The Plan to Turn All of Our Smartphones into a Global Cosmic Ray Detector

One of the first steps to figuring out where the cosmic rays that constantly bombard Earth are actually coming from is to detect when and where they hit. A pair of researchers in California want to take a low-cost approach to solving the mystery by networking the world's smartphones, forming one giant cosmic ray detector.

"There are a billion phones out there. If we could get one million, ten million, we could have a revolutionary new telescope," Daniel Whiteson of UC Irvine said. "It's not like you could do this by buying a few hundred phones, but you don't need a billion. It's tantalizingly feasible."

Cosmic rays are high-energy blasts of radiation that strike the atmosphere, causing particle showers to fall across the Earth's surface. It seems pretty clear that cosmic rays are emitted from all kinds of objects in the universe—black holes, supernovas, even the Big Bang—but understanding how and why they're emitted is a more difficult question.

Current efforts to track cosmic rays like the Auger Experiment in Argentina cost millions of dollars to build due to the cost of detection infrastructure. The Auger Observatory, in particular, boasts a detection area roughly the size of Rhode Island and expanding its capabilities is extremely costly.

But according to Whiteson and Mike Mulhearn of UC Davis, respectively, the silicon in smartphone cameras can detect photons and muons falling from the atmosphere just like the Auger detectors and those at CERN, where Whiteson spends most of his time.

According to the pair, with more than 1 billion smartphones on the planet, the world's largest cosmic ray detector already exists, and all we need is an app to aggregate the data. So they built one.

The pair's app, which runs on iOS and Android, is called CRAYFIS and runs like a screensaver, sending data to their servers. The app is currently in closed beta for testing, and you can sign up to use it on their site. The system is described in a paper awaiting peer review.

When particles from cosmic ray fallout strike the silicon in a phone's camera, they leave traces of energy that appear as hot pixels. By analyzing transmitted pixel and GPS data at 30 frames per second when the phone is in video mode, CRAYFIS pinpoints the time and location of cosmic ray strikes. According to shower and detection simulations run by Whiteson and Mulhearn, just 5 phones can detect a cosmic ray strike.

To achieve 100 percent detection efficiency, 1000 networked phones need to be packed into 1 square kilometer. To surpass the current detection area of the Auger Experiment, according to Whiteson, "Under some optimistic assumptions, it seems like we'd need about a million [phones]."

Barriers to CRAYFIS' eventual efficacy once it moves beyond its closed beta testing period are technical and social in nature. According to Whiteson, server space is the chief technical issue. They need to be able to store and analyze data from millions of phones, and all that server space isn't cheap.

Whiteson and Mulhearn also need to figure out a way to store everyone's data securely, which they hope to do after extensive beta testing. Currently, the app can run in an anonymous mode that doesn't ID devices and randomizes location data to a certain degree—enough to obfuscate the phone's location without degrading the usefulness of the data.

The real deciding factor in CRAYFIS' success or failure, however, will be whether people will actually use it or not. Infrastructure can be bought and designed, after all, but people can be fickle and public taste is often inscrutable.

"The majority of the uncertainty is social; are people interested enough in contributing in science to put this on their phone?" Whiteson said. "Can we lower the barrier to participation and lower the day to day cost to the user to almost zero so the person can say, sure, I'll give them the cycles on my phone. I don't have to notice or worry about it. It's a sociological problem, I think."

Indeed, the future of CRAYFIS could rest on the willingness of the public to give up some data and processing power for the purposes of scientific research. Phone data is already being collected and stored for commercial purposes, with little return for the user. In the case of CRAYFIS, if it takes off, the return on data could be discovering the mysterious origins of cosmic rays.