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Spacetime from bits

#1
C C Offline
https://science.sciencemag.org/content/370/6513/198

Spacetime, reconstructed: Theories of holographic duality feature a correspondence between a gravitational system and a strongly interacting conformal field theory (CFT) living on the system's boundary. Through this correspondence, the CFT encodes the geometry of spacetime in the gravitational system. Van Raamsdonk analyzed the role of entanglement in this theoretical framework. Instead of considering a single CFT, the author's starting point was a collection of CFT “bits” that are mutually entangled but do not interact with one another. The spacetime that these bits collectively encode was then shown to be arbitrarily close to the one encoded by the original CFT, suggesting that entanglement plays a crucial role in the emergence of spacetime.

ABSTRACT: In the anti–de Sitter/conformal field theory approach to quantum gravity, the spacetime geometry and gravitational physics of states in some quantum theory of gravity are encoded in the quantum states of an ordinary nongravitational system. Here, I demonstrate that this nongravitational system can be replaced with an arbitrarily large collection of noninteracting systems (“bits”) placed in a highly entangled state. This construction makes manifest the idea that spacetime geometry emerges from entanglement between the fundamental degrees of freedom of quantum gravity and that removing this entanglement is tantamount to disintegrating spacetime. This setup also reveals that the entangled states encoding spacetimes may be well represented by a certain type of tensor network in which the individual tensors are associated with states of small numbers of bits. (MORE or access to paper)
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#2
Magical Realist Offline
At the macro level, spacetime seems more binary and digital. Things are only at one place, or they are not. Ones and zeros. Either or. Alive or dead. But at the quantum level spacetime becomes more fuzzy and analog. An electron is at this place a little, and at another place a little. A photon is both a particle and a wave. The cat is both alive and dead.
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#3
Syne Offline
(Oct 12, 2020 10:50 PM)Magical Realist Wrote: At the macro level, spacetime seems more binary and digital. Things are only at one place, or they are not. Ones and zeros. Either or. Alive or dead. But at the quantum level spacetime becomes more fuzzy and analog. An electron is at this place a little, and at another place a little. A photon is both a particle and a wave. The cat is both alive and dead.

According to physics, you have that backwards. At the quantum level, everything is discrete quanta, and at the macro level, everything is continuous. Particles jump from one energy state to another, over a gap, like a digital clock jumps from one second to the next. A macro object doesn't jump from one place to another. It smoothly moves through every intervening position on the way.

There is no sense in which an electron is "a little" in multiple places. Wave/particle duality means that such expressions are an expression of a wave state, not the particle state, and quantum objects are not both at the same time. Schrodinger's cat is an expression of the uncertainty of the wave state, and opening the box to observe the actual state of the cat is the particle state.
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#4
confused2 Offline
The way electrons stay by a nucleus is because - and + charges attract. In a little more detail you get that the 'field' consists of virtual photons. Does nature actually do virtual photons - is this in any way a representation of what is really going on? My suspicion is not but it's the best (only) model we have. For gravitation we have the problem of masses affecting each other at a distance with no obvious photon exchange analog. All we have left (that we know of) for action at a distance is 'entanglement' so we have to try entanglement. Even if it can give predictive results it doesn't mean it is in any way a fair representation of the real mechanism.
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#5
Syne Offline
(Oct 15, 2020 01:26 AM)confused2 Wrote: The way electrons stay by a nucleus is because  - and + charges attract. In a little more detail you get that the 'field' consists of virtual photons. Does nature actually do virtual photons - is this in any way a representation of what is really going on? My suspicion is not but it's the best (only) model we have. For gravitation we have the problem of masses affecting each other at a distance with no obvious photon exchange analog. All we have left (that we know of) for action at a distance is 'entanglement' so we have to try entanglement. Even if it can give predictive results it doesn't mean it is in any way a fair representation of the real mechanism.

Too many tested observations verify the short-lived reality of virtual particles. And the laws of QM make them a natural consequence. But even though people speculate there might be a graviton force carrier for gravity, the transfer of gravity between macro objects is very different from interactions between particles. Entanglement isn't known to carry any force. And trying to reify spacetime, itself, as a substance of "bits" is wholly unjustified...except out of theoretical desperation.
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