Nor will we be able to know what the ultimate destiny of the Universe is. Along with dark matter, another similar question came to the fore in the last decade of the twentieth century: dark energy. It had been assumed that the Big Bang must have been driven by a repulsive energy during the creation of the universe, but no one had imagined that such energy could continue to exist in the now-mature Universe.
And since energy is equivalent to mass, that dark energy signified a new contribution to the total mass of the Universe, thought not the same as dark matter. In other words: we thought we knew what the Universe is, and it turns out to be practically unknown to us, because we know the nature and make up of neither dark matter nor dark energy.
One possible explanation of the latter could be found in the term introduced by Einstein in in his field equations for general relativity. As we saw, when applying his theory of gravitational interaction to the entire Universe, Einstein sought a model that would represent a static Universe. That obliged him to introduce a new term into his equations, the previously mentioned cosmological constant, which actually represents a field of repulsive forces that compensate for the attractive effects of gravitation. When relativistic cosmology found solutions that represent an expanding Universe, and that expansion was demonstrated by observation Hubble , Einstein thought that it was no longer necessary to maintain that constant, although it could be included without any difficulty in theoretical expansive models.
Now, it seems necessary to resurrect this term, but it will not be enough to include it in relativist cosmology again; it has to find its place and meaning in quantum theories that attempt to make gravity a part of quantum system. After all, dark energy is the energy of the void, and from a quantum viewpoint, vacuum has a structure. And given that quantum physics has again entered the picture here, let us discuss how the quantum revolution developed and solidified during the second half of the twentieth century.
We also saw that it was Ernest Lawrence who found a new way forward, developing instruments called particle accelerators in his case, cyclotrons , which functioned by accelerating particles to high energy levels and then making them collide with each other or with some predetermined target.
The idea was to examine what was produced by such collisions, that is, what new and smaller components make up such particles if, in fact, there are any The physics of elemental particles, also called high-energy physics, as I indicated above, became one of the main protagonists of the second half of the twentieth century. This is very expensive science it is the epitome of Big Science, which requires large teams of scientists and technicians and large investments , and is becoming ever more expensive, as the size of accelerators grows, making it possible to reach higher energy levels.
After World War II, especially in the United States, high-energy physics drew on the prestige of nuclear physics, which had supplied the powerful atomic bombs. Here, I will mention only the most important accelerators. In , the Cosmotron entered service in Brookhaven, New York. It was for protons and reached 2.
CERN now includes more countries including Spain and with its accelerators it has played an outstanding role in the development of high-energy physics. In fact, it was recently dealt a serious blow by what had been, until then, its strongest supporter: the United States.
This gigantic accelerator, which U. Inside that tunnel, thousands of magnetic superconductor spools would guide two proton beams. After millions of laps, they would reach levels twenty times higher than could be attained with existing accelerators. At various points along the ring, protons from the two beams would collide and enormous detectors would track the results of those collisions. The project would take ten years, and its cost was initially estimated at 6, million dollars. Things got off to a rocky start, but the tunnel excavation was completed.
However, on 19 October , following prolonged, difficult and changing discussions in both houses of Congress, the House of Representatives finally cancelled the project.
Other scientific programs—especially in the field of biomedicine—were more attractive to American congressmen, senators and—why deny it? One of its inhabitants was particularly striking: quarks. Their existence had been theorized in by U. Until quarks appeared in the complex and varied world of elemental particles, it was thought that protons and neutrons were indivisible atomic structures, truly basic, and that their electrical charge was an indivisible unit.
But quarks did not obey this rule, and they were assigned fractional charges. Thus, a proton is made up of two u quarks and one d, while a neutron consists of two d quarks and one u. Therefore, they are composite structures. Later, other physicists proposed the existence of three other quarks: charm c; , bottom b; and top t; To characterize these quarks, scientists say they have six flavors.
Moreover, each of the six types comes in three varieties, or colors: red, yellow or green and blue. And for each quark there is, of course, an antiquark. Needless to say, terms like these—color, flavor, up, down, and so on—do not represent the reality we normally associate with such concepts, although in some cases there can be a certain logic to them, as happens with color.
This is what Gell-Mann , had to say about that term:. There are three colors, called red, green and blue, like the three basic colors in a simple theory of human color vision in the case of painting, the three primary colors are usually red, yellow and blue, but when mixing light instead of pigment, yellow is replaced by green.
The recipe for a neutron or a proton calls for a quark of each color, that is, one red, one green and one blue, so that the sum of the colors cancels out. As in vision, where white can be considered a mixture of red, green and blue, we can metaphorically state that neutrons and protons are white. In short, quarks have color but hadrons do not: they are white. We will never be able to observe a free quark. Now in order for quarks to remain confined, there have to be forces among them that are very different than electromagnetic or other kinds of forces.
About ten years after quarks appeared, a theory, quantum chromodynamics, was formulated to explain why quarks are so strongly confined that they can never escape from the hadron structures they form. With quantum electrodynamics —which, as I already stated, emerged in the first half of the twentieth century—and quantum chromodynamics, we have quantum theories for both electromagnetic and strong interactions.
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But what about the weak interaction, responsible for radioactive phenomena? George Sudarshan b. They independently proposed a theory that unified electromagnetic and weak interactions. Their model included ideas proposed by Sheldon Glashow b. The electroweak theory unified the description of electromagnetic and weak interactions. But could it be possible to take a farther step on the path to unification, formulating a theory that would also include the strong interaction described by quantum chromodynamics? The affirmative answer to this question was provided by Howard Georgi b. From the perspective of GUTs, in the beginning there was only one force, which contained electromagnetic, weak and strong forces.
However, as the Universe cooled, they began to separate. Such theoretical tools make it possible to explain questions such as the existence at least in appearance, and fortunately for us of more matter than antimatter in the Universe. The Japanese physicist, Motohiko Yoshimura used this property to demonstrate that an initial state in which there was an equal amount of matter and antimatter could evolve into one with more protons or neutrons than their respective antiparticles, thus producing a Universe like ours, in which there is more matter than antimatter.
Thanks to the group of theories mentioned above, we have an extraordinary theoretical framework in which to understand what nature is made of. Its predictive capacity is incredible. These theories accept that all matter in the universe is made up of aggregates of three types of elemental particles: electrons and their relatives those called muon and tau , neutrinos electronic, muonic and tauonic neutrinos and quarks, as well as the quanta associated with the fields of the four forces we recognize in nature photons, for electromagnetic interaction, Z and W particles gauge bosons for weak interaction, gluons for strong interaction; and even though gravitation has yet to be included in this framework, the as-yet-unobserved gravitons, for gravitational interaction.
The subset formed by quantum chromodynamics and electroweak theory that is, the theoretical system that includes relativistic and quantum theories of strong, electromagnetic and weak interactions proves especially powerful in its balance of predictions and experimental confirmation.
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It will be remembered—together with general relativity, quantum mechanics, and the unravelling of the genetic code—as one of the most outstanding intellectual advances of the twentieth century. But much more so than general relativity and quantum mechanics, it is the product of a communal effort. That is inevitable: the history of high-energy physics calls not for an entire book, but for several.
Why do those particles have the masses they have? Why, for example, does the tau weigh around 3, times as much as an electron? Why are there four fundamental interactions, instead of three, five, or just one? And why do those interactions have the properties they do such as intensity or range of action? Let us now consider gravitation, the other basic interaction. Can it be unified with the other three? A central problem is the lack of a quantum theory of gravitation that has been subjected to experimental testing.
There are, however, candidates for this splendid unifying dream: complex mathematical structures called string theories. According to string theory, basic particles existing in nature are actually one-dimensional filaments extremely thin strings in spaces with many more dimensions than the three spatial and single temporal one we are aware of. So what kind of materiality do these one-dimensional theoretical constructs have? I said before that string theories are complex mathematical structures, and that is certainly true. And even those approximate equations are so complicated that, to date, they have only partially been solved.
So it is no surprise that one of the great leaders in this field was a physicist with a special gift for mathematics. I am referring to the American, Edward Witten b.
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The reader will get an idea of his stature as a mathematician when I mention that, in , he received one of the four Fields medals alongside Pierre-Louis Lions, Jean-Christophe Yoccoz and Shigefumi Mori that are awarded every four years and are the mathematical equivalent of the Nobel Prize. This eleven-dimensional theory, which Witten called M Theory, has yet to be completely developed But what is that other thing? After all, a vibration is the oscillation of some sort of matter, but as a permanent structure, it is probably more of a mathematical than a material entity.
Physicists would have been working very hard for centuries, or even millennia, only to discover that matter has finally slipped between their fingers, like a net, turning into mathematics, that is, mathematical structures.
In sum, string theory unearths age-old problems, and maybe even ghosts: problems such as the relation between physics and the world and mathematics. Independently of those essentially philosophical aspects of nature, there are others that must be mentioned here. Up to now, string theory has demonstrated very little, especially in light of the fact that science is not only theoretical explanation, but also experiments in which theory is subjected to the ultimate arbiter: experimental testing.
String theories are admired by some, discussed by many, and criticized by quite a few, who insist that its nature is excessively speculative. Thus, the distinguished theoretical physician, Lee Smolin , , pointed out in a book about these theories:. In the last twenty years, a great deal of effort has gone into string theory, but we still do not know if it is certain or not. Even after all the work that has been done, the theory offers no prediction that can be tested through current experiments, or at least, experiments conceivable at the present time.
The few clean predictions they propose have already been formulated by other accepted theories.
Part of the reason why string theory makes no new predictions is that there seem to be an infinite number of versions.