String theory developed over a period of more than 40 years, in a kind of haphazard evolution, with theoretical roots stretching all the way back to the decade of the 1940s.  It lacks a single unifying physical principle (like, for example Einstein's Principle of Equivalence, on which his theory of gravity is based), and has never managed to make contact with actual, concrete space-time physics, or to make testable predictions.  Proponents of String Theory generally point to two achievements: (1) String theory is one of a small handful of proposed viable quantum theories of gravity; and, (2) the new areas of mathematics invented for string theory have proven useful in branches of physics totally removed from fundamental particle physics, for instance condensed matter physics and relativistic heavy ion collisions. It might also be noted that one of the ancestors of string theory was Duality (late 1960s), an approach that provided connections between two apparently totally different physical problems, one difficult to solve, one easy to solve.  A modern version of this kind of situation exists in string theory, the so-called AdS/CFT correspondence, which is the subject of intense current interest, and has led to the concept of "holographic universes."

The very attractive founding idea of string theory was that all “fundamental” particles are in fact different quantum modes of the same physical object, a one-dimensional string with a length of no more than the Planck length (10^-35 meters). The possible modes would be the quantum equivalents of vibrations or rotations. From this alone, you can see that getting strings to behave like fermions was a major problem at first.  In the usual form of string theory, the string is a closed loop.

Critics of string theory point to the fact that string theories are inherently multi-dimensional (9 or 10 space dimensions are needed) and have to be “compacted” to 3 space dimensions to be relevant to physics... but there turn out to be at least 10⁵⁰⁰ different ways to do this, with no way to know which one is the way that leads to the space-time we live in.  They also note that string theory is built on supersymmetry, and that there is no experimental evidence whatsoever that supersymmetry is correct. [String theories originally described only bosons, and supersymmetry is necessary to allow inclusion of fermions.]

In a way, string theory generalizes Feynman's perturbative approach, replacing the point fermions and bosons by closed strings, and thus removing all of the sources of infinities in Feynman's approach (no point particles or point vertices).  But this means there is no non-perturbative version of string theory!  What do you do with strings when facing a problem where a perturbation series cannot converge? Another vexing problem is that the string theory description of gravity does not satisfy background independence: a spacetime has to be input instead of emerging from the calculation.  That is, the string version of gravity is a regular field in space-time, not a quantized space-time.

One type of experiment that can be done at the LHC, and that is relevant to string theories, is a search for so-called KK partners of some existing particle. Such partners, if found, would provide at least some evidence that extra, compacted space dimensions do exist. For an explanation of how this works, at the level of an introductory undergraduate survey physics class, look here, then here and then here. Needless to say, no evidence whatsoever of such KK partners has been observed to date in LHC runs. In string theory, by the way, superpartners are also claimed as evidence for extra dimensions. However, superpartners will result from any theory of fundamental processes that incorporates supersymmetry, and they do not directly provide confirmation for any aspect of string theory.

The distinctive Kaluza-Klein tower is a result of standing waves in a compacted dimension.

As the decade of the 1990s began, leading string theorist Edward Witten pointed out that the six different versions of string theory that were known by that time could probably be viewed as limiting cases of a more general theory with one extra dimension, which he called M-theory. [He said it was premature to decide what the “M” stood for.] Witten's surmise seems to be correct.

What is a "heterotic" string?

The two titans of string theory, Edward Witten (1951, left) and Joe Polchinski (1954 -  2018), together with pioneer Leonard Susskind (1940, right).

Is there a future for string theory? At the moment, it's highly debatable. Here is a somewhat old, but clear, summary of various developments in string theory during its long history. About the most hopeful quote I have seen concerning string theory recently is from string theory pioneer Andrew Strominger, who said in 2016, “String theory may not be the fabled theory of everything, but it is definitely a theory of something.” It remains to find out what.  It would not, for example, make much sense to cut away everything else in the theory, and use strings only for quantum gravity.

Steven Weinberg on the problems of String Theory: here.

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THE CURRENTLY MOST FAMOUS DEVELOPMENT IN STRING THEORY:

Juan Maldacena (1968 - )

One big controversy currently going on within string theory deals with how string theory accommodates Dark Energy, or whether it can even be included correctly within string theory. Here is a brief history of String Theory that is more informed (but also skeptical, so be warned) than the usual very sweeping, general (and vague) accounts.

“I don't like that they are not calculating anything. I don't like that they don't check their ideas. I don't like that for anything that disagrees with an experiment, they cook up an explanation--- a fix-up to say, ‘Well it still might be true.’ For example, the theory requires ten dimensions. Well, maybe there is a way of wrapping up six of the dimensions. Yes, that is possible mathematically, but why not seven? When they write their equations, the equations should decide how many of these things get wrapped up, not the desire to agree with experiment. In other words, there is no reason whatsoever in superstring theory that it is not eight of the ten dimensions that get wrapped up and that the result is two dimensions, which would be completely in disagreement with experience. So the fact that it might disagree with experience is very tenuous, it does not produce anything, it has to be excused most of the time. It does not look right.” [Richard Feynman, when asked for his opinion of string theory]

“It should be clear by now that we probably still do not know what exactly is the problem to which string theories are a solution.” [Enrique Alvarez, 2004]

“We can never know what could be relevant in a given field of physics until the main problems there have been solved.” [Richard Feynman]

"In order to know the truth, it is necessary to imagine a thousand falsehoods." [Sidney Coleman]

"String theory has become an established part of physics departments throughout the world, more so in Europe than in America. We still have some universities which are proudly string-free, like Boston University. We also have an awful lot of string theorists around who are twiddling their thumbs. It is not clear that string theory is going anywhere. I expect that string theorists would disagree with that assessment. But they are actually considering many other circumstances such as black holes in other spaces than ours, and there are all kinds of interesting things being done in mathematics, in physics, elsewhere by string theorists but with no relationship to the questions that interest me.  They cannot answer the questions they set out to answer. That much is clear." [Sheldon Glashow]

Theodor Kaluza (1885 - 1954) & Oskar Klein (1894 - 1977)

“I have one simple idea suggestion for String Theory which I believe should be implemented immediately. We need to stop calling it String Theory. I’ve been a String Theorist for years, but I barely ever touch anything which could be called a string. The subject is incredibly, incredibly, broad. It’s now touching most areas of theoretical physics;  essentially, it’s tangentially related to anything involving Quantum Field Theory. It’s more a set of tools, than a theory in and of itself. Calling yourself a String Theorist is about as specific as calling yourself a Geometer, or a Mechanical Engineer.” [String theorist, who wishes to remain anonymous.]
A personal comment: much of the “science-culture”-based criticism of string theory is that it is treated as the only game in town, and so any university physics department with a group doing fundamental theory is packed with string theorists. The problem with this criticism is that nobody has come up with a viable alternative to string theory, that one could justly claim was being unfairly ignored. For 40 years, string theory has been just about the only game in town. One of very few alternatives is known as “model building,” as practiced mainly at Harvard University. Some dramatic experimental findings, going clearly beyond the Standard Model in ways that suggest new theoretical principles, would be needed to provide the building blocks of popular alternatives to string theory. So far no evidence of extra dimensions has shown up in data from the LHC.  After so many years of effort by extraordinarily competent and creative people, I think it is fair to say that string theory is a failure, at least as a theory of fundamental processes going beyond the Standard Model.  And this leaves us in a very bad fix, with no clues whatsoever, either from theory or experiment, as to how to advance usefully into the unknown realms that the Standard Model cannot include!


Black Hole History

Timeline of the Universe and Solar System

Simulations of Structure formation
Very Early Universe!

HOLOGRAPHIC DUALITY??
Black Hole Primer
Quantum Gravity

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