New! Sign up for our free email newsletter.
Science News
from research organizations

Puzzle of Compton scattering solved: New approach for testing theories in quantum mechanics

April 14, 2020
Goethe University Frankfurt
Light can be used to knock electrons out of atoms, with light particles and electrons bouncing off each other like two billiard balls - Compton scattering. Why electrons can even be ejected from an atom when the light does not actually have enough energy to do so has now been discovered by a team of physicists.

When the American physicist Arthur Compton discovered that light waves behave like particles in 1922, and could knock electrons out of atoms during an impact experiment, it was a milestone for quantum mechanics. Five years later, Compton received the Nobel Prize for this discovery. Compton used very shortwave light with high energy for his experiment, which enabled him to neglect the binding energy of the electron to the atomic nucleus. Compton simply assumed for his calculations that the electron rested freely in space.

During the following 90 years up to the present, numerous experiments and calculations have been carried out with regard to Compton scattering that continually revealed asymmetries and posed riddles. For example, it was observed that in certain experiments energy seemed to be lost when the motion energy of the electrons and light particles (photons) after the collision were compared with the energy of the photons before the collision. Since energy cannot simply disappear, it was assumed that in these cases, contrary to Compton's simplified assumption, the influence of the nucleus on the photon-electron collision could not be neglected.

For the first time in an impact experiment with photons, a team of physicists led by Professor Reinhard Dörner and doctoral candidate Max Kircher at Goethe University Frankfurt have now simultaneously observed the ejected electrons and the motion of the nucleus. To do so, they irradiated helium atoms with X-rays from the X-ray source PETRA III at the Hamburg accelerator facility DESY. They detected the ejected electrons and the charged rest of the atom (ions) in a COLTRIMS reaction microscope, an apparatus that Dörner helped develop and which is able to make ultrafast reactive processes in atoms and molecules visible.

The results were surprising. First, the scientists observed that the energy of the scattering photons was of course conserved and was partially transferred to a motion of the nucleus (more precisely: the ion). Moreover, they also observed that an electron is sometimes knocked out of the nucleus when the energy of the colliding photon is actually too low to overcome the binding energy of the electron to the nucleus. Overall, the electron was only ejected in the direction one would expect in a billiard impact experiment in two thirds of the cases. In all other instances, the electron is seemingly reflected by the nucleus and sometimes even ejected in the opposite direction.

Reinhard Dörner: "This allowed us to show that the entire system of photon, ejected electron and ion oscillate according to quantum mechanical laws. Our experiments therefore provide a new approach for experimental testing of quantum mechanical theories of Compton scattering, which plays an important role, particularly in astrophysics and X-ray physics."

Story Source:

Materials provided by Goethe University Frankfurt. Note: Content may be edited for style and length.

Journal Reference:

  1. Max Kircher, Florian Trinter, Sven Grundmann, Isabel Vela-Perez, Simon Brennecke, Nicolas Eicke, Jonas Rist, Sebastian Eckart, Salim Houamer, Ochbadrakh Chuluunbaatar, Yuri V. Popov, Igor P. Volobuev, Kai Bagschik, M. Novella Piancastelli, Manfred Lein, Till Jahnke, Markus S. Schöffler, Reinhard Dörner. Kinematically complete experimental study of Compton scattering at helium atoms near the threshold. Nature Physics, 2020; DOI: 10.1038/s41567-020-0880-2

Cite This Page:

Goethe University Frankfurt. "Puzzle of Compton scattering solved: New approach for testing theories in quantum mechanics." ScienceDaily. ScienceDaily, 14 April 2020. <>.
Goethe University Frankfurt. (2020, April 14). Puzzle of Compton scattering solved: New approach for testing theories in quantum mechanics. ScienceDaily. Retrieved July 14, 2024 from
Goethe University Frankfurt. "Puzzle of Compton scattering solved: New approach for testing theories in quantum mechanics." ScienceDaily. (accessed July 14, 2024).

Explore More

from ScienceDaily