Our research

Within our research group, a wide range of topics in nonlinear physics is covered. Ionospheric physics, plasma physics, gravitational physics, collective quantum physics, as well as ocean freak waves is within the field of expertise in our group (see below for a description of some of the topics).

Examples

Most of the classical physical systems can be described by nonlinear equations. A prominent example in which nonlinear phenomena are important is the field of hydrodynamics, which tells us what the weather will be over the next few days.

1. Plasma physics: A plasma can be considered as the fourth state of matter. The typical property of a plasma is that it is electrically conducting. Thus, the description of the plasma state need to take into account not only the separate trajectories of the plasma particles, but also the self-consistent electromagnetic interactions between the plasma constituents. Plasma-like behavior can be found in many other systems, such as neutrino-electron systems (where the weak interaction has to be accounted for) or quark-gluon plasmas (where strong interaction are important). The methods for treating the special types of nonlinear effects arising in plasma physics has wide spread applications to other areas of science, such as soliton communication in optical fibers.
   

The investigation of our closest space environment, the solar system, has benefited greatly from knowledge in plasma physics. At the same time, plasma physics has in this environment also found an important laboratory in which theory meets observation.

In astrophysics, plasmas play an important role. It is believed that as much as 99% of the visible baryonic matter content of our Universe is in the plasma state (that said, it should be noted that currently most of the matter and energy in the Universe is of an unknown form).

Our research group has studied many plasma phenomena related to space physics as well as laboratory plasma physics, such as dusty (or complex) plasmas and laser-plasma interactions.

1. Rogue waves: Rogue wave (or freak waves) are extreme water waves, many times higher than the surrounding waves, that occur in rough seas in an unpredictable manner. Over the years, it has been understood that a large part of such waves are due to the nonlinear interactions between the water waves. This nonlinear interaction allow the larger waves to absorb the energy of the smaller surrounding waves in an effective manner, thus creating even bigger waves. Although the dynamics of such waves are described by a very complicated set of equations, certain simplifications, using e.g. the assumption of deep water, can be made. Below out-takes from a simulation is shown.
   

The nonlinear interaction of two water wave systems. In the left figure, a random sea swell is seen. At a later time, the random waves has formed a pattern if large waves (red) and small waves (blue-green), with the maximum peaks being more than three times larger than what one started from (see also Physical Review Focus).

1. Collective quantum phenomena: The event of quantum theory was indeed a mile stone in science of the 20th century, giving an accurate description of the behavior of atoms and molecules and laid the foundation for our understanding of the fundamental interactions between elementary particles. The basic quantum mechanical laws are linear by nature, but as collective interactions are taken into account, or certain degrees of freedom are treated as effective fields, nonlinear effects occur.
   

Our group has done research on light interactions through the quantum vacuum (see Physical Review Focus and Physics News Update), as well as fundamental studies into collective quantum effects in plasmas.

Illustration of the similarities if the Hawking (left) and Unruh (right) effect. In the laboratory, a strong laser pulse would accelerate electrons, subjecting them to a heat bath with a temperature of around 10,000 K, due to the quantum vacuum.