Are Newton’s Laws of Gravitation Wrong: Observation Puzzle Scientists

Findings cannot be explained by classical assumptions.

An international team of astrophysicists has made a puzzling discovery while analyzing certain star clusters. The finding challenges Newton’s laws of gravity, the researchers write in their publication. Instead, the observations are consistent with the predictions of an alternative theory of gravity. However, this is controversial among experts. The results have now been published in the Monthly Notices of the Royal Astronomical Society. The University of Bonn played a major role in the study.

In their work, the researchers examined the so-called open star clusters, which are loosely bound groups of a few tens to a few hundred stars found in spiral and irregular galaxies. Open clusters are formed when thousands of stars are born within a short period of time in a huge cloud of gas. When “ignited”, the galactic newcomers blow away the remnants of the gas cloud. In the process, the cluster expands greatly. This creates a loose formation of several dozen to several thousand stars. The cluster is held together by the weak gravitational forces that act between them.

“In most cases, open star clusters only survive a few hundred million years before dissolving,” explains Prof. Dr. Pavel Kroupa at the Helmholtz Institute of Radiation and Nuclear Physics at the University of Bonn. In the process, they regularly lose stars, which accumulate in two so-called “tidal tails”. One of these tails is trailed behind the cluster as it travels through space. However, the other takes the lead as a spearhead.

Prof. Dr. Pavel Kroupa from the Helmholtz Institute of Radiation and Nuclear Physics at the University of Bonn. Credit: Volker Lannert / University of Bonn

“According to Newton’s laws of gravity, it is a matter of chance in which of the tails a lost star ends up,” explains Dr. Jan Pflamm-Altenburg of the Helmholtz Institute of Radiation and Nuclear Physics. “So both tails should contain roughly the same number of stars. But in our work, we were able to prove for the first time that this is not true: In the clusters we studied, the front tail always contains significantly more stars near the cluster than the back tail.”

New method developed for counting stars

Among the millions of stars near a cluster, it has been almost impossible to tell which belong to its tails – until now. “To do this, you need to look at the speed, direction of motion and age of each of these objects,” explains Dr. Tereza Jerabkova. The co-author of the paper, who did his PhD in Kroupa’s group, recently moved from the European Space Agency (ESA) to the European Southern Observatory in Garching. She developed a method that allowed her to accurately count the stars in the tails for the first time. “So far, five open clusters have been investigated near us, including four of us,” she says. “When we analyzed all the data, we encountered the contradiction with the current theory. The very precise survey data from ESA’s Gaia space mission was indispensable for this.”

In the “Hyades” cluster (top), the number of stars (black) in the front tidal tail is significantly greater than in the back. In the computer simulation with MOND (below) a similar picture emerges. Credit: AG Kroupa/Uni Bonn

The observational data, however, fit much better with a theory that goes by the acronym MOND (“MOdified Newtonian Dynamics”) among experts. “Simply put, according to MOND, stars can leave a cluster through two different doors,” Kroupa explains. “One leads to the back tidal tail, the other to the front. However, the first is much narrower than the second – so it is less likely that a star will leave the cluster through it. Newton’s theory of gravity, on the other hand, predicts that both doors will have the same width.”

Star clusters are shorter than Newton’s laws predict

The team of astrophysicists calculated the expected stellar distribution according to MOND. “The results agree surprisingly well with the observations,” emphasizes Dr. Ingo Thies, who played a key role in the corresponding simulations. “But we had to resort to relatively simple computational methods to do this. We currently lack the mathematical tools for more detailed analyzes of modified Newtonian dynamics.” However, the simulations also matched the observations in another respect: They predicted how long open star clusters would normally survive. And this time period is significantly shorter than what would be expected according to Newton’s laws. “This explains a mystery that has been known for a long time,” Kroupa points out. “Namely, star clusters in nearby galaxies appear to be disappearing faster than they should.”

However, the MOND theory is not uncontested among experts. Since Newton’s laws of gravity would not be valid under certain circumstances, but would have to be modified, this would have far-reaching consequences for other areas of physics as well. “Again, it solves many of the problems that cosmology is facing today,” explains Kroupa, who is also a member of the transdisciplinary research areas “Modeling” and “Matter” at the University of Bonn. The astrophysicists are now exploring new mathematical methods for even more accurate simulations. They could then be used to find further evidence of whether the MOND theory is correct or not.

Reference: “Asymmetrical tidal tails of open star clusters: stars crossing their cluster’s práh challenge Newtonian gravitation” by Pavel Kroupa, Tereza Jerabkova, Ingo Thies, Jan Pflamm-Altenburg, Benoit Famaey, Henri MJ Boffin, Jörg Dabringhausen, Giacomo Pccari, Christian Boily , Hosein Haghi, Xufen Wu, Jaroslav Haas, Akram Hasani Zonoozi, Guillaume Thomas, Ladislav Šubr and Sverre J Aarseth, 26 Oct 2022, Monthly Notices of the Royal Astronomical Society.
DOI: 10.1093/mnras/stac2563

In addition to the University of Bonn, the study involved Charles University in Prague, the European Southern Observatory (

” data-gt-translate-attributes=”[{” attribute=””>ESO) in Garching, the Observatoire astronomique de Strasbourg, the European Space Research and Technology Centre (ESA ESTEC) in Nordwijk, the Institute for Advanced Studies in Basic Sciences (IASBS) in Zanjan (Iran), the University of Science and Technology of China, the Universidad de La Laguna in Tenerife, and the University of Cambridge.

The study was funded by the Scholarship Program of the Czech Republic, the German Academic Exchange Service (DAAD), the French funding organization Agence nationale de la recherche (ANR), and the European Research Council ERC.