TUM physicists achieve solar system’s weakest magnetic field

Prof. Dr. Peter Fierlinger (left) and Mitarbeiter Michael Sturm
Courtesy of Astrid Eckert/TUM

Electromagnetism represents one of the four fundamental interactions of nature, and ever since Maxwell described it mathematically in his equations, electrical engineers have put it to good use. Of course, the downside is that electromagnetism often appears where it’s unwanted, to potentially deleterious effect. Consequently, engineers often find themselves having to simulate and measure electromagnetic interference,¹ generate it to determine the susceptibility of equipment under test,² and ultimately take some steps to minimize it.³

With regard to this last issue—at least with respect to the magnetic component of the EM wave—a group of physicists has implemented record-breaking magnetic shielding to achieve what they call the weakest magnetic field in the solar system, allowing them to conduct high-precision experiments free from magnetic interference. They have presented their work in the Journal of Applied Physics.⁴

The researchers, headed by Professor Peter Fierlinger, physicist at the Technische Universität München (TUM) and researcher of the Cluster of Excellence “Origin and Structure of the Universe,” have successfully created a 4.1 cubic meter space at the Garching research campus in which permanent and temporally variable magnetic fields are reduced over a million-fold at milliHertz frequency ranges. The researchers note that in Central Europe the Earth’s ever-present magnetic field measures 48 microtesla, on top of which occur local EM fields generated by transformers, motors, cranes, and so forth.

The group accomplished the low magnetic field using a magnetic shielding comprising various layers of a highly magentizable alloy trademarked Magnifer. The ensuing magnetic attenuation results in a residual magnetic field inside the shield that is even smaller than that at the depths of our solar system. The approach improves the attenuation of previous setups more than tenfold, they report. To measure the effectiveness of their shielding, they employed a variety of different sensors, including a fluxgate probe, liquid-helium cooled SQUID (superconducting quantum interference device) magnetometers, mercury nuclear spin magnetometers, and cesium atomic vapor magnetometers.⁴

Electric dipole moment of the neutron

The researchers report that reducing electromagnetic noise is a key prerequisite for many high-precision experiments in physics, biology, and medicine. For biology and medicine, they note that effective magnetic shielding could be useful for biomagnetic signal measurements or for the investigation of magnetic nanoparticles for cancer therapy. Their particular area of interest is fundamental physics, for which the highest degree of magnetic shielding is essential for making precision measurements of miniscule effects in phenomena that drove the early development of our universe. Fierlinger’s team currently is developing an experiment to determine the charge distribution in neutrons—referred to by physicists as the electric dipole moment.

Electrically neutral neutrons comprise three quarks, whose charges cancel each other out. However, scientists suspect that neutrons have a tiny electric dipole moment. Unfortunately, past measurements were not sufficiently precise. The new, nearly magnetic-field-free space provides the requisite conditions for improving measurements of the electric dipole moment by a factor of 100, opening the door to a realm of the theoretically predicted scale of the phenomenon.

Physics beyond the Standard Model

“This kind of measurement would be of fundamental significance in particle physics and swing wide open the door to physics beyond the Standard Model of particle physics,” explained Fierlinger in a press release. The Standard Model describes the characteristics of all known elementary particles to a high degree of precision.

Yet, there still are phenomena that cannot be adequately explained: Gravity, for example, is not even considered in this model. The Standard Model also fails to predict the behavior of particles at very high energies as prevailed in the early universe. And, it provides no explanation for why matter and antimatter from the Big Bang did not annihilate each other completely, but rather a small amount of matter remained from which we and our surrounding visible universe are ultimately formed.

Physicists therefore attempt to create short-lived conditions as were prevalent in the early universe using particle accelerators like the Large Hadron Collider at CERN. They smash particles into each other at high energies, in particular to create new particles.

Alternatives to high-energy physics

The experiments of the TUM scientists complement those in high-energy physics: “Our high-precision experiments investigate the nature of particles at energy scales that will likely not be reached by current or future generations of particle accelerators,” said doctoral candidate Tobias Lins, who worked on the magnetic shield setup in Fierlinger’s laboratory.

Exotic and hitherto unknown particles could alter the properties of known particles. Thus, even small deviations in particle characteristics could provide evidence for new, previously unknown particles, the researchers conclude.

In addition to scientists at TUM, physicists of the Physikalisch-Technischen Bundesanstalt Berlin, the University of Illinois at Urbana-Champaign, the University of Michigan, and IMEDCO AG in Switzerland contributed to the experimental setup and measurements of magnetic attenuation. Funding was provided by the German Research Foundation in the context of the Priority Program SPP 1491 and the Custer of Excellence “Origin and Structure of the Universe.”

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