What is light and what is reality?

Scientific advance is nothing more than the spearhead of the evolution of human thought. Few, perhaps no current scientific concepts, even the most advanced such as gravitational waves or messenger RNA, have emerged spontaneously, if not rather they are the result of a long cognitive and creative process, often unsuccessful and doomed to failure, other times great, imaginative, surprising and revolutionary. Today we are dealing with one of those scientific concepts, essential for astrophysics, which is the nature of light. Understanding what light is involves other concepts, nothing simple despite the fact that we talk about them every day, such as matter, particle or mass, and the notion of reality.

Everything is made up of “Atoms”, things are divisible into parts until we arrive at components “that cannot be cut”, which is what atom etymologically means. This is what Democritus of Abdera taught us 2500 years ago.

Today when we talk about the atom we have something else in mind, and our cultural heritage already contains so much this concept as that of particles that make up everything that surrounds us. Here we have introduced another concept, quite difficult to define by the way, perhaps because it is too basic: particle. In principle, a particle has a complementary meaning to that of an atom: it is a small part of matter, according to the RAE. We can speak of dust particles, and atoms would be particles with that definition. As something that makes up matter, one would think that a particle… has mass. But are there massless particles? In fact, can something exist, beyond ideas, Plato would tell us, if it does not have mass or volume? And if the answer is affirmative, what does it mean that something that exists has no mass?

For centuries, even today we do it, the nature of light, which does not seem to be a thing that has mass, was explained in terms of imaginary rays striking surfaces and can pass through, or be reflected or absorbed. A little more than 300 years ago, Isaac Newton, although he is not as well known for his studies on optics as for the Law of Universal Gravitation, spoke that light was composed of “particles of different colors ”. Around the same time, Christiaan Huygens defended, quite vehemently and against Newton’s claims, that light was a wave, something that served him to explain phenomena such as the reflection of light in a rather precise way. mirror. This new concept triumphed in the following centuries, even more so when James Clerk Maxwell predicted the existence of electromagnetic waves, which were identified with light.

But the development of quantum physics at the beginning of the century XX seemed to bring Newton back to reason, with an interpretation of light more in keeping with Democritus. Albert Einstein defended that light was composed of indivisible particles, without mass or volume, packages or quanta of light as they also began to be called at that time: photons. That description helped him to explain the physics behind the so-called photoelectric effect, which is no more (and no less!) Than the description of the absorption of light by matter and the emission of electrons when radiation with a certain energy is applied. . This study helped Einstein win the Nobel Prize for 1921, although he is not as well known for that as for the theory of Relativity.

Anyway , the history of understanding what light is is like the whiting that bites its tail: we have been circling between particles and waves for millennia. Light is the basis of current astrophysics, not in vain all the names mentioned in the previous paragraph, and others closely related to the same works such as Max Planck or Arthur Compton, have served as inspiration to baptize telescopes and exploration missions of the universe. And, in general, the nature of light is one of the most essential questions that we scientists ask ourselves. In fact, that the wave-particle duality of light was extended to all matter, starting with electrons, by Louis de Broglie, which constitutes one of the bases of quantum physics, it returns us to some of the questions of the principle , and it leaves us more perplexed than before: a particle can have no mass and a wave can have it.

Understanding light as a wave is fundamental to our study of the universe today. In fact, we could consider this change in concept as an essential revolution for the birth and development of astrophysics compared to what was astronomy for centuries. It took us several decades to apply these new concepts in a systematic way to explore the universe, but the qualitative leap in astrophysics was extraordinary. We will talk about interferometry or polarization at another time, but today we want to end this article with another reflection that has been among us for millennia.

The de Broglie wave-particle duality is usually explained by saying that properties and behavior of something like light can be described as if it were a wave or a particle. This leads to think that the wave concept and other physical concepts, and physical science more generally, are a mathematical description of reality. Physics describes reality with its mathematical tools, we could say. A wave would not be something real, just a mathematical device, a mere instrument to know what exists in the universe. The alternative is that the universe is physical-mathematical, something that was already discussed as recently 2500 years ago and in Greece, that we know of. Pythagoras, best known for his theorem on triangles, and Plato later, put mathematics as the basis of reality, giving it a metaphysical or ontological value: there are no mathematical descriptions of reality, but reality is made of mathematics. If so, going back to our topic today, that the waves have mass would not have to be so strange, the strange would come from our vision of the universe and the limitations of language, but we would have to find the mathematical reality that is equivalent to what we call dough. Something that perhaps we have already done.

Pablo G. Pérez González 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 is a section in which our knowledge about the universe in a qualitative and quantitative way. It is intended 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 meter cubic, which invites us to reflect on our existence and the presence of life in the universe. The section is made up of Pablo G. Pérez González , researcher at the Center for Astrobiology; Patricia Sánchez Blázquez , teacher holder at the Complutense University of Madrid (UCM); and Eva Villaver , researcher from the Astrobiology Center

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