Piero Scaruffi(Copyright © 2013 Piero Scaruffi | Legal restrictions )
These are excerpts and elaborations from my book "The Nature of Consciousness"
Entropic Gravity: Spacetime as Entanglement
The first one to take Vera Rubin's findings literally (instead of hypothesizing dark matter) was the Israeli astrophysicist Mordehai Milgrom ("A modification of the Newtonian dynamics as a possible alternative to the hidden mass hypothesis", 1983), who formulated "modified Newtonian dynamics," or MOND, according to which gravity undergoes a sort of state transition when the gravitational acceleration drops 0.00000000012 meters per second per second (ours on Earth is about 9.8) and then Newton's equation of gravitation (which is a special case of Einstein's equation) stops working. At the time Milgrom didn't have an explanation for this abrupt irregularity, but Milgrom noticed that this MOND would yield the observed dynamics of matter in galaxies and in galaxy clusters without any need to postulate the existence of dark matter.
A decade later the US astrophysicist Ted Jacobson derived Einstein's equation of gravitation (that matter curves space-time and curved space-time determines the motion of matter) from purely thermodynamic concepts ("Thermodynamics of Spacetime", 1995). He derived Einstein's equation from the proportionality of entropy and horizon area (as shown by Stephen Hawking and Jacob Bekenstein) together with the simple thermodynamic relation that relates heat, entropy, and temperature. Basically, Jacobson viewed every point in space as a tiny black-hole horizon (obeying the Bekenstein-Hawking rules) and didn't need the curvature of space-time to obtain Einstein's equation.
In 1998 Juan Maldacena's holographic model of the universe showed that quantum entanglement is key to the existence of space-time, and the pieces started falling in place.
The Japanese physicists Shinsei Ryu and Tadashi Takayanagi showed that quantum entanglement and spacetime seem to be dual concepts, a result that hinted that spacetime may simply be an emergent property of quantum entanglement ("Holographic Derivation of Entanglement Entropy", 2006).
The Indian astrophysicist Thanu Padmanabhan rewrote Einstein's equation in a way that shows their identity to the laws of thermodynamics ("Thermodynamical Aspects of Gravity", 2010)
The Canadian physicist Mark Van Raamsdonk ("Building up Spacetime with Quantum Entanglement", 2010) proved that the three-dimensional universe in Maldacena╬Ú╬¸s model is being held together by quantum entanglement on the boundary: if you remove the quantum entanglement of all the points on the surface, the universe inside (the space-time subject to gravity) literally disintegrates. Therefore he concluded that quantum entanglement causes space-time to emerge.
The Dutch astrophysics Erik Verlinde ("On the Origin of Gravity and the Laws of Newton", 2010) thinks that gravity is simply a side-effect of the increase in a special kind of entropy/disorder in the universe. The ultimate reality is quantum bits, or "qubits", and "entanglement entropy" counts the number of entangled qubits. Entanglement entropy measures the degree of correlation of qubits. It is called entropy because it reduces to the thermodynamic entropy in the case of thermal states. The network of qubits gives rise to a hologram that is what we observe as space-time geometry and as the distribution of matter in space. Entangled qubits create matter whose mass curves space-time whose curvature causes gravity. Gravity is an emergent thermodynamic effect just like pressure and temperature. Einstein's equation would be a consequence of the very entanglement that he discarded as senseless, and gravity would be a side-effect of the quantum interactions that he called "spooky".
Viewing gravity as thermodynamics is the direct consequence (40 years later) of merging thermodynamics, quantum physics and gravity, which is what Bekenstein and Hawking did.
Mathematically, the US physicist Brian Swingle proved that tensor networks connect entanglement to the geometry of spacetime ("Entanglement Renormalization and Holography", 2012). Swingle showed that higher-degree entanglement behaves just like distance in space: the degree of entanglement is de facto a distance through an extra spatial dimension. Swingle and Van Raamsdonk showed that the universality of gravity is directly related to the universality of entanglement ("Universality of Gravity from Entanglement", 2014).
Thomas Faulkner, Mark Van Raamsdonk and others (" Gravitation from Entanglement in Holographic CFTs", 2013) formulated the "entanglement first law": an exact quantum generalization of the first law of thermodynamics entanglement entropy that yields Einstein's equation, as also shown by Ted Jacobson ("Entanglement Equilibrium and the Einstein Equation", 2015).
Erik Verlinde showed that in this theory the extra gravity attributed to dark matter is simply due to dark energy ("Emergent Gravity and the Dark Universe", 2016). No need for dark matter. There is no need for dark matter to explain the oddities discovered by Vera Rubin. Dark matter is an illusion created by the interaction of matter with the dark energy that permeates the universe. The alternative to dark matter is that gravity does not really exist and Vera Rubin's observations would simply prove that Newton and Einstein were wrong.
Susskind and Maldacena showed that two ideas proposed by Einstein and Nathan Rosen in 1935, namely quantum entanglement and wormholes, might be closely related ("Cool Horizons for Entangled Black Holes", 2013): the radiation emitted by a black hole (the one predicted by Hawking) remains forever entangled with the inside of the black hole via multiple microscopic wormholes. Maybe Einstein was on to something but couldn't see it.
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