String conjectures that on the microscopic scale, Einstein's 4D spacetime is a field of Calabi-Yau manifolds, each containing 6 space dimensions curled up, thus not extended into the 3 space dimensions presented to the classical realm. In string theory, each quantum particle is replaced by a 1D string of vibrating energy whose length is the Planck length. As the string moves, it traces width, and thus becomes 2D, a worldsheet. As a string vibrates and moves within the 6D Calabi-Yau space, the string becomes a quantum particle. With this approach, the hypothetical graviton—predicted to explain general relativity—emerges easily.
String theory began as bosonic string theory, whose 26 dimensions act as many fewer. Yet this modeled only bosons, which are energy particles, while omitting fermions, which are matter particles. So bosonic string theory could not explain matter. Yet by adding supersymmetry to bosonic string theory, fermions were achieved, and string theory became "super"string theory, explaining matter, too.
String theory's claim that all molecules are "strings of energy" has drawn harsh criticism. There are many versions of string theory, none quite successfully predicting the observational data explained by the Standard Model. M theory is now known to have countless solutions, often predicting things strange and unknown to exist. Some allege that string theorists select only the desired predictions.
The allegation that string theory makes no testable predictions is false, as it makes many. No theory—a predictive and perhaps explanatory model of some domain of natural phenomena—is verifiable. All conventional physical theories until the Standard Model have made claims about unobservable aspects of the natural world. Even the Standard Model has various interpretations as to the natural world. When the Standard Model is operated, it is often made a version with supersymmetry, doubling the number of particle species so far identified by particle physicists.
None can literally measure space, yet Newton postulated absolute space and time, and Newton's theory made explicit predictions, highly testable and predictively successful for 200 years, but the theory was still falsified as explanatory of nature. Physicists accept that there exists no such attractive force directly attracting matter to matter, let alone that the force traverses the universe instantly. Nevertheless, Newton's theory is still paradigmatic of science.
The idea of hidden dimensionality of space can seem occult. Some theorists of loop quantum gravity—a contender for quantum gravity—regard string theory as fundamentally misguided by presuming that space even has a shape until particles shape it. That is, they do not doubt that space takes various shapes, simply regard the particles as determining space's shape, not the other way around. The spacetime vortex predicted by general relativity is apparently confirmed.
If interpreted as naturally true, the Standard Model, representing a quantum particle as a 0D point, already indicates that spacetime is a sea of roiling shapes, quantum foam. String theorists tend to believe nature more elegant, a belief that loop theorist Lee Smolin dismisses as romantic while using biology's Modern Synthesis as a rhetorical device. Experiments to detect added spatial dimensions have so far failed, yet there is still the possibility that signs of them can emerge.
M theory has many trillions of solutions. Leonard Susskind, a leader of string theory, interprets string theory's plasticity of solutions as paradoxical support resolving the mystery of why "this" universe exists, as M theory shows it but a variant of a general pattern that always approximately results.
General relativity has brought many discoveries that in 1915 were all but unimaginable except in fiction. A solution of Einstein's equations that sought to explain quantum particles' dynamics, the Einstein-Rosen Bridge predicts a shortcut connecting two distant points in spacetime. Commonly called a wormhole, the Einstein-Rosen Bridge is doubted but not disproved, showing either that not all consequences of a theory must be accurate or that reality is quite bizarre in ways unobservable.
Even the Standard Model of particle physics suggests bizarre possibilities that populist accounts of science either omit or mention as unexplained curiosities. The theory conventionally receives the Copenhagen interpretation, whereby the field is only "possibilities", none real until an observer or instrument interacts with the field, whose wavefunction then collapses and leaves only its particle function, only the particles being real. Yet wavefunction collapse was merely assumed—neither experimentally confirmed nor even mathematically modeled—and no variance from either the wavefunction in the quantum realm or the particle function in the classical realm has been found.
In 1957 Hugh Everett described his "Relative state" interpretation. Everett maintained that the wavefunction does not collapse, and since all matter and interactions are presumed to be built up from quantum waveparticles, all possible variations of the quantum field—indicated by the mathematical equations—are "real" and simultaneously occurring but different courses of history. By this interpretation, whatever interacts with the field joins the field's state that is "relative" to the observer's state—itself a waveform in its own quantum field—while the two simply interact in a universal waveform never collapsing. By now, many physicists' interpretation of the apparent transition from the quantum to the classical realms is not wavefunction collapse, but quantum decoherence.
In decoherence, an interaction with the field takes the observer into only one determinant constellation of the quantum field, and so all observations align with that new, combined quantum state. Everett's thesis has inspired Many worlds interpretation, whereby within our universe are predicted to be virtually or potentially infinite parallel worlds that are real, yet each a minuscule distance from the other worlds. As each world's waveform is universal—not collapsing—and its mathematical relations are invariant, parallel worlds simply fill the gaps and do not touch.
Einstein doubted that black holes, as predicted by the Schwarzschild solution, are real. Some now conjecture that black holes do not exist as such but are dark energy, or that our universe is both—a black hole and dark energy. The Schwarzschild solution of Einstein's equations can be maximally extended to predict a black hole having a flip side—another universe emerging from a white hole. Perhaps our universe's big bang was half of a "big bounce", something's collapse down to a black hole, and our universe popping out its other side as a white hole.
Physicists widely doubt that quantum particles are truly 0D points as represented in Standard Model, which offers "formalism"—mathematical devices whose strokes predict phenomena of interest upon input of data—not "interpretation" of the mechanisms determining those phenomena. Yet string theorists do tend to optimistically conjecture that the strings are both real and explanatory, not merely predictive devices. It is far beyond the capacity of today's particle accelerators to propel any probing particles at energy levels high enough to overcome a quantum particle's own energy and determine whether it is a string. Yet this limitation also exists on testing other theories of quantum gravity. Developments suggest other strategies to "observe" the structure of quantum particles.