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String theory is the branch of modern high-energy physics that attempts to describe the four fundamental forces of nature within a single theoretical framework.
More specifically, string theorists seek to replace the current Standard Model of particle physics, which describes the electromagnetic, weak and strong nuclear forces and general relativity, which describes gravitational interactions. (But note that the Standard Model already contains special relativity.) Under our current understanding, quantum mechanics, from which the Standard Model is constructed, and general relativity appear to be fundamentally incompatible, even though they both give amazingly accurate descriptions of reality as we know it. String theorists claim to be able to bridge this divide by proposing a string-like nature of subatomic particles (such as electrons, quarks, Higgs bosons) and multiple extra spacial dimensions.
Superstring theory (or M-Theory) is a more specific term for modern string theory that incorporates all of the known fundamental particles as well as their (hypothetical) supersymmetric partners. Superstring theory only works in ten dimensions of space and one of time. This is actually down from the total of 26 dimensions of the earlier bosonic string theory developed in the late 1960s.[note 1]
Because string theories as they now stand make few predictions and often lack the ability to be tested with falsifiable experiments, some doubt that they have any relevance to physics at all. Elegance does not guarantee correctness. Nonetheless, while there are other attempts at a Unified Field Theory, string theory has so far received the most attention, both within academia and among the general public.
Electromagnetic, strong and weak forces are well described by quantum field theory. Quantum electrodynamics addresses the electromagnetic interactions, and quantum chromodynamics describes strong interactions. The electroweak theory describes the unification of the weak and electromagnetic forces. This occurs at high energy levels, or the energy density of the Universe at about 10−12 seconds after the Big Bang. At even higher energy levels, (10−36 seconds after the Big Bang), the electroweak and strong forces are predicted to unify. What is not currently understood is whether those three forces will then unify with gravity as energy densities continue to rise. It's believed that the energy densities necessary for this final unification existed at, or very near, 1 Planck time (10−44 seconds) after the Big Bang. Time periods shorter than 1 Plank time are believed to be fundamentally unknowable, if they even exist at all.
Because quantum theory models all sub-atomic particles as 0-dimensional points (i.e, without any size or volume associated with them), rather strange things happen when gravity is applied; the theoretical models collapse into infinities, which essentially means the models no longer work. Clearly, gravity exerts force on matter comprised of these point particles, so theories that cannot account for that interaction (like Relativity and the Standard Model) are considered incomplete. To further complicate things, the uncertainty principle implies that when observing at smaller and smaller chunks of spacetime (the closer you get to 1 Planck length, ≈1.6×10-35 meters), you will see more and more "virtual particles" bubbling-up out of the void. These virtual particles have mass-energy to them, and so warp the fabric of spacetime, to the point that it becomes "foamy" and can therefore no longer be described by general relativity (which assumes that spacetime is locally flat). String theory removes this problem by postulating that elementary particles are not points, but strings of finite length, beneath which it is no longer meaningful to talk about physics at all. Thus, the arbitrary "foaminess" never comes into play.
Many theories exist which attempt to address both the electroweak interaction and the strong interaction (these theories are called "grand unified theories"), and one of them may be proven true by the Large Hadron Collider. Fairly few theories exist which attempt to unify gravity with a Grand Unified Theory — superstring theory is one of these so-called Theories of Everything.
The general idea
String theory postulates that at the scale of the Planck length, the vibrations of 1-dimensional strings of energy give rise to the different properties we observe in subatomic particles (mass, charge, color and spin). These attributes are determined by the respective "frequency" of the string's vibration. This differs from Standard Model physics which states that sub-atomic particles are 0-dimensional points without internal structure. These strings of energy are hypothesized to extend in multiple extra-spacial dimensions (from 5 to 6 to 22, depending on the particular theory) beyond the 3 spacial and 1 time dimension we experience in everyday life. Some of these extra-spacial dimensions are "wrapped up" in microscopic geometric configurations whose shapes are defined by these extra dimensions. Calabi–Yau manifolds are often given as examples of the geometric configurations these extra dimensions may conform to. Other extra dimensions may be macroscopic in size but are unobservable by us in the same way that we humans would be unobservable to a two-dimensional being.[note 2] Theoretically, we should still be able to make predictions and observations about the effects string behavior in these extra dimensions has on the observable energy and matter within our 3+1 dimensions. An example of this would be: the string vibration that gives rise to the theoretical graviton particle's extension into, and interaction with, these extra dimensions may explain why we perceive the force of gravity as orders of magnitude weaker (at short distances) than the other fundamental forces, despite it operating on orders of magnitude longer length scales.
Five different superstring theories exist, each of which postulate different kinds of strings (open ended or closed loops, chirality, type of supersymmetry). Theoretical physicist and mathematician Edward Witten (considered by many to be the greatest living physicist) devised M-Thoery as a way to unify the 5 disparate string theories. While M-Theory utilizes profoundly complicated and esoteric mathematics to unify the different theories, it can be understood essentially as: At higher energy densities (again, at or very near 10-44 seconds after the Big Bang), the 11-dimensional strings are better approximated as 2 and 5 dimensional objects known as branes. As energies fall, the string like nature of these branes emerges and the different emergent aspects of these strings are described by the 5 different string theories. Witten has never stated what the "M" in M-Theory stands for, although "Membrane", "Matrix", "Mystery" and "Magic" have all been used by varying sources.
Or, it all turns out to be bollocks and Loop Quantum Gravity (LQG) or some other attempt at a Theory of Everything takes over the academic world. It's hard to say, as empirical evidence for any of these theories has been incredibly difficult to come by. The energy levels needed to investigate phenomena at the Planck scale are far beyond even our theoretical ability to generate. Currently, the Large Hadron Collider is able to "resolve" down to 2.8-20 meters. For the string like nature of reality (much less branes, or even the spin networks of LQG) to become more apparent, resolutions orders-of-magnitude closer to the 1.6-35 meter Planck length are necessary. That 15 orders-of-magnitude difference is like going from the length of your leg to the diameter of a proton, or the inner radius of the Oort Cloud (~1 ly) to a house.Stephen Hawking on confirming M-theory by observation:
"M-theory is the only possible unified theory, under certain assumptions, the most important of which is that there should be a relation between forces and matter called supersymmetry. This would predict that elementary particles should appear in pairs. It may be possible to observe this in the Large Hadron Collider, which would go a long way towards confirming M-theory experimentally."
To date, the LHC has found no supersymmetric particles, and without those, everything stated above probably doesn't work. Superstring theory is short on falsifiable predictions, except for the prediction of supersymmetry. Given the energy levels necessary to resolve phenomena near 1 Plank length, experiments capable of making those measurements remain purely theoretical. Nevertheless, string theories are considered promising enough to have all but monopolized decently-funded theoretical high-energy physics. This is itself considered a problem by many (proper, non-crank) physicists who think thirty years is quite long enough for string theory to have come up with a verified falsifiable prediction, and who have problems getting funding for research that isn't string theory. These factors make string theory a potential modern protoscience, which may eventually go the way of Luminiferous aether.
String theory has also been the victim of cranks, becoming in their hands a more modern quantum woo, and of course any resemblance with actual string theory and its arcane mathematical foundations is just coincidental. Basically, if you see anything proposing a relationship with string theory outside of academic journals and reputable popular science sources, it's guaranteed to be woo.
- Michio Kaku, who did some work on the subject before he became a full time science communicator.
- Lee Smolin, a dedicated proponent of loop quantum gravity and critic of string theory.
- Roger Penrose: a proponent of String Theory's biggest rival, loop quantum gravity.
- Stephen Hawking, who was obsessed with quantum gravity and a supporter of string theory.
- Subsequent uses of string theory on this page will be in reference to it's current superstring status. M-Theory references a more recent advancement of superstring theory, described under "The General Idea".
- Like many concepts in physics, that's a difficult thing to visualize, but the classic book Flatland by Edward Abbott can help.