PARTICLE physics is the study of the structure of matter to the deepest level that we can probe with our instruments.
It responds to our inborn curiosity to know what the world is made of and to understand how it is built.
It also responds to a deep desire for aesthetics, for learning how beautiful, general, and simple are the laws of nature that preside over the construction of the world.
The motivation of physicists is, however, not in direct applications; it can rather be summarized as "the desire to understand."
Particle physics is part of human culture and should mainly be considered as such even if this research brings technological innovations in its wake. When R. R. Wilson, the director of the largest particle-physics laboratory in the US, was once asked during a congressional hearing, "How can your accelerator contribute to the defense of the United States?," he replied, "I do not know, but it makes the United States more worth defending."
At present, the resolution with which we can analyze the structure of matter corresponds to a billionth of a billionth of a meter (10-18 meters). At this level of scrutiny, the basic constituents of stable matter are three: the up quark, the down quark, and the electron. Arranging them in various numbers, according to the laws of physics that we know, we can understand the structure of all known stable objects in their near-infinite variety.
Particle physics, as it developed as a special branch of physics over the past 40 years, corresponds to this descent into the structure of matter, from 10-15 meters, the size of the proton, to a resolution of 10-18 meters, which we have today. With the Large Hadron Collider, which the European Center for Particle Physics (CERN) hopes to build by the end of the decade, we could go down to 10-19 meters.
We live in a universe of matter. This has not, however, always been the case. In the early universe, antimatter was almost as important as matter. The collision energies that we need to probe the structure of matter bring us to the conditions which prevailed in the very early universe.
As a result, particle physics provides key elements to cosmology. Probing the structure of matter to increasing resolution, we have to study collisions among particles of increasing energy and understand the physics that prevails under such hitherto-unexplored conditions.
We live in a dense sea of very low-energy neutrinos, which are part of the cold remnants of the Big Bang. We are parts of an expanding universe, about 15 billion years old, in which the mean density and the temperature constantly decrease. Trying to reconstruct the conditions which prevailed in the distant past, we have to consider increasing densities and increasing temperatures, which are larger the further one goes back into the past.
Our universe originated from a Big Bang at a time when the equations with which we can follow its global history would give an infinite density and an infinite temperature. But in our attempt to reach this point, we find that what happened in the distant past is governed by rules of high-energy physics that we are only gradually exploring and understanding. The higher the temperature, the higher the mean energy of the particle. At the same time, high densities mean that everything happens very quickly.
In the early universe, the temperature, or the mean energy per particle, falls as the inverse square root of time. With CERN's Large Electron-Positron collider, and the 100-giga-electron-volt collisions that it now provides, we can analyze the physics which prevailed when the universe was 10-10 seconds old. This is the furthest we can now reach experimentally.
The universe was then dominated by quarks and antiquarks, almost in equal amounts. Quarks have different "colors," or hypothetical charges that govern how they combine to form particles. When the universe was 10-5 seconds old, it became opaque to color, meaning that colored quarks could no longer freely propagate, and all surviving quarks had to bind into "white" (color-neutral) particles, such as protons and neutrons. There was a violent carnage of quarks and antiquarks; only one one-billionth of the qu arks survived. Formation of helium
For a short while the universe was then dominated by electrons and positrons, but by the time it was one second old, there was a carnage of them too, leaving only one one-billionth of the electrons surviving. Prior to that, electrons and positrons were already constantly annihilating each other, producing photons; but photon collisions were constantly creating as many electron-positron pairs. As the temperature fell, the annihilation reaction continued, while the production reaction came to a halt, as th e photons were too "cold."
After that, the protons and surviving neutrons bound into helium nuclei. The universe for a while was a fusion reactor. It completely ignited when the universe was 100 seconds old, and everything was over after the first three minutes.
These are but a few important episodes among those which we can now reconstruct with confidence. We know enough particle physics to follow what happened when the universe was 10-10 seconds to one second old. Ratios of time intervals are more relevant than time itself.
Indeed, during that interval of about one second, the temperature fell by a factor of 100,000, while the prevailing physics quickly changed according to temperature in a world of very high density and frequent collisions.
This early evolution bears on direct astrophysical questions, such as the genesis of the light elements. Up to lithium they were essentially made during the Big Bang. Up to iron, they were later slowly "cooked" in stars.
Nature required supernova explosions to make the heavier elements. We are made of the dust of former stars that died before our solar system was born 5 billion years ago.
There are fascinating questions about what took place earlier than 10-10 seconds after the Big Bang. Quarks and leptons were probably born when the universe was 10-35 seconds old. Prior to that, there was probably an inflation period during which the universe inflated tremendously.
As we try to reach 10-44 seconds, we need a quantum theory for gravity, which we do yet not have. By then, time and space lose the meaning that we give them, and we can only read theoretical equations, the physical correctness of which is still not certain.
Yet it is in these early instants, or in the understanding of physics at extremely high energies, that the answers to very important questions are still locked: Why was there a slight excess of matter over antimatter? Why are there now as many protons as electrons? Why are there galaxies and clusters of galaxies in a universe where radiation is so uniformly distributed? Is there, as we expect, much more matter than so far seen?
Understanding the cosmos, this infinity around us, relies to a large extent on the deeper understanding of the structure of matter, the infinity right in front of us.
One of the greatest achievements of physics over the past 40 years is to have understood the overwhelming role of symmetries in nature. These symmetries pertain to the invariance of the equations of motion under a host of transformations, such as space and time translations, rotations, and so forth, while this invariance does not usually apply to the initial conditions from which the equations of physics allow us to predict the outcome of events. One has to abstract these invariance properties, or symmet ries, from the observation of a world that does not show them in an obvious way.
But there is much more to it: Symmetries also have much to do with internal properties of particles, corresponding to the labels with which we distinguish them. The basic laws of physics appear to be invariant when we change the specific "color" and "flavor" labels that we agree to attach to each specific particle, as if such labels were mere conventions.
Here a wonderful thing happens. Imposing these symmetries in a world that does not explicitly display them, we have to include in our theories the photon, the gluon, the W and Z particles, and their role between quarks and leptons. Invariable principles
The existence and the properties of the basic forces, as we can now study them at the deepest level of structure, thus result from invariable principles.
This is physics at its best. The weak force and the electromagnetic force further appear to be two facets of a single mode of interaction. We now see ways of bringing this electro-weak interaction and the strong force into a single global scheme.
Having ascertained the key role of invariance properties, or symmetries, one may wonder why they remain so hidden at first sight. Here comes our recent realization that the vacuum is complicated.
In the usual sense, the vacuum is what remains when we have taken away all forms of energy, whether it appears as matter or as radiation. This is what we call "nothingness." In physics, however, we define the vacuum as the lowest possible state of energy. This may not be identical to nothingness, and indeed, we have theories according to which this is not so.
While we can now establish parameters for the peculiar properties of our vacuum and can thus predict those that prevailed when the universe was still very hot, we do not yet understand the dynamics that are at the origin of such properties, in much the same way that we earlier did not understand the dynamics behind ferromagnetism and superconductivity.
At present, one of the greatest challenges in physics is understanding the dynamics that made the vacuum the way it is. Understanding it is our driving force as we chafe under the desire for the means to probe still further the structure of matter and the conditions that prevailed close to the very beginning of our universe.