An imperceptible shower of cosmic rays

Abundant and imperceptible, like a silent storm, millions of elementary particles fall on us at all hours. There are no umbrellas to stem the torrent of solar neutrinos pouring through every square inch of our planet and our bodies, like spectral images of themselves. From above, by day, and from below, by night. The energy that most of them carry barely reaches one thousandth of the mass of a proton.

Other particles, on the other hand, are much more energetic and come from much further away. They come from other galaxies after traveling through the cosmos for millions of years.

Oil painting recreating the moment Victor Hess and two colleagues made a dangerous balloon ascent from Austria in August 1912. They made the remarkable discovery of cosmic radiation.
Wikimedia commos, CC BY

When they began their journey to Earth, there were no human beings here. While they traveled, the various species succeeded each other that ended up procreating the Homo sapiens. But it was not until 1912 when a specimen of this species got into a hot air balloon during a total eclipse and verified that the most energetic particles that were detected came from above, yes, but not from the Sun.

sheltered from the atmosphere

Over time we understood that some of these particles have enormous energies, ten trillion times that of solar neutrinos. A million times more than the protons in the world’s largest particle accelerator, the Large Hadron Collider (LHC). They must have an electrical charge, otherwise it would be inexplicable that there was a mechanism that could give them such an impetus. And very probably it is about those that are stable and can withstand intact such a long trip: protons or iron nuclei. These violent projectiles do not reach us. The atmosphere protects us.

When one of these particles enters the atmosphere, it sweeps away everything in its path. It tears electrons from the atoms that make up the air and creates a domino effect that spreads from the upper atmosphere to the Earth’s surface, widening along the way, like a shower.

The more energetic the particle, the greater the splashed land area. The most energetic and, therefore, enigmatic, can splash surfaces of several square kilometers.

One particle per century

We can see the particles generated in the atmosphere using cloud chambers, a device that can be assembled at home with little more than alcohol and dry ice. But the only way to know that these are coming from a single extremely energetic particle is to deploy detectors on large surfaces.

Of course: in each square kilometer of the earth’s surface one of these particles impacts… per century!

We need to deploy detectors over a hundred square kilometers if we want to observe one a year, and thirty-six times as much area if impatience leads us to want to observe one every ten days.

And that is what James Cronin, Nobel laureate in physics in 1980, proposed: to lead the quixotic enterprise of detecting and characterizing the most energetic particles, called, for historical reasons, cosmic rays.

One thousand six hundred tanks to hunt them down

To detect cosmic rays, more than 1,600 tanks filled with 12 tons of pure water had to be deployed over 2,000 square miles.

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One of the detection tanks of the Pierre Auger observatory.
Pierre Auge Observatory, CC BY

Each detector had to carry sophisticated electronics that would allow it not only to see some particle of the waterfall but also to record the precise moment in which it was observed. In addition, it would have to communicate it to a central computer that can discern how many detectors were splashed by the shower and in what chronological order. All this, of course, without cables: with solar cells and antennas.

The list of technical difficulties that threaten the operation of such a detector network is very long. But with ingenuity and determination, with a lot of work and talent, it was possible to deploy that gigantic laboratory dreamed of by James Cronin in Malargüe (Argentina), an ideal territory for being quite flat, lying under a pristine atmosphere and being sparsely inhabited. The latter was essential to be able to deploy the tanks over such a vast area, forming an orderly network in which each pair is separated by a kilometer and a half of rustic and difficult-to-pass terrain. This is how the Pierre Auger Observatory was born.

The work of wit

The energy of the particle that gives rise to the shower, that pebble hidden in the persistent rain of neutrinos, can be recognized in two very different ways: by reconstructing it, from the one it deposits in each of the sprayed tanks, or through the direct observation of the fluorescence produced in the atmosphere by the passage of particles, when interacting with nitrogen in the air.

To observe them, the Pierre Auger Observatory has four detectors that stand like sentinels from high promontories on the perimeter of the field. A system of mirrors focus and collect all available light. When atmospheric conditions allow it, these lookouts are able to see the air light up like a dim incandescent light bulb tens of kilometers away.

Our planet is a huge magnet, with its poles, and magnetic fields are ubiquitous in the galactic neighborhood. A charged particle deviates its course in the presence of these fields, the more the lower its speed.

When we try to use the sequence in which the different tanks detect particles that plunge into their interior, to determine the direction of origin of the original pebble, we are condemned by the random sinuosity of the trajectory to which the magnetic fields condemn it. Unless the energy of the incident particle is so immense that the effect of these is negligible.

After two decades of scanning the heavens, the Pierre Auger Observatory has been able to categorically determine that these higher-energy cosmic rays come from other galaxies. Messengers of the cosmos, they travel astral distances before encountering the density of our atmosphere and releasing all their energy, spraying the Earth’s surface like a fluorescent sprinkler.

We have received the message

The curiosity of our species is limitless. As soon as we understood that light is much more than what we can see, we embarked on the adventure of making artificial eyes that were sensitive to infrared and ultraviolet, radio waves, microwaves, X-rays, and gamma rays. And even submerged in a vast ocean of electromagnetic waves, we were not oblivious to the orvalle that falls on us without getting wet.

Subatomic particles, messengers of the cosmos to which providence places a rocky planet on the path that abruptly interrupts its journey. They have the consolation of knowing that the journey was not in vain. It is inhabited by indiscreet beings who have received the message, even though it arrives subtly encrypted in an imperceptible rain.

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