General relativity and quantum mechanics are the two great achievements of 20th century physics . But as Juan Martín Maldacena, an Argentine physicist at the Princeton Institute for Advanced Study, says, “quantum mechanics and gravity are two theories that don’t get along.” And it is that nobody knew how to fit them. Until a great revolution took place in the mid- 1980s : theoretical physicists were convinced that they had found the Theory of Everything. The highly popular string theory had just come into play .
Born in the late 1960s by the Italian Gabriele Veneziano as an attempt to explain the strong force (the force that holds the atomic nucleus together), it was quickly forgotten in the drawer of useless ideas in physics. But between 1984 and 1985 it was discovered that there was a consistent theory to describe the four forces of nature as the product of mysterious one-dimensional objects, strings. Although rather it would be necessary to speak theories, because then there were 5 in operation with different formulations . In 1995 a second revolution came when it was shown that these theories were nothing more than different aspects of a single one, baptized as Theory M: it is as if we were facing a puzzle whose complete aspect we do not know, we only have five pieces and we do not know where are the others
In 1997, the Argentine Juan Maldacena formulated a conjecture, now famous, that gave shape to the holographic principle of Gerard t’Hooft and Leonard Susskind: all the information contained in a volume of space can be represented with a theory that lives on the surface of that space. volume. In other words, we can know what happens inside a room by creating a theory that only takes into account what happens in the walls . Maldacena constructed the first explicit example of this principle in the form of a detailed conjecture and is believed to be a feature of quantum gravity. With this, the Argentine became one of the best paid physicists of the moment.
At its core, the basic idea of string theory is very simple. The entire universe is made of unimaginably small strands of energy called strings , and just as the strings of a violin provide an amazing variety of notes, each subatomic particle is born from one of the vibrational modes of a single string. What has made them so attractive is that they seem to contain within them a theory of gravity that reconciles the incompatibility between general relativity and quantum mechanics. However, it has some important problems that are difficult to solve , among them that it predicts that we live in a world with 10 dimensions – 9 spatial and 1 temporal –. To justify that we do not see 6 of them – our observable universe has three spatial and one temporal – they postulate that they are “coiled” on themselves. That is why theorists say that our observable universe is a 4-dimensional “sheet” – brane – embedded in a 10-dimensional universe.
However, there are physicists who think that the strings are pure speculation; others are more direct and say that it is a waste of time. Sheldon Glashow, Nobel laureate in Physics for his work on the unification of the electromagnetic and weak nuclear forces –responsible for some subatomic disintegrations–, said of the strings: “no experiment can ever confirm what is happening. Theory is always safe. Is that a physical theory or a philosophy? This pioneer in the search for a unified theory focuses on the biggest problem with string theory: the experimental verification of its predictions. No particle accelerator can reach the energies necessary to set up experiments with which to verify the theory.
One of the great critics of string theory is Peter Woit . Remember that the whole debate about strings resembles the one between two giants of early 20th century physics: Heisenberg and Pauli. Heisenberg said that, except for a few details, he had a wonderful theory of unification of the forces of nature. Pauli gave a seminar in which, parodying his friend’s theory, he began by drawing a large white rectangle and added: “This is to show that I can paint like Titian. I have only omitted the details.” If nobody knows what string theory is really like, “its beauty is the same as Pauli’s painting”, adds Woit.
Hope is the last thing to be lost, and despite the drought of ideas that plagues string theory since the mid-1990s, it is still believed that we will be able to find a complete theory of gravity that works both on the scale of galaxies like the atomic nucleus . The problem is that we are facing an elusive force whose existence raises three uncomfortable questions to which we do not know the answer: Why is it only attractive? It is the only one of the four elemental forces that is not also repulsive. Why can’t it be shielded? Unlike the rest, we cannot build a device that removes it from the environment. Why is it so weak? The only answer to this conundrum is to assume that we really live in a 10-dimensional universe: gravity is the only force that “spreads” across all of them, and so we only see a tiny fraction of its full “power.”
Is string theory the answer? Despite the fact that at the end of the 20th century it was sold as the panacea “explain everything”, today there are fewer and fewer who believe that it is the correct theory; At most, it is an approximation. Almost two decades ago César Gómez, who at that time was director of the Madrid Institute of Theoretical Physics, said: “String theory has been with us for 25 years and we don’t really know what it is.” Today we are practically at the same point and we still don’t know if physics will be able to find a Theory of Everything. If it exists.
