Let’s talk about the light particles… Yes, just those that will trigger the production of melanin and will make us brown and attractive this summer. It is inevitable to think that the fundamental particles, among which are the particles of light, are tiny versions of simple and common things like billiard balls. Simple spherical shapes composed of some kind of homogeneous substance with quasi-trivial properties will probably come to our thinking. In this context, it would be enough to say their mass and if anything, if they are rotating or not, so that they are essentially characterized and sharply described. We could also affirm, from our daily experience, that having mass is ‘being’ and not having it is… ‘not being’. But what about the light particles? They are said to have no mass. Therefore, what until now seemed an extremely reasonable construction collapses mercilessly. How is this possible? How can something (light) ‘be’ and not have mass?
Light helps us to see material objects, those that do ‘are’ (or ‘are’) because, as we have stated, they already had mass previously. Light therefore has an instrumental character. Now, when it abounds, we become aware that it also has the power to, for example, warm our face under the sun. Doing that requires, without a doubt, energy (caloric). However, when material objects move, they are said to also have (kinetic) energy. On the other hand, according to our daily experience, light cannot ‘move’ material things (unlike apparently objects do when they interact with each other). As far as it is known, no building has been brought down by a lightning hurricane. In Physics, this “ability to push” is called, as the reader will recall from school, ‘linear momentum’. When one thing pushes another, it is said to ‘transfer momentum’ to it. As we said, in our day to day, we do not observe that light transfers moment to what surrounds us. However, this is not true. We know for a fact (thanks to very meticulous and precise experiments) that light does ‘spur’ matter, although very weakly in most cases. Therefore, if we accept that light is made of particles, these appear to have attributes similar to those of their material cousins, except for the fact that they are massless.
Now, having mass, energy, and momentum is perhaps having too many things. For light, two is enough and what at first seemed like a problem becomes a kind of solution. Light particles are simpler. That is, we need to state less about them so that it is clear what their properties are. The problem is not ‘not having mass’ but ‘having it’ and being forced to explain this ‘exception’. What was that substance from which material particles were made and thanks to which they had mass? Eliminating mysteries is better than creating them! Today we know that particles that have mass have mass thanks to their interaction with the Higgs boson. If this did not exist, they would all be like particles of light: simple, massless, fast.
Furthermore, under this absence of the Higgs, the exception would become the norm and these massless particles could continue to collide with each other, exchanging part of their momentum and energy. All this under a single condition: that the amount of energy and linear momentum of all of them remain unchanged . This fundamental principle is intimately linked with the structure of space and time. Indeed, there are no markers or indicators neither in space nor in time. If we distinguish the different places in space, it is because we use the things in it as references. Similarly, if we can separate several moments in time, it is thanks to using specific clocks as a reference. It is said that space and time are ‘homogeneous’ and that it is the conservation laws, derived from these properties, that allow particles to ‘be’ (and not the fact of ‘having mass’).
It is worth asking, therefore, if in a curved space-time, that is, not homogeneous, the particles would continue to ‘be’ and, if not, what would remain? Modern physics seems to answer this question by proclaiming that the survivors are always the quantum fields and that the particles are only manifestations of them in cases governed by the aforementioned space-time symmetry. Some of these fields are, however, highly exotic and are as abstract in character as assigning a pair of complex numbers to each point in space-time. To the question: what is the world made of? Perhaps we should answer that it is not “made of things” and that this is a category of thought that arises in very concrete (everyday) situations, but that cannot be generalized. As we can see, Physics, far from solving all the mysteries, leads us to places where these are, if possible, even deeper.
Think about all of this when you lie down peacefully in the sun this summer.
Salvador Sánchez is a doctor in Physics and a member of the Faculty of the Degree in Physics at UNIR.
