Gravity is an omnipresent force. Common but, at the same time, extraordinarily mysterious: irrepressible universal attraction, similar to love, to deep friendship. It is the only one that can mutate, as if by magic, becoming pure geometry, the curvature of our space-time. However, its definitive understanding is up in the air.
It is the most familiar of the four fundamental forces, the one with which we fight from birth (until we manage to stand up and start walking), but the most difficult to understand in scientific terms. Even more so than electricity, magnetism and the spectacular nuclear forces.
Current theories about gravity, despite their amazing successes, are called into question. For many reasons. The most recent: clear indications that the expansion of the universe is accelerating non-constantly, which requires a major revision of the general theory of relativity.
Gravity invades everything
Subtly, since it is the weakest force, but the only one that cannot be shielded. In 1687, Isaac Newton dressed it in a party dress, declaring it a universal law. They say that one day, in her garden, she noticed an apple falling on the grass. And he realized that the force with which the Earth attracted the apple would be the same with which it trapped the Moon, forcing it to spin. And the same would happen with the Sun and its planets; and with all heavenly objects. ¡Eureka!
According to his acclaimed law of universal gravitation, all existing objects attract each other, with a force proportional to the product of their masses and inversely to the square of the distance between them.
Beyond Newton
To the surprise of the world, Albert Einstein was able to go much further. In 1915, he gave birth to a wonderful theory, although absolutely incomprehensible, schizophrenic, but irreverently perfect! He demonstrated that gravity can mutate, as if by magic, becoming pure geometry, the curvature of space-time.
How could a human being conceive such a theory? After a hundred years, 99.99% of the population still does not understand it. But the most precise observations and experiments are stubborn in confirming that it is true to the universe.
Or it was, until recently. Gravity is in crisis today!
Einstein: “My theory is not definitive”
If Einstein were still alive, such extraordinary news would not have caused him the slightest surprise. Yes, it would have done so to see that his theory had survived intact to this day. Already when formulating it, he made it very clear that it was approximate, that it was not definitive: “Others will improve it soon,” he said. Even in that he went further than Newton, who always believed that his law of universal gravitation would be eternal.
In order to understand what is now happening with gravity we must step back a bit. There are two types of forces: contact and distance. Gravitation, along with the rest of the fundamental interactions (electromagnetic and nuclear: strong and weak), acts at a distance.
For Newton, action at a distance constituted an unfathomable mystery. He never made assumptions about how gravity was transmitted from one body to another. His phrase is famous “and I don’t make up hypotheses” (“I don’t invent hypotheses”).
How to explain action at a distance?
A way was found, accepted by all: Faraday and Maxwell formulated the concept of force field for electromagnetism and Einstein, for gravity.
The gravitational field is omnipresent: every object creates one around it. Not just a planet, a galaxy or a black hole. An ant or a microbe also creates it, although they are insignificant, almost impossible to measure.
But, Einstein said more: in his theory, the masses and the gravitational field remain replaced by geometry, by the curvature of space-time. Einsteinian gravity is in fact a theory of space-time.
It might seem like everything is resolved. But this wonderful theory is not useful for studying atoms (Einstein never intended to do so). There it is no better than Newton’s gravity: we are fully entering quantum territory.
To enter this nanoworld, gravity would have to be quantified; just as was already done, and with great success, with the other three fundamental forces. But, no matter how much it has been tried, it does not stop.
What the hell is wrong with her?
Seekers of the holy grail
The holy grail of theoretical physics, pursued for decades, is the unified theory of all the forces of nature, sometimes called “the theory of everything.” Einstein failed in his desperate attempts to find it, due to his rejection of orthodox interpretations of quantum mechanics and his indifference to quantum field theory.
Other prominent physicists (the list would be long) have managed to get closer to the goal, unifying electromagnetism and nuclear forces, at very high energies: those that reigned in the universe shortly after the Big Bang. But gravity refuses to enter the fold. The alternatives to achieve this (superstrings, loop theories), although very powerful, remain unrealized. Perhaps because it is very different in nature from the other fundamental forces, despite interacting, like them, at a distance.
Einstein already discovered his escapist nature. And Ted Jacobson, in 1995, found his close kinship with thermodynamics: he derived Einstein’s very equations from thermodynamic equations. There goes that! Reasoning that we extended to modified gravity. So there are even doubts that gravity is a fundamental force at the level of the other three. It could be an emerging phenomenon.
And what about dark energy?
Cosmologists know from experience that dark energy is difficult to explain within Einstein’s theory. In it, the accelerated expansion of the universe can only be obtained based on a universal constant: the famous cosmological constant. Which implies that cosmic acceleration should have remained constant throughout its evolutionary history. And that does not seem to be the case, which would force the theory to be modified. The problem of dark matter could also be solved this way.
Einstein already realized that he had had to truncate his equations to order two. That he hadn’t been able to go any further. And it happens that both the path that leads towards total relativity and the one that could lead us to the quantization of the gravitational field, involves adding more fuel to the fire: by adding terms of curvature of space-time. But wich ones? That’s where we are.