“A new fashion has emerged in physics,” Albert Einstein complained in the early 1930s.
That “fashion” was nothing less than physics or quantum mechanics. Its mere existence endangered the theory of general relativity, Einstein’s greatest creation, published in 1915.
“If all this is true, then it means the end of physics,” the famous scientist even said. The thing is quantum physics and general relativity are incompatible.
Almost 100 years have passed and neither of the two theories has canceled the other. In fact, both are the pillars of all advances in modern physics.
Quantum physics has proven time and again to be the best explanation for the behavior of the smallest particles in the universe.like the electrons, gluons and quarks that make up atoms.
For its part, General relativity, which is the modern theory of gravity, has proven to be the best description of everything that happens on a large scale.from the functioning of the Solar System and black holes to the origin of the universe.
And yet, both remain contradictory to each other. That is, the rules of general relativity work perfectly at the level of galaxies, as well as in everything that surrounds us and is visible: a tree, a cat, a pearl. But as soon as we zoom in and analyze the behavior of something as small as an atom, everything changes.
Researchers can’t even use the same mathematics to explain one theory and the other. Nature somehow manages to make the two coexist, but science does not.
This incompatibility is for many the biggest unanswered question in physics.
Einstein and tens of thousands of researchers around the world have sought to create a theory that unites quantum physics with general relativity.
It is what many call the theory of everythinga name so attractive that it was the title of the award-winning biographical film of Stephen Hawking, one of the renowned scientists who tried—also unsuccessfully—to find the holy grail of physics.
Now a new theory proposes a radical turn in this centuries-old search.
His name is less marketing: It is called post-quantum theory of classical gravity and is led by physicist Jonathan Oppenheim, from the Institute of Quantum Science and Technology at University College London (UCL).
It is so revolutionary that even some of its detractors recognize that it is the first truly original approach to emerge in at least a decade.
The fourth fundamental force
Although it may sound contradictory, one of the most groundbreaking aspects of Oppenheim’s theory is the “classical” part of its name.
Until now, the predominant approach to solving the incompatibility between quantum physics and general relativity has been to try to modify the latter to make it fit the former.
Is what physicists call “quantizing,” because it becomes a quantum theory.
“Quantizing” general relativity makes even more sense if you think that it is something that scientists have already managed to do with the other three fundamental forces that govern the universe: the weak nuclear force, the strong nuclear force, and the electromagnetic force. They just haven’t achieved it with gravity and not for lack of trying..
“It is a very difficult mathematical problem,” Oppenheim tells BBC Mundo. “But it is also conceptually difficult, because These two theories have such fundamental differences that it is very difficult to reconcile them.”.
And he explains: “Almost all attempts have assumed that we must ‘quantize’ gravity. My feeling about why that task has been so difficult is that perhaps it is not possible and that perhaps we are aiming at the wrong thing”.
That is why he and his team decided to change the focus and “modify quantum theory a little, or a lot, so that these two systems can fit together.”
In his theory, published in December 2023 in the magazines Nature Communications and Physical Reviewgeneral relativity remains a non-quantum or classical theory.
Physicist Sabine Hossenfelder from the Center for Mathematical Philosophy in Munich, who is not part of the UCL research, tells BBC Mundo: “It is a very cool. It is very rare in this field to see a new idea born”.
She had been part of a committee that reviewed the theory 6 years ago and, while she found it interesting, she ruled that it was “very speculative, immature and vague.”
“I had so many loose ends that it seemed like I might fail completely, so I was very impressed when I saw what came out several years laterbecause it addressed almost all of those points,” says Hossenfelder, clarifying with a smile: “Although I always have something to complain about.”
Two basic concepts and one “unacceptable”
Before continuing with Oppenheim’s theory, it is important to understand the basic concept of general relativity and one of the characteristics of quantum physics that most disturbed Einstein.
What Einstein did to revolutionize science in 1915 was define gravity as a warp of space-timeas they say.
The easiest way to imagine it is to think of a trampoline where we put a heavy ball, for example, a billiard ball. The fabric then sinks into the place where the ball is.
Now we throw a lighter one, like a marble, trying to make it spin around the edge of the trampoline. What happens is that it moves in smaller and smaller circles, getting closer to the billiard ball.
According to general relativity, This does not happen because the billiard ball exerts an invisible force of attraction, but because the shape of the fabric—or better, its deformation—forces it to make that curvature..
In Einstein’s theory, space-time does the same thing in a four-dimensional way so that, for example, the Earth rotates around the Sun.
Oppenheim explains that in the post-quantum theory of classical gravity “space-time remains that fabric in which quantum particles live, just as Einstein conceived it.”
What changes is that space-time incorporates the chance of quantum physics, that characteristic that gave rise to one of Einstein’s most famous phrases: “God does not play dice”.
Einstein believed that this “fad” of quantum physics lacked information, but what decades of studies have shown is that the randomness is not due to an error in the theory or a flaw in the measurements, but to an inherent characteristic of the behavior of elementary particles.
Oppenheim and his team They bring together quantum physics and general relativity by making space-time inherently random as well..
“We still have this randomness in quantum theory, but it is mediated by space-time itself,” explains the physicist. That is, the fabric itself begins to have random fluctuations.
This is something “unacceptable” to many of his colleagues. And it is likely that it was for Einstein too..
“The random structure of space-time is what, in a sense, is rolling the dice in quantum theory,” Oppenheim says, paraphrasing Einstein.
“Win-win”
“Every time you propose a theory you must do a series of checks to see if it is consistent with observations. What’s exciting is that This theory makes predictions that can be tested experimentally.”explains Oppenheim.
“Taking into account that this theory requires space-time to have fluctuations, we can go look for them,” he adds.
For this, the researchers propose measure the weight of a mass with extreme precision and see if it is constant or if it has certain fluctuations.
For example, France’s International Bureau of Weights and Measures routinely weighs an object of exactly one kilo that was used to create the world standard for what a kilo is.
Using new quantum measurement technologies, according to the post-quantum theory of classical gravity, the apparent weight of said object would no longer be one kilo and would become unpredictable.
“If you find the fluctuations, then you will prove that the theory is true, and if you don’t find them, you will be able to disprove it,” Oppenheim says, adding that “that’s particularly exciting.”
But there is even more.
Oppenheim says that this new theory could answer another of the great unknowns of modern physics: what are dark matter and dark energy.
To understand their importance, you first have to know what they are not.
All planets, stars and visible cosmic objects are made of so-called normal matter. Together, they are considered to represent 5% of the universe. The rest is still a mystery called dark matter and dark energy.
If the fluctuations are intense enough, Oppenheim explains, “they would be very strong candidates for what we think are dark matter and dark energy. They would explain 95% of the evolution of the universeso they could have a big impact.”
For his part, Hossenfelder highlights that the UCL team developed a completely new mathematical framework for this theory and says that its mere existence could be useful for other purposes.
In short, the history of science is full of unexpected applications. Einstein himself lit the spark of quantum physics, which he renounced until the end of his life.
“If there was something that could confirm that these predictions are true, that would be very interesting and would certainly attract a lot of people to take a closer look at them,” says Hossenfelder.
But the theory has only been published for a year and overturning decades of scientific consensus with Einstein at the helm will not be easy.
Hossenfelder is skeptical, something that — in her opinion — puts her in a “win-win” position: she wins if she is right and she wins if she is wrong, because that would mean that she and all of us are witnessing the birth of a scientific revolution.
*With reporting by Max Seitz.
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