Editing Theory of everything (section)

==Historical antecedents==

===Antiquity to 19th century===
Many ancient cultures such as [[Babylonian astronomers]] and [[Indian astronomy]] studied the pattern of the ''Seven Sacred Luminaires''/[[Classical Planets]] against the background of [[stars]], with their interest being to relate celestial movement to human events ([[astrology]]), and the goal being to predict events by recording events against a time measure and then look for recurrent patterns. The debate between the universe having either [[Temporal finitism|a beginning]] or [[Cyclic model|eternal cycles]] can be traced to ancient [[Babylonia]].<ref name="Hodge">{{cite book |last1=Hodge |first1=John C. |title=Theory of Everything: Scalar Potential Model of the Big and the Small |date=2012 |isbn=978-1-4699-8736-1 |pages=1–13, 99 |publisher=CreateSpace Independent Publishing Platform }}</ref> [[Hindu cosmology]] posits that time is infinite with a ''cyclic universe'', where the current universe was preceded and will be followed by an infinite number of universes.<ref>{{cite book |author=Sushil Mittal |author2=Gene Thursby |page=284 |title=Hindu World |publisher=Routledge |year=2012 |isbn=978-1-134-60875-1}}</ref><ref>{{cite book |author=Jones |first=Andrew Zimmerman |title=String Theory For Dummies |publisher=John Wiley & Sons |year=2009 |isbn=978-0-470-59584-8 |page=262}}</ref> Time scales mentioned in [[Hindu cosmology]] correspond to those of modern scientific cosmology. Its cycles run from an ordinary day and night to a day and night of Brahma, 8.64 billion years long.<ref>{{cite book |author=Sagan, Carl |title=Cosmos |year=2006 }}</ref>

The [[natural philosophy]] of [[atomism]] appeared in several ancient traditions. In ancient [[Greek philosophy]], the [[Pre-Socratic philosophy|pre-Socratic philosophers]] speculated that the apparent diversity of observed phenomena was due to a single type of interaction, namely the motions and collisions of atoms. The concept of 'atom' proposed by [[Democritus]] was an early philosophical attempt to unify phenomena observed in nature. The concept of 'atom' also appeared in the [[Nyaya]]-[[Vaisheshika]] school of ancient [[Indian philosophy]].

[[Archimedes]] was possibly the first philosopher to have described nature with axioms (or principles) and then deduce new results from them. Any "theory of everything" is similarly expected to be based on axioms and to deduce all observable phenomena from them.<ref name="Impey2012" />{{rp|340}}

Following earlier atomistic thought, the [[mechanical philosophy]] of the 17th century posited that all forces could be ultimately reduced to [[contact force]]s between the atoms, then imagined as tiny solid particles.<ref name="Burns2001">{{cite book |author=Burns |first=William E. |title=The Scientific Revolution: An Encyclopedia |date=1 January 2001 |publisher=ABC-CLIO |isbn=978-0-87436-875-8}}</ref>{{rp|184}}<ref>
{{cite book
 |first=Steven |last=Shapin
 |date=1996
 |title=The Scientific Revolution
 |url=https://archive.org/details/scientificrevolu00shap_0 |url-access=registration |publisher=[[University of Chicago Press]]
 |isbn=978-0-226-75021-7
}}</ref>

In the late 17th century, [[Isaac Newton]]'s description of the long-distance force of gravity implied that not all forces in nature result from things coming into contact. Newton's work in his ''[[Philosophiæ Naturalis Principia Mathematica|Mathematical Principles of Natural Philosophy]]'' dealt with this in a further example of [[Unification (physics)|unification]], in this case unifying [[Galileo]]'s work on terrestrial gravity, [[Kepler]]'s laws of planetary motion and the phenomenon of [[tide]]s by explaining these apparent actions at a distance under one single law: the law of [[universal gravitation]].<ref>{{cite book |page=255 |url=https://books.google.com/books?id=6EqxPav3vIsC&pg=PA255 |title=The Mathematical Principles of Natural Philosophy |volume=II |last1=Newton |first1=Sir Isaac |date=1729}}</ref> Newton achieved the [[Unification of theories in physics#Unification of gravity and astronomy|first great unification in physics]], and he further is credited with laying the foundations of future endeavors for a grand unified theory.

In 1814, building on these results, [[Laplace]] famously suggested that a [[Laplace's demon|sufficiently powerful intellect]] could, if it knew the position and velocity of every particle at a given time, along with the laws of nature, calculate the position of any particle at any other time:<ref name="Carroll2010">{{cite book <!-- Citation bot deny--> |author=Carroll |first=Sean |title=[[From Eternity to Here: The Quest for the Ultimate Theory of Time]] |publisher=Penguin Group US |year=2010 |isbn=978-1-101-15215-7 |language=en-us}}</ref>{{rp |ch 7}}

{{quote|An intellect which at a certain moment would know all forces that set nature in motion, and all positions of all items of which nature is composed, if this intellect were also vast enough to submit these data to analysis, it would embrace in a single formula the movements of the greatest bodies of the universe and those of the tiniest atom; for such an intellect nothing would be uncertain and the future just like the past would be present before its eyes.|''Essai philosophique sur les probabilités'', Introduction. 1814}}

Laplace thus envisaged a combination of gravitation and mechanics as a theory of everything. Modern [[quantum mechanics]] implies that [[Heisenberg uncertainty|uncertainty is inescapable]], and thus that Laplace's vision has to be amended: a theory of everything must include gravitation and quantum mechanics. Even ignoring quantum mechanics, [[chaos theory]] is sufficient to guarantee that the future of any sufficiently complex mechanical or astronomical system is unpredictable.

In 1820, [[Hans Christian Ørsted]] discovered a connection between electricity and magnetism, triggering decades of work that culminated in 1865, in [[James Clerk Maxwell]]'s theory of [[electromagnetism]], which achieved the [[Unification of theories in physics#Unification of magnetism, electricity, light and related radiation|second great unification in physics]]. During the 19th and early 20th centuries, it gradually became apparent that many common examples of forces – contact forces, [[elasticity (physics)|elasticity]], [[viscosity]], [[friction]], and [[pressure]] – result from electrical interactions between the smallest particles of matter.

In his experiments of 1849–1850, [[Michael Faraday]] was the first to search for a unification of [[gravity]] with electricity and magnetism.<ref>
{{Cite journal
 |first=M.|last=Faraday
 |date=1850
 |title=Experimental Researches in Electricity. Twenty-Fourth Series. On the Possible Relation of Gravity to Electricity
 |journal=Abstracts of the Papers Communicated to the Royal Society of London
 |volume=5 |pages=994–995
 |doi=10.1098/rspl.1843.0267
|doi-access=free
 }}</ref> However, he found no connection.

In 1900, [[David Hilbert]] published a famous list of mathematical problems. In [[Hilbert's sixth problem]], he challenged researchers to find an axiomatic basis to all of physics. In this problem he thus asked for what today would be called a theory of everything.<ref>{{Cite journal |doi=10.1090/S0273-0979-2013-01439-3 |title=Hilbert's 6th Problem: Exact and approximate hydrodynamic manifolds for kinetic equations |journal=Bulletin of the American Mathematical Society |volume=51 |issue=2 |page=187 |year=2013 |last1=Gorban |first1=Alexander N. |last2=Karlin |first2=Ilya|arxiv=1310.0406 |bibcode=2013arXiv1310.0406G |s2cid=7228220 }}</ref>

===Early 20th century===
In the late 1920s, the then new quantum mechanics showed that the [[chemical bond]]s between [[atom]]s were examples of (quantum) electrical forces, justifying [[Paul Dirac|Dirac]]'s boast that "the underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of chemistry are thus completely known".<ref>
{{cite journal
 |last=Dirac |first=P.A.M.
 |date=1929
 |title=Quantum mechanics of many-electron systems
 |journal=[[Proceedings of the Royal Society of London A]]
 |volume=123 |pages=714–733
 |doi=10.1098/rspa.1929.0094
 |bibcode=1929RSPSA.123..714D
 |issue=792|doi-access=free
 }}</ref>

After 1915, when [[Albert Einstein]] published the theory of gravity ([[general relativity]]), the search for a [[unified field theory]] combining gravity with electromagnetism began with a renewed interest. In Einstein's day, the strong and the weak forces had not yet been discovered, yet he found the potential existence of two other distinct forces, gravity and electromagnetism, far more alluring. This launched his 40-year voyage in search of the so-called ''"unified field theory"'' that he hoped would show that these two forces are really manifestations of one grand, underlying principle. During the last few decades of his life, this ambition alienated Einstein from the rest of mainstream of physics, as the mainstream was instead far more excited about the emerging framework of quantum mechanics. Einstein wrote to a friend in the early 1940s, "I have become a lonely old chap who is mainly known because he doesn't wear socks and who is exhibited as a curiosity on special occasions." Prominent contributors were [[Gunnar Nordström]], [[Hermann Weyl]], [[Arthur Eddington]], [[David Hilbert]],<ref>{{cite book |arxiv=physics/0405110 |doi=10.1007/0-8176-4454-7_14 |isbn=978-0-8176-4454-3 | title=Hilbert's "World Equations" and His Vision of a Unified Science |series=Einstein Studies |volume=11 |pages=259–276 |year=2005 |last1=Majer |first1=U. |last2=Sauer |first2=T. |bibcode=2005ugr..book..259M |journal=<!-- Citation bot--> |s2cid=985751 }}</ref> [[Theodor Kaluza]], [[Oskar Klein]] (see [[Kaluza–Klein theory]]), and most notably, Albert Einstein and his collaborators. Einstein searched in earnest for, but ultimately failed to find, a unifying theory<ref name="Pais1982">{{cite book |author=Abraham Pais |title=Subtle is the Lord: The Science and the Life of Albert Einstein: The Science and the Life of Albert Einstein |url=https://archive.org/details/subtleislordscie00pais |url-access=registration |date=23 September 1982 |publisher=Oxford University Press |isbn=978-0-19-152402-8|author-link=Abraham Pais }}</ref>{{rp|ch 17}} (see Einstein–Maxwell–Dirac equations).

===Late 20th century and the nuclear interactions===
In the 20th century, the search for a unifying theory was interrupted by the discovery of the [[strong force|strong]] and [[weak force|weak]] nuclear forces, which differ both from gravity and from electromagnetism. A further hurdle was the acceptance that in a theory of everything, quantum mechanics had to be incorporated from the outset, rather than emerging as a consequence of a deterministic unified theory, as Einstein had hoped.

Gravity and electromagnetism are able to coexist as entries in a list of classical forces, but for many years it seemed that gravity could not be incorporated into the quantum framework, let alone unified with the other fundamental forces. For this reason, work on unification, for much of the 20th century, focused on understanding the three forces described by quantum mechanics: electromagnetism and the weak and strong forces. The first two were [[electroweak interaction|combined]] in 1967–1968 by [[Sheldon Glashow]], [[Steven Weinberg]], and [[Abdus Salam]] into the electroweak force.<ref>Weinberg (1993), Ch. 5</ref>
Electroweak unification is a [[broken symmetry]]: the electromagnetic and weak forces appear distinct at low energies because the particles carrying the weak force, the [[W and Z bosons]], have non-zero masses ({{val|80.4|u=GeV/c2}} and {{val|91.2|u=GeV/c2}}, respectively), whereas the [[photon]], which carries the electromagnetic force, is massless. At higher energies W bosons and Z bosons can be [[matter creation|created]] easily and the unified nature of the force becomes apparent.

While the strong and electroweak forces coexist under the [[Standard Model]] of particle physics, they remain distinct. Thus, the pursuit of a theory of everything remained unsuccessful: neither a unification of the strong and electroweak forces&nbsp;– which Laplace would have called 'contact forces'&nbsp;– nor a unification of these forces with gravitation had been achieved.