Brian Swingle was a graduate student studying the physics of matter at the Massachusetts Institute of Technology when he decided to take a few classes in string theory to round out his education—“because, why not?” he recalled—although he initially paid little heed to the concepts he encountered in those classes. But as he delved deeper, he began to see unexpected similarities between his own work, in which he used so-called tensor networks to predict the properties of exotic materials, and string theory’s approach to black-hole physics and quantum gravity. “I realized there was something profound going on,” he said.

PrintOriginal story reprinted with permission from Quanta Magazine, an editorially independent division of SimonsFoundation.org whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.

Tensors crop up all over physics—they’re simply mathematical objects that can represent multiple numbers at the same time. For example, a velocity vector is a simple tensor: It captures values for both the speed and the direction of motion. More complicated tensors, linked together into networks, can be used to simplify calculations for complex systems made of many different interacting parts—including the intricate interactions of the vast numbers of subatomic particles that make up matter.

Swingle is one of a growing number of physicists who see the value in adapting tensor networks to cosmology. Among other benefits, it could help resolve an ongoing debate about the nature of space-time itself. According to John Preskill, the Richard P. Feynman professor of theoretical physics at the California Institute of Technology in Pasadena, many physicists have suspected a deep connection between quantum entanglement—the “spooky action at a distance” that so vexed Albert Einstein—and space-time geometry at the smallest scales since the physicist John Wheeler first described the latter as a bubbly, frothy foam six decades ago. “If you probe geometry at scales comparable to the Planck scale” — the shortest possible distance—“it looks less and less like space-time,” said Preskill. “It’s not really geometry anymore. It’s something else, an emergent thing [that arises] from something more fundamental.”

Physicists continue to wrestle with the knotty problem of what this more fundamental picture might be, but they strongly suspect that it is related to quantum information. “When we talk about information being encoded, [we mean that] we can split a system into parts, and there is some correlation among the parts so I can learn something about one part by observing another part,” said Preskill. This is the essence of entanglement.

It is common to speak of a “fabric” of space-time, a metaphor that evokes the concept of weaving individual threads together to form a smooth, continuous whole. That thread is fundamentally quantum. “Entanglement is the fabric of space-time,” said Swingle, who is now a researcher at Stanford University. “It’s the thread that binds the system together, that makes the collective properties different from the individual properties. But to really see the interesting collective behavior, you need to understand how that entanglement is distributed.”

Tensor networks provide a mathematical tool capable of doing just that. In this view, space-time arises out of a series of interlinked nodes in a complex network, with individual morsels of quantum information fitted together like Legos. Entanglement is the glue that holds the network together. If we want to understand space-time, we must first think geometrically about entanglement, since that is how information is encoded between the immense number of interacting nodes in the system.

Many Bodies, One Network

It is no easy feat to model a complex quantum system; even doing so for a classical system with more than two interacting parts poses a challenge. When Isaac Newton published his Principia in 1687, one of the many topics he examined became known as the “three-body problem.” It is a relatively simple matter to calculate the movement of two objects, such as the Earth and the sun, taking into account the effects of their mutual gravitational attraction. However, adding a third body, like the moon, turns a relatively straightforward problem with an exact solution into one that is inherently chaotic, where long-term predictions require powerful computers to simulate an approximation of the system’s evolution. In general, the more objects in the system, the more difficult the calculation, and that difficulty increases linearly, or nearly so—at least in classical physics.

Now imagine a quantum system with many billions of atoms, all of which interact with each other according to complicated quantum equations. At that scale, the difficulty appears to increase exponentially with the number of particles in the system, so a brute-force approach to calculation just won’t work.

Consider a lump of gold. It is comprised of many billions of atoms, all of which interact with one another. From those interactions emerge the various classical properties of the metal, such as color, strength or conductivity. “Atoms are tiny little quantum mechanical things, and you put atoms together and new and wonderful things happen,” said Swingle. But at this scale, the rules of quantum mechanics apply. Physicists need to precisely calculate the wave function of that lump of gold, which describes the state of the system. And that wave function is a many-headed hydra of exponential complexity.

Even if your lump of gold has just 100 atoms, each with a quantum “spin” that can be either up or down, the total number of possible states totals 2100, or a million trillion trillion. With every added atom the problem grows exponentially worse. (And worse still if you care to describe anything in addition to the atomic spins, which any realistic model would.) “If you take the entire visible universe and fill it up with our best storage material, the best hard drive money can buy, you could only store the state of about 300 spins,” said Swingle. “So this information is there, but it’s not all physical. No one has ever measured all these numbers.”

Tensor networks enable physicists to compress all the information contained within the wave function and focus on just those properties physicists can measure in experiments: how much a given material bends light, for example, or how much it absorbs sound, or how well it conducts electricity. A tensor is a “black box” of sorts that takes in one collection of numbers and spits out a different one. So it is possible to plug in a simple wave function — such as that of many non-interacting electrons, each in its lowest-energy state — and run tensors upon the system over and over, until the process produces a wave function for a large, complicated system, like the billions of interacting atoms in a lump of gold. The result is a straightforward diagram that represents this complicated lump of gold, an innovation much like the development of Feynman diagrams in the mid-20th century, which simplified how physicists represent particle interactions. A tensor network has a geometry, just like space-time.

The key to achieving this simplification is a principle called “locality.” Any given electron only interacts with its nearest neighboring electrons. Entangling each of many electrons with its neighbors produces a series of “nodes” in the network. Those nodes are the tensors, and entanglement links them together. All those interconnected nodes make up the network. A complex calculation thus becomes easier to visualize. Sometimes it even reduces to a much simpler counting problem.

There are many different types of tensor networks, but among the most useful is the one known by the acronym MERA (multiscale entanglement renormalization ansatz). Here’s how it works in principle: Imagine a one-dimensional line of electrons. Replace the eight individual electrons — designated A, B, C, D, E, F, G and H — with fundamental units of quantum information (qubits), and entangle them with their nearest neighbors to form links. A entangles with B, C entangles with D, E entangles with F, and G entangles with H. This produces a higher level in the network. Now entangle AB with CD, and EF with GH, to get the next level in the network. Finally, ABCD entangles with EFGH to form the highest layer. “In a way, we could say that one uses entanglement to build up the many-body wave function,” Román Orús, a physicist at Johannes Gutenberg University in Germany, observed in a paper last year.

Why are some physicists so excited about the potential for tensor networks—especially MERA—to illuminate a path to quantum gravity? Because the networks demonstrate how a single geometric structure can emerge from complicated interactions between many objects. And Swingle (among others) hopes to make use of this emergent geometry by showing how it can explain the mechanism by which a smooth, continuous space-time can emerge from discrete bits of quantum information.

Space-Time’s Boundaries

Condensed-matter physicists inadvertently found an emergent extra dimension when they developed tensor networks: the technique yields a two-dimensional system out of one dimension. Meanwhile, gravity theorists were subtracting a dimension—going from three to two—with the development of what’s known as the holographic principle. The two concepts might connect to form a more sophisticated understanding of space-time.

In the 1970s, a physicist named Jacob Bekenstein showed that the information about a black hole’s interior is encoded in its two-dimensional surface area (the “boundary”) rather than within its three-dimensional volume (the “bulk”). Twenty years later, Leonard Susskind and Gerard ’t Hooft extended this notion to the entire universe, likening it to a hologram: Our three-dimensional universe in all its glory emerges from a two-dimensional “source code.” In 1997, Juan Maldacena found a concrete example of holography in action, demonstrating that a toy model describing a flat space without gravity is equivalent to a description of a saddle-shaped space with gravity. This connection is what physicists call a “duality.”

Mark Van Raamsdonk imagines entanglement creating space-time gradually: Along the outside of the figure, individual particles (dots) become entangled with each other. These entangled pairs then become entangled with other pairs. As more particles become entangled, the three-dimensional structure of space-time emerges.Click to Open Overlay Gallery

Mark Van Raamsdonk imagines entanglement creating space-time gradually: Along the outside of the figure, individual particles (dots) become entangled with each other. These entangled pairs then become entangled with other pairs. As more particles become entangled, the three-dimensional structure of space-time emerges. OLENA SHMAHALO/QUANTA MAGAZINE

Mark Van Raamsdonk, a string theorist at the University of British Columbia in Vancouver, likens the holographic concept to a two-dimensional computer chip that contains the code for creating the three-dimensional virtual world of a video game. We live within that 3-D game space. In one sense, our space is illusory, an ephemeral image projected into thin air. But as Van Raamsdonk emphasizes, “There’s still an actual physical thing in your computer that stores all the information.”

The idea has gained broad acceptance among theoretical physicists, but they still grapple with the problem of precisely how a lower dimension would store information about the geometry of space-time. The sticking point is that our metaphorical memory chip has to be a kind of quantum computer, where the traditional zeros and ones used to encode information are replaced with qubits capable of being zeros, ones and everything in between simultaneously. Those qubits must be connected via entanglement — whereby the state of one qubit is determined by the state of its neighbor — before any realistic 3-D world can be encoded.

Similarly, entanglement seems to be fundamental to the existence of space-time. This was the conclusion reached by a pair of postdocs in 2006: Shinsei Ryu (now at the University of Illinois, Urbana-Champaign) and Tadashi Takayanagi (now at Kyoto University), who shared the 2015 New Horizons in Physics prize for this work. “The idea was that the way that [the geometry of] space-time is encoded has a lot to do with how the different parts of this memory chip are entangled with each other,” Van Raamsdonk explained.

Inspired by their work, as well as by a subsequent paper of Maldacena’s, in 2010 Van Raamsdonk proposed a thought experiment to demonstrate the critical role of entanglement in the formation of space-time, pondering what would happen if one cut the memory chip in two and then removed the entanglement between qubits in opposite halves. He found that space-time begins to tear itself apart, in much the same way that stretching a wad of gum by both ends yields a pinched-looking point in the center as the two halves move farther apart. Continuing to split that memory chip into smaller and smaller pieces unravels space-time until only tiny individual fragments remain that have no connection to one another. “If you take away the entanglement, your space-time just falls apart,” said Van Raamsdonk. Similarly, “if you wanted to build up a space-time, you’d want to start entangling [qubits] together in particular ways.”

Combine those insights with Swingle’s work connecting the entangled structure of space-time and the holographic principle to tensor networks, and another crucial piece of the puzzle snaps into place. Curved space-times emerge quite naturally from entanglement in tensor networks via holography. “Space-time is a geometrical representation of this quantum information,” said Van Raamsdonk.

And what does that geometry look like? In the case of Maldacena’s saddle-shaped space-time, it looks like one of M.C. Escher’s Circle Limit figures from the late 1950s and early 1960s. Escher had long been interested in order and symmetry, incorporating those mathematical concepts into his art ever since 1936 when he visited the Alhambra in Spain, where he found inspiration in the repeating tiling patterns typical of Moorish architecture, known as tessellation.

His Circle Limit woodcuts are illustrations of hyperbolic geometries: negatively curved spaces represented in two dimensions as a distorted disk, much the way flattening a globe into a two-dimensional map of the Earth distorts the continents. For instance, Circle Limit IV (Heaven and Hell) features many repeating figures of angels and demons. In a true hyperbolic space, all the figures would be the same size, but in Escher’s two-dimensional representation, those near the edge appear smaller and more pinched than the figures in the center. A diagram of a tensor network also bears a striking resemblance to the Circle Limit series, a visual manifestation of the deep connection Swingle noticed when he took that fateful string theory class.

To date, tensor analysis has been limited to models of space-time, like Maldacena’s, that don’t describe the universe we inhabit—a non-saddle-shaped universe whose expansion is accelerating. Physicists can only translate between dual models in a few special cases. Ideally, they would like to have a universal dictionary. And they would like to be able to derive that dictionary directly, rather than make close approximations. “We’re in a funny situation with these dualities, because everyone seems to agree that it’s important, but nobody knows how to derive them,” said Preskill. “Maybe the tensor-network approach will make it possible to go further. I think it would be a sign of progress if we can say — even with just a toy model—‘Aha! Here is the derivation of the dictionary!’ That would be a strong hint that we are onto something.”

Over the past year, Swingle and Van Raamsdonk have collaborated to move their respective work in this area beyond a static picture of space-time to explore its dynamics: how space-time changes over time, and how it curves in response to these changes. Thus far, they have managed to derive Einstein’s equations, specifically the equivalence principle—evidence that the dynamics of space-time, as well as its geometry, emerge from entangled qubits. It is a promising start.

“‘What is space-time?’ sounds like a completely philosophical question,” Van Raamsdonk said. “To actually have some answer to that, one that is concrete and allows you to calculate space-time, is kind of amazing.”

Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.Alex The Great Kalel @ManOfTommorrow 8m8 minutes ago

@GaryABro @liberalhamlet @TheExpanseWR Gary you should really read this, it’s fascinating http://www.wired.com/2015/05/spooky-quantum-action-might-hold-universe-together/ …

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Alex The Great Kalel @ManOfTommorrow 8m8 minutes ago

@GaryABro @liberalhamlet @TheExpanseWR the information about a black hole’s interior is encoded in its two-dimensional surface area

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Alex The Great Kalel @ManOfTommorrow 7m7 minutes ago

@GaryABro @liberalhamlet @TheExpanseWR 20yrs later, Leonard Susskind extended this notion to the entire universe, likening it to a hologram

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Alex The Great Kalel @ManOfTommorrow 7m7 minutes ago

@GaryABro @liberalhamlet @TheExpanseWR Our three-dimensional universe in all its glory emerges from a two-dimensional “source code.”

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Alex The Great Kalel @ManOfTommorrow 6m6 minutes ago

@GaryABro @liberalhamlet @TheExpanseWR flat space without gravity is equivalent to description of saddle-shaped space with gravity.

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Alex The Great Kalel @ManOfTommorrow 5m5 minutes ago

@GaryABro @liberalhamlet @TheExpanseWR This connection is what physicists call a “duality.”

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Alex The Great Kalel @ManOfTommorrow 4m4 minutes ago

@GaryABro @liberalhamlet @TheExpanseWR They talk about how a lower dimension stores all the universe’s information via entanglement

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Alex The Great Kalel @ManOfTommorrow 2m2 minutes ago

@GaryABro @liberalhamlet @TheExpanseWR If we ever get between two distant points in space quickly entanglement is how it will be done

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Alex The Great Kalel @ManOfTommorrow 2m2 minutes ago

@GaryABro @liberalhamlet @TheExpanseWR Reading that, you can see how entanglement controls space-time’s curvature

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Alex The Great Kalel @ManOfTommorrow 1m1 minute ago

@GaryABro @liberalhamlet @TheExpanseWR Should also work as a time travel mechanism- again, if we can ever find a way to control entanglement

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Alex The Great Kalel @ManOfTommorrow 1m1 minute ago

@GaryABro @liberalhamlet @TheExpanseWR Likening the universe to a black hole also has implications for what other black holes actually are

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https://twitter.com/GaryABro/status/597383682236768256

http://sploid.gizmodo.com/nasa-reveals-new-impossible-engine-can-change-space-t-1614549987

Last year, NASA’s advanced propulsion research wing made headlines by announcing the successful test of a physics-defying electromagnetic drive, or EM drive. Now, this futuristic engine, which could in theory propel objects to near-relativistic speeds, has been shown to work inside a space-like vacuum.

NASA: New “impossible” engine works, could change space travel forever

Until yesterday, every physicist was laughing at this engine and its inventor, Roger Shawyer.…

Read more sploid.gizmodo.com

Illustration: “Dreamscape IV,” by jamajurabaev, via Deviantart

NASA Eagleworks made the announcement quite unassumingly via NASASpaceFlight.com. There’s also a major discussion going on about the engine and the physics that drives it at the site’s forum.

New Test Suggests NASA’s “Impossible” EM Drive Will Work In Space

COMSOL Magnetic Field Surface Distribution (NASA Eagleworks).

The EM drive is controversial in that it appears to violate conventional physics and the law of conservation of momentum; the engine, invented by British scientist Roger Sawyer, converts electric power to thrust without the need for any propellant by bouncing microwaves within a closed container. So, with no expulsion of propellant, there’s nothing to balance the change in the spacecraft’s momentum during acceleration. Hence the skepticism. But as stated by NASA Eagleworks scientist Harold White:

[T]he EM Drive’s thrust was due to the Quantum Vacuum (the quantum state with the lowest possible energy) behaving like propellant ions behave in a MagnetoHydroDynamics drive (a method electrifying propellant and then directing it with magnetic fields to push a spacecraft in the opposite direction) for spacecraft propulsion.

New Test Suggests NASA’s “Impossible” EM Drive Will Work In Space

The trouble with this theory, however, is that it might not work in a closed vacuum. After last year’s tests of the engine, which weren’t performed in a vacuum, skeptics argued that the measured thrust was attributable to environmental conditions external to the drive, such as natural thermal convection currents arising from microwave heating.

The recent experiment, however, addressed this concern head-on, while also demonstrating the engine’s potential to work in space. (Image: NASA Eagleworks.)

The NASASpaceflight.com group has given consideration to whether the experimental measurements of thrust force were the result of an artifact. Despite considerable effort within the NASASpaceflight.com forum to dismiss the reported thrust as an artifact, the EM Drive results have yet to be falsified.

After consistent reports of thrust measurements from EM Drive experiments in the US, UK, and China – at thrust levels several thousand times in excess of a photon rocket, and now under hard vacuum conditions – the question of where the thrust is coming from deserves serious inquiry.

Serious inquiry, indeed. It’s crucial now that these tests be analyzed, replicated, and confirmed elsewhere. A peer-review and formal paper would also seem to be in order lest we get too carried away with these results. But wow. Just wow.

Related: Don’t Get Too Excited About NASA’s New Miracle Engine

It’s still early days, but the implications are mind-boggling to say the least. A full-fledged EM drive could be used on everything from satellites working in low Earth orbit, to missions to the Moon, Mars, and the outer solar system.

New Test Suggests NASA’s “Impossible” EM Drive Will Work In Space

(Image: Mark Rademaker)

EM drives could also be used on multi-generation spaceships for interstellar travel. A journey to Alpha Centauri, which is “just” 4.3 light-years away, suddenly wouldn’t be so daunting. An EM drive working under a constant one milli-g acceleration would propel a ship to about 9.4% the speed of light, resulting in a total travel time of 92 years. But that’s without the need for deceleration; should we wish to make a stop at Alpha Centauri, we’d have to add another 38 years to the trip. Not a big deal by any extent of the imagination.

Much more at NASASpaceFlight.com.

http://io9.com/5963263/how-nasa-will-build-its-very-first-warp-drive

http://io9.com/heres-nasas-new-design-for-a-warp-drive-ship-1588948192

http://io9.com/we-should-be-able-to-detect-spaceships-moving-near-the-1693540956

http://www.wired.co.uk/news/archive/2014-07/31/nasa-validates-impossible-space-drive

http://forum.nasaspaceflight.com/index.php?topic=36313.2080

http://sploid.gizmodo.com/nasa-reveals-new-impossible-engine-can-change-space-t-1614549987

http://sploid.gizmodo.com/nasa-reveals-new-impossible-engine-can-change-space-t-1614549987

http://io9.com/are-we-ever-going-to-develop-faster-than-light-travel-1703108613

http://io9.com/stars-get-all-lopsided-and-slosh-around-before-explodin-1703081120

http://io9.com/5963263/how-nasa-will-build-its-very-first-warp-drive

http://gizmodo.com/the-future-of-energy-is-hidden-in-the-guts-of-insects-1702970203

http://gizmodo.com/more-students-refusing-to-pay-loan-debts-to-fraudulent-1702147930

http://dx.doi.org/10.1103/PhysRevLett.61.1446

http://www.cosmicyarns.com/2015/04/wormholes-galactic-subway-system_21.html

http://io9.com/can-galactic-empires-exist-without-faster-than-light-tr-1555347751

Causality actually doesn’t exist on the quantum level (there are evn examples of retrocausality), and if you view time as a dimension, you can take it a step further like Einstein did, and say that the past, present and future co-exist. Add that to the well accepted theory that the universe is actually a three dimensiona projection of the interior of the two dimensional surface of a black hole with space and time bound together by entanglement and you have a “way out.” It also implies that other universes can exist in other black holes.

https://theconversation.com/faster-than-light-travel-are-we-there-yet-41112