Cosmic rays have captivated scientists due to their enigmatic origins, imperceptibility, and natural abundance. Originating from celestial bodies ranging in distances from as close as our sun to as far as distant galaxies, these particles bombard our Earth at rates close to the speed of light. While these particles are responsible for the aurora borealis displays in the arctic, for the most part they go unnoticed and have been mainly researched in the context of astronomy and astrophysics (Howell 2018). However, recent development in muon tomography and research from Professor Hiroyuki Tanaka’s research group from the University of Tokyo has developed a wireless muometric navigation system (MuWNS) capable of using muons to create an indoor positioning system (Tanaka 2022).
Muons are natural subatomic particles that are created from cosmic rays interacting with atoms in the atmosphere. With their mass around 207 times that of electrons, muons are capable of penetrating solid materials and water (Gururaj 2023). This unique property of muons has allowed for their use in mapping the interiors of hard-to-access places such as volcanoes, tropical storm cells, and even Egyptian pyramids (Morishima, 2017). Professor Tanaka’s team has now focused on improving the currently limited GPS system with a wireless muon detection system capable of navigation in places where radio waves used in GPS can not reach. This makes it an ideal technology for underground and underwater navigation, natural disaster relief, exploration of caves in planets, and much more.
While the initial principle behind MuWNS involves the precise measurement of the timing and direction of cosmic-ray-generated muons through reference detectors, Professor Tanaka’s team had issues with the synchronization of time between the reference and receiver detectors (Tanaka, 2022). This precise time synchronization issue was displayed in their 2022 MuWNS prototype that had a navigation accuracy between 2-14 m, which Professor Tanaka claims is “far from the level required for the practical indoor navigation applications.” In a more recent article published in September 2023, Professor Tanaka has shifted his focus from using the timing of muons to measuring the directional vectors of incoming muons. Thus, instead of using the time of muon travel between the reference and receiver detectors for navigation, the next generation vector muPS (muometric positioning system) uses the angles of incoming muons through the reference and receiver detectors to locate the detector’s positioning. In essence, matching the angles of muons entering the two detectors confirms the same muon event. By identifying the same muon event, the angle and path of the muon is then used to determine the position of the receiver detector without relying on timing mechanisms. This approach minimizes the effects of time synchronization resulting in what he predicts as centimeter-level accuracy (Tanaka 2023). This new development has been greeted with excitement, earning Professor Tanaka’s team a spot in Time Magazine’s “The Best Inventions Of 2023” (Stokel-Walker 2023).
After being intrigued by Professor Tanaka’s work published in Nature (Tanaka 2023), I reached out to him asking a few questions for this article. The first question I asked was about the presence of muons and whether muon tomography could work on other celestial bodies. His response highlighted that muons are in fact generated in dust deposits on top of the surface of the Moon and Mars. Specifically, Professor Tanaka discussed how muons could be used to explore caves within the Moon. This would involve deploying a muPS navigating robot that uses muons generated in the regolith for navigation underground. This could allow us to explore hard to examine places on other planets without the physical presence of human exploration.
The second question involves the application of muPS within cell phones. Tanaka explains that our phones currently have a GPS receiver inside of them, allowing us to track their location when they are lost. However, if the cellphone is lost in an elevator, basement, cave, or room that has limited GPS signals, muPS could locate the phone instead. With 6.92 billion smartphone users worldwide, this application could be useful in natural disasters where individuals may be trapped under rubble and GPS signals cannot locate their phones (Zippia 2023).
Finally, I asked Professor Tanaka what made him excited about muPS. He responded by discussing the current limitations with our present indoor/underground navigation systems and how they all rely on laser, sound, or radio waves to guide them through obstacles. This method he claims is not technically navigation because it does not provide coordinate information and thus is un-programmable. Tanaka states that “muPS is [the] only technique that provides the coordinate information besides GPS” and it can be used in locations where GPS is unavailable.
In future technology, muon-based positioning systems may provide the opportunity to open new navigational and observational possibilities, propelling us into a world of new discoveries and exploration on Earth and beyond.
- Gururaj, T. (2023, June 16). World’s first cosmic-ray GPS can detect underground movement. Interesting Engineering. https://interestingengineering.com/innovation/cosmic-ray-gps-underground-movement-disaster-management-muons
- Howell, E. (2018, May 11). What are cosmic rays?. Space.com. https://www.space.com/32644-cosmic-rays.html
- Morishima, K., Kuno, M., Nishio, A. et al. (2017). Discovery of a big void in Khufu’s Pyramid by observation of cosmic-ray muons. Nature 552, 386–390.. https://doi.org/10.1038/nature24647
- Stokel-Walker, C. (2023, October 24). Muon Positioning System: The 200 best inventions of 2023. Time. https://time.com/collection/best-inventions-2023/6326412/muon-positioning-system/
- Tanaka, H.K.M. Wireless muometric navigation system. Sci Rep 12, 10114 (2022). https://doi.org/10.1038/s41598-022-13280-4
- Tanaka, H.K.M. Muometric positioning system (muPS) utilizing direction vectors of cosmic-ray muons for wireless indoor navigation at a centimeter-level accuracy. Sci Rep 13, 15272 (2023). https://doi.org/10.1038/s41598-023-41910-y
- Vlasov, A. (2023, April 14). Muon Imaging: How Cosmic Rays help us see inside pyramids and volcanoes. IAEA. https://www.iaea.org/newscenter/news/muon-imaging-how-cosmic-rays-help-us-see-inside-pyramids-and-volcanoes
- Zippia. 20 Vital Smartphone Usage Statistics : Facts, Data, and Trends On Mobile Use In The U.S. Zippia.com. Apr. 3, 2023, https://www.zippia.com/advice/smartphone-usage-statistics/