The search for a new theory combining general relativity and quantum mechanics [0]
At the moment we have one theory supported by a copious amount of experimental data for things that are quite massive – general relativity – and a separate and distinct theory for things when they are very small – quantum mechanics - and on the face of it they are irreconcilable. Quantum mechanics is a great theory of all the forces and particles but it ignores gravity. General relativity is a great theory of gravity, but in most cases it ignores quantum mechanics[1]. As we have seen, this conflict is only evident on minute Planck scale distances (10-33 cm), but what about where things are both very massive and very small: near the central point of a black hole or the universe at the moment of the big bang? In these instances, the theories of classical physics break down and a separate unified theory of everything (TOE) encompassing all matter and all forces (including gravity) is required to explain and interpret these phenomena. One possible candidate is ‘superstring theory[2] (‘string theory’ for short), a unified theory of the universe which postulates that the fundamental ingredients of nature are not zero dimensional point particles, but tiny one dimensional filaments called strings - somewhat like infinitely thin rubber bands vibrating to and fro.
Before going any further, let's see a brief overview as to how the theory works:
www.youtube.com/watch?v=0CeLRrBAI60
The “super” in superstring theory refers to supersymmetry, a mathematical feature which postulates that for every known particle species there should be a heavier and as yet undetected partner species but having the same electrical and nuclear force properties and adorned with such whimsical names as selectrons and squarks. Theorists introduced supersymmetry in the 1960s to connect the two basic types of particles seen in nature: fermions and bosons. Broadly speaking, fermions are the constituents of matter (electrons, for example), whereas bosons are the carriers of the fundamental forces (the photon in the case of electromagnetism). Supersymmetry would give every known boson a heavy “superpartner” that is a fermion and every known fermion a heavy partner that is a boson. “It is the next step up towards the ultimate view of the world where everything is symmetrical and beautiful”[3].
If these superpartners exist, they have the property of naturally canceling out the tiny quantum jiggles that would drive the weak force away from its observed range. "That's one of the things that makes supersymmetry so attractive: It can keep the scales separate," says Lawrence Krauss [4].
[0] See also Black holes and information theory: the link between quantum physics and general relativity?
[1] “What’s the energy density of the vacuum?”, John Baez, 10 June 2011: http://math.ucr.edu/home/baez/vacuum.html
[2] Greene (2011), 94.
[3] Davide Castelvecchi, “Is Supersymmetry Dead?’ Scientific American, May 2012, 9. However, Castelvecchi goes on to suggest that if the LHC fails to turn up anything on supersymmetry, which to date (March 2016) it hasn't, interest in the theory may die away, even though strictly, it cannot be disproved. See www.wired.com/2012/07/supersymmetry-explained/
[4] "Supersymmetry: the future of science explained": www.wired.com/2012/07/supersymmetry-explained/