Publicado en agosto 2012 |30
The humanity of physics
In light of the recent firm claim of the discovery of the Higgs particle, physicist Paul D. Núñez reflects on the “flesh-and-bones” theories that have led us to the understanding of the universe that surrounds us.
by Paul D. Núñez
Many of us try to develop an almost detached description of reality in order to have an objective understanding of nature. However, after stepping back and looking closely at our constructs we can see that they reflect as much nature itself as our own human approach to understanding. This can be particularly true in physics, a beautiful attempt to understand our surroundings. Many descriptions of seemingly unrelated phenomena are made with the same mathematical constructs, with the same language; and language can exist independently from natural phenomena. In fact, many fields of mathematics were developed long before they were used in our attempt to describe nature, and similar descriptions are used for describing different phenomena. For example, the equation that describes the behavior of light waves in a vacuum, shares common features with the (quantum-mechanical) equations that describe the probability of a particle to exist in a certain position in space, or with the equation that describes the flow of material undergoing diffusion (across a membrane, for example). The existence of these similarities can either reflect nature itself or the fact that the concepts were formulated with the same type of creative thought by creatures made of flesh and bone. However, it is difficult to demonstrate one or the other.
These similarities are not always coincidental. They often arise because they are born from a single unifying concept … the beauty of physics lies herein. Finding this unifying idea or starting point is one of the biggest challenges of physics, since none of its predictions may conflict with what we observe in nature. Accordingly, predictions sometimes arise as the need for something to be true so that our axiomatic unifying idea can be used to describe natural phenomena.
This is how some new elementary particles have been predicted and discovered time and time again, or, equivalently, how theories that may be aesthetically pleasing but that do not reproduce reality have been disproved. A concrete example is the recent firm claim of the discovery of the Higgs particle, a crucial, and until recently missing, part of the standard model of elementary particles. To grasp the importance of such an achievement it is useful to go over a bit of history and related ideas.
The theory of electrodynamics, one of the great triumphs of physics, was formulated in the mid-1800s by several renowned scientists, of whom James Clerk Maxwell is undoubtedly the most notorious. Before this time, electricity and magnetism were fairly understood as separate phenomena. The great discovery was not only that the two phenomena are intimately related, but also that the propagation of light is an electromagnetic phenomenon: The motion of electric charges creates disturbances or waves that propagate through space at a speed that can be accurately extracted from Maxwell’s equations. Maxwell’s equations have inspired many physicists for more than a century, and on a more practical point of view, we would be in a very different world had these equations not been formulated: a world with no radio, TV, computers or Internet.
A crucial feature of Maxwell’s equations remained unnoticed for a few decades after their formulation, namely, that the theory remains unchanged after a certain type of transformation, known as a Gauge transformation, is applied. The exact definition of a Gauge transformation is beyond the scope of this article, and an attempt to do so without recourse to other definitions and a bit of mathematics, would most likely mislead the reader. However, a simple geometrical analogy can be made: The equation of a circle remains unchanged if we rotate the coordinate system in which it is described. In the case of the circle, the transformation we considered was a rotation, and a physicist would say that the circle is invariant under rotations. We could equivalently start by saying that there is a geometrical object that is invariant under rotations and derive the equation of a circle. In the same way, we could start by assuming that the laws of electromagnetism are invariant under these more abstract Gauge transformations and derive Maxwell’s equations. Therefore, a general principle of invariance allows us to derive electrodynamics and have an almost geometrical understanding of it.
Physicists went even further to assume that all theories should be Gauge invariant, and this assumption worked extremely well for describing the world at the smallest scales. Elementary particle physics describes the world in terms of its most basic components (particles) and the interactions between them; there are so far four known fundamental interactions (or forces): the electromagnetic force, the weak force, the strong force, and the gravitational force. Each of the fundamental interactions is mediated by “messenger” particles. In the case of the electromagnetic interaction, the mediating particle is called the photon, and Gauge invariance is consistent with the photon being massless, which is true as far as we know. However, experiments show that other interactions have mediating particles that are in fact very massive. This seems to be in contradiction with the assumption of Gauge invariance. During the ’60s, a few physicists, including Peter Higgs, independently thought of a way out of this problem. The result: The simplest possible mechanism to preserve Gauge invariance necessitates a new particle. This solution, although thought to be somewhat ad hoc by some, was accepted by most.
The effect of this predicted extra particle, now known as the Higgs particle, is to give mass to other particles. The Higgs particle can be thought to “hover” around other particles making it harder for them to move, and also harder to stop once they are in motion, so that they have essentially acquired mass. The final verdict on this theory can only be given by an actual detection of such a particle. This has been one of the main goals of particle physics experiments for the past four decades. These experiments have the difficult task of recreating the conditions present during the first few seconds of the Big Bang, the moment when it is thought the Higgs particle was born.
The Higgs may seem like an artificial construct, made up so that our theories are consistent with observations. However, there are also other situations in which particles acquire mass through a mechanism similar to the Higgs mechanism, namely, when charged particles such as an electron travel through some macroscopic material. It is well known by solid state physicists that when electrons move through certain material, they may effectively acquire more mass. As the electron travels through a medium, the positively charged nuclei that are close to the electron tend to become closer to the negatively charged electron, and, as a result, an outside observer would see a more massive particle. In this example, the medium plays a role very similar to the Higgs particle, or more precisely, the Higgs field. Again one wonders whether these similarities are real or just simply have the human imprint. Is the universe really that self-similar? There is at least one human part that is difficult to take away: Our constant need to relate to other more familiar ideas. This is something that constantly drives our pursuit of knowledge. In this sense, science is not all that different from the way we experience our everyday life: We constantly compare the landscape, the people, and the overall experience with more familiar ones. For this reason we have become proficient at detecting self-similarities in nature, and feel we have gained understanding when unifying principles are found.
There will always be some human imprint in our effort to understand nature. However, what brings us closer to understanding reality is not so much our attempt at flawless reasoning, but our attempt to find ways to test our reasoning. Testing our reasoning with observations is in some sense a way to “detach” our theories from humanity and make them more objective. Our reasoning allows us to propose unifying “simple” ideas (such as that of an invariance) from which we can also make predictions. What is remarkable is when theoretical predictions are confirmed; when we succeed at slightly detaching our theories from humanity and are allowed to have a small glimpse of reality. If the recent claim of the discovery of the Higgs particle is confirmed, it would be one of the greatest achievements of mankind. Not only would we better understand why we have mass, but it would help stabilize the ground that supports much of the standard elementary particle physics model. However, not all is glory in physics and the work is far from over; all of what has been described above only accounts for a very small fraction of the universe. That is, ordinary matter made of known elementary particles accounts for less than five percent of the mass of the known universe. There is still plenty of room to use our imagination and test our “flesh-and-bone” theories.