knee

The night sky looks serene, but it is actually pierced by an invisible rain of charged particles: cosmic rays. Scientists have long tried to figure out where the most energetic ones are born. Until recently, it was thought that our Galaxy supplies particles up to about 4 EeV, the so-called "ankle" in the spectrum.

Cosmic rays are particles that race across our Galaxy at enormous speeds. Their energies can be millions of times higher than those of particles in the most powerful accelerator on Earth. But the higher the energy, the fewer such particles there are: the plot of their numbers shows a noticeable kink, which is called the knee.

Cosmic rays do not travel in straight lines. Charged particles accelerated to enormous speeds loop through the Galaxy's magnetic field like dust motes in a gust of wind. It's usually assumed that they simply scatter along and across field lines.

The mystery of the main break in the spectrum of galactic cosmic rays, the knee at 3 PeV, has persisted for nearly a century.

In an orchestra, each musician plays their own part, but at a certain moment all instruments pause on a single note. Cosmic rays behave somewhat like this — particles flying toward us from distant stars.

An X-ray of a knee with osteoarthritis looks like a road map where the roads have narrowed and bumps have grown along the shoulders. Doctors look at this map and assign a grade from zero to four — how worn out the joint is. The problem is that two experienced specialists often see the map differently: one gives a three, the other a four.

When a heavy nucleus slams into interstellar gas at enormous speed, it shatters into pieces. Those pieces (lithium, beryllium, boron) then fly onward, and their abundance in the cosmic-ray flux depends on how the breakup unfolded. Conventionally, one expected the proportions to shift smoothly with energy, like the taste of soup when you keep adding water.

In a dark chamber where the temperature sits just four degrees above absolute zero, a sapphire crystal the size of a grain of rice lies waiting. When a neutrino, a ghostly particle that barely notices matter, flies through it, the crystal shudders and warms by a tiny fraction of a degree.

Cosmic rays are charged particles that constantly bombard Earth from deep space. When one of these particles enters the atmosphere, it collides with air nuclei and spawns a whole cascade of secondary particles: electrons, muons, photons.

A thin plate stands across a flow of water or air. Behind it, a wake forms — a region where the flow becomes tangled and mixed. Usually, such a wake becomes turbulent when the flow speed is high enough, quantified by the Reynolds number.

Cosmic rays are charged particles that fly to us from the depths of the Galaxy. If you plot how many particles arrive at each energy, the line runs smooth, then suddenly drops sharply.

High above Antarctica, a balloon carried an instrument that caught electrons from space. These particles come from distant stellar explosions and other powerful events. The instrument measured their energy, like the force of an impact. And in one energy range, there were more impacts than calculations predicted.