Scientists Report Second Highest-Energy Cosmic Ray Ever Detected

12:16 minutes

Helicopters lowering cosmic ray detectors.
The surface detector array of the Telescope Array experiment, deployed by helicopter. Credit: Institute For Cosmic Ray Research, University Of Tokyo

Around 30 years ago, scientists in Utah were monitoring the skies for cosmic rays when they detected a surprising particle. It struck the atmosphere with much more energy than they had previously seen—enough energy to cause the researchers to dub it the “Oh My God Particle.”

Over the years, a collaboration of researchers in Utah and Japan has detected other powerful rays—about 30 a year—but none that rival the OMG. In 2021, however, a second particle was detected. It was only slightly less powerful than OMG, but still many times more powerful than can be created on Earth. That 2021 particle was named “Amaterasu,” after a sun goddess from the Japanese Shinto religion. The researchers described their observations in a recent issue of the journal Science.

The researchers believe the particle must have come from relatively nearby, cosmically speaking, as otherwise it would likely have collided with something in space and lost its energy. However, when they tried to trace the particle back to its origin in space, they were unsuccessful. Both the OMG particle and the new Amaterasu particle seem to have come from empty regions of space, with no violent events or massive structures to create them.

Dr. John Matthews, a research professor in physics and astronomy and manager of the Cosmic Ray Physics Program at the University of Utah, joins Ira to talk about cosmic rays, how they’re detected, and the challenges of finding the origin of particles like Amaterasu.

Segment Guests

John Matthews

Dr John Matthews is a research professor in Physics And Astronomy and manager of the Cosmic Ray Physics Program at the University of Utah in Salt Lake, Utah.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Later in the hour, recycling cocoa pods to make a slew of useful materials. Plus the tricky legal landscape of syringe exchange programs. But first, have you ever heard of the “Oh my God” particle? Me neither, but physicists love to give subatomic particles cool names.

And back in 1992 when they discovered the highest energy cosmic ray ever discovered, they christened it as the “Oh my God” particle because it was really powerful and really mysterious. They had no explanation for it, hence, OMG.

Fast forward to around two years ago, sensors in Utah desert detect the arrival of a second very high energy cosmic ray, the second most powerful they’ve ever seen. And while the researchers are convinced that the cosmic ray was real, many aspects of the event don’t really make sense. It’s more powerful than anything we can make on Earth and seems to come from nowhere in the sky. The researchers recently described their observations in the journal Science.

Joining me now to talk about the find is one of the co-authors of that report, Dr. John Matthews, research professor in physics and astronomy, manager of the Cosmic Ray Physics Program at the University of Utah in Salt Lake City. Welcome to Science Friday.

JOHN MATTHEWS: Hello. Good to be here. Thanks for having me.

IRA FLATOW: This new cosmic ray find, would it be the, “Oh my god.2” of particles?

JOHN MATTHEWS: [LAUGHS] Yeah, that’s one way to describe it. The “Oh my God” particle, somebody was looking at an event to say and said, oh my God, what was that? In this case, we found it a little bit later because of the different kind of detectors that were observing it. And people said, wow, that’s really an impressive energy. That needs a name.

IRA FLATOW: Let’s start with the basics. What is a cosmic ray? And where does it come from, all those kinds of things?

JOHN MATTHEWS: So a cosmic ray is a particle from space. Some can come from within the galaxy, some from outside the galaxy. They can be photons or particles of light. They can be electrons. At this energy, they’re often subatomic particles, like a proton. They could be a helium nucleus. In this case, we think it’s a proton probably.

IRA FLATOW: And how do you detect them? What does your detector look like?

JOHN MATTHEWS: Particles at this energy, they’re relatively rare, so sort of one particle per square kilometer per century, so really rare.


JOHN MATTHEWS: So you detect them in an indirect way. The particle comes in from space. It collides with the nucleus of an atom high up in the atmosphere. It smashes apart that nucleus, and you get a bunch of secondary particles. They travel a short distance and do the same thing. And

You get what’s called an extensive air shower. So you sprinkle the desert with detectors that are about the size of a ping pong table. And when those charged particles reach the Earth’s surface, they pass through those detectors and generate light, which our detectors measure. And then that’s a sample of how many charged particles passed.

IRA FLATOW: So around two years ago, you detected this unusual particle. How unusual was it?

JOHN MATTHEWS: Well, it’s so unusual that 30 years later, it’s the second one in this energy range. [LAUGHS] At low energies, we’re seeing them several a night or more, depending on the energy range. At high energies, they’re really, really rare. At low energies, two are passing through your head every second. But at these high energies, it’s one per square kilometer per century, so it’s really a rare event.

IRA FLATOW: When we say that they’re powerful, though, when you talk about how powerful this is, what kind of power are we talking about?

JOHN MATTHEWS: This event comes in with an energy of like 40 joules. This particle, if you read the paper, is 244 EeV exa-electronvolts, which that is really not a relatable term. But if you say 40 joules, then what that means for an ordinary object is like a four kilogram object dropped from one meter on Earth. So if a 10 pound object dropped it on the floor, it’s that much energy. But instead of being in a brick or something, that’s all contained in a single proton.

IRA FLATOW: So you’re saying if you hold a brick at waist height and drop it on your foot, that’s what you’d be feeling.

JOHN MATTHEWS: That’s what you’d be feeling if you absorbed all of that energy from that particle.

IRA FLATOW: Wow. Then that’s all packed into one tiny subatomic particle.

JOHN MATTHEWS: Exactly, which is pretty amazing amount of energy. It’s millions of times more energy than we can generate in protons here at Earth, for example, at the Large Hadron Collider.

IRA FLATOW: No kidding. And one of the mysteries about this is that it’s coming from, as I said, nowhere.

JOHN MATTHEWS: Yeah, exactly. That’s one of the real mysteries is, in the last 20-ish years, since 2008 when we’ve been operating the Telescope Array, we’ve detected 30 particles that are in not too much farther from this but above 10 to the 20th electronvolts. And they all appear to come from nowhere. And in specific, these really, really high ones, high energy ones, that come from the great void next to us or the “Oh my God” particle comes from someplace else, none of them really looks like they come from any place you might expect.

IRA FLATOW: Do we know what produced them, then? Obviously, not if we don’t know where they came from.

JOHN MATTHEWS: That’s the mystery, and that’s why we have experiments like the Telescope Array and the Pierre Auger experiment in Argentina because people would really love to know where these things are coming from. But so far, we’re not able to really identify the sources.

IRA FLATOW: Any conjecture? Anybody have any bets going on of what it might be?

JOHN MATTHEWS: Well, the conjecture is sort of things like what’s called an active galactic nuclei, which is a big supermassive black hole with stuff swirling around it and really energetic jets of particles shooting out of it, stuff that didn’t quite get sucked in. That’s one possibility. More fanciful ideas are things like, well, maybe it’s a decay of a dark matter particle that nobody can see. And it decays and then shoots out these really energetic particles. If it was something like that, that might explain why it’s coming from nowhere or everywhere.

IRA FLATOW: Could it also mean that we need new physics to talk about this?

JOHN MATTHEWS: If it was dark matter decays, we would need new physics, and we would need to be able to find something like dark matter. And that’s something people are really looking hard to find but so far have been unsuccessful.

IRA FLATOW: So this really must be keeping physicists up late at night or scratching their heads.

JOHN MATTHEWS: Exactly. Well, we’re up late at night all the time


That’s the nature of our business.

IRA FLATOW: How far away are these possible? Can you determine– if you don’t know where they’re coming from, can you determine how far away that nothing is?

JOHN MATTHEWS: So that’s a big part of the mystery. They should come from someplace, quote, unquote, close because otherwise they’d collide with the microwave background. And then they would lose their energy.

IRA FLATOW: So what do you have to do to figure this out? Do you need new equipment? Do you need new theories?

JOHN MATTHEWS: We need new equipment and more data, so new equipment in the sense that it’d be nice to have bigger detectors. Right now, the Telescope Array is about 1000 square-kilometers. We’d like to finish our expansion to 3000 square-kilometers, which is like the size of Rhode Island. Ideally, it would be nice to have muon detectors, but those would be really expensive. So really, the answer is more bigger, better detectors. People talk about putting detectors out in space that would do similar things.

IRA FLATOW: There are other groups, right, with detectors looking at different areas.

JOHN MATTHEWS: Well, the other main group at the moment is the Pierre Auger project, which is down in Argentina, sort of in a space that’s very similar to our space in Delta, Utah. There’s other groups like EUSO, the Extreme Universe Space Observatory, or POEMMA, which would launch satellites up into space or put detectors on the Space Station and try to find events that way.

IRA FLATOW: Do all the groups around the world that are looking for the particles, are they seeing the same ones or the same amounts?

JOHN MATTHEWS: That’s actually a good question because, if you look at what we see here in Utah, you can see the spectrum or the flux of how these things arrive at different energies as a function of time. And in Argentina, they look, and they see something very similar. However, when you look at the details at the very highest energies, the Pierre Auger experiment sees an energy cutoff that’s at a significantly lower energy than what we’re seeing at Telescope Array.

So we’ve been looking and looking for a reason for that. And so far, we haven’t been able to explain it. But one explanation is there’s just different sources in the Northern Hemisphere than in the Southern Hemisphere. In the Southern Hemisphere, they’re looking more at the center of the galaxy. And here in the Northern Hemisphere, we’re looking more away from the galactic center.

IRA FLATOW: I get it. Are there any theories about what these could be, where they’re coming from, that could be tested to see if they’re correct or not?

JOHN MATTHEWS: Well, the test is, can you point it back to some object? And at a little bit lower energies, we’re starting to see hints of some things but not at these really high energies.

IRA FLATOW: And is that frustrating or fun?

JOHN MATTHEWS: Oh, it’s both frustrating and fun. It’s frustrating in the sense you’d really like to find the sources of these events. And that would be very fulfilling if we could really nail down what these things would be.

On the other hand, there’s a lot of mystery and fun in the chase. And you keep working at it, trying to find new and better ways to figure out what these things are and where they’re coming from.

IRA FLATOW: Well, you’re not the first physicist who said that the chase is more fun than the actual discovery, so.

JOHN MATTHEWS: Well, you got to be an optimist when you’re in this kind of business.

IRA FLATOW: [LAUGHS] Well, we’ll be checking back in with you when you get some new stuff. Is that OK?

JOHN MATTHEWS: Oh, that would be great. Thank you very much.

IRA FLATOW: We have run out of time with Dr. Matthews. Dr. John Matthews is a research professor in physics and astronomy, manager of the Cosmic Ray Physics Program at the University of Utah in Salt Lake City. Thank you for taking time to be with us today.

JOHN MATTHEWS: Oh, thanks for having me. This has been great.

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