Ten String Theory Skeptics
String theory may be the dominant theory to reconcile quantum physics and general relativity, but not all physicists have embraced it. In fact, some have become specifically marked by speaking out against it often, and in public venues, where others can hear. These dissenters against the conventional wisdom may ultimately be proved wrong, or they may be viewed as insightful pioneers in finding the true path.

Richard P. Feynman, Skeptic Emeritus: Probably the most famous skeptic of string theory was Nobel laureate Richard P. Feynman, whose lectures at CalTech were later recorded and published as The Feynman Lectures on Physics, becoming instructive to a whole generation of physicists.
The popular Feynman diagrams depict the behavior of subatomic particles within quantum electrodynamics (QED). The development of QED earned him a Nobel Prize in Physics.

Lee Smolin: Physicist Lee Smolin became a prominent voice of dissent against string theory supremacy with the publication of his 2006 bestselling book The Trouble with Physics: The Rise of String Theory, the Fall of Science, and What Comes Next.
Smolin is a founding member of the Perimeter Institute for Theoretical Physics and also author of the 2001 book Three Roads to Quantum Gravity, which argued that string theory and loop quantum gravity (of which Smolin is arguably a cofounder) were two approaches to the same fundamental theory.

Peter Woit: In 2006, the Columbia University mathematics lecturer Peter Woit, who has a PhD in theoretical physics from Princeton University, published Not Even Wrong: The Failure of String Theory and the Search for Unity in Physical Law. The title comes from the idea by Wolfgang Pauli that some ideas were so illdefined that they were “not even wrong.” In Woit’s view, string theory has gone beyond that, and adherents refuse to even contemplate the idea that it might be incorrect.

Robert B. Laughlin: Robert B. Laughlin was awarded a PhD from MIT in 1979 and earned the 1998 Nobel Prize in Physics for the discovery of the fractional quantum Hall effect. (Don’t ask.)
Laughlin and some of his solidstate physics colleagues have been critical of string theory’s approach (and the approach of particle physics in general), believing that the focus needs to be on the macroscopic level rather than zooming in at smaller and smaller scales that have no testable consequences.

Roger Penrose: As a mathematical physicist, he has worked on diverse scientific topics, but is probably best known for his work on black holes in conjunction with Stephen Hawking. He created a form of diagram, called a Penrose diagram, that depicts the behavior of black holes, and has done extensive work on theorems describing singularities in general relativity.
He remains highly critical of string theory, and in his 2005 tome The Road to Reality: A Complete Guide to the Laws of the Universe, which outlines the basic principles of twistor theory, he argues that compactified dimensions in string theory reduce to singularities, based on general relativity theorems that he had earlier developed with Stephen Hawking.

Lawrence Krauss: Astrophysicist Lawrence Krauss of Arizona State University is a bestselling author of many popular science books, including Hiding in the Mirror: The Quest for Alternate Realities, from Plato to String Theory (by Way of Alice in Wonderland, Einstein, and the Twilight Zone).
Krauss makes his skepticism about string theory evident, although it’s also clear that he’s still somewhat of a fencesitter. According to him, his views on the validity of string theory oscillated frequently between agreement and disagreement during the course of writing the book.

Sheldon Glashow: Sheldon Glashow was one of three physicists who determined the relationship between the weak nuclear force and quantum electrodynamics, resulting in the unification of the electroweak force (and Glashow’s 1979 Nobel Prize in Physics).
Glashow now teaches at Boston University, and there are (unsubstantiated) rumors that his departure from Harvard may have had to do with the recent rise of string theory there as well.

John Moffat: John Moffat of the Perimeter Institute for Theoretical Physics has spent nearly three decades investigating alternate gravity theories. He presented modified gravity (MOG) and variable speed of light (VSL) theories. More details can be found in his 2008 book Reinventing Gravity: A Physicist Goes Beyond Einstein.

João Magueijo: There’s hardly a more rebellious figure in contemporary physics than João Magueijo, the Portuguese cosmologist who in 1998 proposed (together with Aldreas Albrecht) the variable speed of light (VSL) cosmology theory, in order to overturn nearly two decades of inflation theory.
Magueijo also seems to hold no special place in his heart for string theory, as he explains in colorful language.

Philip Anderson: Condensed matter physicist Philip Anderson was awarded the 1977 Nobel Prize in Physics for work in magnetic and disordered systems that led to the development of electronic switching devices necessary for the operation of computers.
Anderson has made the point that the method followed by string theorists is the same process used in medieval times, where mathematical ideals were followed without any recourse to experiment.