String theory Crossword Puzzle for Kids

String theory tries to use math to explain the four known fundamental forces—gravitation, electromagnetism, strong nuclear force, weak nuclear force—together in one theory. This tries to solve the problems of having both classical physics and quantum physics.

Einstein wanted a unified field theory, a single theory to explain the fundamental forces. Today's scientists want a unified field theory that also explains matter. This is called the search for a theory of everything (TOE). The most famous theory used as a TOE is called superstring theory. It says there are 6 higher dimensions as well as the four dimensions of Height, Length, Width and Time).

Some superstring theories think that it is about the geometry of space. This idea is a big part of a theory called M-theory. Many string theorists believe that M-theory explains the universe and might explain how other universes, if they exist, are as part of the "multiverse". M theory/supergravity theory has 7 higher dimensions and the four dimensions of Height, Length, Width and Time.

Background

Introductions to string theory that are designed for the general public must first explain physics. Some of the controversies over string theory are because of people not understanding physics. A common misunderstanding even is the idea that a theory is proven true whenever something it says will happen happens. Another misunderstanding is that earlier physical scientists, including chemists, have already explained the world. This leads to the misunderstanding that string theorists began making up new ideas after they became "set free from truth".

Newton's law of universal gravitation (UG) was published in 1687. Newton's theory successfully explained objects big enough for us to see them. Coulomb's law explained electricity. Maxwell's electromagnetic field theory explained electricity and magnetism. This led to the creation of optics, the study of light.

Light's speed usually stayed the same when measured by an observer travelling almost as fast as the light. Scientists thought that that would change based on how fast the observer was moving but it didn’t. This did not violate Galileo’s Principle of relativity that says the laws of mechanics work the same for all objects showing inertia.

Inertia means that when no force is applied to an object, the object keeps its velocity, which means it’s as fast as it is moving and going in the direction that it is going. An object is moving at a constant speed in the same direction, which isn’t changing at all, has inertia. This is called Galilean invariance — it does what it normally does without changing —and is also called Galilean relativity, since it’s impossible to tell whether it is not moving or moving all the time at the same speed.

In 1905, Einstein's special theory of relativity explained both Maxwell's ideas about Electromagnetism and Galilean relativity by saying that the Speed of light is always the same but Space and Time aren’t. This means that when an object travels almost as fast as light, Time slows down. This is called Time dilation. Special relativity meant that Newton's theory—which said that space and time never change—couldn’t explain gravity.

Concept

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 superstring theory, explaining matter, too.

(In versions of quantum field theory that include supersymmetry (SUSY), each boson has a corresponding fermion, and vice versa. That is, each energy particle has a corresponding matter particle, and each matter particle has a corresponding energy particle, yet the unobservable partner is more massive and thus super. These superpartners might seem an extravagant prediction, yet many theorists and experimentalists favor supersymmetric versions of the Standard Model, whose equations must otherwise be tweaked extravagantly and sometimes arbitrarily to maintain predictive success or mathematical consistency, but with the superpartners align.)

Controversies

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.

Developments

An inspiring development is discovery of mirror symmetry, whereby Calabi-Yau spaces tend to come in pairs such that solutions previously difficult within the extreme vibrational mode of one string can be solved by through the mirror Calabi-Yau space's geometry in its opposite range.

String theory is usually solved through conformal field theory, a quantum field theory on 2D space. It is confirmed that molecules can collapse to 2D. And the electron, long presumed an elementary particle, apparently splits into three entities separately carrying the electron's three degrees of freedom when the molecules that contain the electrons are channeled through a 1D pathway.

Text adapted from the Simple English Wikipedia article “String theory” (revision 10443290, 2025-08-07), authors, licensed under CC BY-SA 4.0.