Our mind has a special admiration for the invisible. We know that there are things that escape our eyes and remain hidden, which also leads us to other feelings, such as curiosity, fantasy, and even (dis)belief or terror. If I don’t see it with my eyes, if I don’t touch it with my fingers like Saint Thomas, it can’t exist, I don’t believe it. Believing in science does not have much room, doubt about the certainty of something also has to be supported by evidence. But, in any case, there are many things around us that are invisible.
For something to be invisible means that it cannot be perceived with sight. But invisible is different from undetectable, also from transparent and, of course, it does not mean non-existent either. What perhaps is part of its meaning is the strangeness caused by the invisible. We have mentioned many concepts together, and they are very similar and therefore very related, so let’s start with one: there are many things that we cannot see at times, that are transparent, but, nevertheless, are detectable under certain circumstances. This means that its existence can be proven.
Air, for example, is something invisible, simple and everyday. Air is a medium composed of molecules of nitrogen, oxygen, carbon dioxide, water, and more in small quantities. The mixture has a relatively low density. Consider that almost all of those molecules that I have named are heavier than the water molecule, and while a liter of water has a mass of one kilogram, a liter of air is only one gram. If we look at the number of particles in a given volume, the comparison is similar: water has about a thousand times more particles per unit volume than air. With such a low density and due to the type of particles that make it up, we would say that the air in our atmosphere is invisible. But that is only true in certain circumstances and in a certain sense. Let’s give two examples that tell us that air is not as invisible, nor transparent as we suppose.
The air in our atmosphere is quite transparent to red light, but not so much to blue light, for which it is like a conglomerate of small mirrors that reflect it (it is said, rather, that it diffuses it), which explains that in In reality, we can “see” the large amount of air that makes up the sky with a bluish color. The air in our atmosphere is, on the other hand, opaque for ultraviolet light and also for certain infrared light, electromagnetic wavelengths (of light, to understand ourselves) that are “strange” to humans, since only with technology We have been able to access its detection in the last 200 years. So the concept of invisibility of air is not so simple: depending on which “eyes” (if we had eyes sensitive to many types of photons) you look at it, it is translucent, other times it is opaque.
But that’s not the whole story with air. Under certain circumstances air is visible even to the human eye. I could talk about indirect ways of “seeing” the air, such as the effect of turbulence near hot surfaces, for example asphalt in summer, a circumstance in which distorted images are seen, making the presence of air noticeable. But I want to focus on a more direct way of looking at air. When particles of what is called the solar wind interact with the Earth’s magnetic field, they are directed towards the poles. We are referring to the magnetic poles, which are close to the poles defined by the Earth’s axis of rotation, which in turn are identified as geographic poles (although none of the three things are exactly the same). These solar wind particles collide with air molecules in the highest areas of the atmosphere, which can cause the nitrogen atoms to lose an electron, they are said to be ionized, until some electron of the oxygen atoms suffers an electron. increase in energy, it is said to be excited. But just as the goat goes to the mountain, and I go to the couch, the electron tries to be as rested as possible, at the lowest energy level. This implies that the nitrogen atoms tend to capture the electron that has been stolen from them by a collision with the solar wind, and the oxygen electrons tend to go to their initial state, they become de-excited. The result is a loss of energy that cannot be lost, it is emitted in the form of light, and that is when we can see the air (that’s what we were going for!) directly through the northern lights. Green or orange and red aurora means we are seeing oxygen; Blue auroras, also red, mean that we are seeing nitrogen. The air is no longer invisible.
I accomplished my first objective for this section of cosmic void, which is teaching physics, we move on to the second: describing how amazing the universe is. We have seen auroras on planets like Saturn or Jupiter, which are themselves large gaseous planets where we see their “air” in different ways. Auroras are also seen on Venus or Mars itself, the latter with a super tenuous “air” (not very dense) and with a very different composition from our air (almost all of it is carbon dioxide, 95%, compared to at least 0.1 % terrestrial).
But the invisible shows us the unknown far beyond, and reveals a strange universe. An emission by gas clouds, which should be invisible to us due to their density, typically a billion times lower than our terrestrial air, allows us to see and confirm the existence of distant galaxies, already existing when the universe was 2% of its current age, a distance record recently broken by the James Webb telescope. These galaxies already have large quantities of excited, even ionized, oxygen, which emits light in what we call auroral lines like those we described before, and forbidden, somewhat stranger ones, also made of carbon or silicon. If there is oxygen, and assuming that in the universe 91% of the atoms are hydrogen (as it weighs little compared to others, hydrogen accounts for 71% of the mass of all the atoms in the universe), there can be water , in fact we have detected it in gas clouds present in galaxies that existed when the universe was 5% of its current age. And carbon compounds.
The invisible, which is not so invisible if you know it and can look at it, is extremely curious, they reveal to us a fantastic universe, incredible from its origins, and I will not say terrifying, but it is dizzying when you understand how big it is and how limited. which is our vision of its nature. Disbelief, perhaps out of innocence, we leave for when we talk about things that are not really perceptible with photons.
Pablo G. Pérez González He is a researcher at the Astrobiology Center, dependent on the Higher Council for Scientific Research and the National Institute of Aerospace Technology (CAB/CSIC-INTA).
Cosmic Void It is a section in which our knowledge about the universe is presented 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 vacuum” refers to the fact that the universe is and is, for the most part, empty, with less than 1 atom per cubic meter, despite the fact that in our environment, paradoxically, there are quintillions of atoms per meter cubic, which invites us to reflect on our existence and the presence of life in the universe.
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