The theory group works on a wide variety of topics with diverse interests ranging from astrophysics and condensed matter to physics at the Planck scale. Theoretical advances frequently rely on the intimate connections between seemingly different areas of physics. Also, the nature of theoretical research encourages a multidisciplinary approach and makes it easy for people to move from topic to topic. So this page should not be regarded in terms of rigid categories; rather it is intended to provide an overview of some of the main themes of theoretical work at Columbia.

The theory group has particular strengths in the areas of: string theory, quantum field theory, particle and nuclear theory, lattice gauge theory, condensed matter theory, and astrophysics and cosmology. It is made up of about 15 physics faculty, about a dozen postdocs (many of whom are associated with the Institute for Strings, Cosmology and Astroparticle Physics (ISCAP), and the lattice gauge groups), and about 16 graduate students. Some of the areas that Columbia is actively involved in are described in more detail below.

The theory group has a close relationship with several members of the Mathematics and Astronomy Departments, as well as the Barnard College Physics Department. The math department has a number of people working on string theory and related topics while the astronomy department has several theoretical astrophysicists.

**Facilities:**

While the traditional facilities for theoretical physicists are blackboards, some fields of theoretical physics utilize powerful computers. Lattice gauge theory is one such field and Columbia is home to one of the most powerful parallel processors for lattice gauge computations in the world. In addition, ISCAP provides a venue where theoretical physicists with cross-disciplinary interests in those fields can easily interact.

*This drawing illustrates the three different length scales involved in the decay of a kaon into two pions, a process whose calculation was carried out for the first time by a group of Columbia physicists and their collaborators and was announced last year. The lowest layer is a picture showing the tracks of the decay particles as they move through the liquid hydrogen of a “bubble chamber” — a particle detectorused in the 1950s and 60s. The next layer is a diagrammatic interpretation of what’s happening in the bubble-chamber picture — how the kaon (K) is produced and “breaks apart” to form two other particles: the positive pion (p+) and negative pion (p-). This process is viewed on the familiar scale of a fraction of a meter. The next scale of a few femtometers is shown on the third layer, where the lattice of points and paths represents the supercomputer calculation, which incorporates the binding of quarks and antiquarks as they form the particles being studied. (A femtometer is one millionth of one billionth of a meter: 0.000000000000001 of one meter.) Finally the top layer shows what is known as a Feynman diagram of the shortest scale —1/1000 of a femtometer — the scale at which a quark undergoes a metamorphosis from one flavor into another which allows the kaon to decay.