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 (in)credulity or terror. If I don't see it with my eyes, if I don't touch it with my fingers like St. Thomas, it can't exist, I don't believe it. Believing in science does not have much place, 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.
That something is invisible means that it cannot be perceived with the eye. But invisible is different from undetectable, also transparent and, of course, it doesn't mean non-existent either. What is perhaps part of its meaning is the strangeness caused by the invisible. We've mentioned a lot of concepts together, and they're very similar and therefore very related, so let's start with one: there's a lot of things that we can't see sometimes, that are transparent, but nevertheless detectable under certain circumstances. This means that their existence can be proven.
Air, for example, is something invisible, simple and everyday. Air is a medium made up of molecules of nitrogen, oxygen, carbon dioxide, water, and more in small amounts. The mixture has a relatively low density. Think that almost all those molecules I have named are heavier than the molecule of water, 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 because of the type of particles that compose it, the air in our atmosphere is invisible. But that's only true in certain circumstances and in a certain sense. Let's take two examples that tell us that air is not as invisible, nor is it transparent as we suppose.
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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 reality the large amount of air that makes up the sky can be "seen" with a bluish color. The air in our atmosphere is, on the other hand, opaque to ultraviolet light and also to certain infrared light, electromagnetic wavelengths (of light, to be clear) that are "foreign" to us humans, since only with technology have we been able to access their detection in the last 200 years. So the concept of air invisibility is not so simple: depending on which "eyes" (if we had eyes sensitive to many types of photons) you look at it is translucent, other times it is opaque.
But that's not the whole story with the air. Under certain circumstances, the 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, 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 the air. When particles from what's called the solar wind interact with Earth's magnetic field, they are directed toward the poles. We're talking about 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 are exactly the same). These solar wind particles collide with air molecules in the highest areas of the atmosphere, which can cause nitrogen atoms to lose an electron, they are said to ionize, until some electron in the oxygen atoms suffers an increase in energy, it is said to be excited. But just as the goat throws the mountain, and I throw the couch, the electron tries to be as rested as possible, at the lowest energy level. That implies that nitrogen atoms tend to pick up the electron that has been stolen from them by a collision with the solar wind, and the electrons in oxygen tend to go back to their initial state, they become deexcited. The result is a loss of energy that can't be lost, it's emitted in the form of light, and that's when we can see the air (that's what we were going for!) directly through the Northern Lights. Green or orange and red auroras mean we're looking at oxygen; Blue auroras, also red, mean we're looking at nitrogen. The air is no longer invisible.
Having accomplished my first objective for this section of Cosmic Void, which is to teach physics, let's move on to the second: to describe how amazing the universe is. We have seen auroras on planets such as 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-thin "air" (not very dense) and with a very different composition from our air (almost everything is carbon dioxide, 95%, compared to at least 0.1% on Earth).
But the invisible shows us the unknown far beyond, and reveals a strange universe. An emission from clouds of gases, which should be invisible to us due to their density, typically a thousand trillion times lower than our Earth's air, allows us to see and confirm the existence of distant galaxies, existing already when the universe was 2% of its current age, a distance record recently broken by the James Webb telescope. These galaxies already have large amounts of excited, even ionized, oxygen that emits light in what we call auroral lines like the ones we described earlier, and forbidden, somewhat stranger, also made of carbon or silicon. If there is oxygen, and considering that 91% of the atoms in the universe are hydrogen (as it weighs little compared to others, hydrogen accounts for 71% of the mass of all the atoms in the universe), there may be water, in fact we have detected it in clouds of gas present in galaxies existing when the universe was 5% of its current age. And carbon composites.
The invisible, which is not so invisible if you know it and can look at it, is extremely curious, they reveal a fantastic universe, incredible from its origins, and I won't say terrifying, but it does make you dizzy to understand how big it is and how limited our vision of its nature is. Disbelief, perhaps out of innocence, we leave for when we talk about things that are not really perceptible with photons.
Pablo G. Pérez González is a researcher at the Center for Astrobiology, under the Spanish National Research Council and the National Institute of Aerospace Technology (CAB/CSIC-INTA)
Cosmic Void is a section that presents our knowledge of 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 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 quintillion atoms per cubic meter, which invites a reflection on our existence and the presence of life in the universe.
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