In 2021, scientists detected a cosmic ray entering Earth’s atmosphere with as much kinetic energy as a fast-moving tennis ball, all contained in a single subatomic particle. Named the “Amaterasu particle” after the Japanese sun goddess, the event left researchers questioning its origin because its arrival direction pointed to a region of space with no known source capable of producing such energy.

Researchers at Penn State now suggest that the puzzle may stem more from assumptions about the particle’s composition rather than its missing source.

A recent study in Physical Review Letters suggests that the Amaterasu particle, along with other extremely energetic cosmic rays, may actually be atomic nuclei heavier than iron. If correct, the idea could change how researchers search for the sources of cosmic rays.

A Particle of Unknown Origins

Ultrahigh-energy cosmic rays are some of the rarest and most violent phenomena in nature. These particles slam into Earth’s atmosphere with more than 100 exa-electron volts of energy, which is about 10 million times more than what the Large Hadron Collider can produce. The Amaterasu particle, found by the Telescope Array in Utah in May 2021 and reported in Science in 2023, had about 244 exa-electron volts of energy. This makes it one of the most energetic natural events ever recorded.

The mystery extended not only to the particle’s energy, but also to its origin. It came from the direction of the Local Void, which is an almost empty area of space next to the Milky Way, where there is no known source that could create such a powerful particle.

“The origins and acceleration mechanisms of ultrahigh-energy cosmic rays have been among the biggest mysteries in the field for more than 60 years, since the first example was reported,” said Kohta Murase, professor of physics and astronomy at Penn State’s Eberly College of Science and the study’s lead researcher.

The Heavy Nucleus Hypothesis

The Penn State team ran computational simulations to study how particles of different masses travel through intergalactic space. They found that atomic nuclei heavier than iron, known as ultraheavy nuclei, lose energy much more slowly than protons or lighter nuclei as they move through the radiation fields of deep space.

Most research on cosmic rays has assumed that the most extreme events involve protons, which are the simplest and lightest atomic nuclei. Ultraheavy nuclei lose energy much more slowly during their journey, allowing them to reach Earth carrying far more energy than lighter particles traveling similar distances. That makes them strong candidates for the most powerful cosmic ray events we’ve seen, including the Amaterasu particle.

“When we detect individual cosmic-ray particles such as the Amaterasu particle here on Earth, we can often use their energies, arrival directions and expected magnetic deflections to infer their possible cosmic sources,” Murase said.

New Theoretical Limits

The hypothesis that these particles are ultraheavy nuclei also changes the range of possible sources. If the particles are indeed heavier nuclei rather than protons, their production would require some of the most extreme environments in the universe. The researchers identify collapsars, which are massive stars collapsing directly into black holes, and binary neutron-star mergers as likely candidates. Both phenomena are also associated with gamma-ray bursts, which are among the most energetic explosions known to science.

The team’s calculations set new theoretical limits on how much ultraheavy nuclei may contribute to the overall population of ultrahigh-energy cosmic rays. Murase also noted that these sources could help explain a possible asymmetry in the ultrahigh-energy cosmic-ray spectrum between the northern and southern hemispheres.

“The most promising sites for producing and accelerating such ultraheavy nuclei are massive star deaths involving explosive collapse into black holes or strongly magnetized neutron stars, as well as binary neutron-star mergers known to be powerful gravitational-wave emitters,” Murase said.

Testing the Idea

The researchers stress that they are not saying all ultrahigh-energy cosmic rays are ultraheavy nuclei, but some of the most extreme ones might be. This could change what scientists look for in future studies. New observatories and upgrades, like AugerPrime in Argentina and the planned Global Cosmic Ray Observatory, could test these ideas. If ultraheavy nuclei are important, future observations should find evidence that protons alone cannot explain.

More than six decades after scientists first detected ultrahigh-energy cosmic rays, researchers may finally have a new clue about what the most extreme particles in the universe are made of.

Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds an MBA, a Bachelor of Science in Business Administration, and a data analytics certification. His work focuses on breaking scientific developments, with an emphasis on emerging biology, cognitive neuroscience, and archaeological discoveries.

Einstein’s equations have withstood every experimental test for more than a century. However, according to one philosopher, they have not withstood careful scrutiny of the language used to describe them.

Daryl Janzen, an astronomer and instructor at the University of Saskatchewan, recently published a philosophical challenge in The Conversation. Janzen’s critique does not question the mathematics but instead focuses on how physicists and philosophers describe the nature of space-time itself. He argues that over the past century, the language used to describe space-time has gradually merged two separate concepts into one.

Existence vs. Occurrence

The main focus of this argument is the idea of the “block universe.” In this view, also known as eternalism, space-time is a fixed, four-dimensional structure. All past, present, and future events are equally real within this block, according to this interpretation. Time does not flow, and there is no single present moment. The block contains everything that has ever occurred.

Janzen does not object to the geometry itself. His concern lies with the way the word “exists” is used in this context.

Janzen distinguishes between existence and occurrence. Objects exist because they endure through time. Events occur at a specific place and moment, and then they are over. For example, an elephant standing in a room exists. A cross-sectional snapshot of that elephant, representing a single instant removed from its timeline and frozen in place, does not exist in the same way; it only occurs.

In the block universe model, space-time holds every moment of the elephant’s existence as a four-dimensional world line. This world line shows the animal’s full path through space and time. A single cross-section, or one slice of that path, is not the elephant itself; it is only a record of what the elephant was doing at a certain moment. Janzen says the block is better understood as a representational framework rather than a physical object in the usual sense.

A Problem of When

This confusion leads to a fundamental problem. The entire block universe is said to exist, but it is unclear when it exists. A timeless structure that never unfolds cannot endure as the elephant endures from moment to moment. Meanwhile, if the whole block occurs at once, like a single flash, there is no internal experience of time that could be considered an illusion. 

To keep the sense of time passing in a block universe, the four-dimensional structure would need to endure in some sense. This would mean adding a fifth dimension, a second time axis outside the block, for the block itself to exist within; however, this extra axis is not part of Einstein’s original theory.

Janzen uses a simpler analogy. He compares the situation to describing a song that exists all at once, complete but unperformed. A song only becomes a song when it unfolds over time.

Austrian-British philosopher Ludwig Wittgenstein warned that philosophical problems arise when “language goes on holiday.” Janzen argues that physics has been in this situation for decades, reusing words such as “timeless,” “exist,” and “become” in technical senses without considering their everyday connotations.

Implications for Physics and Philosophy

One of the biggest unsolved problems in physics is how to bring together general relativity, which explains gravity and the large-scale universe, with quantum mechanics, which describes matter at the smallest scales. These two frameworks do not fit together, and every real attempt to unify them demands a clear definition of what space-time actually is.

If the language physicists use to describe space-time has been internally inconsistent for more than a century, Janzen suggests the resulting confusion may extend beyond philosophy. Clarifying what physicists mean when they talk about space-time could influence how researchers think about attempts to reconcile general relativity and quantum mechanics.

Janzen does not propose a replacement for the current framework but instead suggests that the fundamental question is not how space-time curves, but how to properly define them in the first place.

Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds an MBA, a Bachelor of Science in Business Administration, and a data analytics certification. His work focuses on breaking scientific developments, with an emphasis on emerging biology, cognitive neuroscience, and archaeological discoveries.