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How Quantum Pairs Stitch Space-Time
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The Quantum Fabric of Space-Time
How Quantum Pairs Stitch Space-Time
By
Jennifer Ouellette
April 28, 2015
New tools may reveal how quantum information builds the structure of space.
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Hannes Hummel for Quanta Magazine
Introduction
By Jennifer Ouellette
Contributing Writer
April 28, 2015
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black hole information paradox
black holes
cosmology
entanglement
general relativity
physics
quantum gravity
quantum physics
space-time
The Quantum Fabric of Space-Time
All topics
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.
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.”
Video: Brian Swingle explains what quantum entanglement and gravity have to do with each other.
Natalie Wolchover/Quanta Magazine
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Video: Brian Swingle explains what quantum entanglement and gravity have to do with each other.
Natalie Wolchover/Quanta Magazine
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...