Contact with experiment

The mathematics of string theory may lead to new insights on quantum chromodynamics, a gauge theory which is the fundamental theory of the strong nuclear force. To this end, it is hoped that a gravitational theory dual to quantum chromodynamics will be found.

This means that the discovery of a new gauge group with a small quantum of charge and only heavy charged particles would falsify string theory. Since this argument is very general — relying only on black-hole evaporation and the holographic principle, it has been suggested that this prediction would be true of any consistent holographic theory of quantum gravity, although the phrase “consistent holographic theory of quantum gravity” might very well be synonymous with “String Theory”.

It is notable that all the gross features of the Standard model can be embedded within String theory, so that the standard model is not in the swampland. This includes features such as non-abelian gauge groups and chiral fermions which are hard to incorporate in non-string theories of quantum gravity.

— Wikipedia on String theory

2011.01.26 Wednesday ACHK

Physics Today, created in 1948, is the membership journal of the American Institute of Physics. It is provided to 130,000 members of twelve physics societies, including the American Physical Society. Over the last 60 years many famous physicists have written for the magazine, including Albert Einstein, Niels Bohr, and Richard Feynman.

Although its content is scientifically rigorous and up to date, it is not a true scholarly journal in the sense of being a primary vehicle for communicating new results. Rather, it is more of a hybrid magazine that informs readers about important developments in the form of overview articles written by experts, shorter review articles written internally by staff, and also discusses the latest issues and events of importance to the science community such as science politics.

The physics community’s main vessel for new results is the Physical Review suite of scientific journals published by the American Physical Society and Applied Physics Letters published by the American Institute of Physics.

— Wikipedia on Physics Today

2011.01.25 Tuesday ACHK

Gauge-gravity duality

Gauge-gravity duality is a conjectured duality between a quantum theory of gravity in certain cases and gauge theory in a lower number of dimensions. This means that each predicted phenomenon and quantity in one theory has an analogue in the other theory, with a “dictionary” translating from one theory to the other.

Description of the duality

In certain cases the gauge theory on the D-branes is decoupled from the gravity living in the bulk; thus open strings attached to the D-branes are not interacting with closed strings. Such a situation is termed a decoupling limit. In those cases, the D-branes have two independent alternative descriptions.

As discussed above, from the point of view of closed strings, the D-branes are gravitational sources, and thus we have a gravitational theory on spacetime with some background fields.

From the point of view of open strings, the physics of the D-branes is described by the appropriate gauge theory.

Therefore in such cases it is often conjectured that the gravitational theory on spacetime with the appropriate background fields is dual (i.e. physically equivalent) to the gauge theory on the boundary of this spacetime (since the subspace filled by the D-branes is the boundary of this spacetime).

So far, this duality has not been proven in any cases, so there is also disagreement among string theorists regarding how strong the duality applies to various models.

— Wikipedia on String theory

2011.01.24 Monday ACHK

But the theory also describes universes like ours, with four observable spacetime dimensions, as well as universes with up to 10 flat space dimensions, and also cases where the position in some of the dimensions is not described by a real number, but by a completely different type of mathematical quantity. So the notion of spacetime dimension is not fixed in string theory: it is best thought of as different in different circumstances.

This can be better understood by noting that a photon included in a consistent theory (technically, a particle carrying a force related to an unbroken gauge symmetry) must be massless.

— Wikipedia on String theory

2011.01.23 Sunday ACHK

Investigating how a string theory may include fermions in its spectrum led to the invention of supersymmetry, a mathematical relation between bosons and fermions.

T-duality relates the large and small distance scales between string theories, whereas S-duality relates strong and weak coupling strengths between string theories. U-duality links T-duality and S-duality.

— Wikipedia on String theory

2011.01.22 Saturday ACHK