On March 1, the McGill physics department  hosted a public lecture by Nobel laureate Dr. Theodor W. Hänsch of the Max-Planck-Institute of Quantum Optics, as part of this year’s Anna I. McPherson lecture series. During his talk, called “Passion for Precision,” Hänsch discussed the importance of precise measurements in his field, and his lifelong pursuit to develop breakthroughs in fundamental physics through precise laser spectroscopy of simple atoms.

Hänsch shared a Nobel Prize in Physics in 2005 with Roy J. Glauber and John L. Hall “for their contributions to the development of laser-based precision spectroscopy, including the optical frequency comb technique.” Hänsch began the lecture by providing some insight into the importance of pursuing increased precision in physical measurements.

“Precise measurements are very important,” he said. “If you can measure more … digits than people before you, you could find something surprising, and if you’re lucky, you might find something new and fundamental.”

Hänsch and his team have long pushed the limits of precise measurement. One of their most recent projects involved trying to measure the radius of a proton more accurately than ever before.

“Proton size is a dominant source of uncertainty, and we pondered for a very long time on how to measure this well [beyond] mainstream techniques of measurement,” he said. “Ten years ago, we started the quest to measure the size of a proton using laser spectroscopy.”

Physicists have been measuring proton size for more than 90 years. But in 2010, a team led by Randolf Pohl from Hänsch’s laboratory derived a new experimental value for the proton radius that is 10 times more precise than the accepted official CODATA (the Committee on Data for Science and Technology) value. The measurement was about four per cent smaller than the previous estimate. The finding rocked the physics world, as it potentially undermines some of the most trusted laws in physics. According to Hänsch, even his team did not expect the discovery. They thought at first that the measurements were not in the predicted range because of some experimental failure.

“Based on the accepted proton radius, we had anticipated the range of frequency that the results would produce, but the experiment failed. We decided to look outside the range and we finally found it way outside the expected frequencies. This might be an experimental error, or [it may] indicate a gap in the quantum electrodynamic theory,” he said.

Hänsch also looked back on his 1970 invention of a very precise type of laser called a frequency comb. The tool can measure spectral line emissions of a hydrogen atom with higher precision than was previously possible. A hydrogen atom has many energy levels, and when the atom moves between energy states, a wave of light is emitted. The energy in this wave of light is equal to the energy change in the atom. The light wave has a specific colour and wavelength. The set of all possible observable light emissions are collectively referred to as spectral line emissions, and this set is unique to each atom.

Increased precision measurements in this area proved to be game-changing. Hydrogen atom properties form the basis of many fundamental quantum physics constants, such as the Rydberg constant, which is precisely inferred through analysis of hydrogen’s light emissions. As of 2010, the Rydberg constant was the most accurately measurable of the fundamental constants.

“Before, we were only able to measure the spectral lines of hydrogen up until six decibels. Now, we’re able to measure up to 15 decibels. We are pushing the limits of what is humanly possible, which might lead to a deeper understanding of fundamental physics,” Hänsch told his audience.

Most recently, Hänsch’s discovery of the size of the proton was published in Nature, and was listed as one of Physics World‘s top 10 breakthroughs of 2011. It is a great example of the impact precise measurements can make.