The man who challenges Einstein

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Gravity is an illusion. Dark matter does not exist. And the universe works like a hologram. Erik Peter Verlinde, a theoretical physicist at the University of Amsterdam, came to these radical conclusions after a six-year study, whose ideas on gravity are radically different from those accepted, which refer to Newton and Einstein.

Risky statements? Provocations? The European Research Council does not think so, the most important institution of the European Union in the field of science, which has funded the research with over 2 million euros. And from astronomy the first results come in favor of Verlinde. There are already those who acclaim him as the Einstein of our day.

Focus asked him to explain his revolutionary interpretation of the universe.

Cause or effect. "Mine is a new theory of gravity, " says the Dutch physicist. «For Isaac Newton was the force with which two bodies with mass, for example the Earth and the Moon, attract each other. Instead the general relativity of Albert Einstein describes gravity thanks to geometry: a mass deforms space-time in its vicinity and therefore, for example, the Moon orbits the Earth because it moves along this curvature ». A bit like a marble follows the track drawn on the sand.

The two different visions are united by the fact that they consider gravity a fundamental force. Instead Verlinde believes that gravity is … an illusion. Attention, this does not mean that it is not real, otherwise the proverbial apple would not fall into our heads! But it would no longer be the cause of the movements of stars and planets, but an effect: "We think of temperature. Considered for a long time a fundamental greatness, it was then understood that it derives from the movements of each of the myriad of atoms and molecules that compose us ». But by touching a teapot full of boiling water we feel the heat of the container (and maybe we burn ourselves), without perceiving the motions of the individual particles. Similarly, according to Verlinde, gravity is the product of microscopic processes.

Information. What's underneath then? The answer lies in one of the most avant-garde concepts of contemporary physics: quantum bits . To understand what it is we need to start from the "bits", that is those sequences of 0 and 1 that are the basic unit of information for computers and telecommunications (in fact the speed of our internet connection is measured in bits per second). Elementary particles follow the rules of quantum mechanics, which for example allow them to be simultaneously in different places and states (a sort of "ubiquity", see Focus n ° 293). Therefore also their characteristics must be expressed in terms of quantum bits, or qubits, with which one can perform operations inaccessible with the classic bits (see Focus 295).

Verlinde has developed a complex mathematical treatment to analyze the transmission of qubits at the microscopic level. "The information associated with the matter and its position influences each other, " he explains. «At our human scale we do not perceive the flow of information, which changes moment by moment, but only the result, that is, material that changes position over time. Here is how gravity emerges ». This vision brings with it another oddity: the cosmos can be described as a huge hologram, that is, a two-dimensional representation of a 3D object. Verlinde, in fact, expresses the universe and the relationships between the particles that compose it in terms of qubits arranged on a plane: in practice, it is like saying that the cosmos can be transcribed into a huge sheet.

Dark. The theory is bold, but it has a strong point: it explains in a natural way the astronomical observations without calling into question the dark matter, now considered necessary by the majority of scholars to explain certain phenomena. For example, in a galaxy the stars are thicker towards the center. Therefore one would expect that, in the peripheral areas, the force of gravity becomes weaker and the speed with which the stars orbit around the center decreases, because they are less attracted (see drawings below ).

The article continues after the drawings

that explain how dark matter would act

Image The problem according to Newton and Einstein. In a "normal" galaxy, that is, without dark matter, the speed at which the stars orbit around the galactic center, from a certain distance onwards, must steadily decrease as you go towards the periphery. Thus do the planets of the Solar System, whose orbital velocities decrease with distance from the Sun. | Image Observed behavior. Instead, in most spiral galaxies, the orbital velocity of the stars, in regions far from the center, first decreases, but then, moving further away, remains approximately constant. As if around there was invisible matter that "pulls" the stars on its own orbits. | Image The possible solution hypothesizing dark matter. To date, dark matter (if it exists) is not known of what it is composed of. The hypotheses speak of particles not yet discovered, called Wimps.
The presence of a halo of dark matter would allow us to explain why the stars close to the galactic nucleus (in the green area of ​​the drawing) behave in a "normal" way while those more distant (in the blue region) orbit at a speed greater than that foreseen by the laws of the dynamics of Newton and those of relativity of Einstein: it is the force of gravity exerted by the dark matter to pull the stars of the galactic periphery. Those towards the center, on the other hand, are too thickened together to be affected. |

This is the case in the Solar System, where the planets far from the Sun move more slowly than the neighboring ones. But the observations say that beyond a certain distance from the galactic center the orbital velocity of the stars does not drop, as if there were a large amount of invisible matter capable of maintaining high gravity and giving a "boost" to the stars.

A similar argument applies to galaxy clusters, where the sum of the gravity due to the individual component galaxies is less than that required to hold them together in clusters ( see photo below ). This is why astrophysicists have introduced dark matter, so called because it does not emit light or other electromagnetic radiation, but only reveals its presence thanks to its gravitational effects.

A question of scale. Everything's fine, then? Not at all, because despite decades of research it has not yet been understood what dark matter is made of. And since it would be almost six times more abundant than normal, it means that we don't know what forms 85% of the matter in the universe! Verlinde solves the problem at the root: «My latest calculations show that gravity changes considering increasingly larger dimensions. At the scale of the universe as a whole it assumes precisely the characteristics that explain the movement of the stars in the galaxies and the structure of the clusters. So there is no longer any need to artificially introduce dark matter, which simply does not exist ".

And that's not all: the variations of gravity are associated with an energy that has properties similar to the other great protagonist of modern cosmology, dark energy, hypothesized to explain why the cosmos is expanding in an accelerated way. "Even dark energy has been added ad hoc to explain the observations. Instead in my theory it results spontaneously from the calculations », emphasizes Verlinde.

Image The Bullet Cluster cluster of galaxies, 4.5 billion light years away. In blue, where dark matter would be found. |

Trials are sought. At this point one might wonder if there is a way to verify such bold theses. A group led by Margot Brouwer, from the University of Leiden in the Netherlands, tried it and studied 33, 000 galaxies that produce gravitational lenses. This phenomenon occurs because the mass of a galaxy curves the surrounding space so that the light emitted by another that is behind (and therefore further away) can "bypass" it and reach our telescopes. In fact, the nearest galaxy concentrates the light of the most distant one towards us, like a lens. This effect is already described by relativity. But the deviation of light rays predicted by Einstein's theory matches the values ​​measured by Brouwer and his colleagues only assuming, again, the existence of a right amount of dark matter. The data are in good agreement with what was predicted by Verlinde. The interested party, however, does not sing victory: "It will still take years of work for my theory to explain the large-scale structure of the universe with sufficient precision."

What's coming up? Recent observations made with the Very Large Telescope in Chile suggest that about 10 billion years ago the galaxies, then very young, were devoid of dark matter: the speed of rotation of the stars farthest from the center dropped dramatically, instead of remaining constant. The dark matter would be added later, and this "latecomer" behavior surprised everyone: at the moment there is no clear explanation.

There is another novelty. A few months ago, two research teams identified the possible traces of a hypothetical particle, the "sterile neutrino", which could be one of the constituents of dark matter, in the data of the Chandra NASA and Xmm-Newton satellites of ESA. Meanwhile, the advanced version of Virgo, the European gravitational wave hunter envisioned by Einstein, is coming into action and will give indications on the abundance in the universe of black holes, other candidates for dark matter. Who knows that Verlinde is not forced to revise his ideas thanks to a satellite with the name of Newton and a tool designed to verify Einstein's theory. The challenge of gravity is still open.

Andrea Bernagozzi for Focus