Department of Physics & Astronomy

UC Riverside physicist Umar Mohideen has achieved what few physicists have in their careers: his research has made the “standard literature,” meaning it has become part of the key experiments of physics.

“Since it is in textbooks, the experiment is part of the established key experiments,” said Mohideen, a distinguished professor of physics. “It also means the experiment is clear for graduate students to understand.”

Mohideen’s experiment focuses on the Casimir effect, first predicted in 1948 by physicist Hendrik Casimir. The Casimir effect is typified by the small attractive force that acts between two close parallel uncharged conducting plates.

Mohideen’s experiment proved that virtual photons lead to the Casimir effect. He was able to use a modification of the technique called Atomic Force Microscopy to make the measurements. While a photon is the carrier particle of electromagnetic interactions, a virtual photon is a particle that exists for so brief an instant as an intermediary in a process that it can never be directly observed. 

In the late 1990s, Mohideen and a colleague positioned an aluminum-coated sphere less than a thousandth of a millimeter away from a plate and measured the force between them. The force they measured — the Casimir force — was within 1% of the predicted value. This experiment is now included in the best and most popular graduate-level textbooks in quantum mechanics.

“The experiment is a clear demonstration of the Casimir effect, which verifies both the presence and the role of virtual photons,” Mohideen said.

The Casimir force results when the properties of virtual photons are modified. Because virtual photons are ever-present in empty space, studying the Casimir force allows physicists to learn the properties of the quantum nature of space. The force plays an important role in condensed matter physics, quantum field theory, atomic and molecular physics, gravitation and cosmology, mathematical physics, and nanotechnology.

Mohideen has coauthored a book on the Casimir force along with colleagues titled “Advances in the Casimir Effect.” The book is part of the “International Series of Monographs on Physics,” a very selective publication list from Oxford University Press.

Fellow physicist José Wudka, the divisional dean of physical sciences and mathematics in the College of Natural and Agricultural Sciences, put Mohideen’s achievement into context.

“In physics there are classic experiments that provide clear and unassailable evidence of particular theoretical predictions,” he said. “They provide the evidence on which theories are based. Among these is the Michelson-Morley experiment for the Special Theory of Relativity, and the Tonomura experiments for the Aharonov-Bohm effect. One of the more bizarre predictions of quantum mechanics is the Casimir effect, and though Umar’s experiments did not provide the first confirmation, they are becoming the classical ones. His research is likely to be one of the few results at UCR that will be remembered a century from now.”