Alien bombardment: What comes our way from space

On September 19, 1783, a sheep, a duck, and a rooster traveled for the first time in a hot air balloon called Réveillon, which apparently translates as “dinner party.” They reached an altitude of almost 500 meters and landed safely. I don’t know if things got worse for the passengers that night, given the balloon’s name. For the record, I may be thinking a little incorrectly; the balloon’s name was the same as that of the wallpaper producer who collaborated with Joseph-Michel and Jacques-Étienne Montgolfier, brothers living in Paris, entrepreneurs specializing in wallpaper, and inventors of the hot air balloon, among other things.

That interest in inventing new things, using physical properties of the universe — like the fact that a hot gas expands, decreases its density, and can rise within a mass of colder air — led a few years later to the first military uses of balloons (mainly for reconnaissance of enemy territory), a utility that even lasted into World War II. But I don’t want to focus on those violent uses that we sadly give to many inventions, or that even encourage human invention, but on another application of hot air balloons, which came later and which demonstrates the essence of basic science: research that can lead to unthinkable discoveries, even paradigm shifts, but on quite long timescales, and involving heterogeneous efforts by many people.

Before getting to the history of the balloon, I want to dwell on several other interesting works. In 1895, Wilhelm Conrad Roentgen discovered that a plate made of a material that fluoresces when bathed in sunlight behaved in the same way after being placed in a room containing a tube through which an electric current was passed, even when covered with cardboard. This was the discovery of X-rays, which passed through opaque material, and even through the flesh of Roentgen’s wife’s hand, where he later tested his discovery.

A year later, Antoine-Henri Becquerel discovered a natural material, a uranium salt, that exhibited similar behavior, although its effect disappeared if a magnetic field was created, something that did not occur with Roentgen’s X-rays. In both cases, the power of certain materials to ionize, that is, to remove electrons from atoms through a certain type of radiation, was being examined. Ionizing (that is, being affected by this radiation) is relatively easy for some materials, which then tend to regain electrons (recombine) and in the process emit light (fluorescence).

Theodore Wulf was also interested in ionization and argued that if some materials ionize others, as we move away from the former, we should measure less ionization. Applied to the atmosphere, and knowing that certain salts, as we mentioned earlier, found in some rocks, are ionizing sources, “common sense” might indicate that if we move away from the Earth’s surface where those rocks are, the ionization of the air should decrease. Wulf climbed the Eiffel Tower in 1910 and saw that 300 meters above the city of Paris, ionization was lower than at ground level, but not as much as he had predicted; it should have decreased much more up there. Faced with something like this, a scientist thinks his calculations and predictions are wrong because the theory is completely flawed, or perhaps, more likely, the theory needs to be refined. That’s the thing about data (which describe reality); they should force us to reconsider our prejudices; it’s not just a matter of scientists.

And we come to Victor Franz Hess, who continued researching this matter and found that the atmosphere has a certain degree of ionization that actually decreases, but only up to a height of about one kilometer, where the air shows virtually no ionization. But then it starts to rise! Using a hydrogen balloon (much better than a hot air balloon), equipped with a measuring device, an electroscope, which measures whether something is electrically charged, he ascended to heights of about five kilometers in 1911-1912 and saw that the ionization of the air was twice that measured at sea level. He concluded that the source of ionization came from outer space. He also measured ionization at those heights during a solar eclipse and saw no changes, so it seemed the Sun had nothing to do with the problem.

In 1925, Robert Andrews Millikan confirmed the extraterrestrial origin of radiation and called them “cosmic rays.” I find it curious that although he coined the name, he actually argued that cosmic rays were energetic photons — hence the word rays — which must originate from the continuous creation of atoms in space, necessary to avoid the so-called “heat death of the universe,” which would occur when the temperature of the universe homogenized, increasing entropy until it reached its maximum. I know many readers like this stuff about entropy, the arrow of time, the laws of thermodynamics, etc., but to talk about it, more articles would be needed, which we will do. The relevant thing here is that Millikan’s vision of the nature of cosmic rays was incorrect (it was complicated and imaginative, yes). In reality, it is mainly charged particles, as another great physicist, Arthur Compton, argued just under 100 years ago, that are behind these cosmic rays.

And after all this historical background, we come to astrophysics. Indeed, outer space is full of cosmic rays. They originate in stars like ours, in interstellar space, in small or large black holes in nearby or distant galaxies existing in the far reaches of space-time.

Cosmic rays, now more accurately called astroparticles from a physical perspective, are primarily atomic nuclei, without electrons and therefore electrically charged. Almost everything that reaches us from the outside world is, in fact, hydrogen nuclei, or protons; they account for 90% of astroparticles. Another 9% are helium atoms (two protons and two neutrons, called α (alpha) particles), and the rest are nuclei of heavier elements. A very small portion of the astroparticles that reach us are actually antimatter, primarily positrons (the antimatter of electrons) and antiprotons. We have been searching for anti-alpha particles reaching us from outer space for several years, without success so far.

The speeds of these cosmic rays are impressive, close to the speed of light in many cases, so they carry a large amount of energy, which they release when they collide with atoms in the upper layers of the atmosphere. Measurements indicate that, every second and per square meter, we receive the equivalent of 1,000 protons, or, more specifically, a proton with a kinetic energy 1,000 times greater than the energy equivalent of its mass. The most energetic cosmic ray measured, dubbed the “Oh-My-God particle,” was interpreted as a single proton with the kinetic energy equivalent of a hard-hit golf ball.

At these or lower energies, the typical cosmic ray is billions of times less energetic. In these collisions with the material of our atmosphere, atoms are broken, others are ionized, and new particles are created, such as neutrinos, pions, and muons. And these last, the muons, which are very unsociable and do not interact with matter (almost), reach the Earth's surface very easily. It is estimated that one muon passes through our heads every second. These muons are responsible for much of the ionization of the air at ground level, which is not only due to radioactive material present in Earth's rocks. Curiously, the initial assumption of the theory that explained atmospheric ionization was actually quite incorrect, but it led to the discovery of cosmic rays. The universe always surprises.

Cosmic Void is a section that presents our knowledge about the universe in a qualitative and quantitative way. It aims to explain the importance of understanding the cosmos not only from a scientific point of view, but also from a philosophical, social, and economic point of view. The name “cosmic void” refers to the fact that the universe is, for the most part, empty, with less than one atom per cubic meter, despite the fact that in our environment, paradoxically, there are quintillions of atoms per cubic meter, which invites a reflection on our existence and the presence of life in the universe. The section is composed of Pablo G. Pérez González, researcher at the Center for Astrobiology, and Eva Villaver, deputy director of the Institute of Astrophysics of the Canary Islands.

Sign up for our weekly newsletter to get more English-language news coverage from EL PAÍS USA Edition