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Physiker fanden starke Hinweise darauf, dass die Schwerkraft keinegrundlegende Kraft ist-sondern womogiich nur eine hartnacklge Illusion.

vo n Rudiger Vaas

Die eldgleichungender r vit tionh bendenselben Status wie die Gieichungen der Elastizitatoder Stromungslehre. Dieser Satz aus einem

wissenschaftlichen Ubersichtsartikelvor wenigen Mona-ten, mag fur Laien nicht sonderlich aufregend klingen,vieileicht sogar kryptisch. Aber er birgt Sprengstoff insich, der die physikalischenVorsteilungen von Raum,Zeitund Schwerkraft sowie die Grundfesten des Universumsgeradezu zerfetzen kann. Natiirlich nicht praktisch,aber in der Theorie. Unddas warebrachial und revolutio-n a r g e n u g .

Man kann diesen Satzals eine Hypothese lesen. Dochfur seinen Autor ist er mehr- gewissermalSen die Quint-

essenz aus zwolf Jahren Forschung. Und dieser Autorist kein extravaganter

Kompaktf

fe Grafhungen der AligemeinenReiativitatstheorie ahnein seltsamerweise

denen fiir Energie, Entropie und Arbeit.

Das deutet darauf bin, dass Raum undZeit nicht fundamental, sondern aus kiel-

neren ..Bestandteilen aufgebautsind-viellelcht ein Wegweiser zur Weltformel.

-v

4 6 b i ld d e r w i s s e n sc h a f t 11 2 1 4

Esoteriker, sondern einmehrfach ausgezeichne-ter Physik-Professor amInter-University Centrefor Astronomy and Astrophysics im westindi-schen Pune, vormaligerPresident der Cosmo

logy Commission derInternationalen Astrono-mischen Union sowie ein

Experte fur Allgemeine Reiativitatstheorie und Kosmolo-gie mit grofiem Renommee: Thanu Padmanabhan.

Begabung und Nonkonformismus waren dem Indergleichsam in die wissenschaftliche Wiege gelegt; Bereitsmit 20 publizierte der heute57-Jahrige seinen ersten For-schungsartikel zu einem Themader Aligemeinen Reiativitatstheorie. Wenig spater promovierte er beijayant Narli-kar, der seine Dissertation in den 1960er-Jahren bei demberiihmten britischen AstrophysikerFred Hoyle geschrie-ben hatte und mit diesem an kosmoiogischen Modellenohne Urknall forschte, was er noch immer tut.

D as G an ze u nd seine Telle

Wie Narlikar attackiert auch Padmanabhan eingeschliffeneVorsteilungen - ohne sich jedoch in eine Aufienseiterpositionzu manovrieren. Wir brauchen einefundamentale Revisionin der Betrachtung der Gravitation , sagt er. Die Schwerkraft konnte lediglich ein emergentes Phanomen sein.

Emergent heil?en in der Physik Systemeigenschaften,die sichim Prinzipauf dieKomponenten und Wechselwir-kungen des Systems zuriickfuhren lassen, in der Praxis jedochauf einer hoheren Beschreibungsebene mit einer effektivenTheorie erfasst werden, nicht mit einer fundamentalen.

Ein soiches emergentes Phanomen ist etwa die Eigen-

schaftvon Wasser, bei bestimmten Druck-und Temperatur-

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verhaitnissenflussig und bei anderen fest oder gasformig zusein. Das kann man einem einzelnen H^O-Molekiil nichtansehen, obwohles sich aus der genauen Kenntnisseiner igenschaften und Wechseiwirkungentheoretisch durchausableiten lie(?e. Insofern istdas Ganze mehrais seine Teile.

Andere Beispiele fiir Emergenzsind Elastizitat und Gas-dynamik. Dafiir haben Physiker Formein gefunden, dieetwa beim Bau von Briicken oder der Beschreibung derLuftbewegung an einem Flugzeugflugel zur Anwendungkommen. Diese seit Langem bekannten Gieichungen funk-tionieren gut. Aber sie verweisen nichtauf die tiefere Rea-litat: dieMolekule und Atome. Fiirderen Beschreibung isteine fundamentalereTheorie notig, die Quantentheorie.

Oberflachliche Reiativitatstheorie

Padmanabhan sieht hier Parallelen zur Aligemeinen Reiativitatstheorie. Auch diese halt er nicht fiir fundamental,sondern bloSfureine effektive Beschreibung,wie Physiker

sagen. Entsprechend waredieSchwerkraft emergent-unddie Raumzeit selbst. Das hatte drastische Konsequenzenfur die seit Langem gesuchte Theorie der Quantengravi-tation, zuweilen etwas iibertrieben auch Weltformelgenannt .

Wenn das Emergenz-Konzept richtigist, dann gingendie meisten bisherigen Versuche, Einsteins Gleichung zuquantisieren, in die falsche Richtung - ahnlich wie wennjemand probieren wurde,die Atomphysik zu finden, in-

^ dem er die Gesetze der Elastizitatquantisiert , meint Pad-I manabhan. VariablenwieMetrikundKrummungbeider1 Beschreibung der Raumzeit inder Reiativitatstheorie sindI analog zu Dichte, Geschwindigkeit und so weiter in der

Hoch hinaus : Der Schwerkraf t sche inbar en thoben

schweb t die se r HelBluftballon in de n Himmel . Tatsach i i ch

ist die Wirkung der Gravjtation triigerisch- sie ist einabgeleitetes Phanomen wie der Druck eines 6ase der

nichts Fundamentales darstellt, sondern auf die Bewegungvon Moiekiiien reduziertwerc^n kann. Erstaunlicherweise

ahnein die Gieichungen der Allgemeinen Relativitats-theorie jenen der Thermodynamik, die auch das Verhalten

. von bunten Montgolfieren beschreibt.

D i e A t o m e

d e r R a u m z e i tStromungslehre, un d siehaben au f de r mikrosko-

pischen Beschreibungsebene keine Bedeu-

tung. Mehr noch; DieQuantisierung der Met-rik hilft genauso wenig

dabei, die mikroskopi-sche S t ru k tu r d e r R a um

zeit zu enthulien, wie dieQuantisierung der Dichte und Geschwindigkeiteiner Flussigkeit hilft,die molekulare Dyna-mi k zu verstehen .

Da s ist eine radikaleAuffassung. Denn da-raus folgt, dass dieRaumzei t selbst aus klei-

neren Einheiten besteht un d die Gravi tation keiirc fundamentaleKraft ist, sondern eine abgeleitete GrolSe - alsoletztlich eine Illusion. Das ist fiir den Alltagsverstand einefast grotesk anmutende Schlussfolgerung.SchlieGlich spiirtjeder die Anziehungskraft der Erde - besonders stark amfriihen Morgen, wenn der Wecker klingelt. Aber die Ideeist eigentlich nicht neu. Denn im Rahmen der Reiativitatstheorie wird di e Gravitation nicht als Kraft beschrieben,

sondern als geometrische Ei^enschaftder Raumzeit.Einstein selbst lehrte uns, dass es keine Schwerkraft

gibt , sagt Padmanabhan. Materie kriimmt die Raumzeit, was uns eben als eine Kraft erscheint. Daher ist alles,was erklart werden muss, wiesichdie Beziehung zwischen

Er blickt hinter di e Schwerkraft : T ha nu P ad

manabhan halt die Gravitation fiir eine Illusion,

erzeugtdurcheine Kornung der Raumzeit.

bild der wissenschaf t 11-2014 4 7 ?

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(Translated from German to English)

< Translation No. 2014TGE131 >

THE ATOMS OF SPACE-TIME

Physicists have a strong belief that the gravitation is not a fundamental force but possibly just a

persistent illusion.

By Ruediger Vaas

The field equations of gravitation have similarity with the equations of elasticity or

hydrodynamics. This statement from a scientific review article published few months back, maynot sound very exciting to laymen, rather it may sound cryptic. But it harbours within it explosives

that can virtually blow up the physical notions of space, time and gravity along with the

fundamentals of the universe. Not practically of course, but theoretically. And that would be fierce

and revolutionary enough.

This statement can be read as a hypothesis. For its author, however, it is more than that in a way,

the quintessence of twelve years of research. This author is not an extravagant esoteric, but a

multiple award-winning physics professor at the Inter-University Centre for Astronomy andAstrophysics in a western Indian state of Pune, former president of Cosmology Commission of

International Astronomical Union, an expert of the theory of general relativity and cosmology; and

a man of great renown: Thanu Padmanabhan.

Talent and nonconformity were gifted to this Indian right from the birth: The 57-year old physicist

had already published his first research article on general relativity when he was merely 20. A few

years later, he completed his doctorate under Jayant Narlikar, who wrote his thesis under the

High up in the sky: Apparently against the gravity, this hot air balloon floats in the sky. Thegravitational effect is in fact deceptive. Gravity is actually a derived phenomenon such as pressure of agas, which is something not fundamental, but can be reduced to the movement of molecules.Surprisingly, the equations of general relativity are similar to those of thermodynamics, which also

describe the functioning of colourful Montgolfier balloons.

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The Superficial Relativity Theory

Padmanabhan sees parallels to the general relativity theory here. He does not consider these

parallels to be fundamental either, but as an effective description, in the manner of speaking of

Physicists. Accordingly, the gravity and space-time would be emergent. This would have drasticconsequences on the long sought-after theory of quantum gravity, which is also known in a bit

exaggerated terms as the theory of everything.

If the emergent-concept is correct, then most of the previously performed experiments to quantify

Einsteins equation are on the wrong path same as if one were to seek atomic physics by

quantifying the laws of elasticity, explained Padmanabhan. Variables like metrics and curves

used for describing space-time in the theory of relativity, are analogous to thickness, velocity etc.,

in hydrodynamics, and are insignificant at microscopic level of description. Further: thequantification of metrics helps as little to unfold the microscopic structure of space-time, as does

the quantification of thickness and velocity of a liquid in understanding molecular dynamics.

This is a radical concept, since it implies that space-time itself is composed of small particles, and

that gravitation is not a fundamental force, rather a derived quantity and an illusion in the end. For

common sense, that is an almost grotesque sounding conclusion. After all, everyone feels the

gravitation of the earth the feeling is intense especially in the morning, when the alarm goes off.

But the idea is actually not new. This is because, in the context of the theory of relativity,

gravitation is described not as a force, but as a geometrical property of space-time.

"Einstein himself taught us that there is no gravity", says Padmanabhan. "Matter bends space-time,

which appears as a force to us. That is why all that needs to be explained is how the relation

between matter and space-time bending can be understood within thermodynamics."

The basic idea behind the relation between gravitation, thermodynamics and "granular" space-time

has been known for a while. The Soviet physicist and later dissident Andrew Sacharov had already

speculated about this in his two-page long article in 1967. In the 1980's, Kip Thorne and Thibault

Damour drew the same conclusion in a different way as they discovered an analogy between the

"superficial" properties of the black holes and hydrodynamics. In 1995, Ted Jacobson caused quite

a stir as he proposed a thermodynamic derivation of the general relativity. Erik Verlinde from the

University of Amsterdam, with the perspective of string theory, has been arguing that gravitation is

an "entropic force" since 2009, and thus calls it to be an emerging phenomenon. Verlinde's model

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was, however, strongly criticised on pure mathematical grounds by Matt Viser from Wellington

University, New Zealand, and Padmanabhan considers that to be "algebraically inadequate".

Hot Horizons

Thanu Padmanabhan has further developed Jacobson's approach since 2002. It started off with the

findings of Bill Unruh, Paul Davies, Stephen Hawking and other physicists in the field of

thermodynamics of horizons in the theory of relativity. Such horizons emerge on the outer edges of

black holes, but also during accelerated movements. Unruh and Davies independently calculated

that a fictitious observer could register a temperature of space that is proportional to its acceleration

(ref. to the box on the right "The temperature of vacuum"). Admittedly, it is infinitely small in

practice: the value of acceleration due to gravity on earth (9.8 m/s 2) would be equal to only 10 -46

Kelvin in vacuum. But in principle, that is a real quantum effect, which could slightly heat up a

glass of water.

"When a space-time horizon moves by a small degree, it is analogous to the change in volume of a

gas", this is how Padmanabhan sums up his discovery. "Einstein's equations, which describe space-

time, then exactly correspond to the first law of thermodynamics." This law of conservation of

energy relates to the changes in mechanical work, internal energy and entropy, which is a physicalmeasure of the disorder in a system. Thus, the pressure of a gas in a vessel is able to do work by

moving a piston, e.g. in a combustion engine. Moreover, energy can be exchanged with the

environment, e.g., when the heat "flows", thereby changing the entropy of the system.

Even space-time horizons, for instance the imaginary surface of a black hole, have a certain entropy

and therefore a temperature. At the same time, the greater the horizon, the greater is the entropy

because the horizon virtually harbours the disorder or the volume of information within it.

The Density of Space-Time

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Hence, according to Padmanabhan, a logic applies here: that if something can be heated, then it has

a microstructure. This logic had already been successfully applied by Ludwig Boltzmann in

thermodynamics and in statistical mechanics co-established by him. Thus, the Viennese physicist

interpreted temperature as the result of random movement of discrete microscopic particles the

molecules or the atoms. The swifter they move e.g., in a gas, the hotter it becomes. Only because of

this microstructure can a system save energy and exchange it with its environment. Entropy

describes the microscopic information content of such a system.

"There must exist well-defined microscopic degrees of freedom within space-time, which are

responsible for its thermal behaviour", is how Padmanabhan translates Boltzmann's logic. The

Unruh-effect quasi refers to the heating up of space-time. "The correlation between

thermodynamics and gravity is not a mathematical curiosity, but a physical reality. An appropriate

description of gravitation should assume the entropy density of space-time, which is equivalent to

the density of atoms of space-time."

Padmanabhan is even convinced that the density of microscopic degrees of freedom can be

estimated from the microscopic dynamics. That too is analogous to thermodynamics since at a

specific temperature, each degree of freedom saves the amount of energy proportional to that

temperature. As early as the 19th century, this made it possible to estimate the number of particles

in a certain amount of gas which is also known as the Avogadro number. Frankly, nobody knewwhat that meant back then. "Same is the case with the atoms of space-time", Padhmanabhan

comments. According to him, one can directly explain this more fundamental reality only with a

quantum gravitational theory.

According to Padmanabhan, gravity can more or less be

explained thermodynamically, depending upon the

entropy density and the number of microscopic degrees of

freedom similar to how a gas can be characterised by

the macroscopic variables - volume, pressure and

temperature - and how the kinetic energy of the molecules

can be deduced from it.

If a thermodynamic system is in equilibrium, then its entropy is at maximum. Same applies to

gravitational systems, as demonstrated by Padmanabhan and Aseem Paranjape: "Space-time

complies with Einstein's equations because the atoms of space-time maximize the entropy same

The world made of pixels: Space-timecould be granular like a digital photo

but we dont see it because the resolvingpower of our eyes and instruments is too

little.

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as a gas obeys the gas laws because its atoms maximize the entropy". The abstract correlation with

thermodynamics is therefore more than an analogy, as concluded by both the physicists: It allows

you to take a look at a deeper reality in the manner in which Boltzmann with his statistical

thermodynamics could conclude the existence of atoms in the 19 th century, although the atoms were

unobservably small for the state of technology at that time.

"Instead of talking about space-time atoms, one can talk about the physical degrees of freedom.

Both expressions are mathematically equivalent, if there is a pre-defined space and if not, the

description can still be given with the help of degrees of freedom", says Padmanabhan. "The

number of degrees of freedom can be calculated with respect to the theory of relativity and other

alternative gravitational theories as well.

As speculated by other approaches to the theory of quantum gravitation, this granulation of space-time lies in the magnitude of 10 -33 cm and 10 -43 s on the Planck's scale. This size is so tiny that

space-time appears to be a single unit to both - normal vision and the most advanced atom colliders

worldwide. Space-time can be compared to a photo, which seems homogenous if seen from a

distance, but if one looks at it closely, one notices that it is composed of individual pixels.

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Quantum Gravitation and Dark Energy

Padmanabhan is convinced: since one has not yet identified the atoms of space-time and

described them with the help of a new quantum theory, the centuries-long experiments to quantify

gravity were doomed to fail from the very start. With the current understanding, he still considers itdifficult to make statements about the density of space-time atoms or their modification. However,

in his current works, the physicist has included observations on how the emergence of space-time

can be understood in the context of expansion of the universe.

Furthermore, the mysterious dark energy, which is at present accelerating this expansion, can also

possibly be explained in the context of Padmanabhans approach. The fact that this dark energy

for instance Einsteins cosmological constant exists, but is very small, is one of the biggest

puzzles of Physics. If Padmanabhans ideas are correct, then there has to be a small cosmologicalconstant as a relic of quantum gravitation. With a theoretically sharpened perspective, one could

therefore consider both the realm of the tiniest particles and the dynamics of the universe as a

whole.

More about the topic:

FOR READING

[Theory of relativity, cosmology, dark matter and

dark energy:]

[Review articles about modified gravitational

theories:]

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[Excellent introduction to theory of relativity]

[Clarification about anti-relativity theories

and Einsteins opponents:]

[Jordan-Brans-Dicke-theory:]

[MOON Theory and arguments:]

[TeVeS Theory and arguments:]

[Homepage of Thanu Padmanabhan on

Emergent Gravitation:]

< Boxes in the original text >

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He looks behind the veil of gravity: Thanu Padmanabhan

considers gravity to be an illusion, created by the granulation of

space-time.

In Brief

The equations of general relativity are curiously similar to those of energy, entropy, and

work.

This suggests that space and time are not fundamental notions, but are made up of small

constituents which probably points towards a theory of everything.

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The Fire behind Einsteins Equations

The laws of thermodynamics are surprisingly similar to those of general relativity: entropy, energy and

work can be described in the same way as gravitation. According to that, gravity would be a derived

amount like temperature. That is because there is a deeper thermodynamic relation between horizons in

the theory of relativity such as those at the outer boundary of a black hole and the volume of a gas

at a specific pressure. If the volume increases, so will the entropy density same as in case of a black

hole whose surface increases proportionally with its entropy.

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The Temperature of Vacuum

The discovery of correlations between areas of physics that are apparently far apart from each other has

time and again proved to be the key to a deeper understanding of the world. Therefore, Thanu

Padmanabhans interpretation of field equations of gravitation as equilibrium conditions of space-time is

very promising. At first glance, the theory of relativity seems to be quite different than thermodynamics

and quantum theory. Thus, the classical field equations of gravitation do not have a Plancks constant h,

which is essential for the quantum theory. The Unruh-temperature T, which in principle could be

measured by an observer accelerated in quantum vacuum, is based, on the other hand, on h. T =

ha/2ck B, where k B is the Boltzmann constant, a is the local acceleration of the observer and c is the

speed of light. (Hawkings temperature of a black hole is described with the same formula, but a is here

the gravitational acceleration at the event-horizon). Since T depends on h, and the entropy of the horizon

is inversely proportional to h, this value gets cancelled. The correlation between gravitation and

thermodynamics can be established at all for this reason alone.

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Higher Dimensions

The analogy between thermodynamics and gravitation applies not only to the general relativity,

wherein entropy is proportional to the surface of a horizon, but also to other gravitational theories

which are higher dimensional. (The temperature of the horizon is independent of the respective

gravitational theory, but not entropy). Einsteins field equations can namely be generalised to more

than 3 spatial dimensions. This was proved by the British physicist David Lovelock in 1971, based on

the works of Hungarian physicist Cornelius Lanczos in the 1930s. This applicability to Lanczos-

Lovelock theories suggests that the idea of space-time atoms encounters something physically real,says Thanu Padmanabhan. Due to the same reason he is, however, sceptical that there is a connection

to loop quantum gravity here, which likewise assumes that space-time is made up of more

fundamental particles. This is because it functions only in 3 spatial dimensions. Padmanabhan is more

sympathetic towards the higher dimensional string theory with its additional spatial dimensions. The

more so as it also describes a correlation between a surface such as a horizon and a space

surrounding it or within it (holographic principle), which also plays a role in Padmanabhans

approach, but not in loop quantum gravity.

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