Magnetic fields occur wherever magnets are active. The Earth itself is surrounded by a magnetic field and its orientation can be readily determined using a compass, for example. Cities also have magnetic fields and can be clearly distinguished from each other thanks to their unique magnetic signatures. This is the conclusion of a multidisciplinary study comparing two US urban areas – Berkeley in California and Brooklyn Borough of New York City. Researchers from both the USA and Germany were tasked with continuously recording data over a four-week period, supplemented with additional measurements and analyses. "We found that these areas have what you could call an individual magnetic pulse," said Professor Dmitry Budker, a UC Berkeley Professor Emeritus currently working at Johannes Gutenberg University Mainz (JGU) and the Helmholtz Institute Mainz (HIM). Berkeley, interestingly, exhibits negligible magnetic field activity at night, while Brooklyn's magnetic activity remains at a high level overnight – confirming, as the team expected, that New York really is The City That Never Sleeps.
Experts from Mainz University contribute their knowledge in measuring magnetic fields to the comparison of urban settlements
The comparison of the two urban areas was preceded by an initial prototype study at Berkeley. Here, a line of the Bay Area Rapid Transit (BART) public transport system was identified as the dominant source of magnetic field activity during the daytime. "At Berkeley, we find a lot of different magnetic background signals during both the day and at night, but the main factor contributing to this is the BART system. When the line stops operating at night, this is reflected in our measurements," explained Professor Dmitry Budker. His research group has contributed to the project, notably through its expertise in using magnetometers to measure magnetic fields. At Mainz University, the team headed by Budker is mainly concerned with developing atomic magnetometry techniques to facilitate research into the fundamental questions of physics, such as the search for dark matter.
According to Budker, the multidisciplinary nature of cities is also of interest to the participating researchers at the Center for Urban Science and Progress at New York University. They want to learn more about how cities work, looking at various aspects such as energy efficiency, environmental pollution, and the social organization of a city.
Magnetometry and urban development
The recently published analysis is intended to provide a starting point for further research in this field and to lay the foundations for future developments. "It's a small study and we get the impression that we're only just scratching the surface," Budker added. "This type of investigation offers great potential and we hope that we can take it much further in the future," said the physicist.
"Apart from the anticipated result that 'New York never sleeps,' our measurements indicate that each city has distinct magnetic signatures that can, perhaps, be exploited for the analysis of anomalies in city operation and long-term trends of the development of cities," stated the authors in their paper published in the Journal of Applied Physics. The journal selected the study as its cover article.
Potential applications for magnetometry in urban studies include post-disaster assessments, monitoring of infrastructure such as bridges, and monitoring the stability of the power grid. The researchers pose an interesting multidisciplinary question that could be addressed in future: they wonder how and if an anomalous event, such as an epidemic or a pandemic, could influence an urban magnetic signature.
In addition to Dr. Arne Wickenbrock and Professor Dmitry Budker from Mainz, the study entitled "Do cities have a unique magnetic pulse?" also involved researchers from the Lawrence Berkeley National Laboratory, the University of California, Berkeley, the University of Delaware, and the Center for Urban Science and Progress in New York City.