String theory is an attempt at a deeper description of nature by thinking of an elementary particle not as a little pointbut as a little loop of vibrating string. Ed Witten (1951-)
Unification: Consistency is required. Actual high-energy unification is not.
String Theory and M-Theory by Leonard Susskind (Stanford, Fall 2010) [1 |2 |3 |4 |5 |6 |7 |8 |9 |10 ] Topics in String Theory by Leonard Susskind (Stanford, Winter 2011) [1 |2 |3 |4 |5 |6 |7 |8 |9 ]
In order of appearance : Steven Weinberg, Michael Green, Jim Gates, Peter Galison,Marcia Bartusiak,Joseph Polchinski,Walter Lewin,Amanda Peet,Nima Arkani-Hamed,Edward Farhi,Michael Duff,Ed Witten,Joe Lykken,Sheldon Glashow,Gabriele Veneziano, Lenny Susskind, John Schwarz, Cumrun Vafa,David Gross,Savas Dimopoulos,Burt Ovrut,Nathan Seiberg,Gary Horowitz,Alan Guth,Paul Steinhardt, Maria Spiropulu.
flaunted the above version of their logo worldwide. The discovery of the Higgs boson was announced 4 years later (2012-07-04). The 2012 Nobel prize was then widely expected to go to Peter Higgs (it went toSerge Haroche and David Wineland instead, for unrelated work). Peter Higgs andFrançois Englert were duly awarded the2013 Nobel prize for physics.
(2008-07-23) Why use the same units for all kinds of distances, large and small?
Theancient Egyptians measured largehorizontal distances by rolling a wheel whose diameter was measured inthe units they used for small vertical distances... The countless appearances of the number (the ratio of the circumferenceof a circle to its diameter) in the Egyptianpyramids can be puzzling to whoever has been exposed to unified Euclidean space since childhood. To Egyptian architects, the relevant dimensions were integers!
Blurring the distinction between horizontal and verticaldistances is philosophically pleasing, although this does notmake practical differences go away... There's very little difference between right and left butthere's a huge difference between up and down (just try falling up). The distinction between the horizontal andvertical directions (near the surface of the Earth) seems to vanish at high energies : If you shoot a gunindoors, the bullet always moves in a straight line and atthe same speed no matter where you aim. Outdoors,distances are larger and there's enough time for the pull of gravityto influence the bullet noticeably. Very fast "bullets" (photons or particles moving atnearly the speed of light) are not noticeably influenced by gravity,except over astronomical distances.
"Unifying" two physical concepts is not at all a denial of their differences,it's the creation of a common consistent frameworkwhere those differences can be charted and where their interplay becomes clear. If you have arigid stick with one fixedend, our "unified" notion of Euclidean distance will tell youhow the vertical position of the moving end varieswhen its horizontal position changes.
Similarly, the unification of space and time in the context ofSpecial Relativity does not equatethe two notions but it describes circumstances (motion of the observer) where one is traded for the other. Loosely speaking, the speed of light (Einstein's constant c) is built into relativistic spacetime in very much the same way was built into the architectureof the ancient Egyptians... The numerical value of c is merely a consequence of our traditional ways to measure spatialdistances, on one hand, and time intervals, on the other. Rulers and clocks.
Historically, unifying separate physical concepts has always resulted ina deeper understanding of Nature. Arguably, the most satisfying such event was the unification of electricityand magnetism byMaxwell (1861) ashe found a simple way toamendthe law of Ampère into aconsistent picture of electromagnetism which demanded the existence of electromagnetic waves propagatingat a constant speed c. The fact that this ought to be so for all observers in uniform motionwith respect to each other directly led tospecial relativity.
Unifying quests are so appealing that many mathematicalphysicist share a blind faith: The forces of nature mustultimately be unified; at high enough energies all interactions ought to lookalike (just like the aforementioned great speed of bullets blurs the distinctionbetween horizontal and vertical directions). Some evidence indicates that it may well be so. However, the greatest goal of physics will be achieved if we have consistent descriptions of all physicalphenomena, not necessarily unified ones.
(2008-07-09) A universe with 5 dimensions to unify gravity and electromagnetism.
When Oskar Klein told of his ideas which would not only unify the Maxwell with theEinsteinequations but also bring in the quantum theory, I felt a kind of ecstasy: Now, one understands the world ! GeorgeUhlenbeck (1900-1988) Summer of 1926.
Classical relativistic spacetime has 4 dimensions;one dimension of time and 3 dimensions of space. In 1919, the German physicist Theodor Kaluza (1885-1954) suggested that one extra geometrical dimension could be added to account forelectromagnetic phenomena and describe them inpurely geometrical terms, in much the same way Einstein'sGeneral Relativity describes gravity.
In the summer of 1926,Paul Ehrenfest (1880-1933) invited to Leiden the Swedish physicistOskar Klein (1894-1977)to present his refinement of the Kaluza theoryand the idea that extra spatial dimensions might be a good way of unifying Relativity with Quantum Theory. Klein envisioned that a topologically curled extra dimensionwouldn't be perceived as a spatial dimensionon a normal scale, pretty much like the two-dimensional surface of a garden hosemay look like a single-dimensional wire, if observed from a large enough distance.
For a while, this stirred the enthusiasm ofAlbert Einstein (1879-1955) himselfand caused the special type of ecstasy described in theabove quote by George Uhlenbeck (who was Ehrenfest's assistant at Leiden in 1926).
However,the excitement over this 5-dimensional physical universe of 1926 (the so-called Kaluza-Klein Universe) was short-lived, since some consequences ofOskar Klein's proposal turn out to be entirely off-base.
What's still with us today is Klein's fundamental ideas about how extra dimensionsmight provide a quantum theory compatible with General Relativity. The concept was revived in the 1970s and in the 1980s,as extra geometrical dimensions are a prerequisite forwhat's now called String Theory. Such dimensions are still visualized as rolled up, although they need not have a compact topology.
(2011-04-20) The light-cone frame ("infinite momentum") & constant-tension strings.
In 1968,Gabriele Veneziano (1942-)took a boat trip from Israel to Italy en route to hisfirst postdoctoral job at CERN.
At the time, Veneziano had already been working for about a year (with M. Ademollo, H. Rubinstein and M. Virasono) on the complementary duality of theReggeand resonance descriptionof pion-nucleon exchange, which had been proposed by R. Dolen, D. Hornand C. Schmid.
Veneziano and his three colleagues had been putting together a modelof the relevant scattering amplitude in the process. On the boat, Veneziano realized that the essential features of thatscattering amplitude would be captured by a simple expressioninvolving Euler'sBeta function andGamma function,namely:
A ( s , t ) B ( 1 (s) ,1 (t) ) =
( 1 (s) ) ( 1 (t) )
( 2 (s)(t) )
The attractive closed form of the Veneziano amplitude contrastedsharply with the usual intractability which physicists had to deal with forstrong nuclear interactions. The formula created a widespread stir.
(2007-08-17) Leonard Susskind (1940-) Nielsen and Nambu.
Lenny Susskind has beenatStanford since 1979 (Felix Bloch Professor of Theoretical Physics, since 2000).
In 1969, Susskind pondered Veneziano's formula for months,trying to make some clear physical sense out of it. He finally came to the conclusionthat an entity was described which could stretch and vibrate just likean open-ended elastic string. (I'm told that the expression is now interpreted asthe scattering amplitude for four open-string tachyons).
Two other physicists working on the same premises arrived independentlyat similar conclusions shortly thereafter: Holger Nielsen (1941-)at the Niels Bohr Institute, and Yoichiro Nambu (1921-2005)the inventor of the color charge (University of Chicago).
(2007-08-17) Joël Scherk (1946-1979) & John H. Schwarz (1941-).
As they were trying to use the newly minted string theory to describethe strong nuclear force, Joël Scherk and John Schwarzkept bumping into a massless elementary particle of spin 2 which did notfit whatever was known about strong interactions. After failing to conjure up ways to get rid of this nuisance,they came to the conclusion that this unavoidable entity could very well be the graviton itself (that same idea is also credited to the Japanese physicistTamiaki Yoneya, b. 1947)... Thus, string theory had to encompass gravityand seemed destined to describe fundamental strings with a much smaller size and a much greater tension thanpreviously thought (in the restricted context of strong interactions).
After the death of Scherk, Schwartz found only one person willing to help withthe work they had started together: Michael Green...
(2007-08-17) Michael B. Green (1946-) & John H. Schwarz (1941-).
Arguably, Superstring Theory was born in the Summer of 1984,when Michael Green and John Schwarzfinally established the consistency of a theory rich enough to encompassall known forces of nature. This was the first credible candidate for a Theory of Everything (TOE).
At the time, it appeared that the ultimate puzzle was being solved for good.
At Princeton, Ed Witten built immediately on the breakthrough of Green & Schwarz. On Monday, November 12, 1984, for the annual MarstonMorse Memorial lecture, Witten delivered a fast-paced speech entitled "Index Theorems and Superstrings" at the Institute for Advanced Study. Witten was speaking for the record, not for the immediatebenefit of the 200 top-level scientists who were attending (there were no questions from them).
When Stephen Hawking (1942-2018) stepped down as Lucasian Professor of Mathematics in Cambridge (on 2009-09-30) Michael B. Green was appointed (on 2009-10-19, as of 2009-11-01) to theprestigious chair, once held by Newton,Airy,Babbage,Stokes,Larmor andDirac. With effect from 2015-07-01, Green's successor isMichael Cates (b. 1961).
(2017-08-07) Hybrids of a closed superstring and a bosonic string.
Heterotic string theory was first developed in 1985, by the so-called Princeton String Quartet composed of:
(2008-08-29) Too much of a good thing: 5 consistent string theories.
No fewer than five consistent string theorieshave been devised:
Type I : The earliest theory. It allows both open and closed strings (the other theories allow only closed strings).
Type II A : The only nonchiral string theory.
Type II B : The chiral version of the previous one. Both of them feature two supersymmetries between fermions and bosons (the other three superstring theories have only one such supersymmetry).
SO(32)Heterotic Strings : The term "heterotic" means thatthe two directions along a string represent two different particles.
E8 x E8 Heterotic Strings : Based on two copies of the largest exceptional Lie group (E8).
(2007-08-17) Is "M" magic,mystery,matrix,murky or membrane ?
In 1995,Edward Witten (1951-)combined into a single 11-dimensional framework the 5 competing 10-dimensional string theoriesand the 11-dimensional theory of supergravity which had been devised in 1978 by Joël Scherk (1946-1979) Eugène Cremmer (1942-2019) and Bernard Julia (1952-).
Even before that tour de force, Ed Witten was widely recognized as the dominant string theoristof that era. (He became aFields Medalist in 1990.)
(2009-10-22) In M-Theory,branes are the membranes of which parallel universes are made of.
(2023-08-42) It proves that General Relativity and Quantum principles are not incompatible.
At the very least, the existence of string theory proves that General Relativity and thequantum principles on which our standard model is based can coexist logically. Unification is thus not impossiblea priori.
