sabato 16 giugno 2012

Space, time and gravity are "emergent" reality.

In a recent article published in Scientific American is commented a new article of the string theorist Erik Verlinde that has aroused unusual reactions in the physics community. From mathematical point of view the treatment does not go beyond a simple algebra. From logical and physical standpoint things change.  
”Theoretical physicists consulted said that they could not follow him, an answer that seems like a nice way of saying that their colleague had gone crazy. Some bloggers were more explicit and gave openly of loosening. But who knows him knows that he deserves the title: Verlinde is a brilliant theoretical physicist, and the amount of threads caused by his article suggests that most of his colleagues will have found something interesting”. 

Anyone who aspires to the unification of physics is facing a major problem. Theories attempting to unify quantum field theory and Einstein's general theory of relativity are based on solid foundations and excellent experimental verification, but are incompatible with each other. Reconcile them will require the abandonment of some deep-seated intuitions, the first of which is that the world exists in space-time. Space and time are not fundamental entities but "emerging": the universe we see exist in space and time might be just the surface level on which we float like boats, while the leviathans stir the depths. 

Interviewed on the subject Verlinde says: "Let's start with an example: the motion of gas in a room. At microscopic distances, is the result of collisions between individual gas molecules. But at macroscopic distances, it is much more practical to describe the gas status in terms of pressure and temperature. These two quantities do not exist at the microscopic level, but only when we make an average behavior of molecules on a large scale. Gravity works in the same way. A more accurate analogy is osmosis. We separate a mixture of two types of gas molecules with a membrane, which allows the passage of only one type of molecules. If the concentration of the latter molecule is higher on one side of the membrane than the other, we can measure a net force. You can calculate this force by using statistical methods. In my articl, I explain how to obtain the Newton's law of gravity in a similar way. In the case of gravity, the force is the result of the change of probability, when two heavy objects are moved relative to one another and the membrane becomes instead a holographic screen. Usually, you think of gravity as one of the four fundamental forces. Among physicists, it is believed that to get the last description is necessary to unify quantum mechanics with Einstein's general relativity. But there are indications that gravity is not essential. The matter and even space and time are composed of bricks, which obey the laws that do not contain microscopic severity. These degrees of freedom and the laws that govern them are invisible to us, as we can not see with the naked eye the individual gas molecules. The gravity that we see is the net result of all these forces, which emerges only at great distances. "  

In the Verlinde model all matter, both ordinary and dark, consists of vibrations of the underlying degrees of freedom of the universe and, therefore, is created and destroyed continuously. Verlinde's article applies this line of reasoning to the same laws of gravity. Instead of being a fundamental form of nature, as all physicists have thought from Newton, gravity would be an "entropic force", a product of a sort of dynamic at finer scale, a bit as the pressure of a gas emerges from collective motion of molecules. To explain the observed anomalous internal dynamics in galaxies and more extensive star systems, astronomers think that our universe should be filled by an invisible form of matter, which exceeds the ordinary matter in abundance by a factor of five. However this matter has never been detected directly and to be something so prominent dark matter has a surprisingly thin effect. Consequently some physicists and astronomers suspect that dark matter may not exist at all. The main alternative to dark matter is known as "MOND" Modified Newtonian Dynamics. Verlinde reinterpreted MOND not only as a revision of the laws of physics, but as evidence of the existence of a large substrate. He derivates MOND formula assuming that dark matter is not a new type of particle, but the event of vibrations of some freedom degrees below, namely vibrations produced by random thermal fluctuations. These fluctuations are softened and become intense where the average thermal energy is low as at the outskirts of galaxies. Amazingly Verlinde managed to get five to one ratio of the mass of dark matter than ordinary. 

Here's an abstract from Verlinde article. 
On the Origin of Gravity and the Laws of Newton 
Erik Verlinde. Institute for Theoretical Physics University of Amsterdam. 

Starting from first principles and general assumptions Newton's law of grav itation is shown to arise naturally and unavoidably in a theory in which space is emergent through a holographic scenario. Gravity is explained as an entropic force caused by changes in the information associated with the positions of material bodies. A relativistic generalization of the presented arguments directly leads to the Einstein equations. When space is emergent even Newton's law of inertia needs to be explained. The equivalence principle leads us to conclude that it is actually this law of inertia whose origin is entropic. Of all forces of nature gravity is clearly the most universal. Gravity influences and is influenced by everything that carries an energy, and is intimately connected with the structure of space-time. The universal nature of gravity is also demonstrated by the fact that its basic equations closely resemble the laws of thermodynamics and hydrodynamics. So far there has not been a clear explanation for this resemblance. Gravity dominates at large distances, but is very weak at small scales. In fact its basic laws have only been tested up to distances of the order of a millimeter. Gravity is also considerably harder to combine with quantum mechanics than all the other forces. The quest for unification of gravity with these other forces of nature, at a microscopic level, may therefore not be the right approach. It is known to lead to many problems, paradoxes and puzzles. String theory has to a certain extent solved some of these, but not all. Many physicists believe that gravity, and space-time geometry are emergent. Also string theory and its related developments have given several indications in this direction. The universality of gravity suggests that its emergence should be understood from general principles that are independent of the specific details of the underlying microscopic theory. In this paper we will argue that the central notion needed to derive gravity is information. More precisely, it is the amount of information associated with matter and its location, in whatever form the microscopic theory likes to have it, measured in terms of entropy. Changes in this entropy when matter is displaced leads to an entropic force, which as we will show takes the form of gravity. Its origin therefore lies in the tendency of the microscopic theory to maximize its entropy. We present a holographic scenario for the emergence of space and address the origins of gravity and inertia, which are connected by the equivalence principle. Starting from first principles, using only space independent concepts like energy, entropy and temperature, it is shown that Newton's laws appear naturally and practically unavoidably. Gravity is explained as an entropic force caused by a change in the amount of information associated with the positions of bodies of matter. A crucial ingredient is that only a finite number of degrees of freedom are associated with a given spatial volume, as dictated by the holographic principle. The energy, that is equivalent to the matter, is distributed evenly over the degrees of freedom, and thus leads to a temperature. The product of the temperature and the change in entropy due to the displacement of matter is shown to be equal to the work done by the gravitational force. In this way Newton's law of gravity emerges in a surprisingly simple fashion. valid. An entropic force is an effective macroscopic force that originates in a system with many degrees of freedom by the statistical tendency to increase its entropy. The force equation is expressed in terms of entropy differences, and is independent of the details of the microscopic dynamics. In particular there is no fundamental field associated with an entropic force. Entropic forces occur typically in macroscopic systems such as in colloid or bio-physics. Big colloid molecules suspended in an thermal environment of smaller particles, for instance, experience entropic forces due to excluded volume effects. Osmosis is another phenomenon driven by an entropic force. Perhaps the best known example is the elasticity of a polymer molecule. A single polymer molecule can be modeled by joining together many monomers of fixed length, where each monomer can freely rotate around the points of attachment and direct itself in any spatial direction. Each of these configurations has the same energy. When the polymer molecule is immersed into a heat bath, it likes to put itself into a randomly coiled configuration since these are entropically favored. There are many more such configurations when the molecule is short compared to when it is stretched into an extended configuration. The statistical tendency to return to a maximal entropy state translates into a macroscopic force, in this case the elastic force. By using tweezers one can pull the endpoints of the polymer apart, and bring it out of its equilibrium configuration by an external force F. By the balance of forces, the external force F should be equal to the entropic force, that tries to restore the polymer to its equilibrium position. An entropic force is recognized by the facts that it points in the direction of increasing entropy, and, secondly, that it is proportional to the temperature. For the polymer the force can be shown to obey Hooke's law. This example makes clear that at a macroscopic level an entropic force can be conservative at least when the temperature is kept constant. The corresponding potential has no microscopic meaning, however, and is emergent. Space is in the first place a device introduced to describe the positions and movements of particles. Space is therefore literally just a storage space for information. This information is naturally associated with matter. Given that the maximal allowed information is finite for each part of space, it is impossible to localize a particle with infinite precision at a point of a continuum space. In fact, points and coordinates arise as derived concepts. One could assume that information is stored in points of a discretized space (like in a lattice model). But if all the associated information would be without duplication, one would not obtain a holographic description. In fact, one would not recover gravity. Thus we are going to assume that information is stored on surfaces, or screens. Screens separate points, and in this way are the natural place to store information about particles that move from one side to the other. Thus we imagine that this information about the location particles is stored in discrete bits on the screens. The dynamics on each screen is given by some unknown rules, which can be thought of as a way of processing the information that is stored on it. Hence, it does not have to be given by a local field theory, or anything familiar. Gravity has given many hints of being an emergent phenomenon, yet up to this day it is still seen as a fundamental force. The similarities with other known emergent phenomena, such as thermodynamics and hydrodynamics, have been mostly regarded as just suggestive analogies. It is time we not only notice the analogy, and talk about the similarity, but finally do away with gravity as a fundamental force. Of course, Einstein's geometric description of gravity is beautiful, and in a certain way compelling. Geometry appeals to the visual part of our minds, and is amazingly powerful in summarizing many aspects of a physical problem. Presumably this explains why we, as a community, have been so reluctant to give up the geometric formulation of gravity as being fundamental. But it is inevitable we do so. If gravity is emergent, so is space time geometry. Einstein tied these two concepts together, and both have to be given up if we want to understand one or the other at a more fundamental level. The results of this paper suggest gravity arises as an entropic force, once space and time themselves have emerged. If the gravity and space time can indeed be explained as emergent phenomena, this should have important implications for many areas in which gravity plays a central role. It would be especially interesting to investigate the consequences for cosmology. For instance, the way redshifts arise from entropy gradients could lead to many new insights. The reason why gravity has to keep track of energies as well as entropy differences is now clear. It has to, because this is what causes motion! The presented arguments have admittedly been rather heuristic. One can not expect otherwise, given the fact that we are entering an unknown territory in which space does not exist to begin with. The profound nature of these questions in our view justifies the heuristic level of reasoning. 

Verlinde's theory is a good example of how, although difficult to "digest", only giving up to consider gravity as effect of space – time curvature can be a logically consistent approach in the modern observational astronomy. As the spin networks of Loop Quantum Gravity by Carlo Rovelli and Lee Smolin, the Verlinde’s "phase space" is the "substrate" necessary to originate and transmit gravity force that is, more 'than force, a manifestation of this physical entity in the underlying that in the MT theory presented in this blog is a kind of non quantized "aether" consisting in electromagnetic standing waves whose dynamics arises from broken internal symmetry. 

Shocking? Not for who, like Marius, the founder of MT, didn’t get Einstein spacetime as an intangible ideological totem. 

Stefano Gusman.

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