Mar 26, 2018 06:07 PM
(This post was last modified: Mar 26, 2018 06:59 PM by C C.)
AUDIO VERSION: https://soundcloud.com/thebjps/christian...space-time
INTRO: Christian Wüthrich delivered one of the plenary talks at this summer's BSPS conference in Edinburgh and lo! It was recorded (future is now!). For your listening pleasure, here is his 'The Temporal and Atemporal Emergence of (Space-)Time' with a link to the slides below.
EXCERPT: General relativity (GR) can’t be the last word: it assumes classical matter. We need a quantum theory of gravity, theory that combines quantum effects and (strong) gravitational fields. A quantum theory of gravity will be fundamental, at least relative to currently held theories of physics. [...] Now, many approaches to formulating such a theory either presuppose or entail that fundamentally, there is neither space nor time (even though that denial comes in degrees). Spacetime according to quantum theories of gravity in general, and according to loop quantum gravity/cosmology in particular, is not fundamental.
PDF snippets: http://thebjps.typepad.com/2017mBSPS_Atemporal.pdf
Are Space and Time Fundamental?
http://www.pbs.org/wgbh/nova/blogs/physi...ndamental/
EXCERPT: [...] “There aren’t many things in quantum gravity that everyone agrees on,” says Eleanor Knox, a philosopher at King’s College London who specializes in the philosophy of physics. “Yet the one thing many people seemed to agree on in quantum gravity was that we were going to have to cope with space and time not being fundamental.”
It sounds radical, but physics has a long and proud history of spearheading exactly this kind of coup. “Historically, whenever we thought something was fundamental, it turns out that it is not,” says Nathan Seiberg, a theoretical physicist at the Institute for Advanced Study. [...]
So why are physicists picking on space? Relativity delivered the first strike. [...] More alarmingly to theorists, our ability to measure features in space is intrinsically limited. [...] “It’s not because we don’t have the budget to build a powerful enough machine,” explains Seiberg. If we somehow tried to make an infinitely small measuring device, that device would become so dense that it would warp the fabric of space. The conclusion: “Space itself is ambiguous,” says Seiberg. Strike two.
Space also took a hit from an unlikely foe: the hologram. [...] It turns out that you can write equations that describe our universe perfectly well using different combinations of spatial dimensions, creating mathematical holograms that are indistinguishable from reality. Like a book that can be translated into many disparate languages without losing a syllable of meaning, our universe seems to tell a story that is independent of the words in which we have always chosen to express it.
Finally, physicists have known for some time that their descriptions of space start to break down when they’re applied to the strange-but-true environments [...] “Something else should kick in,” says Seiberg.
But what is that something else? “I don’t think I have an answer to that,” says Seiberg. Knox also leaves the door open to as-yet-unknown possibilities: “Whatever it is that’s fundamental, it’s not the stuff we have a handle on right now.” Morever, Seiberg adds that though theorists have assembled a strong case that space is emergent, time presents a more difficult problem. “In order to understand emergent time, we need a complete revolution in the way we think about physics.”
Letting go of space and time without ready replacements may seem like a surefire way to plunge into the abyss of abstraction. But it may be only by loosening our grip that we can come to grasp what is truly fundamental.
MORE: http://www.pbs.org/wgbh/nova/blogs/physi...ndamental/
Spooky Action at a Distance' Author George Musser Talks Physics Loopholes
https://www.space.com/31066-spooky-actio...rview.html
EXCERPT: . . . The real message of the spooky action is that space and time are not fundamental. The particles are rooted in that deeper layer where space and time don’t yet exist. And that’s the way to explain them. So the building blocks of the world may not be tiny things. What could these building blocks be if they’re not tiny because that’s kind of what we think of as a building block; a Lego is smaller than the thing you build out of the Legos. They may be what seems to us enormous things. Things that span the entire universe and are somehow acting in concert with one another to produce these phenomena of space.
- - -
Musser: A New Way of Thinking About Spacetime That Turns Everything Inside Out
https://gizmodo.com/a-new-way-of-thinkin...1741498475
EXCERPT: . . . John Wheeler, the renowned gravity theorist, speculated that space-time is built out of “pregeometry,” but admitted that this was nothing more than “an idea for an idea.” Even someone as irrepressible as Arkani-Hamed has had his doubts: “These problems are very hard. They’re outside our usual language for talking about them.”
What keeps Arkani-Hamed going is that he and his colleagues have found just the sort of methods Einstein said they’d have to — ways to describe physics in the absence of space, to breathe in the vacuum. He has put these efforts into historical perspective: “For 2,000-plus years, people asked about the deep nature of space and time, but they were premature. We’ve finally arrived at the epoch where you can pose the questions and hope for some meaningful answer.”
How Quantum Pairs Stitch Space-Time
https://www.quantamagazine.org/tensor-ne...-20150428/
EXCERPT: . . . 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.
[...] 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. [...] 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.
MORE: https://www.quantamagazine.org/tensor-ne...-20150428/
- - -
Quantum gravity's time problem
https://www.quantamagazine.org/quantum-g...-20161201/
EXCERPT: . . . “I think we now understand that space-time really is just a geometrical representation of the entanglement structure of these underlying quantum systems,” said Mark Van Raamsdonk, a theoretical physicist at the University of British Columbia.
[...] the space-time fabric has a “de Sitter” geometry, stretching as you look into the distance. The fabric stretches until the universe hits a very different sort of boundary from the one in AdS space: the end of time. At that point, in an event known as “heat death,” space-time will have stretched so much that everything in it will become causally disconnected from everything else, such that no signals can ever again travel between them. The familiar notion of time breaks down. From then on, nothing happens.
On the timeless boundary of our space-time bubble, the entanglements linking together qubits (and encoding the universe’s dynamical interior) would presumably remain intact, since these quantum correlations do not require that signals be sent back and forth. But the state of the qubits must be static and timeless.
This line of reasoning suggests that somehow, just as the qubits on the boundary of AdS space give rise to an interior with one extra spatial dimension, qubits on the timeless boundary of de Sitter space must give rise to a universe with time — dynamical time, in particular. Researchers haven’t yet figured out how to do these calculations. “In de Sitter space,” Swingle said, “we don’t have a good idea for how to understand the emergence of time.”
[...] Don Page and William Wootters [...] discovered that an entangled system that is globally static can contain a subsystem that appears to evolve from the point of view of an observer within it. [...] In other words, time doesn’t exist globally, but an effective notion of time emerges for the subsystem.
A team of Italian researchers experimentally demonstrated this phenomenon in 2013. In summarizing their work, the group wrote: “We show how a static, entangled state of two photons can be seen as evolving by an observer that uses one of the two photons as a clock to gauge the time-evolution of the other photon. However, an external observer can show that the global entangled state does not evolve.”
[...] The bottom line, in Swingle’s words, is that “somehow, you can emerge time from timeless degrees of freedom using entanglement.”
- - -
Theoretical physics: The origins of space and time
https://www.nature.com/news/theoretical-...me-1.13613
EXCERPT: . . . Meanwhile, Van Raamsdonk has proposed a very different idea about the emergence of space-time, based on the holographic principle. Inspired by the hologram-like way that black holes store all their entropy at the surface, this principle was first given an explicit mathematical form by Juan Maldacena, a string theorist at the Institute of Advanced Study in Princeton, New Jersey, who published his influential model of a holographic universe in 1998. In that model, the three-dimensional interior of the universe contains strings and black holes governed only by gravity, whereas its two-dimensional boundary contains elementary particles and fields that obey ordinary quantum laws without gravity.
Hypothetical residents of the three-dimensional space would never see this boundary, because it would be infinitely far away. But that does not affect the mathematics: anything happening in the three-dimensional universe can be described equally well by equations in the two-dimensional boundary, and vice versa.
In 2010, Van Raamsdonk studied what that means when quantum particles on the boundary are 'entangled' — meaning that measurements made on one inevitably affect the other. He discovered that if every particle entanglement between two separate regions of the boundary is steadily reduced to zero, so that the quantum links between the two disappear, the three-dimensional space responds by gradually dividing itself like a splitting cell, until the last, thin connection between the two halves snaps. Repeating that process will subdivide the three-dimensional space again and again, while the two-dimensional boundary stays connected. So, in effect, Van Raamsdonk concluded, the three-dimensional universe is being held together by quantum entanglement on the boundary — which means that in some sense, quantum entanglement and space-time are the same thing.
10 spacetime mysteries that quantum gravity could solve
https://www.forbes.com/sites/startswitha...8dde682ba3
EXCERPT: . . . It might be that to combine quantum theory with gravity, we do not have to update gravity, but quantum theory itself. If that is so, the consequences could be far-reaching. Because quantum theory underlies all electronic devices and if has to be changed, this might open entirely new possibilities.
~
INTRO: Christian Wüthrich delivered one of the plenary talks at this summer's BSPS conference in Edinburgh and lo! It was recorded (future is now!). For your listening pleasure, here is his 'The Temporal and Atemporal Emergence of (Space-)Time' with a link to the slides below.
EXCERPT: General relativity (GR) can’t be the last word: it assumes classical matter. We need a quantum theory of gravity, theory that combines quantum effects and (strong) gravitational fields. A quantum theory of gravity will be fundamental, at least relative to currently held theories of physics. [...] Now, many approaches to formulating such a theory either presuppose or entail that fundamentally, there is neither space nor time (even though that denial comes in degrees). Spacetime according to quantum theories of gravity in general, and according to loop quantum gravity/cosmology in particular, is not fundamental.
PDF snippets: http://thebjps.typepad.com/2017mBSPS_Atemporal.pdf
Are Space and Time Fundamental?
http://www.pbs.org/wgbh/nova/blogs/physi...ndamental/
EXCERPT: [...] “There aren’t many things in quantum gravity that everyone agrees on,” says Eleanor Knox, a philosopher at King’s College London who specializes in the philosophy of physics. “Yet the one thing many people seemed to agree on in quantum gravity was that we were going to have to cope with space and time not being fundamental.”
It sounds radical, but physics has a long and proud history of spearheading exactly this kind of coup. “Historically, whenever we thought something was fundamental, it turns out that it is not,” says Nathan Seiberg, a theoretical physicist at the Institute for Advanced Study. [...]
So why are physicists picking on space? Relativity delivered the first strike. [...] More alarmingly to theorists, our ability to measure features in space is intrinsically limited. [...] “It’s not because we don’t have the budget to build a powerful enough machine,” explains Seiberg. If we somehow tried to make an infinitely small measuring device, that device would become so dense that it would warp the fabric of space. The conclusion: “Space itself is ambiguous,” says Seiberg. Strike two.
Space also took a hit from an unlikely foe: the hologram. [...] It turns out that you can write equations that describe our universe perfectly well using different combinations of spatial dimensions, creating mathematical holograms that are indistinguishable from reality. Like a book that can be translated into many disparate languages without losing a syllable of meaning, our universe seems to tell a story that is independent of the words in which we have always chosen to express it.
Finally, physicists have known for some time that their descriptions of space start to break down when they’re applied to the strange-but-true environments [...] “Something else should kick in,” says Seiberg.
But what is that something else? “I don’t think I have an answer to that,” says Seiberg. Knox also leaves the door open to as-yet-unknown possibilities: “Whatever it is that’s fundamental, it’s not the stuff we have a handle on right now.” Morever, Seiberg adds that though theorists have assembled a strong case that space is emergent, time presents a more difficult problem. “In order to understand emergent time, we need a complete revolution in the way we think about physics.”
Letting go of space and time without ready replacements may seem like a surefire way to plunge into the abyss of abstraction. But it may be only by loosening our grip that we can come to grasp what is truly fundamental.
MORE: http://www.pbs.org/wgbh/nova/blogs/physi...ndamental/
Spooky Action at a Distance' Author George Musser Talks Physics Loopholes
https://www.space.com/31066-spooky-actio...rview.html
EXCERPT: . . . The real message of the spooky action is that space and time are not fundamental. The particles are rooted in that deeper layer where space and time don’t yet exist. And that’s the way to explain them. So the building blocks of the world may not be tiny things. What could these building blocks be if they’re not tiny because that’s kind of what we think of as a building block; a Lego is smaller than the thing you build out of the Legos. They may be what seems to us enormous things. Things that span the entire universe and are somehow acting in concert with one another to produce these phenomena of space.
- - -
Musser: A New Way of Thinking About Spacetime That Turns Everything Inside Out
https://gizmodo.com/a-new-way-of-thinkin...1741498475
EXCERPT: . . . John Wheeler, the renowned gravity theorist, speculated that space-time is built out of “pregeometry,” but admitted that this was nothing more than “an idea for an idea.” Even someone as irrepressible as Arkani-Hamed has had his doubts: “These problems are very hard. They’re outside our usual language for talking about them.”
What keeps Arkani-Hamed going is that he and his colleagues have found just the sort of methods Einstein said they’d have to — ways to describe physics in the absence of space, to breathe in the vacuum. He has put these efforts into historical perspective: “For 2,000-plus years, people asked about the deep nature of space and time, but they were premature. We’ve finally arrived at the epoch where you can pose the questions and hope for some meaningful answer.”
How Quantum Pairs Stitch Space-Time
https://www.quantamagazine.org/tensor-ne...-20150428/
EXCERPT: . . . 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.
[...] 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. [...] 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.
MORE: https://www.quantamagazine.org/tensor-ne...-20150428/
- - -
Quantum gravity's time problem
https://www.quantamagazine.org/quantum-g...-20161201/
EXCERPT: . . . “I think we now understand that space-time really is just a geometrical representation of the entanglement structure of these underlying quantum systems,” said Mark Van Raamsdonk, a theoretical physicist at the University of British Columbia.
[...] the space-time fabric has a “de Sitter” geometry, stretching as you look into the distance. The fabric stretches until the universe hits a very different sort of boundary from the one in AdS space: the end of time. At that point, in an event known as “heat death,” space-time will have stretched so much that everything in it will become causally disconnected from everything else, such that no signals can ever again travel between them. The familiar notion of time breaks down. From then on, nothing happens.
On the timeless boundary of our space-time bubble, the entanglements linking together qubits (and encoding the universe’s dynamical interior) would presumably remain intact, since these quantum correlations do not require that signals be sent back and forth. But the state of the qubits must be static and timeless.
This line of reasoning suggests that somehow, just as the qubits on the boundary of AdS space give rise to an interior with one extra spatial dimension, qubits on the timeless boundary of de Sitter space must give rise to a universe with time — dynamical time, in particular. Researchers haven’t yet figured out how to do these calculations. “In de Sitter space,” Swingle said, “we don’t have a good idea for how to understand the emergence of time.”
[...] Don Page and William Wootters [...] discovered that an entangled system that is globally static can contain a subsystem that appears to evolve from the point of view of an observer within it. [...] In other words, time doesn’t exist globally, but an effective notion of time emerges for the subsystem.
A team of Italian researchers experimentally demonstrated this phenomenon in 2013. In summarizing their work, the group wrote: “We show how a static, entangled state of two photons can be seen as evolving by an observer that uses one of the two photons as a clock to gauge the time-evolution of the other photon. However, an external observer can show that the global entangled state does not evolve.”
[...] The bottom line, in Swingle’s words, is that “somehow, you can emerge time from timeless degrees of freedom using entanglement.”
- - -
Theoretical physics: The origins of space and time
https://www.nature.com/news/theoretical-...me-1.13613
EXCERPT: . . . Meanwhile, Van Raamsdonk has proposed a very different idea about the emergence of space-time, based on the holographic principle. Inspired by the hologram-like way that black holes store all their entropy at the surface, this principle was first given an explicit mathematical form by Juan Maldacena, a string theorist at the Institute of Advanced Study in Princeton, New Jersey, who published his influential model of a holographic universe in 1998. In that model, the three-dimensional interior of the universe contains strings and black holes governed only by gravity, whereas its two-dimensional boundary contains elementary particles and fields that obey ordinary quantum laws without gravity.
Hypothetical residents of the three-dimensional space would never see this boundary, because it would be infinitely far away. But that does not affect the mathematics: anything happening in the three-dimensional universe can be described equally well by equations in the two-dimensional boundary, and vice versa.
In 2010, Van Raamsdonk studied what that means when quantum particles on the boundary are 'entangled' — meaning that measurements made on one inevitably affect the other. He discovered that if every particle entanglement between two separate regions of the boundary is steadily reduced to zero, so that the quantum links between the two disappear, the three-dimensional space responds by gradually dividing itself like a splitting cell, until the last, thin connection between the two halves snaps. Repeating that process will subdivide the three-dimensional space again and again, while the two-dimensional boundary stays connected. So, in effect, Van Raamsdonk concluded, the three-dimensional universe is being held together by quantum entanglement on the boundary — which means that in some sense, quantum entanglement and space-time are the same thing.
10 spacetime mysteries that quantum gravity could solve
https://www.forbes.com/sites/startswitha...8dde682ba3
EXCERPT: . . . It might be that to combine quantum theory with gravity, we do not have to update gravity, but quantum theory itself. If that is so, the consequences could be far-reaching. Because quantum theory underlies all electronic devices and if has to be changed, this might open entirely new possibilities.
~
