It seems inherent in the nature of human beings to question everything around them. History suggests that, since the beginning of time, we have searched our environment for patterns and models that explain the phenomena that happen around us in order to somehow find a foothold to cling to our own existence.
We begin by looking at the heavens, measuring the positions of the stars in order to seek answers, and yet it is this, astronomy —or rather, cosmology—, perhaps the oldest of the sciences, the one that today today raises more questions.
It was the laws that govern the firmament itself that consecrated Sir Isaac Newton (1643-1727) as one of the greatest minds of all time and that, at the same time, pointed the way to an even deeper understanding of reality. In the preface to his Principia , the possibility already appears that, just as there were laws and forces that governed the celestial movements, there must be other similar ones that explain everything else: « I wish that it were possible to deduce the phenomena of nature from of mechanical principles with the same kind of argument, since many things lead me to suspect that they may all depend on certain forces with which the particles of bodies, for reasons still unknown, either attract one another, uniting according to regular figures, well they flee and separate from each other».
He was not misguided, and today those forces have names: gravitational, electromagnetic, strong nuclear and weak nuclear . There are four types of interactions that cannot be broken down or explained in terms of other simpler ones and that, to date, have been able to describe most of the phenomena that we observe, which is why we call them “fundamental”. We have also shown that these four fundamental forces are not disconnected and independent entities, but that there are interrelationships between —almost— all of them. This leads us to think that, surely, the last word has not been said regarding our conception and description of the universe.
De Faraday a Maxwell
Until Michael Faraday (1791-1867) discovered the phenomenon of electromagnetic induction at the beginning of the 19th century, electricity and magnetism were understood as two manifestations of a different nature. James Clerk Maxwell (1831-1879) went a step further and shortly afterwards demonstrated, with his equations, that they were not only two sides of the same coin, but also closely related to another discipline: optics. Electricity, magnetism and light could be explained through the same basic principles: those of electromagnetism.
At this time, when the description of the world was limited to the theories of Newton and Maxwell , there was already a first attempt to encompass both under the same set of laws. In 1849, Faraday himself wrote in his diary: « Gravity: Surely this force must maintain an experimental relationship with electricity, magnetism and the other forces, in such a way that it can be related to them in a reciprocal action » ,and, with the experimental spirit that always characterized him, he tried to demonstrate this relationship by dropping coils from high places in such a way as to vary the flow of gravitational field lines through them in the hope of observing an induced current. He watched it, but not for the reason he expected, but because of the Earth’s magnetic field. He abandoned the idea in 1859 and it would take a few more decades for someone to consider addressing the issue again.
By the end of the 19th century, the truth is that a kind of generalized optimism had settled in physics that led many scientists to think that this was a practically finished discipline. In 1875, Phillip von Jolly (1809-1884), a professor at the University of Munich, tried to dissuade one of his students, Max Planck (1858-1947), from studying science on the grounds that there was hardly anything left to discover. . And, although there were inconsistencies between the proposals of Newton and Maxwell, especially in relation to the limit of the speed of light, it seems that the one who lost most of his sleep was a young and unknown physicist named Albert Einstein , who he worked on clock patents for railway stations in Bern (1879-1955). The rest is history: quantum mechanics and the theory of relativity brought about an unprecedented revolution in physics, destroying the existing paradigm. Far from coming to an end, it seemed that physics had just begun.
The general theory of relativity ended the Newtonian supremacy in the heavens. It extended and generalized the classical theory and reconciled it, in some respects, with certain premises of Maxwell’s theories. The forces gave way to the fields and the speed of light ended the possibility of instant action at a distance. It seemed that now the pieces of the puzzle could fit together better, however, when Einstein tried to emulate Faraday’s steps to unify the two theories, he made the mistake of doing so with a certain disdain for new and counterintuitive approaches to quantum mechanics. Curiously, the latter has been the one that has given us the most joy in terms of the construction of a theoretical framework that encompasses the four fundamental forces.
The revelation of the secrets that governed the atoms began with the discovery of radioactivity in 1896 by Henri Becquerel (1852-1908). The explanation of this phenomenon culminated in 1933 with the investigations of Enrico Fermi on beta decay, the emission of an electron or a positron by an unstable atomic nucleus, and the description of a third type of force that would materialize, in the 1950s, in the so-called weak nuclear interaction.
Quarks, the fundamental particles
By then, quantum mechanics had already taken off and Paul Dirac (1902-1984) had taken the first steps in the attempt to quantize the electromagnetic field with the aim of describing the interactions between light and matter. In the 1940s, the contributions of Shin’ichirō Tomonaga, Julian Schwinger, Freeman Dyson, and Richard Feynman (1918-1988) materialized in quantum electrodynamics , whose formalism was instrumental in developing subsequent discoveries.
In another order of things, the reason why the neutrons and protons that constituted the atomic nuclei remained cohesive and did not yield to the electromagnetic repulsion forces was, for a long time, a mystery to scientists. In 1964, Murray Gell-Mann (1929-2019) and George Zweig (1937-), and Yuval Ne’eman (1925-2006) independently proposed that there were particles even more fundamental than neutrons and protons to which was called quarks . The further development of quantum chromodynamics explained the interactions between quarks and thereby revealed the workings of the fourth fundamental force: the strong nuclear force.
In the midst of this salad of particles, energy, radiation, fields and interactions, however, everything began to make sense and it seemed that these different theories that unified some aspects or others of reality irremediably led to the same point. In 1967, Sheldon Lee Glashow, Abdus Salam and Steven Weinberg showed that the weak interaction and electromagnetism could be explained in terms of a single model: that of the electroweak force, which, in turn, could be unified with quantum chromodynamics that explained strong interaction. The current standard model of particles was born.
Developed primarily from empirical data, the Standard Model is, to date, the best theory we have for describing and explaining the world of subatomic particles. He has successfully predicted the existence of the W and Z bosons , carriers of the weak interaction; the gluons , carriers of the strong interaction; the top and charm quarks , as well as some of their properties, and the celebrated Higgs boson, essential for explaining how some particles acquire their mass. Even so, it is not exempt from certain drawbacks, among them, the fact that it has almost twenty parameters that must be adjusted based on the experimental data and, the greatest of all, that, despite not showing any problem in what concerns special relativity, so far it has been unable to incorporate general relativity, or, what amounts to the same thing, the fourth fundamental force. And not because I don’t want to, but because of the difficulty of expressing it in the same language as the other three: quantum field theory .
a theory of everything
When we talk about quantum field theories we no longer talk about particles and interactions in the Newtonian sense, but about force fields in which the particles appear as a manifestation of these. The field corresponding to each force also has its own messenger particle associated with it: photons (electromagnetism), Z and W bosons (weak interaction) and gluons (strong interaction). If gravity had a quantum field associated with its carrier particle, it would be the graviton.
The principle that underlies quantum field theories, and therefore the standard model, is that of symmetry: mathematical transformations that leave certain quantities invariant—we say they are “conserved.” Chen Ning Yang (1922-) and Robert Mills (1928-1999), in 1954, generalized the symmetry rules of quantum electrodynamics, electroweak theory and quantum chromodynamics to a higher order that encompassed them all: the model standard. The challenge in this approach lies in finding a theory, or group of symmetries that,in addition to including everything included in the standard model, include gravity. A theory of everything. The key to why the reconciliation between quantum and relativity has not yet been achieved is mainly related to how space and time are defined in each of them. On the one hand, the equations of the first are determined for a flat and static space that is not affected by the particles that are in it, while the second tells us about a curved and dynamic space modeled by the bodies that inhabit it. . In other words, when we have quantized the fields associated with the electromagnetic, weak and strong interactions, we have done it over an absolute, Newtonian space, and it has worked very well. But to do the same with gravity we would have to quantize space-time itself, since the gravitational field, as Einstein conceived it, is not something alien to it. On the other hand, quantum mechanics treats time as a variable separate from the rest and absolute, as well as symmetric —the equations work towards the past and towards the future—. However, in general relativity, time is one of the main dimensions and, furthermore, it is a malleable and sometimes non-reversible quantity, as happens in the presence of a black hole —matter can enter it, but not we can rewind and make it come out. Given this panorama, where could we start looking for meeting points between both visions?
string theory
If we assume that the quantum mechanics we know is not a definitive theory and that gravity could just be a consequence of its effects, we arrive at string theory. One of the best-known contenders for a theory of everything. Its main idea appeared for the first time in the work that Gabriele Veneziano (1942) was developing on the strong interaction at CERN in 1968. Although initially promising, it is currently suffering from a stagnation that seems to have led it to a dead end. In it, the particles take the form of one-dimensional vibrating strings instead of being punctual elements and it is in their different modes of vibration where the characteristics of each one would reside. Gravity, in this context, would be nothing more than an emergent property.
The success of quantum gravity
For everything to make sense, however, it is necessary to suppose up to eleven dimensions of which we would only be able to perceive four —the three spatial and the temporal—, while the rest would be found squashed to scales of the order of the Planck length ( 1.616199 × 10-35 m) and would be inaccessible to us. String theory eliminates many of the annoying infinities that appear in mathematics when we try to unite quantum and relativity, it also includes the force of gravity and even appears what would be its carrier particle, the graviton. However, it is extremely difficult to limit the wide range of possible solutions to those that may be useful to us to describe reality and, to date, it has not been possible to use it to make any predictions or calculate any fundamental physical property, such as, for example, the mass of the electron.
The other option would be to assume that general relativity accurately describes the nature of space-time , and quantize it directly, and moreover, do it in such a way that the infinities and inconsistencies that appear in the equations can be avoided. This was attempted in 1967 by John Wheeler (1911-2008) and Bryce De Witt (1923-2004), who proposed a sort of time-independent Schrödinger equation that did not describe the evolution of any particle, but that of the metric itself that defines space-time. The Wheeler-De Witt equation was considered unsolvable until the 1980s. The first solution would be the result of the work of Amitabha Sen, in 1982; Abhay Ashtekar (1949), in 1986, and Ted Jacobson, Carlo Rovelli (1956-) and Lee Smolin (1955-), in 1988, who thus introduced loop quantum gravity. In this approach, the quantum states would not be found in space-time, but would be space-time itself expressed through a series of mathematical connections – the loops – that would form a network of interactions. The success of loop quantum gravity lies in the fact that it does not need to modify any of the fundamental principles of quantum mechanics or general relativity, in addition to the fact that it would predict phenomena such as Hawking radiation or the entropy of black holes. It could also be demonstrated experimentally, since it establishes that the value of the speed of light would depend, very slightly, on the energy of the photons, in such a way that the most energetic ones would move more slowly —and vice versa. — due to its propagation through the grainy structure of space-time.
“All that is or was or will ever be”: the universe
We will try to make that measurement, for example, in bursts of gamma rays that arrive from the universe. However, at the moment it is a three-dimensional theory that we do not know if it can be extended to the four that general relativity uses and does not solve the problem of the nature of time in which the quantum and relativistic visions also contradict each other.
All of physics throughout history has been structured around time or, rather, the changes we observe with respect to it. So it is striking that loop quantum gravity doesn’t need it, and if time turns out not to be needed in a final theory describing the universe, what is its role? What is it really? Why do we see it always flowing in the same direction? What relationship exists between time and ourselves? John Wheeler would say in 1986, in an article for American Scientist in which he raised similar questions, that «unveiling the deep and hidden connection between time and existence […] is a task for the future». Perhaps in it is the key to unraveling this mess.
At the moment, we are taking only the first steps in the direction that equations and experiments point us to, but we are only at the beginning of the search for a theory of everything that explains, without resorting to even more fundamental principles. , the workings of, as Carl Sagan would say, “everything that is or was or ever will be”: the universe.
Gisela Baños is a theoretical physicist from the University of Leipzig. Professor of the Department of Human Sciences at IMMUNE Technology Institute. Scenio promoter and science, technology and science fiction streamer on Twitch.
