies. Planets closer to Earth tend to bend light that’s coming from planets farther away, thus creating a lens effect. This can be used to establish a galaxy’s mass.

Normally, at distances that are up to a hundred times the radius of the galaxy, Einstein’s theory of gravity actually doesn’t account for the strength of the force of gravity. The existence of the hypothetical dark matter is invoked to make the numbers work. But Verlinde’s theory actually predicts how much gravity there would be without relying on dark matter, using only the mass of the visible matter.

test of Verlinde's theory

Measuring the distribution of gravity using gravitational lensing.  

Brouwer used Verlinde’s theory to calculate a prediction for the gravity of 33,613 galaxies and found that it compares well with the numbers from the measurements via gravitational lensing. The scientist cautions, however, that dark matter could still be an explanation for the additional gravitational force but as a free, unobserved parameter. The trouble with “free parameters” is that they can be tweaked to adjust for differences between observations and hypotheses.

“The dark matter model actually fits slightly better with the data than Verlinde’s prediction,” Brouwer explained to the New Scientist. “But then if you mathematically factor in the fact that Verlinde’s prediction doesn’t have any free parameters, whereas the dark matter prediction does, then you find Verlinde’s model is actually performing slightly better.

As this test only looks at the validity of Verlinde’s theory in a very specific situation, more work needs to be done to prove its worth more broadly.

“The question now is how the theory develops, and how it can be further tested. But the result of this first test definitely looks interesting, “ said Brouwer.

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Verlinde’s new theory of gravity passes first test

Verlindes new theory of gravity passes first test
The gravity of galaxies bends space, such that the light traveling through this space is bent. This bending of light allows astronomers to measure the distribution of gravity around galaxies, even up to distances a hundred times larger than the galaxy itself.

A team led by astronomer Margot Brouwer (Leiden Observatory, The Netherlands) has tested the new theory of theoretical physicist Erik Verlinde (University of Amsterdam) for the first time through the lensing effect of gravity. Brouwer and her team measured the distribution of gravity around more than 33,000 galaxies to put Verlinde’s prediction to the test. She concludes that Verlinde’s theory agrees well with the measured gravity distribution. The results have been accepted for publication in the British journal Monthly Notices of the Royal Astronomical Society.

The gravity of galaxies bends space, such that the light traveling through this space is bent, as through a lens. Background galaxies that are situated far behind a foreground galaxy (the lens), thereby seem slightly distorted. This effect can be measured in order to determine the distribution of gravity around a foreground-galaxy. Astronomers have measured, however, that at distances up to a hundred times the radius of the galaxy, the force of gravity is much stronger than Einstein’s of gravity predicts. The existing theory only works when invisible particles, the so-called dark matter, are added.

Verlinde now claims that he not only explains the mechanism behind gravity with his alternative to Einstein’s theory, but also the origin of the mysterious extra gravity, which astronomers currently attribute to dark matter. Verlinde’s new theory predicts how much gravity there must be, based only on the mass of the .

Brouwer calculated Verlinde’s prediction for the gravity of 33,613 galaxies, based only on their visible mass. She compared this prediction to the distribution of gravity measured by gravitational lensing, in order to test Verlinde’s theory. Her conclusion is that his prediction agrees well with the observed distribution, but she emphasizes that dark matter could also explain the extra gravitational force. However, the mass of the dark matter is a free parameter, which must be adjusted to the observation. Verlinde’s theory provides a direct , without free parameters.

The new theory is currently only applicable to isolated, spherical and static systems, while the universe is dynamic and complex. Many observations cannot yet be explained by the new theory, so is still in the race. Brouwer: “The question now is how the theory develops, and how it can be further tested. But the result of this first test definitely looks interesting.”

Remarkable New Theory Says There’s No Gravity, No Dark Matter, and Einstein Was Wrong.

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Gravity is something all of us are familiar with from our first childhood experiences. You drop something – it falls. And the way physicists have described gravity has also been pretty consistent – it’s considered one of the four main forces or “interactions” of nature and how it works has been described by Albert Einstein’s general theory of relativity all the way back in 1915.

But Professor Erik Verlinde, an expert in string theory from the University of Amsterdam and the Delta Institute of Theoretical Physics, thinks that gravity is not a fundamental force of nature because it’s not always there. Instead it’s “emergent” – coming into existence from changes in microscopic bits of information in the structure of spacetime.

Verlinde first articulated this groundbreaking theory in his 2010 paper, which took on the laws of Newton and argued that gravity is “an entropic force caused by changes in the information associated with the positions of material bodies”.  He famously stated then that “gravity is an illusion,” elaborating further that:

“Well, of course gravity is not an illusion in the sense that we know that things fall. Most people, certainly in physics, think we can describe gravity perfectly adequately using Einstein’s General Relativity. But it now seems that we can also start from a microscopic formulation where there is no gravity to begin with, but you can derive it. This is called ‘emergence’.”

What’s more, the Dutch professor now published an elaboration of his previous work in “Emergent Gravity and the Dark Universe”, which argues there’s no “dark matter” – a mysterious kind of matter that along with dark energy theoretically makes up 95% of the universe, but has not really been discovered yet. Dark matter alone is thought to account for nearly 27% of the universe’s mass-energy.

There has undoubtedly been something scientifically disconcerting about giving so much significance to a force that’s never been detected directly. It’s existence has only been inferred through gravitational effects. Interestingly, it’s existence has been first suggested by another Dutch scientist – the astronomer Jacobus Kapteyn in 1922.

One way the existence of dark matter was used was to explain why stars in outer regions of space seem to rotate faster around the center of their galaxy than theory suggested. What Verlinde proposes is that gravity just works differently from how we previously understood it, and creating the concept of dark matter is irrelevant. He is able to predict the velocity of outer-rim stars and their “excess gravity” within his new theory.

“We have evidence that this new view of gravity actually agrees with the observations,” said Verlinde. “At large scales, it seems, gravity just doesn’t behave the way Einstein’s theory predicts.”

This aspect of Verlinde’s theory was actually tested recently with success by a team of Dutch scientists.

One great outcome of Verlinde’s work is that it pushes us further towards reconciling quantum physics with general relativity.

“Many theoretical physicists like me are working on a revision of the theory, and some major advancements have been made. We might be standing on the brink of a new scientific revolution that will radically change our views on the very nature of space, time and gravity, “ explained Verlinde.

Entanglement: Gravity’s long-distance connection

When Albert Einstein scoffed at a “spooky” long-distance connection between particles, he wasn’t thinking about his general theory of relativity.

Einstein’s century-old theory describes how gravity emerges when massive objects warp the fabric of space and time. Quantum entanglement, the spooky source of Einstein’s dismay, typically concerns tiny particles that contribute insignificantly to gravity. A speck of dust depresses a mattress more than a subatomic particle distorts space.

Yet theoretical physicist Mark Van Raamsdonk suspects that entanglement and spacetime are actually linked. In 2009, he calculated that space without entanglement couldn’t hold itself together. He wrote a paper asserting that quantum entanglement is the needle that stitches together the cosmic spacetime tapestry.

Multiple journals rejected his paper. But in the years since that initial skepticism, investigating the idea that entanglement shapes spacetime has become one of the hottest trends in physics. “Everything points in a really compelling way to space being emergent from deep underlying physics that has to do with entanglement,” says John Preskill, a theoretical physicist at Caltech.

In 2012, another provocative paper presented a paradox about entangled particles inside and outside a black hole. Less than a year later, two experts in the field proposed a radical resolution: Those entangled particles are connected by wormholes — spacetime tunnels imagined by Einstein that nowadays appear as often in sci-fi novels as in physics journals. If that proposal is correct, then entanglement isn’t the spooky long-distance link that Einstein thought it was — it’s an actual bridge linking distant points in space.

Many researchers find these ideas irresistible. Within the last few years, physicists in seemingly unrelated specialties have converged on this confluence of entanglement, space and wormholes. Scientists who once focused on building error-resistant quantum computers are now pondering whether the universe itself is a vast quantum computer that safely encodes spacetime in an elaborate web of entanglement. “It’s amazing how things have been progressing,” says Van Raamsdonk, of the University of British Columbia in Vancouver.

Physicists have high hopes for where this entanglement-spacetime connection will lead them. General relativity brilliantly describes how spacetime works; this new research may reveal where spacetime comes from and what it looks like at the small scales governed by quantum mechanics. Entanglement could be the secret ingredient that unifies these supposedly incompatible views into a theory of quantum gravity, enabling physicists to understand conditions inside black holes and in the very first moments after the Big Bang.

Holograms and soup cans

Van Raamsdonk’s 2009 insight didn’t materialize out of thin air. It’s rooted in the math of the holographic principle, the idea that the boundary enclosing a volume of space can contain all the information about what’s inside. If the holographic principle applied to everyday life, then a nosy employee could perfectly reconstruct the inside of a coworker’s office cubicle — piles of papers, family photos, dust bunnies in the corner, even files on the computer’s hard drive — just by looking at the cubicle’s outer walls. It’s a counterintuitive idea, considering walls have two dimensions and a cubicle’s interior has three. But in 1997, Juan Maldacena, a string theorist then at Harvard, perceived an intriguing example of what the holographic principle could reveal about the universe (SN: 11/17/07, p. 315).

He started with anti-de Sitter space, which resembles the universe’s gravity-dominated spacetime but also has some quirky attributes. It is curved in such a way that a flash of light emitted at any location eventually returns to where it started. And while the universe is expanding, anti-de Sitter space neither stretches nor contracts. Because of these features, a chunk of anti-de Sitter spacetime with four dimensions (three spatial, one time) can be surrounded by a three-dimensional boundary.

Maldacena considered a cylinder of anti-de Sitter spacetime. Each horizontal slice of the cylinder represented the state of its space at a given moment, while the cylinder’s vertical dimension represented time. Maldacena surrounded his cylinder with a boundary for the hologram; if the anti-de Sitter space were a can of soup and its contents, then the boundary was the label.

Just as nobody would mistake a Campbell’s label for the actual soup, the boundary seemingly shared nothing in common with the cylinder’s interior. The boundary “label,” for instance, observed the rules of quantum mechanics, with no gravity. Yet gravity described the space inside containing the “soup.” Maldacena showed, though, that the label and the soup were one and the same; the quantum interactions on the boundary perfectly described the anti-de Sitter space it enclosed. “They are two theories that seem completely different but describe exactly the same thing,” Preskill says.

Maldacena added entanglement to the holographic equation in 2001. He considered the space within two soup cans, each containing a black hole. Then he created the equivalent of a tin can telephone by connecting the black holes with a wormhole — a tunnel through spacetime first proposed by Einstein and Nathan Rosen in 1935. Maldacena looked for a way to create the equivalent of that spacetime connection on the cans’ labels. The trick, he realized, was entanglement.

Like a wormhole, quantum entanglement links entities that share no obvious relationship. The quantum world is a fuzzy place: An electron can seemingly be spinning up and down simultaneously, a state called superposition, until a measurement provides a definitive answer. But if two electrons are entangled, then measuring the spin of one enables an experimenter to know what the spin of the other will be — even though the partner electron is still in a superposition state. This quantum link remains if the electrons are separated by meters, kilometers or light-years.

QUANTUM SKEPTICS A New York Times article on May 4, 1935, highlighted Einstein’s concerns about quantum mechanics, especially its feature now known as entanglement. Today physicists are exploring links between entanglement and Einstein’s general theory of relativity.


Maldacena demonstrated that by entangling particles on one can’s label with particles on the other, he could perfectly describe the wormhole connection between the cans in the language of quantum mechanics. In the context of the holographic principle, entanglement is equivalent to physically tying chunks of spacetime together.

Inspired by this entanglement-spacetime link, Van Raamsdonk wondered just how large a role entanglement might play in shaping spacetime. He considered the blandest quantum soup-can label he could think of: a blank one, which corresponded to an empty disk of anti-de Sitter space. But he knew that because of quantum mechanics, empty space is never truly empty. It is filled with pairs of particles that blink in and out of existence. And those fleeting particles are entangled.

So Van Raamsdonk drew an imaginary line bisecting his holographic label and then mathematically severed the quantum entanglement between particles on one half of the label and those on the other. He discovered that the corresponding disk of anti-de Sitter space started to split in half. It was as if the entangled particles were hooks that kept the canvas of space and time in place; without them, spacetime pulled itself apart. As Van Raamsdonk decreased the degree of entanglement, the portion connecting the diverging regions of space got thinner, like the rubbery thread that narrows as a chewed wad of gum is pulled apart. “It led me to suggest that the origin of having space at all is having this entanglement,” he says.

That was a bold claim, and it took a while for Van Raamsdonk’s paper, published in General Relativity and Gravitation in 2010, to garner serious attention. The spark came in 2012, when four physicists at the University of California, Santa Barbara wrote a paper challenging conventional wisdom about the event horizon, a black hole’s point of no return.

Insight behind a firewall

In the 1970s, theoretical physicist Stephen Hawking showed that pairs of entangled particles — the same kinds Van Raamsdonk later analyzed on his quantum boundary — can get split up at the event horizon. One falls into the black hole, and the other escapes as what’s known as Hawking radiation. The process gradually saps the mass of a black hole, ultimately leading to its demise. But if black holes disappear, then so would the record of everything that ever fell inside. Quantum theory maintains that information cannot be destroyed.

By the 1990s several theoretical physicists, including Stanford’s Leonard Susskind, had proposedresolutions of the issue. Sure, they said, matter and energy fall into a black hole. But from the perspective of an outside observer, that stuff never quite makes it past the event horizon; it seemingly teeters on the edge. As a result, the event horizon becomes a holographic boundary containing all the information about the space inside the black hole. Eventually, as the black hole shrivels away, that information will leak out as Hawking radiation. In principle, the observer could collect the radiation and piece together information about the black hole’s interior.

In their 2012 paper, Santa Barbara physicists Ahmed Almheiri, Donald Marolf, James Sully and Joseph Polchinski claimed something was wrong with that picture. For an observer to assemble the puzzle of what’s inside a black hole, they noted, all the individual puzzle pieces — the particles of Hawking radiation— would have to be entangled with each other. But each Hawking particle also has to be entangled with its original partner that fell into the black hole.

Unfortunately, there is not enough entanglement to go around. Quantum theory dictates that the entanglement required to link all the particles outside the black hole precludes those particles from also linking up with particles inside the black hole. Compounding the problem, the physicists found that severing one of those entanglements would create an impenetrable wall of energy, called a firewall, at the event horizon (SN: 5/31/14, p. 16).

Many physicists doubted that black holes actually vaporize everything trying to enter. But the mere possibility that firewalls exist had disturbing implications. Previously, physicists had wondered what the space inside a black hole looked like. Now they weren’t sure whether black holes even had an inside. “It was kind of humbling,” Preskill says.

Susskind was not so much humbled as restless. He had spent years trying to show that information wasn’t lost inside a black hole; now he was just as convinced that the firewall idea was wrong, but he couldn’t prove it. Then one day he received a cryptic email from Maldacena: “It had very little in it,” Susskind says, “except for ER = EPR.” Maldacena, now at the Institute for Advanced Study in Princeton, N.J., had thought back to his 2001 paper on interconnected soup cans and wondered whether wormholes could resolve the entanglement mess raised by the firewall problem. Susskind quickly jumped on the idea.

In a paper in the German journal Fortschritte der Physik in 2013, Maldacena and Susskind argued that a wormhole — technically, an Einstein-Rosen bridge, or ER — is the spacetime equivalent of quantum entanglement. (EPR stands for Einstein, Boris Podolsky and Rosen, authors of the 1935 paper that belittled entanglement.) That means that every particle of Hawking radiation, no matter how far away it is from where it started, is directly connected to a black hole’s interior via a shortcut through spacetime. “Through the wormhole, the distant stuff is not so distant,” Susskind says.

Susskind and Maldacena envisioned gathering up all the Hawking particles and smushing them together until they collapse into a black hole. That black hole would be entangled, and thus connected via wormhole, with the original black hole. That trick transformed a confusing mess of Hawking particles — paradoxically entangled with both a black hole and each other — into two black holes connected by a wormhole. Entanglement overload is averted, and the firewall problem goes away.

Not everyone has jumped aboard the ER = EPR bandwagon. Susskind and Maldacena admit they have more work to do to prove the equivalence of wormholes and entanglement. But after pondering the implications of the firewall paradox, many physicists agree that the spacetime inside a black hole owes its existence to entanglement with radiation outside. That’s a major insight, Preskill says, because it also implies that the entire universe’s spacetime fabric, including the patch on which we reside, is a product of quantum spookiness.

Cosmic computer

It’s one thing to say the universe constructs spacetime through entanglement; it’s another to show how the universe does it. The trickier of those assignments has fallen on Preskill and colleagues, who have come to view the cosmos as a colossal quantum computer. For two decades scientists have worked on building quantum computers that use information encoded in entangled entities, such as photons or tiny circuits, to solve problems intractable on traditional computers, such as factoring large numbers. Preskill’s team is using knowledge gained in that effort to predict how particular features inside a soup can would be depicted on the entanglement-filled label.

Quantum computers work by exploiting components that are in superposition states as data carriers — they can essentially be 0s and 1s at the same time. But superposition states are very fragile. Too much heat, for example, can destroy the state and all the quantum information it carries. These information losses, which Preskill compares to having pages torn out of a book, seem inevitable.

But physicists responded by creating a protocol called quantum error correction. Instead of relying on one particle to store a quantum bit, scientists spread the data among multiple entangled particles. A book written in the language of quantum error correction would be full of gibberish, Preskill says, but its entire contents could be reconstructed even if half the pages were missing.

Quantum error correction has attracted a lot of attention in recent years, but now Preskill and his colleagues suspect that nature came up with it first. In the June Journal of High Energy Physics, Preskill’s team showed how the entanglement of multiple particles on a holographic boundary perfectly describes a single particle being pulled by gravity within a chunk of anti-de Sitter space. Maldacena says this insight could lead to a better understanding of how a hologram encodes all the details about the spacetime it surrounds.

Physicists admit that their approximations have a long way to go to match reality. While anti-de Sitter space offers physicists the advantage of working with a well-defined boundary, the universe doesn’t have a straightforward soup-can label. The spacetime fabric of the cosmos has been expanding since the Big Bang and continues to do so at an increasing clip. If you shoot a pulse of light into space, it won’t turn around and come back; it will just keep going. “It is not clear how to define a holographic theory for our universe,” Maldacena wrote in 2005. “There is no convenient place to put the hologram.”

Yet as crazy as holograms, soup cans and wormholes sound, they seem to be promising lenses in the search for a way to meld quantum spookiness with spacetime geometry. In their paper on wormholes, Einstein and Rosen discussed possible quantum implications but didn’t make a connection to their earlier entanglement paper. Today that link may help reconcile quantum mechanics and general relativity in a theory of quantum gravity. Armed with such a theory, physicists could dig into mysteries such as the state of the infant universe, when matter and energy were packed into an infinitesimally small space. “We don’t really know the answers yet by any means,” Preskill says. “But we’re excited to find a new way of looking at things.”


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Alfonso Cuarón must have felt pretty certain that nobody would be coming to Gravity for the script. Though his reputation as a writer sings of creativity and deviation from the typical Hollywood fodder, every beat in his surprisingly linear outer space film feels not so much like an exploration of a fascinating story, but more like a means to transport an audience (that’s us) to the next harrowing explosion of IMAX technology. On the surface, this probably sounds cheap — you signed up for a movie, not a roller coaster. But if it is the principal purpose of any movie to offer its audience an emotional experience, then Gravity is an unquestionable triumph.

In fact, it should say a great deal that the moreover “typical” narrative that throughlines this movie doesn’t undercut the experience. Through the film’s dazzling effects and a profoundly immersive directorial style, Gravity gives us something that feels altogether new.Sandra Bullock’s new-to-space scientist Dr. Ryan Stone doesn’t break the mold on action-adventure heroes of either gender, but you’ll be adhered desperately to her every move thanks to the veritable space simulator that Gravity really is.

It’s far more than just the benefits of IMAX technology that keep us feeling like we’re inches from life-threatening danger at all times. It is Cuarón’s flare for the construction of genuine tension. We open on a painfully slow climb up a mountain of dread, with a nauseated Stone struggling to repair a faction of the ship while a pseudo-nihilistic astronaut Matt Kowalski (George Clooney, who can be paid credit for all of this film’s moments of comic relief) jet-packs around her recounting stories of Marti Gras and romantic infidelity. All the while, aimless conversation and pleasant radio melodies notwithstanding, our chests grow heavier with anticipation of what is about to follow this mammoth single take. Disaster.

Gravity© 2013 Warner Bros. Entertainment Inc.

And once it hits, we’re gone. Drowning, treading for dear life for the hour and change to follow, thrown a leaky life preserver on occasion when Stone (our consierge through this unforgiving nightmare) manages some semblance of momentary sanctuary from the insatiable abyss all around her. Our anxiety never dips below “barely sustainable” as Stone efforts to lay waist to probability and fight her way back to safety. At no point in the entire real-time adventure do we feel liberated from Stone’s danger. The magic of this movie makes us feel everything that she does, without allowing for even a second of comfort to be drawn from the fact that we, and Bullock, are in no real harm.

To reiterate, it is nearly miraculous that we can’t, even if and when we really want to, grip at the refuge of the “it’s just a movie” mentality, especially in the face of a plotline you might find occupied by a Ron Howard epic. No, we’re far too deep by the time the danger strikes to conceive of a world beyond the one Cuarón forces upon us. He’s strategic generous in his inclusion of Clooney’s loquacious playboy: without a few trembling smiles, we might succumb to full-on nervous breakdown. But Cuarón peppers the pleasantries in just seldom enough to keep the titular sentiment so painfully alive.

Gravity is the sort of movie that demands as big a screen and as focused a pair of IMAX-framed eyes as possible. It doesn’t offer much dramatic surprise — in fact, we’re prepared for just about every big turn — but the shocks, the screams, the moments that make you cower and whimper and hope to dear God that Stone is going to be okay are plentiful. Beyond plentiful, in fact. They’re the whole way through. So a great story, it might not be, but in its achievement of this degree of emotional immersion, Gravity is an unbelievable piece of work.


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