In un recente articolo pubblicato su Scientific American viene commentato un nuovo articolo del teorico delle stringhe Erik Verlinde che ha destato singolari reazioni nella comunità dei fisici. Dal punto di vista matematico la trattazione non va oltre un’algebra da scuola superiore. Dal punto di vista logico e fisico, invece, le cose cambiano.
"I fisici teorici consultati dissero che non riuscivano a seguirlo ; una risposta che sembra un modo gentile per dire che il loro collega era andato fuori di testa. Alcuni blogger furono più espliciti e gli diedero apertamente dello svitato. Ma chi lo conosce sa che non merita l'appellativo: Verlinde è un brillante fisico teorico, e la quantità di discussioni provocate dal suo articolo fa pensare che la maggior parte dei suoi colleghi vi abbia trovato qualcosa di stimolante”.
Chiunque aspiri all’unificazione della fisica si trova di fronte a un problema di fondo. Le teorie che cercano di unificare la teoria quantistica dei campi e la teoria della relatività generale di Einstein poggiano su solide basi e ottime verifiche sperimentali, ma sono incompatibili tra loro. Riconciliarle richiederà l'abbandono di alcune intuizioni profondamente radicate, la prima delle quali è che il mondo esiste nello spazio-tempo.
Spazio e tempo non sono enti fondamentali ma "emergenti": l’universo che vediamo esistere nello spazio e nel tempo potrebbe essere soltanto il livello di superficie, sul quale galleggiamo come barchette, mentre i leviatani si agitano nelle profondità.
Intervistato sull’argomento Verlinde afferma :
“Partiamo da un esempio: il moto del gas in una stanza. A distanze microscopiche, è la conseguenza degli urti tra le singole molecole di gas. Ma a distanze macroscopiche, è molto più pratico descrivere lo stato termodinamico del gas in funzione della pressione e della temperatura. Queste due quantità non esistono a livello microscopico ma soltanto quando facciamo una media del comportamento delle molecole a grande scala. La gravità funziona allo stesso modo.
Un’analogia più precisa è l’osmosi. Separiamo una miscela di due tipi di molecole di gas con una membrana che permette il passaggio di un tipo solo di molecole. Se la concentrazione di quest’ultima molecola è più elevata da una parte della membrana rispetto all’altra, possiamo misurare una forza netta. Si può calcolare questa forza usando metodi statistici. Nel mio articolo, spiego come ottenere la legge della gravità di Newton in un modo molto simile. Nel caso della gravità, la forza è la conseguenza del cambiamento di probabilità, quando due oggetti pesanti vengono spostati l’uno rispetto all’altro e la membrana diventa invece uno schermo olografico.
Di solito si pensa alla gravità come a una delle quattro forze fondamentali. Tra i fisici, si crede che per ottenere la descrizione ultima sia necessario unificare la meccanica quantistica con la relatività generale di Einstein. Ma ci sono indicazioni che la gravità non sia fondamentale. La materia e persino lo spazio e il tempo sono composti da mattoncini che obbediscono a delle leggi microscopiche che non contengono la gravità. Questi gradi di libertà e le leggi che li governano sono invisibili per noi, così come non riusciamo a vedere a occhio nudo le singole molecole di gas. La gravità che vediamo è il risultato netto di tutte queste forze, che emerge solo a grandi distanze”.
Nel modello di Verlinde tutta la materia, sia ordinaria sia oscura, consiste di vibrazioni dei gradi di libertà dell'universo sottostante e, perciò, viene creata e distrutta in continuazione
L’articolo di Verlinde applica questa linea di ragionamento alle stesse leggi della gravità. Invece di essere una forza fondamentale della natura, come tutti i fisici hanno pensato, a partire da Newton, la gravità potrebbe essere una “forza entropica”, un prodotto di una sorta di dinamica a scala più fine, un po' come la pressione di un gas emerge dal moto collettivo delle molecole.
Per spiegare l’anomala dinamica interna osservata nelle galassie e nei sistemi stellari più vasti, gli astronomi pensano che il nostro universo debba essere riempito da una forma invisibile di materia, che supera in abbondanza la materia ordinaria di un fattore cinque. Tuttavia questa materia non è mai stata rivelata direttamente e per essere una cosa tanto predominante la materia oscura ha un effetto sorprendentemente sottile.
Di conseguenza, alcuni astronomi e fisici sospettano che la materia oscura potrebbe non esistere affatto.
La principale alternativa alla materia oscura è nota come "MOND", Modified Newtonian Dynamics. Verlinde ha reinterpretato la MOND non solo come una revisione delle leggi della fisica, ma come la prova dell'esistenza di un vasto substrato. Ha derivato la formula della MOND assumendo che la materia oscura non sia un nuovo tipo di particella ma la manifestazione delle vibrazioni di alcuni gradi di libertà sottostanti e, precisamente, le vibrazioni prodotte da fluttuazioni termiche casuali. Tali fluttuazioni sono attenuate e diventano intense solo dove l’energia termica media è bassa, come alla periferia delle galassie. Sorprendentemente Verlinde è riuscito a ottenere il rapporto cinque a uno della massa della materia oscura rispetto a quella ordinaria.
Ecco un estratto dell’articolo di Verlinde.
On the Origin of Gravity
and the Laws of Newton
Erik
Verlinde
Institute for Theoretical Physics
University of Amsterdam
A partire dai principi primi e dalle ipotesi generali della legge gravitazionale di Newton si dimostra che questi sorgono naturalmente e inevitabilmente da una teoria in cui lo spazio emerge da uno scenario olografico. La gravità è spiegata come una forza entropica causata da cambiamenti nelle informazioni associate con le posizioni dei corpi materiali. Una generalizzazione relativistica delle argomentazioni presentate conduce direttamente alle equazioni di Einstein. Quando emerge lo spazio si spiega anche la legge di inerzia di Newton. Il Principio di Equivalenza ci porta a concludere che, in realtà, l’origine di questa legge è entropica.
In natura, di tutte le forze, la gravità è chiaramente la più universale. Essa influenza ed è influenzata da tutto ciò che porta una energia ed è intimamente connessa con la struttura dello spazio-tempo.
La natura universale della gravità è dimostrata anche dal fatto che le sue equazioni di base sono simili alle leggi della termodinamica e dell’ idrodinamica. Finora, non c'è stata una chiara spiegazione per questa somiglianza. La gravità domina a grandi distanze ma è molto debole alle scale piccole. In realtà le sue leggi fondamentali sono state testate solo fino a distanze dell'ordine di un millimetro. La gravità è anche notevolmente più difficile da coniugare con la meccanica quantistica rispetto a tutte le altre forze. La ricerca di unificazione della gravità con le altre forze della natura, a livello microscopico, quindi, puo’ non essere il giusto approccio. Tale approccio, come noto, causa molti problemi e paradossi. La teoria delle stringhe in certa misura ha risolto alcuni di essi, ma non tutti.
Molti fisici ritengono che la gravità e la geometria spazio-temporale siano “emergenti”. Anche la teoria delle stringhe e i suoi relativi sviluppi hanno dato diverse indicazioni in tal senso.
L'universalità della gravità suggerisce che la sua comparsa sia indipendenti dai dettagli specifici della sottostante teoria microscopica. In questo lavoro si sostiene che la nozione centrale necessaria per ricavare la forza di gravità è l'informazione. Più precisamente è la quantità di informaziona associata alla materia e alla sua posizione.
Per la nascita dello spazio si ipotizza uno scenario olografico dove la gravità e l’inerzia sono collegate dal Principio di Equivalenza.
A partire da principi primi, usando solo concetti spaziali indipendenti come l'energia, l’entropia e la temperatura, si dimostra che le leggi di Newton appaiono in modo naturale e quasi inevitabilmente. La gravità è spiegata come una forza entropica causata da un cambiamento nella quantità di informazioni associate alle posizioni di corpi materiali. Un ingrediente essenziale è che solo un numero finito di gradi di libertà è associato a un dato volume di spazio, come dettato dal principio olografico. L'energia, che è equivalente alla materia, viene distribuita uniformemente secondo i gradi di libertà e porta così a una temperatura. Si dimostra che il prodotto della temperatura e della variazione di entropia dovuta allo spostamento della materia è uguale al lavoro svolto dalla forza gravitazionale. In questo modo la Legge di Newton emerge in modo sorprendentemente semplice.
Una forza entropica è una forza efficace macroscopica che ha origine in un sistema con molti gradi di libertà per la tendenza statistica del sistema ad aumentare la sua entropia. La forza è espressa in termini di differenze entropia ed è indipendente dai particolari delle dinamiche microscopiche. In particolare, non c'è un campo fondamentale associato a una forza entropica. Forze entropiche si sviluppano, per esempio, in sistemi macroscopici come quelli biofisici colloidali. Grandi molecole colloidali sospese in un ambiente termico di particelle più piccole dove, a causa degli effetti di volume escluso, si generano forze entropiche che guidano, ad esempio, il fenomeno dell’osmosi.
Forse l'esempio più noto di forze entropiche è l'elasticità di una molecola di polimero. Una molecola di polimero singolo può essere modellato unendo insieme molti monomeri di lunghezza fissa in modo che ogni monomero possa ruotare liberamente attorno ai punti di attacco e disporsi secondo qualsiasi direzione dello spazio. Ciascuna di queste configurazioni ha la stessa energia. Quando la molecola di polimero viene immersa in un bagno di calore si dispone secondo configurazioni spaziali casuali a spirale poiché queste sono entropicamente favorite. Di queste configurazioni ce ne sono molte di piu’ quando la molecola è corta rispetto a quando si è allungata. La tendenza statistica a tornare in uno stato di massima entropia si traduce in una forza macroscopica che, in questo caso, è la forza elastica.
Usando delle pinzette si possono tirare i punti estremi del polimero da un lato con una forza esterna F che deve essere uguale alla forza entropica che tenta di ripristinare il polimero alla sua posizione di equilibrio. Una forza entropica si riconosce da un lato dal fatto che punta nella direzione di aumento dell'entropia, e dall'altro che è proporzionale alla temperatura. Per il polimero si dimostra che tale forza obbedisce alla legge di Hooke.
Questo esempio rende chiaro che a livello macroscopico una forza entropica può essere conservativa almeno quando la temperatura è mantenuta costante. Il corrispondente potenziale non ha alcun significato microscopico e, tuttavia, “emerge”.
Lo spazio è in primo luogo un dispositivo introdotto per descrivere le posizioni e i movimenti di particelle. Lo spazio è, quindi, letteralmente solo uno spazio di archiviazione per informazioni. Questa informazione è, naturalmente, associata alla materia. Dato che per ogni volume di spazio il numero massimo di informazioni consentite è finito è impossibile localizzare una particella con precisione infinita in un punto di uno spazio continuo. In effetti i punti e le coordinate nascono come concetti derivati. Si potrebbe ipotizzare che le informazioni sono memorizzate in punti di uno spazio discretizzato (come in un modello reticolare). Ma se tutte le informazioni associate esistessero senza duplicazioni, non si otterrebbe una descrizione olografica. In realtà, non esisterebbe la gravità. Così presumiamo che le informazioni siano memorizzate su superfici o schermi. Schermi separano le particelle e, quindi, sono il luogo naturale per memorizzare le informazioni sulle particelle che si muovono da uno all’altro. Quindi immaginiamo che queste informazioni sulla localizzazione delle particelle siano memorizzati in bit discreti sugli schermi. Le dinamiche su ogni schermo scaturiscono da regole sconosciute che possono essere pensate come un modo di elaborare le informazioni memorizzate su di esso. Non si tratta, quindi, una teoria di campo o, comunque, a noi familiare.
La gravità fornisce molti indizi sul suo essere un fenomeno emergente, ma fino ad oggi essa è ancora considerata una forza fondamentale. Le analogie con altri noti fenomeni emergenti come la termodinamica e l’idrodinamica sono state per lo più considerate solo suggestive somiglianze. E' tempo di parlare non solo di analogie e somiglianze e, finalmente, farla finita con la gravità come forza fondamentale. La descrizione geometrica della gravità di Einstein è bellissima e in un certo modo convincente. La geometria si appella alla parte visiva della nostra mente ed è sorprendentemente potente nel riassumere molti aspetti di un problema fisico. Presumibilmente questo spiega perché noi, come comunità, siamo stati così riluttanti a rinunciare alla formulazione geometrica della gravità come fondamentale. Ma è inevitabile che se la gravità è emergente, cosi’ lo è anche la geometria dello spazio tempo. Einstein aveva legato questi due concetti insieme, ma entrambi devono essere abbandonati se vogliamo comprendere l’uno o l'altro a un livello più profondo. I risultati di questo studio suggeriscono che la gravità si manifesta come una forza entropica una volta che sono emersi lo spazio e il tempo. Se gravità, spazio e tempo possono essere intesi come fenomeni emergenti, questo avrebbe grandi implicazioni per molte aree in cui la gravità gioca un ruolo centrale. Sarebbe particolarmente interessante indagare le conseguenze per la cosmologia. Ad esempio il modo in cui i redshift derivano dai gradienti di entropia potrebbe portare a scoprire nuove realtà.
Il motivo per cui la gravità deve portar traccia delle energie cosi’ come delle differenze di entropia è ormai chiaro. Deve, perché questo è ciò che provoca il movimento!
Gli argomenti presentati sono certamente di natura piuttosto euristica. Non poteva essere diversamente in considerazione del fatto che stiamo entrando in un territorio sconosciuto dove non esiste uno spazio da cui cominciare.
E’ la natura profonda di queste domande che, a nostro avviso, giustifica il carattere euristico del ragionamento.
La teoria di Verlinde è un ottimo esempio di come, per quanto difficile da “digerire”, solo rinunciando a considerare la gravità come effetto della curvatura dello spazio – tempo si possano approcciare in maniera logicamente consistente le evidenze osservative dell’astronomia moderna.
Come lo spin network della Loop Quantum Gravity di Carlo Rovelli e Lee Smolin, lo “spazio delle fasi” di Verlinde è quel “substrato” necessario a originare e trasmettere la forza di gravità che, piu’ che una forza, è una manifestazione di questa entità fisica sottostante che nella MT presentata in questo blog è una sorta di “etere” non quantizzato costituito da onde stazionarie di natura elettromagnetica la cui dinamica scaturisce da rotture di simmetria interne.
Sconvolgente ? Non per chi come Marius, l’ideatore della MT, non ha fatto dello spazio tempo di Einstein un intangibile totem ideologico.
Stefano Gusman.
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.
”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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