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.
Biographies | Viewpoints | Glossary | Dimensions | Resonance
The current epistemological status of String Theory is probably best grasped by
comparing it with another theory devised in simpler times:
The Dirac equation was a clever example of a theory
consistent with the axioms of both quantum mechanics and special relativity.
Some of the concepts involved in its interpretation may not have survived
the test of time but it was instrumental in predicting the existence
of antimatter and it helped demonstrate that any quantum theory consistent
with special relativity would likewise involve antimatter (which was duly
observed, by Carl Anderson, a couple of years after
P.A.M. Dirac predicted it).
Similarly, String Theory is a quantum theory consistent with General Relativity.
Unlike the Dirac equation, however, it has failed to make any definite physical
prediction so far.
Therefore, it's currently a legitimate target for critics who call it
"nonphysical" or "unscientific"
(arguably, a theory must have falsifiable consequences
to be call "scientific" outside of the realm of pure mathematics).
Over the years, this state of affairs has slowly transformed "String Theory"
into a general study of all possible quantum theories compatible with
General Relativity. It's not just a theory of "strings" anymore.
It is hoped that, sooner or later, String Theory will achieve at least the
same philosophical status as either Newtonian mechanics or Dirac's equation.
It may or may not turn out to be the advertised "theory of everything"
but its logical structure would at least reveal some testable features
of our physical universe.
The social impact of String Theory among the "community" of
theoretical physicists can hardly be overstated...
An entire generation of many of the brightest theoretical physicists
have been lured by its mathematical appeal away from other pursuits.
Yet, nobody knows how relevant to physics the resulting body of knowledge
really is. So far,
this has been a gamble of unprecedented magnitude without any definite payoff
in sight.
LHC-inspired Google Logo (Used by Google on 2008年09月10日)
On the day LHC was supposed to starts operations (2008年09月10日) Google flaunted the above version of their logo worldwide. The discovery of the Higgs boson was announced 4 years later (2012年07月04日). The 2012 Nobel prize was then widely expected to go to Peter Higgs (it went to Serge Haroche and David Wineland instead, for unrelated work). Peter Higgs and François Englert were duly awarded the 2013 Nobel prize for physics.
The ancient Egyptians measured large horizontal distances by rolling a wheel whose diameter was measured in the units they used for small vertical distances... The countless appearances of the number p (the ratio of the circumference of a circle to its diameter) in the Egyptian pyramids can be puzzling to whoever has been exposed to unified Euclidean space since childhood. To Egyptian architects, the relevant dimensions were integers!
Robert Copley, called Grosseteste 1168-1253Robert Grosseteste (1168-1253) is credited with the idea that the homogeneous and isotropic space of Euclidean geometry can be a backdrop for light and matter (De Luce = About Light, c. 1235).
Blurring the distinction between horizontal and vertical distances is philosophically pleasing, although this does not make practical differences go away... There's very little difference between right and left but there's a huge difference between up and down (just try falling up). The distinction between the horizontal and vertical directions (near the surface of the Earth) seems to vanish at high energies : If you shoot a gun indoors, the bullet always moves in a straight line and at the same speed no matter where you aim. Outdoors, distances are larger and there's enough time for the pull of gravity to influence the bullet noticeably. Very fast "bullets" (photons or particles moving at nearly 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 framework where those differences can be charted and where their interplay becomes clear. If you have a rigid stick with one fixed end, our "unified" notion of Euclidean distance will tell you how the vertical position of the moving end varies when its horizontal position changes.
Similarly, the unification of space and time in the context of Special Relativity does not equate the 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 p was built into the architecture of the ancient Egyptians... The numerical value of c is merely a consequence of our traditional ways to measure spatial distances, on one hand, and time intervals, on the other. Rulers and clocks.
Historically, unifying separate physical concepts has always resulted in a deeper understanding of Nature. Arguably, the most satisfying such event was the unification of electricity and magnetism by Maxwell (1861) as he found a simple way to amend the law of Ampère into a consistent picture of electromagnetism which demanded the existence of electromagnetic waves propagating at a constant speed c. The fact that this ought to be so for all observers in uniform motion with respect to each other directly led to special relativity.
Unifying quests are so appealing that many mathematical physicist share a blind faith: The forces of nature must ultimately be unified; at high enough energies all interactions ought to look alike (just like the aforementioned great speed of bullets blurs the distinction between 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 physical phenomena, not necessarily unified ones.
George Uhlenbeck 1900-1988
When Oskar Klein told of his ideas which would not only unify the
Maxwell with the Einstein
equations but also bring in the quantum
theory, I felt a kind of ecstasy:
Now, one understands the world !
George
Uhlenbeck (1900-1988) Summer of 1926.
For a while, this stirred the enthusiasm of Albert Einstein (1879-1955) himself and caused the special type of ecstasy described in the above 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 of Oskar Klein's proposal turn out to be entirely off-base.
What's still with us today is Klein's fundamental ideas about how extra dimensions might 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 for what's now called String Theory. Such dimensions are still visualized as rolled up, although they need not have a compact topology.
The 2009 Oskar Klein Memorial Lecture :
My Life as a Boson
by Peter Higgs
Hidden Dimensions: Exploring Hyperspace
World Science Festival (2010年06月05日).
[画像: Come back later, we're still working on this one... ]
Tullio Regge (1931-) | Reggeology by Lubos Motl
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 the Regge and resonance description of pion-nucleon exchange, which had been proposed by R. Dolen, D. Horn and C. Schmid.
Veneziano and his three colleagues had been putting together a model of the relevant scattering amplitude in the process. On the boat, Veneziano realized that the essential features of that scattering amplitude would be captured by a simple expression involving Euler's Beta function and Gamma function, namely:
The attractive closed form of the Veneziano amplitude contrasted sharply with the usual intractability which physicists had to deal with for strong nuclear interactions. The formula created a widespread stir.
Lenny Susskind has been at Stanford since 1979 (Felix Bloch Professor of Theoretical Physics, since 2000).
susskindsblogphysicsforeveryone.blogspot.com
In 1969, Susskind pondered Veneziano's formula for months, trying to make some clear physical sense out of it. He finally came to the conclusion that an entity was described which could stretch and vibrate just like an open-ended elastic string. (I'm told that the expression is now interpreted as the scattering amplitude for four open-string tachyons).
Two other physicists working on the same premises arrived independently at 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).
In 2008, Nambu was awarded the Nobel prize in Physics for his introduction (in 1960) of mass-producing spontaneous symmetry breaking in particle physics (he had been inspired by an analogy with the theory of superconductivity).
Working with Eugène Cremmer and Bernard Julia, Scherk devised an 11-dimensional theory of supergravity and proposed (with Cremmer) the mechanism of spontaneous compactification in quantum field theory. Scherk was a diabetic and he died in tragic circumstances, as he passed out when nobody was around to give him a shot of insulin.
After the death of Scherk, Schwartz found only one person willing to help with the work they had started together: Michael Green...
John H. Schwarz:
String People
|
JHS / 60
Murray Gell-Mann and String Theory (27:11)
by John Schwarz (2013年10月12日).
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 Marston Morse 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 immediate benefit 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年09月30日) Michael B. Green was appointed (on 2009年10月19日, as of 2009年11月01日) to the prestigious chair, once held by Newton, Airy, Babbage, Stokes, Larmor and Dirac. With effect from 2015年07月01日, Green's successor is Michael Cates (b. 1961).
Heterotic string theory was first developed in 1985, by the so-called Princeton String Quartet composed of:
No fewer than five consistent string theories have been devised:
The difference between the various string theories by Randall Scalise
In 1995, Edward Witten (1951-) combined into a single 11-dimensional framework the 5 competing 10-dimensional string theories and 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 theorist of that era. (He became a Fields Medalist in 1990.)
Video : A New Look at the Path-Integral of Quantum Mechanics by Ed Witten (2010年08月16日).
still working on this one... ">
Membranes (M-Theory) | Brane Cosmology
At the very least, the existence of string theory proves that General Relativity and the quantum principles on which our standard model is based can coexist logically. Unification is thus not impossible a priori.
