The fate of the fifth interaction in physics hangs in the balance

"Look, Mr. Galileo has calculated everything correctly." This conclusion was not based on the most accurate experiment, but it was one of the most spectacular - as it took place on the moon.

In 1971, the cosmonaut of the Apollo 15 mission, David Scott, dropped a feather and a hammer from the same height and discovered that they simultaneously reached the lunar surface. The acceleration imparted by gravity does not depend on the composition or mass of the body, as Galileo assumed in his (apocryphal) experiment with the Tower of Pisa.
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Or does it depend? Fast forward to the first page of The New York Times in January 1986: " Hints at the fifth force in the Universe change the discoveries of Galileo ." The newspaper described the scientific work of the respected journal Physical Review Letters, done by physicist Ephraim Fischbach and his colleagues. It cited evidence that the acceleration imparted by gravity depends on the chemical composition of the object in question. It turned out that gravity was not the same as we thought: according to the authors, its effect is affected by what The New York Times reporter John Noble Wilford called the “fifth interaction”, adding it to the four forces already known to us.



For more than 30 years, many experiments have been carried out, trying to confirm the presence of the alleged fifth force. Despite their extremely high accuracy, none have provided conclusive evidence of its presence. But the search does not stop. Only last year a new seductive hint at the existence of such a force appeared in experiments of nuclear physics, which led to new speculations and unrest.

On a thread hang the fundamental principles of modern physics. Some physicists believe that the fifth force is possible, and even necessary, for the expansion and unification of existing theories today. Others hope that such power will shed light on the mysterious dark matter, outweighing all ordinary matter in the universe. If it exists, says physicist Jonathan Feng of the University of California, Irvine, "this would mean that our attempts to unite certain forces were premature, since now it is necessary to unite with the fifth as well."

But what about the new fundamental interaction, if he has no evidence? The initial motivation was clear in the days of Galileo. Mass can be described in two ways. One is inertia: the mass of an object is resistance to movement, and the greater the mass, the greater the resistance. The other is gravity: according to Newton's law of gravity, the force of attraction experienced by two objects is proportional to the product of their masses, divided by the square of the distance between them. This force causes the falling apple to accelerate. And only if the two definitions of mass are identical, the gravitational acceleration does not depend on the amount of accelerated mass.

But are they identical? If not, then different masses will fall under the influence of gravity at different speeds. The intuitive idea that a large mass should fall faster, inspired people to test long before Galileo. Simon Stevin , a Flemish naturalist, dropped the lead balls from the clock tower in Delft in 1586, and found no difference in the time he needed to reach the ground. Newton himself tested this idea in 1680 by measuring whether the oscillation period of the pendulums of different masses, but of equal length, coincides - and they must coincide if the gravitational acceleration does not depend on the mass. His research was repeated with greater accuracy by the German scientist Friedrich Wilhelm Bessel in 1832. They found no apparent difference.

The idea of ​​the coincidence of inertial and gravitational masses is known as the " weak equivalence principle " (EIT). The question became critical when Einstein formulated his general theory of relativity in 1912-1916, based on the idea that the forces experienced by an object due to gravity do not differ from those experienced by acceleration. If it is not, then GTR will not work.

“The equivalence principle is one of the basic assumptions of GR,” says Stephan Schlamminger, who works in the holy of holies of the world of accurate measurements, at the National Institute of Standards and Technology at Gaithersburg. “And so it needs to be carefully checked. Verification of the equivalence principle is relatively cheap and simple, but the discovery of its violation can have serious consequences. It would be careless not to conduct such experiments. ”

If a POC fails, we will have two options. Either the Newtonian expression for the attraction of two masses (present in GTR for not very large masses) is a bit inaccurate, and it needs to be corrected. Either gravity is fine, but there is a new, fifth interaction affecting it. The fifth interaction would be added to the four already known to us: gravity, electromagnetism, and strong and weak interactions that control the interactions of subatomic particles in the nuclei of atoms. Modified gravity or fifth interaction is the difference, according to Fischbach, semantic.

In any case, says Feng, "there is no reason why the fifth interaction that we had not previously noticed would not exist."

By the time Einstein linked the PSE with his new theory of gravity, this principle was already quite meticulously tested several times. At the end of the 19th century, the representative of the Hungarian nobility, Baron Lorand Etvös , who worked at the University of Budapest, realized that it can be checked with the help of a two-mass balance.

Evesh used torsion scales. He attached two objects to the ends of a pole hanging on a rope. If the weight of the objects is the same - that is, they have the same gravitational mass - then the pole is balanced horizontally. But the masses also experience centrifugal force due to the rotation of the Earth, depending on their inertial mass. If the inertial mass is equivalent to gravitational, then all forces will be balanced, and the pole will not move. Otherwise, the masses will have to deviate from the horizontal due to the rotation of the Earth.

And if the deviation of the two masses is different - for example, if the deviation from the EIT depends on the composition of the mass - then the pole will experience a torque. Even if the rotation is very small, it can be measured, for example, with the help of a beam reflected from a mirror attached to a pole.

But the fact is that the force of gravity on Earth varies depending on the terrain. Our planet is not a flat and homogeneous sphere. Stones have different density, and they exert different gravitational forces on objects. Due to the accuracy of Etosha's experiment, even the presence of nearby university buildings could spoil the results. One way to eliminate this influence was to take measurements in two different orientations of the pole - for example, when it is directed from west to east, and then from north to south. In both positions, the effects of gravity should work the same, but the centrifugal forces will be different - therefore, any deviation from the performance of the EIT will result in a difference in torque at different positions of the pole. This approach is consistent with the overall strategy for conducting experiments with balancers - no need to worry about local effects or the accuracy of measuring absolute values.

Local perturbations may also change over time - even a passing truck can have a small gravitational effect. Researchers had to work on the exclusion of such variables. Even the presence of the experimenter can make a difference. Therefore, the Hungarian scientists were at a respectful distance, while the balancer calmed down, and then rushed headlong into the laboratory to take readings, until he changed positions (the period of his turn was 40 minutes).

Ötvös has built torsion scales so that they become a piece of precise engineering art. At one end of the pole was a standard weight of platinum, and at the other end other materials were fixed. The pole stood on a tripod, capable of turning to adjust its orientation. The turns of the pole were tracked with a telescope and a mirror mounted on the pole. Small temperature drops could distort the apparatus and create parasitic rotation, so the whole structure was enclosed in a closed and isolated room. For greater accuracy, the researchers conducted further experiments in a dark room so that the light did not lead to temperature fluctuations. The device itself was under an awning, insulated with algae.


Feel the disturbance of power: Ewosch's torsional scales were very sensitive to the turning point, which could indicate the presence of a fifth interaction

Hungarian scientists began their experiments in 1889, and did not find any visible rotation associated with deviations from the principle of equivalence among the masses of several different materials with an accuracy of 1 part per 20 million.

So, by the end of the XIX century there was no reason to doubt the POC. But by this time other reasons began to appear. For example, the discovery of radioactivity spoke of the presence of an unknown source of energy inside the atoms. Moreover, Einstein's GTR gave a new look at matter and mass. Everything looked so that the mass could be converted into energy - and also it depended on speed, increasing as the speed of the object approached the speed of light. Mindful of all this, in 1906 the Royal Society of Göttingen in Germany established a prize of 4,500 marks for conducting more sensitive checks on the equivalence principle of "inertia and gravity", offering Ettwös experiments as a reference.

Even Etwos himself could not resist the competition. “He was a world expert on such experiments,” says Fischbach. He and his students Detsso Pekar and Geno Fekete brushed dust from their experiment with torsional weights, and spent thousands of hours checking other materials: copper, water, asbestos, dense wood, and so on. They sent their findings in 1909, announcing an increase in the accuracy of the experiment to 1 part for 200 million. But the full report was published only three years after Etvyos’s death, in 1922. Another of his students, Janos Renner, continued his work and published it in 1935, announcing the verification of EIT with an accuracy of 1 part for 2-5 billion.

Was such accuracy possible then? Physicist Robert Dick, an expert on general relativity, expressed doubts about this, taking up a similar issue in the 1960s. Regardless of whether his criticism was correct, he and his colleagues used more complex torsion scales and achieved accuracy of one part per 100 billion. They managed to do this by measuring the acceleration of test masses not only by the gravity of the Earth, but also by the attraction of the Sun. With this approach, there was no need to disturb the balance by rotating the device: the direction of gravitational attraction itself rotated as the Earth moved in orbit around the Sun. Any deviation from the POC would show changes in the signal, consistent with the Earth’s rotation period of 24 hours, which made it possible to accurately distinguish useful data from spurious signals arising due to local gravitational changes and other factors. Dick and his colleagues did not see any signs of such a deviation: no signs that Newton's law must be corrected by the fifth interaction.

Are physicists satisfied? And when are they generally satisfied?

Fischbach became interested in the fifth interaction after hearing about the experiment conducted by his colleague from Perdue, Roberto Colela and his collaborators in 1975. They tried to detect traces of Newtonian gravity on subatomic particles. Fischbach wondered whether it was possible to conduct such experiments with subatomic particles in a situation where gravity is strong enough for the appearance of relativistic effects, and not just Newtonian ones, which do not exactly describe gravity. Such an experiment could offer a completely new way to test Einstein's theory.

He began to estimate the possibility of using exotic particles of " kaons ", and their antiparticles, antikaons, arising in particle accelerators. Studying the work on kaons done in the Fermilab accelerator, Fischbach began to suspect that a certain new force, sensitive to such a parameter as the baryon number , B, could influence their behavior.

This property of fundamental particles, unlike mass or energy, has no clear, everyday meaning. It is equal to the simple sum of the number of even more fundamental components, quarks and antiquarks that make up the protons and neutrons that are in the nuclei of atoms. But here's the thing: if a new force depends on the baryon number, it should depend on the chemical composition of the materials, since different chemical elements have different numbers of protons and neutrons. More precisely, it would depend on the ratio of the number B to the masses of the constituent atoms. At first glance, this ratio should be constant, since atomic masses are obtained from the sum of protons and neutrons. But in fact, a small part of the total mass of all these components is converted into energy, which binds them together, and varies depending on the atoms. So each element has its own unique relation of B to mass.

Strength, depending on the composition. Isn't that what Etvyos was looking for? Fischbach decided to rewind history and carefully study the results of the experiments of the Hungarian baron. In the fall of 1985, he and his student Karick Talmadge [Carrick Talmadge] calculated the B / mass ratio for the substances used by Ethos. What they found surprised them.

The Hungarian team found extremely small deviations for the measured gravitational acceleration of various substances, but, in the absence of a clear outline of these deviations, they were written off as errors. But when Fischbach and Talmadzh plotted the deviations as a function of the B / mass ratio, they found a straight line indicating the existence of a small repulsion of the masses, reducing their gravitational attraction.


Fischbach, E. The fifth force: A personal history. The European Physical Journal H 40, 385-467 (2015).

The chemical composition of the objects used by Ethosha was not always easy to restore - five different plant species are called the “snakewood” tree, and how to determine the composition of “sheep's internal fat” is not clear at all - but according to their calculations, the interconnection of the values ​​was maintained. In one of the most surprising cases, the deviations for platinum and copper sulfate crystals turned out to be almost the same. It turned out that almost all the properties of these materials (density, etc.) are different, and the B / mass ratios are almost identical.

Fischbach and Talmadge presented these discoveries in their acclaimed article in 1986, with the help of Peter Buck, a postdoc whose German possession allowed him to translate the original report of the Athos team from 1922. The reviewer was Dick, who expressed some doubts, but in the end voted for the publication. Dick later published his work, which stated that the anomalies in the measurements of Ötvösz can be explained by the effect on the device of temperature. But it was still quite difficult to see how these effects would lead to such a convincing correlation with such an exotic property as the baryon number.

After publication, many wrote about the work - not only The New York Times, but also the legendary physicist Richard Feynman. Fischbach, to whom Feynman called home four days after the publication of the work, at first even decided that it was some kind of joke. Feynman was not particularly impressed with the discovery, as he stated to both Fischbach and the Los Angeles Times. But his very reaction to the work already speaks about the impression she made on the persons concerned.

“Given that our work hinted at the existence of a new interaction in nature,” wrote Fischbach, “it may seem surprising that the review process went so smoothly.” It is possible that this smoothness was due to the fact that there were already theoretical and experimental reasons for suspicion about the existence of the fifth interaction.

In 1955, American physicists of Chinese origin, Li Zhendao and Yang Zhenning , who separated the Nobel Prize for working on the interaction of fundamental particles two years later, were interested in the idea of ​​having a new interaction depending on the baryon number, and even used the work of Ettwesh to denote restrictions on its strength. Lee met with Fischbach just a week after the publication of his work and congratulated him on this.

Moreover, in the 1970s, two Australian geophysicists, Frank Stacy and Gary So, very accurately measured the gravitational constant in a deep mine, which determines the ratio of masses and forces in the Newtonian equation of gravitational attraction. They got a value that was very different from that previously obtained in laboratories. It was possible to explain this discrepancy, including by introducing a new force that operated at a distance of several kilometers. Stacy and Taka's measurements were partially inspired by the work of the early 1970s by Japanese physicist Yasunori Fujii, who investigated the possibility of non-Newtonian gravity.

After 1986, the hunting season continued. If the fifth interaction acts at distances of tens and hundreds of meters, it will be possible to detect deviations from the values ​​predicted by Newtonian gravity when objects fall at a high altitude from the surface of the Earth. In the late 1980s, a team from the US Air Force laboratory at Hansky in Bedford, pc. Massachusetts, measured gravitational acceleration using a 600-meter television tower in North Carolina, and reported signs of the existence of a “sixth interaction”, which, unlike the Fishbach repulsion, seemed to increase gravity. But after a thorough analysis of the work, these statements were rejected.

The most thorough studies were conducted at the University of Washington in Seattle by a team of physicists who decided to play with words and because of the sound of the Hungarian surname Eőtvős who took the name Eot-Wash. A nuclear physicist Eric Adelberger took part in their work, who by that time “had become the world's best experimenter in the search for deviations from the predictions of Newtonian gravity,” as Fishbach said. The Eot-Wash team used cutting-edge torsion scales, and took many precautions to eliminate possible artifacts. They found nothing.

One of the most memorable and promising experiments began immediately after the announcement in 1986, and was performed by Peter Tyberger from the Brookhaven National Laboratory in Upton, pc. New York.In his experiment, a hollow copper sphere floated in a container with water over a precipice. In 1987, Tyberger reported that the sphere was constantly moving toward a precipice, where the gravitational pull of the surrounding stone was smaller — that was the behavior one could expect if there was a repulsive force opposed to gravity. And this was the only evidence of the existence of the fifth interaction, published in a famous scientific journal. Why did this experiment lead to this result? No one knows so far. “It is not clear what exactly was wrong in Tyberger’s experiment, and whether there was anything wrong there at all,” wrote Fischbach.

By 1988, Fischbach had already counted 45 experiments searching for the fifth interaction. But after five years, only Tyberger’s experiment showed something similar to her. Making a report in honor of a decade published in 1986, Fischbach admitted that: “At present there is no convincing experimental evidence of any deviations from the predictions of Newtonian gravity. The preponderance of existing experimental data does not correspond to the presence of any new interactions acting on medium or long distances. ”

It seemed, as Fishbach sadly stated, that he had become the discoverer of something non-existent. The general mood was caught by physicist Lawrence Kraus, then working at Yale University, who, in response to the 1986 job, officially sent a play to the Physical Review Letters, in which he allegedly re-analyzed Galileo’s experiments with balls accelerated down the hill, described in the book 1638 “Mathematical evidence concerning two new branches of science related to mechanics and local motion”, and allegedly found evidence of the presence of a “third interaction” (in addition to gravity and electromagnetism). The journal rejected the work, stating the refusal in the spirit of the work itself: on the basis that the six reviews of this work were clearly written by the author himself.

After several decades of universal non-discovery of the fifth interaction, it can be decided that the game is over. But physicists are looking for ways to expand the foundations of their science, and therefore the desire to believe in the existence of the fifth interaction seems to be more and more attractive, and there are more and more reasons for this. “Now you can find thousands of works describing new fundamental interactions that can be the source of the fifth,” says Fischbach. “There is more than enough theoretical motivation.”

For example, later theories trying to expand physics beyond the “standard model” describing all known particles and their interactions offer several possibilities for new interactions, trying to reveal the next layer of reality. Some of them predict the existence of particles capable of working as carriers of previously unknown interactions, just as electromagnetic, strong and weak interactions are associated with particles-carriers, such as a photon.

A group of models that predict deviation from Newtonian gravity is called modified Newtonian dynamics (MOND). They are trying to explain some features of the motion of stars in galaxies, which are usually explained with the help of a hypothetical "dark matter", interacting with the usual only (or almost only) through gravity. There is no evidence of the MOND models yet, but some physicists find them more and more attractive, since an active search for dark matter particles leads nowhere.

In addition, according to Feng, the fifth interaction can help us sort out dark matter. As far as we know, it interacts with ordinary matter only through gravity. But if she suddenly feels the fifth interaction, “this can provide us with a certain 'portal', through which we can finally interact with dark matter not only with the help of gravity, and understand what it represents.”

Moreover, some theories that use more than three measurements that are familiar to us — for example, the most favorite versions of string theory by physicists — predict that at distances up to a millimeter there may exist forces similar to gravity, but significantly superior in strength.

That is the scale scientists are investigating now. And this means measuring forces with extreme precision, operating between small masses separated by very small distances. Three years ago, Fischbach and colleagues took measurements with particles located at distances from 40 to 8,000 ppm. The problem with such measurements is that between such close objects there is already an attracting force that arises due to the Casimir effect . Its nature is the same as that of the van der Waals forces.working at even smaller distances and connecting molecules between themselves. They arise due to the synchronous movement of clouds of electrons in objects, which leads to electrostatic attraction due to the presence of a charge on electrons. The Casimir effect is what van der Waals forces become when objects are divorced far enough — more than a few nanometers — so that the time delay in electron fluctuations plays a role.

Fischbach and his colleagues found a way to suppress the Casimir effect, reducing it a million times, covering the test mass with a layer of gold. They attached a sapphire ball with gold plating and a radius of 1/150 000 mm to the plate, whose movements can be controlled electronically. Then they organized the rotation of a microscopic disk with areas covered with gold and silicon, exactly under the ball. If there is a difference in the interaction between gold and silicon, this should lead to a ball's vibration. Such an effect was not found, from which it follows that it is possible to impose even stronger restrictions on the possible strength of the fifth interaction that depends on the material on a microscopic scale.

Torsion scales can be used in such experiments. Researchers from the Cosmic Ray Research Institute at Tokyo University have used such a device in search of deviations from the standard Casimir effect, due to the fifth interaction. So far, they have found only more stringent restrictions on the strength of this interaction.

In addition to the direct detection of the fifth interaction, it is still possible to find it the way Fischbach originally wanted to do: through high-energy collisions of fundamental particles. In 2015, a team from the Institute for Nuclear Research in Debrecen, Hungary, led by Attila Krasznahorkay, reported unexpected results from the experiment. The unstable form of beryllium atoms, obtained by proton bombarding lithium foil, decays and emits a pair of electrons and their antiparticles, positrons. The number of electron-positron pairs released by the sample at an angle of 140 degrees exceeded the other indicators, which standard theories of nuclear physics cannot explain.

These results were, in fact, ignored, until Feng and his colleagues last year suggested that they could be explained by the appearance in the experiment of a new particle of interaction that quickly decays into an electron and a positron. In other words, this particle can be the carrier of the fifth interaction at a short distance, of several trillionths of a millimeter.

This experience has not yet been reproduced by other researchers, but the findings of Hungarian scientists look reliable. The chances of this being a random statistical fluctuation are small: 1 out of 100 billion. “Moreover, the data perfectly correspond to the hypothesis that takes into account the new particle,” he says. “If it exists, that’s how it can be found.” Schlaminger agrees that the interpretation of Hungarian observations by Feng was “one of the most fascinating things that happened in 2016”.

“We have yet to confirm the fact of the existence of a new particle,” Feng admits, “but such a confirmation would be revolutionary, it would be the greatest discovery in particle physics in the last 40 years.” His theoretical work predicts that his estimated particle is only 33 times as heavy as an electron. In this case, it would be quite simple to get it in collisions of particles - but it is difficult to see. “It interacts very poorly, and we have shown that it would not have been detected in all previous experiments,” says Feng. Perhaps it can be searched at the Large Hadron Collider at CERN.

So the hypothesis about the existence of the fifth interaction is not completely exhausted. It can be said that all observations in fundamental physics or cosmology that cannot be explained through existing theories — through the Standard Model or GTR — should force physicists to reason about new interactions or new types of matter, such as dark matter and dark energy. This is how physics has always worked: when everything else does not fit, you place a new piece on the board and watch how it moves. Of course, we have not yet seen convincing evidence for the existence of the fifth interaction, but no one has also observed direct evidence of dark matter, supersymmetry, or additional dimensions, but we have been looking for them. We have already eliminated many territories where the fifth interaction could occur,but a vast area remained unexplored.


Restrictions on the possible force of the fifth interaction α on large (left) and small (right) scales. The yellow areas show the excluded zones, and the boundary marks refer to individual experiments. The broken lines on small scales show the possible magnitudes of the fifth interaction predicted by different theories.

In any case, the search continues. In April 2016, the European Space Agency launched the French satellite Microscope, which should verify the weak principle of equivalence in space with unprecedented accuracy. It uses two pairs of cylinders inserted into each other in a free fall: one pair is made of the same alloy of platinum and rhodium, the other pair has an outer cylinder made of a lighter titanium-vanadium-aluminum alloy. If the cylinders fall at a speed depending on the material - and deviations from the POC reach 1 share per thousand trillion (which is 100 times less than can be measured on Earth), then they can be determined using electronic sensors.

“The string theory models predict EIT violations on scales smaller than one part out of 10 trillion,” said Joel Berge, a scientist at ONERA, the French aerospace research center responsible for the Microscope project. He says that the mission’s scientific work began last November, and the first results will appear this summer.

Despite all these high-tech experiments, Fischbach continues to return to the experiments with Etwös torsion weights. Then the Hungarians had no theoretical motivation to expect the appearance of the fifth interaction, depending on the material - nothing that could subconsciously incline them to distort the results of their extremely accurate work. And yet, they discovered something like this - not a random variation of results, but a systematic deviation. “I still think, maybe I’m missing something about what they did there,” says Fischbach. - While this remains a mystery.