Relativity and the Quantum / Elementary Tour part 5: Superstrings and universal harmony

If you go by the number of researchers involved in it, string theory is the most actively studied candidate for a theory of quantum gravity. String theory is descended from the ordinary models of particle physics, but with a crucial difference: Its basic constituents are not point-like particles, but one-dimensional objects – strings.

Strings can do what point particles cannot: They can oscillate in intricate patterns, as sketched in the following animation:

Strings

One basic postulate of string theory is that the plethora of possible oscillatory states of a string is responsible for the variety of different elementary matter and force particles that occur within the standard model of particle physics. Put simply: there is only one species of string, but depending on how this tiny string oscillates, it acquires the same properties as an electron, a photon, a specific kind of quark or some other elementary particle. It can be shown quite generally that one of the possible oscillation states of a string will automatically correspond to the properties of a graviton – a carrier particle of the gravitational force – but without any of the attendant difficulties that arise from trying to include gravity in the ordinary quantum theories of forces. This makes string theory a natural candidate for a quantum theory of gravity, and much more. String theory promises nothing less than a complete unified description of all forces and all matter particles – the whole world as a consequence of the harmony of strings.

However, this promise comes with a price attached. For instance, in order for string theory to be consistent, space must have six or even seven space dimensions in addition to the three we know from everyday life. There are several possible explanations for why we don´t see these extra dimensions: They could be “rolled up” in a complicated way, or it could be that we live on what string theorists call a “brane” – a three-dimensional space embedded into a higher-dimensional space just as a two-dimensional surface like that of the earth is embedded into ordinary, three-dimensional space.

Previous part: Loop quantum gravity

Next part: Conclusion

Tour index

Definitioner

String

Ansatz für eine Theorie der Quantengravitation; eine Quantentheorie, in der die elementaren Bestandteile winzige, eindimensionale schwingende Saiten (englisch: Strings) sind.

Einen kurzen Überblick über Ideen der Stringtheorie bietet die Seite Strings und kosmische Harmonie im Kapitel Relativität und Quanten von Einstein für Einsteiger.

surface

Geometric space with two dimensions. Examples include the plane or the surface of a sphere.

string theory

Candidate for a theory of quantum gravity; a quantum theory, where the fundamental building blocks are tiny, one-dimensional, oscillating entities, called strings. A brief description can be found on the page String theory in the chapter Relativity and the quantum of Elementary Einstein.

string (string theory)

strings (string theory)

standard model of particle physics (standard model of elementary particle physics)

The current state of the art for describing the basic properties of matter and forces. The standard model theories are based on special relativity and quantum theory and they describe the behaviour of elementary matter particles such as electrons, neutrinos and quarks as well as their anti-particles. It also describes three quantum forces acting between them: electromagnetism, the weak nuclear force and the strong nuclear force. These forces act by the exchange of force particles. There is one elementary force for which no such quantum description exists, and which is not part of the standard model: gravity.

space

In a strict sense: Space as we know it from everyday life: the totality of all locations in which objects can sit, with three dimensions. In a more general sense used by mathematicians, all kinds of sets of points are spaces - a line for instance, which has but a single dimension, or a two-dimensional surface, but also higher-dimensional spaces. Also, in such more general spaces, geometry can be different from the standard Euclidean geometry taught in high schools - such spaces can be curved.

quark

Elementary particle that is subject to the strong interaction. Quarks come in six species: up, down, strange, charme, bottom and top (with the last two sometimes called beauty and truth). Quarks are the constituents of protons and neutrons, and thus the material atomic nuclei are made of.

quantum theory

The framework for formulating the physical laws that govern the world at microscopic length-scales - the physics of the micro-world, for instance of atoms, atomic nuclei or elementary particles, but also the physics of ultra-precise measurements such as those made by gravitational wave detectors.

The laws of quantum theory are fundamentally different from our everyday experience and from those of classical physics.

The first unusual feature is that, in many cases, quantum theory merely allows statements about probabilities. For instance, in classical physics, one can assign to every particle, at every point in time, a location and a velocity. Whosoever can measure those quantities precisely can, in principle, predict where the particle in question can be found at every point in the future. In quantum theory, all one can assign to a system of particles is an abstract quantum state from which can be derived no precise predictions, but merely the probabilities for detecting a specific particle at a given time in a given place. Whether or not one will really find the particle at that location is governed by chance.

The second unusual feature is a fundamental restriction placed on the exactness of certain measurements (Heisenberg uncertainty relation). For instance, the more precise one measures a particle's location, the less definite any statements one can make about its velocity.

The third feature is how quantum theory came by its name: A number of physical quantities in nature come in little packets, in quanta. For instance, according to quantum theory, electromagnetic radiation is made of tiny quanta of energy called photons.

Examples for quantum theories are quantum mechanics and relativistic quantum field theories such as Quantum electrodynamics or other parts of the standard model of particle physics.

point

Elementary "building block" of geometrical entities such as surfaces or more general spaces. For instance, a surface is the set of all its points, of all possible locations on the surface, and all geometrical objects in that surface are defined by the points that belong to them - for instance, a line on the surface is the set of (infinitely many) points.

photon

Synonym: light particle, light quantum. In quantum theory, light is not a continuous electromagnetic wave, but a steady stream of tiny energy packets - photons.

particle physics

The branch of physics that deals with particles that are, to the best of today's knowledge, not made up of more fundamental sub-units, for instance with electrons, quarks or neutrinos. Also included is the study of some species of particles that do have more elementary constitutents, such as protons or neutrons, but not of larger systems such as atomic nuclei (that would be nuclear physics) or, even worse, whole atoms. Questions such as, whether the particles nowadays thought to be elementary really are elementary, or are, for instance, different manifestations of one and the same species of string, fall within the scope of particle physics. The theoretical tools of particle physics are the so-called quantum field theories which allow the description of elementary particles on the basis of both quantum theory and special relativity, while the main experimental tools are particle accelerators in which particles are accelerated and then brought to collision.

matter

In general relativity: All contents of spacetime that contribute to its curvature: particles, dust, gases, fluids, electromagnetic and other fields. In particle physics: All elementary particles with half-integer spin, such as electrons and quarks, as well as their composites such as protons and neutrons, in contrast with force particles.

gravity

See gravitation

graviton

Hypothetical carrier particle in a quantum theory of gravity. However, as of yet physicists have but a rough idea of how a complete theory of quantum gravity will look like.

gravity (gravitation)

In classical physics: An action-at-a-distance force by which all bodies that possess mass attract each other (see Newtonian theory of gravity), synonym: gravitational force. In Einstein's general theory of relativity: The fact that matter that possesses mass, energy, pressure or similar properties distorts spacetime, and that this distortion in turn influences whatever matter might be present. An introduction to the basic ideas of general relativity is provided by the section General relativity of Elementary Einstein. More information about the nature of gravity in general relativity can be found in the spotlight text Gravity: From weightlessness to curvature.

force

In mechanics: Influence acting on a body, trying to accelerate it.

More generally: All influences by which elementary or other particles can interact; in this sense, force and interaction are synonymous. In the standard model of particle physics, there are three elementary forces: electromagnetism, the weak (nuclear) force and the strong (nuclear) force, while there is no quantum description of the fourth fundamental interaction, gravity.

extra dimensions (extra dimension)

According to some of the candidate models for a theory of quantum gravity, notably in string theory, our world should have extra dimensions - space dimensions in addition to the three we know from everday life. More information about extra dimensions, possibilities of observing their effects and their use for building physical models can be found in our section Spotlights on relativity, namely Extra dimensions and how to hide them, The hunt for extra dimensions and Simplicity in higher dimensions.

elementary particle

Within the commonly accepted models of physics, certain particles are not built of even more fundamental sub-particles - they are themselves elementary. Examples for elementary particles are electrons, quarks and neutrinos, while particles such as protons or neutrons consist of sub-units and are thus not elementary. The study of elementary particles is called particle physics, under which keyword some more information about the theoretical framework of elementary particles can be found.

electron

Low-mass elementary particle with negative electric charge. The atoms that are the constituents of everyday matter each consists of a nucleus surrounded by a shell of electrons.

carrier particle

In the framework of relativistic quantum field theories (which form the theoretical basis of the physics of elementary particles, the forces by which matter particles interact are transmitted by so-called carrier particles travelling back and forth between them. For instance, the electric force between two electrons would come about through the exchange of photons, the carrier particles of the electromagnetic interaction. Carrier particles always have integer spin, such as spin 1 or 2 (which means they belong to the class of particles called bosons). Synonym: force particles.

brane

In string theory: An object that is the analogue of a two-dimensional membrane embedded in three-dimensional space - an entity with a certain number of dimensions (one-brane, two-brane, three-brane...) embedded in the higher-dimensional space of string theory. A one-brane or 1-brane has one spatial dimension, a two-brane has two, and so on.

Relativity and the Quantum / Elementary Tour part 5: Superstrings and universal harmony

Further Articles

Tag Cloud