The Particle That Broke a Cosmic Speed Limit | Quanta Magazine

On the night of October 15, 1991, the “Oh-My-God” particle streaked across the Utah sky.

A cosmic ray from space, it possessed 320 exa-electron volts (EeV) of energy, millions of times more than particles attain at the Large Hadron Collider, the most powerful accelerator ever built by humans. The particle was going so fast that in a yearlong race with light, it would have lost by mere thousandths of a hair. Its energy equaled that of a bowling ball dropped on a toe. But bowling balls contain as many atoms as there are stars. “Nobody ever thought you could concentrate so much energy into a single particle before,” said David Kieda, an astrophysicist at the University of Utah.

Five or so miles from where it fell, a researcher worked his shift inside an old, rat-infested trailer parked atop a desert mountain. Earlier, at dusk, Mengzhi “Steven” Luo had switched on the computers for the Fly’s Eye detector, an array of dozens of spherical mirrors that dotted the barren ground outside. Each of the mirrors was bolted inside a rotating “can” fashioned from a section of culvert, which faced downward during the day to keep the sun from blowing out its sensors. As darkness fell on a clear and moonless night, Luo rolled the cans up toward the sky.

“It was a pretty crude experiment,” said Kieda, who operated the Fly’s Eye with Luo and several others. “But it worked — that was the thing.”

University of Utah
The Fly’s Eye array operated out of Dugway Proving Ground, a military base in the desert of western Utah, from 1981 to 1993; it pioneered the “air fluorescence technique” for determining the energies and directions of ultrahigh-energy cosmic rays based on faint light emitted by nitrogen air molecules as the cosmic-ray air shower traverses the atmosphere. In 1991, the Fly’s Eye detected a cosmic ray that still holds the world record for highest-energy particle.
The faintly glowing contrail of the Oh-My-God particle (as the computer programmer and Autodesk founder John Walker dubbed it in an early Web article) was spotted in the Fly’s Eye data the following summer and reported after the group spent an extra year convincing themselves the signal was real. The particle had broken a cosmic speed limit worked out decades earlier by Kenneth Greisen, Georgiy Zatsepin and Vadim Kuzmin, who argued that any particle energized beyond approximately 60 EeV will interact with background radiation that pervades space, thereby quickly shedding energy and slowing down. This “GZK cutoff” suggested that the Oh-My-God particle must have originated recently and nearby — probably within the local supercluster of galaxies. But an astrophysical accelerator of unimagined size and power would be required to produce such a particle. When scientists looked in the direction from which the particle had come, they could see nothing of the kind.

“It’s like you’ve got a gorilla in your backyard throwing bowling balls at you, but he’s invisible,” Kieda said.

Where had the Oh-My-God particle come from? How could it possibly exist? Did it really? The questions motivated astrophysicists to build bigger, more sophisticated detectors that have since recorded hundreds of thousands more “ultrahigh-energy cosmic rays” with energies above 1 EeV, including a few hundred “trans-GZK” events above the 60 EeV cutoff (though none reaching 320 EeV). In breaking the GZK speed limit, these particles challenged one of the farthest-reaching predictions ever made. It seemed possible that they could offer a window into the laws of physics at otherwise unreachable scales — maybe even connecting particle physics with the evolution of the cosmos as a whole. At the very least, they promised to reveal the workings of extraordinary astrophysical objects that had only ever been twinkles in telescope lenses. But over the years, as the particles swept brushstrokes of light across sensors in every direction, instead of painting a telltale pattern that could be matched to, say, the locations of supermassive black holes or colliding galaxies, they created confusion. “It’s hard to explain the cosmic-ray data with any particular theory,” said Paul Sommers, a semiretired astrophysicist at Pennsylvania State University who specializes in ultrahigh-energy cosmic rays. “There are problems with anything you propose.”

Olena Shmahalo/Quanta Magazine.
Original data via S. Swordy, U. Chicago.
A logarithmic plot showing the flux of cosmic rays as a function of energy. The line has two bends (where its slope changes), known as the cosmic-ray energy spectrum’s “knee” and “ankle.”
Only recently, with the discovery of a cosmic ray “hotspot” in the sky, the detection of related high-energy cosmic particles, and a better understanding of physics at more familiar energies, have researchers secured the first footholds in the quest to understand ultrahigh-energy cosmic rays. “We’re learning things very rapidly,” said Tim Linden, a theoretical astrophysicist at the University of Chicago.

Ankle Problems

Thousands of cosmic rays bombard each square foot of Earth’s atmosphere every second, and yet they managed to elude discovery until a series of daring hot-air-balloon rides in the early 1910s. As the Austrian physicist Victor Hess ascended miles into the atmosphere, he observed that the amount of ionizing radiation increased with altitude. Hess measured this buzz of electrically charged particles even during a solar eclipse, establishing that much of it came from beyond the sun. He received a Nobel Prize in physics for his efforts in 1936.

Cosmic rays, as they became known, arc through Earth’s magnetic field from every direction, and with a smooth spread of energies. (At sea level, we experience the low-energy, secondary radiation produced as the cosmic rays crash through the atmosphere.) Most cosmic rays are single protons, the positively charged building blocks of atomic nuclei; most of the rest are heavier nuclei, and a few are electrons. The more energetic a cosmic ray is, the rarer it is. The rarest of all, those that are labeled “ultrahigh-energy” and exceed 1 EeV, strike each square kilometer of the planet only once per century.

Plotting the number of cosmic rays that sprinkle detectors according to their energies produces a downward-sloping line with two bends — the energy spectrum’s “knee” and “ankle.” These seem to mark transitions to different types of cosmic rays or progressively larger and more powerful sources. The question is, which types, and which sources?

Like many experts, Karl-Heinz Kampert, a professor of astrophysics at the University of Wuppertal in Germany and spokesperson for the Pierre Auger Observatory, the world’s largest ultrahigh-energy cosmic ray detector, believes cosmic rays are accelerated by something like the sonic booms from supersonic jets, but on grander scales. Shock acceleration, as it’s called, “is a fundamental process which you find on any scale in the universe,” Kampert said, from solar flares to star explosions (supernovas) to rapidly spinning stars called pulsars to the enormous lobes emanating from mysterious, super-bright galaxies known as active galactic nuclei. All are cases of heated matter (or “plasma”) flowing faster than the speed of sound, producing an expanding shock wave that accumulates a crust of protons and other particles. The particles reflect back and forth across the shock wave, trapped between the magnetic field of the plasma and the vacuum of empty space like little balls ping-ponging between table and paddle. A particle gains energy with every bounce. “Then it will escape,” Kampert said, “and move through the universe and be detected by an experiment.”

Emily Fuhrman for Quanta Magazine
Cosmic rays are most likely energized through “shock acceleration,” reflecting back and forth across a shock wave that is produced when plasma flows faster than the speed of sound. The stronger and larger the magnetic field of the plasma, the more energy it can impart to a particle. Ultrahigh-energy cosmic rays surpass 1 exa-electron volt (EeV).
Trying to match different shock waves to parts of the cosmic-ray energy spectrum puts astrophysicists on shaky ground, however. They would expect the knee and ankle to mark the highest points to which protons and heavier nuclei (respectively) can be energized in the shock waves of supernovas — the most powerful accelerators in our galaxy. Calculations suggest the protons should max out around 0.001 EeV, and indeed, this aligns with the knee. Heavier nuclei from supernova shock waves are thought to be capable of reaching 0.1 EeV, making this number the expected transition point to more powerful sources of “extragalactic” cosmic rays. These would be shock waves from singular objects that aren’t found in the Milky Way or in most other galaxies, and which could well be galaxy-size themselves. However, the measured ankle of the spectrum — “the only place where it looks like there’s a clear transition,” Sommers said — lies around 5 EeV, an order of magnitude past the theoretical maximum for galactic cosmic rays. No one is sure what to make of the discrepancy.

Past the ankle, at around 60 EeV, the line dips toward zero, forming a sort of toe. This is probably the GZK cutoff, the point beyond which cosmic rays can only tarry for so long before losing energy to ambient cosmic microwaves generated by a phase transition in the early universe. The existence of the cutoff, which Kampert calls “the only firm prediction ever made” about cosmic rays, was established in 2007 by the Fly’s Eye’s successor — the High Resolution


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