Lee Smolin: Three roads to quantum gravity.

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4 prinsipls.

Quantum theory chanj'd the asumptions about the relation betwyn observer and observ'd but retain'd Newton's viw of spas and tIm. Jeneral relativity chanj'd the later but not the former. So, Lee Smolin said, in the yer 2000, of the serch for the unifying theory of quantum gravity.

Thry main grups reserch this, by way of string theory, wich is mainly a development of quantum theory; or lwp quantum gravity, bais'd on jeneral relativity with quantum modifikations; and a third smal grup of orijinals. Smolin is houpful that the thry grups ar konverjing to enhans ych other's understanding.

String theory is introdus'd as the third roud to quantum gravity. Having reviw'd Brian Greene's bwk on The Elegant Universe, I'v said no mor about it hyr.

Smolin out-lIns 4 prinsipls as a basis for progres. First, konsistensy, with the definition of a univers, requirs that 'there is nothing outside the universe'. So far, he agrys with his frend and kolyg, Julian Barbour ( hws bwk is also reviw'd on this sIt ). He agrys that tIm only maks sens in terms of chanj. But Smolin dosnt tryt tIm as an ilusion.
Wen we lwk into spas we ar lwking bak, also, in tIm. The lIt from further away koms from further bak in the history of the univers.

Hens, Smolin's sekond prinsipl: in the futur we shal no mor. Nothing kan travel faster than lIt. But as tIm pases by, the spot-lIt, we ar in, grows biger. How-ever, the spot-lIt is the limit to wich we kan sy. This spot-lIt is diferent for diferent parts of the univers, depending on the tIm lIt has had to rych a given spot.
( The spot-lIts ar an other word for the 'lIt-kouns' familiar to ryders of popular bwks on jeneral relativity. )

No-uon kan hav akses to total nolej about events in the univers. So, we kan not always say wether a thing is tru or fals, as Aristotle's klasikal lojik asums. Nw systems of lojik, aknolejing only partial information, dependent on the observer's situation, reflekt the natur of sosiety. Uon of thys systems, topos theory was found, by Fotini Markopoulou-Kalamara, to suit kosmolojy.

The quantum theory paradox of Schrödinger's kat, and so forth, mak no sens in terms of klasikal lojik or komon sens. This is the 'super-position prinsipl' that a kat in a box, subjekt to the chans of a fatal aksident, is in super-pos'd staits of being aliv and ded, until an observer opens the box. Then, the konventional interpretation gos, ther is a 'kolaps of the wav funktion' of super-pos'd staits, resulting in either of the definit staits: ded or aliv - but not both! ( Tho, this paradox begs the question of being 'haf ded'. )

The paradox givs a vivid idea of the observer being out-sId the observ'd system. But kombining quantum theory with kosmolojy myns that the observer kan not be konsyv'd as existing out-sId the system, wen it is the houl univers.
The Wheeler-DeWitt equations supos the quantum konstraints on the univers. The author play'd a part in hiting upon ther exakt solutions, saying it twk an other ten yers to find out wat they ment.

Leiter, Smolin ads, lIk Douglas Adams' galaktik hitch-hiker syking the myning of lIf:

So conventional quantum cosmology seems to be a theory in which we can formulate the answers, but not the questions.

He gos on that this is not surprising, sins the houl univers is not within our purviw lIk a quantum experiment in the laboratory. Kontext-dependent theorys, such as Markopoulou's kosmolojikal lojik aplI'd to quantum theory, provid a ryson for observers' diferent points of viw, from wich the super-position paradox folows. Uon may observ a system, that inkluds an other observer in a super-position of staits. But that observer never so deskribs himself, remaining out-sId the system he deskribs.
And this is never prekluded: the system observ'd kan never be the totality of the univers, bekaus of the lIt-spyd limits on the sIz of the observabl univers.

A slogan for this point of viw is: 'Uon univers, syn by many observers, rather than many universes, syn by uon mythikal observer out-sId the univers'. And this is Smolin's third prinsipl.

His forth prinsipl is: The univers is mayd of proseses not things. Hyr he klyrly difers from Julian Barbour. The world is not mayd up of a lot of statik snap-shots put together lIk a movy. Taking the analojy further, he points out that ryl snap-shots dekompos. Every-thing we observ is always chanjing mor or les. In direkt kontrast to Barbour, Smolin spyks of 'the ilusion of the frozen moment'.

Smolin says we lern about things, just as we do about pepl, from ther storys, wich ar esentialy about kauses.

The fundamental idea in general relativity is that the causal structure of events can itself be influenced by those events...The laws that determine how the causal structure of the universe grows in time are called the Einstein equations. They are very complicated, but when there are big, slow-moving klutzes of matter around, like stars and planets, they become much simpler. Basically, what happens then is that the light cones tilt towards the matter... ( This is what is often described as the curvature, or distortion of the geometry of space and time. ) As a result matter tends to fall towards massive objects. This is...the gravitational force. If matter moves around, then waves travel through the causal structure and the light cones oscillate back and forth...These are the gravitational waves.

Never the les, Smolin says fysisists tend to think ther is a limit to the number of events in a proses. And that spas and tIm ar not kontinuus but form into fundamental diskryt units ( rather lIk the quantum, h, is such ). ( Barbour's 'snap-shot' reality may yet get a lwk in. )

Blak hol thermo-dynamiks.

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This is the first roud to quantum gravity.

In akord with Einstein's equivalens prinsipl, a spas-ship kan maintain a position out-sId the event horizon of a blak hol, with a fors of akseleration matching the hol's fors of gravitational atraktion. The horizon is a 'kurtain' of unsyn fotons that ar just unabl to eskaip the blak hol. Hens the naim: this kurtain and every-thing behind it is a hiden rejon.

In jeneral, wer-ever a lIt sors kan not rych an observer, that is in a hiden rejon to that observer. Even far away from any blak hols, a spas-ship's akseleration wil kreat a hiden rejon behind a horizon of fotons that kan not be syn from the ship, bekaus its akseleration has byn enuf to put them out of rych, even tho the ship itself kan not rych lIt spyd.

Bill Unruh predikted that the enerjy sors provided by the akseleration wil aktivat the ship's partikl detektors to rejister quantum fluktuations, in the vakuum of spas, betwyn electrik and magnetik fylds. By Heisenberg's unsertainty relation, both fylds kan not be mesur'd, in a rejon, as zero.

An other prinsipl, that of quantum korelation, predikts the fluktuations wil be random, wich implIs hyt, detektabl as a temperatur proportional to the ship's akseleration. A pair of spontanusly kreated partikls, lIk fotons, within the limits alow'd by the unsertainty relation, ar, in efekt a system, wich kan only be properly understud as a houl. A chanj in the kondition of uon foton, such as its polarisation, wil afekt the polarisation of the other, konserving the pair as a system, even tho they may hav mov'd tu far apart for a lIt signal to hav byn quik enuf to efekt this korelation.

The akselerating spas-ship detekts fotons korelated with fotons in its hiden rejon, denIing ther systemik information and, in efekt, surounding the ship with a random 'gas' of fotons.
This is Unruh's law, the study of quantum gravity's first prediktion.

The entropy of the gas is the mesur of al the positions and motions of molekuls in the gas. This mesur is mayd in terms of information theory that kounts in sequenses of bits, as the number of ansers to yes/no questions, lIk a dijital komputer. This information is mis'd wen taking only temperatur and density averajes yus'd in statistikal mekaniks or thermo-dynamiks.

The foton gas randomnes results from the mising information, in the akselerating spas-ship's hiden rejon, wich the entropy mesurs as exaktly proportional to the area of the horizon boundary betwyn the ship and its hiden rejon.
This is Bekenstein's law, the sekond prediktion of quantum gravity.
'Bekenstein's bound' is a limit on the information that kan be kontain'd in any rejon. This finit kapasity for information implIs spas is diskryt, on the Planck skail.

Thermo-dynamiks staits an entropy law of the over-al inkrysing dis-order of things, wich givs a sens of tIm being ireversibl. Blak hols hav this karakter, bekaus nothing faling in uon kan ever get out. Konsequently, Stephen Hawking show'd, that, lIk entropy, the area of a blak hol kan never dekrys.

In the kais of blak hols, the random foton gas is nown as Hawking radiation, wen pairs of virtual partikls, kreated from the quantum fluktuations of spas, ar split nyr the event horizon. Uon partner may fal in the blak hol, the other be shot off into spas. The random radiation ment a blak hol wud giv off ( very minimal ) hyt, the result of mising information from blak-hol'd partner partikls, wich show'd a blak hol kud hav entropy.

Hawking's law is a third prediktion, that the temperatur of a blak hol is inversly proportional to its mas. Hawking radiation myns a blak hol wil lws mas, therfor lws area, and lws entropy. The out-sId world shud gain the entropy, so ther is no over-al los, kontrary to thermo-dynamik law.

A blak hol of the sun's mas wud tak ten, to the power of 57, tIms the 14 bilion yer-aij of our univers, to evaporat. So, the natur of the information trap'd in the blak hol, and posibly relys'd by evaporation, is of desIdedly theoretikal interest.

The information lost in a blak hol, is mesur'd in diskryt units of atoms and fotons. But the mesur of the blak hol's entropy is in terms of the kontinuus area of its horizon. The thry rouds to quantum gravity ar konverjing on an atomis'd or quantis'd konsept of spas and tIm having fundamental units.

The Bekenstein surpris that information kapasity of spas is proportional to a rejon's area, and not its volum, maks uon think of a holograf. This is a tw dimensional piktur that enkouds thry dimensions depending on wich angl yu lwk at it.
The wyk holografik prinsipl tryts the surfases of things as skryns with finit kapasitys to chanel information from observer to observer.

The last part of Smolin's bwk, on frontiers of nolej, mentions several versions of a holografik prinsipl, for wich ther ar greit houps as a nw founding prinsipl of quantum gravity, as the unsertainty relation is for quantum mekaniks, and the equivalens prinsipl for jeneral relativity.

Lwp quantum gravity.

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The atomik natur of thermo-dynamiks was not aksepted in the nIntynth sentury. Einstein's 1905 paper explain'd Brownian motion in terms of kolisions from the random motions of atoms or molekuls. An other of his 1905 papers also explain'd lIt atomikly in terms of lIt-quanta, karying a unit of enerjy proportional to the lIt's frequensy.
A theory of quantum gravity wil lIkwIs quantis spas and tIm.

Chapters nIn and ten ar the kor of Smolin's bwk, bekaus they deskrib him working with many kolygs, the world over, to kom to lwp quantum gravity -- the sekond roud. Bekaus he is teling a story, this ryder was given an ilusion of understanding ther progres, wich klyrly depends so much on profesional ko-operation.

With apolojys, this amater reviwer myrly maks a fw nouts, by way of memorandums, hoping they ar not tu mis-lyding.

Of the 4 nown forses of natur, the strong fors binds the thry quark konstituents of partikls, lIk protons and nutrons, that themselvs mak up an atomik nukleus, hws kloud of elektrons, in turn maks up an atom. Elektrons ar mor or les ysily strip'd from atoms.

But enerjy direkted at frying quarks from protons only syms to ad to the lenth of an aparent string, joining the quarks, without diminishing its strenth. This 'quark konfInment' has an analog in super-konduktiv metals at temperaturs a fw degrys abov absolut zero. Normaly, magnetik lIns of fors ar kontinuus, tho the sIz of iron fIlings, put on paper around a magnet, show diskryt lIns. Only in a super-konduktor ar magnetik fyld lIns quantis'd, karying a houl number multipl of a basik unit of magnetik flux.

The electrik fors is klosly related to the magnetik fors. Together they kompris uon of the for forses of natur. And the theory of the strong fors was bais'd on an analojy with electrik charj, exept that quarks ar distinguish'd by having thry distinkt charjes ( thry 'kolors' ).
( Quantum Kromo-dynamiks, is the analjus theory to Quantum Elektro-dynamiks, QED. )

The kolor-elektrik lIns of fors, holding the kolor charjes of quarks together, kud bekom diskryt lIk a lIn of magnetik flux in a super-konduktor. The ges is that 'empty spas is a kolor-elektrik super-konduktor'. The komplyt lak of elektrikal resistans found in super-konduktivity is as if the temporary quantum fluktuations of enerjy in a vakuum also had larj-skail efekts.

The stretch'd 'strings', betwyn quarks, hav byn thot basik entitys, rather than just lIns from fors fylds. Other fysisists thot both points of viw valid. Uon of the later was Smolin after hyring of 'Wilson's lwps'. Ken Wilson asum'd a diskryt spas bais'd on a grid or latis, of units far smaler than a proton's diameter. Quarks kud only be on the nouds and strings on the ejes of the latis.

Yusing simpl ruls, the thry-kolor-elektrik fyld was deskrIb'd by the movment of diskryt fyld lIns along the diskryt spas. Given only uon charj, lIk normal elektrisity, the fyld lIns tended to lws ther diskrytnes by joining to behaiv lIk kontinuus elektrik fyld lIns. But, given thry charjes, as with quarks, the fyld lIns always stay'd diskryt, no mater how big they got.

The next step wud be to dispens with the grid, as a fix'd bak-ground, lyving only the 'quantis'd lwps of elektrik flux' to karakteris a diskryt spas. Bilding on the work of many kolygs, as always, the author's work inkluded yusing Polyakov's expresions for the quantis'd lwps of elektrik fylds as the quantum staits for a jeometry of spas-tIm given in a simplify'd version of the Wheeler-DeWitt equations.

It wud not mater wer thys bak-ground independent lwps ar in spas. That wud hav no myning, bekaus spas itself wud be defin'd by the inter-relations of the lwps, ther inter-sektions, nots, links and kinks.
The idea of diskryt lIns of fors, taken from that in a super-konduktor's magnetik fyld, quantis'd areas into diskryt units, on the Planck skail, ych karying finit amounts of area; lIkwIs for volum.

Lwp staits kud be aranj'd in 'spin net-works', previusly deriv'd by Roger Penrose, uon of the orijinals, amongst the grups reserching quantum gravity. The varius lenths, of the join'd lIns in the net, ar integers koming from quantum theory's alow'd spin staits of partikls.
Arduus translation, of lwp quantum gravity into spin net-works, revyl'd:

...each spin network gives a possible quantum state for the geometry of space. The integers on each edge of a network correspond to units of area carried by that edge. Rather than carrying a certain amount of electric or magnetic flux, the lines of a spin network carry units of area. The nodes...correspond to quantised units of volume. The volume contained in a simple spin network, when measured in Planck units, is basically equal to the number of nodes of the network.

Theorems show 'that the spin network picture of quantum geometry... follows directly from combining the basic principles of quantum theory with those of relativity.'

'Connections have been discovered to... such as Alain Connes' non-commutative approach to geometry, Roger Penrose's twistor theory and string theory.'

Giovanni Amelino-Camelia sujested a test wether the jeometry of spas is diskryt on the Planck skail. A foton's path shud be deviated, from its expekted klasikal path, by interferens efekts of its asociated wav being skater'd by the diskryt nouds of the quantum jeometry. Altho the efekt is extrymly smal, it is kumulativ and mIt be detektabl over larj fraktions of the observabl univers.

How probabl wud it be that this atomik struktur of spas yilded the Euclidean spas we sy? The univers under-went a feiz transition, lIk a gas turning to liquid. The erly plasma of fotons 'froz' into mater. A smwthly fyturles thry-dimensional jeometry resembls the krystalIn atomik struktur of a metal with its smwth surfas. For the atoms of spas to organis themselvs over the kosmos, so hIly, syms fantastikly improbabl.

( Reviw'd bryfly on this sIt's paj on theorys and methods of natural selektion ) Lee Smolin's former bwk, The Life of the Cosmos, sujests a kosmolojikal theory of natural selektion of universes, to mak the imposibl sym inevitabl.

Richard Lung.

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