British journal Nature publishes UMR study of 3-D atomic process imaging

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On March 5, 2003

Physicists at the University of Missouri-Rolla have published the first-ever three-dimensional images of atomic collision processes. The images, which promise to further understanding of theoretical physics, accompany a paper by the physicists in the March 6 issue of the British journal Nature.

The paper, "Three-dimensional imaging of atomic four-body processes," by three UMR physicists and colleagues at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, has enormous implications to theoretical physics, the authors say, because it offers scientists a new look at how ions react when they collide with atoms. Previous studies have shown only two-dimensional images of the collisions, says Dr. Michael Schulz, professor of physics at UMR, and one of the authors.

The research promises to help atomic physicists better understand the "few-body problem," which has kept scientists from accurately calculating the properties of certain subatomic systems.

"What does it take to understand nature? That’s really what physics is all about," says Schulz. "In order to understand nature, we must build on our knowledge of the forces acting in nature." The four fundamental forces are called strong, electromagnetic, weak and gravitational. "All other forces are merely different manifestations of these four," Schulz explains.

"An important property shared by all of these forces is that they always act between pairs of only two particles at a time," Schulz says. "As a second step, one has to investigate how systems containing more than two particles develop in space and time under the influence of these pairwise acting forces." This second step is known as the few-body problem.

The work conducted by Schulz and his colleagues, Dr. Don H. Madison, Curators’ Professor of physics at UMR, and Steven Jones, post-doctoral fellow at UMR, along with Dr. Robert Moshammer, Daniel Fischer, Dr. Holger Kollmus, and Dr. Joachim Ullrich from the Max Planck Institute for Nuclear Physics, is used like a "super-microscope," making reactions visible which occur within distances that are about a factor of 10 smaller than the size of an average atom. This research provides the most sensitive test for treatment of the few-body problem, a dilemma in the world of physics that frequently keeps scientists from being able to accurately calculate the properties of systems involving more than one pair of particles reacting to each other.

Fundamental forces cannot act between more than one pair of particles simultaneously because they are mediated by the two particles exchanging another particle, Schulz explains. For example, two electrons can exert a Coulomb force upon each other by exchanging a photon, the particle which constitutes light.

"Imagine you are standing on roller skates with a friend throwing a basketball to each other," Schulz explains. "The moment you throw the ball you are pushed backward, so a force is acting on you. Likewise when your buddy catches the ball, he also is thrown backward." In this instance, the basketball is the particle mediating the force; in the case of the two electrons, the photon is mediating the force.

"Because most relevant systems in nature contain more than two particles," Schulz says, "one has to understand the forces acting between pairs of particles and the few-body problem simultaneously."

Schulz uses the analogy of a basketball team to illustrate his point.

"Players are always moving around, throwing the ball in all directions, so at any given instance there is a force between two players as they throw the ball," Schulz says. "But the other players, sooner or later, will also be involved." What a player at one end of the court does depends also on what the players are doing at the other end. "Although the force acts only in pairs, various pairs in the system are linked to each other and because of this linkage, the problem of treating more than two particles is actually analytically unsolvable."

At this point, theory is used to come up with models using approximations. Because of the difficulty of studying forces in multi-particle systems, the validity of the approximations being used in the theoretical models of atomic systems have to be critically tested by experiments, ideally measuring where every particle of the system is going and how fast it gets there.

Since theorists cannot come up with an exact analytical solution, they have to use approximation and experimentalists have to provide them with data which allows them to verify the validity of those approximations. Madison and Jones provided the theoretical input while Schulz and the Heidelberg researchers performed the experiments.

In previous experiments, it was taken for granted that under certain conditions certain forces acting between selected pairs of particles are not important. The new work shows those approximations are not always justified.

Those previous experiments looked at only electrons moving in one specified plane. The experiments performed by the UMR researchers and their German colleagues look at electrons in the entire three-dimensional space.

"By this limitation to a two-dimensional plane, the earlier experiments missed a lot of information which we see in three-dimensional space," says Schulz. "It’s like searching for treasure in a castle. If you stay only on the main floor restrict yourself to one plane you will never find a treasure hidden in the basement," he says. "To find the treasure you have to search the entire three-dimensional space."

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On March 5, 2003. Posted in News